Assessment of the Effects 

of Conservation Practices on 

Conservation Effects 

Assessment Project 

(CEAP) Cultivated Cropland in the 

FEBRUARY 2011 Chesapeake Bay Region 

100% 70 ws 

80% 

90% 

s 

60

rface flo

70%

 acre

50

ubsu

50% 

60%

cropped

40

(pounds/acre)

loss in s

40%

Percent of 

30

gen 

20% 

30% 20 Average nitro

10% 10 

0% Sediment loss Nitrogen lost 

with runoff 

Phosphorus 

loss to surface 

water 

Nitrogen loss in 

subsurface 

flows 

One or more 

High treatment need Moderate treatment need 

0 

Low Moderate High 

Level of need for additional conservation treatment 

 

 

 

 

Cover photos are by (clockwise from top left) Tim McCabe, Bob Nichols, Lynn Betts, Tim McCabe, USDA 

Natural Resources Conservation Service. 

CEAP—Strengthening the science base for natural resource conservation 

The Conservation Effects Assessment Project (CEAP) was initiated by USDA’s Natural Resources Conservation 

Service (NRCS), Agricultural Research Service (ARS), and Cooperative State Research, Education, and Extension 

Service (CSREES—now National Institute of Food and Agriculture [NIFA]) in response to a general call for better 

accountability of how society would benefit from the 2002 Farm Bill’s substantial increase in conservation program 

funding (Mausbach and Dedrick 2004). The original goals of CEAP were to estimate conservation benefits for 

reporting at the national and regional levels and to establish the scientific understanding of the effects and benefits 

of conservation practices at the watershed scale. As CEAP evolved, the scope was expanded to provide research and 

assessment on how to best use conservation practices in managing agricultural landscapes to protect and enhance 

environmental quality. 

CEAP activities are organized into three interconnected efforts: 

	 Bibliographies, literature reviews, and scientific workshops to establish what is known about the 

environmental effects of conservation practices at the field and watershed scale. 

	 National and regional assessments to estimate the environmental effects and benefits of conservation practices 

on the landscape and to estimate conservation treatment needs. The four components of the national and 

regional assessment effort are Cropland; Wetlands; Grazing lands, including rangeland, pastureland, and 

grazed forest land; and Wildlife. 

	 Watershed studies to provide in-depth quantification of water quality and soil quality impacts of conservation 

practices at the local level and to provide insight on what practices are the most effective and where they are 

needed within a watershed to achieve environmental goals. 

Research and assessment efforts were designed to estimate the effects and benefits of conservation practices through 

a mix of research, data collection, model development, and model application. Duriancik et al. (2008) summarize the 

accomplishments of CEAP through 2007. A vision for how CEAP can contribute to better and more effective 

delivery of conservation programs in the years ahead is addressed in Maresch, Walbridge, and Kugler (2008). 

Additional information on the scope of the project can be found at http://www.nrcs.usda.gov/technical/nri/ceap/. 

1 
 

 

 

 

 

 

This report was prepared by the Conservation Effects Assessment Project (CEAP) Cropland Modeling Team and published by the 

United States Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS). The modeling team consists of 

scientists and analysts from NRCS, the Agricultural Research Service (ARS), Texas AgriLife Research, and the University of 

Massachusetts. 

Natural Resources Conservation Service, USDA 

Daryl Lund, Project Coordinator, Beltsville, MD, Soil Scientist 

Jay D. Atwood, Temple, TX, Agricultural Economist 

Joseph K. Bagdon, Amherst, MA, Agronomist and Pest Management Specialist 

Jim Benson, Beltsville, MD, Program Analyst 

Jeff Goebel, Beltsville, MD, Statistician 

Kevin Ingram, Beltsville, MD, Agricultural Economist 

Robert L. Kellogg, Beltsville, MD, Agricultural Economist 

Jerry Lemunyon, Fort Worth, TX, Agronomist and Nutrient Management Specialist 

Lee Norfleet, Temple, TX, Soil Scientist 

Agricultural Research Service, USDA, Grassland Soil and Water Research Laboratory, Temple, TX 

Jeff Arnold, Agricultural Engineer 

Mike White, Agricultural Engineer 

Blackland Center for Research and Extension, Texas AgriLife Research, Temple, TX 

Tom Gerik, Director 

Santhi Chinnasamy, Agricultural Engineer 

Mauro Di Luzio, Research Scientist 

Arnold King, Resource Conservationist 

David C. Moffitt, Environmental Engineer 

Kannan Narayanan, Agricultural Engineer 

Theresa Pitts, Programmer 

Evelyn Steglich, Research Assistant 

Xiuying (Susan) Wang, Agricultural Engineer 

Jimmy Williams, Agricultural Engineer 

University of Massachusetts Extension, Amherst, MA 

Stephen Plotkin, Water Quality Specialist 

The study was conducted under the direction of Douglas Lawrence, Deputy Chief for Soil Survey and Resource Assessment, Michele 

Laur, Director for Resources Assessment Division, and Wayne Maresch, William Puckett, and Maury Mausbach, former Deputy 

Chiefs for Soil Survey and Resource Assessment, NRCS. Executive support was provided by the current NRCS Chief, Dave White, 

and former NRCS Chiefs Arlen Lancaster and Bruce Knight. 

Acknowledgements 

The team thanks Gregory McCarty, USDA Agricultural Research Service, for comments and suggestions on the cover crop model 

simulation and comments on an earlier version of the report; the Chesapeake Bay modeling team, U.S. EPA Chesapeake Bay 

Program Office, for providing NRCS with point source loadings for the Chesapeake Bay watershed and comments on an earlier 

version of the report; Alex Barbarika, Rich Iovanna, and Skip Hyberg USDA-Farm Service Agency, for providing data on 

Conservation Reserve Program (CRP) practices and making contributions to the report; Harold Coble and Danesha Carley, North 

Carolina State University, for assisting with the analysis of the integrated pest management (IPM) survey data; Dania Fergusson, 

Eugene Young, and Kathy Broussard, USDA-National Agricultural Statistics Service, for leading the survey data collection effort; 

Mark Siemers and Todd Campbell, CARD, Iowa State University, for providing I-APEX support; NRCS field offices for assisting 

in collection of conservation practice data; Dean Oman, USDA-NRCS, Beltsville, MD, for geographic information systems (GIS) 

analysis support; Melina Ball, Texas AgriLife Research, Temple, TX, for HUMUS graphics support; Peter Chen, Susan Wallace, 

George Wallace, and Karl Musser, Paradigm Systems, Beltsville, MD, for graphics support, National Resources Inventory (NRI) 

database support, Web site support, and calculation of standard errors; and many others who provided advice, guidance, and 

suggestions throughout the project. 

The team also acknowledges the many helpful and constructive suggestions and comments by reviewers who participated in the peer 

review of earlier versions of the report, including Lynne Hoot, Maryland Grain Producers Association; Earl F. Hance, Secretary of 

the Maryland Department of Agriculture; Ana Maria Garcia, U.S. Geological Survey; Peter Kleinman, USDA Agricultural 

Research Service; and M. Todd Walter and Zachary M. Easton, Cornell University. 

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Foreword 

The United States Department of Agriculture has a rich tradition of working with farmers and ranchers to enhance agricultural 

productivity and environmental protection. Conservation pioneer Hugh Hammond Bennett worked tirelessly to establish a nationwide 

Soil Conservation Service along with a system of Soil and Water Conservation Districts. The purpose of these entities, now as then, is 

to work with farmers and ranchers and help them plan, select, and apply conservation practices to enable their operations to produce 

food, forage, and fiber while conserving the Nation’s soil and water resources. 

USDA conservation programs are voluntary. Many provide financial assistance to producers to help encourage adoption of 

conservation practices. Others provide technical assistance to design and install conservation practices consistent with the goals of the 

operation and the soil, climatic, and hydrologic setting. By participating in USDA conservation programs, producers are able to— 

 install structural practices such as riparian buffers, grass filter strips, terraces, grassed waterways, and contour farming to reduce 

erosion, sedimentation, and nutrients leaving the field; 

 adopt conservation systems and practices such as conservation tillage, comprehensive nutrient management, integrated pest 

management, and irrigation water management to conserve resources and maintain the long-term productivity of crop and pasture 

land; and 

 retire land too fragile for continued agricultural production by planting and maintaining on them grasses, trees, or wetland 

vegetation. 

Once soil conservation became a national priority, assessing the effectiveness of conservation practices has been important. Over the 

past several decades, the relationship between crop production and the landscape in which it occurs has become better understood in 

terms of the impact on sustainable agricultural productivity and the impact of agricultural production on other ecosystem services that 

the landscape has potential to generate. Accordingly, the objectives of USDA conservation policy have expanded along with the 

development of conservation practices to achieve them. 

This report on the Chesapeake Bay region is the second in a series of regional reports that continues the tradition within USDA of 

assessing the status, condition, and trends of natural resources to determine how to improve conservation programs to best meet the 

Nation’s needs. These reports use a sampling and modeling approach to quantify the environmental benefits that farmers and 

conservation programs are currently providing to society, and explore prospects for attaining additional benefits with further 

conservation treatment. Subsequent reports on cultivated cropland will be prepared for regions shown in the following map. 

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Assessment of the Effects 

of Conservation Practices on Cultivated Cropland 

in the Chesapeake Bay Region 

Contents Page 

Key Findings 6 

Executive Summary 6 

Chapter 1: Land Use and Agriculture in the Chesapeake Bay Region 

Land Use 13
 

Agriculture 13
 

Watersheds 16
 

Chapter 2: Overview of Sampling and Modeling Approach 

Scope of Study 17
 

Sampling and Modeling Approach 17
 

The NRI and the CEAP Sample 18 

The NRI-CEAP Cropland Survey 19
 

Simulating the Effects of Weather 19
 

Chapter 3: Evaluation of Conservation Practice Use—the Baseline Conservation Condition
 

Historical Context for Conservation Practice Use 22 

Summary of Practice Use 22
 

Structural Conservation Practices 23 

Residue and Tillage Management Practices 25
 

Conservation Crop Rotation 28
 

Cover Crops 28
 

Irrigation Management Practices 28
 

Nutrient Management Practices 29
 

Pesticide Management Practices 34
 

Conservation Cover Establishment 36
 

Chapter 4: Onsite (Field-Level) Effects of Conservation Practices 

The Field-Level Cropland Model—APEX 37
 

Simulating the No-Practice Scenario 38 

Effects of Practices on Fate and Transport of Water 43 

Effects of Practices on Wind Erosion 46 

Effects of Practices on Water Erosion and Sediment Loss 47
 

Effects of Practices on Soil Organic Carbon 51 

Effects of Practices on Nitrogen Loss 54 

Effects of Practices on Phosphorus Loss 61
 

Effects of Practices on Pesticide Residues and Environmental Risk 65
 

Chapter 5: Assessment of Conservation Treatment Needs 

Conservation Treatment Levels 70
 

Inherent Vulnerability Factors 73
 

Evaluation of Conservation Treatment 78
 

Chapter 6: Assessment of Potential Field-Level Gains from Further Conservation Treatment 

Simulation of Additional Water Erosion Control Practices 90 

Simulation of Additional Nutrient Management Practices 91
 

Potential for Field-Level Gains 92 

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Chapter 7: Offsite Water Quality Effects of Conservation Practices 

The National Water Quality Model—HUMUS/SWAT 105 

Source Loads and Instream Loads 109 

Conservation Practice Effects on Water Quality 112 

Assessment of Potential Water Quality Gains from Further Conservation Treatment 128 

Summary of Conservation Practice Effects on Water Quality in the Chesapeake Bay Watershed 137 

References 141 

Appendix A: Estimates of Margins of Error for Selected Acres Estimates 143 

Appendix B: Model Simulation Results for the Baseline Conservation Condition for the Four 

Subregions in the Chesapeake Bay Region 147 

Appendix C: Simulation of Full Adoption of Cover Crops in the Chesapeake Bay Region 149 

Documentation Reports 

There are a series of documentation reports and associated publications by the modeling team posted on the CEAP website at: 

http://www.nrcs.usda.gov/technical/nri/ceap. (Click on “Cropland” and then click on “documentation reports and associated 

publications.”) Included are the following reports that provide details on the modeling and databases used in this study: 

 The HUMUS/SWAT National Water Quality Modeling System and Databases 

 Calibration and Validation of CEAP-HUMUS 

 Delivery Ratios Used in CEAP Cropland Modeling 

 APEX Model Validation for CEAP 

 Pesticide Risk Indicators Used in CEAP Cropland Modeling 

 Integrated Pest Management (IPM) Indicator Used in CEAP Cropland Modeling 

 NRI-CEAP Cropland Survey Design and Statistical Documentation 

 Transforming Survey Data to APEX Model Input Files 

 Modeling Structural Conservation Practices for the Cropland Component of the National Conservation Effects Assessment 

Project 

 APEX Model Upgrades, Data Inputs, and Parameter Settings for Use in CEAP Cropland Modeling 

 APEX Calibration and Validation Using Research Plots in Tifton, Georgia 

 The Agricultural Policy Environmental EXtender (APEX) Model: An Emerging Tool for Landscape and Watershed 

Environmental Analyses 

 The Soil and Water Assessment Tool: Historical Development, Applications, and Future Research Directions 

 Historical Development and Applications of the EPIC and APEX Models 

 Assumptions and Procedures for Simulating the Natural Vegetation Background Scenario for Cropland 

 Manure Loadings Used to Simulate Pastureland and Hayland in CEAP HUMUS/SWAT modeling 

 Adjustment of CEAP Cropland Survey Nutrient Application Rates for APEX Modeling 

This report was initially drafted in August 2010, and was distributed to about 20 

researchers and scientists familiar with the Chesapeake Bay for review and comment. 

The report was subsequently modified to address reviewers’ comments and posted on 

the Internet for public review and comment in October 2010. One of the suggestions 

was to add a “what if” simulation on the effects of cover crops. To conduct this 

analysis, the field-level process model was upgraded with improved modeling 

routines for winter plant growth. Use of these revised modeling routines resulted in 

changes to most of the model outcomes, which is why results reported in this final 

report differ from earlier drafts. While most changes were relatively moderate, the 

changes in model routines significantly decreased estimates of acres with a high need 

for additional treatment (critical under-treated acres). Estimates of all under-treated 

acres, however, were relatively unaffected. 

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Assessment of the Effects of Conservation Practices on 

Cultivated Cropland in the Chesapeake Bay Region 

Key Findings 

The voluntary, incentives-based conservation approach is working. Farmers have made good progress in reducing 

sediment, nutrient, and pesticide losses from farm fields through conservation practice adoption throughout the 

Chesapeake Bay region. Most cropland acres have structural or management practices—or both—in place to control 

erosion. Nearly half the cropland acres are protected by one or more structural practices, such as buffers or terraces. 

Reduced tillage is used in some form on 88 percent of the cropland. Adoption of conservation practices has reduced 

edge-of-field sediment loss by 55 percent, losses of nitrogen with surface runoff by 42 percent, losses of nitrogen in 

subsurface flows by 31 percent, and losses of phosphorus (sediment attached and soluble) by 41 percent. 

Opportunities exist to further reduce sediment and nutrient losses from cropland. The assessment of conservation 

treatment needs presented in this study identifies opportunities to contribute to improved water quality in the Bay. 

The study found that 19 percent of cropped acres (810,000 acres) have a high level of need for additional 

conservation treatment. Acres with a high level of need consist of the most vulnerable acres with the least 

conservation treatment and the highest losses of sediment and nutrients. Model simulations show that adoption of 

additional conservation practices on these 810,000 acres would, compared to the 2003–06 baseline, further reduce 

edge-of-field sediment loss by 37 percent, losses of nitrogen with surface runoff by 27 percent, losses of nitrogen in 

subsurface flows by 20 percent, and losses of phosphorus (sediment-attached and soluble) by 25 percent. 

Targeting enhances effectiveness and efficiency. Targeting critical acres significantly improves the effectiveness of 

conservation practice implementation. Use of additional conservation practices on acres that have a high need for 

additional treatment—acres most prone to runoff or leaching and with low levels of conservation practice use—can 

reduce sediment and nutrient per-acre losses by over twice as much as treatment of acres with a low or moderate 

conservation treatment need. 

Comprehensive conservation planning and implementation is essential. The most critical conservation concern 

related to cropland in the region is the need to reduce nutrient losses from farm fields, especially nitrogen in 

subsurface flows. Suites of practices that include soil erosion control and comprehensive nutrient management— 

appropriate rate, form, timing, and method of application—are required to simultaneously address soil erosion, 

nutrient losses in runoff, and loss of nitrogen through leaching. 

Executive Summary 

Overview of the Chesapeake Bay Region

The Chesapeake Bay is the largest estuary in the United States. The Bay is about 200 miles long, and the Bay and its 

tributaries cover about 4,500 square miles of open water. The Chesapeake Bay watershed covers about 68,500 

square miles in parts of six states (Delaware, Maryland, New York, Pennsylvania, Virginia, and West Virginia) and 

the District of Columbia. Agricultural land makes up less than 30 percent of the area of the watershed (10 percent 

cultivated cropland and 18 percent grazing land and hayland). Forest land covers about 59 percent and urban land 

about 8 percent of the region. The balance of the area is in wetlands or is open water. 

Agriculture plays an important role in the economy of the Bay Watershed. The 2007 Census of Agriculture reported that 

there were nearly 84,000 farms in the Chesapeake Bay region and that the value of agricultural sales in 2007 was about 

$9.5 billion—24 percent from crops and 76 percent from livestock. About 51 percent of Chesapeake Bay region farms 

primarily raise crops, about 42 percent are primarily livestock operations, and the remaining 7 percent produce a mix of 

livestock and crops. Most of the farms (74 percent) in 2007 were small operations with less than $50,000 in total farm 

sales. About 7 percent of the farms had total farm sales greater than $500,000. Forty-three percent of the farms in the 

Chesapeake Bay region are smaller than 50 acres. Corn, soybeans, and hay are the principal crops grown. 

Livestock production in the region is dominated by poultry production, followed by dairy. Livestock operations in the 

region produced 10 percent of all poultry and egg sales in the United States in 2007, exceeding $3.7 billion in value. Sales 

of dairy products ranked second in the region at $2.2 billion, representing 7 percent of the U.S. total. Populations of 

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pastured cattle, horses, and ponies are also significant, representing about one-third of the total livestock population in the 

region in 2007. 

Challenges Faced by Agriculture 

Cropped acres in the Chesapeake Bay region have a high vulnerability profile compared to other agricultural regions 

in the country. A comparison of the vulnerability factors in the Upper Mississippi River Basin and Chesapeake Bay 

watersheds related to the loss of sediment and nutrients from cropped acres reveals that per-acre losses are higher in 

the Chesapeake Bay region, on average. Compared to the Upper Mississippi River Basin, the Chesapeake Bay 

region has— 

 higher annual precipitation, averaging 8 more inches per year;
 

 a higher percentage of cropped acres with slopes greater than 2 percent (60 percent compared to 42 

percent); 

 a higher percentage of cropped acres that are highly erodible land (44 percent compared to 18 percent); 

 a higher percentage of cropped acres with soils prone to surface water runoff (23 percent compared to 13 

percent); 

 a higher percentage of cropped acres with a “high” or “moderately high” soil leaching potential (46 percent 

compared to 9 percent); and 

 a higher percentage of cropped acres with manure applied (38 percent compared to 18 percent). 

Because of the higher vulnerability factors, the Chesapeake Bay region has higher per-acre average annual losses of 

sediment, nitrogen, and phosphorus from fields than does the Upper Mississippi River Basin. For the baseline 

conservation condition (based on the 2003–06 farmer survey)— 

 sediment loss from fields averages 1.2 tons per acre in the Chesapeake Bay region compared to 0.9 ton per 

acre in the Upper Mississippi River Basin; 

 total nitrogen loss from fields averages 50 pounds per acre in the Chesapeake Bay region compared to 40 

pounds per acre in the Upper Mississippi River Basin; and 

 total phosphorus loss from fields averages 3.8 pounds per acre in the Chesapeake Bay region compared to 

3.2 pounds per acre in the Upper Mississippi River Basin. 

Overview of Data Collection and Modeling 

The focus of the CEAP Chesapeake Bay study is on the 10 percent of the watershed (4.38 million acres) that is 

cultivated cropland. The study was designed to quantify the effects of conservation practices commonly used on 

cultivated cropland in the Chesapeake Bay region, evaluate the need for additional conservation treatment in the 

region, and estimate the potential gains that could be attained with additional conservation treatment. 

Simulation models were used to estimate the effects of conservation practices that were in use during the period 

2003–06. The National Resources Inventory, a statistical survey of conditions and trends in soil, water, and related 

resources on U.S. non-Federal land conducted by USDA’s Natural Resources Conservation Service, provides the 

statistical framework. 

Information on farming activities and conservation practices was obtained primarily from a farmer survey conducted 

as part of the study. The assessment includes not only practices associated with Federal conservation programs but 

also the conservation efforts of States, independent organizations, and individual landowners and farm operators. 

Farmer Survey Summary

A farmer survey obtained information on the extent of conservation practice use in the Chesapeake Bay region for 

the period 2003–06. The survey results define the “baseline conservation condition.” 

 Structural practices for controlling water erosion are in use on 46 percent of cropped acres, including 63 

percent of the highly erodible land. 

 About 88 percent of the acres have a conservation tillage system in use including no-till (48 percent) or 

mulch till (40 percent). 

 Producers use residue and tillage management practices, structural practices, or both, on nearly all (96 

percent) cropped acres in the region. 

 Appropriate rates of nitrogen application are in use on about 35 percent of the acres receiving nitrogen 

(including manure) for all crops in the rotation. 

 Appropriate timing of nitrogen application is in use on about 54 percent of the acres receiving nitrogen 

(including manure) for all crops in the rotation. 

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 Good nitrogen management practices (rate, timing and method) are in use on about 13 percent of the acres 

receiving nitrogen (including manure) for all crops during every year of production. 

 Good phosphorus management practices (rate, timing, and method) are in use on 17 percent of the acres 

receiving phosphorus (including manure) for all crops during every year of production. 

 While most acres have evidence of some nitrogen or phosphorus management, there is an opportunity to 

enhance existing nutrient management practices on most acres, especially those receiving manure. 

	 Land in long-term conserving cover, as represented by enrollment in the CRP General Signup, consists of 

about 100,000 acres in the region (2 percent of cultivated cropland acres), of which 67 percent is highly 

erodible land. 

Since 2003–06, when the farmer survey was conducted, the six States in the Chesapeake Bay watershed have 

continued to work with farmers to enhance conservation practice adoption. In Maryland, for example, the State 

offered expanded incentive payments for the planting of cover crops starting in the 2008–09 growing season. State 

and Federal programs in the region have expanded incentives for manure incorporation, use of variable rate 

applications, side-dressing of nutrients, and other production techniques targeted at reducing losses of nutrients from 

farm fields. In addition, the 2008 Farm Bill increased Federal resources targeted to addressing nutrients and 

sediment in the Chesapeake Bay watershed. As a result, cover crops and nutrient management practices are probably 

in wider use within the watershed than the CEAP survey shows for 2003–06. 

Distribution of Land Uses and Sources of Sediment and Nutrients 

Land use. The majority (59 percent) of the land in the Chesapeake Bay watershed is forested. Pasture and hayland 

cover 18.5 percent of the watershed, cultivated cropland 10.5 percent, and urban land 8.6 percent. (Another 3.5 

percent is in other small land uses, combined with forestland in the figure below.) 

Land Use (exclusive of water) 

10.5% 

5.9% 

12.6% 

8.6%

62.5% 

Cultivated cropland 

Hayland 

Pasture and grazing land 

Urban land 

Forest and other 

Sediment sources. Urban land is the largest source of sediment delivered to rivers and streams, delivering 39.5 

percent of the sediment. Another 23 percent comes from forest land and other minor land uses, and 21.5 percent 

comes from cultivated cropland. These three land use categories—urban land, forest land, and cultivated cropland— 

contribute 84 percent of all sediment delivered to rivers and streams in the watershed. 

Sediment delivered to rivers and streams 

21.5% 

10.1% 

5.9% 39.5% 

23.0% Cultivated cropland 

Hayland 

Pasture and grazing land 

Urban point and non-point 

Forest and other 

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Nitrogen Sources. Urban land is also the largest source of nitrogen delivered to rivers and streams in the watershed, 

contributing 37.7 percent of the nitrogen. Cultivated cropland contributes 30.8 percent, and forest land and other 

minor land uses contributes 13.3 percent. Urban areas, forests, and cultivated cropland together account for nearly 

82 percent of the nitrogen delivered to rivers and streams in the Chesapeake Bay watershed. 

Total nitrogen delivered to rivers and streams 

30.8% 

6.3% 

11.8% 

37.7% 

13.3% 

Cultivated cropland 

Hayland 

Pasture and grazing land 

Urban point and non-point 

Forest and other 

Phosphorus Sources. Over half of the phosphorus delivered to streams and rivers comes from urban land. Cropland 

contributes 28.3 percent and forest land and other minor land uses contribute 6.8 percent. About 85 percent of total 

phosphorus delivered to rivers and streams in the watershed are from urban land, cultivated cropland, and forest 

land. 

Total phosphorus delivered to rivers and streams 

28.3% 

7.4% 

7.4% 

50.1% 

6.8% 

Cultivated cropland 

Hayland 

Pasture and grazing land 

Urban point and non-point 

Forest and other 

Contribution on a per acre basis. On a per-acre basis, loadings of sediment and nutrients from urban land and 

cultivated cropland are disproportionately higher than are loadings from other land uses. The following table 

summarizes the per-acre sediment and nutrient loadings by major land use. Forest land has the lowest per-acre 

contribution, while urban areas have the highest per-acre contribution. 

Per-acre contributions of sediment and nutrients to rivers and streams in the 

Chesapeake Bay region, by land use 

Pounds per acre delivered to 

rivers and streams annually 

Land use Sediment Nitrogen Phosphorus 

Cultivated cropland 897 23.1 1.5 

Hayland 743 8.3 0.7 

Pasture 206 7.4 0.3 

Urban 2,011 34.6 3.2 

Forest and other 160 1.7 0.1 

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Conservation Accomplishments 

Compared to a model scenario with all conservation practices removed, conservation practices in use during the 

period 2003–06 were shown to have— 

 reduced sediment loss from fields by 55 percent; 

 reduced nitrogen lost with surface runoff (attached to sediment and in solution) by 42 percent; 

 reduced nitrogen loss in subsurface flows by 31 percent; 

 reduced total phosphorus loss from fields by 40 percent; 

 reduced pesticide loss from fields to surface water, resulting in a 22 to 24 percent reduction in edge-of-field 

pesticide risk (all pesticides combined) for humans and aquatic ecosystems; and 

 increased the percentage of acres gaining or maintaining soil organic carbon. 

The high losses of nitrogen in subsurface flows result from a combination of incomplete nutrient management and 

the re-routing of surface water runoff to subsurface flows by water erosion control practices on some acres in the 

region. On 17 percent of the cropped acres, nitrogen losses in subsurface flows increase slightly as a result of 

conservation practices. Structural erosion control practices, residue management practices, and reduced tillage slow 

the flow of surface water runoff and allow more of the water to infiltrate into the soil. This re-routing of surface 

water to subsurface flows not only redirects the soluble nitrogen into subsurface flows but also can extract additional 

nitrogen from the soil as the water passes through the soil profile. 

For land in long-term conserving cover (100,000 acres), soil erosion and sediment loss have been almost completely 

eliminated. Compared to a cropped condition without conservation practices, average annual total nitrogen loss has 

been reduced by 88 percent, average annual total phosphorus loss has been reduced by 92 percent, and soil organic 

carbon has been increased by an average of more than 333 pounds per acre per year. 

Potential for Gains Related to Cover Crop Use 

About 4 percent of cropped acres used cover crops during 2003–06 time period used to evaluate conservation 

practices in this study. In recent years, however, the use of cover crops in the Chesapeake Bay region has increased, 

particularly where State incentive programs have been implemented. To estimate the potential for cover crops to 

reduce sediment and nutrient loss from fields in this region, a “what if” scenario was created that simulated the use 

of cover crops on all cropped acres. Results indicate that a hypothetical full adoption of cover crops in this region 

would, compared to the baseline conservation in 2003 to 2006— 

 further reduce sediment loss by an average of 59 percent; 

 further reduce total nitrogen loss (all loss pathways combined) by an average of 19 percent and nitrogen 

loss in subsurface flows by an average of 31 percent; 

 further reduce total phosphorus loss by an average of 32 percent; 

 reduce the acres with a high level of treatment need from 19 percent to 11 percent; and 

 reduce the acres with a moderate level of treatment need from 61 percent to 42 percent. 

Conservation Treatment Needs 

Evaluation of the baseline conservation condition. The adequacy of conservation practices in use for the period 

2003–06 in the Chesapeake Bay region was evaluated to identify remaining conservation treatment needs. Undertreated acres are those where an imbalance exists between the level of conservation treatment and the level of 

inherent vulnerability. Acres with characteristics such as steeper slopes and soil types that promote surface water 

runoff are more vulnerable than other acres to sediment and nutrient losses beyond the edge of the field. In addition, 

acres that are flat with porous soil types are more prone to nutrient losses through subsurface flow pathways. 

Conservation treatment needs for the Chesapeake Bay Region: 

 Nineteen percent of cropped acres (0.8 million acres) have a high level of need for additional conservation 

treatment; acres with a “high” level of need consist of the most vulnerable of the under-treated acres with 

the least conservation treatment and the highest losses of sediment and/or nutrients. 

 Sixty-one percent of cropped acres (2.6 million acres) have a moderate level of need for additional 

conservation treatment; acres with a “moderate” level of need consist of under-treated acres that generally 

have lower levels of inherent vulnerability or have more existing conservation practice use than do acres 

with a high level of need. 

 Twenty percent of cropped acres (0.9 million acres) have no or a low level of need for additional treatment 

and are considered to be adequately treated. 

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The greatest conservation need in the region is complete and consistent use of nutrient management—appropriate 

rate, form, timing, and method of application. In some cases, minor adjustments to an existing nutrient management 

plan are required to bring it up to current standards (590 practice code for Nutrient Management), whereas other 

acres require more extensive adjustments that could also include edge-of-field filters or buffers. While most cropped 

acres have some nutrient management practices in use (see table 9, chapter 3), the study found that 18 percent have a 

high need for additional treatment to better control nitrogen or phosphorus loss from fields, based on practices in use 

during 2003–06. About 62 percent of cropped acres have a high or moderate treatment need to address excessive 

levels of nitrogen loss in subsurface flow pathways, most of which returns to surface water through drainage ditches, 

tile drains, natural seeps, and groundwater return flow. 

Additional soil erosion control is also needed in the region, though the need is less than for nutrient management. 

Based on practice use during 2003–06, 5 percent of cropped acres have a high need for additional soil erosion 

control and 20 percent have a moderate need. 

Simulation of the effects additional conservation treatment. Model simulations demonstrated that a suite of 

practices that includes both soil erosion control and comprehensive nutrient management is required to 

simultaneously address soil erosion and nutrient loss through all loss pathways. In practice, a comprehensive 

planning process is used to identify the appropriate combination of practices needed to address multiple resource 

concerns by taking into account the specific inherent vulnerabilities associated with each field. 

Model simulation demonstrated that treatment of the 810,000 acres with a high need for additional treatment would 

achieve the following gains for the region as a whole compared to the 2003–2006 baseline: 

 Sediment loss from fields would be reduced by 37 percent. 

 Nitrogen lost from fields with surface runoff would be reduced by 27 percent. 

 Nitrogen loss from fields in subsurface flows would be reduced by 20 percent. 

 Total phosphorus loss from fields would be reduced by 25 percent. 

Treatment of all 3.4 million under-treated acres (acres with either a high or moderate need for treatment) would 

achieve the following gains for the region as a whole compared to the 2003–2006 baseline: 

 Sediment loss from fields would be reduced by 87 percent. 

 Nitrogen lost from the field with surface runoff would be reduced by 66 percent. 

 Nitrogen loss from the field in subsurface flows would be reduced by 53 percent. 

 Total phosphorus loss would be reduced by 57 percent. 

The nutrient management used in these scenarios to simulate additional conservation treatment represents 

feasible and proven conservation practices that can be successfully applied using today’s technology. There are, 

however, emerging conservation technologies that have the potential to further reduce nutrient loss from farm 

fields and provide even greater conservation benefits once the technologies become more widespread. These 

include— 

 variable rate technology for precise nutrient application rates and placement methods; 

 nitrogen use efficiency enhancers (time release and ammonia loss inhibitors); 

 drainage water management which reduces late fall and early spring flushes of nitrate-laden drainage water; 

and 

 constructed wetlands that receive surface water runoff from fields prior to discharge to streams and 

rivers. 

Targeting. Not all acres provide the same benefit from conservation treatment. The more vulnerable acres, such as 

highly erodible land and soils prone to leaching, inherently lose more sediment or nutrients, therefore greater benefit 

can be attained with additional conservation treatment. The gains in efficiency by first treating acres with a high 

level of treatment need are demonstrated in the table below using results from the treatment simulations. For 

example, treatment of acres with a high level of treatment need would result in an average reduction in sediment loss 

of 2.3 tons per acre per year for those 810,000 acres. Treatment of acres with a moderate level of treatment need 

would result in an average reduction in sediment loss of only 1.0 ton per acre per year. Treatment of acres with a 

low level of need for additional treatment would result in an average reduction in sediment loss of only 0.3 ton per 

acre per year. 

11 
 

 

 

 

Average annual per-acre reductions in losses from treatment of designated acres 

with additional erosion control and nutrient management 

2.6 million acres Remaining 

810,000 acres with with a moderate 872,000 acres 

a high level of level of treatment with a low level 

Resource concern treatment need need of treatment need 

Sediment loss at edge of field due to water erosion 

(tons/acre) 2.3 1.0 0.3 

Total nitrogen loss for all pathways (pounds/acre) 53 26 2 

Loss of nitrogen with surface runoff, including 

waterborne sediment (pounds/acre) 13 6 2 

Loss of nitrogen in subsurface flows 

(pounds/acre) 35 17 <1 

Total phosphorus loss for all pathways 

(pounds/acre) 4.9 2.0 0.3 

Effects of Conservation Treatment on Water Quality in the Chesapeake Bay 

Reductions in field-level losses due to conservation practices, including land in long-term conserving cover, 

translate into improvements in water quality in streams and rivers in the region. Transport of sediment, nutrients, 

and pesticides from farm fields to streams and rivers involves a variety of processes and time-lags, and not all of the 

potential pollutants leaving fields contribute to instream loads. 

What agriculture accomplished through 2006. When considered along with loads from all other sources, 

conservation practices in use on cultivated cropland in 2003–06, including land in long-term conserving cover, are 

responsible for reducing total loads delivered to the Bay (all sources) by— 

 10 percent for sediment, 

 14 percent for phosphorus, and 

 14 percent for nitrogen. 

Opportunities for additional reductions in sediment and nutrient losses. If all the under-treated acres (3.4 million 

acres with either a high or moderate need for treatment) were fully treated with the appropriate soil erosion control 

or nutrient management practices, total loads delivered to the Bay (all sources) would be reduced from baseline 

levels by— 

 6 percent for sediment (bringing loads from cultivated cropland down very close to “background levels”), 

 12 percent for phosphorus, and 

 15 percent for nitrogen.
 

Background levels represent loads that would be expected if no acres in the watershed were cultivated and were 

derived by running an additional model scenario that simulated a grass and tree mix cover without any tillage or 

addition of nutrients or pesticides for all cultivated cropland acres in the watershed. Background loads include loads 

from all other land uses—hayland, pastureland, forestland, and urban land—as well as point sources. 

12 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Chapter 1 

Land Use and Agriculture in the 

Chesapeake Bay Region 

Land Use 

The Chesapeake Bay region covers about 68,500 square miles 

and includes parts of New York, Pennsylvania, Maryland, 

Delaware, Virginia, and West Virginia. The majority of the 

land cover in the Chesapeake Bay region is forest land, which 

covers about 59 percent of the region (table 1 and fig. 1). The 

forests consist primarily of deciduous trees with conifers and 

mixed stands in some areas. Pastureland and hay land make up 

about 18 percent of the land cover in the region. About 10 

percent of the area is used for crop production. About 6 

percent of the area is in water and wetlands. 

Urban areas make up about 8 percent of the area. The major 

metropolitan areas are Washington, DC; Baltimore, MD; 

Richmond, VA; Norfolk VA; and Harrisburg, PA. 

. 

Table 1. Distribution of land cover in the Chesapeake Bay 

region 

Land use Acres* Percent 

Cultivated cropland and land enrolled 

in the CRP General Signup 4,588,332 10 

Forest deciduous 19,106,747 44 

Hay/Pasture not in rotation with crops 7,738,805 18 

Urban 3,651,000 8 

Water 1,152,262 3 

Wetland forested 793,516 2 

Range grasses 142,690 <1 

Wetland non-forested 517,632 1 

Forest evergreen 2,999,538 7 

Forest mixed 2,421,677 6 

Range brush 266,807 1 

Horticulture and barren 473,994 1 

Total 43,853,000 100 

Source: 2001 National Land Cover Database for the Conterminous United 

States (Homer et al., 2007).
 

*Acreage estimates for cultivated cropland differ slightly from those provided 

elsewhere in this report because of differences in sources and methods.
 

Agriculture

The 2007 Census of Agriculture reported 83,775 farms in the 

Chesapeake Bay region, about 4 percent of the total number of 

farms in the United States (table 2). Farms in the Chesapeake 

Bay region make up about 1 percent of all farmland in the 

nation. According to the 2007 Census of Agriculture, the value 

of Chesapeake Bay region agricultural sales in 2007 was about 

$9.5 billion—24 percent from crops and 76 percent from 

livestock. 

About 51 percent of Chesapeake Bay region farms primarily 

raise crops, about 42 percent are primarily livestock 

operations, and the remaining 7 percent produce a mix of 

livestock and crops (table 3). 

Most of the farms (74 percent) in 2007 were small operations 

with less than $50,000 in total farm sales. About 7 percent of 

the farms had total farm sales greater than $500,000 (table 3). 

Forty-three percent of the farms in the Chesapeake Bay region 

are smaller than 50 acres, and 51 percent are between 50 and 

500 acres. Only 6 percent of the farms have more than 500 

acres (table 3). 

Crop production 

The Chesapeake Bay region accounts for about 2 percent of all 

U.S. crop sales (table 2). Corn, soybeans, and hay are the 

principal crops grown. Wheat is an important secondary crop 

in terms of acres harvested. 

Farmers in the region produced 2 percent of the corn harvested 

for grain in the United States in 2007—163 million bushels— 

on about 1.5 million acres. Hay, grass silage, haylage, and 

greenchop were harvested on 2.2 million acres. Farms in the 

region also produced 2 percent of the national soybean crop 

(31.9 million bushels) on 1.1 million acres. 

Commercial fertilizers and pesticides are widely used on 

cultivated cropland throughout the region (table 2). In 2007, 

4.1 million acres of cropland were fertilized, 3.3 million acres 

of cropland and pasture were treated with chemicals for weed 

control, and 1.7 million acres of cropland were treated for 

insect control. About 1.7 million acres had manure applied in 

2007. 

Irrigation is used on some acres to supplement rainfall during 

dry periods. According to the Census of Agriculture, about 4 

percent of the harvested acres were irrigated in 2007. 

Livestock operations 

Livestock production in the region is dominated by poultry 

production, followed by dairy. Livestock operations in the 

region produced 10 percent of all poultry and egg sales in the 

United States in 2007, exceeding $3.7 billion in value (table 

2). Sales of dairy products ranked second in the region at $2.2 

billion, representing 7 percent of the U.S. total. Populations of 

pastured cattle, horses, and ponies are also significant, 

representing about one-third of the total livestock population 

in the region in 2007 (table 2). 

Although 66 percent of the farms in the Chesapeake Bay 

region (55,600 farms) reported livestock sales in 2007, the 

majority are small operations. About 29,000 of these farms 

had fewer than 30 animal units on the farm; a small number of 

these had specialty livestock such as rabbits, bison, mink, or 

deer (table 3). (An animal unit is 1,000 pounds of live animal 

weight.) Pastured livestock (cattle, horses, sheep, or goats) 

predominate on about 14,000 farms; 81 percent of these farms 

raised fewer than 100 animal units in 2007. About 13,700 

farms could be defined as animal feeding operations (AFOs). 

AFOs are typically livestock operations with confined 

livestock, such as poultry, swine, cattle on feed, or dairies. 

Sixteen percent of the farms in the Chesapeake Bay region are 

AFOs, although the bulk of these are relatively small 

operations. Only about 600 of the livestock operations (4 

percent of the AFOs) are large, with livestock numbers in 

2007 above the threshold for a concentrated animal feeding 

operation (CAFO). 

13 
 

 

 

 

 

 

 

 

 

Table 2. Profile of farms in the Chesapeake Bay region, 2007 

Percent of

Characteristic Value national total 

Number of farms 83,775 4 

Acres on farms 12,826,065 1 

Average acres per farm 153 

Cropland harvested, acres 6,027,682 2 

Cropland used for pasture, acres 606,584 2 

Cropland on which all crops failed, acres 73,359 1 

Cropland in summer fallow, acres 39,109 <1 

Cropland idle or used for cover crops, acres 447,020 1 

Woodland pastured, acres 383,612 1 

Woodland not pastured, acres 2,609,960 6 

Permanent pasture and rangeland, acres 1,925,684 <1 

Other land on farms, acres 713,055 2 

Principal crops grown 

 Field corn for grain harvested, acres 1,546,362 2 

 Field corn for silage harvested, acres 551,955 9 

 Soybeans harvested, acres 1,066,151 2 

 Wheat harvested, sum acres 455,516 1 

 Alfalfa hay harvested, acres 436,156 2 

 Grass silage, haylage, and greenchop harvested, acres 308,028 9 

 Tame and wild hay harvested, acres 1,506,020 4 

Irrigated harvested land, acres 240,438 <1 

Irrigated pastureland or rangeland, acres 3,707 <1 

Cropland fertilized, acres 4,103,629 2 

Pastureland fertilized, acres 410,840 2 

Land treated for insects on hay or other crops, acres 1,701,146 2 

Land treated for nematodes in crops, acres 111,728 1 

Land treated for diseases in crops and orchards, acres 267,317 1 

Land treated for weeds in crops and pasture, acres 3,320,537 1 

Crops on which chemicals for defoliation applied, acres 77,940 1 

Acres on which manure was applied 1,716,448 8 

Total grains and oilseeds sales, million dollars 915,631,290 1 

Total fruit and berry sales, million dollars 197,357,734 1 

Total vegetable, melons sales, million dollars 279,696,733 2 

Total nursery, greenhouse, and floriculture sales, million dollars 611,617,415 4 

Total hay other crop sales, million dollars 239,943,957 1 

Total crop sales, million dollars 2,004,303,172 2 

Total dairy sales, million dollars 2,150,033,444 7 

Total hog and pigs sales, million dollars 343,587,556 2 

Total poultry and eggs sales, million dollars 3,711,452,954 10 

Total cattle sales, million dollars 843,470,000 1 

Total sheep, goats, and their products sales, million dollars 15,958,047 2 

Total horses, ponies, and mules sales, million dollars 107,465,458 5 

Total other livestock sales, million dollars 99,358,748 4 

Total livestock sales, million dollars 7,271,326,207 5 

Animal units on farms 

 All livestock types 3,221,469 3 

Swine 187,118 2 

Dairy cows 853,938 7 

Fattened cattle 107,140 1 

Other cattle, horses, sheep, goats 1,229,906 2 

Chickens, turkeys, and ducks 831,182 10 

Other livestock 12,185 3 

Source: 2007 Census of Agriculture, National Agricultural Statistics Service, USDA 

Note: Information in the Census of Agriculture was used to estimate animal units using methods and assumptions described in USDA-NRCS (2003).
 

14 
 

 

 

Figure 1. Land cover in the Chesapeake Bay region 

Source: National Agricultural Statistics Service (NASS 2007). 

Table 3. Characteristics of farms in the Chesapeake Bay region, 2007 

Number of Percent of farms in 

farms Chesapeake Bay region 

Farming primary occupation 39,584 47 

Farm size: 

<50 acres 36,142 43 

50–500 acres 42,795 51 

500–2,000 acres 4,399 5 

>2,000 acres 439 1 

Farm sales: 

<$10,000 45,013 54 

$10,000–50,000 16,754 20 

$50,000–250,000 11,217 13 

$250,000–500,000 4,657 6 

>$500,000 6,134 7 

Farm type: 

Crop sales make up more than 75% of farm sales 42,630 51 

Livestock sales make up more than 75% of farm sales 35,334 42 

Mixed crop and livestock sales 5,811 7 

Farms with no livestock sales 28,187 34 

Farms with few livestock or specialty livestock types 27,751 33 

Farms with pastured livestock and few other livestock types 14,143 17 

Farms with animal feeding operations (AFOs)* 13,694 16 

Source: 2007 Census of Agriculture, National Agricultural Statistics Service, USDA 

* AFOs, as defined here, typically have a total of more than 12 animal units consisting of fattened cattle, dairy cows, hogs and pigs, chickens, ducks, and turkeys. 

15 
 

 

 

 

 

 

 

 

 

Watersheds 

A hydrologic accounting system consisting of water resource 

regions, major subregions, and smaller watersheds has been 

defined by the U.S. Geological Survey (USGS) (1980). In this 

study, the Chesapeake Bay region is represented by four 

subregions within the Mid-Atlantic Water Resource Region. 

Each water resource region is designated with a 2-digit code, 

which is further divided into 4-digit subregions and then into 

8-digit watersheds, or Hydrologic Unit Codes (HUCs). 

The four subasins within the Chesapeake Bay region are 

shown in figure 2, and agricultural land use within each 

subregion is summarized in table 4. The highest concentration 

of cultivated cropland, 24 percent) is in subregion 0206—the 

Upper Chesapeake Bay subregion. The Susquehanna River 

subregion has about 11 percent of the land base in cultivated 

cropland. The remaining two subregions have 5.5 to 6.5 

percent of the land base in cultivated cropland. About threefourths of the cultivated cropland in the region is in the 

Susquehanna River and Upper Chesapeake Bay subregions. 

Estimates presented in this report for field-level effects of 

conservation practices (chapters 3-5) are for the Chesapeake 

Bay region, whereas estimates of instream water quality 

effects (chapter 6) are for the Chesapeake Bay watershed. The 

Chesapeake Bay watershed excludes two 8-digit watersheds in 

the Upper Chesapeake Bay subregion that drain to the Atlantic 

Ocean (8-digit HUCs 02060010 and 02080110). The area that 

includes these two watersheds is referred to as the Chesapeake 

Bay region. 

Figure 2. Percent cultivated cropland, including land in long-term conserving cover, for the four subregions in the Chesapeake Bay 

region 

Table 4. Agricultural land use in the four subregions in the Chesapeake Bay region 

Percent of Hayland 

cultivated not in Pastureland 

Percent Percent of cropland rotation not in 

Total Cultivated cultivated cultivated acres in with rotation 

Sub- land cropland cropland cropland in long-term crops with crops 

region (1,000 (1,000 in Chesapeake conserving (1,000 (1,000 

code Subregion name acres) acres)* subregion Bay region cover acres) acres) 

0205 Susquehanna River 17,596 2,008 11.4 43.8 4.0 1,315 1,438 

0206 Upper Chesapeake Bay 5,773 1,361 23.6 29.7 1.1 54 879 

0207 Potomac River 9,404 612 6.5 13.3 2.0 670 1,566 

0208 Lower Chesapeake Bay 11,080 608 5.5 13.2 4.2 461 1,356 

Total 43,853 4,588 10.5 100.0 2.9 2,500 5,239 

Source: 2001 National Land Cover Database for the Conterminous United States (Homer et al. 2007) and the 1997 National Resources Inventory (USDA-NRCS, 2002). 

* Acres of cultivated cropland include land in long-term conserving cover. Estimates of cultivated cropland were obtained from HUMUS databases on land use, 

differing slightly from acreage estimates obtained with the NRI-CEAP sample. 

16 
 

 

 

 

 

 

 

 

 

Chapter 2 

Overview of Sampling and Modeling 

Approach 

Scope of Study

This study was designed to evaluate the effects of 

conservation practices at the regional scale to provide a better 

understanding of how conservation practices are benefiting the 

environment and to determine what challenges remain. The 

report does the following. 

 Evaluates the extent of conservation practice use in the 

region; 

 Estimates the environmental benefits and effects of 

Conservation practices in use; 

 Estimates conservation treatment needs for the region; 

and 

 Estimates potential gains that could be attained with 

additional conservation treatment. 

The study was designed to quantify the level the effects of 

commonly used conservation practices on cultivated cropland, 

regardless of how or why the practices came to be in use. This 

assessment is not an evaluation of Federal conservation 

programs, because it is not restricted to only those practices 

associated with Federal conservation programs. 

The Chesapeake Bay region has about 4.4 million acres of 

cultivated cropland—4.3 million cropped acres and about 0.1 

million acres in long-term conserving cover. For purposes of 

this report, cultivated cropland includes land in row crops or 

close-grown crops, hay and pasture in rotation with row crops 

and close-grown crops, and land in long-term conserving 

cover. Cultivated cropland does not include agricultural land 

that has been in hay, pasture, or horticulture for 4 or more 

consecutive years. Acres enrolled in the General Signup of the 

Conservation Reserve Program (CRP) were used to represent 

cultivated cropland currently in long-term conserving cover. 

Sampling and Modeling Approach

The assessment uses a statistical sampling and modeling 

approach to estimate the environmental effects and benefits of 

conservation practices (fig. 3). 

	 A subset of 771 National Resources Inventory (NRI) 

sample points provides a statistical sample that represents 

the diversity of soils and other conditions for cropped 

acres in the Chesapeake Bay region. The sample also 

includes 61 additional NRI sample points designated as 

CRP acres to represent land in long-term conserving 

cover. NRI sample points are linked to NRCS Soil Survey 

databases and were linked spatially to climate databases 

for this study. 

	 A farmer survey—the NRI-CEAP Cropland Survey—was 

conducted at these sample points during the period 2003– 

06 to determine what conservation practices were in use 

and to collect information on farming practices. 

	 The field-level effects of the conservation practices were 

assessed using a field-scale physical process model—the 

Agricultural Policy Environmental Extender (APEX)— 

which simulates the day-to-day farming activities, wind 

and water erosion, loss or gain of soil organic carbon, and 

edge-of-field losses of soil, nutrients, and pesticides. 

	 A watershed model and system of databases—the 

Hydrologic Unit Model for the United States 

(HUMUS)—was used to simulate how reductions of field 

losses have reduced instream concentrations and loadings 

of sediment, nutrients, and pesticides within the 

Chesapeake Bay region. The SWAT model (Soil and 

Water Assessment Tool) was used to simulate nonpoint 

source loadings from land uses other than cropland and to 

route instream loads from one watershed to another. 

Figure 3. Statistical sampling and modeling approach used to simulate the effects of conservation practices 

17 
 

 

 

 

 

 

 

 

 

The modeling strategy for estimating the effects of 

conservation practices consists of two model scenarios that are 

produced for each sample point. 

1.	 A baseline scenario, the “baseline conservation condition” 

scenario, provides model simulations that account for 

cropping patterns, farming activities, and conservation 

practices as reported in the NRI-CEAP Cropland Survey 

and other sources. 

2.	 An alternative scenario, the “no-practice” scenario, 

simulates model results as if no conservation practices 

were in use but holds all other model inputs and 

parameters the same as in the current conservation 

condition scenario. 

The effects of conservation practices are obtained by taking 

the difference in model results between the two scenarios (fig. 

4).1

 For example, to simulate “no practices” for sample points 

where some type of residue management is used, model 

simulations were conducted as if continuous conventional 

tillage had been used. Similarly, for sample points with 

structural conservation practices (buffers, terraces, grassed 

waterways, etc.), the no-practice scenario was simulated as if 

the practices were not present. The no-practice representation 

for land in long-term conserving cover was derived from 

model results for cropped acres as simulated in the no-practice 

scenario, representing how the land would have been managed 

had crops been grown without the use of conservation 

practices. 

The approach captures the diversity of land use, soils, climate, 

and topography from the NRI; accounts for site-specific 

farming activities; estimates the loss of materials at the field 

scale where the science is most developed; and provides a 

statistical basis for aggregating results to the national and 

regional levels. 

The NRI and the CEAP Sample

The approach is an extension of the NRI, a longitudinal, 

scientifically-based survey designed to gauge natural resource 

status, conditions, and trends on the Nation’s non-Federal land 

(Goebel 1998; USDA-NRCS 2002). NRCS has previously 

used the NRI for modeling to address issues related to natural 

resources and agriculture (Goebel and Kellogg 2002).2 

1

 This modeling strategy is similar to how the NRI produces estimates of soil 

erosion and the intrinsic erosion rate used to identify highly erodible land. The 

NRI uses the Universal Soil Loss Equation (USLE) to estimate sheet and rill 

erosion at each sample point on the basis of site-specific factors. Soil loss per 

unit area is equal to R*K*L*S*C*P. The first four factors—R, K, L, S— 

represent the conditions of climate, soil, and topography existing at a site. 

(USDA 1989). The last two factors—C and P—represent the degree to which 

management influences the erosion rate. The product of the first four factors is 

sometimes called the intrinsic, or potential, erosion rate. The intrinsic erosion 

rate divided by T, the soil loss tolerance factor, produces estimates of EI, the 

erodibility index. The intrinsic erosion rate is thus a “no-practice” 

representation of sheet and rill erosion, since C=1 represents smooth-tilled 

continuous fallow and P=1 represents no supporting practices.

2

 Previous studies have used this NRI micro-simulation modeling approach to 

estimate soil loss, nutrient loss, and change in soil organic carbon (Potter et al. 

2006), to estimate pesticide loss from cropland (Kellogg et al. 1992, 1994, 

2002; Goss et al. 1998), and to identify priority watersheds for water quality 

protection from nonpoint sources related to agriculture (Kellogg 2000, 

Kellogg et al. 1997). 

Figure 4. Modeling strategy used to assess effects of 

conservation practices 

The NRI sampling design implemented in 1982 provided a 

stratified, two-stage, unequal probability area sample of the 

entire country (Goebel and Baker 1987; Nusser and Goebel 

1997). Nominally square areas/segments were selected within 

geographical strata on a county-by-county basis; specific point 

locations were selected within each selected segment. The 

segments ranged in size from 40 to 640 acres but were 

typically half-mile square areas, and most segments contained 

three sample points. At each sample point, information is 

collected on nearly 200 attributes; some items are also 

collected for the entire segment. The sampling rates for the 

segments were variable, typically from 2 to 6 percent in 

agricultural strata and much lower in remote nonagricultural 

areas. The 1997 NRI Foundation Sample contained about 

300,000 sample segments and about 800,000 sample points. 

NRCS made several significant changes to the NRI program 

over the past 10 years, including transitioning from a 5-year 

periodic survey to an annual survey. The NRI’s annual design 

is a supplemented panel design. A core panel of 41,000 

segments is sampled each year, and rotation (supplemental) 

panels of 31,000 segments each vary by inventory year and 

allow an inventory to focus on an emerging issue. The core 

panel and the various supplemental panels are unequal 

probability subsamples from the 1997 NRI Foundation 

Sample.3 

3

 For more information on the NRI sample design, see 

www.nrcs.usda.gov/technical/NRI/. 

18 
 

 

 

 

 

 

 

 

 

 

 

 

 

The CEAP cultivated cropland sample is a subset of NRI 

sample points from the 2003 NRI (USDA/NRCS 2007). The 

2001, 2002, and 2003 Annual NRI surveys were used to draw 

the sample.4

 The sample is statistically representative of 

cultivated cropland and formerly cultivated land currently in 

long-term conserving cover. Nationally, the NRI-CEAP 

sample consists of about 18,700 NRI points representing 

cropped acres, and about 13,000 NRI points representing land 

enrolled in the General Signup of the CRP. 

The Chesapeake Bay region portion of the NRI-CEAP sample 

consists of 771 sample points representing 4.3 million cropped 

acres and 61 sample points representing 0.1 million acres of 

agricultural land in long-term conserving cover. Table 5 

provides a breakdown of sample sizes for the dominant 

cropping systems that occur in the Chesapeake Bay region. 

About 89 percent of the cultivated cropland acres include corn 

or soybean or both in the crop rotation. 

Nationally, there were over 30,000 samples in the original 

sample draw. A completed farmer survey was required to 

include the sample point in the CEAP sample. Some farmers 

declined to participate in the survey, others could not be 

located during the time period scheduled for implementing the 

survey, and other sample points were excluded for 

administrative reasons such as overlap with other USDA 

surveys. Some sample points were excluded because the 

surveys were incomplete or contained inconsistent 

information, land use found at the sample point had recently 

changed and was no longer cultivated cropland, or because the 

crops grown were uncommon and model parameters for crop 

growth were not available. 

The CEAP sample was designed to allow reporting of results 

for the four subregions (4-digit HUCs) within the region. The 

acreage weights were derived so as to approximate total 

cropped acres by 4-digit HUC as estimated by the full 2003 

NRI. The sample size is too small, in most cases, for reliable 

and defensible reporting of results for areas below the 

subregion level. Acres reported using the CEAP sample are 

“estimated” acres because of the uncertainty associated with 

the statistical sample. Margins of error for estimated acres 

used in this report are provided in appendix A. 

The NRI-CEAP Cropland Survey 

A farmer survey—the NRI-CEAP Cropland Survey—was 

conducted to obtain the additional information needed for 

modeling the 771 sample points with crops.5

 The USDA 

National Agricultural Statistics Service (NASS) administered 

the survey. Farmer participation was voluntary, and the 

information gathered is confidential. The survey content was 

specifically designed to provide information on farming 

activities for use with a physical process model to estimate 

field-level effects of conservation practices. 

4

 Information about the CEAP sample design is in “NRI-CEAP Cropland 

Survey Design and Statistical Documentation,” available at 

http://www.nrcs.usda.gov/technical/nri/ceap. 

5

 The surveys, the enumerator instructions, and other documentation can be 

found at www.nrcs.usda.gov/technical/nri/ceap. 

The survey obtained information on— 

 crops grown for the previous 3 years, including double 

crops and cover crops; 

 field characteristics, such as proximity to a water body or 

wetland and presence of tile or surface drainage systems; 

 conservation practices associated with the field; 

 crop rotation plan; 

 application of commercial fertilizers (rate, timing, 

method, and form) for crops grown the previous 3 years; 

 application of manure (source and type, consistency, 

application rate, method, and timing) on the field over the 

previous 3 years; 

 application of pesticides (chemical, rate, timing, and 

method) for the previous 3 years; 

 pest management practices; 

 irrigation practices (system type, amount, and frequency); 

 timing and equipment used for all field operations (tillage, 

planting, cultivation, harvesting) over the previous 3 

years, and; 

 general characteristics of the operator and the operation. 

In a separate survey, NRCS field offices provided information 

on the practices specified in conservation plans. 

Because of the large size of the sample, it was necessary to 

spread the data collection process over a 4-year period, from 

2003 through 2006. In each year, surveys were obtained for a 

separate set of sample points. The final CEAP sample was 

constructed by pooling the set of usable, completed surveys 

from all four years. 

Simulating the Effects of Weather

Weather is the predominant factor determining the loss of soil, 

nitrogen, phosphorus, and pesticides from farm fields, as well 

as the effects of conservation practices. To capture the effects 

of weather, each scenario was simulated using 47 years of 

actual daily weather data for the time period 1960 through 

2006. The 47-year record is the extent of a serially complete 

daily data set of weather station data from weather station 

records available from the NCDC (National Climatic Data 

Center), for the period 1960 to 2006, including precipitation, 

temperature maximum, and temperature minimum (Eischeid et 

al. 2000). These data were combined with the respective 

PRISM (Parameter–Elevation Regressions on Independent 

Slopes Model; Daly et al. 1994) monthly map estimates to 

construct daily estimates of precipitation and temperature (Di 

Luzio et al. 2008). The same 47-year weather data were used 

in the HUMUS/SWAT simulations and in the APEX model 

simulations. 

Annual precipitation over the 47-year simulation averaged 

about 42 inches for cropped acres in this region. Annual 

precipitation varied substantially in the model simulations, 

both within the region and from year to year, as shown in 

figures 5 and 6. 

19 
 

 

 

Table 5. Cultivated cropland in the Chesapeake Bay region 

Number of Percent 

Cropping System CEAP samples Estimated acres of acres 

Corn-soybean only 246 1,174,736 27 

Corn-soybean with close grown crops 180 830,308 19 

Corn only 103 690,403 16 

Soybean only 40 161,087 4 

Soybean-wheat only 22 124,649 3 

Corn and close grown crops 46 295,685 7 

Vegetable or tobacco with or without other crops 24 139,064 3 

Hay-crop mix (rotations include corn or soybean) 85 688,255 16 

Remaining mix of crops 25 175,713 4 

Sub-total for cropped acres 771 4,279,900 98 

CRP General Signup, representing cultivated cropland in long-term 

conserving cover 61 100,300 2 

Total 832 4,380,200 100 

Note: Estimates are from the 2003 NRI and the NRI-CEAP Cropland Survey. Cultivated cropland acres in this table differ slightly from estimates presented in table 1 

because of differences in sources and methods. 

Figure 5. Cumulative distributions of annual precipitation used in the model simulations for cropped acres in the Chesapeake Bay 

region 

70 

Annual precipitation (inches/year) 

65 

60 

55 

50 

45 

40 

35 

30 

25 

0 10 20 30 40 50 60 70 80 90 100 

Cumulative percent acres 

Note:. Each of the 47 curves shown above represents a single year of data and shows how annual precipitation varies over the region in that year, starting with the driest 

acres within the region and increasing to the wettest acres for each year. The family of curves shows how annual precipitation varies from year to year. Annual 

precipitation over the 47-year simulation averaged about 42 inches for cropped acres. 

20 
 

Figure 6. Mean, minimum, and maximum levels of annual precipitation used in the model simulations for cropped acres in the 

Chesapeake Bay region Annual precipitation (inches/year) 

70 

65 

60 

55 

50 

45 

40 

35 

30 

25 

20 

Minimum to mean 

Each curve in figure 5 shows how annual precipitation varied 

over the region in one of the 47 years. The family of curves 

shows the variability from year to year. The top curve shown 

is for the year 2003, the wettest year in this region during the 

47 years. The curve for 2003 shows that precipitation 

exceeded 56 inches for about 82 percent of cropped acres in 

the Chesapeake Bay region. 

The drier parts of the region received about 33 inches of 

precipitation per year, on average, and the wettest parts of the 

region received about 51 inches per year, on average. Year-toyear variability is especially pronounced—the annual average 

precipitation amount (representing all cropped acres) ranged 

from 31 inches in 1965 to 59 inches in 2003 over the 47-years 

(fig. 6). 

Mean Mean to maximum 

Throughout most of this report model results are presented in 

terms of the 47-year averages where weather is the only input 

variable that changes year to year. Since we used the cropping 

patterns and practices for the 2003–06 period, we did not 

simulate actual loses for each of these years. Rather, we 

provide estimates of what model outputs would average over 

the long term if weather varied as it has over the past 47 years. 

Similarly, estimates of the average effects of conservation 

practices include effectiveness in extreme weather years, such 

as floods and prolonged droughts, as represented in the 47-

year weather record. 

21 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Chapter 3 

Evaluation of Conservation Practice 

Use—the Baseline Conservation 

Condition 

This study assesses the use and effectiveness of conservation 

practices in the Chesapeake Bay region for the period 2003 to 

2006 to determine the baseline conservation condition for the 

region. The baseline conservation condition provides a 

benchmark for estimating the effects of existing conservation 

practices as well as projecting the likely effects of alternative 

conservation treatment. Conservation practices that were 

evaluated include structural practices, annual practices, and 

long-term conserving cover. 

Structural conservation practices, once implemented, are 

usually kept in place for several years. Designed primarily for 

erosion control, they also mitigate edge-of-field nutrient and 

pesticide loss. Structural practices evaluated include— 

 in-field practices for water erosion control, divided into 

two groups: 

o	 practices that control overland flow (terraces, contour 

buffer strips, contour farming, stripcropping, contour 

stripcropping), and 

o	 practices that control concentrated flow (grassed 

waterways, grade stabilization structures, diversions, 

and other structures for water control); 

	 edge-of-field practices for buffering and filtering surface 

runoff before it leaves the field (riparian forest buffers, 

riparian herbaceous cover, filter strips, field borders); and 

	 wind erosion control practices (windbreaks/shelterbelts, 

cross wind trap strips, herbaceous wind barriers, 

hedgerow planting). 

Annual conservation practices are management practices that 

are conducted as part of the crop production system each year. 

These practices are designed primarily to promote soil quality, 

reduce in-field erosion, and reduce the availability of
 

sediment, nutrients, and pesticides for transport by wind or 

water. They include—
 

 residue and tillage management; 

 nutrient management practices;
 

 pesticide management practices; and
 

 cover crops.
 

Long-term conservation cover establishment consists of
 

planting suitable native or domestic grasses, forbs, or trees on
 

environmentally sensitive cultivated cropland. 

Historical Context for Conservation 

Practice Use 

The use of conservation practices in the Chesapeake Bay 

region closely reflects the history of Federal conservation 

programs and technical assistance. In the beginning the focus 

was almost entirely on reducing soil erosion and preserving 

the soil’s productive capacity. In the 1930s and 1940s, Hugh 

Hammond Bennett, the founder and first chief of the Soil 

Conservation Service (now Natural Resources Conservation 

Service) instilled in the national ethic the need to treat every 

acre to its potential by controlling soil erosion and water 

runoff. Land shaping structural practices (such as terraces, 

contour farming, and strip cropping) and sediment control 

structures were widely adopted. Conservation tillage emerged 

in the 1960s and 1970s as a key management practice for 

enhancing soil quality and further reducing soil erosion. 

Conservation tillage, along with use of crop rotations and 

cover crops, was used either alone or in combination with 

structural practices. The conservation compliance provisions 

in the 1985 Farm Bill sharpened the focus to treatment of the 

most erodible acres, tying farm commodity payments to 

conservation treatment of highly erodible land. The 

Conservation Reserve Program was established to enroll the 

most erodible cropland acres in multi-year contracts to plant 

acres in long-term conserving cover. 

During the 1990s, the focus of conservation efforts began to 

shift from soil conservation and sustainability to reducing 

pollution impacts associated with agricultural production. 

Prominent among new concerns were the environmental 

effects of nutrient export from farm fields. Traditional 

conservation practices used to control surface water runoff and 

erosion control were mitigating a significant portion of these 

nutrient losses. Additional gains were being achieved using 

nutrient management practices—application of nutrients 

(appropriate timing, rate, method, and form) to minimize 

losses to the environment and maximize the availability of 

nutrients for crop growth. 

Summary of Practice Use 

Given the long history of conservation in the Chesapeake Bay 

region, it is not surprising to find that nearly all cropped acres 

in the region have evidence of some kind of conservation 

practice, especially erosion control practices. The conservation 

practice information collected during the study was used to 

assess the extent of conservation practice use. Key findings 

are the following. 

	 Structural practices for controlling water erosion are in 

use on 46 percent of cropped acres. On the 44 percent of 

the acres designated as highly erodible land, structural 

practices designed to control water erosion are in use on 

63 percent of those acres. 

	 Reduced tillage is common in the region; 88 percent of 

the cropped acres meet criteria for either no-till (48 

percent) or mulch till (40 percent). All but 7 percent of the 

acres had evidence of some kind of reduced tillage on at 

least one crop. 

	 About 43 percent of cropped acres are gaining soil 

organic carbon. An additional 33 percent of cropped acres 

are considered to be “maintaining” soil organic carbon 

(average annual loss less than 100 pounds per acre). 

Overall, 76 percent of cropped acres are maintaining or 

enhancing soil organic carbon. 

	 Producers use either residue and tillage management 

practices or structural practices, or both, on 96 percent of 

the acres. 

	 While most acres have evidence of some nitrogen or 

phosphorus management, the majority of the acres in the 

region lack consistent use of appropriate rates, timing, 

22 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

and method of application on each crop in every year of 

production, including nearly all of the acres receiving 

manure. 

o	 Appropriate timing of nitrogen applications is in use 

on about 54 percent of the acres for all crops in the 

rotation. 

o	 About 35 percent of cropped acres meet criteria for 

appropriate nitrogen application rates for all crops in 

the rotation. 

o	 Appropriate nitrogen application rates, timing of 

application, and application method for all crops 

during every year of production, however, are in use 

on only about 13 percent of cropped acres. 

o	 Good phosphorus management practices (appropriate 

rate, timing, and method) are in use on 17 percent of 

the acres on all crops during every year of 

production. 

o	 Only about 9 percent of cropped acres meet full 

nutrient management criteria for both phosphorus and 

nitrogen management, including acres not receiving 

nutrient applications. 

	 Cover crop use during the 2003–06 period of data 

collection was restricted to only about 4 percent of the 

acres in the region. 

	 An Integrated Pest Management (IPM) indicator showed 

that only about 8 percent of the acres were being managed 

at a relatively high level of IPM. 

	 Land in long-term conserving cover, as represented by 

enrollment in the CRP General Signup, consists of about 

100,000 acres in the region (2 percent of cultivated 

cropland), of which 67 percent is highly erodible land. 

Structural Conservation Practices 

Data on structural practices for the farm field associated with 

each sample point were obtained from four sources: 

1.	 The NRI-CEAP Cropland Survey included questions 

about the presence of 12 structural practices: terraces, 

grassed waterways, vegetative buffers (in-field), 

hedgerow plantings, riparian forest buffers, riparian 

herbaceous buffers, windbreaks or herbaceous wind 

barriers, contour buffers (in-field), field borders, filter 

strips, critical area planting, and grade stabilization 

structures. 

2.	 For fields with conservation plans, NRCS field offices 

provided data on all structural practices included in the 

plans. 

3.	 The USDA-Farm Service Agency (FSA) provided 

practice information for fields that were enrolled in the 

Continuous CRP for these structural practices: contour 

grass strips, filter strips, grassed waterways, riparian 

buffers (trees), and field windbreaks (Alex Barbarika, 

USDA/FSA, personal communication). 

4.	 The 2003 NRI provided additional information for 

practices that could be reliably identified from overhead 

photography as part of the NRI data collection process. 

These practices include contour buffer strips, contour 

farming, contour stripcropping, field stripcropping, 

terraces, cross wind stripcropping, cross wind trap strips, 

diversions, field borders, filter strips, grassed waterways 

or outlets, hedgerow planting, herbaceous wind barriers, 

riparian forest buffers, and windbreak or shelterbelt 

establishment. 

Overland flow control practices are designed to slow the 

movement of water across the soil surface to reduce surface 

water runoff and sheet and rill erosion. NRCS practice 

standards for overland flow control include terraces, contour 

farming, stripcropping, in-field vegetative barriers, and field 

borders. This is the most prevalent group of structural 

practices in the Chesapeake Bay region; these practices are 

found on about 34 percent of the cropped acres in the region; 

including 51 percent of the highly erodible land (table 6). 

Concentrated flow control practices are designed to prevent 

the development of gullies along flow paths within the field. 

NRCS practice standards for concentrated flow control 

practices include grassed waterways, grade stabilization 

structures, diversions, and water and sediment control basins. 

About 17 percent of the cropped acres have one or more of 

these practices, including 29 percent of the highly erodible 

land (table 6). 

Edge-of-field buffering and filtering practices, consisting of 

grasses, shrubs, and/or trees, are designed to capture the 

surface runoff losses that were not avoided or mitigated by the 

in-field practices. NRCS practice standards for edge-of-field 

mitigation practices include edge-of-field filter strips, riparian 

herbaceous buffers, and riparian forest buffers. CRP’s buffer 

practices are included in this category. Edge-of-field buffering 

and filtering practices are in use on about 10 percent of all 

cropped acres in the region (table 6). 

Overall, about 46 percent of the cropped acres in the 

Chesapeake Bay region are treated with one or more water 

erosion control structural practices (table 6). The treated 

percentage for highly erodible land acres is higher—63 

percent. 

At each sample point, structural conservation practices for 

water erosion control were classified as either a high, 

moderately high, moderate, or low level of treatment 

according to criteria presented in figure 7. About 5 percent of 

cropped acres in the region have a high level of treatment 

(combination of edge-of-field buffering or filtering and at least 

one in-field structural practice). About 54 percent of the acres 

do not have structural practices for water erosion control; 

however, about 40 percent of these acres have slopes less than 

2 percent, some of which may not need to be treated with 

structural practices. These treatment levels are combined with 

soil risk classes to estimate acres that appear to be undertreated for water erosion control in chapter 5 

Wind erosion control practices are designed to reduce the 

force of the wind on the field. NRCS practice standards for 

wind erosion control practices include cross wind ridges, cross 

wind trap strips, herbaceous wind barriers, and 

windbreak/shelterbelt establishment. Wind erosion is not a 

resource concern for most acres in this region. Only about 7 

percent of the cropped acres in the region are treated for wind 

erosion using structural practices (table 6). 

. 

23 
 

Table 6. Structural conservation practices in use for the baseline conservation condition, Chesapeake Bay region 

Percent Percent of 

Structural practice of non- Percent of cropped 

category Conservation practice in use HEL HEL acres 

Overland flow control Terraces, contour buffer strips, contour farming, stripcropping, 

practices contour stripcropping, field border, in-field vegetative barriers 20 51 34 

Concentrated flow Grassed waterways, grade stabilization structures, diversions, 

control practices other structures for water control 8 29 17 

Edge-of-field buffering 

and filtering practices Riparian forest buffers, riparian herbaceous buffers, filter strips 12 8 10 

One or more water 

erosion control 

practice Overland flow, concentrated flow, or edge-of-field practice 33 63 46 

Wind erosion control Windbreaks/shelterbelts, crosswind trap strips, herbaceous 

practices windbreak, hedgerow planting 5 9 7 

Note: About 44 percent of cropped acres in the Chesapeake Bay region are highly erodible land (HEL). Soils are classified as HEL if they have an erodibility index (EI) 

score of 8 or higher. A numerical expression of the potential of a soil to erode, EI considers the physical and chemical properties of the soil and climatic conditions 

where it is located. The higher the index, the greater the investment needed to maintain the sustainability of the soil resource base if intensively cropped. 

Figure 7. Conservation treatment levels for structural practices, baseline conservation condition, Chesapeake Bay region 

Low Moderate Moderately 

high High 

Slope 2 percent or less 29.1 5.2 4.2 1.8 

Slope greater than 2 percent 24.5 22.5 9.4 3.3 

0 

10 

20 

30 

40 

50 

60 

Percent of cropped acres 

Criteria for four levels of treatment with structural conservation practices are: 

	 High treatment: Edge-of-field mitigation and at least one in-field structural practice (concentrated flow or overland flow 

practice) required. 

	 Moderately high treatment: Either edge-of-field mitigation required or both concentrated flow and overland flow practices 

required. 

	 Moderate treatment: No edge-of-field mitigation, either concentrated flow or overland flow practices required. 

	 Low treatment: No edge-of-field or in-field structural practices. 

24 
 

 

 

 

 

 

 

 

Residue and Tillage Management Practices

Simulations of the use of residue and tillage management 

practices were based on the field operations and machinery 

types reported in the NRI-CEAP Cropland Survey for each 

sample point. The survey obtained information on the timing, 

type, and frequency of each tillage implement used during the 

previous 3 years, including the crop to which the tillage 

operation applied. 

The Soil Tillage Intensity Rating (STIR) (USDA-NRCS 2007) 

was used to determine the soil disturbance intensity for each 

crop at each sample point. The soil disturbance intensity is a 

function of the kinds of tillage, the frequency of tillage, and 

the depth of tillage. STIR values were calculated for each crop 

and for each of the 3 years covered by the NRI-CEAP 

Cropland Survey (accounting for multiple crops or cover 

crops). By combining the STIR values for each crop year with 

model output on the long-term trend in soil organic carbon 

gain or loss, eight categories of residue and tillage 

management were identified, as defined in table 7. 

Overall, 88 percent of cropped acres in the Chesapeake Bay 

region meet the tillage intensity rating for no-till or mulch till 

(table 7). About 48 percent meet the criteria for no-till—27 

percent with gains in soil organic carbon and 21 percent with 

soil organic carbon loss. About 40 percent meet the tillage 

intensity criteria for mulch till—13 percent with gains in soil 

organic carbon and 26 percent with soil organic carbon loss. 

No-till is used on a slightly higher percentage of highly 

erodible land than of non-highly erodible land. Only 7 percent 

of the acres are conventionally tilled for all crops in the 

rotation. 

Most of the cropped acres (96 percent) in the Chesapeake Bay 

region have some kind of water erosion control practice— 

either reduced tillage or structural practices or both (table 8). 

About 40 percent meet tillage intensity for no-till or mulch till 

and have structural practices, including 57 percent of highly 

erodible land. Only 4 percent have no water erosion control 

practices. 

Four levels of treatment for residue and tillage management 

practices were derived according to criteria presented in figure 

8. These treatment levels are combined with soil risk classes to 

estimate acres that appear to be under-treated for water erosion 

control in chapter 5. The high and moderately high treatment 

levels represent the 40 percent of cropped acres that meet 

tillage intensity criteria for either no-till or mulch till with 

gains in soil organic carbon. The high treatment level (35 

percent of the acres) includes only those acres where the 

tillage intensity criteria are met for each crop in the rotation. 

The majority of the acres have a moderate level of treatment 

because soil organic carbon is not being enhanced. Only 5 

percent of the acres have a low treatment level, consisting of 

continuous conventional tillage for all crops in the rotation and 

loss of soil organic carbon. 

The evaluation of conservation practices and associated estimates of conservation 

treatment needs are based on practice use derived from a farmer survey conducted 

during the years 2003–06. Since that time, however, the six States in the Chesapeake Bay 

watershed have continued to work with farmers to enhance conservation practice 

adoption in a joint effort to reduce nonpoint source pollution contributing to water 

quality issues in the Bay. In Maryland, for example, the state offered expanded incentive 

payments for the planting of cover crops starting in the 2008–09 growing season. State 

and Federal programs in the region have expanded incentives for manure incorporation, 

use of variable rate applications, side-dressing of nutrients, and other production 

techniques targeted at reducing losses of nutrients from farm fields. As a result, cover 

crops and nutrient management practices are probably in wider use within the watershed 

than the CEAP survey shows for 2003–06. 

25 
 

 

 

 

 

Table 7. Residue and tillage management practices for the baseline conservation condition, Chesapeake Bay region 

Percent of Percent of Percent of 

Residue and tillage management practice in use non-HEL HEL all acres 

Acres with carbon gain 43 43 43 

Average annual tillage intensity for crop rotation meets criteria for no-till* 25 29 27 

Average annual tillage intensity for crop rotation meets criteria for mulch till** 15 12 13 

Reduced tillage on some crops in rotation but average annual tillage intensity greater than 

criteria for mulch till 2 1 1 

Continuous conventional tillage in every year of crop rotation*** 2 2 2 

Acres with carbon loss 57 57 57 

Average annual tillage intensity for crop rotation meets criteria for no-till* 21 22 21 

Average annual tillage intensity for crop rotation meets criteria for mulch till** 27 26 26 

Reduced tillage on some crops in rotation but average annual tillage intensity greater than 

criteria for mulch till 4 4 4 

Continuous conventional tillage in every year of crop rotation*** 5 5 5 

All acres 

Average annual tillage intensity for crop rotation meets criteria for no-till* 46 51 48 

Average annual tillage intensity for crop rotation meets criteria for mulch till** 42 38 40 

Reduced tillage on some crops in rotation but average annual tillage intensity greater than 

criteria for mulch till 5 5 5 

Continuous conventional tillage in every year of crop rotation*** 7 7 7 

* Average annual Soil Tillage Intensity Rating (STIR) over all crop years in the rotation is less than 30. 

** Average annual Soil Tillage Intensity Rating (STIR) over all crop years in the rotation is between 30 and 100. 

*** Soil Tillage Intensity Rating (STIR) for every crop year in the rotation is more than 100. 

Note: A description of the Soil Tillage Intensity Rating (STIR) can be found at http://stir.nrcs.usda.gov/. 

Note: HEL = highly erodible land. About 44 percent of cropped acres in the Chesapeake Bay region are highly erodible land (HEL). 

Note: Percents may not add to totals because of rounding. 

Table 8. Percent of cropped acres with water erosion control practices for the baseline conservation condition, Chesapeake Bay 

region 

Percent of Percent of all 

Conservation treatment non-HEL Percent of HEL cropped acres 

No-till or mulch till with carbon gain, no structural practices 26 14 21 

No-till or mulch till with carbon loss, no structural practices 33 18 26 

Some crops with reduced tillage, no structural practices 3 2 2 

Structural practices and no-till or mulch till with carbon gain 14 27 19 

Structural practices and no-till or mulch till with carbon loss 14 30 21 

Structural practices and some crops with reduced tillage 3 2 3 

Structural practices only 2 4 3 

No water erosion control treatment 5 3 4 

All acres 100 100 100 

Note: Percents may not add to totals because of rounding. 

26 
 

 

Figure 8. Conservation treatment levels for residue and tillage management, baseline conservation condition, Chesapeake Bay region 

Low Moderate Moderately high High 

Losing carbon 5.2 51.5 0.0 0.0 

Gaining carbon 0.0 2.7 5.5 35.0 

0 

10 

20 

30 

40 

50 

60 

Percent of cropped acres 

Criteria for four levels of treatment with residue and tillage management are: 

	 High treatment: All crops meet tillage intensity criteria for either no-till or mulch till and crop rotation is gaining soil organic 

carbon. 

	 Moderately high treatment: Average annual tillage intensity meets criteria for mulch till or no-till and crop rotation is gaining 

soil organic carbon; some crops in rotation exceed tillage intensity criteria for mulch till. 

	 Moderate treatment: Some crops have reduced tillage but do not meet criteria for high or moderately high treatment or crop 

rotation is gaining soil organic carbon; most acres in this treatment level are losing soil organic carbon. 

	 Low treatment: Continuous conventional tillage and crop rotation is losing soil organic carbon. 

27 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Conservation Crop Rotation 

In the Chesapeake Bay region, crop rotations that meet NRCS 

criteria (NRCS practice code 328) occur on about 77 percent 

of the cropped acres. This practice consists of growing 

different crops in a planned rotation to manage nutrient inputs, 

enhance soil quality, or reduce soil erosion. Including hay or a 

close grown crop in the rotation can have a pronounced effect 

on long-term average field losses of sediment and nutrients, as 

well as enhancement of soil quality. 

The model outputs reported in chapter 4 reflect the benefits of 

conservation crop rotations. However, the benefits of 

conservation crop rotation practices could not be assessed in 

this study for two reasons. First, it was not possible to 

differentiate conservation crop rotations from crop rotations 

for other purposes, such as the control of pests or in response 

to changing markets. Second, the “no-practice scenario” 

would require simulation of continuous cropping systems. Not 

only was there inadequate information on chemical use and 

other farming practices for widespread continuous crop 

production, but arbitrary decisions about which crops to 

simulate at each sample point would be required to preserve 

the level of regional production. 

Cover Crops 

Cover crops are planted when the principal crops are not 

growing. The two most important functions of cover crops are 

(1) to provide soil surface cover and reduce soil erosion, and 

(2) to utilize and convert excess nutrients remaining in the soil 

from the preceding crop into plant biomass, thereby reducing 

nutrient leaching and minimizing the amount of soluble 

nutrients in runoff during the non-crop growing season. Cover 

crops also contribute to soil quality by capturing atmospheric 

carbon in plant tissue and adding it to the soil carbon. 

The presence or absence of cover crops was determined from 

farmer responses in the NRI-CEAP Cropland Survey. The 

following criteria were used to identify a cover crop. 

	 A cover crop must be a close-grown crop that is not 

harvested as a principal crop, or if it is harvested, must 

have been specifically identified in the NRI-CEAP 

Cropland Survey as a cover crop as an indicator that the 

harvest was for an acceptable purpose (such as biomass 

removal or use as mulch or forage material). 6 

	 Spring-planted cover crops are inter-seeded into a 

growing crop or are followed by the seeding of a summer 

or late fall crop that may be harvested during that same 

year or early the next year. 

	 Late-summer-planted cover crops are followed by the 

harvest of another crop in the same crop year or the next 

spring. 

	 Fall-planted cover crops are followed by the spring 

planting of a crop for harvest the next year. 

6

 Except for the 2003 survey, the questionnaire allowed the respondent to list 

the purpose for which a crop was grown, including cover crop. This 

information was not a reliable indicator of a cover crop for conservation 

purposes for all sample points, based on other information in the survey on 

crops planted and field operations. 

Some cover crops are planted for soil protection during 

establishment of spring crops such as sugar beets and potatoes. 

Early spring vegetation protects young crop seedlings. 

In the Chesapeake Bay region, cover crops were not 

commonly used as a conservation practice during the period 

covered by the farmer survey (2003–06). Only about 4 percent 

of the acres (31 sample points) met the above criteria for a 

cover crop. 

(Since the CEAP survey was conducted, participation in the 

Maryland Department of Agriculture cover crop program has 

increased substantially. As a result, cover crops are currently 

in wider use in Maryland than the CEAP survey shows for 

2003–06.) 

Irrigation Management Practices

Irrigation in the United States has its roots in the arid West 

where precipitation is insufficient to meet the needs of 

growing crops. In other parts of the United States, rainfall 

totals are sufficient in most years to produce optimum yields 

of the crops grown. The distribution of the rainfall during the 

crop growing season, however, is sometimes problematic, 

especially in years when precipitation is below average. In the 

Chesapeake Bay region, irrigation applications are sometimes 

used to supplement natural rainfall. 

Irrigation applications are made either with a pressure or a 

gravity system. Gravity systems, as the name implies, utilizes 

gravitational energy to move water from higher elevations to 

lower elevations, such as moving water from a ditch at the 

head of a field, across the field to the lower end. Pumps are 

most often used to create the pressure in pressure systems, and 

the water is applied under pressure through nozzles of one 

form or another. There are also many variations such as where 

water is diverted at higher elevations and the pressure created 

by gravity is substituted for the energy of a pump. 

Proper irrigation involves applying appropriate amounts of 

water to the soil profile to reduce any plant stress while at the 

same time minimizing water losses through evaporation, deep 

percolation, and runoff. The conversion of much of the gravity 

irrigated area to pressure systems and the advent of pressure 

systems in rain-fed agricultural areas has reduced the volumes 

of irrigation water lost to deep percolation and end-of-field 

runoff, but has greatly increased the volume of water lost to 

evaporation in the sprinkling process. Modern sprinklers 

utilize improved nozzle technology to increase droplet size as 

well reduce the contact time from the nozzle to the ground. 

Irrigation specialists consider the center pivot or linear move 

sprinkler with low pressure heads as the current state of the 

art. 

About 5 percent of the cropped acres—209,000 acres— 

receive irrigation water in the Chesapeake Bay region. 

Irrigation in the region is almost exclusively by pressure 

systems, however, some 5,300 acres or 2.5 percent of the 

irrigated area is served by gravity systems. Most common 

pressure systems are center-pivot or linear-move systems with 

impact sprinkler heads (43.9 percent) followed by center-pivot 

or linear-move systems with more efficient low-pressure spray 

28 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

or near-ground emitters (34 percent). Big gun sprinklers make 

up 9.1 percent. In the Chesapeake Bay region, 80,800 acres 

(39 percent of the irrigated acres) already have systems with 

efficiencies at or better than the current state of the art. 

Nutrient Management Practices 

Nitrogen and phosphorus are essential inputs to profitable crop 

production. Farmers apply these nutrients to the land as 

commercial fertilizers and manure to promote plant growth 

and increase crop yields. Not all of the nutrients applied to the 

land, however, are taken up by crops; some are lost to the 

environment and, when combined with naturally occurring 

levels of these elements or other pollution sources, can create 

offsite water quality problems. 

Sound nutrient management systems can minimize nutrient 

losses from the agricultural management zone while providing 

adequate soil fertility and nutrient availability to ensure 

realistic yields. (The agricultural management zone is defined 

as the zone surrounding a field that is bounded by the bottom 

of the root zone, edge of the field, and top of the crop canopy.) 

Such systems are tailored to address the specific cropping 

system, nutrient sources available, and site characteristics of 

each field. Nutrient management systems have four basic 

criteria for application of commercial fertilizers and manure. 

1.	 Apply nutrients at the appropriate rate based on soil and 

plant tissue analyses and realistic yield goals. 

2.	 Apply the appropriate form of fertilizer and organic 

material with compositions and characteristics that resist 

nutrient losses from the agricultural management zone. 

3.	 Apply at the appropriate time to supply nutrients to the 

crop when the plants have the most active uptake and 

biomass production, and avoid times when adverse 

weather conditions can result in large losses of nutrients 

from the agricultural management zone. 

4.	 Apply using the appropriate application method that 

provides nutrients to the plants for rapid, efficient uptake 

and reduces the exposure of nutrient material to forces of 

wind and water. 

Depending on the field characteristics, these nutrient 

management techniques can be coupled with other 

conservation practices such as conservation crop rotations, 

cover crops, residue management practices, and structural 

practices to minimize the potential for nutrient losses from the 

agricultural management zone. Even though nutrient transport 

and losses from agricultural fields cannot be completely 

eliminated, they can be minimized by careful management and 

kept within an acceptable level. 

The presence or absence of nutrient management practices 

was based on information on the timing, rate, and method of 

application for manure and commercial fertilizer as reported 

by the producer in the NRI-CEAP Cropland Survey. The 

appropriate form of nutrients applied was not evaluated 

because the survey was not sufficiently specific about the 

material formulations that were applied. The following criteria 

were used to identify the appropriate rate, time, and method of 

nutrient application for each crop or crop rotation. 

	 All commercial fertilizer and manure applications are 

within 3 weeks prior to plant date, at planting, or within 

60 days after planting. 

	 The method of application for commercial fertilizer or 

manure is some form of incorporation or banding or spot 

treatment or foliar applied. 

	 The rate of nitrogen application, including the sum of 

both commercial fertilizer and manure nitrogen available 

for crops in the year of application, is— 

o	 less than 1.4 times the amount of nitrogen removed in 

the crop yield at harvest for each crop, except for 

cotton and small grain crops; 

o	 less than 1.6 times the amount of nitrogen removed in 

the crop yield at harvest for small grain crops (wheat, 

barley, oats, rice, rye, buckwheat, emmer, spelt, and 

triticale); 

o	 less than 60 pounds of nitrogen per bale of cotton 

harvested. 

	 The rate of phosphorus application summed over all 

applications and crops in the rotation, including both 

commercial fertilizer and manure phosphorus, is less than 

1.1 times the amount of phosphorus removed in the crop 

yields at harvest summed over all crops in the rotation. 

Phosphorus application rate criteria apply to the full crop 

rotation to account for infrequent applications intended to 

provide phosphorus for multiple crops or crop years, which is 

often the case with manure applications. Nitrogen application 

rate criteria apply to each crop in the rotation. 

Criteria used here to identify the occurrence of nutrient 

management practices, while consistent with NRCS standards, 

do not necessarily represent the best possible set of nutrient 

management practices. These nutrient management criteria are 

intended to represent practice recommendations commonly 

found in comprehensive nutrient management conservation 

plans. 

As shown in table 9, the majority of acres in the Chesapeake 

Bay region meet one or more of the criteria for nutrient 

management: 

 73 percent of cropped acres meet criteria for timing of 

nitrogen applications for one or more crops and 76 

percent meet criteria for timing of phosphorus 

applications for one or more crops; 

	 88 percent of cropped acres meet criteria for method of 

nitrogen application for one or more crops and 92 percent 

meet criteria for method of phosphorus application for 

one or more crops; 

 88 percent of cropped acres meet criteria for nitrogen 

application rate for one or more crops; 

 3 percent of cropped acres have no nitrogen applied and 1 

percent have no phosphorus applied. 

29 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 9. Nutrient management practices for the baseline conservation condition, Chesapeake Bay region 

Percent Percent 

of acres of acres Percent 

without with of all 

manure manure cropped 

applied applied acres 

Nitrogen* 

No N applied to any crop in rotation 4 0 3 

For samples where N is applied: 

Time of application 

All crops in rotation have application of N (manure and/or fertilizer) within 3 weeks before planting or after planting 77 16 54 

Some but not all crops have application of N (manure and/or fertilizer) within 3 weeks before planting or after planting 8 36 19 

No crops in rotation have application of N (manure and/or fertilizer) within 3 weeks before planting or after planting 10 48 24 

Method of application 

All crops in rotation have N applied with incorporation or banding/foliar/spot treatment 37 32 35 

Some but not all crops in rotation have N applied with incorporation or banding/foliar/spot treatment 45 67 53 

No crops in rotation have N applied with incorporation or banding/foliar/spot treatment 14 0 9 

Rate of application 

All crops in rotation meet the nitrogen rate criteria described in text 39 30 35 

Some but not all crops in rotation meet the nitrogen rate criteria described in text 51 57 53 

No crops in rotation meet the nitrogen rate criteria described in text 6 13 9 

Timing and method and rate of application 

All crops meet the nitrogen rate criteria described in text and application within 3 weeks before planting or after 

planting with incorporation or banding/foliar/spot treatment 18 3 13 

Some but not all crops meet the nitrogen rate criteria described in text or application within 3 weeks before planting or 

after planting or use of incorporation or banding/foliar/spot treatment 56 63 59 

No crops meet the nitrogen rate criteria described in text or application within 3 weeks before planting or after 

planting or use of incorporation or banding/foliar/spot treatment 21 34 26 

Phosphorus* 

No P applied to any crop in rotation 1 0 1 

For samples where P is applied: 

Time of application 

All crops in rotation have application of P (manure and/or fertilizer) within 3 weeks before planting or after planting 82 17 58 

Some but not all crops have application of P (manure and/or fertilizer) within 3 weeks before planting or after planting 8 36 18 

No crops in rotation have application of P (manure and/or fertilizer) within 3 weeks before planting or after planting 9 47 23 

Method of application 

All crops in rotation have P applied with incorporation or banding/foliar/spot treatment 49 47 49 

Some but not all crops in rotation have P applied with incorporation or banding/foliar/spot treatment 37 53 43 

No crops in rotation have P applied with incorporation or banding/foliar/spot treatment 12 0 7 

Rate of application 

Crop rotation has P applied at a rate less than 1.1 times the removal of P in the yield at harvest for the crop rotation 48 19 37 

Crop rotation has P applied at a rate more than 1.1 times the removal of P in the yield at harvest for the crop rotation 50 81 62 

Timing and method and rate of application 

Crop rotation has P rate less than 1.1 times removal at harvest and applications within 3 weeks before planting or after 

planting with incorporation or banding/foliar/spot treatment 26 2 17 

Crop rotation has P rate less than 1.1 times removal at harvest and some crops had application within 3 weeks before 

planting or after planting and/or some crops used incorporation or banding/foliar/spot treatment 19 12 17 

Crop rotation has P rate more than 1.1 times removal at harvest and no crops had applications within 3 weeks before 

planting or after planting and no crops used incorporation or banding/foliar/spot treatment 54 85 66 

Nitrogen and Phosphorus 

Crop rotation P rate less than 1.1 and N rate criteria described in text and all applications within 3 weeks before 

planting or after planting with incorporation or banding/foliar/spot treatment, including acres with no N or P applied 14 1 9 

Crop rotation P rate less than 1.1 and N rate criteria appropriate for full conservation treatment (see text) and all 

application within 3 weeks before planting or after planting with incorporation or banding/foliar/spot treatment , 

including acres with no N or P applied 9 1 6 

All sample points 100 100 100 

Note: About 38 percent of cropped acres (1.6 million acres) have manure applied. Percents may not add to 100 because of rounding. 

* These estimates include adjustments made to the reported data on nitrogen and phosphorus application rates from the survey because of missing data and data-entry 

errors. In the case of phosphorus, the 3-year data period for which information was reported was too short to pick up phosphorus applications made at 4- and 5-year 

intervals between applications, which is a common practice for producers adhering to sound phosphorus management techniques. Since crop growth, and thus canopy 

development which decreases erosion, is a function of nitrogen and phosphorus, it was necessary to add additional nitrogen and phosphorus when the reported levels 

were insufficient to support reasonable crop yields throughout the 47 years in the model simulation. The approach taken was to first identify crop samples that have 

application rates recorded erroneously or were under-reported in the survey. The model was used to identify these samples by running the simulation at optimal levels 

of nitrogen and phosphorus for crop growth. The set of crop samples identified were treated as if they had missing data. Additional nitrogen or phosphorus was added to 

these crop samples so that the total nitrogen or phosphorus use was similar to that for the unadjusted set of crop samples. About 29 percent of the acres received a 

nitrogen adjustment for one or more crops. About 26 percent of the acres received a phosphorus adjustment for one or more crops. Nitrogen and phosphorus were added 

by increasing the existing applications (thus preserving the reported timing and methods), when present, or were applied at plant. (For additional information on 

adjustment of nutrient application rates, see “Adjustment of CEAP Cropland Survey Nutrient Application Rates for APEX Modeling,” available at 

http://www.nrcs.usda.gov/technical/nri/ceap 

30 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fewer acres, however, meet criteria for all crops in the 

rotation: 

	 54 percent of cropped acres meet criteria for timing of 

nitrogen applications on all crops and 58 percent meet 

criteria for timing of phosphorus applications on all crops. 

	 35 percent of cropped acres meet criteria for method of 

nitrogen application on all crops and 49 percent meet 

criteria for method of phosphorus application on all crops. 

	 35 percent of cropped acres meet criteria for nitrogen 

application rate on all crops and 37 percent meet criteria 

for phosphorus application rates for the full crop rotation. 

Acres with manure applied—about 38 percent of cropped 

acres—generally meet the criteria for method of application at 

about the same frequency as acres with only commercial 

fertilizer applications. Criteria for timing and rate, however, 

are met less frequently for acres receiving manure: 

	 16 percent of cropped acres receiving manure meet 

criteria for timing of nitrogen applications on all crops, 

compared to 77 percent for acres not receiving manure; 

	 17 percent of cropped acres receiving manure meet 

criteria for timing of phosphorus applications on all crops, 

compared to 82 percent for acres not receiving manure; 

	 30 percent of cropped acres receiving manure meet 

criteria for nitrogen application rates on all crops, 

compared to 39 percent for acres not receiving manure; 

	 19 percent of cropped acres receiving manure meet 

criteria for phosphorus application rates, compared to 48 

percent for acres not receiving manure. 

Overall, few acres meet all nutrient management criteria: 

	 only 13 percent of the acres meet all criteria for nitrogen 

applications, while another 3 percent of cropped acres 

have no nitrogen applied; 

	 only17 percent of the acres meet all criteria for 

phosphorus applications, while another 1 percent of the 

acres have no phosphorus applied; 

	 only 9 percent of cropped acres meet criteria for both 

phosphorus and nitrogen management (table 9), including 

acres not receiving nutrient applications. 

Lower nitrogen rate criteria are appropriate for acres that meet 

application timing and method criteria and also are fully 

treated for soil erosion control because more of the nitrogen 

applied is retained on the field and is therefore available for 

crop growth. In the simulation of additional soil erosion 

control and nutrient management (full treatment) in chapter 6, 

the rates of nitrogen application, including both commercial 

fertilizer and manure nitrogen, were proportionately reduced 

to the following levels— 

 1.2 times the amount of nitrogen removed in the crop 

yield at harvest for each crop, except for cotton and small 

grain crops; 

 1.5 times the amount of nitrogen removed in the crop 

yield at harvest for small grain crops; and 

 50 pounds of nitrogen per bale of cotton harvested. 

As shown in table 9, only 6 percent of cropped acres in the 

region meet all nutrient management criteria including these 

lower nitrogen rate criteria and including acres not receiving 

nutrient applications. 

Four levels of treatment for nitrogen and phosphorus 

management were derived for use in evaluating the adequacy 

of nutrient management. These treatment levels are combined 

with soil risk classes to estimate acres that appear to be undertreated in chapter 5. Criteria for the treatment levels are 

presented in figures 9 and 10. The high treatment level 

represents consistent use of appropriate rate, timing, and 

method for all crops, including the lower nitrogen application 

rate criteria appropriate for full conservation treatment 

conditions. 

Based on these treatment levels, about 11 percent of the acres 

in the Chesapeake Bay region have a high level of nitrogen 

management and about 18 percent have a high level of 

phosphorus management (figs. 9 and 10). Few acres with 

manure applied meet the criteria for the high treatment levels. 

About 22 percent of cropped acres have a low level of 

nitrogen management and 48 percent of the acres have a low 

level of phosphorus management. 

31 
 

 

 

Figure 9. Conservation treatment levels for nitrogen management, baseline conservation condition, Chesapeake Bay region 

Low Moderate Moderately 

high High 

Manure applied 16.2 10.3 10.4 0.9 

0 

10 

20 

30 

40 

50 

Percent cropped acres 

No manure applied 6.1 29.4 16.9 9.8 

Criteria for four levels of nitrogen management are: 

	 High treatment: All crops have: (1) total nitrogen application rates (including manure) less than 1.2 times the nitrogen in the 

crop yield for crops other than cotton and small grains, less than 1.5 times the nitrogen in the crop yield for small grains, and less 

than 50 pounds of nitrogen applied per cotton bale; (2) all applications occur within 3 weeks before planting or after planting; and 

(3) all applications are incorporated or banding/foliar/spot treatment is used. 

	 Moderately high treatment: All crops have total nitrogen application rates (including manure) less than 1.4 times the nitrogen in 

the crop yield for crops other than cotton and small grains, less than 1.6 times the nitrogen in the crop yield for small grains, and 

less than 60 pounds of nitrogen applied per cotton bale for all crops. Timing and method of application criteria may or may not be 

met. 

 Moderate treatment: All crops meet either the above criteria for timing or method, but do not meet criteria for rate. 

 Low treatment: Some or all crops in rotation exceed criteria for rate and either timing or method. 

32 
 

 

 

 

Figure 10. Conservation treatment levels for phosphorus management, baseline conservation condition, Chesapeake Bay region 

50 

40 

30 

20 

10 

0 Moderately Low Moderate High high 

Manure applied 28.2 2.5 6.2 0.8 Percent of cropped acres 

No manure applied 20.0 11.4 13.8 17.0 

Criteria for four levels of phosphorus management are: 

	 High treatment: (1) total phosphorus application rates (including manure) summed over all crops are less than 1.1 times the 

phosphorus in the crop yields for the crop rotation, (2) all applications occur within 3 weeks before planting or after planting, and 

(3) all applications are incorporated or banding/foliar/spot treatment was used. (Note that phosphorus applications for individual 

crops could exceed 1.1 times the phosphorus in the crop yield but total applications for the crop rotation could not.) 

 Moderately high treatment: Total phosphorus application rates (including manure) are less than 1.1 times the phosphorus in the 

crop yield for the crop rotation. No application rate or timing of application criteria is applied. 

 Moderate treatment: Sample points that do not meet the high or moderately high criteria but all phosphorus applications for all 

crops have appropriate time and method of application. 

	 Low treatment: All acres have excessive application rates over the crop rotation and inadequate method or timing of application 

for at least one crop in the rotation. 

33 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Pesticide Management Practices

The presence or absence of pesticide management practices 

was based on an Integrated Pest Management (IPM) indicator 

developed using producer responses to the set of IPM-related 

questions in the NRI-CEAP Cropland Survey (table 10).7 

Adoption of IPM systems normally occurs along a continuum 

from largely reliant on prophylactic control measures and 

pesticides to multiple-strategy, biologically intensive 

approaches, and is not usually an either/or situation. The 

practice of IPM is site-specific in nature, with individual 

tactics determined by the particular crop/pest/environment 

scenario. Where appropriate, each site should have in place a 

management strategy for Prevention, Avoidance, 

Monitoring, and Suppression of pest populations (the PAMS 

approach) (Coble 1998). In order to qualify as IPM 

practitioners, growers would use tactics in all four PAMS 

components. 

Prevention is the practice of keeping a pest population from 

infesting a field or site, and should be the first line of defense. 

It includes such tactics as using pest-free seeds and 

transplants, preventing weeds from reproducing, irrigation 

scheduling to avoid situations conducive to disease 

development, cleaning tillage and harvesting equipment 

between fields or operations, using field sanitation procedures, 

and eliminating alternate hosts or sites for insect pests and 

disease organisms. 

Avoidance may be practiced when pest populations exist in a 

field or site but the impact of the pest on the crop can be 

avoided through some cultural practice. Examples of 

avoidance tactics include crop rotation in which the crop of 

choice is not a host for the pest, choosing cultivars with 

genetic resistance to pests, using trap crops or pheromone 

traps, choosing cultivars with maturity dates that may allow 

harvest before pest populations develop, fertilization programs 

to promote rapid crop development, and simply not planting 

certain areas of fields where pest populations are likely to 

cause crop failure. 

Monitoring and proper identification of pests through surveys 

or scouting programs, including trapping, weather monitoring 

and soil testing where appropriate, are performed as the basis 

for suppression activities. Records are kept of pest incidence 

and distribution for each field or site. Such records form the 

basis for crop rotation selection, economic thresholds, and 

suppressive actions. 

Suppression of pest populations may be necessary to avoid 

economic loss if prevention and avoidance tactics are not 

successful. Suppressive tactics include cultural practices such 

as narrow row spacing or optimized in-row plant populations, 

alternative tillage approaches such as no-till or strip-till 

systems, cover crops or mulches, or using crops with 

allelopathic potential in the rotation. Physical suppression 

tactics include cultivation or mowing for weed control, baited 

7 For a full documentation of the derivation of the IPM indicator, see 

“Integrated Pest Management (IPM) Indicator Used in the CEAP Cropland 

Modeling,” available at http://www.nrcs.usda.gov/technical/nri/ceap. 

or pheromone traps for certain insects, and temperature 

management or exclusion devices for insect and disease 

management. Biological controls, including mating disruption 

for insects, are alternatives to conventional pesticides, 

especially where long-term control of a troublesome pest 

species can be attained. Naturally occurring biological 

controls exist, where they exist, are important IPM tools. 

Chemical pesticides are applied as a last resort in suppression 

systems using a sound management approach, including 

selection of pesticides with low risk to non-target organisms. 

An IPM index was developed to determine the level of IPM 

activity for each sample point. The index was constructed as 

follows. 

 Scores were assigned to each question by a group of IPM 

experts. 

 Scores for each PAMS category were normalized to have 

a maximum score of 100. 

 The four PAMS categories were also scored in terms of 

relative importance for an IPM index: prevention = 1/6, 

avoidance = 1/6, monitoring = 1/3, and suppression = 1/3. 

 The IPM indicator was calculated by multiplying the 

normalized PAMS category by the category weight and 

summing over the categories. 

An IPM indicator score greater than 60 defined sample points 

with a high level of IPM activity. Sample points with an IPM 

indicator score of 35 to 60 were classified as moderately high 

IPM treatment and sample points with an IPM score less than 

35 were classified as low IPM treatment. 

About 9 percent of the acres in the Chesapeake Bay region 

have a high level of IPM activity (fig. 11). About 38 percent 

have a moderate level of IPM activity, and 53 percent have a 

low level of IPM activity. 

Figure 11. Integrated Pesticide Management indicator for the 

baseline conservation condition, Chesapeake Bay region 

60 

52.5 

50 

38.1
 

40
 

30 

20 

9.4
 10
 

0 

Percent of cropped acres 

Low Moderate High 

34 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 10. Summary of survey responses to pest management questions, Chesapeake Bay region 

Number samples Percent of 

Survey question* with “yes” response cropped acres 

Prevention 

Pesticides with different action rotated or tank mixed to prevent resistance 252 33 

Plow down crop residues 133 20 

Chop, spray, mow, plow, burn field edges, etc. 264 33 

Clean field implements after use 272 35 

Remove crop residue from field 72 10 

Water management used to manage pests (irrigated samples only) 12 1 

Avoidance 

Rotate crops to manage pests 531 66 

Use minimum till or no-till to manage pests 482 56 

Choose crop variety that is resistant to pests 299 34 

Planting locations selected to avoid pests 109 12 

Plant/harvest dates adjusted to manage pests 53 6 

Monitoring 

Scouting practice: general observations while performing routine tasks 278 36 

Scouting practice: deliberate scouting 366 44 

 --Established scouting practice used 156 19 

 --Scouting due to pest development model 62 8 

 --Scouting due to pest advisory warning 99 9 

Scouting done by: (only highest of the 4 scores is used)

 --Scouting by operator 221 27 

 --Scouting by employee 2 <1 

 --Scouting by chemical dealer 86 10 

 --Scouting by crop consultant or commercial scout 63 7 

Scouting records kept to track pests? 140 17 

Scouting data compared to published thresholds? 221 26 

Diagnostic lab identified pest? 58 6 

Weather a factor in timing of pest management practice 250 31 

Suppression 

Pesticides used? 747 94 

Weather data used to guide pesticide application 479 60 

Biological pesticides or products applied to manage pests 87 9 

Pesticides with different mode of action rotated or tank mixed to prevent resistance 252 33 

Pesticide application decision factor (one choice only): 

--Routine treatments or preventative scheduling 393 50 

--Comparison of scouting data to published thresholds 67 8 

--Comparison of scouting data to operator's thresholds 71 8 

--Field mapping or GPS 2 0 

--Dealer recommendations 112 13 

--Crop consultant recommendations 54 8 

--University extension recommendations 5 1 

--Neighbor recommendations 1 <1 

--"Other" 13 2 

Maintain ground covers, mulch, or other physical barriers 317 41 

Adjust spacing, plant density, or row directions 153 16 

Release beneficial organisms 13 1 

Cultivate for weed control during the growing season 42 6 

Number of respondents 771 100 

Note: The scores shown in this table were used to develop an IPM indicator as discussed in the text. 

35 
 

 

 

 

 

 

 

 

Conservation Cover Establishment 

Establishing long-term cover of grass, forbs, or trees on a site 

provides the maximum protection against soil erosion. 

Conservation cover establishment is often used on cropland 

with soils that are vulnerable to erosion or leaching. The 

practice is also effective for sites that are adjacent to 

waterways, ponds, and lakes. Because these covers do not 

require annual applications of fertilizer and pesticides, this 

long-term conserving cover practice greatly reduces the loss of 

nitrogen and phosphorus from the site, and nearly eliminates 

pesticide loss. Because conservation covers are not harvested, 

they generate organic material that decomposes and increases 

soil organic carbon. For this study, the effect of a long-term 

conserving cover practice was estimated using acres enrolled 

in the General Signup of the CRP. The CRP General Signup is 

a voluntary program in which producers with eligible land 

enter into 10- to 15-year contracts to establish long-term cover 

to reduce soil erosion, improve water quality, and enhance 

wildlife habitat. 

Landowners receive annual rental payments and cost-share 

assistance for establishing and maintaining permanent 

vegetative cover. To be eligible for enrollment in the CRP 

General Signup, the field (or tract) must meet specified crop 

history criteria. 

Other factors governing enrollment in the CRP include natural 

resource-based eligibility criteria, an Environmental Benefits 

Index (EBI) used to compare and rank enrollment offers, 

acreage limits, and upper limits on the proportion of a 

county’s cropland that can be enrolled (USDA Farm Service 

Agency 2004; Wiebe and Gollehon 2006). Initially, the 

eligibility criteria included only soil erosion rates and inherent 

soil erodibility. During the 1990s and to date, the eligibility 

criteria have continued to evolve, with increasing emphasis 

placed on issues other than soil erodibility. For contract offer 

ranking, weight was given to proposals that also benefited 

wildlife, air and water quality, and other environmental 

concerns. 

As of 2003, about 31.5 million acres were enrolled in the CRP 

General Signup nationally, including about 100,000 in the 

Chesapeake Bay region (USDA/NRCS 2007). Approximately 

two-thirds of the cropland acres enrolled in the CRP in the 

Chesapeake Bay region is classified as highly erodible land. 

The inclusion of non-highly erodible land is due to both the 

expansion of enrollment eligibility criteria beyond soil erosion 

issues and the fact that farmers were allowed to enroll entire 

fields in the CRP if a specified portion of the field (varied by 

signup and eligibility criterion) met the criteria. 

In the Chesapeake Bay region, 65 percent of the CRP land is 

planted to introduced grasses, 29 percent to trees, 5 percent to 

wildlife habitat and 1 percent to native grasses. The plantings 

designated in the NRI database for each sample point were 

simulated in the model. However, in all cases the simulated 

cover was a mix of species and all points included at least one 

grass and one clover species. 

In addition to the General CRP Signup, cultivated 

cropland acres (whole field) can also be enrolled in longterm conserving cover under the Conservation Reserve 

Enhancement Program (CREP). The Maryland 

Department of Agriculture reports that CREP acreage 

enrollment outpaces CRP General Signup in that state by 

11 to 1 because CREP is better suited to regional 

conditions and provides better incentive rates than CRP. 

The Farm Service Agency estimates that about an 

additional 100,000 acres of land in long-term conserving 

cover have been enrolled in CREP throughout the 

Chesapeake Bay region (Alex Barbarika, USDA/FSA, 

personal communication). 

These additional acres of land in long-term conserving 

cover could not be included in the assessment of the 

effects of conservation practices because the acreage is 

not represented in the NRI, which is the source of 

information on soils, slopes, and cover types required to 

simulate fate and transport of sediment and nutrients. 

36 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

Chapter 4 

Onsite (Field-Level) Effects of 

Conservation Practices 

The Field-Level Cropland Model—APEX

A physical process model called APEX was used to simulate 

the effects of conservation practices at the field level 

(Williams et al. 2006; Williams et al. 2008; Gassman et al. 

2009 and 2010).8

 The I_APEX model run management 

software developed at the Center for Agricultural and Rural 

Development, Iowa State University, was used to perform the 

simulations in batch mode.9 

The APEX model is a field-scale, daily time-step model that 

simulates weather, farming operations, crop growth and yield, 

and the movement of water, soil, carbon, nutrients, sediment, 

and pesticides (fig. 12). The APEX model and its predecessor, 

EPIC (Environmental Policy Impact Calculator), have a long 

history of use in simulation of agricultural and environmental 

processes and of the effect of agricultural technology and 

government policy (Izaurralde et al. 2006;Williams 1990; 

Williams et al. 1984; Gassman et al. 2005).10 

Figure 12. Daily hydrologic processes simulated by APEX 

On a daily basis, APEX simulates the farming operations used 

to grow crops, such as planting, tillage before and after 

planting, application of nutrients and pesticides, application of 

manure, irrigation, and harvest. Weather events and their 

interaction with crop cover and soil properties are simulated; 

these events affect crop growth and the fate and transport of 

water and chemicals through the soil profile and over land to 

8

 The full theoretical and technical documentation of APEX can be found at 

http://epicapex.brc.tamus.edu/downloads/user-manuals.aspx. 

9

 The I_APEX software steps through the simulations one at a time, extracting 

the needed data from the Access input tables, executes APEX, and then stores 

the model output in Access output files. The Web site for that software is 

http://www.card.iastate.edu/environment/interactive_programs.aspx. 

10 Summaries of APEX model validation studies on how well APEX simulates 

measured data are presented in Gassman et al. (2009) and in “APEX Model 

Validation for CEAP” found at http://www.nrcs.usda.gov/technical/nri/ceap. 

the edge of the field. Over time, the chemical makeup and 

physical structure of the soil may change, which in turn affect 

crop yields and environmental outcomes. Crop residue 

remaining on the field after harvest is transformed into organic 

matter. Organic matter may build up in the soil over time, or it 

may degrade. 

APEX simulates all of the basic biological, chemical, 

hydrological, and meteorological processes of farming 

systems and their interactions. Soil erosion is simulated over 

time, including wind erosion, sheet and rill erosion, and the 

loss of sediment beyond the edge of the field. The nitrogen, 

phosphorus, and carbon cycles are simulated, including 

chemical transformations in the soil that affect their 

availability for plant growth or for transport from the field. 

Exchange of gaseous forms between the soil and the 

atmosphere is simulated, including losses of gaseous nitrogen 

compounds. 

The NRI-CEAP Cropland Survey was the primary source of 

information on all farming activities simulated using APEX. 

Crop data were transformed for the model into a crop rotation 

for each sample point, which was then repeated over the 47-

year simulation. The 3 years of data reported in the survey 

were represented in the model simulation as 1-, 2-, 3-, 4-, or 5-

year crop rotations. For example, a 2-year corn-soybean 

rotation was used if the operator reported that corn was grown 

in the first year, soybeans in the second year, and corn again in 

the third year. In this case, only 2 of the reported 3 years of 

survey data were used. If management differed significantly 

for the 2 years that corn was grown (manure was applied, for 

example, or tillage was different), the rotation was expanded 

to 4 years, retaining the second year of corn and repeating the 

year of soybeans. In addition, some rotations with alfalfa or 

grass seed were simulated as 5-year rotations. Specific rules 

and procedures were established for using survey data to 

simulate cover crops, double crops, complex systems such as 

intercropping and nurse crops, perennial hay in rotations, 

abandoned crops, re-planting, multiple harvests, manure 

applications, irrigation, and grazing of cropland before and 

after harvest.11 

Use of conservation practices in the Chesapeake Bay region 

was obtained from four sources: (1) NRI-CEAP Cropland 

Survey, (2) NRCS field offices, (3) USDA Farm Service 

Agency (FSA), and (4) the 2003 NRI. For each sample point, 

data from these four sources were pooled and duplicate 

practices discarded.12 

11 For a detailed description of the rules and procedures, see “Transforming 

Survey Data to APEX Model Input Files,” 

http://www.nrcs.usda.gov/technical/nri/ceap. 

12 For a detailed description of the rules and procedures for simulation of 

structural conservation practices, see “Modeling Structural Conservation 

Practices in APEX,” http://www.nrcs.usda.gov/technical/nri/ceap. 

37 
 

 

 

 

 

 

 

 

 

 

 

Simulating the No-Practice Scenario

The purpose of the no-practice scenario is to provide an 

estimate of sediment, nutrient, and pesticide loss from farm 

fields under conditions without the use of conservation 

practices. The benefits of conservation practices in use within 

the Chesapeake Bay region were estimated by contrasting 

model output from the no-practice scenario to model output 

from the baseline conservation condition (2003–06). The only 

difference between the no-practice scenario and the baseline 

conservation condition is that the conservation practices are 

removed or their effects are reversed in the no-practice 

scenario simulations. There were usually several alternatives 

that could be used to represent “no practices.” The no-practice 

representations derived for use in this study conformed to the 

following guidelines. 

	 Consistency: It is impossible to determine what an 

individual farmer would be doing if he or she had not 

adopted certain practices, so it is important to represent all 

practices on all sample points in a consistent manner that 

is based on the intended purpose of each practice. 

	 Simplicity: Complex rules for assigning “no-practice” 

activities lead to complex explanations that are difficult to 

substantiate and sometimes difficult to explain and accept. 

Complexity would not only complicate the modeling 

process but also hamper the interpretation of results. 

	 Historical context avoided: The no-practice scenario is a 

technological step backward for conservation, not a 

chronological step back to a prior era when conservation 

practices were not used. Although the advent of certain 

conservation technologies can be dated, the adoption of 

technology is gradual, regionally diverse, and ongoing. It 

is also important to retain the overall crop mix in the 

region, as it in part reflects today’s market forces. 

Therefore, moving the clock back to 1950s (or any other 

time period) agriculture is not the goal of the no-practice 

scenario. Taking away the conservation ethic is the goal. 

	 Moderation: The no-practice scenario should provide a 

reasonable level of “poor” conservation so that a 

believable benefit can be determined, where warranted, 

but not so severe as to generate exaggerated conservation 

gains by simulating the worst-case condition. Tremendous 

benefits could be generated if, for example, nutrients were 

applied at twice the recommended rates with poor timing 

or application methods in the no-practice simulation. 

Similarly, large erosion benefits could be calculated if the 

no-practice representation for tillage was fall plowing 

with moldboard plows and heavy disking, which was 

once common but today would generally be considered 

economically inefficient. 

	 Maintenance of crop yield or efficacy. It is impossible 

to avoid small changes in crop yields, but care was taken 

to avoid no-practice representations that would 

significantly change crop yields and regional production 

capabilities. The same guideline was followed for pest 

control—the suite of pesticides used was not adjusted in 

the no-practice scenario because of the likelihood that 

alternative pesticides would not be as effective and would 

result in lower yields under actual conditions. 

A deliberate effort was made to adhere to these guidelines to 

the same degree for all conservation practices so that the 

overall level of representation would be equally moderate for 

all practices. 

Table 11 summarizes the adjustments to conservation 

practices used in simulation of the no-practice scenario. 

No-practice representation of structural practices 

The no-practice field condition for structural practices is 

simply the removal of the structural practices from the 

modeling process. In addition, the soil condition is changed 

from “Good” to “Poor” for the determination of the runoff 

curve number for erosion prediction. 

Overland flow. This group includes such practices as terraces 

and contouring which slow the flow of water across the field. 

For the practices affecting overland flow of water and 

therefore the P factor of the USLE-based equations, the P 

factor was increased to 1. Slope length is also changed for 

practices such as terraces to reflect the absence of these slopeinterrupting practices. 

Concentrated flow. This group of practices is designed to 

address channelized flow and includes grassed waterways and 

grade stabilization structures. These practices are designed to 

prevent areas of concentrated flow from developing gullies or 

to stabilize gullies that have developed. The no-practice 

protocol for these practices removes the structure or waterway 

and replaces it with a “ditch” as a separate subarea. This ditch, 

or channel, represents a gully; however, the only sediment 

contributions from the gully will come from downcutting. 

Headcutting and sloughing of the sides are not simulated in 

APEX. 

Edge of field. These practices include buffers, filters, and 

other practices that occur outside the primary production area 

and act to mitigate the losses from the field. The no-practice 

protocol removes these areas and their management. When the 

practices are removed, the slope length is also restored to the 

undisturbed length that it would be if the practices were not in 

place. (When simulating a buffer in APEX, the slope length 

reported in the NRI is adjusted.) 

Wind control. Practices such as windbreaks or shelterbelts, 

cross wind ridges, stripcropping or trap strips, and hedgerows 

are examples of practices used for wind control. The 

unsheltered distance reflects the dimensions of the field as 

modeled, 400 meters or 1,312 feet. Any practices reducing the 

unsheltered distance are removed and the unsheltered distance 

set to 400 meters. 

38 
 

 

Table 11. Construction of the no-practice scenario for the Chesapeake Bay region 

Criteria used to determine if a practice was in use Adjustment made to create the no-practice scenario 

Practice adjusted 

Structural practices 1. Overland flow practices present 

2.	 Concentrated flow—managed structures or 

waterways present 

3.	 Edge-of-field mitigation practices present 

4.	 Wind erosion control practices present 

Residue and tillage management STIR ≤100 for any crop within a crop year 

Cover crop Cover crop planted for off-season protection 

Irrigation Pressure systems 

Nitrogen rate Total of all applications of nitrogen (commercial 

fertilizer and manure applications) ≤1.4 times harvest 

removal for non-legume crops, except for cotton and 

small grain crops 

Total of all applications of nitrogen (commercial 

fertilizer and manure applications) ≤1.6 times harvest 

removal for small grain crops 

Total of all applications of nitrogen (commercial 

fertilizer and manure applications) for cotton ≤60 pounds 

per bale 

Phosphorus rate Applied total of fertilizer and manure P over all crops in 

the crop rotation ≤ 1.1 times total harvest P removal over 

all crops in rotation. 

Commercial fertilizer application Incorporated or banded 

method 

Manure application method Incorporated, banded, or injected 

Commercial fertilizer application Within 3 weeks prior to planting, at planting, or within 

timing 60 days after planting. 

Pesticides 1. Practicing high level of IPM 

2. Practicing moderate level of IPM 

3. Spot treatments 

4. Partial field treatments 

1.	 USLE P-factor changed to 1 and slope length increased for 

points with terraces, soil condition changed from good to 

poor. 

2.	 Structures and waterways replaced with earthen ditch, soil 

condition changed from good to poor. 

3.	 Removed practice and width added back to field slope 

length. 

4.	 Unsheltered distance increased to 400 meters 

Add two tandem diskings 1 week prior to planting 

Remove cover crop simulation (field operations, fertilizer, grazing, 

etc.) 

Change to hand-move sprinkler system except where the existing 

system is less efficient 

Increase rate to 1.98 times harvest removal (proportionate increase 

in all reported applications, including manure) 

Increase rate to 2.0 times harvest removal (proportionate increase 

in all reported applications, including manure) 

Increase rate to 90 pounds per bale (proportionate increase in all 

reported applications, including manure) 

Increase commercial P fertilizer application rates to reach 2.2 

times harvest removal for the crop rotation (proportionate increase 

in all reported applications over the rotation), accounting also for 

manure P associated with increase to meet nitrogen applications 

for no practice scenario. Manure applications were NOT increased 

to meet the higher P rate for the no-practice scenario. 

Change to surface broadcast 

Change to surface broadcast 

Moved to 3 weeks prior to planting. Manure applications were not 

adjusted for timing in the no-practice scenario. 

1. All incorporated applications changed to surface application. 

For each crop, the first application event after planting and 30 

days prior to harvest replicated twice, 1 week and 2 weeks 

later than original. 

2. Same as for high level of IPM, except replication of first 

application only 1 time, 1 week after original 

3. Application rates for spot treatments were adjusted upward 

relative to the baseline rate to represent whole-field application 

(see text) 

4. Application rates for partial field treatments were adjusted 

upward relative to the baseline rate to represent whole-field 

application (see text)
 

39 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

No-practice representation of conservation tillage 

The no-practice tillage protocols are designed to remove the 

benefits of conservation tillage. For all crops grown with some 

kind of reduced tillage, including cover crops, the no-practice 

scenario simulates conventional tillage, based on the STIR 

(Soil Tillage Intensity Rating) value. Conventional tillage for 

the purpose of estimating conservation benefits is defined as 

any crop grown with a STIR value above 100. (To put this in 

context, no-till or direct seed systems have a STIR of less than 

30, and that value is part of the technical standard for Residue 

Management, No-Till/Strip Till/Direct Seed [NRCS Practice 

Standard 329]). Those crops grown with a STIR value of less 

than 100 in the baseline conservation condition had tillage 

operations added in the no-practice scenario. 

Simulating conventional tillage for crops with a STIR value of 

less than 100 requires the introduction of additional tillage 

operations in the field operations schedule. For the no-practice 

scenario, two consecutive tandem disk operations were added 

prior to planting. In addition to adding tillage, the hydrologic 

condition for assignment of the runoff curve number was 

changed from good to poor on all points receiving additional 

tillage. Points that are conventionally tilled for all crops in the 

baseline condition scenario are also modeled with a “poor“ 

hydrologic condition curve number. 

The most common type of tillage operation in the survey was 

disking, and the most common disk used was a tandem disk 

for nearly all crops, in all parts of the region, and for both 

dryland and irrigated agriculture. The tandem disk has a STIR 

value of 39 for a single use. Two consecutive disking 

operations will add 78 to the existing tillage intensity, which 

allows for more than 90 percent of the crops to exceed a STIR 

of 100 and yet maintain the unique suite and timing of 

operations for each crop in the rotation. Although a few 

sample points will have STIR values in the 80s or 90s after 

adding the two disking operations, the consistency of an 

across-the-board increase of 78 is simple and provides the 

effect of a distinctly more intense tillage system. 

These additional two tillage operations were inserted in the 

simulation one week prior to planting, one of the least 

vulnerable times for tillage operations because it is close to the 

time when vegetation will begin to provide cover and 

protection. 

No-practice representation of cover crops 

The no-practice protocol for this practice removes the planting 

of the crop and all associated management practices such as 

tillage and fertilization. In a few cases the cover crops were 

grazed; when the cover crops were removed so were the 

grazing operations. 

No-practice representation of irrigation practices 

The no-practice irrigation protocols were designed to remove 

the benefits of better water management and the increased 

efficiencies of modern irrigation systems. Irrigation 

efficiencies are represented in APEX by a combination of 

three coefficients that recognize water losses from the water 

source to the field, evaporation losses with sprinkler systems, 

percolation losses below the root-zone during irrigation, and 

runoff at the lower end of the field. These coefficients are 

combined to form an over-all system efficiency that varies 

with soil type and land slope. 

The quantity of water applied for all scenarios was simulated 

in APEX using an “auto-irrigation” procedure that applied 

irrigation water when the degree of plant stress exceeded a 

threshold. “Auto-irrigation” amounts were determined within 

pre-set single event minimums and maximums, and an annual 

maximum irrigation amount. APEX also used a predetermined minimum number of days before another irrigation 

event regardless of plant stress. 

In the no-practice representation, all conservation practices, 

such as Irrigation Water Management and Irrigation Land 

Leveling, were removed and samples with pressurized 

systems, such as center pivot, side roll, and low flow (drip), 

were changed to “hand move sprinklers,” which represents an 

early form of pressure system. The “Big Gun” systems, which 

comprise 9.1 percent of the irrigated acres, are by and large 

already less efficient than the “hand move sprinklers,” and 

most were not converted. However, 1.3 percent of the 

irrigated acres served by “Big Gun” systems are more efficient 

than the “hand move sprinklers,” and these were converted in 

the no-practice representation. “Open discharge” gravity 

systems are used on approximately 5,300 acres or 2.5 percent 

of the irrigated area. The no-practice representation of gravity 

systems would use a ditch system with portals which is more 

efficient than the open discharge configuration, so these also 

were not converted. 

For the no-practice scenario, the percentage of irrigated 

acreage with hand-move lines with impact sprinkler heads was 

increased to 89.7 percent (from 43.9 percent in the baseline 

conservation condition), 7.8 percent retained the Big Gun 

systems that were in use, and 2.5 percent were simulated with 

open discharge flood irrigation. 

No-practice representation of nutrient management 

practices 

The no-practice nutrient management protocols are designed 

to remove the benefits of proper nutrient management 

techniques. 

The NRCS Nutrient Management standard (590) allows a 

variety of methods to reduce nutrient losses while supplying a 

sufficient amount of nutrient to meet realistic yield goals. The 

standard addresses nutrient loss in one of two primary ways: 

(1) by altering rates, form, timing, and methods of application, 

or (2) by installing buffers, filters, or erosion or runoff control 

practices to reduce mechanisms of loss. The latter method is 

covered by the structural practices protocols for the nopractice scenario. The goals of the nutrient management nopractice protocols are to alter three of the four basic aspects of 

nutrient application—rate, timing, and method. The form of 

application was not addressed because of the inability to 

determine if proper form was being applied. 

40 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Nitrogen rate. For the no-practice scenario, the amount of 

commercial nitrogen fertilizer applied was— 

 increased to 1.98 times harvest removal for non-legume 

crops receiving less than or equal to 1.40 times the 

amount of nitrogen removed at harvest in the baseline 

scenario, except for cotton and small grain crops; 

 increased to 2.0 times harvest removal for small grain
 

crops receiving less than or equal to 1.60 times the 

amount of nitrogen removed at harvest in the baseline 

scenario, and 

 increased to 90 pounds per bale for cotton crops receiving 

less than 60 pounds of nitrogen per bale in the baseline 

scenario. 

The ratio of 1.98 for the increased nitrogen rate was 

determined by the average rate-to-yield-removal ratio for 

crops exceeding the application-removal ratio of 1.4. Where 

nitrogen was applied in multiple applications, each application 

was increased proportionately. For sites receiving manure, the 

threshold for identifying good management was the total 

nitrogen application rate, both manure and fertilizer, and both 

fertilizer and manure were increased proportionately to reach 

the no-practice scenario rate. The assessment for using 

appropriate nitrogen application rates was made on an average 

annual basis for each crop in the rotation using average annual 

model output on nitrogen removed with the yield at harvest in 

the baseline conservation condition scenario. 

Phosphorus rate. The threshold for identifying proper 

phosphorus application rates was 1.1 times the amount of 

phosphorus taken up by all the crops in rotation and removed 

at harvest. The lower threshold for phosphorus was used 

because phosphorus is not lost through volatilization to the 

atmosphere and much less is lost through other pathways 

owing to strong bonding of phosphorus to soil particles. For 

the no-practice scenario, the amount of commercial 

phosphorus fertilizer applied was increased to 2.2 times the 

harvest removal rate. For crops receiving manure, any increase 

in phosphorus from manure added to meet the nitrogen criteria 

for no-practice was taken into account in setting the nopractice application rate. However, no adjustment was made to 

manure applied at rates below the P threshold because the 

appropriate manure rate was based on the nitrogen level in the 

manure. The ratio of 2.2 for the increased phosphorus rate was 

determined by the average rate-to-yield-removal ratio for 

crops with phosphorus applications exceeding 1.1 times the 

amount of phosphorus taken up by all the crops in rotation and 

removed at harvest. Multiple commercial phosphorus fertilizer 

applications were increased proportionately to meet the 2.2 

threshold. 

Timing of application. Nutrients applied closest to the time 

when a plant needs them are the most efficiently utilized and 

least likely to be lost to the surrounding environment. All 

commercial fertilizer applications occurring within 3 weeks 

prior to planting, at planting, or within 60 days after planting 

were moved back to 3 weeks prior to planting for the nopractice scenario. For example, split applications that occur 

within 60 days after planting are moved to a single application 

3 weeks before planting. Timing of manure applications was 

not adjusted in the no-practice scenario. 

Method of application. Nutrient applications, including 

manure applications, that were incorporated or banded were 

changed to a surface broadcast application method. 

No-practice representation of pesticide management 

practices 

Pesticide management for conservation purposes is a 

combination of three types of interrelated management 

activities: 

1.	 A mix of soil erosion control practices that retain 

pesticide residues within the field boundaries. 

2.	 Pesticide use and application practices that minimize the 

risk that pesticide residues pose to the surrounding 

environment. 

3.	 Practice of Integrated Pest Management (IPM), including 

partial field applications and spot treatment. 

The first activity is covered by the no-practice representation 

of structural practices and residue and tillage management. 

The second activity, for the most part, cannot be simulated in 

large-scale regional modeling because of the difficulty in 

assuring that any changes in the types of pesticides applied or 

in the method or timing of application would provide 

sufficient protection against pests to maintain crop yields.13 

Farmers, of course, have such options, and environmentally 

conscientious farmers make tradeoffs to reduce environmental 

risk. But without better information on the nature of the pest 

problem both at the field level and in the surrounding area, 

modelers have to resort to prescriptive and generalized 

approaches to simulate alternative pesticides and application 

techniques, which would inevitably be inappropriate for many, 

if not most, of the acres simulated. 

The no-practice representation for pesticide management is 

therefore based on the third type of activity—practicing IPM. 

One of the choices for methods of pesticide application on the 

survey was “spot treatment.” Typically, spot treatments apply 

to a small area within a field and are often treated using a 

hand-held sprayer. Spot treatment is an IPM practice, as it 

requires scouting to determine what part of the field to treat 

and avoids treatment of parts of the field that do not have the 

pest problem. The reported rate of application for spot 

treatments was the rate per acre treated. For the baseline 

simulation, it was assumed that all spot treatments covered 5 

percent of the field. Since the APEX model run and associated 

acreage weight for the sample point represented the whole 

field, the application rate was adjusted downward to 5 percent 

of the per-acre rate reported for the baseline scenario. For the 

no-practice scenario, the rate as originally reported was used, 

simulating treatment of the entire field rather than 5 percent of 

the field. In the Chesapeake Bay region, there were four 

sample points with spot treatments, representing less than 1 

percent of cropped acres. 

13 The APEX model can simulate pesticide applications, but it does not 

currently include a pest population model that would allow simulation of the 

effectiveness of pest management practices. Thus, the relative effectiveness of 

pesticide substitution or changes in other pest management practices cannot be 

evaluated. 

41 
 

 

 

 

 

Partial field treatments were simulated in a manner similar to 

spot treatments. For the baseline scenario, application rates 

were reduced proportionately according to how much of the 

field was treated. For the no-practice scenario, the rate as 

reported in the survey was used, simulating treatment of the 

entire field. However, this adjustment for the no-practice 

scenario was only done for partial field treatments less than 

one-third of the field, as larger partial field treatments could 

have been for reasons unrelated to IPM. In the Chesapeake 

Bay region, there were eight sample points with partial field 

treatments, representing about 1 percent of cropped acres. 

The IPM indicator, described in the previous chapter, was 

used to adjust pesticide application methods and to increase 

the frequency of applications to represent “no IPM practice.” 

For samples classified as having either high or moderate IPM 

use, all soil-incorporated pesticide applications in the baseline 

condition were changed to surface applications in the nopractice scenario. For high IPM cases, the first application 

event between planting and 30 days before harvest was 

replicated twice for each crop, one week and two weeks after 

its original application. For moderate IPM cases, the first 

application event was replicated one time for each crop, one 

week after its original application. 

No-practice representation of land in long-term 

conserving cover 

The no-practice representation of land in long-term conserving 

cover is cultivated cropping with no conservation practices in 

use. For each CRP sample point, a set of cropping simulations 

was developed to represent the probable mix of management 

that would be applied to the point if it were cropped. Cropped 

sample points were matched to each CRP sample point on the 

basis of slope, soil texture, soil hydrologic group, and 

geographic proximity. The cropped sample points that 

matched most closely were used to represent the cropped 

condition that would be expected at each CRP sample point if 

the field had not been enrolled in CRP. In most cases, seven 

“donor” points were used to represent the crops that were 

grown and the various management activities to represent 

crops and management for the CRP sample point “as if” the 

acres had not been enrolled in CRP. The crops and 

management activities of each donor crop sample were 

combined with the site and soil characteristics of the CRP 

point for the no-practice representation of land in long-term 

conserving cover. 

42 
 

 

 

 

 

 

 

 

 

Effects of Practices on Fate and Transport Figure 13. Estimates of average annual water lost through 

three loss pathways for cropped acres in the Chesapeake Bay of Water 

Water is a potent force that interacts with or drives almost all region 

environmental processes acting within an agricultural 

production system. The hydrologic conditions prevalent in the 

0 

5 

10 

15 

20 

25 

30 

35 

40

Average annual water loss (inches) 

Chesapeake Bay region are critical to understanding the 

estimates of sediment, nutrient, and pesticide loss presented in 

subsequent sections. The APEX model simulates hydrologic 

processes at the field scale—precipitation, irrigation, 

evapotranspiration, surface water runoff, infiltration, and 

percolation beyond the bottom of the soil profile. 

Baseline condition for cropped acres 

Precipitation and irrigation are the sources of water for a field. 

Annual precipitation over the 47-year simulation averaged 

about 42 inches in this region (table 12). (Also see figs. 5 and 

6.) Only about 5 percent of cropped acres are irrigated, at an 

average application of 12 inches per year. 0 10 20 30 40 50 60 70 80 90 100 

Cumulative percent acres Most of the water that leaves the field is lost through 

evaporation and transpiration (evapotranspiration) (fig. 13). Evapotranspiration Surface water runoff Evapotranspiration is the dominant loss pathway for 98 

percent of cropped acres. On average, about 64 percent of the 

water loss for cropped acres in this region is through 

evapotranspiration (table 12). Model results indicate that 

evapotranspiration losses vary, however, according to soil 

characteristics and land cover; evapotranspiration ranges from 

about 40 percent to 80 percent of the total amount of water 

that leaves the field (fig. 14). 

Subsurface flow 

Figure 14. Cumulative distributions of the proportion of 

water lost through three loss pathways for cropped acres, 

Chesapeake Bay region 

0.0 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

0.7 

0.8 

0.9 

1.0

Proportion of water loss pathways 

Loss of water through subsurface flow pathways is the second 

largest source of water loss at an average of about 11 inches 

per year for cropped acres (table 12). Subsurface flow 

pathways include: (1) deep percolation to groundwater, 

including groundwater return flow to surface water, (2) 

subsurface flow into a tile or ditch drainage system, (3) lateral 

subsurface outflow, and (4) quick-return subsurface flow. The 

percentage of water loss represented by subsurface flows 

averages about 25 percent for cropped acres (table 12). 

However, this percentage varies from less than 10 percent to 

over 50 percent for cropped acres in the Chesapeake Bay 

region, as shown in figure 14. 

Surface water runoff averages about 12 percent of water loss 0 10 20 30 40 50 60 70 80 90 100 
 

for cropped acres (table 12), ranging from less than 5 percent 

to 20 percent (fig. 14). Average surface water loss for cropped 

acres is about 5 inches per year (table 12). The amount of 

surface water runoff varies from acre to acre, ranging from an 

annual average of about 2 inches per year for some acres to 

over 10 inches per year. 

Effects of conservation practices on cropped acres 

Structural water erosion control practices, residue 

management practices, and reduced tillage slow the flow of 

surface water runoff and allow more of the water to infiltrate 

into the soil. Model simulations indicate that conservation 

practices have reduced surface water runoff by about 0.7 inch 

per year averaged over all acres, representing a 12-percent 

reduction for the region (table 12). 

. 

Cumulative percent acres 

Evapotranspiration Surface water runoff 

Subsurface flow 

Note: The horizontal axis consists of percentiles for each pathway; a given 

percentile for one curve will not represent the same acres on another curve. 

The re-routing of surface water to subsurface flows is shown 

graphically in figures 15 and 16 for cropped acres The nopractice scenario curve in figure 15 shows what the 

distribution of surface water runoff would be if there were no 

conservation practices in use—more surface water runoff and 

thus less subsurface flow. 

43 
 

 

 

 

 

Table 12. Field-level effects of conservation practices on water loss pathways for cultivated cropland in the Chesapeake Bay region 

Baseline conservation No-practice Reduction due Percent 

Model simulated outcome condition scenario to practices reduction 

Cropped acres (4.3 million acres) 

Water sources 

Non-irrigated acres 

Average annual precipitation (inches) 42.2 42.2 0.0 0 

Irrigated acres 

Average annual precipitation (inches) 42.8 42.8 0.0 0 

Average annual irrigation water applied (inches)* 12.3 22.7 10.4 46 

Water loss pathways 

Average annual evapotranspiration (inches) 27.7 28.0 0.2 1 

Average annual surface water runoff (inches) 5.1 5.7 0.7 12 

Average annual subsurface water flows (inches)** 10.7 9.9 -0.8*** -8*** 

Land in long-term conserving cover (0.1 million acres) 

Water sources* 

Average annual precipitation (inches) 41.9 41.9 0.0 0 

Average annual irrigation water applied (inches)* 0.0 0.1 0.1 100 

Water loss pathways 

Average annual evapotranspiration (inches) 25.1 26.7 1.7 6 

Average annual surface water runoff (inches) 4.3 5.9 1.6 27 

Average annual subsurface water flow (inches)** 12.7 9.5 -3.2*** -33*** 

* About 5 percent of the cropped acres in the Chesapeake Bay region are irrigated. Land in long-term conserving cover was not irrigated, but some farming practices 

used to simulate a cropped condition to represent the no-practice scenario included irrigation. Values shown in the table for land in long-term conserving cover are 

averages over all acres, including non-irrigated acres. 

** Subsurface flow pathways include: (1) deep percolation to groundwater, including groundwater return flow; (2) subsurface flow into a drainage system; (3) lateral 

subsurface outflow; and (4) quick-return subsurface flow.
 

*** Represents an average gain in subsurface flows of 0.8 inch per year (8 percent increase) for cropped acres due to the use of conservation practices; represents an 

average gain of 3.2 inches in subsurface flow for land in long-term conserving cover.
 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text.
 

Note: Model simulation results for the baseline conservation condition are presented in appendix B for the four subregions.
 

Figure 15. Estimates of average annual surface water runoff Figure 16. Estimates of average annual reduction in surface 

for cropped acres in the Chesapeake Bay region water runoff due to the use of conservation practices on 

cropped acres in the Chesapeake Bay region 12

Average annual surface water runoff 

(inches) 

11 

10 

9 

8 

7
 

6
 

5
 

4
 

3
 

2
 

1
 

0
 

0 10 20 30 40 50 60 70 80 90 100 
 

3.0

surface water runoff (inches) -0.5 

0 10 20 30 40 50 60 70 80 90 100 

Cumulative percent acres 

Note: About 12 percent of the acres had less surface water runoff in the nopractice scenario than the baseline conservation condition. In general, these 

gains in surface water runoff due to practices occur on soils with low to 

moderate potential for surface water runoff together with: (1) higher nutrient 

application rates in the no-practice scenario that result in more biomass 

production, which can reduce surface water runoff (typically rotations with 

hay or continuous corn); or (2) the additional tillage simulated in the nopractice scenario provided increased random roughness of the surface 

reducing runoff on nearly level landscapes with low crop residue rotations. Average annual reduction in 

2.5
 

2.0
 

1.5
 

1.0
 

0.5
 

0.0
 

Cumulative percent acres 

Baseline conservation condition 

No-practice scenario 

44 
 

 

 

 

 

 

 

 

Reductions in surface water runoff due to conservation Figure 17. Estimates of average annual reduction in surface 

practices range to over 3 inches per year (fig. 16).The water runoff due to conversion to long-term conserving cover 

variability in reductions due to practices reflects different in the Chesapeake Bay region 

levels of conservation treatment as well as differences in 5 precipitation and inherent differences among acres for water to 

run off. 

4
 

Use of improved irrigation systems in the Chesapeake Bay 

region increases overall system efficiency from 42 percent in 

the no-practice scenario to 68 percent in the baseline scenario. 

This change in efficiency represents an annual decreased need 

of water diversions of about 10 inches where irrigation is used 

(table 12). 

Land in long-term conserving cover 

Model simulations further show that land in long-term 

conserving cover (baseline conservation condition) in the 

region also has, on average, less surface water runoff and 

more subsurface flow than would occur if the land was 

cropped (table 12). Evapotranspiration is slightly lower for 

land in long-term conserving cover, as well. 

Average annual reduction in surface water 

runoff for acres in long-term conserving 

cover (inches) 

3
 

2
 

1
 

0
 

-1
 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres

Reductions in surface water runoff due to conversion to longterm conserving cover average 1.6 inches per year in this 

region (table 12), but range to above 4 inches per year for 

some acres (fig. 17). 

Cumulative Distributions Show How Effects of Conservation Practices Vary 

Throughout the Region
 

The design of this study provides the opportunity to examine not only the overall mean value for a given outcome, 

but also the entire distribution of outcomes. This is possible because outcomes are estimated for each of the 771 

sample points used to represent cropped acres in the Chesapeake Bay region and for each of the 61 sample points 

used to represent land in long-term conserving cover. Cumulative distributions show the full set of estimates and thus 

demonstrate how conditions and the effects of conservation practices vary throughout the region. 

Cumulative distributions shown in this report are plots of the value for each percentile. In figure 15, for example, the 

curve for average annual surface water runoff for the baseline conservation condition consists of each of the 

percentiles of the distribution of 771 surface water runoff estimates, weighted by the acres associated with each 

sample point. The 10th percentile for the baseline conservation condition is 3.1 inches per year, indicating that 10 

percent of the acres have 3.1 inches or less of surface water runoff, on average. Similarly, the same curve shows that 

25 percent of the acres have surface water runoff less than 3.9 inches per year. The 50th percentile—the median—is 

4.8 inches per year, which in this case is close to the mean value of 5.1 inches per year. At the high end of the 

distribution, 90 percent of the acres in this region have surface water runoff less than 7.3 inches per year; and 

conversely, 10 percent of the acres have surface water runoff greater than 7.3 inches per year. 

Thus, the distributions show the full range of outcomes for cultivated cropland acres in the Chesapeake Bay region. 

The full range of outcomes for the baseline condition is compared to that for the no-practice scenario in figure 15 to 

illustrate the extent to which conservation practices reduce surface water runoff throughout the region. 

Figure 16 shows the effects of conservation practices on surface water runoff using the distribution of the reduction in 

surface water runoff, calculated as the outcome for the no-practice scenario minus the outcome for the baseline 

conservation condition at each of the 771 cropped sample points. This distribution shows that, while the mean 

reduction is 0.7 inch per year, 5 percent of the acres have reductions due to conservation practices greater than 1.5 

inches per year and 12 percent of the acres actually have small increases in surface water runoff (i.e., negative 

reductions) as a result of soil erosion control conservation practice use. (See footnote to figure 16 for an explanation 

of the conditions that result in gains in surface water runoff due to conservation practices.) 

45 
 

 

 

Effects of Practices on Wind Erosion Table 13. Average annual wind erosion (tons/acre) for 

Wind erosion removes the most fertile parts of the soil such as 

the lighter, less dense soil constituents including organic 

matter, clays, and silts. Wind erosion occurs when the soil is 

unprotected and wind velocity exceeds about 13 miles per 

hour near the surface. Wind velocity, tillage, vegetative cover, 

and the texture and structure of the soil are primary 

determinants of wind erosion. Wind erosion is estimated in 

APEX using the Wind Erosion Continuous Simulation 

(WECS) model. The estimated wind erosion rate is the amount 

of eroded material leaving the downwind edge of the field. 

Wind erosion is a relatively minor resource concern in the 

Chesapeake Bay region. The greatest concern with wind 

erosion in the Chesapeake Bay region is crop damage to 

young seedlings exposed to windblown material. Wind 

erosion rates as low as 0.5 ton per acre are known to cause 

physical damage to young seedlings. 

For all cropped acres, model simulations show that the 

average annual rate of wind erosion is only 0.03 ton per acre 

(table 13). Model simulations further show, however, that 

wind erosion can be an issue in some years for some acres 

(fig. 18). In the most extreme year included in the model 

simulations (representing 1997), annual wind erosion 

exceeded 0.5 ton per acre for 6 percent of the cropped acres. 

cultivated cropland in the Chesapeake Bay region 

Baseline No- Reduction 

conservation practice due to Percent 

condition scenario practices reduction 

Cropped acres 0.027 0.056 0.029 52 

Land in longterm 

conserving 

cover <0.001 0.008 0.008 100 

Note: Percent reductions were calculated prior to rounding the values for 

reporting in the table and the associated text. Model simulation results for the 

baseline conservation condition are presented in appendix B for the 4 

subregions. 

Structural practices for wind erosion control are in use on only 

7 percent of the cropped acres in the Chesapeake Bay region. 

However, other practices common in the region, such as 

residue and tillage management, reduced tillage, and various 

water erosion control practices, are also effective in reducing 

wind erosion. Model simulations indicate that conservation 

practices have reduced the average wind erosion rate by 52 

percent in the region (table 13). Even though wind erosion is 

not a major resource concern in the Chesapeake Bay region, 

these reductions in wind erosion rates are still significant. 

Figure 18. Distribution of annual wind erosion rate for each year of the 47-year model simulation, Chesapeake Bay region Annual wind erosion (tons/acre) 

1.6 

1.4 

1.2 

1.0 

0.8 

0.6 

0.4 

0.2 

0.0 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres 

Note: This figure shows how annual wind erosion (tons per acre per year) varies within the region and from year to year in the model simulation for cropped acres. 

Each of the 47 curves shown above represents a single year of data and shows how annual wind erosion varies over the region in that year, starting with the acres with 

the lowest rates and increasing to the acres with the highest rates. The family of curves shows how annual wind erosion rates vary from year to year. 

46 
 

 

 

 

 

 

 

 

 

 

 

 

Effects of Practices on Water Erosion and 

Sediment Loss 

Sheet and rill erosion is the detachment and movement of soil 

particles within the field that occurs during rainfall events. 

Controlling sheet and rill erosion is important for sustaining 

soil productivity and preventing soil from leaving the field. 

Sediment loss is the sediment that is transported beyond the 

edge of the field by water, where the field includes any edgeof-field filtering and buffering conservation practices. Soil 

erosion and sedimentation are separate but interrelated 

resource concerns. Soil erosion is the detachment and 

transport of soil particles, while sedimentation is that portion 

of the eroded material that settles out in areas onsite or offsite. 

Sediment loss, as estimated in this study, includes the portion 

of the sheet and rill eroded material that settles offsite as well 

as some sediment that originates from gully erosion 

processes.14 Sediment is composed of detached and 

transported soil minerals, organic matter, plant and animal 

residues, and associated chemical and biological compounds. 

The Chesapeake Bay region has one of the highest proportions 

of cropland classified as highly erodible for water erosion of 

all the basins studied (44 percent). Most of these soils occur in 

the Piedmont, Appalachian, or Allegheny mountain or plateau 

physiographic regions. Soils in this region tend to occur on 

moderately sloping to steep landscapes. They are often 

relatively shallow agricultural soils with approximately half of 

the HEL lands classified with a soil loss tolerance (T) of 3 

tons/acre/year. 

Sheet and rill erosion 

Model simulations show that sheet and rill erosion in the 

Chesapeake Bay region averages about 1.0 ton per acre per 

year (table 14). Sheet and rill erosion rates are higher for 

highly erodible land, averaging 1.6 tons per acre per year 

compared to the average annual rate for non-highly erodible 

land of 0.5 ton per acre. 

Model simulation results also show that conservation practices 

have reduced sheet and rill erosion on cropped acres in the 

Chesapeake Bay region by an average of 0.78 ton per acre per 

year, representing a 44-percent reduction on average (table 

14). While the average annual reduction in sheet and rill 

erosion for highly erodible land is more than three times that 

for non-highly erodible acres (table 14), the percent reduction 

due to conservation practices is about the same. 

For land in long-term conserving cover, sheet and rill erosion 

has been reduced from 2.48 tons per acre per year if cropped 

without conservation practices to 0.02 ton per acre (table 14), 

on average. 

14 For this study, the APEX model was set up to estimate sediment loss using 

a modified version of USLE, called MUSS, which uses an internal sediment 

delivery ratio to estimate the amount of eroded soil that actually leaves the 

boundaries of the field. A large percentage of the eroded material is 

redistributed and deposited within the field or trapped by buffers and other 

conservation practices and does not leave the boundary of the field, which is 

taken into account in the sediment delivery calculation. The estimate also 

includes some gully erosion and some ephemeral gully erosion. For this 

reason, sediment loss rates can exceed sheet and rill erosion rates. 

Sediment loss due to water erosion 

Baseline condition for cropped acres. The average annual 

sediment loss for cropped acres in the Chesapeake Bay region 

is 1.2 tons per acre per year, according to the model simulation 

(table 14). As seen for sheet and rill erosion, sediment loss for 

highly erodible land is much higher than for non-highly 

erodible land, even though a higher proportion of highly 

erodible acres have structural water erosion control practices 

in use. 

On an annual basis, sediment loss can vary considerably. 

Figure 19 shows that, with the conservation practices currently 

in use in the Chesapeake Bay region, annual sediment loss is 

below 2 tons per acre for about 60 percent of the acres under 

all conditions, including years with high precipitation. In 

contrast, sediment loss exceeds 12 tons per acre in one or 

more years on about 10 percent of the cropped acres. 

Figure 19 also illustrates the extent to which high sediment 

losses are restricted to a minority of acres within the region, 

even during years with high precipitation. These are the acres 

that have the highest inherent vulnerability to water erosion 

and have inadequate soil erosion control. 

Effects of conservation practices on cropped acres. Model 

simulations indicate that the use of conservation practices in 

the Chesapeake Bay region has reduced average annual 

sediment loss due to water erosion by 55 percent for cropped 

acres in the region, including both treated and untreated acres 

(table 14). Without conservation practices, the average annual 

sediment loss for these acres would have been 2.64 tons per 

acre per year compared to 1.18 tons per acre average for the 

baseline conservation condition. Figure 20 shows that about 

61 percent of the acres would have less than 2 tons per acre 

per year sediment loss without practices, on average, 

compared to 83 percent with conservation practices. 

Reductions in sediment loss due to conservation practices are 

much higher for some acres than others, reflecting both the 

level of treatment and the inherent erodibility of the soil. For 

about 60 percent of cropped acres, the average annual 

sediment loss reduction due to practices is less than 1 ton per 

acre (fig. 21). The top 10 percent of the acres had reductions 

in average annual sediment loss greater than 3.8 tons/acre. 

Acres with a combination of structural practices and residue 

and tillage management have the highest percent reduction in 

sediment loss (table 15). Acres that are treated with structural 

practices, meet tillage intensity criteria for no-till or mulch till, 

and are gaining soil organic carbon (about 19 percent of 

cropped acres) have reduced sediment loss by 80 percent, on 

average. For these treated acres, annual sediment loss averages 

only about 0.5 ton per acre. 

47 
 

 

 

 

 

 

 

 

 

 

Table 14. Field-level effects of conservation practices on erosion and sediment loss for cultivated cropland in the Chesapeake Bay 

region 

Baseline Reduction 

conservation No-practice due to Percent 

Model simulated outcome condition scenario practices reduction 

Cropped acres (4.3 million acres) 

Average annual sheet and rill erosion (tons/acre)* 0.99 1.77 0.78 44 

Average annual sediment loss at edge of field due to water 

erosion (tons/acre) 1.18 2.64 1.46 55 

Highly erodible land (44 percent of cropped acres) 

Average annual sheet and rill erosion (tons/acre)* 1.62 2.87 1.25 44 

Average annual sediment loss at edge of field due to water 

erosion (tons/acre) 2.07 4.63 2.56 55 

Non-highly erodible land (56 percent of cropped acres) 

Average annual sheet and rill erosion (tons/acre)* 0.50 0.92 0.41 45 

Average annual sediment loss at edge of field due to water 

erosion (tons/acre) 0.49 1.08 0.59 55 

Land in long-term conserving cover (0.1 million acres) 

Average annual sheet and rill erosion (tons/acre)* 0.02 2.48 2.46 99 

Average annual sediment loss at edge of field due to water 

erosion (tons/acre) 0.02 4.26 4.24 100 

* Estimated using the Revised Universal Soil Loss Equation. 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text.
 

Note: Model simulation results for the baseline conservation condition are presented in appendix B for the 4 subregions.
 

Figure 19. Distribution of annual sediment loss for each year of the 47-year model simulation, Chesapeake Bay region Annual sediment loss (tons/acre) 

40 

38 

36 

34 

32 

30 

28 

26 

24 

22 

20 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres 

Note: This figure shows how annual sediment loss (tons per acre per year) varies within the region and from year to year in the model simulation for cropped acres. 

Each of the 47 curves shown above represents a single year of data and shows how annual sediment loss varies over the region in that year, starting with the acres with 

the lowest sediment loss and increasing to the acres with the highest sediment loss. The family of curves shows how annual sediment loss varies from year to year. 

48 
 

Figure 20. Estimates of average annual sediment loss for Figure 21 Estimates of average annual reduction in sediment 

cropped acres in the Chesapeake Bay region loss due to the use of conservation practices on cropped acres 

in the Chesapeake Bay region 22
 

15
 

13
 

20
 

18

Average annual sediment loss (tons/acre) 16
 

14
 

12
 

10
 

8
 

6
 

4
 

2
 

0
 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual reduction in sediment loss 

11
 

9
 

(tons/acre) 7
 

5
 

3
 

1
 

-1
 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres Cumulative percent acres 

Baseline conservation condition 

Note: About 2 percent of the acres had less sediment loss in the no-practice No-practice scenario scenario than the baseline conservation condition, resulting from the increase 

is surface water runoff on some acres due to conservation practices. See 

footnote to figure 16. 

Table 15. Estimates of effects of combinations of structural practices and residue and tillage management on average annual sediment 

loss for cropped acres in the Chesapeake Bay region 

Average annual sediment loss (tons/acre) 

Percent of Baseline Reduction 

cropped conservation No-practice due to Percent 

Conservation treatment acres condition scenario practices reduction 

No-till or mulch till with carbon gain, no 

structural practices 21 0.34 0.81 0.46 57 

No-till or mulch till with carbon loss, no 

structural practices 26 1.30 2.00 0.70 35 

Some crops with reduced tillage, no structural 

practices 2 1.25 1.68 0.43 26 

Structural practices and no-till or mulch till with 

carbon gain 19 0.50 2.55 2.05 80 

Structural practices and no-till or mulch till with 

carbon loss 21 2.19 5.24 3.05 58 

Structural practices and some crops with reduced 

tillage 3 0.69 2.80 2.11 75 

Structural practices only 3 2.16 4.06 1.90 47 

No water erosion control treatment 4 2.22 2.29 0.08 3* 

All acres 100 1.18 2.64 1.46 55 

* For non-irrrigated sample points, the reduction due to practices for these acres with no water erosion control treatment was close to zero, as expected. For irrigated 

sample points, additional irrigation water was added to simulate lower water use efficiencies in the no-practice scenario, which contributes to higher sediment loss in the 

no-practice scenario. In addition, sediment loss was slightly affected by the higher nutrient application rates simulated in the no-practice scenario to estimate the 

benefits of nutrient management practices where they occurred. 

Note: Differences in slope, soil texture, hydrologic group, and precipitation for acres in different treatment groups account for some of the differences shown in this 

table. Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Percents may not add to totals because of 

rounding. 

49 
 

 

 

 

 

 

Effects of cover crops on sediment loss. A “what if” scenario 

that simulated the use of cover crops on all cropped acres was 

conducted to demonstrate the potential for cover crops to 

reduce sediment and nutrient loss from fields in this region 

(see Appendix C). Results showed that full adoption of cover 

crops in this region would further reduce sediment loss by an 

average of 59 percent, compared to the 2003–06 baseline 

condition, bringing the average annual sediment loss for the 

region to less than 0.5 ton per acre per year. 

However, as shown in figures 22 and 23, the effects of cover 

crops on reducing sediment loss is modest for the majority of 

cropped acres—generally those with the lowest losses in the 

2003–06 baseline condition. The median sediment loss 

reduction would be only about 0.2 ton per acre per year. About 

60 percent of the acres would have a reduction in sediment 

loss of 0.35 tons per acre or less (fig. 23). In contrast, 18 

percent of the acres would have average annual reductions of 

1 ton per acre or more. Nonetheless, figure 22 shows that full 

adoption of cover crops would bring average annual sediment 

loss to below 2 tons per acre for 93 percent of the acres in the 

region, compared to 83 percent for the 2003–06 baseline 

Land in long-term conserving cover. Acres in long-term 

conserving cover have very little erosion or sediment loss, and 

thus show nearly 100 percent reductions when compared to a 

cropped condition (table 14). If these 100,000 acres were still 

being cropped without any conservation practices, sediment 

loss would average about 4.3 tons per acre per year. 

Reductions in sediment loss for land in long-term conserving 

cover compared to the same acres with crops and no 

conservation practices vary considerably, as shown in figure 

24. While the average reduction over all acres in long-term 

conserving cover is 4.2 tons per acre per year, 40 percent of 

the acres in long-term conserving cover have reductions of
 

less than 2 tons per acre per year. Reductions greater than 10 

tons per acre per year occur on about 11 percent of the acres 

with long-term conserving cover. 

Figure 23. Estimates of the average annual potential 

reduction in sediment loss if cover crops were used on all 

cropped acres in the Chesapeake Bay region 

14
 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual reduction in sediment 

13
 

12
 

11
 

10
 

condition. 

Figure 22. Estimates of average annual sediment loss for 

cropped acres, comparing the baseline scenario to a “what if” 

scenario with cover crops added to all cropped acres, 

Chesapeake Bay region 

loss (tons/acre) 

9
 

8
 

7
 

6
 

5
 

4
 

3
 

2
 

20
 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual sediment loss (tons/acre) 18
 

16
 

14
 

12
 

10
 

1
 

0
 

8 Cumulative percent acres 
 

6
 Figure 24. Estimates of average annual reduction in sediment 

4
 loss due to conversion to long-term conserving cover in the 

Chesapeake Bay region 2
 

0
 24
 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual reduction in 

sediment loss for acres in 

long-term conserving cover

22
 

20
 Cumulative percent acres 18(tons/acre)16
 

14
 

12
 

10
 

8
 

6
 

4
 

2
 

0
 

Cumulative percent acres 

Baseline Baseline with cover crops added 

50 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Effects of Practices on Soil Organic Carbon 

The landscape and climate in the Chesapeake Bay region is 

much less conducive to maintaining and enhancing soil 

organic carbon relative to landscapes and climate of the soils 

in the Midwest. The combination of higher rainfall on more 

sloping soils and milder winters that allow for more 

degradation of organic materials make carbon accumulation 

far more challenging. The soils in this region developed 

residuum from igneous and metamorphic bedrock, glacial 

outwash or sandy coastal plain sediments. These materials are 

highly weathered with mixed or siliceous mineralogy, causing 

them to be inherently less fertile. The highly weathered, less 

reactive nature of these soils makes them less able to 

withstand even moderately intense tillage and maintain or 

enhance carbon stores relative to regions of the country like 

the Mississippi River drainage basin. 

In this study, estimation of soil organic carbon change is based 

on beginning soil characteristics that reflect the effects of 

years of traditional conventional tillage practices and older, 

lower yielding crop varieties. These effects generally resulted 

in soils with organic carbon levels at or near their low steady 

state. Modern high yielding crop varieties with and without 

the adoption of conservation tillage tend to readily improve 

the status of carbon in many soils, especially those with 

beginning stocks far less than the steady state representation of 

the present management. Beginning the simulations at a lower 

steady state for carbon allows for a more equitable comparison 

of conservation practices, particularly conservation tillage. 

Because of this, however, model estimates of soil organic 

carbon change may be somewhat larger than shown in other 

studies. Nevertheless, model estimates obtained in this study 

fall within the expected range for the continuum of adoption 

of new crop genetics and tillage practices. 

Baseline condition for cropped acres 

Model simulation shows that for the baseline conservation 

condition the average annual soil organic carbon change is a 

loss of about 27 pounds per acre per year, on average (table 

16), with about 43 percent of cropped acres gaining annually 

in soil organic carbon and 57 percent losing soil organic 

carbon, on average. These estimates account for losses of 

carbon with sediment removed from the field by wind and 

water erosion. Loss of soil organic carbon due to wind and 

water erosion averages about 152 pounds per acre per year for 

the baseline conservation condition (table 16). 

Cropped acres that are gaining soil organic carbon every year 

provide soil quality benefits that enhance production and 

reduce the potential for sediment, nutrient, and pesticide 

losses. Soil organic carbon improves the soil’s ability to 

function with respect to nutrient cycling, improves water 

holding capacity, and reduces erodibility. However, 

enhancement of carbon stores on a scale seen in the 

Midwestern basins could only occur in this region with 

significant shifts in crop mixes toward rotations with hay or 

pasture as components. 

Given the challenging nature of the inherent conditions of this 

region, maintenance of soil organic carbon is also an 

important benchmark. Cropping systems can be considered to 

be maintaining soil organic carbon if average annual losses do 

not exceed 100 pounds per acre per year; this rate of change is 

typically too small to detect via typical soil sampling over a 

20-year period. Applying this criterion, about 33 percent of the 

acres in the region would be considered to be maintaining (but 

not enhancing) soil organic carbon. When combined with 

acres enhancing soil organic carbon, a total of 76 percent of 

the acres in the region would be either maintaining or 

enhancing soil organic carbon. This achievement is in large 

part due to the high rate of conservation tillage adoption, 

particularly no-till and the high residue crop rotations on most 

of the acres. 

Effects of conservation practices on cropped acres 

Without conservation practices, the annual change in soil 

organic carbon would be an average loss of 68 pounds per acre 

per year, compared to an average loss of 27 pounds per acre 

for the baseline (table 16). Thus, conservation practices in the 

region have resulted in an average annual gain in soil organic 

carbon of 41 pounds per acre per year on cropped acres. 

Table 16. Field-level effects of conservation practices on soil organic carbon for cultivated cropland in the Chesapeake Bay region 

Baseline Reduction 

conservation No-practice due to Percent 

Model simulated outcome condition scenario practices reduction 

Cropped acres (4.3 million acres) 

Average annual loss of carbon with wind and water erosion 

(pounds/acre) 152 180 28 15 

Average annual change in soil organic carbon, including loss of 

carbon with wind and water erosion (pounds/acre) -27 -68 41* --

Land in long-term conserving cover (0.1 million acres) 

Average annual loss of carbon with wind and water erosion 

(pounds/acre) 75 256 281 71 

Average annual change in soil organic carbon, including loss of 

carbon with wind and water erosion (pounds/acre) 233 -100 333* -- * Gain in soil organic carbon due to conservation practices.
 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text.
 

Note: Model simulation results for the baseline conservation condition are presented in appendix B for the 4 subregions.
 

51 
 

 

 

 

 

 

 

 

However, average annual change in soil organic carbon varies Figure 26. Estimates of average annual gain in soil organic 

considerably among acres in the region, as shown in figure 25. carbon due to the use of conservation practices on cropped 

For the baseline conservation condition, the 43 percent of acres in the Chesapeake Bay region 

acres gaining soil organic carbon have an average annual gain 

of 82 pounds per acre per year. If conservation practices were 

not in use, only 33 percent of the acres would be gaining soil 

-100 

0 

100 

200 

300 

400

organic carbon (pounds/acre) 

Average annual gain in soil 

organic carbon and the annual rate of gain would be only 68 

pounds per acre per year on those acres. 

The average annual gain in soil organic carbon due to 

practices varies among acres, as shown in figure 26, 

depending on the extent to which residue and nutrient 

management is used, as well as the soil’s potential to sequester 

carbon. 

0 10 20 30 40 50 60 70 80 90 100 
 Some of the increased gain in soil organic carbon due to 

Cumulative percent acres 

Note: About 18 percent of the acres have a higher soil organic carbon increase 

in the no-practice scenario than the baseline conservation condition because of 

conservation practices is the result of soil erosion control— 

keeping soil organic carbon on the field promotes soil quality. 

If conservation practices were not in use, loss of soil organic 

carbon due to wind and water erosion would average 180 

pounds per acre per year, compared to 152 pounds per acre the higher fertilization rates, including manure application rates, used in the 

no-practice scenario to simulate the effects of nutrient management practices. 

with conservation practices (table 16). 

For air quality concerns, the analysis centers on the decrease 

in CO2 emissions. Soils gaining carbon are obviously 

diminishing emissions, but so are soils that continue to lose 

carbon but at a slower rate. For all cropped acres, the 

reduction in soil organic carbon of 41 pounds per acre due to 

conservation practice use is equivalent to a CO2 emission 

reduction of 0.32 million U.S. tons of carbon dioxide for the 

Chesapeake Bay region. 

Figure 25. Estimates of average annual change in soil organic 

carbon for cropped acres in the Chesapeake Bay region 

600
 

500
 

Effects of cover crops on soil organic carbon 

change

The “what if” scenario that simulated the full adoption of 

cover crops on all cropped acres showed that soil organic 

carbon levels would increase on more cropped acres and 

decrease more slowly on other cropped acres (see Appendix 

C). The annual change in soil organic carbon would average 

about 97 pounds per acre higher than the 2003–06 baseline. 

Figure 27 shows that if cover crops were added to all cropped 

acres, 27 percent of cropped acres would move from losing 

soil organic carbon to gaining soil organic carbon, bringing 

acres that are gaining soil organic carbon to 70 percent of 

cropped acres in the region. 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual change in soil organic carbon 

(pounds/acre) 

Figure 27. Estimates of average annual change in soil organic 

400 carbon for cropped acres, comparing the baseline scenario to a 

“what if” scenario with cover crops added to all cropped acres, 300 

Chesapeake Bay region 200 

600 100
 

0
 

-100
 

-200
 

-300
 

-400
 

-500
 

Average annual change in soil organic 

500
 

400
 

300
 

200
 

100
 

0
 

-100
 

-200
 

-300
 

-400
 

carbon (pounds/acre)

Cumulative percent acres 

Baseline conservation condition -500
 

No-practice scenario 0 10 20 30 40 50 60 70 80 90 100 

Cumulative percent acres 

Baseline 

Baseline with cover crops added 

52 
 

 

 

 

 

Land in long-term conserving cover Figure 28. Estimates of average annual gain in soil organic 

For land in long-term conserving cover, the annual change in carbon due to conversion to long-term conserving cover in the 

soil organic carbon for the baseline conservation condition Chesapeake Bay region 

averages 233 pounds per acre per year (table 16). If these 1,400

acres were still being cropped without any conservation 

practices, the annual average change in soil organic carbon 1,200 

would be a loss of 100 pounds per acre per year. 

Average annual gain in soil organic 

carbon for acres in long-term conserving 

0 10 20 30 40 50 60 70 80 90 100 

1,000 

800

cover (pounds/acre)

For these 100,000 acres, the gain in soil organic carbon 

averages 333 pounds per acre compared to a cropped 

condition without conservation practices. This is equivalent to 

a CO2 emission reduction of 0.06 million U.S. tons of carbon 

dioxide for the region. However, the rate of emission 

reduction due to conservation practices varies considerably 

among acres in long-term conserving cover, as shown in 

figure 28. 

600 

400 

200 

0 

-200 

Cumulative percent acres 

Note: About 2 percent of the acres in long-term conserving cover have 

decreases in annual carbon gain compared to a cropped condition. Biomass 

production under long-term conserving cover is typically nitrogen limited. 

The higher biomass production and resulting crop residue from the 

fertilization of cropped acres can exceed the carbon benefits of long-term 

conserving cover under some conditions. 

53 
 

 

 

 

 

 

 

Effects of Practices on Nitrogen Loss acres not receiving manure (table 17). Losses were also higher 

for highly erodible land (44 percent of cropped acres) than for 

Baseline condition for cropped acres non-highly erodible land. 

Plant-available nitrogen sources include application of 

commercial fertilizer, application of manure, nitrogen Figure 29. Average annual nitrogen loss by loss pathway, 

produced by legume crops (soybeans, alfalfa, beans, and Chesapeake Bay region 

peas), a small amount of manure deposited by grazing 35 Average annual nitrogen loss (pounds/acre) 

livestock, and atmospheric nitrogen deposition. In total, these 

sources provide about 130 pounds of nitrogen per acre per 

year for cropped acres in the Chesapeake Bay region (table 

17). Model simulations show that about 64 percent of this (84 

pounds per acre) is taken up by the crop and removed at 

harvest in the crop yield, on average, and the remainder is lost 

from the field through various pathways. 

For the baseline conservation condition, the annual average 

amount of total nitrogen lost from the field, other than the 

nitrogen removed from the field at harvest, is about 50 pounds 

per acre. These nitrogen loss pathways are (fig. 29)— 

 nitrogen lost due to volatilization associated primarily 

with fertilizer and manure application (average of 6.9 

pounds per acre per year);
 

 nitrogen returned to the atmosphere through
 

denitrification processes (average of 1.6 pounds per acre 

per year); 

	 nitrogen lost with windborne sediment (average of 0.2 

pounds per acre per year); 

	 nitrogen lost with surface runoff, including nitrogen lost 

with waterborne sediment (average of 8.8 pounds per acre 

per year); and 

	 nitrogen loss in subsurface flow pathways (average of 

32.7 pounds per acre per year). 

30
 

25
 

20
 

15
 

10
 

5
 

0
 

Figure 30. Cumulative distributions of average annual The two pathways that impact water quality directly—surface nitrogen lost through various loss pathways, Chesapeake Bay water and subsurface flows (average of 41.5 pounds/acre per region year) —account for 83 percent of the total nitrogen loss in this 

region. Most of the nitrogen loss in subsurface flows returns to 200

surface water through drainage ditches, tile drains, natural 

seeps, and groundwater return flow. 

Model simulation results showed that nitrogen loss to specific 

pathways varies from acre to acre, as shown in figures 30 and 

31. However, loss of nitrogen in subsurface flows is the 

dominant loss pathway for 80 percent of the cropped acres in 

the region. Nitrogen loss with waterborne sediment is the 

dominant loss pathway for 12 percent of the acres, and 

volatilization is the dominant loss pathway for 7 percent of the 

acres. Nitrogen loss in surface water (soluble) was the 

dominant loss pathway for only 1 percent of the cropped acres. 

Windborne nitrogen loss and denitrification were not 

Average annual nitrogen loss (pounds/acre) 180
 

160
 

140
 

120
 

100
 

80
 

60
 

40
 

20
 

0
 

0 10 20 30 40 50 60 70 80 90 100 
 

dominant loss pathways for any cropped acres in the region. 

Loss of nitrogen in subsurface flows can be quite high for 

some acres (fig. 30). Average annual losses of nitrogen in 

subsurface flows exceed 100 pounds per acre per year for the 

7 percent of acres with the highest losses. 

Acres receiving manure (38 percent of cropped acres) have, on 

average, twice the nitrogen loss as acres not receiving manure. 

Total nitrogen loss for acres receiving manure was 75 pounds 

per acre per year, compared to 35 pounds per acre per year for 

Cumulative percent acres 

Waterborne sediment 

Surface water runoff (soluble) 

Subsurface flows 

Windborne sediment 

Volatilization 

Denitrification 

54 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 31. Cumulative distributions of proportions of nitrogen Model simulations for the baseline conservation condition 

lost through six loss pathways, Chesapeake Bay region indicate that some cropped acres in the Chesapeake Bay Proportion of nitrogen loss

region are much more susceptible to the effects of weather 1.0 than other acres and lose much higher amounts of nitrogen 

(fig. 32). About 30 percent of the acres lose less than 40 

pounds per acre per year through the various loss pathways 

0.8 

0.6 under all weather conditions. About 25 percent of the acres, 

0.4 on the other hand, lose more than 100 pounds per acre in at 

least some years, and more than 40 pounds per acre in almost 

0.2 every year. In years with the most extreme weather, up to 5 

percent of the acres lose over 300 pounds of nitrogen. Figure 0.0 32 also shows that nitrogen loss for the 20 percent of the 0 10 20 30 40 50 60 70 80 90 100 cropped acres with the highest losses varies dramatically from 

Cumulative percent acres year to year when compared to the 40 percent with the lowest 

Waterborne sediment total nitrogen loss. 

Surface water runoff (soluble) 

The average annual total nitrogen loss for the baseline is Subsurface flows shown in figure 33. Acres with the highest nitrogen losses 

Windborne sediment have the highest inherent vulnerability combined with 

Volatilization inadequate nutrient management and runoff controls. About 

59 percent of cropped acres lose less than 40 pounds per acre Denitrification 

per year, while 10 percent lose more than 100 pounds per acre 

Note: The horizontal axis consists of percentiles for each pathway; a given per year. percentile for one curve will not represent the same acres on another curve. 

Table 17. Field-level effects of conservation practices on nitrogen sources and nitrogen loss pathways for cropped acres in the 

Chesapeake Bay region 

Average annual values in pounds per acre 

Baseline conservation No-practice Reduction due Percent 

Model simulated outcome condition scenario to practices reduction 

All cropped acres 

Nitrogen sources 

Atmospheric deposition 8.7 8.7 0.0 0 

Bio-fixation by legumes 26.4 23.8 -2.6 -11 

Nitrogen applied as commercial fertilizer and manure 95.3 123.8 28.5 23 

All nitrogen sources 130.4 156.3 25.9 17 

Nitrogen in crop yield removed at harvest 84.1 93.9 9.8* 10* 

Nitrogen loss pathways

 Nitrogen loss by volatilization 6.9 6.1 -0.8** -13**

 Nitrogen loss through denitrification processes 1.6 1.2 -0.5** -39**

 Nitrogen lost with windborne sediment 0.2 0.3 0.1 42

 Nitrogen loss with surface runoff, including waterborne sediment 8.8 15.1 6.3 42 

Nitrogen loss with surface water (soluble) 1.2 3.1 1.9 61 

Nitrogen loss with waterborne sediment 7.6 12.0 4.4 37

 Nitrogen loss in subsurface flow pathways 32.7 47.3 14.6 31 

Total nitrogen loss for all loss pathways 50.2 70.0 19.8 28 

Change in soil nitrogen -5.2 -8.7 -3.5 --

Highly erodible land (44 percent of cropped acres) 

All nitrogen sources 133 158 25.1 16

 Total nitrogen loss for all loss pathways 60 83 22.9 27
 

Non-highly erodible land (56 percent of cropped acres) 

All nitrogen sources 128 155 26.6 17

 Total nitrogen loss for all loss pathways 42 60 17.4 29 

Acres with manure applied (38 percent of cropped acres) 

All nitrogen sources 160 193 32.9 17

 Total nitrogen loss for all loss pathways 75 102 27.9 27 

Acres without manure applied (62 percent of cropped acres) 

All nitrogen sources 112 134 21.7 16 

Total nitrogen loss for all loss pathways 35 50 14.9 30 

* The reduction in yield reflects the increase in nutrients in the representation in the no-practice scenario for nutrient management.
 

** On about half of the cropped acres, more nitrogen volatilization and denitrification occurs with practices than without practices, resulting in only a small change in 

nitrogen volatilization and denitrification on average for the region due to conservation practices. In preventing nitrogen loss to other loss pathways, conservation 

practices keep more of the nitrogen compounds on the field longer, where it is exposed to wind and weather conditions that promote volatilization and denitrification. 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Model simulation results for the baseline 

conservation condition are presented in appendix B for the 4 subregions.
 

55 
Figure 32. Distribution of annual total nitrogen loss for each year of the 47-year model simulation, Chesapeake Bay region 

0 

20 

40 

60 

80 

100 

120 

140 

160 

180 

200 

220 

240 

260 

280 

300 

Annual total nitrogen loss (pounds/acre) 

0 10 20 30 40 50 60 70 80 90 100 

Cumulative percent acres 

Note: This figure shows how annual total nitrogen loss (pounds per acre per year) varied within the region and from year to year in the model simulation for cropped 

acres. Each of the 47 curves shown above represents a single year of data and shows how annual total nitrogen loss varied over the region in that year, starting with the 

acres with the lowest total nitrogen loss and increasing to the acres with the highest total nitrogen loss. The family of curves shows how annual total nitrogen loss varied 

from year to year. The average annual curve for the baseline is shown in figure 33 (below). 

Figure 33. Estimates of average annual total nitrogen loss for cropped acres in the Chesapeake Bay region 

0 

20 

40 

60 

80 

100 

120 

140 

160 

180 

200 

220 

240 

260 

280 

300

Average annual total nitrogen loss (pounds/acre) 

0 10 20 30 40 50 60 70 80 90 100 

Cumulative percent acres 

Baseline conservation condition No-practice scenario 

56 
 

 

 

 

 

 

Effects of conservation practices on cropped acres 

Model simulations show that the conservation practices in use 

in the region have reduced total nitrogen loss from cropped 

acres by an average of 20 pounds per acre per year, 

representing a 28 percent reduction, on average (table 17). 

Without conservation practices, about 64 percent of the 

cropped acres would have average annual total nitrogen loss 

exceeding 40 pounds per acre per year; with conservation 

practices, 40 percent of acres exceed this level of loss (fig. 

33). 

The effects of conservation practices vary from acre to acre 

(fig. 34). About half of the acres have average annual 

reductions in total nitrogen loss below 11 pounds per acre. In 

contrast, about 17 percent of the acres have reduced total 

nitrogen loss by an average of over 40 pounds per acre per 

year. These are acres with higher levels of treatment and often 

higher levels of nitrogen use in the no-practice scenario. 

Figure 34 also shows that about 16 percent of the acres have 

an increase in total nitrogen loss due to conservation practice 

use. Most of these gains are small; only 2 percent of the acres 

have gains of more than 4 pounds per acre. This result 

primarily occurs on soils with relatively high soil nitrogen 

content and generally with low slopes where the surface water 

runoff is re-directed to subsurface flow by soil erosion control 

practices. The higher volume of water moving through the soil 

profile extracts more nitrogen from the soil than under 

conditions without conservation practices. Cropping systems 

that include legumes also have a higher soil nitrogen stock in 

Nitrogen lost with surface runoff. Model simulations 

show that, on average, nitrogen lost with surface runoff has 

been reduced 42 percent due to use of conservation practices 

in the region (table 17). Without conservation practices, about 

35 percent of the cropped acres would have nitrogen lost with 

surface runoff in excess of an average of 15 pounds per acre 

per year, compared to only 18 percent of the acres in the 

baseline conservation condition (fig. 35). Figure 36 shows that 

about 20 percent of the cropped acres have reductions in 

nitrogen lost with surface runoff greater than 10 pounds per 

acre due to conservation practice use. Figure 36 also shows, 

however, that about 60 percent of the acres have reductions 

less than 5 pounds per acre due to conservation practices. 

Figure 35 Estimates of average annual nitrogen lost with 

surface runoff (including waterborne sediment) for cropped 

acres in the Chesapeake Bay region 

90
 

80
 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual nitrogen lost with 

surface runoff (pounds/acre) 70
 

60
 

50
 

40
 

30
 

20
 

10
 

0
 

the baseline conditions because legumes produce Cumulative percent acres proportionately less biofixation of nitrogen under the higher 

fertilization rates simulated in the no-practice scenario. Baseline conservation condition 

Figure 34. Estimates of average annual reduction in total No-practice scenario

nitrogen loss due to the use of conservation practices on 

cropped acres in the Chesapeake Bay region Figure 36. Estimates of average annual reduction in nitrogen 

lost with surface runoff and reduction in nitrogen loss in 

subsurface flows due to the use of conservation practices on 

Average annual reduction in total 

nitrogen loss (pounds/acre) 

-10 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

110 

120 

130 

140 

150 

cropped acres in the Chesapeake Bay region Average annual reduction in nitrogen loss (pounds/acre) 

120
 

100
 

80
 

60
 

40
 

20
 

0
 

-20
 

0 10 20 30 40 50 60 70 80 90 100 
 

0 10 20 30 40 50 60 70 80 90 100 
 Cumulative percent acres Cumulative percent acres 

N lost with surface runoff 

Note: See text for discussion of conditions that result in lower total nitrogen 

loss in the no-practice scenario than in the baseline conservation condition for N loss in subsurface flows 

16 percent of the acres. 

Note: See text for discussion of negative reductions for loss of nitrogen in 

subsurface flows. 

57 
 

 

 

 

 

 

 

 

 

 

 

 

Nitrogen loss in subsurface flows. Conservation 

practices are effective in reducing nitrogen loss in subsurface 

flows on many acres, but make little difference on other acres 

and even result in small gains in nitrogen loss in subsurface 

flows for 17 percent of cropped acres (figs. 36 and 37). (Gains 

in nitrogen loss in subsurface flows are represented in figure 

36 as negative reductions.) On average, conservation practices 

have reduced nitrogen loss in subsurface flows from 47 

pounds per acre without practices to 33 pounds per acre with 

practices, representing an average reduction of 15 pounds per 

acre per year (31-percent reduction) (table 17). Figure 36 

shows that reductions in nitrogen loss in subsurface flows 

exceed 30 pounds per acre for 15 percent of the cropped acres, 

on average, but are less than 5 pounds per acre for 42 percent 

of the acres. 

Figure 37. Estimates of average annual nitrogen loss in 

subsurface flows for cropped acres in the Chesapeake Bay 

region Average annual nitrogen loss in subsurface flows (pounds/acre) 

250 

225 

200 

175 

150 

125 

100 

75 

50 

25 

0 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres 

Baseline conservation condition 

No-practice scenario 

The gains in nitrogen loss due to conservation practices for 17 

percent of the cropped acres are largely due to relatively weak 

nutrient management practices on acres with erosion control 

treatment. A portion of the reduction in nitrogen lost with 

surface runoff is re-routed to subsurface loss pathways, 

resulting in gains or only small reductions in nitrogen loss in 

subsurface flows. This re-routing of surface water runoff to 

subsurface flow pathways results in additional nitrogen being 

leached from the soil, diminishing and sometimes offsetting 

the overall positive effects of conservation practices on total 

nitrogen loss. 

These model simulation results underscore the importance of 

pairing water erosion control practices with effective nutrient 

management practices so that the full suite of conservation 

practices will provide the environmental protection needed. 

Tradeoffs in Conservation Treatment 

Conservation practices applied on cropland are, for the most 

part, synergistic. The benefits accumulate as more practices 

are added to the designed systems. However, when only a 

single resource concern is addressed (such as soil erosion), 

antagonism between the practices and other resource 

concerns may occur. That is why it is essential that all 

resource concerns be considered during the conservation 

planning process. Most of the time the tradeoffs are much 

smaller than the magnitude of the primary resource concerns. 

Common examples are: 

	 Terraces and conservation tillage are planned to solve a 

serious water erosion problem. However, in some areas 

there may be concern about saline seeps at the lower part 

of the field. The planned practices will solve the erosion 

problem, but could exacerbate the saline seep problem 

under some conditions. Ignoring that fact does not make 

for an adequate conservation plan. 

	 Conservation tillage is planned for erosion control on a 

cropland field with a high water table. The reduction in 

runoff may increase leaching of nitrates into the shallow 

water table. This potential secondary problem requires 

additional nutrient management practices to address the 

concern. 

	 A nutrient management plan reduces the amount of 

manure added to a field to reduce the loss of nutrients to 

surface or groundwater. However, the reduction in 

organic material added to the field may reduce the soil 

organic matter or reduce the rate of change in soil 

organic matter. 

	 Figure 34 shows that about 16 percent of the acres have 

an increase in total nitrogen loss due to conservation 

practice use. This result occurs primarily on soils with 

relatively high soil nitrogen content and generally low 

slopes where the surface water runoff is re-directed to 

subsurface flow by soil erosion control practices. The 

higher volume of water moving through the soil profile 

extracts more nitrogen from the soil than under 

conditions without conservation practices. For these 

fields, the nutrient management component of a farmer’s 

conservation plan would need to be enhanced to reduce 

or eliminate the negative effects of soil erosion control 

practices on nitrogen loss. 

A comprehensive planning process is used to identify the 

appropriate combination of practices needed to address 

multiple resource concerns by taking into account the 

specific inherent vulnerabilities associated with each field. 

To ensure that proper consideration is given to the effects of 

conservation practices on all of the resource concerns, 

USDA/NRCS developed a comprehensive planning tool 

referred to as CPPE (Conservation Practice Physical Effects). 

The CPPE is included in the Field Office Technical Guide. 

Conservation planners are expected to use CPPE as a 

reference to ensure that all resource concerns are addressed 

in conservation plans. 

58 
 

 

 

 

 

 

 

 

 

Effects of cover crops on nitrogen loss. A “what if” 

scenario that simulated the use of cover crops on all cropped 

acres was conducted to demonstrate the potential for cover 

crops to reduce sediment and nutrient loss from fields in this 

region (see Appendix C). Results showed that full adoption of 

cover crops in this region would further reduce total nitrogen 

loss by 19 percent compared to the 2003–06 baseline 

condition, bringing the average annual total nitrogen loss for 

the region to about 40.5 pounds per acre per year. Cover crops 

were most effective in reducing the two loss pathways that 

impact water quality. Full adoption of cover crops on all acres 

would— 

 reduce nitrogen loss with surface runoff, including 

waterborne sediment, an average of 4.0 pounds per acre 

per-year (from 8.8 to 4.8 pounds per acre), representing a 

46 percent decrease; and 

 reduce nitrogen loss in subsurface flow pathways an 

average of 10 pounds per acre per year (from 33 to 23 

Land in long-term conserving cover 

Total nitrogen loss has been reduced by about 88 percent on 

the 0.1 million acres in long-term conserving cover, compared 

to conditions that would be expected had the acres remained in 

crops. Converting cropped acres to long-term conserving 

cover is very effective in reducing total nitrogen loss, as 

demonstrated in figure 40 and table 18, although the 

reductions are much higher for some acres than others. 

Conversion of cropped acres to long-term conserving cover in 

the region has reduced nitrogen loss in subsurface flows from 

these acres by an average loss of 59 pounds per acre per year 

to about 3 pounds per acre per year, a reduction of 95 percent. 

Figure 39. Estimates of the average annual potential 

reduction in nitrogen loss in subsurface flows if cover crops 

were used on all cropped acres in the Chesapeake Bay region 

50 

0 10 20 30 40 50 60 70 80 90 100 
 

pounds per acre), representing a 31-percent decrease. 

The largest reductions due to full adoption of cover crops was 

for nitrogen loss in subsurface flow pathways (figs. 38 and 

39). While all reductions in nitrogen loss due to cover crop use 

are significant, they are not uniform across all cropped acres. 

For nitrogen loss in subsurface flows, 30 percent of the acres 

would be expected to have loss reductions less than 5 pounds 

per acre, while about 10 percent of the acres would be 

expected to have loss reductions greater than 20 pounds per 

acre. 

nitrogen loss in subsurface flows 

45
 

40
 

35

Average annual reduction in 

(pounds/acre) 30
 

25
 

20
 

15
 

10
 

5
 

0
 

Even after full adoption of cover crops, however, about 20 -5

percent of cropped acres in the region will have average 

annual losses of nitrogen in subsurface flows above 25 pounds Cumulative percent acres 

per acre. These acres will require comprehensive nutrient 

management to bring losses down to acceptable levels. 

Figure 38. Estimates of average annual nitrogen loss in Figure 40. Estimates of average annual total nitrogen loss for 

subsurface flows for cropped acres, comparing the baseline land in long-term conserving cover in the Chesapeake Bay 

scenario to a “what if” scenario with cover crops added to all region 

cropped acres, Chesapeake Bay region 

0 

25 

50 

75 

100 

125 

150 

175 

200

Average annual nitrogen loss in 

subsurface flows (pounds/acre) 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual total nitrogen loss for 

acres in long-term conserving cover 

(pounds/acre) 

500
 

450
 

400
 

350
 

300
 

250
 

200
 

150
 

100
 

50
 

0
 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres 

Cumulative percent acres 

Baseline Baseline with cover crops added Baseline conservation condition 

No-practice scenario 

59 
 

 

 

 

 

 

 

Table 18. Effects of conservation practices on nitrogen sources and nitrogen loss pathways for land in long-term conserving cover, 

Chesapeake Bay region 

Average annual values in pounds per acre 

Baseline Reduction 

conservation No-practice due to Percent 

Model simulated outcome condition scenario practices reduction 

Nitrogen sources 

Atmospheric deposition 9.8 9.8 0.0 0 

Bio-fixation by legumes 11.5 21.7 10.2 47 

Nitrogen applied as commercial fertilizer and manure 0.0 132.8 132.8 100 

All nitrogen sources 21.3 164.3 143.0 87 

Nitrogen in crop yield removed at harvest 1.5* 84.7 83.2 98 

Nitrogen loss pathways

 Nitrogen loss by volatilization 5.2 6.2 0.9 15 

 Nitrogen loss through denitrification processes 2.4 2.8 0.4 15 

 Nitrogen lost with windborne sediment 0.00 0.03 0.03 100 

 Nitrogen loss with surface runoff, including waterborne sediment 0.6 21.7 21.1 97 

Nitrogen loss with surface water (soluble) 0.3 3.2 2.9 91 

Nitrogen loss with waterborne sediment 0.4 18.5 18.2 98 

 Nitrogen loss in subsurface flow pathways 2.9 59.3 56.3 95 

 Total nitrogen loss for all pathways 11.2 90.0 78.8 88 

Change in soil nitrogen 7.6 -11.8 -19.4 --

* Harvest was simulated on acres planted to trees where expected tree age is less than the 47-years included in the model simulation. At tree harvest time, the grass also 

is removed and replanted.
 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text.
 

60 
 

 

 

 

 

 

 

 

 

 

Effects of Practices on Phosphorus Loss 

Phosphorus, like nitrogen, is an essential element needed for 

crop growth. Unlike nitrogen, however, phosphorus rarely 

occurs in a gaseous form so the agricultural model has no 

atmospheric component. Only phosphorus compounds that are 

soluble in water are available for plants to use. Although total 

phosphorus is plentiful in the soil, only a small fraction is 

available at any one time for plant uptake. Farmers apply 

commercial phosphate fertilizers to supplement low quantities 

acres (figs. 43 and 44). About half of the acres lose less than 4 

pounds per acre per year through the various loss pathways 

under all weather conditions. About one-fourth of the acres, 

on the other hand, lose more than 12 pounds per acre in at 

least some years. 

Figure 41. Estimates of average annual phosphorus lost 

through various loss pathways, Chesapeake Bay region 

of plant-available phosphorus in the soil. 

Baseline condition for cropped acres 

In the model simulations for the Chesapeake Bay region, 

about 25 pounds per acre of phosphorus were applied as 

commercial fertilizer or in manure to cropped acres, on 

average, in each year of the model simulation (table 19). 

About half of the phosphorus applied is taken up by the crop 

and removed at harvest—13 pounds per acre per year, on 

average. 

Total phosphorus loss for all loss pathways averaged 3.8 

pounds per acre per year in the baseline conservation 

condition (table 19). These phosphorus loss pathways are— 

 phosphorus lost with windborne sediment (average of 

0.03 pound per acre per year); 

Average annual phosphorus loss 

(pounds/acre) 

20
 

18
 

16
 

14
 

12
 

10
 

8
 

6
 

4
 

2
 

0
 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres 

 phosphorus lost with waterborne sediment (average of 1.8 

pounds per acre per year); 

 soluble phosphorus lost to surface water, including 

Waterborne sediment 

Surface water runoff (soluble) 

soluble phosphorus in surface water runoff, and soluble 

phosphorus that infiltrates into the soil profile but quickly 

returns to surface water either through quick return lateral 

flow or intercepted by drainage systems (average of 1.9 

pounds per acre per year); and 

	 soluble phosphorus that percolates through the soil profile 

into the groundwater (average of 0.07 pound per acre per 

year). 

Nearly all (98 percent) of phosphorus loss from fields in the 

Chesapeake Bay region is either with waterborne sediment (55 

percent) or soluble phosphorus lost to surface water (43 

percent) (fig. 41), and these two loss pathways are 

approximately equal in importance. The percent of phosphorus 

lost in each loss pathway varies from acre to acre, as shown in 

figure 42 for cropped acres. Soluble phosphorus loss with 

surface water runoff and lateral flow (including discharge to 

drainage ditches) was the dominant loss pathway for 59 

percent of cropped acres. The dominant loss pathway for the 

remaining 41 percent of cropped acres is phosphorus lost with 

waterborne sediment. A very small amount of soluble 

phosphorus is lost through percolation into groundwater—2 

percent of the total phosphorus loss. 

Percolation 

Windborne sediment 

Figure 42. Cumulative distributions of the proportion of 

phosphorus lost through various loss pathways, Chesapeake 

Bay region 

1.0 

0.9 

Proportion of phosphorus loss 0.8
 

0.7
 

0.6
 

0.5
 

0.4
 

0.3
 

0.2
 

0.1
 

As shown for nitrogen, total phosphorus losses are much 

higher for acres receiving manure than for acres that did not 

receive manure (table 19). Phosphorus losses are also much 

higher for highly erodible land than for non-highly erodible 

land. 

Model simulations for the baseline conservation condition 

indicate that some cropped acres in the Chesapeake Bay 

region lose much higher amounts of phosphorus than other 

0.0 

0 10 20 30 40 50 60 70 80 90 100 

Cumulative percent acres 

Waterborne sediment 

Surface water runoff (soluble) 

Percolation 

Windborne sediment 

61 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 19. Field-level effects of conservation practices on phosphorus sources and phosphorus loss pathways for cultivated cropland in 

the Chesapeake Bay region 

Average annual values in pounds per acre 

Baseline No- Reduction 

conservation practice due to Percent 

Model simulated outcome condition scenario practices reduction 

Cropped acres (4.3 million acres) 

Phosphorus sources

 Phosphorus applied as commercial fertilizer and manure 25.2 33.7 8.5 25 

Phosphorus in crop yield removed at harvest 13.28 14.40 1.12 8 

Phosphorus loss pathways 

Phosphorus lost with windborne sediment 0.03 0.08 0.05 59 

Phosphorus lost to surface water (sediment attached and soluble)* 3.67 6.17 2.51 41 

Soluble phosphorus lost to surface water* 1.89 2.64 0.74 28 

Phosphorus loss with waterborne sediment 1.77 3.54 1.76 50 

Soluble phosphorus loss to groundwater 0.07 0.07 0.00 0 

Total phosphorus loss for all loss pathways 3.77 6.33 2.56 40 

Change in soil phosphorus 8.11 12.97 4.86 --

Highly erodible land (44 percent of cropped acres) 

Phosphorus applied as commercial fertilizer and manure 27.6 34.0 6.4 19 

Total phosphorus loss for all loss pathways 5.3 8.3 3.06 37 

Non-highly erodible land (56 percent of cropped acres) 

Phosphorus applied as commercial fertilizer and manure 23.4 33.5 10.1 30 

Total phosphorus loss for all loss pathways 2.6 4.7 2.16 45 

Acres with manure applied (38 percent of cropped acres) 

Phosphorus applied as commercial fertilizer and manure 39.3 46.8 7.5 16 

Total phosphorus loss for all loss pathways 6.2 9.2 3.03 33 

Acres without manure applied (62 percent of cropped acres) 

Phosphorus applied as commercial fertilizer and manure 16.7 25.8 9.1 35 

Total phosphorus loss for all loss pathways 2.3 4.6 2.27 50 

Land in long-term conserving cover (0.1 million acres) 

Phosphorus sources

 Phosphorus applied as commercial fertilizer and manure 0.0 35.4 35.4 100 

Phosphorus in crop yield removed at harvest 0.51** 12.98 12.47 96 

Phosphorus loss pathways 

Phosphorus lost with windborne sediment 0.00 0.01 0.01 100 

Phosphorus lost to surface water (sediment attached and soluble)* 0.49 7.89 7.39 94 

Soluble phosphorus lost to surface water* 0.46 2.75 2.30 83 

Phosphorus loss with waterborne sediment 0.04 5.13 5.09 99 

Soluble phosphorus loss to groundwater 0.12 0.08 -0.03 -40 

Total phosphorus loss for all loss pathways 0.61 7.98 7.37 92 

Change in soil phosphorus -1.48 14.44 15.92 --

* Soluble phosphorus lost to surface water includes phosphorus in water moving laterally within the soil into drainage systems (tile and surface drainage) and natural 

seeps. 

** Harvest was simulated on acres planted to trees where expected tree age is less than the 47-years included in the model simulation. At tree harvest time, the grass
 

also is removed and replanted.
 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text.
 

Note: Model simulation results for the baseline conservation condition are presented in appendix B for the 4 subregions.
 

62 
Figure 43. Distribution of annual total phosphorus loss for each year of the 47-year model simulation, Chesapeake Bay region 

0 

4 

8 

12 

16 

20 

24 

28 

32 

36 

40 

44 

48 

52 

56 

60 

64 

68 

72 

Annual total phosphorus loss (pounds/acre) 

0 10 20 30 40 50 60 70 80 90 100 

Cumulative percent acres 

Note: This figure shows how annual total phosphorus loss (pounds per acre per year) varied within the region and from year to year in the model simulation on cropped 

acres. Each of the 47 curves shown above represents a single year of data and shows how annual total phosphorus loss varied over the region in that year, starting with 

the acres with the lowest total phosphorus loss and increasing to the acres with the highest total phosphorus loss. The family of curves shows how annual total 

phosphorus loss varied from year to year. 

Figure 44. Estimates of average annual phosphorus lost to surface water (sediment attached and soluble)*for cropped acres in the 

Chesapeake Bay region 

32 

Average annual phosphorus lost to 

surface water (pounds/acre) 28 

24 

20 

16 

12 

8 

4 

0 

Cumulative percent acres 

Baseline conservation condition No-practice scenario 

0 10 20 30 40 50 60 70 80 90 100 

* Soluble phosphorus lost to surface water includes phosphorus in water moving laterally within the soil into drainage systems (tile and surface drainage) and natural 

seeps. 

63 
 

 

 

 

 

 

 

 

 

Effects of conservation practices on cropped acres Figure 45. Estimates of average annual reduction in 

Conservation practices have reduced total phosphorus lost to phosphorus lost to surface water (sediment attached and 

surface water for cropped acres by 41 percent, reducing the soluble) due to conservation practices on cropped acres in the 

average loss from 6.2 pounds per acre per year if conservation Chesapeake Bay region 

practices were not in use to 3.7 pounds per acre per year for 20 the baseline conservation condition (table 19). The effects of 

conservation practices on phosphorus lost to surface water 18
 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual reduction in 

phosphorus lost to surface water 

16
 

14 (soluble and sediment attached) are shown in figures 44 and 

45 for cropped acres. With the conservation practices in use as 

represented by the baseline conservation condition, about 30 

percent of cropped acres exceed 4 pounds per acre per year, on 

average. Without those practices in use, phosphorus lost to 

surface water would exceed 4 pounds per acre for 52 percent 

of the acres. 

The effects of conservation practices on phosphorus lost to 

surface water vary considerably throughout the Chesapeake 

Bay region, as shown in figure 45. Reductions due to practices 

(pounds/acre)12
 

10
 

8
 

6
 

4
 

2
 

0
 

-2
 

-4
 are less than 2 pounds per acre for half of the cropped acres. 

At the high end, reductions exceed 6 pounds per acre for about 

10 percent of the acres. 

Effects of cover crops on phosphorus loss 

The “what if” scenario simulating the use of cover crops on 

all cropped acres (see Appendix C) found that cover crops 

were also helpful in reducing phosphorus losses. Results 

showed that full adoption of cover crops in this region would 

further reduce total phosphorus loss from an average of 3.8 

pounds per acre per year to 2.6 pounds per acre per year, 

representing a 32-percent decrease compared to the 2003–06 

baseline condition. About half of the acres would be expected 

Cumulative percent acres 

Note: Acres with an overall increase in surface water runoff due to 

conservation practices (see figure 16) causes gains (negative reductions) 

greater than 0.1 pound per acre in phosphorus lost to surface water due to 

conservation practices for about 11 percent of the cropped acres. 

Figure 46. Estimates of average annual reduction in total 

phosphorus loss due to conversion to long-term conserving 

cover in the Chesapeake Bay region 

to have loss reductions less than 0.5 pound per acre, while 60

about 15 percent of the acres would be expected to have loss 

reductions greater than 2 pounds per acre. (See figs. C11 and 

C12 in Appendix C.) 

Land in long-term conserving cover 

For land in long-term conserving cover, total phosphorus loss 

is 92 percent less than it would have been if crops had been 

grown and no conservation practices used, reducing total 

phosphorus loss by 7.4 pounds per acre per year, on average 

(table 19 and figure 46). Reductions vary among the acres in 

the region; reductions are less than 2 pounds per acre for about 

20 percent of the acres in long-term conserving cover. 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual reduction in total 

phosphorus loss for acres in longterm conserving cover (pounds/acre) 55
 

50
 

45
 

40
 

35
 

30
 

25
 

20
 

15
 

10
 

5 

0 

Cumulative percent acres 

64 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Effects of Practices on Pesticide Residues 

and Environmental Risk 

Use of pesticides to protect crops from weeds, insects, and 

diseases is an integral part of crop production. While 

pesticides are essential for large-scale agriculture, pesticide 

residues can migrate from the application site and lead to 

unintentional risk to humans and non-target plants and 

animals. Most pesticides are applied at much lower rates than 

nutrients. The fraction of pesticides applied that migrates 

offsite with water is generally less than 1 to 2 percent. 

Nevertheless, small amounts of pesticide residue can create 

water quality concerns depending on the toxicity of the 

pesticide residues to non-target species and even exceed EPA 

drinking water standards at times. 

Baseline condition for pesticide loss 

The APEX model tracks the mass loss of pesticides dissolved 

in surface water runoff, adsorbed to sediment loss from water 

erosion, and dissolved in subsurface flow pathways.15 The 

distribution of losses through each of these three pathways is 

contrasted in figure 47. All three pathways are important in the 

transport of pesticide residues from fields. Waterborne 

sediment accounted for about 37 percent of the total mass loss, 

followed by pesticides dissolved in surface water runoff at 36 

percent and pesticides in subsurface flows at 27 percent. 

In the Chesapeake Bay region, the dominant loss pathway for 

37 percent of cropped acres was pesticides dissolved in 

surface water runoff. Waterborne sediment was the dominant 

pesticide loss pathway for 34 percent of the acres, and 

subsurface flow s were the dominant pesticide loss pathway 

for 20 percent of the acres. The remaining 9 percent of the 

acres had no pesticide loss. 

The average annual amount of pesticide lost from farm fields 

in the Chesapeake Bay region is about 15 grams of active 

ingredient per hectare per year (table 20).16 As was observed 

for sediment and nutrient loss, the majority of pesticide loss 

occurs on a minority of acres within the Chesapeake Bay 

region (fig. 47). About 64 percent of the acres have total mass 

loss less than the mean value of 15 grams per hectare. The 

median loss is 9.4 grams per hectare. 

In the model simulations, the pesticide applied in the largest 

amount throughout the region was atrazine at 24 percent of the 

total weight of pesticides applied, followed closely by 

glyphosate at 21 percent (table 21). The herbicide Smetolachlor represented 14 percent of the total weight of 

pesticides applied in the region. These three pesticides 

accounted for 59 percent of the pesticides applied in the 

region, by weight. 

15 The APEX model currently does not estimate pesticides lost in spray drift or 

volatilization. 

16 Grams per hectare is the standard reporting unit for pesticide active 

ingredients. 

The most common pesticide residues lost from farm fields are 

atrazine (33 percent of total mass loss), paraquat dichloride 

(15 percent of total mass loss), and S-metolachlor (11 percent 

of total mass loss) (table 21). Pendimethalin, glyphosate, and 

Simazine each represented over 5 percent of the total mass 

loss. These six pesticides represented 80 percent of all 

pesticide residues lost from fields in the model simulations for 

the Chesapeake Bay region. 

Pesticide loss for land in long-term conserving cover was not 

simulated because the survey did not provide information on 

pesticide use on land enrolled in CRP General Signups. It was 

assumed that there was no pesticide residues lost from land in 

long-term conserving cover. 

Figure 47. Estimates of average annual pesticide loss (mass 

loss of all pesticides combined) for three loss pathways, 

Chesapeake Bay region 

60 

50 

40 

30 

20 

10 

0 

Cumulative percent acres 

Waterborne sediment 

Soluble in surface water runoff 

Soluble in subsurface flows 

Average annual pesticide mass loss 

(grams/hectare) 

0 10 20 30 40 50 60 70 80 90 100 
 

65 
 

 

 

 

 

 

 

 

Table 20. Field-level effects of conservation practices on pesticide loss and associated edge-of-field environmental risk for cropped 

acres in the Chesapeake Bay region 

Baseline Reduction 

conservation No-practice due to Percent 

Model simulated outcome condition scenario practices reduction 

Pesticide sources 

Average annual amount of pesticides applied (grams of 

active ingredient/hectare) 2000 2285 285 12 

Pesticide loss 

Average annual mass loss of pesticides for all pathways 

(grams of active ingredient/hectare) 15 20 5 26 

Edge-of-field pesticide risk indicator 

Average annual surface water pesticide risk indicator for 

aquatic ecosystems 1.76 2.32 0.56 24 

Average annual surface water pesticide risk indicator for 

humans 0.42 0.54 0.12 22 

Average annual groundwater pesticide risk indicator for 

humans 0.36 0.48 0.11 24 

Note: It was assumed that no pesticides were applied to land in long-term conserving cover and there was no data on residual pesticides in the soil for these acres; thus, 

the assessment of the effects of this practice on pesticide loss was not done. 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Model simulation results for the baseline 

conservation condition are presented in appendix B for the 4 subregions.
 

Table 21. Dominant pesticides applied in model simulations and contributing to losses, Chesapeake Bay region 

Pesticide (active ingredient name) Pesticide type 

Percent of total applied in the region 

Pesticide application* 

Atrazine Herbicide 24 

Glyphosate, isopropylamine salt Herbicide 21 

S-Metolachlor Herbicide 14 

Simazine Herbicide 6 

Pendimethalin Herbicide 5 

Metolachlor Herbicide 4 

Acetochlor Herbicide 4 

Paraquat dichloride Herbicide 3 

2,4-D, 2-ethylhexyl ester Herbicide 1 

Glyphosate Herbicide 1 

Alachlor Herbicide 1 

Metam-sodium Multi-purpose 1 

Glyphosate-trimesium Insecticide 1 

1,3-Dichloropropene Fungicide 1 

Total 88 

Percent of total pesticide loss in the region** 

Pesticide loss from farm fields* 

Atrazine Herbicide 33 

Paraquat dichloride Herbicide 15 

S-Metolachlor Herbicide 11 

Pendimethalin Herbicide 8 

Glyphosate, isopropylamine salt Herbicide 7 

Simazine Herbicide 6 

Metolachlor Herbicide 5 

Sulfentrazone Herbicide 3 

Acetochlor Herbicide 2 

2,4-D 2-ethylhexyl ester Herbicide 1 

Trifluralin Herbicide 1 

Total 91 

* Pesticides not listed each represented less than 1 percent of the total. Percents may not add to total due to rounding.
 

** Includes loss of pesticides dissolved in surface water runoff, adsorbed to sediment loss from water erosion, and dissolved in subsurface flow pathways.
 

66 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Effects of conservation practices on pesticide 

residues and risk 

Management practices that reduce the potential for loss of 

pesticides from farm fields consist of a combination of 

Integrated Pesticide Management (IPM) techniques and water 

erosion control practices. Water erosion control practices 

mitigate the loss of pesticides from farm fields by reducing 

surface water runoff and sediment loss, both of which carry 

pesticide residues from the farm field to the surrounding 

environment. IPM is site-specific in nature, with individual 

tactics determined by the particular crop/pest/environmental 

condition. IPM consists of a management strategy for 

prevention, avoidance, monitoring, and suppression of pest 

populations. When the use of pesticides is necessary to protect 

crop yields, selection of pesticides that have the least 

environmental risk is an important aspect of the suppression 

component of IPM. 

Model simulations show that conservation practices— 

primarily water erosion control practices—are effective in 

reducing the loss of pesticide residues from farm fields. Use of 

conservation practices has reduced the loss of pesticides 

(summed over all pesticides) by an average of 5 grams of 

active ingredient per hectare per year, a 26-percent reduction 

from the 20 grams per hectare for the no-practice scenario 

(table 20). 

However, the total quantity of pesticide residues lost from the 

field is not the most useful outcome measure for assessing the 

environmental benefits of conservation practices. The 

environmental impact is specific to the toxicity of each 

pesticide to non-target species that may be exposed to the 

pesticide. 

Pesticide risk indicators were therefore developed to represent 

risk at the edge-of-the field (bottom of soil profile for 

groundwater). These edge-of-field risk indicators are based on 

the ratio of pesticide concentrations in water leaving the field 

to safe concentrations (toxicity thresholds) for each pesticide. 

As such, these risk indicators do not have units. The pesticide 

risk indicators were developed so that the relative risk for 

individual pesticides could be aggregated over the 130 

pesticides in use on cropped acres in the Chesapeake Bay 

region.17 

Risk indicator values of less than 1 are considered “safe” 

because the concentration is below the toxicity threshold for 

exposure at the edge-of-the field.18 

17 For a complete documentation of the development of the pesticide risk 

indicators, see “Pesticide risk indicators used in the CEAP cropland 

modeling,” found at http://www.nrcs.usda.gov/technical/nri/ceap. 

18 A threshold value of 1 for the pesticide risk indicator applies when 

assessing the risk for a single pesticide. Since the indicator is summed over all 

pesticides in this study, a threshold value of 1 would still apply if pesticide 

toxicities are additive and no synergistic or antagonistic effects are produced 

when non-target species are exposed to a mix of pesticides. 

Three edge-of-field risk indicators are used here to assess the 

effects of conservation practices: (1) surface water pesticide 

risk indicator for aquatic ecosystems, (2) surface water 

pesticide risk indicator for humans, and (3) groundwater 

pesticide risk indicator for humans. The surface water risk 

indicator includes pesticide residues in solution in surface 

water runoff and in all subsurface water flow pathways that 

eventually return to surface water (water flow in a surface or 

tile drainage system, lateral subsurface water flow, and 

groundwater return flow). The pesticide risk indicator for 

aquatic ecosystems was based on chronic toxicities for fish 

and invertebrates, and acute toxicities for algae and vascular 

aquatic plants. The pesticide risk indicators for humans were 

based on drinking water standards or the equivalent for 

pesticides where standards have not been set. 

These indicators provide a consistent measure that is 

comparable from field to field and that represents the effects 

of farming activities on risk reduction without being 

influenced by other landscape factors. In most environmental 

settings, however, non-target species are exposed to 

concentrations that have been diluted by water from other 

sources, even when those environments are located adjacent to 

a field. Consequently, these edge-of-field risk indicators 

cannot be used to predict actual environmental impacts. 

Atrazine was the dominant pesticide contributing to all three 

risk indicators (table 22). Based on the model simulations, the 

edge-of-field risk indicator for atrazine exceeded 1 for 36 

percent of the cropped acres for risk to aquatic ecosystems, 8 

percent of the cropped acres for surface water risk to humans, 

and 7 percent of the cropped acres for groundwater risk to 

humans. Atrazine's dominance in the risk indicators is due to 

its widespread use, its mobility (solubility = 30 mg/L; 

Koc = 100 g/ml), its persistence (field half-life = 60 days), 

its toxicity to aquatic ecosystems (aquatic plant toxicity = 1 

ppb), and the human drinking water standard (EPA Maximum 

Contaminant Level = 3 ppb). 

The pesticide risk indicator for aquatic ecosystems averaged 

1.76 over all years and cropped acres (table 20) for the 

baseline conservation condition. (The 1.76 value indicates that 

pesticide concentrations in water leaving cropped fields in the 

Chesapeake Bay region are, on average, 1.76 times the “safe” 

concentration for non-target plant and animal species.) The 

median value, however, is only 0.89 (fig. 49), indicating that 

the risk indicator for half of the acres is less than 0.89 and 

greater than 0.89 for half of the acres. Figure 48 shows that for 

most years the overall risk for aquatic ecosystems is low, in 

part because of the conservation practices in use. But in some 

years the edge-of-field concentrations can be high relative to 

"safe" thresholds for some acres. 

67 
 

 

 

 

 

Table 22. Dominant pesticides determining edge-of-field environmental risk, Chesapeake Bay region 

Percent of cropped acres in the region with 

average annual edge-of-field risk indicator 

Pesticide (active ingredient name) Pesticide type greater than 1 

Risk indicator for aquatic ecosystem 

Atrazine Herbicide 36 

Metolachlor Herbicide 4 

2,4-D 2-ethylhexyl ester Herbicide 3 

Sulfentrazone Herbicide 2 

Phostebupirim Insecticide <1 

Linuron Herbicide <1 

Risk indicator for humans, surface water 

Atrazine Herbicide 8 

Simazine Herbicide <1 

Alachlor Herbicide <1 

Dimethoate Insecticide <1 

Terbofos Insecticide <1 

Risk indicator for humans, groundwater 

Atrazine Herbicide 7 

Simazine Herbicide <1 

Figure 48. Distribution of annual values of the edge-of-field surface water pesticide risk indicator for aquatic ecosystems for each 

year of the 47-year model simulation, Chesapeake Bay region Annual edge-of-field pesticide risk indicator for aquatic ecosystem 

26 

24 

22 

20 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

0 10 20 30 40 50 60 70 80 90 100 

Cumulative percent acres 

Note: This figure shows how the annual values of the risk indicator varied within the region and from year to year in the model simulation on cropped acres. Each of 

the 47 curves shown above represents a single year of data and shows how annual values of the risk indicator varied over the region in that year, starting with the acres 

with the lowest value and increasing to the acres with the highest value. The family of curves shows how annual values vary from year to year. 

68 
 

 

 

 

The pesticide risk indicators for humans were much lower, 

averaging 0.42 for surface water and 0.36 for groundwater 

(table 20). The median values are 0.17 for surface water and 

0.08 for groundwater. Only about 11 percent of the cropped 

acres have an average annual edge-of-field surface water 

pesticide risk indicator for humans greater than 1 (fig. 50), and 

only 8 percent of the acres have an average annual bottom-ofthe-rootzone groundwater pesticide risk indicator greater than 

1. 

The use of conservation practices in the Chesapeake Bay 

region has reduced the pesticide risk indicators by 30 to 34
 

percent (table 20), averaged over all years, all pesticides, and 

all cropped acres. 

Figure 51 shows the distribution of the reductions in the two 

pesticide risk indicators due to conservation practices. 

Significant risk reductions for aquatic ecosystems occur on 

about 25 percent of the acres, while significant risk reductions 

for humans occur on only about 5 percent of the acres. The 

benefits of conservation practices were significant for both 

aquatic risks and human risks on the acres that had those risks, 

but aquatic risks were more widespread than human risks so 

conservation practices have greater total benefit for aquatic 

ecosystems than for human drinking water. 

Figure 50. Estimates of average annual edge-of-field surface 

water pesticide risk indicator for humans in the Chesapeake 

Bay region 

0 

1 

2 

3 

4 

5 

0 10 20 30 40 50 60 70 80 90 100 

Average annual edge-of-field surface 

water pesticide risk indicator for 

humans

Cumulative percent acres 

Baseline conservation condition 

No-practice scenario 

Figure 49. Estimates of average annual edge-of-field surface 

water pesticide risk indicator for aquatic ecosystem in the 

Chesapeake Bay region 

16
 15
 14
 13
 12
 11
 10
 

9
 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual edge-of-field surface water 

pesticide risk indicator for aquatic 

8
 

7
 

6
 

5
 

4
 

ecosystem 

3

2
 

1
 

0
 

Figure 51. Estimates of average annual reductions in the Cumulative percent acres edge-of-field surface water pesticide risk indicators for aquatic 

ecosystems in the Chesapeake Bay region 

Baseline conservation condition 16
 

14
 

12
 

10
 

8
 

6
 

4
 

2
 

0
 

-2
 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres
 

Aquatic ecosystem Humans 

Average annual reduction in edgeof-field surface water pesticide risk 

indicators 

No-practice scenario 

Note: Negative reductions in pesticide risk indicators result primarily from an 

increase in surface water runoff due to conservation practices (see figure 16). 

69 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

Chapter 5 

Assessment of Conservation 

Treatment Needs 

The adequacy of conservation practices in use in the 

Chesapeake Bay region was evaluated to identify remaining 

conservation treatment needs for controlling sediment and 

nutrient loss from fields. The evaluation was based on 

conservation practice use for the time period 2003 through 

2006. 

In summary, findings for the Chesapeake Bay Region indicate 

that: 

 19 percent of cropped acres (810,000 acres) have a high 

level of need for additional conservation treatment, 

 61 percent of cropped acres (2,598,000 acres) have a 

moderate level of need for additional conservation 

treatment, and 

	 20 percent of cropped acres (872,000 acres) have a low 

level of need for additional treatment and are considered 

to be adequately treated. 

Field-level model simulation results for the baseline 

conservation conditions were used to make the assessment. 

Four resource concerns were evaluated for the Chesapeake 

Bay region: 

1.	 Sediment loss due to water erosion 

2.	 Nitrogen loss with surface runoff (nitrogen attached to 

sediment and in solution) 

3.	 Nitrogen loss in subsurface flows 

4.	 Phosphorus lost to surface water (phosphorus attached to 

sediment and in solution, including soluble phosphorus in 

subsurface lateral flow pathways) 

The conservation treatment needs for controlling pesticide loss 

were not evaluated because it requires information on pest 

infestations, which was not available for the CEAP sample 

points. A portion of the pesticide residues are controlled by 

soil erosion control practices; meeting soil erosion control 

treatment needs would provide partial protection against loss 

of pesticide residues from farm fields. Integrated Pest 

Management (IPM) practices are also effective in reducing the 

risk associated with pesticide residues leaving the farm field. 

Determination of adequate IPM, however, is highly dependent 

on the specific site conditions and the nature and extent of the 

pest problems. 

Adequate conservation treatment consists of combinations of 

conservation practices that treat the specific inherent 

vulnerability factors associated with each field. Not all acres 

require the same level of conservation treatment. Acres with a 

high level of inherent vulnerability require more treatment 

than less vulnerable acres to reduce field-level losses to 

acceptable levels. Acres with characteristics such as steeper 

slopes and soil types that promote surface water runoff are 

more vulnerable to sediment and nutrient losses beyond the 

edge of the field. Acres that are essentially flat with porous 

soil types are more prone to nutrient losses through subsurface 

flow pathways, most of which return to surface water through 

drainage ditches, tile drains, natural seeps, and groundwater 

return flow. 

Under-treated acres were identified by an imbalance between 

the level of conservation treatment and the level of inherent 

vulnerability. 

Conservation Treatment Levels 

Four levels of conservation treatment (high, moderately high, 

moderate, and low) were defined. For sediment loss due to 

water erosion, conservation treatment levels were defined by a 

combination of structural practices and residue and tillage 

management practices, as defined in figure 52. For nitrogen 

loss with surface runoff, conservation treatment levels were 

defined by a combination of structural practices, residue and 

tillage management practices, and nitrogen management 

practices, as defined in figure 53. For phosphorus lost to 

surface water, conservation treatment levels were defined by a 

combination of structural practices, residue and tillage 

management practices, and phosphorus management practices, 

as defined in figure 54. The nitrogen management level 

presented in figure 9 (see chapter 3) was used to evaluate the 

adequacy of conservation treatment for nitrogen loss in 

subsurface flows. 

A “high” level of treatment was shown by model simulations 

(see chapter 6) to reduce sediment and nutrient losses to low 

levels for nearly all cropped acres in the Chesapeake Bay 

region. 

	 A high level of water erosion control treatment is in use 

on about 16 percent of cropped acres (fig. 52), primarily 

on non-highly erodible land. Only 1 percent of cropped 

acres are highly erodible and have a high level of 

treatment. An additional 11 percent of the highly erodible 

acres have a moderately high level of water erosion 

control treatment. 

	 A high level of treatment for nitrogen runoff is in use on 

less than 1 percent of the acres (fig. 53). About 23 percent 

of the acres have combinations of practices that indicate a 

moderately high level of treatment. 

	 A high level of treatment for phosphorus runoff is in use 

on only 2 percent of the acres (fig. 54). About 22 percent 

of the acres have a moderately high level of treatment for 

controlling phosphorus loss with surface runoff. 

	 A high level of treatment for nitrogen loss in subsurface 

flows is in use on 11 percent of the acres (fig. 9). About 

27 percent of the acres have a moderately high level of 

treatment for controlling nitrogen loss in subsurface 

flows. 

70 
 

Figure 52. Conservation treatment levels for water erosion control in the baseline conservation condition, Chesapeake Bay region 

Low Moderate Moderately 

high High 

HEL 9.9 21.4 11.2 1.4 

0 

10 

20 

30 

40 

50 

60 

Percent of cropped acres 

Non-HEL 8.4 28.8 4.7 14.3 

Criteria for water erosion control treatment levels were derived using a combination of structural practice treatment levels and residue and tillage
 

management treatment levels (see figs. 7 and 8). Scores were first assigned to these treatment levels as follows: High=4, Moderately high=3,
 

Moderate=2, and Low=1. If slope was 2 percent or less, the water erosion control treatment level is the same as the residue and tillage management 

level. If slope was greater than 2 percent, the water erosion control treatment level is determined as follows: 

 High treatment: Sum of scores is equal to 8. (High treatment level for both structural practices and residue and tillage management practices). 

 Moderately high treatment: Sum of scores equal to 6 or 7. 

 Moderate treatment: Sum of scores equal to 4 or 5. 

 Low treatment: Sum of scores equal to 2 or 3.
 

Note: About 44 percent of cropped acres in the Chesapeake Bay region is highly erodible land. 

Figure 53. Conservation treatment levels for nitrogen runoff control in the baseline conservation condition, Chesapeake Bay region 

Low Moderate Moderately 

high High 

HEL 8.2 28.5 7.2 0.0 

0 

10 

20 

30 

40 

50 

60

Percent of cropped acres 

Non-HEL 9.5 30.1 15.8 0.7 

Criteria were derived using a combination of structural practice treatment levels, residue and tillage management treatment levels, and nitrogen 

management treatment levels (see figs. 7-9). Scores were first assigned to these treatment levels as follows: High=4, Moderately high=3, 

Moderate=2, and Low=1. 

If slope was 2 percent or less, the nitrogen runoff control treatment level is determined as follows: 

	 High treatment: Sum of residue and tillage management score and nitrogen management score is equal to 8. (High treatment level for both 

structural practices and nitrogen management practices). 

	 Moderately high treatment: Sum of scores equal to 6 or 7. 

	 Moderate treatment: Sum of scores equal to 4 or 5. 

	 Low treatment: Sum of scores equal to 2 or 3. 

If slope was greater than 2 percent, the nitrogen runoff control treatment level is determined as follows: 

	 High treatment: Sum of structural practice score, residue and tillage management score, and nitrogen management score is equal to 12. (High 

treatment level for all three treatment types.) 

	 Moderately high treatment: Sum of scores equal to 9, 10, or 11. 

	 Moderate treatment: Sum of scores equal to 6, 7 or 8. 

	 Low treatment: Sum of scores equal to 3, 4, or 5. 

Note: About 44 percent of cropped acres in the Chesapeake Bay region is highly erodible land. 

71 
 

Figure 54. Conservation treatment levels for phosphorus runoff control in the baseline conservation condition, Chesapeake Bay region 

Low Moderate Moderately high High 

HEL 15.6 21.7 6.3 0.2 

Non-HEL 14.7 23.8 15.7 1.9 

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

50 

Percent of cropped acres 

Criteria were derived using a combination of structural practice treatment levels, residue and tillage management treatment levels, and phosphorus 

management treatment levels (see figs. 7, 8, and 10) in the same manner as the nitrogen runoff control treatment level. Scores were first assigned to 

these treatment levels as follows: High=4, Moderately high=3, Moderate=2, and Low=1. 

If slope was 2 percent or less, the phosphorus runoff control treatment level is determined as follows: 

	 High treatment: Sum of residue and tillage management score and phosphorus management score is equal to 8. (High treatment level for both 

structural practices and phosphorus management practices). 

	 Moderately high treatment: Sum of scores equal to 6 or 7. 

	 Moderate treatment: Sum of scores equal to 4 or 5. 

	 Low treatment: Sum of scores equal to 2 or 3. 

If slope was greater than 2 percent, the phosphorus runoff control treatment level is determined as follows: 

	 High treatment: Sum of structural practice score, residue and tillage management score, and phosphorus management score is equal to 12. 

(High treatment level for all three treatment types.) 

	 Moderately high treatment: Sum of scores equal to 9, 10, or 11. 

	 Moderate treatment: Sum of scores equal to 6, 7 or 8. 

	 Low treatment: Sum of scores equal to 3, 4, or 5. 

Note: About 44 percent of cropped acres in the Chesapeake Bay region is highly erodible land. 

72 
 

 

 

 

 

 

 

Inherent Vulnerability Factors 

Not all acres require the same level of conservation treatment 

because of differences in inherent vulnerabilities due to soils 

and climate. Inherent vulnerability factors for surface runoff 

include soil properties that promote surface water runoff and 

erosion—soil hydrologic group, slope, and K-factor. Inherent 

vulnerability factors for loss of nutrients in subsurface flows 

include soil properties that promote infiltration—soil 

hydrologic group, slope, K-factor, and coarse fragment 

content of the soil. 

Soil runoff and leaching potentials were estimated for each 

sample point on the basis of vulnerability criteria. A single set 

of criteria was developed for all regions and soils in the US to 

allow for regional comparisons. Thus, some soil runoff and 

leaching potentials are not well represented in every region. 

The criteria for the soil runoff potential are presented in figure 

55, followed by the spatial distribution of the soil runoff 

potential within the Chesapeake Bay region in figure 56. The 

criteria and spatial distribution for the soil leaching potential 

are presented in figures 57 and 58. 

The maps show the soil potentials for all soils and land uses in 

the region. For the assessment of conservation treatment 

needs, however, only the soil potentials for cropped acres were 

used. 

Cropped acres in the Chesapeake Bay region are a mix of 

vulnerable and non-vulnerable acres. About 47 percent of 

cropped acres in the Chesapeake Bay region have a low soil 

runoff potential (fig. 55). However, 23 percent of the acres 

have a high soil runoff potential, consisting almost entirely of 

highly erodible land, and 19 percent have a moderately high 

soil runoff potential. 

About 17 percent of the cropped acres in the region have a 

high soil leaching potential (fig. 57). About 29 percent have a 

moderately high soil leaching potential and 48 percent have a 

moderate soil leaching potential. Only 6 percent of cropped 

acres have a low soil leaching potential in this region. 

Estimates of sediment and nutrient losses for the no-practice 

scenario (without conservation practices) demonstrate how 

vulnerability factors influence losses in the Chesapeake Bay 

region. 

	 Sediment loss for the low soil runoff potential would have 

averaged 0.9 ton per acre per year without conservation 

practices, compared to 6.5 tons per acre per year for the 

high soil runoff potential. 

	 Nitrogen loss with surface runoff for the low soil runoff 

potential would have averaged 7 pounds per acre per year, 

compared to 32 pounds per acre per year for the high soil 

runoff potential. 

	 Nitrogen loss in subsurface flows for the low soil leaching 

potential would have averaged 29 pounds per acre per 

year, compared to 62 pounds per acre per year for the 

high soil leaching potential. 

	 Phosphorus lost to surface water for the low soil runoff 

potential would have averaged 4 pounds per acre per year, 

compared to 10 pounds per acre per year for the high soil 

runoff potential. 

73 
 

 

Figure 55. Soil runoff potential for cropped acres in the Chesapeake Bay region 

Low Moderate Moderately high High 

HEL 5.0 3.0 14.8 21.1 

Non-HEL 41.6 8.6 4.2 1.8 

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

50 

Percent of cropped acres 

Criteria for four classes of soil runoff potential were derived using a combination of soil hydrologic group, slope, and K-factor, as shown in the table 

below: 

Soil runoff potential 

Acres with 

soil hydrologic group A 

Acres with 

soil hydrologic group B 

Acres with 

soil hydrologic group C 

Acres with 

soil hydrologic 

group D 

Low All acres Slope<4 Slope<2 

Slope<2 and 

K-factor<0.28 

Moderate None 

Slope >=4 and <=6 

and K-factor<0.32 

Slope >=2 and <=6 

and K-factor<0.28 

Slope<2 and 

K-factor>=0.28 

Moderately high None 

Slope >=4 and <=6 

and K-factor>=0.32 

Slope >=2 and <=6 

and K-factor>=0.28 Slope >=2 and <=4 

High None Slope>6 Slope>6 Slope>4 

Hydrologic soil groups are classified as: 

 Group A—sand, loamy sand, or sandy loam soils that have low runoff potential and high infiltration rates even when thoroughly wetted.
 

 Group B—silt loam or loam soils that have moderate infiltration rates when thoroughly wetted. 

 Group C—sandy clay loam soils that have low infiltration rates when thoroughly wetted.
 

 Group D—clay loam, silty clay loam, sandy clay, silty clay, or clay soils that have very low infiltration rates when thoroughly wetted. 

K-factor is a relative index of susceptibility of bare, cultivated soil to particle detachment and transport by rainfall. It is determined by the 

composition of the soil, saturated hydraulic conductivity, and soil structure. 

Note: About 44 percent of cropped acres in the Chesapeake Bay region is highly erodible land. 

74 
 

Figure 56. Soil runoff potential for soils in the Chesapeake Bay region 

Note: The soil runoff potential shown in this map was derived using the criteria presented in figure 55 applied to soil characteristics for SSURGO polygons. All soils 

and land uses are represented. 

75 
 

 

 

 

 

Figure 57. Soil leaching potential for cropped acres in the Chesapeake Bay region 

Low Moderate Moderately high High 

HEL 1.2 18.3 17.7 6.7 

Non-HEL 5.2 29.3 11.4 10.1 

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

50 

Percent of cropped acres 

Criteria for four classes of soil leaching potential were derived using a combination of soil hydrologic group, slope, and K-factor, as shown in the 

table below: 

Soil leaching potential 

Acres with 

soil hydrologic group A 

Acres with 

soil hydrologic group B 

Acres with 

soil hydrologic group C 

Acres with 

soil hydrologic 

group D 

Low None None None 

All acres except 

organic soils 

Moderate None 

Slope <=12 and 

 K-factor>=0.24 

or slope>12 

All acres except 

organic soils None 

Moderately high Slope>12 

Slope >=3 and <=12 

and K-factor<0.24 None None 

High 

Slope<=12 or acres 

classified as

 organic soils 

Slope<3 and K-factor 

<0.24 or acres classified 

as organic soils 

Acres classified 

as organic soils 

Acres classified 

as organic soils 

Coarse fragments (stones and rocks) in the soil make it easier for water to infiltrate rather than run off. If the coarse fragment content of the soil was 

greater than 30 percent, the soil leaching potential was increased two levels (moderate and moderately high to high, and low to moderately high). If
 

the coarse fragment content was greater than 10 percent but less than 30 percent, the soil leaching potential was increased one level.
 

Hydrologic soil groups are classified as: 

 Group A—sand, loamy sand, or sandy loam soils that have low runoff potential and high infiltration rates even when thoroughly wetted.
 

 Group B—silt loam or loam soils that have moderate infiltration rates when thoroughly wetted. 

 Group C—sandy clay loam soils that have low infiltration rates when thoroughly wetted.
 

 Group D—clay loam, silty clay loam, sandy clay, silty clay, or clay soils that have very low infiltration rates when thoroughly wetted. 

K-factor is a relative index of susceptibility of bare, cultivated soil to particle detachment and transport by rainfall. It is determined by the 

composition of the soil, saturated hydraulic conductivity, and soil structure. 

Note: About 44 percent of cropped acres in the Chesapeake Bay region is highly erodible land. 

76 
Figure 58. Soil leaching potential for soils in the Chesapeake Bay region 

Note: The soil leaching potential shown in this map was derived using the criteria presented in figure 57 applied to soil characteristics for SSURGO polygons. All soils 

and land uses are represented. 

77 
 

 

 

 

 

 

 

Evaluation of Conservation Treatment Figure 60. Trend in average annual nitrogen loss with surface 

The “matrix approach” runoff for increasing levels of soil runoff potential at two 

A “matrix approach” was used to identify acres where the levels of conservation treatment, Chesapeake Bay Region 

level of conservation treatment was inadequate relative to the 30 

level of inherent vulnerability due to soils and climate. These 25 

acres are referred to as “under-treated acres.” Levels of 

20 conservation treatment in the baseline conservation condition Average annual nitrogen lost with surface runoff (pounds/acre) 15 

10 

5 

0 

were evaluated using both the levels of conservation treatment 

and the soil runoff/leaching potentials. Cropped acres were 

divided into 16 groups—four soil vulnerability potentials and 

four treatment levels. 

The high or moderately high treatment levels are effective in 

reducing losses for all soil potentials, as shown in figures 59 

Low Moderate Moderately High 

high 

through 62. 

The matrixes are presented in tables 23 through 26. Acres and 

model results for each of the 16 groupings are presented in the 

first five matrixes in each table. This matrix approach was 

very effective in segregating acres with high losses from acres 

with low losses. 

	 Estimates of sediment and nutrient loss for the no-practice 

scenario consistently increased from small losses for the 

low soil runoff or leaching potential to large losses for the 

Soil runoff potential 

Low treatment level
 

Moderately high treatment level*
 

* There was not sufficient sample size to report values for the high treatment 

class. 

Figure 61. Trend in average annual nitrogen loss in 

subsurface flows for increasing levels of soil leaching 

potential at two levels of conservation treatment, Chesapeake 

Bay Region 

120 

high soil runoff or leaching potential. As the no-practice 

scenario represents crop production without conservation 

practices, there is no consistent relationship in loss 

estimates among the four conservation treatment levels. 

The differences in losses among conservation treatment 

levels reflect the underlying variability, which is also 

influenced by the number of acres in each group. 

Average annual nitrogen loss in 

subsurface flows (pounds/acre) 100 

80 

60 

40 

20 

0 

Low Moderate Moderately High

Figure 59. Trend in average annual sediment loss for high 

increasing levels of soil runoff potential at two levels of Soil leaching potential conservation treatment, Chesapeake Bay Region 

Low treatment level High treatment level 4.0 

Average annual sediment loss (tons/acre)

Low Moderate Moderately 

3.5 

Figure 62. Trend in average annual phosphorus lost to 

3.0 surface water for increasing levels of soil runoff potential at 

two levels of conservation treatment, Chesapeake Bay Region 2.5 

9 

2.0 

1.5 

1.0 

0.5 

0.0 

Average annual phosphorus lost to 

surface water (pounds/acre) 

8 

7 

6 

5 

4 

3 

2 

1 

0 

Low Moderate Moderately 

High 

high High 

Soil runoff potential high 

Low treatment level High treatment level Soil runoff potential 

Low treatment level 

Moderately high treatment level* 

* There was not sufficient sample size to report values for the high treatment 

class. 

78 
 

 

 

 

 

 

 

 

 

	 Estimates of sediment and nutrient loss for the baseline 

conservation condition exhibit a nearly consistent trend of 

decreasing loss with increasing treatment level within 

each soil runoff or leaching potential. 

	 The highest losses in the baseline conservation condition 

were for groups of acres where the conservation treatment 

level was one step or more below the soil leaching or 

runoff potential. 

The evaluation of conservation treatment needs was conducted 

by identifying which of the 16 groups of acres are 

inadequately treated with respect to the soil runoff or soil 

leaching potential. Three levels of conservation treatment need 

were identified. 

	 Acres with a “high” level of need for conservation 

treatment consist of the most critical under-treated acres 

in the region. These are the most vulnerable of the undertreated acres with the least conservation treatment and 

have the highest losses of sediment and/or nutrients. 

	 Acres with a “moderate” level of need for conservation 

treatment consist of under-treated acres that generally 

have lower levels of vulnerability and/or have more 

conservation practice use than acres with a high level of 

need. The treatment level required is not necessarily less, 

although it can be, but rather the sediment and nutrient 

losses are lower and thus there is less potential on a peracre basis for reducing agricultural pollutant loadings 

with additional conservation treatment. 

	 Acres with a “low” level of need for conservation 

treatment consist of acres that are adequately treated with 

respect to the level of inherent vulnerability. While gains 

can be obtained by adding conservation practices to some 

of these acres, additional conservation treatment would 

reduce field losses by only a small amount. 

The last two matrixes in each of the tables 23 through 26 

shows how conservation treatment needs were identified. 

Specific criteria were used to identify the groups of acres that 

fall into each of the three levels of conservation treatment 

need. Criteria were not tailored to a specific region, but were 

derived for use in all regions of the country to allow for 

comparisons of under-treated acres across regions using a 

consistent analytical framework. The criteria and steps in the 

process are as follows. 

1.	 The percentage of acres that exceeded a given level of 

loss was estimated for each cell in the matrix as a guide to 

determining the extent of excessive losses, shown in 

tables 23 through 26. These are referred to as “acceptable 

levels.” Losses above these levels were treated as 

unacceptable levels of loss. “Acceptable levels” for fieldlevel losses used in this study are— 

o	 Average of 2 tons per acre per year for sediment loss 

o	 Average of 15 pounds per acre per year for nitrogen 

loss with surface runoff (soluble and sediment 

attached) 

o	 Average of 25 pounds per acre per year for nitrogen 

loss in subsurface flows 

o	 Average of 4 pounds per acre per year for phosphorus 

lost to surface water (soluble and sediment attached) 

2.	 Groups of acres with less than 30 percent of the acres 

exceeding acceptable levels were defined as adequately 

treated acres and designated as having a low level of 

conservation treatment need. 

3.	 Groups of acres with more than 60 percent of the acres in 

excess of acceptable levels were designated as having a 

high level of conservation treatment need, indicated by 

darker shaded cells in the matrixes. 

4.	 The remaining acres were designated as having a 

moderate level of conservation treatment need, 

indicated by lighter shaded cells in the matrix. 

Under-treated acres—those groups of acres with either a high 

or moderate level of conservation treatment need—are shown 

in the last matrix in each table. In most cases, under-treated 

acres consisted of acres where the conservation treatment level 

was one step or more below the soil leaching or runoff 

potential (indicated by the red boundary shown in the baseline 

conservation condition matrix). 

Acceptable levels were initially derived through a series of 

forums held at professional meetings of researchers working 

on fate and transport of sediment and nutrients in agriculture. 

Those meetings produced a range of estimates for edge-offield sediment loss, nitrogen loss, and phosphorus loss, 

representing what could be realistically achieved with today’s 

production and conservation technologies. The range was 

narrowed by further examination of APEX model output, 

which also showed that the levels selected were agronomically 

feasible in all agricultural regions of the country. In the 

Chesapeake Bay region, for example, percentages of acres that 

can attain these acceptable levels with additional soil erosion 

control and nutrient management practices on all under-treated 

acres are (see the next chapter): 

 99 percent of cropped acres for sediment loss, 

 99 percent of cropped acres for nitrogen loss with surface 

runoff, 

 88 percent of cropped acres for nitrogen loss in 

subsurface flows, and 

 91 percent of cropped acres for phosphorus lost to surface 

water. 

The criteria used to identify acres that need additional 

treatment, including acceptable levels, are not intended to 

provide adequate protection of water quality, although for 

some environmental settings they may be suitable for that 

purpose. Evaluation of how much additional conservation 

treatment is needed to meet Federal, State, and/or local 

water quality goals in the region is beyond the scope of this 

study. 

79 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 23. Identification of under-treated acres for sediment loss due to water erosion in the Chesapeake Bay region 

Conservation treatment levels for water erosion control 

Moderately 

Soil runoff potential Low Moderate high High All 

Estimated cropped acres

 Low 238,374 958,787 230,998 564,256 1,992,414

 Moderate 153,509 242,434 65,585 34,493 496,021 

 Moderately high 186,902 447,148 131,542 46,549 812,140 

 High 202,678 497,055 252,723 26,869 979,325 

All 781,462 2,145,424 680,848 672,167 4,279,900 

Percent of cropped acres

 Low 6 22 5 13 47

 Moderate 4 6 2 1 12

 Moderately high 4 10 3 1 19

 High 5 12 6 1 23

 All 18 50 16 16 100 

Sediment loss estimates without conservation practices 

(no-practice scenario, average annual tons/acre) 

Low 1.39 0.85 1.10 0.54 0.85

 Moderate 2.98 1.86 3.01 0.60 2.27

 Moderately high 3.57 2.23 2.88 1.57 2.61

 High 5.79 7.22 5.72 4.90 6.48 

All 3.37 2.73 3.34 0.79 2.64 

Sediment loss estimates for the baseline conservation condition (average annual tons/acre)

 Low 1.00 0.43 0.19 0.13 0.38

 Moderate 2.25 0.72 0.54 0.25 1.14

 Moderately high 2.66 1.03 0.55 0.13 1.27

 High 3.75 3.40 0.93 0.11 2.75 

All 2.36 1.27 0.57 0.14 1.18 

Percent reduction in sediment loss due to conservation practices 

Low 28 50 83 75 55

 Moderate 25 61 82 58 50

 Moderately high 26 54 81 92 51

 High 35 53 84 98 58 

All 30 53 83 82 55 

Percent of acres in baseline conservation condition with average annual sediment loss more than 2 tons/acre

 Low 4 6 0 0 3 

 Moderate 48 6 0 0 18

 Moderately high 42 13 4 0 18

 High 64 54 2 0 41 

All 37 18 2 0 16 

Estimate of under-treated acres 

Low 0 0 0 0 0 

 Moderate 153,509 0 0 0 153,509 

 Moderately high 186,902 0 0 0 186,902 

 High 202,678 497,055 0 0 699,733 

 All 543,088 497,055 0 0 1,040,143 

Note: Cells below the red boundary shown for the baseline conservation condition are the acres where the level of conservation treatment is one step or more below the 

soil runoff potential. These cells consistently had the highest losses in the model simulations. 

Note: Color-shaded cells indicate under-treated acres; groups of acres with more than 30 percent of the acres exceeding acceptable levels were defined as under-treated 

acres. Darker color-shaded cells indicate critical under-treated acres; critical under-treated acres were defined as groups of acres with more than 60 percent of the acres 

in excess of acceptable levels. 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Percents may not add to totals because of 

rounding. 

80 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 24. Identification of under-treated acres for nitrogen loss with surface runoff (sediment attached and soluble) in the Chesapeake 

Bay region 

Conservation treatment levels 

for nitrogen runoff control 

Moderately 

Soil runoff potential Low Moderate high High All 

 Moderate 11.4 6.4 5.9 NA 7.8

 Moderately high 14.0 9.6 4.5 NA 9.5

 High 27.1 18.1 7.9 NA 18.3 

Estimated cropped acres

 Low 302,053 1,028,547 631,632 30,182 1,992,414 

 Moderate 147,417 260,991 87,613 0 496,021 

 Moderately high 147,863 513,303 150,974 0 812,140 

 High 158,399 705,199 115,727 0 979,325 

All 755,732 2,508,040 985,946 30,182 4,279,900 

Percent of cropped acres

 Low 7 24 15 <1 47 

 Moderate 3 6 2 0 12

 Moderately high 3 12 4 0 19

 High 4 16 3 0 23

 All 18 59 23 <1 100 

Estimates of nitrogen loss with surface runoff without conservation practices 

(no-practice scenario, average annual pounds/acre) 

Low 11.8 6.6 6.7 3.3 7.4

 Moderate 17.6 11.9 12.0 NA 13.6

 Moderately high 18.9 15.3 12.1 NA 15.4

 High 41.9 30.6 22.9 NA 31.5 

All 20.6 15.7 9.9 3.3 15.1 

Estimates of nitrogen loss with surface runoff for the baseline conservation condition (average annual 

pounds/acre)

 Low 7.8 3.6 3.3 2.0 4.1

All 13.8 9.2 4.3 2.0 8.8 

Percent reduction in nitrogen loss with surface runoff due to conservation practices 

Low 34 46 50 39 44

 Moderate 35 46 51 NA 43

 Moderately high 26 37 62 NA 38

 High 35 41 66 NA 42 

All 33 41 57 39 42 

Percent of acres in baseline conservation condition with average annual nitrogen loss with surface runoff 

more than 15 pounds/acre 

Low 12 2 0 0 3 

 Moderate 28 4 0 NA 10

 Moderately high 35 19 0 NA 18

 High 73 50 11 NA 49 

All 32 19 1 0 17 

Estimate of under-treated acres for nitrogen loss with surface runoff 

Low 0 0 0 0 0 

 Moderate 0 0 0 0 0 

 Moderately high 147,863 0 0 0 147,863 

 High 158,399 705,199 0 0 863,598 

All 480,466 705,199 0 0 1,011,461 

Note: Cells below the red boundary shown for the baseline conservation condition are the acres where the level of conservation treatment is one step or more below the 

soil runoff potential. These cells consistently had the highest losses in the model simulations. 

Note: Color-shaded cells indicate under-treated acres; groups of acres with more than 30 percent of the acres exceeding acceptable levels were defined as under-treated 

acres. Darker color-shaded cells indicate critical under-treated acres; critical under-treated acres were defined as groups of acres with more than 60 percent of the acres 

in excess of acceptable levels. 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Percents may not add to totals because of 

rounding. 

Note: NA indicates not applicable because there were no acres in the category. 

81 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 25. Identification of under-treated acres for nitrogen loss in subsurface flows in the Chesapeake Bay region 

Conservation treatment levels for nitrogen management 

Moderately 

Soil leaching potential Low Moderate high High All 

Estimated cropped acres

 Low 63,413 94,946 75,796 40,884 275,040 

 Moderate 466,151 836,068 533,735 202,304 2,038,260

 Moderately high 277,231 450,068 408,474 113,392 1,249,166

 High 147,787 317,466 149,428 102,753 717,434 

All 954,583 1,698,549 1,167,435 459,334 4,279,900 

Percent of cropped acres

 Low 1 2 2 1 6 

 Moderate 11 20 12 5 48

 Moderately high 6 11 10 3 29

 High 3 7 3 2 17

 All 22 40 27 11 100 

Estimates of nitrogen loss in subsurface flows without conservation practices 

(no-practice scenario, average annual pounds/acre) 

Low 48 32 15 18 29

 Moderate 72 40 26 25 42

 Moderately high 89 47 38 27 51

 High 128 54 39 24 62 

All 84 44 31 25 47 

Estimates of nitrogen loss in subsurface flows for the baseline conservation condition (average annual 

pounds/acre)

 Low 33 28 8 8 21

 Moderate 58 31 10 10 30

 Moderately high 65 36 12 11 32

 High 109 42 17 14 47 

All 67 34 11 11 33 

Percent reduction in nitrogen loss in subsurface flows due to conservation practices 

Low 31 13 49 54 29

 Moderate 19 22 63 61 30

 Moderately high 27 22 69 60 37

 High 15 21 57 44 24 

All 21 21 64 56 31 

Percent of acres in baseline conservation condition with average annual nitrogen loss in subsurface flows 

more than 25 pounds/acre 

Low 41 41 6 0 25

 Moderate 53 39 6 4 30

 Moderately high 66 54 6 3 36

 High 89 59 23 6 50 

All 61 47 8 4 35 

Estimate of under-treated acres for nitrogen loss in subsurface flows

 Low 63,413 94,946 0 0 158,359 

 Moderate 466,151 836,068 0 0 1,302,220

 Moderately high 277,231 450,068 0 0 727,300 

 High 147,787 317,466 0 0 465,254 

All 954,583 1,698,549 0 0 2,653,132 

Note: Cells below the red boundary shown for the baseline conservation condition are the acres where the level of conservation treatment is one step or more below the 

soil leaching potential. These cells consistently had the highest losses in the model simulations. 

Note: Color-shaded cells indicate under-treated acres; groups of acres with more than 30 percent of the acres exceeding acceptable levels were defined as under-treated 

acres. Darker color-shaded cells indicate critical under-treated acres; critical under-treated acres were defined as groups of acres with more than 60 percent of the acres 

in excess of acceptable levels. 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Percents may not add to totals because of 

rounding. 

82 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 26. Identification of under-treated acres for phosphorus lost to surface water (phosphorus attached to sediment and in solution, 

including soluble phosphorus in subsurface lateral flow pathways) in the Chesapeake Bay region 

Conservation treatment levels for phosphorus runoff control 

Moderately 

Soil runoff potential Low Moderate high High All 

Estimated cropped acres

 Low 433,234 842,870 636,510 79,799 1,992,414 

 Moderate 208,900 228,271 58,850 * 496,021 

 Moderately high 270,049 411,749 120,304 * 812,140 

 High 386,555 465,724 127,045 * 979,325 

 All 1,298,738 1,948,614 942,710 89,838 4,279,900 

Percent of cropped acres

 Low 10 20 15 2 47

 Moderate 5 5 1 * 12

 Moderately high 6 10 3 * 19

 High 9 11 3 * 23

 All 30 46 22 2 100 

Phosphorus lost to surface water without conservation practices 

(no-practice scenario, average annual pounds/acre) 

Low 6.7 3.0 3.5 4.7 4.0

 Moderate 8.3 5.1 5.9 * 6.5 

 Moderately high 8.1 6.6 6.0 * 7.0 

 High 11.6 9.0 6.2 * 9.7 

All 8.7 5.4 4.3 4.9 6.2 

Phosphorus lost to surface water for the baseline conservation condition (average annual pounds/acre)

 Low 5.4 1.8 0.7 0.6 2.2

 Moderate 6.3 2.7 1.2 * 4.0 

 Moderately high 6.7 3.9 1.3 * 4.4 

 High 8.3 5.0 1.7 * 5.8 

All 6.7 3.1 1.0 0.7 3.7 

Percent reduction in phosphorus lost to surface water due to conservation practices 

Low 19 38 79 87 45

 Moderate 24 47 79 * 38

 Moderately high 18 41 79 * 37

 High 29 45 72 * 40 

All 24 43 78 87 41 

Percent of acres in baseline conservation condition with average annual phosphorus lost to surface water more than 

4 pounds/acre

 Low 43 14 0 0 15

 Moderate 56 23 0 * 34

 Moderately high 56 32 4 * 35

 High 78 42 9 * 52

 All 58 26 2 0 30 

Estimate of under-treated acres for phosphorus lost to surface water 

Low 433,234 0 0 0 433,234 

 Moderate 208,900 0 0 0 208,900 

 Moderately high 270,049 411,749 0 0 681,798 

 High 386,555 465,724 0 0 852,280 

 All 1,298,738 877,473 0 0 2,176,211 

Note: Cells below the red boundary shown for the baseline conservation condition are the acres where the level of conservation treatment is one step or more below the 

soil runoff potential. These cells consistently had the highest losses in the model simulations. 

Note: Color-shaded cells indicate under-treated acres; groups of acres with more than 30 percent of the acres exceeding acceptable levels were defined as under-treated 

acres. Darker color-shaded cells indicate critical under-treated acres; critical under-treated acres were defined as groups of acres with more than 60 percent of the acres 

in excess of acceptable levels. 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Percents may not add to totals because of 

rounding. 

* Estimate not reported because there were only 4 or fewer sample points available in the category. 

83 
 

 

 

 

 

 

 

Conservation treatment needs by resource concern 

The proportion of cropped acres with a high or moderate need 

for additional conservation treatment was determined to be 

(fig. 63)— 

	 24 percent for sediment loss (5 percent with a high need 

for treatment), 

	 24 percent for nitrogen loss with runoff (4 percent with a 

high need for treatment), 

	 51 percent for phosphorus lost to surface water (9 percent 

with a high need for treatment), and 

	 62 percent for nitrogen loss in subsurface flows (10 

percent with a high need for treatment), most of which 

returns to surface water through drainage ditches, tile 

management practices in use, 18 percent have a high need for 

additional treatment to better control nitrogen and/or 

phosphorus loss from fields. About 79 percent have a high or 

moderate need for additional nutrient management for 

nitrogen and/or phosphorus (table 27). 

Figure 63. Percent of cropped acres that are under-treated in 

the Chesapeake Bay Region, by resource concern 

70% 

60% 

drains, natural seeps, and groundwater return flow. 

Under-treated acres in the Chesapeake Bay region are 

presented by combinations of resource concerns in table 27. 

About 36 percent of cropped acres are under-treated for only 

one of the four resource concerns, usually nitrogen leaching: 

 28 percent of cropped acres are under-treated only for 

nitrogen leaching, 

 8 percent of cropped acres are under-treated only for 

phosphorus runoff, and 

Percent of cropped acres 

0% 

Sediment Nitrogen Phosphorus Nitrogen 

50%
 

40%
 

30%
 

20%
 

 less than 1 percent of cropped acres are under-treated only 

for sediment loss. 

10%
 

Nitrogen runoff and phosphorus runoff was the most loss lost with loss to loss in 

frequently occurring combination of resource concerns, runoff surface subsurface 

water flows representing 15 percent cropped acres. About 12 percent of 

cropped acres were determined to be under-treated for all four High treatment need 

resource concerns. 

Moderate treatment need 

The most critical conservation concern in the region is the 

need for complete and consistent use of nutrient 

management—appropriate rate, form, timing, and method of 

application. While most cropped acres have some nutrient 

Table 27. Under-treated acres with resource concerns needing treatment in the Chesapeake Bay region 

Estimated acres Percent Percent 

needing of cropped of underReason for treatment need treatment acres treated acres 

Nitrogen leaching only 1,178,363 27.5 34.6 

Nitrogen and phosphorus runoff 649,438 15.2 19.1 

Sediment, nitrogen and phosphorus runoff, and nitrogen leaching 521,727 12.2 15.3 

Phosphorus runoff only 347,165 8.1 10.2 

Sediment, nitrogen runoff and phosphorus runoff 288,472 6.7 8.5 

Nitrogen leaching, nitrogen runoff, and phosphorus runoff 164,796 3.9 4.8 

Sediment, phosphorus runoff, and nitrogen leaching 108,342 2.5 3.2 

Sediment and phosphorus runoff 96,271 2.2 2.8 

Nitrogen leaching and nitrogen runoff 28,019 0.7 0.8 

Sediment only 14,438 0.3 0.4 

Sediment and nitrogen runoff 8,446 0.2 0.2 

Sediment and nitrogen leaching 2,447 0.1 0.1 

All under-treated acres 3,407,924 79.6 100.0

 Note: This table summarizes the under-treated acres identified in tables 37-40 and reports the joint set of acres that need treatment according to combinations of
 

resource concerns.
 

Note: Percents may not add to totals because of rounding. 

84 
 

 

 

 

 

 

 

 

Conservation treatment needs for one or more 

resource concern 

Some acres require additional treatment for only one of the 

four resource concerns, while other acres require additional 

treatment for two or more resource concerns. After accounting 

for acres that need treatment for multiple resource concerns, 

the evaluation of treatment needs for the Chesapeake Bay 

region determined the following (fig. 64 and table 28): 

 19 percent of cropped acres (810,000 acres) have a high 

level of need for additional conservation treatment. 

	 61 percent of cropped acres (2,598,000 acres) have a 

moderate level of need for additional conservation 

treatment. 

	 20 percent of cropped acres (872,000 acres) have a low 

level of need for additional treatment and are considered 

to be adequately treated. 

Figure 64. Percent of cropped acres with a high, moderate, or 

low level of need for additional conservation treatment for one 

or more resource concern in the Chesapeake Bay Region 

100% 

Percent of cropped acres 

80% 

60% 

40% 

20% 

0% 

High treatment Moderate Low treatment 

need treatment need need 

High level of need for conservation treatment. Acres with a 

“high” level of need for conservation treatment consist of the 

most critical under-treated acres in the region (table 28 and 

figs. 65 through 68). These are the most vulnerable of the 

under-treated acres with the least conservation treatment and 

have the highest losses of sediment and/or nutrients. Eightyseven percent of these acres have losses higher than the 

acceptable level criteria used in the matrix approach for either 

sediment or nutrients. These acres lose (per acre per year, on 

average)— 

o	 2.5 tons of sediment, 

o	 7.6 pounds of phosphorus, 

o	 16 pounds of nitrogen with surface runoff, and 

o	 60 pounds of nitrogen in subsurface flows. 

Acres with a high level of treatment need have the greatest 

potential for reducing agricultural pollutant loadings with 

additional conservation treatment. 

Moderate level of need for conservation treatment. Acres 

with a “moderate” level of need for conservation treatment 

consist of under-treated acres that generally have lower levels 

of vulnerability and/or have more conservation practice use 

than acres with a high level of need (table 28 and figs. 65 

through 68).The sediment and nutrient losses are lower than 

those with a high need for additional treatment and thus there 

is less potential on a per-acre basis for reducing agricultural 

pollutant loadings with additional conservation treatment. 

Fifty-five percent of these acres have losses higher than the 

acceptable level criteria used in the matrix approach for either 

sediment or nutrients. These acres lose (per acre per year, on 

average)— 

o	 1.0 ton of sediment, 

o	 3.5 pounds of phosphorus, 

o	 8 pounds of nitrogen with surface runoff, and 

o	 32 pounds of nitrogen in subsurface flows. 

While the benefit of additional treatment of acres with a 

moderate level of treatment need is less than for acres with a 

high level of treatment need, a portion of these acres may need 

to be treated to meet water quality goals in the region. For 

example, it may be that some of the acres with a moderate 

treatment need for nitrogen loss will need additional treatment 

to meet goals for reducing nitrogen loads to the Bay. 

(Evaluation of conservation treatment needed to meet water 

quality goals in the region is beyond the scope of this study.) 

Low level of need for conservation treatment. Acres with a 

low level of need for conservation treatment consist of acres 

that are adequately treated with respect to the level of inherent 

vulnerability (table 28 and figs. 65 through 68). Only 7 

percent of these acres have losses higher than the acceptable 

level criteria used in the matrix approach, almost all of which 

are for a single resource concern. These acres lose (per acre 

per year, on average)— 

o	 0.3 ton of sediment, 

o	 1 pound of phosphorus, 

o	 4 pounds of nitrogen with surface runoff, and 

o	 11 pounds of nitrogen in subsurface flows. 

While gains can be obtained by adding conservation practices 

to some of these acres, additional conservation treatment 

would reduce field losses by only a small amount. 

85 
Figure 65. Average per-acre sediment loss for three levels of Figure 67. Average per-acre phosphorus lost to surface water 

conservation treatment need for one or more resource for three levels of conservation treatment need for one or more 

concerns, Chesapeake Bay Region resource concerns, Chesapeake Bay Region 

3.0 8.0 

7.0

2.5
 

Low Moderate High 

Average sediment loss (tons/acre) 

Average total phosphorus loss 

(pounds/acre) 

6.0 

5.0 

4.0 

3.0 

2.0 

1.0 

Low Moderate High 

2.0
 

1.5
 

1.0
 

0.5
 

0.0 

0.0 

Level of need for additional conservation 

Level of need for additional conservation treatment 

treatment 

Note: The average phosphorus lost to surface water for all cropped acres is 

Note: The average sediment loss for all cropped acres is 1.18 tons per acre per 3.67 pounds per acre per year, shown in red. 

year, shown in red. 

Figure 66. Average per-acre nitrogen loss with surface runoff Figure 68. Average per-acre nitrogen loss in subsurface flow 

for three levels of conservation treatment need for one or more pathways for three levels of conservation treatment need for 

resource concerns, Chesapeake Bay Region one or more resource concerns, Chesapeake Bay Region 

18
 70

Average nitrogen loss with surface 

runoff (pounds/acre) 

Average nitrogen loss in subsurface 

flows (pounds/acre) 

16
 

14
 

12
 

10
 

8
 

6
 

4
 

2
 

0
 

60
 

50
 

40
 

30
 

20
 

10
 

0
 

Low Moderate High Low Moderate High 

Level of need for additional conservation Level of need for additional conservation 

treatment treatment 

Note: The average nitrogen loss with surface runoff for all cropped acres is Note: The average nitrogen loss in subsurface flow pathways for all cropped 

8.8 pounds per acre per year, shown in red. acres is 32.7 pounds per acre per year, shown in red. 

86 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 28. Baseline conservation condition model simulation results for subsets of under-treated and adequately treated acres in the 

Chesapeake Bay region 

Acres with a Acres with a Acres with a 

low need moderate need high need 

Model simulated outcome for treatment for treatment for treatment All acres 

Cultivated cropland acres in subset 871,976 2,597,927 809,997 4,279,900 

Percent of acres 20.4% 60.7% 18.9% 100% 

Water flow 

Average annual surface runoff (inches) 4.9 5.1 5.2 5.1 

Average annual subsurface water flow (inches) 11.1 10.7 10.3 10.7 

Erosion and sediment loss 

Average annual wind erosion (tons/acre) 0.026 0.031 0.016 0.027 

Average annual sheet and rill erosion (tons/acre) 0.4 1.0 1.7 1.0 

Average annual sediment loss at edge of field due to 

water erosion (tons/acre) 0.3 1.0 2.5 1.2 

Soil organic carbon 

Average annual change in soil organic carbon, including 

loss of carbon with wind and water erosion 

(pounds/acre) -12 -17 -75 -27 

Nitrogen loss 

Nitrogen applied (pounds/acre) 57 97 133 95 

Nitrogen in crop yield removed at harvest (pounds/acre) 86 84 81 84 

Total nitrogen loss for all pathways (pounds/acre) 20 49 87 50 

Average annual loss of nitrogen through volatilization 

(pounds/acre) 5 7 8 7 

Average annual nitrogen returned to the atmosphere 

through denitrification processes (pounds/acre) 1 1 3 2 

Average annual loss of nitrogen with surface runoff, 

including waterborne sediment (pounds/acre) 4 8 16 9 

Average annual nitrogen loss in subsurface flows 

(pounds/acre) 11 32 60 33 

Phosphorus loss 

Phosphorus applied (pounds/acre) 15.3 24.2 39.3 25.2 

Total phosphorus loss for all pathways (pounds/acre) 1.0 3.5 7.6 3.8 

Loss of phosphorus with surface runoff, including 

waterborne sediment (pounds/acre) 0.9 3.4 7.5 3.7 

Pesticide loss 

Average annual mass loss of pesticides for all pathways 

(grams of active ingredient/hectare) 7.6 14.4 22.5 14.6 

Average annual surface water pesticide risk indicator for 

aquatic ecosystem 1.0 1.7 2.7 1.8 

Average annual surface water pesticide risk indicator for 

humans 0.2 0.4 0.6 0.4 

87 
 

 

 

 

 

 

 

 

 

 

Conservation treatment needs if cover crops were 

used 

The “what if” scenario simulating full adoption of cover crops 

(see appendix C) indicated that if all acres in the region had 

cover crops in use, conservation treatment needs would have 

been much lower. Acres with a high level of treatment need 

would have been only 11 percent of cropped acres instead of 

19 percent, and acres with a moderate level of treatment need 

would have been 42 percent of cropped acres instead of 61 

percent. The breakdown of under-treated acres by resource 

concern is shown in figure 69 for comparison to the same 

breakdown for the baseline conservation condition in figure 

63. To the extent that cover crops are being used more now 

than during 2003–06, conservation treatment needs will be 

less today than the levels estimated in this study. 

Figure 69. Percent of cropped acres that would have been 

determined to be under-treated in the Chesapeake Bay Region 

if all acres in the region had cover crops in use, by resource 

concern Percent of cropped acres 

70% 

60% 

50% 

40% 

30% 

20% 

10% 

0% 

Sediment Nitrogen lost Phosphorus Nitrogen loss 

loss with runoff lost to surface in subsurface 

water flows 

High treatment need 

Moderate treatment need 

Conservation treatment needs by cropping systems 

The breakdown of under-treated acres by cropping system 

showed a proportionate distribution of under-treated acres 

among cropping systems, shown in table 29. 

For the critical under-treated acres (acres with a high need for 

treatment), however, a disproportionately higher percentage 

occurs for two cropping systems—hay-crop mixes and corn 

only—indicating that these cropping systems tend to occur 

more frequently on the more vulnerable acres within the 

region. 

Conservation treatment needs by subregions 

Percentages of the under-treated acres in the Chesapeake Bay 

region that are in each subregion are close to the same 

percentages of the region’s cultivated cropland in each 

subregion, indicating that under-treated acres are spread 

proportionately throughout the region (table 30). 

Critical under-treated acres, however, are disproportionately 

high in the Susquehanna River subregion relative to the 

percentage of cropped acres. This subregion has 41 percent of 

the cropped acres in the watershed and 69 percent of the 

critically under-treated acres. Critical under-treated acres are 

disproportionately low in the Upper Chesapeake subregion 

relative to the percentage of cropped acres. 

88 
Table 29. Under-treated acres by cropping system in the Chesapeake Bay region 

Critical under-treated acres All under-treated acres 

Percent of 

cropped acres Percent of Percent of Percent of Percent of 

in acres in acres in acres in acres in 

Chesapeake Chesapeake cropping Chesapeake cropping 

Subregion name Bay region Acres Bay region system Acres Bay region system 

Corn-soybean only 27 208,111 26 18 933,499 27 79 

Corn-soybean with close grown crops 19 137,046 17 17 683,444 20 82 

Corn only 16 212,227 26 31 595,083 17 86 

Hay-crop mix 16 170,146 21 25 545,460 16 79 

Corn and close grown crops 7 63,386 8 21 263,845 8 89 

Remaining mix of crops 4 11,681 1 7 139,192 4 79 

Soybean only 4 0 0 0 40,184 1 25 

Vegetable or tobacco with or without 

other crops 3 7,400 1 5 125,164 4 90 

Soybean-wheat only 3 0 0 0 82,053 2 66 

Total 100 809,997 100 19 3,407,924 100 80 

Note: Percents may not add to totals because of rounding. 

Table 30. Under-treated acres for the four subregions in the Chesapeake Bay region 

Critical under-treated acres All under-treated acres 

Percent of 

cropped Percent of Percent of 

Sub- acres in acres in Percent of acres in Percent of 

region Chesapeake Chesapeake acres in Chesapeake acres in 

code Subregion name Bay region Acres Bay region subregion Acres Bay region subregion 

0205 Susquehanna River 41 558,625 69 32 1,464,251 43 84 

0206 Upper Chesapeake Bay 28 92,499 11 8 862,878 25 73 

0207 Potomac River 16 120,517 15 18 590,263 17 86 

0208 Lower Chesapeake Bay 16 38,356 5 6 490,533 14 73 

Total 100 809,997 100 19 3,407,924 100 80 

Note: Percents may not add to totals because of rounding. 

89 
 

 

 

 

 

 

 

 

Chapter 6 

Assessment of Potential Field-Level 

Gains from Further Conservation 

Treatment 

Four conservation treatment scenarios were simulated to 

evaluate the potential gains from further conservation 

treatment in the Chesapeake Bay region: 

	 Treatment of the 0.8 million critical under-treated acres 

(acres with a high need for conservation treatment) with 

water erosion control practices. 

	 Treatment of all 3.4 million under-treated acres (acres 

with a high or moderate need for conservation treatment) 

with water erosion control practices. 

	 Treatment of the 0.8 million critical under-treated acres 

with nutrient management practices in addition to water 

erosion control practices to address nutrient losses. 

	 Treatment of all 3.4 million under-treated acres with 

nutrient management practices in addition to water 

erosion control practices to address nutrient losses. 

The simulated levels of conservation treatment were designed 

to add the additional practices needed to complete the existing 

suite of practices so as to reduce sediment and nutrient losses 

at the edge of the field to acceptable levels. The existing 

practices were augmented with additional practices to— 

 avoid or limit the potential for loss by using nutrient 

management practices (appropriate rate, timing, and 

method) on all crops in the rotation; 

 control overland flow where needed; and 

 trap materials leaving the field using appropriate edge-offield mitigation where absent. 

The simulated additional treatment consists of traditional 

conservation practices and treatment options that have been 

implemented over the past 10 years and would be expected to 

be found in current NRCS conservation plans. 

The simulated treatment levels are intended to maintain the 

production capacity within the region to produce crops for 

food, fiber, forage, and fuel. The simulated practices produced 

small decreases in acres in crop production and crop yields. 

The specific conservation practices used in the simulated 

treatments are not intended to be a prescription for how to 

construct conservation plans, but rather are a general 

representation of sets or suites of conservation practices that 

could be used to address multiple resource concerns. In actual 

planning situations a variety of alternative practice scenarios 

would be presented to the producer and selections would be 

based on the level of treatment need, cost of conservation 

implementation, impact on production goals, and preferences 

of the farm operator. 

The level of conservation treatment is simulated to show 

potential environmental benefits, but is not designed to 

achieve specific environmental protection goals. Treatment 

scenarios were also not designed to represent actual program 

or policy options for the Chesapeake Bay region. Economic 

and programmatic aspects--such as producer costs, 

conservation program costs, and capacity to deliver the 

required technical assistance--were not considered in the 

design of the treatment scenarios. 

Conservation crop rotations were not included in the treatment 

scenarios because of the criteria to maintain crop acres and 

preserve current market value and yield for the region. 

Nevertheless, crop rotations that are conducted specifically for 

the purpose of reducing average annual losses of sediment and 

nutrients from farm fields have a high potential to further 

improve crop nutrient efficiency and reduce contaminant 

loadings. 

For the same reason, long-term conserving cover was not 

included in the treatment scenarios. Long-term conserving 

cover represents the ultimate conservation treatment for acres 

that are highly vulnerable to sediment and nutrient loss. 

Enrolling more cultivated cropland acres in programs that 

provide the economic incentives for long-term conserving 

cover may be necessary in some areas to meet water quality 

goals for environmental protection. 

Pesticide management was also not addressed in the treatment 

scenarios. While erosion control practices influence pesticide 

loss, significant reductions in pesticide risk within the region 

will require more intensive Integrated Pest Management (IPM) 

practices, including pesticide substitutions. Simulation of 

additional IPM and any associated pesticide substitutions is 

site specific and requires more information about the sample 

fields than was available from the farmer survey. 

Simulation of Additional Water Erosion 

Control Practices 

Erosion and surface water runoff treatment consists of 

structural and vegetative practices that slow runoff water and 

capture contaminants that it may carry. Practices were added 

where needed (summarized in table 31) according to the 

following rules. 

	 In-field mitigation: 

o	 Terraces were added to all sample points with slopes 

greater than 6 percent, and to those with slopes 

greater than 4 percent and a high potential for 

excessive runoff (hydrologic soil groups C or D). 

Although terraces may be too expensive or 

impractical to implement in all cases, they serve here 

as a surrogate for other practices that control surface 

water runoff. 

o	 Contouring or stripcropping (overland flow practices) 

was added to all other fields with slope greater than 2 

percent that did not already have those practices and 

did not have terraces. 

o	 Concentrated flow practices were not applied since 

they occur on unique landscape situations within the 

field; landscape data other than slope and slope 

length were not available for CEAP sample points. 

90 
 

 

 

 

 

 

 

 

 

 

 

	 Edge-of-field mitigation: 

o	 Fields adjacent to water received a riparian buffer, if 

one was not already present. 

o	 Fields not adjacent to water received a filter strip, if 

one was not already present. 

In addition, the implementation of structural and vegetative 

practices is simulated by an adjustment in the land condition 

parameter used to estimate the NRCS Runoff Curve Number 

(RCN). The RCN is an empirical parameter used in surface 

hydrology for predicting direct runoff or infiltration. The 

hydrologic condition (a component in the determination of the 

RCN) was adjusted from “poor” to “good” for sample points 

where these additional practices were simulated. 

Cover crops were not added. Similarly, tillage management 

was not altered in the simulation of conservation treatment. 

Simulation of Additional Nutrient 

Management Practices

The nutrient management treatment scenario consists of 

additional nutrient management practices where needed in 

addition to the water erosion control practices. The nutrient 

management practices simulated the application of nutrients at 

an appropriate rate, in an appropriate form, at appropriate 

times, and using an appropriate method to provide sufficient 

nutrients for crop growth while minimizing losses to the 

environment. Simulation of nutrient management required 

changes to nutrient applications for one or more crops on all 

but about 6 percent of the acres (see table 9). 

Specific rules for application timing 

The goal for appropriate timing is to apply nutrients close to 

the time when the plant is likely to require them, thereby 

minimizing the opportunity for loss from the field. Rules for 

the timing of nutrient applications (both nitrogen and 

phosphorus) are: 

o	 All commercial fertilizer applications were adjusted to 14 

days prior to planting, except for acres susceptible to 

leaching loss. 

o	 For acres susceptible to leaching loss (hydrologic soil 

group A, soils with sandy textures, or tile drained fields), 

nitrogen was applied in split applications, with 25 percent 

of the total application 14 days before planting and 75 

percent 30 days after planting. 

o	 Manure applications during winter months (December, 

January, February, and March) were moved to 14 days 

pre-plant or April 1, whichever occurs first. This rule 

allows for late March applications of manure in the 

warmer climates of the Chesapeake Bay region. April 1 is 

near the period when the soils warm and become 

biologically active. However, this late date could begin to 

pressure manure storage capacities and it is recognized 

that this could create storage problems. 

In the baseline condition, about 20 percent of the cropped 

acres in the Chesapeake Bay region receive fertilizer 

applications in the fall for at least one spring-planted row crop 

in the rotation. The only fall application of nutrients simulated 

in the nutrient management treatment scenario was for fall 

seeded crops that received a starter fertilizer at planting time. 

Table 31. Summary of additional structural practices simulated for under-treated acres to assess the potential for gains from additional 

conservation treatment in the Chesapeake Bay region 

Critical under-treated acres Non-critical under-treated acres 

(acres with a high level of (acres with a moderate level of 

treatment need) treatment need) All under-treated acres 

Percent of Percent of total Treated Percent 

Additional practice Treated acres total Treated acres acres of total 

Overland flow practice only 5,143 <1 41,971 2 47,115 1 

Terrace only 19,744 2 41,940 2 61,684 2 

Filter only 55,724 7 767,816 30 823,540 24 

Filter plus overland flow practice 96,083 12 535,839 21 631,922 19 

Filter plus Terrace 504,826 62 615,910 24 1,120,736 33 

Buffer only 20,756 3 201,730 8 222,486 7 

Buffer plus overland flow practice 56,858 7 131,006 5 187,864 6 

Buffer plus Terrace 50,864 6 145,134 6 195,997 6 

 One or more additional practices 809,997 100 2,481,347 96 3,291,344 97 

No structural practices 0 0 116,580 4 116,580 3 

Total 809,997 100 2,597,927 100 3,407,924 100 

Note: Percents may not add to totals because of rounding. 

91 
 

 

 

 

 

 

 

 

 

 

 

 

Specific rules for method of application 

If the method of application was other than incorporation then 

fertilizer and manure applications became incorporated or 

injected. Incorporation reduces the opportunity for nutrients 

on the soil surface to be carried away in the soluble form or 

attached to eroding particles. For manure applications on notill fields, if the manure was in liquid or slurry form and had 

been sprayed/broadcast applied it was changed to injected or 

placed under the soil surface. Manure of solid consistency was 

incorporated by disking without regard to the tillage 

management type. If the tillage type had been originally notill, the incorporation of the manure changed the tillage type to 

mulch tillage. 

Specific rules for the form of application 

If the tillage type was no-till, commercial fertilizer was 

changed to a form that could be knifed or injected below the 

soil surface. The change in form did not change the ammonia 

or nitrate ratio of the fertilizer. 

Specific rules for the rate of nutrient applied 

All nitrogen application rates for all crops except cotton and 

small grain crops were reduced to 1.2 times the crop removal 

rate. The 1.2 ratio is in the range of rates recommended by 

many of the Land Grant Universities. This rate replaces some 

of the environmental losses that occur during the cropping 

season, and also accounts for the savings in nutrients due to 

improved application timing and implementation of water 

erosion control practices. 

For small grain crops (wheat, barley, oats, rice, rye, 

buckwheat, emmer, spelt, and triticale), nitrogen applications 

were reduced to 1.5 times the crop removal rate. 

For cotton, nitrogen applications were reduced to 50 pounds 

per bale for sample points with application rates exceeding 50 

pounds per bale. 

For phosphorus, the application rates were adjusted to be 

equal to 1.1 times the amount of phosphorus removed in the 

crop at harvest over the crop rotation. 

Potential for Field-Level Gains 

Treatment of the 0.8 million critical under-treated 

acres 

According to the model simulation, treatment of the 0.8 

million critical under-treated acres with water erosion control 

practices would nearly eliminate sediment loss for these acres 

and dramatically reduce nitrogen and phosphorus lost to 

surface water, as shown in table 32. Sediment loss would be 

reduced to an annual average of about 0.2 ton per acre per year 

for these acres, a 93-percent reduction. Nitrogen loss with 

surface runoff would be reduced to 4.5 pounds per acre per 

year on average (73-percent reduction), and phosphorus lost to 

surface water would be reduced to 3.9 pounds per acre per 

year (48-percent reduction). However, the re-routing of 

surface water to subsurface flow pathways would reduce 

nitrogen loss in subsurface flows by only 1 percent, on 

average, for these acres. 

The addition of nutrient management had little additional 

effect on sediment loss or nitrogen loss with surface runoff, 

but was effective in reducing nitrogen loss in subsurface flows 

and phosphorus lost to surface water (table 32). Nitrogen loss 

in subsurface flows for these acres would be reduced 59 

percent compared to losses simulated for the baseline 

conservation condition. Phosphorus lost to surface water 

would be reduced 65 percent compared to the baseline 

condition for these acres. 

These results support the conclusion drawn from the 

assessment of the effects of conservation practices that 

nutrient management practices need to be paired with erosion 

control practices to obtain significant reductions in the loss of 

soluble nutrients. 

Table 33 presents estimates of how treatment of only the 0.8 

million critical under-treated acres in the region would reduce 

overall edge-of-field losses for the region as a whole. These 

results were obtained by combining treatment scenario model 

results for the 0.8 million acres with model results from the 

baseline conservation condition for the remaining acres. 

Treating the 0.8 million critical under-treated acres with soil 

erosion control practices and nutrient management practices 

would, for the region as a whole— 

 reduce sediment loss in the region by 37 percent on 

average, compared to the baseline conservation condition; 

 reduce total nitrogen loss by 20 percent: 

o	 reduce nitrogen loss with surface runoff (sediment 

adsorbed and soluble) by 27 percent, and 

o	 reduce nitrogen loss in subsurface flows by 20 

percent; 

 reduce phosphorus lost to surface water (sediment 

adsorbed and soluble) by 25 percent, and 

	 reduce environmental risk from loss of pesticide residues 

by 4 percent. 

Treatment of all 3.4 million under-treated acres 

Simulation results for only the 3.4 million under-treated acres 

are presented in table 34 and results for the region as a whole 

are presented in table 35. 

Table 34 shows that per-acre percent reductions of sediment 

and nutrient loss due to additional practices would be less on 

average than those for the 0.8 million most vulnerable undertreated acres. The 3.4 million under-treated acres include 2.6 

million acres with a moderate need for treatment that are less 

vulnerable than the critical under-treated acres. The percent 

reductions for the region as a whole by treating 2.6 million 

additional acres, however, would be much higher, as shown in 

table 35. 

92 
 

 

 

 

 

 

 

 

Table 32. Conservation practice effects for additional treatment of 0.8 million critical under-treated acres (acres with a high need for 

conservation treatment) in the Chesapeake Bay region 

Baseline Treatment with erosion control 

conservation Treatment with erosion control and nutrient management 

condition practices practices 

Average annual Average Percent Average Percent 

Model simulated outcome amount annual amount reduction annual amount reduction 

Water flow 

Surface water runoff (inches) 5.2 4.4 15% 4.4 15% 

Subsurface water flow (inches) 10.3 11.0 -6% 11.0 -6% 

Erosion and sediment loss 

Wind erosion (tons/acre) 0.016 0.014 11% 0.014 10% 

Sheet and rill erosion (tons/acre) 1.71 0.58 66% 0.59 66% 

Sediment loss at edge of field due to water 

erosion (tons/acre) 2.52 0.17 93% 0.18 93% 

Soil organic carbon 

Change in soil organic carbon, including 

loss of carbon with wind and water 

erosion (pounds/acre) -75 1 -- -20 --

Nitrogen 

Nitrogen applied (pounds/acre) 133 128* 3% 79 40% 

Nitrogen in crop yield removed at harvest 

(pounds/acre) 81 79 1% 74 8% 

Total nitrogen loss for all pathways except 

harvest (pounds/acre) 87.2 74.3 15% 34.6 60% 

Loss of nitrogen with surface runoff, 

including waterborne sediment 

(pounds/acre) 16.3 4.5 73% 3.7 77% 

Nitrogen loss in subsurface flows 

(pounds/acre) 59.5 58.7 1% 24.6 59% 

Phosphorus 

Phosphorus applied (pounds/acre) 39.3 38.2 3% 28.6 27% 

Total phosphorus loss for all pathways 

except harvest (pounds/acre) 7.6 4.0 48% 2.7 64% 

Loss of phosphorus to surface water, 

including waterborne sediment 

(pounds/acre) 7.5 3.9 48% 2.6 65% 

Pesticide loss 

Mass loss of pesticides for all pathways 

(grams of active ingredient/hectare) 22.5 12.0 47% 12.2 46% 

Surface water pesticide risk indicator for 

aquatic ecosystems 2.74 2.34 15% 2.39 13% 

Surface water pesticide risk indicator for 

humans 0.55 0.46 17% 0.47 14% 

* Total nitrogen applied was less in the treatment scenario than in the baseline because a small fraction of the field was removed from production to simulate use of 

additional edge-of-field buffer and filtering practices. This also explains the small decrease in nitrogen in the crop yield at harvest. 

Note: Values reported in this table are for the 0.8 million critical under-treated acres only. Percent reductions are with respect to the baseline conservation condition. 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text.
 

93 
 

 

 

 

 

 

 

Table 33. Conservation practice effects for the region as a whole* after additional treatment of 0.8 million critical under-treated acres 

(acres with a high need for conservation treatment) in the Chesapeake Bay region 

Baseline Treatment with erosion control 

conservation Treatment with erosion control and nutrient management 

condition practices practices 

Average annual Average Percent Average Percent 

Model simulated outcome amount annual amount reduction annual amount reduction 

Water flow 

Surface water runoff (inches) 5.1 4.9 3% 4.9 3% 

Subsurface water flow (inches) 10.7 10.8 -1% 10.8 -1% 

Erosion and sediment loss 

Wind erosion (tons/acre) 0.027 0.027 1% 0.027 1% 

Sheet and rill erosion (tons/acre) 0.99 0.78 22% 0.78 21% 

Sediment loss at edge of field due to water 

erosion (tons/acre) 1.18 0.74 38% 0.74 37% 

Soil organic carbon 

Change in soil organic carbon, including 

loss of carbon with wind and water 

erosion (pounds/acre) -27 -12 -- -16 --

Nitrogen 

Nitrogen applied (pounds/acre) 95 94** 1% 85 11% 

Nitrogen in crop yield removed at harvest 

(pounds/acre) 84 84 0% 83 1% 

Total nitrogen loss for all pathways except 

harvest (pounds/acre) 50.2 47.7 5% 40.2 20% 

Loss of nitrogen with surface runoff, 

including waterborne sediment 

(pounds/acre) 8.8 6.6 25% 6.4 27% 

Nitrogen loss in subsurface flows 

(pounds/acre) 32.7 32.6 0% 26.1 20% 

Phosphorus 

Phosphorus applied (pounds/acre) 25.2 25.0 1% 23.2 8% 

Total phosphorus loss for all pathways 

except harvest (pounds/acre) 3.8 3.1 18% 2.8 25% 

Loss of phosphorus to surface water, 

including waterborne sediment 

(pounds/acre) 3.7 3.0 19% 2.7 25% 

Pesticide loss 

Mass loss of pesticides for all pathways 

(grams of active ingredient/hectare) 14.6 12.6 14% 12.6 13% 

Surface water pesticide risk indicator for 

aquatic ecosystems 1.76 1.68 4% 1.69 4% 

Surface water pesticide risk indicator for 

humans 0.42 0.41 4% 0.41 4% 

* Results presented for the region as a whole combine model output for the 0.8 million treated acres with model results from the baseline conservation condition for the 

remaining acres. 

** Total nitrogen applied was less in the treatment scenario than in the baseline because a small fraction of the field was removed from production to simulate use of 

additional edge-of-field buffer and filtering practices. 

Note: Percent reductions are with respect to the baseline conservation condition.
 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text.
 

94 
 

 

 

 

 

 

 

Table 34. Conservation practice effects for additional treatment of 3.4 million under-treated acres (acres with a high or moderate need 

for conservation treatment) in the Chesapeake Bay region 

Baseline Treatment with erosion control 

conservation Treatment with erosion control and nutrient management 

condition practices practices 

Average annual Average Percent Average Percent 

Model simulated outcome amount annual amount reduction annual amount reduction 

Water flow 

Surface water runoff (inches) 5.1 4.3 15% 4.3 15% 

Subsurface water flow (inches) 10.6 11.2 -6% 11.2 -6% 

Erosion and sediment loss 

Wind erosion (tons/acre) 0.027 0.025 10% 0.025 9% 

Sheet and rill erosion (tons/acre) 1.14 0.44 61% 0.44 61% 

Sediment loss at edge of field due to water 

erosion (tons/acre) 1.39 0.11 92% 0.11 92% 

Soil organic carbon 

Change in soil organic carbon, including 

loss of carbon with wind and water 

erosion (pounds/acre) -31 11 -- -5 --

Nitrogen 

Nitrogen applied (pounds/acre) 105 102* 3% 67 36% 

Nitrogen in crop yield removed at harvest 

(pounds/acre) 83 82 2% 76 9% 

Total nitrogen loss for all pathways except 

harvest (pounds/acre) 57.9 50.6 13% 25.3 56% 

Loss of nitrogen with surface runoff, 

including waterborne sediment 

(pounds/acre) 10.1 3.3 67% 2.7 73% 

Nitrogen loss in subsurface flows 

(pounds/acre) 38.3 38.0 1% 16.7 56% 

Phosphorus 

Phosphorus applied (pounds/acre) 27.8 27.3 2% 21.3 23% 

Total phosphorus loss for all pathways 

except harvest (pounds/acre) 4.5 2.6 42% 1.8 60% 

Loss of phosphorus to surface water, 

including waterborne sediment 

(pounds/acre) 4.4 2.5 43% 1.7 62% 

Pesticide loss 

Mass loss of pesticides for all pathways 

(grams of active ingredient/hectare) 16.4 10.3 37% 10.5 36% 

Surface water pesticide risk indicator for 

aquatic ecosystems 1.95 1.71 12% 1.75 10% 

Surface water pesticide risk indicator for 

humans 0.47 0.42 12% 0.43 9% 

* Total nitrogen applied was less in the treatment scenario than in the baseline because a small fraction of the field was removed from production to simulate use of 

additional edge-of-field buffer and filtering practices. This also explains the small decrease in nitrogen in the crop yield at harvest. 

Note: Values reported in this table are for the 3.4 million under-treated acres only. Percent reductions are with respect to the baseline conservation condition.
 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text.
 

95 
 

 

 

 

 

 

 

Table 35. Conservation practice effects for the region as a whole* after additional treatment of 3.4 million under-treated acres (acres 

with a high or moderate need for conservation treatment) in the Chesapeake Bay region 

Baseline Treatment with erosion control 

conservation Treatment with erosion control and nutrient management 

condition practices practices 

Average annual Average Percent Average Percent 

Model simulated outcome amount annual amount reduction annual amount reduction 

Water flow 

Surface water runoff (inches) 5.1 4.4 12% 4.4 12% 

Subsurface water flow (inches) 10.7 11.2 -5% 11.2 -5% 

Erosion and sediment loss 

Wind erosion (tons/acre) 0.027 0.025 8% 0.025 7% 

Sheet and rill erosion (tons/acre) 0.99 0.44 56% 0.44 56% 

Sediment loss at edge of field due to water 

erosion (tons/acre) 1.18 0.16 87% 0.16 87% 

Soil organic carbon 

Change in soil organic carbon, including 

loss of carbon with wind and water 

erosion (pounds/acre) -27 6 -- -6 --

Nitrogen 

Nitrogen applied (pounds/acre) 95 93** 3% 65 32% 

Nitrogen in crop yield removed at harvest 

(pounds/acre) 84 83 2% 78 7% 

Total nitrogen loss for all pathways except 

harvest (pounds/acre) 50.2 44.4 12% 24.2 52% 

Loss of nitrogen with surface runoff, 

including waterborne sediment 

(pounds/acre) 8.8 3.4 61% 3.0 66% 

Nitrogen loss in subsurface flows 

(pounds/acre) 32.7 32.4 1% 15.5 53% 

Phosphorus 

Phosphorus applied (pounds/acre) 25.2 24.8 2% 20.0 21% 

Total phosphorus loss for all pathways 

except harvest (pounds/acre) 3.8 2.3 40% 1.6 57% 

Loss of phosphorus to surface water, 

including waterborne sediment 

(pounds/acre) 3.7 2.2 41% 1.5 59% 

Pesticide loss 

Mass loss of pesticides for all pathways 

(grams of active ingredient/hectare) 14.6 9.8 33% 9.9 32% 

Surface water pesticide risk indicator for 

aquatic ecosystems 1.76 1.57 11% 1.60 9% 

Surface water pesticide risk indicator for 

humans 0.42 0.38 10% 0.39 8% 

* Results presented for the region as a whole combine model output for the 3.4 million treated acres with model results from the baseline conservation condition for the 

remaining acres. 

** Total nitrogen applied was less in the treatment scenario than in the baseline because a small fraction of the field was removed from production to simulate use of 

additional edge-of-field buffer and filtering practices. This also explains the small decrease in nitrogen in the crop yield at harvest. 

Note: Percent reductions are with respect to the baseline conservation condition.
 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text.
 

96 
 

 

 

 

 

 

 

 

 

 

 

 

Treating all 3.4 million under-treated acres with soil erosion 

control practices and nutrient management practices would,
 

for the region as a whole (table 35)—
 

 reduce sediment loss in the region by 87 percent on
 

average, compared to the baseline conservation condition; 

 reduce total nitrogen loss by 52 percent: 

o	 reduce nitrogen loss with surface runoff (sediment 

attached and soluble) by 66 percent, and 

o	 reduce nitrogen loss in subsurface flows by 53 

percent; 

 reduce phosphorus lost to surface water by 59 percent; 

and 

	 reduce environmental risk from loss of pesticide residues 

by 8-9 percent. 

Comparison of treatment scenario results 

The distributions of sediment and nutrient losses for the two 

levels of treatment are compared to the baseline conservation 

condition in the Chesapeake Bay region in figures 70 through 

74. For perspective, the distribution of loss estimates if no 

conservation practices were in use, represented by the nopractice scenario, is also shown. 

The distributions show how the number of acres with high 

losses could be reduced dramatically in the region, by treating 

the under-treated acres. For example, 16 percent of the acres 

in the Chesapeake Bay region exceed an annual average loss 

of sediment of 2 tons per acre per year in the baseline 

conservation condition. Model simulations indicate that 

treating the most vulnerable of the under-treated acres (0.8 

million acres) with water erosion control practices would 

reduce the acres exceeding sediment loss of 2 tons per acre per 

year to 9 percent. Expanding the treatment to include all 

under-treated acres (3.4 million acres) would further reduce 

the acres exceeding annual sediment loss of 2 tons per acre to 

1 percent (fig. 70). 

Similar effects of additional treatment are shown for nitrogen 

lost with surface runoff and phosphorus lost to surface water. 

Treatment of critical under-treated acres with water erosion 

control and nutrient management would reduce the acres 

exceeding 15 pounds per acre of nitrogen lost with runoff 

from 18 percent for the baseline to 9 percent (fig. 72); 

treatment of all 3.4 million under-treated acres would nearly 

eliminate losses exceeding 15 pounds per acre. Acres 

exceeding 4 pounds per acre of phosphorus lost to surface 

water would be reduced from 30 percent for the baseline to 21 

percent by treating the critical acres and to 9 percent by 

treating all under-treated acres (fig. 74). 

For nitrogen loss in subsurface flow pathways, however, 

treatment of all 3.4 million under-treated acres would be 

required to reduce the overall regional edge-of-field losses to 

acceptable levels (fig. 73). About 34 percent of the acres in the 

region have nitrogen loss in subsurface flows greater than 25 

pounds per acre per year for the baseline conservation 

condition. Treating the 0.8 million critical under-treated acres 

with nutrient management practices would reduce this 

percentage to 29 percent. Treatment of all 3.4 million undertreated acres would reduce the percentage to 12 percent. 

Soil organic carbon would be minimally affected by the 

additional soil erosion control and nutrient management 

practices. Increases in soil organic carbon would occur largely 

because of savings of carbon that would otherwise be lost 

from the field through wind and water erosion. Figure 75 

shows that the percentage of acres building soil organic carbon 

would increase from 43 percent for the baseline conservation 

condition to 48 percent with additional conservation treatment 

of the 3.4 million under-treated acres. 

One of the objectives in constructing the treatment scenarios 

was to maintain the level of regional crop production. The 

removal of nitrogen at harvest serves as a useful proxy for 

crop yields and allows for aggregation over the mix of crops. 

The average annual amount of nitrogen removed at harvest 

would be reduced about 9 percent for the 3.4 million acres 

treated with additional soil erosion control and nutrient 

management practices (table 34), which represents a 7-percent 

reduction for the region as a whole (table 35). Figure 76 shows 

that the distribution of nitrogen removed at harvest would be 

slightly lower for the treatment scenario with nutrient 

management, but otherwise similar to the distribution for the 

baseline conservation condition. 

Emerging technologies for reducing nitrogen loss 

from farm fields 

The nutrient management treatment level simulated in this 

study represents feasible and proven conservation practices 

that are currently being successfully applied. There are, 

however, emerging conservation technologies that have the 

potential to further reduce nutrient loss from farm fields 

and provide even greater conservation benefits once the 

technologies become more widespread. These include— 

 variable rate technology for precise nutrient application 

rates and placement methods; 

 nitrogen use efficiency enhancers (time release and 

ammonia loss inhibitors); 

 drainage water management that reduces late fall and 

early spring flushes of nitrate-laden drainage water; and 

 constructed wetlands receiving surface water runoff 

from farm fields prior to discharge to streams and 

rivers. 

New technologies that have the potential to increase crop 

yields without increasing nutrient inputs could further improve 

crop nutrient use efficiency and reduce offsite transport of 

nutrients relative to the level of crop production. 

97 
 

 

Figure 70. Estimates of average annual sediment loss for under-treated acres treated with water erosion control and nutrient 

management compared to the baseline conservation condition and the no-practice scenario, Chesapeake Bay region Average annual sediment loss (tons/acre)22 

20 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres 

No-practice scenario 

Baseline conservation condition 

Treatment of acres with a High need for treatment 

Treatment of acres with a High or Moderate need for treatment 

Figure 71. Estimates of average annual total nitrogen loss for under-treated acres treated with water erosion control and nutrient 

management compared to the baseline conservation condition and the no-practice scenario, Chesapeake Bay region Average annual total nitrogen loss (pounds/acre) 

280 

240 

200 

160 

120 

80 

40 

0 

0 10 20 30 40 50 60 70 80 90 100 

Cumulative percent acres 

No-practice scenario 

Baseline conservation condition 

Treatment of acres with a High need for treatment 

Treatment of acres with a High or Moderate need for treatment 

98 
 

Figure 72. Estimates of average annual loss of nitrogen with surface runoff for under-treated acres treated with water erosion control 

and nutrient management compared to the baseline conservation condition and the no-practice scenario, Chesapeake Bay region Average annual nitrogen lost with surface runoff (pounds/acre) 

90 

75 

60 

45 

30 

15 

0 

Cumulative percent acres 

No-practice scenario 

Baseline conservation condition 

Treatment of acres with a High need for treatment 

Treatment of acres with a High or Moderate need for treatment 

0 10 20 30 40 50 60 70 80 90 100 
 

Figure 73. Estimates of average annual loss of nitrogen in subsurface flows for under-treated acres treated with water erosion control 

and nutrient management compared to the baseline conservation condition and the no-practice scenario, Chesapeake Bay region Average annual nitrogen loss in subsurface flows (pounds/acre) 

250 

225 

200 

175 

150 

125 

100 

75 

50 

25 

0 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres 

No-practice scenario 

Baseline conservation condition 

Treatment of acres with a High need for treatment 

Treatment of acres with a High or Moderate need for treatment 

99 
 

Figure 74. Estimates of average annual phosphorus lost to surface water (sediment attached and soluble)* for under-treated acres 

treated with water erosion control and nutrient management compared to the baseline conservation condition and the no-practice 

scenario, Chesapeake Bay region Average annual phosphorus lost to surface water (pounds/acre) 

32 

28 

24 

20 

16 

12 

8 

4 

0 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres 

No-practice scenario 

Baseline conservation condition 

Treatment of acres with a High need for treatment 

Treatment of acres with a High or Moderate need for treatment 

* Soluble phosphorus lost to surface water includes phosphorus in water moving laterally within the soil into drainage systems (tile and surface drainage) and natural 

seeps. 

Figure 75. Estimates of average annual change in soil organic carbon for under-treated acres treated with water erosion control and 

nutrient management compared to the baseline conservation condition and the no-practice scenario, Chesapeake Bay region Average annual change in soil organic carbon (pounds/acre) 

500 

400 

300 

200 

100 

0 

-100 

-200 

-300 

-400 

-500 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres 

No-practice scenario 

Baseline conservation condition 

Treatment of acres with a High need for treatment 

Treatment of acres with a High or Moderate need for treatment 

100 
Figure 76. Estimates of average annual removal of nitrogen with crop yield at harvest for under-treated acres treated with water 

erosion control and nutrient management compared to the baseline conservation condition and the no-practice scenario, Chesapeake 

Bay region 

220
 

200
 

180
 

160
 

140
 

120
 

100
 

80
 

60
 

40
 

20
 

0
 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres 

No-practice scenario
 

Baseline conservation condition
 

Treatment of acres with a High need for treatment
 

Treatment of acres with a High or Moderate need for treatment
 

Average annual removal of nitrogen with crop yield at 

harvest (pounds/acre) 

101 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

Diminishing returns from additional conservation 

treatment 

Tables 32 through 35 and figures 70 through 74 suggest 

diminishing returns from additional conservation treatment 

when the most vulnerable acres are treated first. These 

diminishing returns are shown explicitly in table 36, which 

includes estimates of the effects of additional conservation 

practices on the 0.9 million adequately treated acres in the 

Chesapeake Bay region. Diminishing returns to additional 

conservation treatment is demonstrated by comparing the 

average annual per-acre reductions in loss among the three 

groups of acres. 

For example, conservation treatment of the 0.8 million critical 

under-treated acres would reduce sediment loss an average of 

2.3 tons per acre per year on those acres. In comparison, 

additional treatment of the remaining 2.6 million under-treated 

acres would reduce sediment loss by about 1 ton per acre per 

year on those acres, and treatment of the remaining 0.9 million 

acres would reduce sediment loss by only 0.3 ton per acre per 

year on those acres, on average. 

Similarly, diminishing returns were pronounced for nitrogen 

and phosphorus loss. Total nitrogen loss would be reduced by 

an average of 53 pounds per acre per year on the 0.8 million 

critical under-treated acres, compared to a reduction of 26 

pounds per acre for the remaining 2.6 million under-treated 

acres and only 2 pounds per acre for the remaining 0.9 million 

acres. 

Total phosphorus loss would be reduced by an average of 4.9 

pounds per acre per year on the 0.8 million critical undertreated acres, compared to a reduction of 2.0 pounds per acre 

for the remaining 2.6 million under-treated acres and only 0.3 

pound per acre for the remaining 0.9 million acres. 

Nitrogen loss in subsurface flows would be reduced by an 

average of 35 pounds per acre per year on the 0.8 million 

critical under-treated acres, compared to a reduction of 17 

pounds per acre for the remaining 2.6 million under-treated 

acres. However, the reduction for treatment of the remaining 

0.9 million acres would average only 0.3 pound per acre. 

Diminishing returns for reduction in environmental risk for 

pesticides are also evident, in spite of the fact that pesticide 

risk was not taken into account in the identification of undertreated acres and the assessment of conservation treatment 

needs. 

Estimates of edge-of-field sediment and nutrient 

savings due to use of conservation practices

A convenient way to envision the potential gains from further 

conservation treatment is to contrast the potential sediment 

and nutrient savings to estimated savings for the conservation 

practices currently in use. 

The no-practice scenario represents the maximum losses that 

would be expected without any conservation practices in use. 

Treatment of all acres with nutrient management and water 

erosion control practices was used to represent a “fulltreatment” condition. 

The difference in sediment and nutrient loss between these 

two scenarios was used to represent the maximum amount of 

savings possible for conservation treatment, which totaled 

10.9 million tons of sediment, 99 thousand tons of nitrogen, 

and 10.2 thousand tons of phosphorus for the Chesapeake Bay 

region (fig. 77). 

As shown in figure 77, about 57 percent of the potential 

sediment savings are accounted for by the conservation 

practices already in use, as represented by the baseline 

conservation condition. Additional treatment of the 0.8 million 

critical under-treated acres would account for another 17 

percent of the potential sediment savings. Additional treatment 

of the remaining 2.6 million under-treated acres would 

account for about 23 percent of the potential savings. Further 

treatment of the 0.9 million adequately treated acres would 

account for the last 2 percent of potential savings. 

For total phosphorus, 54 percent of the potential savings are 

accounted for by the conservation practices already in use, as 

represented by the baseline conservation condition. Additional 

treatment of the 0.8 million critical under-treated acres would 

account for another 19 percent of the potential phosphorus 

savings. Additional treatment of the remaining 2.6 million 

under-treated acres would account for another 26 percent of 

the potential savings. Only 1 percent of potential phosphorus 

savings could be realized through additional treatment of the 

0.9 million adequately treated acres. 

Less progress is evident for total nitrogen, and therefore the 

potential for savings with additional treatment is greater. The 

baseline conservation condition accounts for 43 percent of the 

potential savings from conservation treatment. Treatment of 

the 0.8 million critical under-treated acres would account for 

an additional 22 percent of the potential nitrogen savings. 

Treatment of the remaining 2.6 million under-treated acres 

would account for another 35 percent of the potential nitrogen 

savings. Only 1 percent of potential nitrogen savings could be 

realized through additional treatment of the 0.9 million 

adequately treated acres. 

102 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 36. Effects of additional conservation treatment with erosion control practices and nutrient management practices for three 

groups of acres comprising the 4.3 million cropped acres in the Chesapeake Bay region 

Additional treatment for 0.8 million 

critical under-treated acres* 

Additional treatment for 2.6 million 

non-critical under-treated acres* 

Additional treatment for remaining 

0.9 million acres 

Baseline Treatment scenario Baseline Treatment scenario Baseline Treatment scenario 

Average 

annual 

amount 

Average 

annual 

amount Reduction 

Average 

annual 

amount 

Average 

annual 

amount Reduction 

Average 

annual 

amount 

Average 

annual 

amount Reduction 

Water flow 

Surface water runoff (inches) 5.2 4.4 0.8 5.1 4.3 0.8 4.9 4.2 0.7 

Subsurface water flow (inches) 

Erosion and sediment loss 

10.3 11.0 -0.6 10.7 11.3 -0.7 11.1 11.7 -0.6 

Wind erosion (tons/acre) 0.016 0.014 0.002 0.031 0.028 0.003 0.026 0.024 0.002 

Sheet and rill erosion (tons/acre) 

Sediment loss at edge of field due to 

1.71 0.59 1.12 0.96 0.40 0.56 0.43 0.21 0.22 

water erosion (tons/acre) 

Soil organic carbon 

Change in soil organic carbon, 

including loss of carbon with wind 

2.52 0.18 2.34 1.04 0.09 0.95 0.35 0.07 0.28 

and water erosion (pounds/acre) 

Nitrogen 

-75 -20 55** -17 0 16** -12 -1 10** 

Nitrogen applied (pounds/acre) 

Nitrogen in crop yield removed at 

133 79 53 97 63 33 57 53 3 

harvest (pounds/acre) 

Total nitrogen loss for all pathways 

81 74 7 84 77 8 86 83 3 

except harvest (pounds/acre) 

Loss of nitrogen with surface 

runoff, including waterborne 

87.2 34.6 52.6 48.8 22.3 26.4 20.1 17.7 2.3 

sediment (pounds/acre) 

Nitrogen loss in subsurface flows 

16.3 3.7 12.6 8.2 2.4 5.7 3.8 2.0 1.8 

(pounds/acre)

Phosphorus 

 59.5 24.6 35.0 31.7 14.3 17.4 10.8 10.5 0.3 

Phosphorus applied (pounds/acre) 

Total phosphorus loss for all pathways 

39.3 28.6 10.6 24.2 19.0 5.2 15.3 14.4 0.8 

except harvest (pounds/acre) 

Loss of phosphorus to surface 

water, including waterborne 

7.6 2.7 4.9 3.5 1.5 2.0 1.0 0.7 0.3 

sediment (pounds/acre) 

Pesticide loss 

Mass loss of pesticides for all 

pathways (grams of active 

7.5 2.6 4.9 3.4 1.4 2.0 0.9 0.6 0.3 

ingredient/hectare) 

Surface water pesticide risk indicator 

22.5 12.2 10.3 14.4 10.0 4.4 7.6 6.2 1.3 

for aquatic ecosystem 

Surface water pesticide risk indicator 

2.74 2.39 0.35 1.70 1.55 0.15 1.03 0.94 0.09 

for humans 0.55 0.47 0.08 0.44 0.41 0.03 0.24 0.22 0.01 

*Critical under-treated acres have a high need for additional treatment. Non-critical under-treated acres have a moderate need for additional treatment. 

** Gain in soil organic carbon. 

103 
 

 

 

 

2 1 

23 

35 

26 

17 

19 

22 

57 

43 

54 

Figure 77. Comparison of estimated sediment, nitrogen, and phosphorus savings (field-level) that are due to practices in use in the 

baseline conservation condition and potential savings with additional water erosion control and nutrient management treatment of 

cropped acres in the Chesapeake Bay region 

1

Percent of total estimated and potential savings from 

conservation treatment 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

Sediment loss Total nitrogen loss Total phosphorus loss 

Baseline conservation condition 

Treatment of 0.8 million critical under-treated acres 

Treatment of 2.6 million additional under-treated acres 

Treatment of remaining 0.9 million acres 

Tons of sediment, nitrogen, and phosphorus saved or potentially saved due to conservation practices 

Potential 

Potential savings savings from 

Estimated savings due Potential savings from treatment of treatment of Total estimated and 

to conservation practice from treatment of 0.8 2.6 million remaining 0.9 potential savings 

use (baseline million critical under- additional under- million from conservation 

conservation condition) treated acres* treated acres* acres* treatment 

Sediment 6,231,976 1,895,157 2,480,544 244,258 10,851,935 

Nitrogen 42,395 21,312 34,305 1,014 99,027 

Phosphorus 5,470 1,981 2,632 125 10,208 

*Treatment with erosion control practices and nutrient management practices on all acres. 

Note: Calculations do not include land in long-term conserving cover.
 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

104 
 

 

 

 

 

 

 

 

 

Chapter 7 

Offsite Water Quality Effects of 

Conservation Practices 

The National Water Quality Model— 

HUMUS/SWAT 

Offsite estimates of water quality benefits were assessed using 

HUMUS/SWAT, a combination of the SWAT model and 

HUMUS (Hydrologic Unit Modeling for the United States) 

databases required to run SWAT at the watershed scale for all 

watersheds in the United States (Arnold et al. 1999; Srinivasan 

et al. 1998). SWAT simulates the transport of water, sediment, 

pesticides, and nutrients from the land to receiving streams 

and routes the flow downstream to the next watershed and 

ultimately to estuaries and oceans (fig. 78). 

contiguous land area, but rather represents the percentage of 

the watershed that has the HRU characteristics. In this study, 

SWAT is used to simulate the fate and transport of water, 

sediment, nutrients, and pesticides for the following land use 

categories, referred to as HRUs: 

 Pastureland 

 Range shrub 

 Range grass 

 Urban 

 Mixed forest 

 Deciduous forest 

 Evergreen forest 

 Horticultural lands 

 Forested wetlands 

 Non-forested wetlands 

Upland processes were modeled for each of these HRUs in 

Figure 78. Sources of water flows, sediment, and agricultural 

chemicals simulated with HUMUS/SWAT 

Like APEX, SWAT is a physical process model with a daily 

time step (Arnold and Fohrer 2005; Arnold et al. 1998; 

Gassman et al. 2007).19 The hydrologic cycle in the model is 

divided into two parts. The land phase of the hydrologic cycle, 

or upland processes, simulates the amount of water, sediment, 

nutrients, and pesticides delivered from the land to the outlet 

of each watershed. The routing phase of the hydrologic cycle, 

or channel processes, simulates the movement of water, 

sediment, nutrients, and pesticides from the outlet of the 

upstream watershed through the main channel network to the 

watershed outlet. 

Upland processes 

The water balance is the driving force for transport and 

delivery of sediment, nutrients, and pesticides from fields to 

streams and rivers. For this study, upland processes for noncultivated cropland were modeled using SWAT, while source 

loads for cultivated cropland are estimated by APEX. 

In SWAT, each watershed is divided into multiple Hydrologic 

Response Units (HRUs) that have homogeneous land use, 

management, and soil characteristics. An HRU is not a 

each watershed (8-digit HUC) (fig. 79). The model simulates 

surface runoff estimated from daily rainfall; percolation 

modeled with a layered storage routing technique combined 

with a subsurface flow model; lateral subsurface flow; 

groundwater flow to streams from shallow aquifers, potential 

evapotranspiration; snowmelt; transmission losses from 

streams; and water storage and losses from ponds. 

Figure 79. SWAT model upland simulation processes 

Upland processes for cultivated cropland (including land in 

long-term conserving cover) were modeled using APEX as 

described in previous chapters. The weighted average of peracre APEX model output for surface water delivery, sediment, 

nutrients, and pesticides was multiplied by the acres of 

cultivated cropland in the HUMUS database and used as 

SWAT model inputs for cultivated cropland for each 8-digit 

HUC. The acreage weights for the CEAP sample points were 

used to calculate the per-acre loads. (Several of the 8-digit 

watersheds in each region had too few CEAP sample points to 

reliably estimate edge-of-field per-acre loads. In these cases, 

the 6-digit per acre loads and sometimes the 4-digit per-acre 

loads were used to represent cultivated cropland.) 

19 A complete description of the SWAT model can be found at 

http://www.brc.tamus.edu/swat/index.html. 

105 
 

 

 

 

 

 

 

 

 

 

 

Some land management activities were modeled in SWAT. 

No management was simulated for rangeland, forestland, 

urban land, or horticulture. For permanent hayland, the 

following management activities were simulated: 

	 Three hay cuttings were simulated per crop year. 

	 Hay was fertilized with nitrogen according to the crop 

need as determined by an auto-fertilization routine, which 

was set to grow the crop without undue nitrogen stress. 

	 For legume hay, phosphorus was applied at the time of 

planting (every fourth year) at a rate of 50 pounds per 

acre, followed by applications of 13 pounds per acre 

every other year. 

	 Manure was applied to hayland at rates estimated from 

probable land application of manure, estimated using the 

methods described in USDA/NRCS (2003). 

	 For hayland acres that land-use databases indicated were 

irrigated, water was applied at a frequency and rate 

defined by an auto-irrigation routine. 

For pastureland, the following management activities were 

simulated: 

	 Four grass cuttings were simulated per year to simulate 

grazing. 

	 Pastureland was fertilized with nitrogen according to the 

crop need as determined by an auto-fertilization routine, 

which was set to grow grass without undue nitrogen 

stress. 

	 Manure was applied to pastureland at rates estimated from 

probable land application of manure as estimated in 

USDA/NRCS (2003). 

	 Manure nutrients from grazing animals were simulated 

for pastureland according to the density of pastured 

livestock as reported in the 2002 Census of Agriculture. 

Non-recoverable manure was estimated by subtracting 

recoverable manure available for land application from 

the total manure nutrients representing all livestock 

populations. Non-recoverable manure nutrients include 

the non-recoverable portion from animal feeding 

operations. Estimates of manure nutrients were derived 

from data on livestock populations as reported in the 2002 

Census of Agriculture, which were available for each 6-

digit HUC and distributed among the 8-digit HUCs on a 

per-acre basis. 

A summary of the total amount of nitrogen and phosphorus 

applied to agricultural land in the model simulation, including 

nitrogen and phosphorus applied to cultivated cropland in the 

APEX modeling, is presented in table 37.20 Manure nutrients 

from wildlife populations are not included 

Urban Sources 

Discharges from industrial and municipal wastewater 

treatment plants can be major sources of sediment and 

nutrients in some watersheds. For this study, the point source 

20 For information on how manure nutrients were calculated for use in 

HUMUS modeling, see “Manure Loadings Used to Simulate Pastureland and 

Hayland in CEAP HUMUS/SWAT Modeling,” available at: 

http://www.nrcs.usda.gov/technical/nri/ceap. 

database developed for use in the Chesapeake Bay model was 

used. Point source loads are aggregated within each watershed 

and average annual loads input into SWAT at the watershed 

outlet. 

Urban runoff is estimated separately for three categories of 

cover: 1) impervious surfaces such as buildings, parking lots, 

paved streets, etc., 2) impervious surfaces hydraulically 

connected to drainage systems such as storm drains, and 3) 

impervious surfaces not hydraulically connected to drainage 

systems. For estimating surface water runoff, a runoff curve 

number of 98 was used for impervious surfaces connected 

hydraulically to drainage systems and a composite runoff 

curve number was used for impervious surfaces not 

hydraulically connected to drainage systems. Sediment and 

nutrients carried with storm water runoff to steams and rivers 

were estimated using regression equations developed by 

Driver and Tasker (1988). 

Sediment produced from construction sites was also simulated 

in SWAT. Construction areas were assumed to represent 3 

percent of urban areas. Parameters in the soil input file were 

modified to produce surface runoff and sediment yield that 

mimicked the average sediment load from published studies 

on construction sites. 

Not included in the point source data are 1) pseudo-point 

sources, such as confined animal feeding operations and 

fertilizer handling and distribution centers, 2) urban 

applications of nutrients and chemicals (lawns, golf-courses, 

etc.), or 3) small communities and homes not connected to 

sewer systems. 

Atmospheric nitrogen deposition 

Atmospheric deposition of nitrogen can be a significant 

component of the nitrogen balance. Nitrogen deposition data 

(loads and concentrations) were developed from the National 

Atmospheric Deposition Program/National Trends Network 

database (NAPD 2004). When a rainfall event occurs in the 

model simulation, the amount of rainfall is multiplied by the 

average ammonium and nitrate concentrations calculated for 

the watershed to account for wet deposition. An additional 

amount of ammonium and nitrate are added on a daily basis to 

account for dry deposition. 

Routing and channel processes 

SWAT simulates stream/channel processes including channel 

flood routing, channel sediment routing, nutrient and pesticide 

routing, and transformations modified from the QUAL2E 

model (fig. 80). 

	 Flood routing. As water flows downstream, some may be 

lost due to evaporation and transmission through the 

channel bed. Another potential loss is removal of water 

from the channel for agricultural or human use. Flow may 

be supplemented by rainfall directly on the channel and/or 

addition of water from point source discharges. 

106 
 

 

 

 

 

 

 

 

 

Table 37. Summary of commercial fertilizer and manure nutrients applied to agricultural land in HUMUS/SWAT (pastureland and 

hayland) and APEX (cultivated cropland) model simulations, Chesapeake Bay watershed. 

Commercial Commercial 

Sub- nitrogen Nitrogen from phosphorus Phosphorus Total 

region fertilizer manure Total nitrogen fertilizer from manure phosphorus 

code Subregion name (tons/year) (tons/year) (tons/year) (tons/year) (tons/year) (tons/year) 

Cultivated cropland 

0205 Susquehanna River 50,296 38,805 89,102 11,035 14,422 25,456 

0206 Upper Chesapeake Bay 34,097 13,611 47,708 6,122 4,192 10,314 

0207 Potomac River 22,969 12,444 35,413 4,933 5,616 10,549 

0208 Lower Chesapeake Bay 23,054 121 23,175 5,363 71 5,434 

 Total 130,416 64,982 195,398 27,452 24,301 51,753 

Hayland 

0205 Susquehanna River 22,681 3,196 25,876 2,774 1,446 4,220 

0206 Upper Chesapeake Bay 787 309 1,096 110 142 252 

0207 Potomac River 14,913 5,136 20,049 632 2,448 3,080 

0208 Lower Chesapeake Bay 13,479 1,065 14,544 181 514 695 

 Total 51,860 9,706 61,566 3,698 4,549 8,247 

Pastureland and rangeland 

0205 Susquehanna River 8,532 36,160 44,693 3,150 13,496 16,646 

0206 Upper Chesapeake Bay 1,880 9,091 10,971 822 4,000 4,821 

0207 Potomac River 6,928 33,652 40,580 3,386 16,362 19,748 

0208 Lower Chesapeake Bay 3,394 14,382 17,777 1,927 8,080 10,008 

 Total 20,734 93,285 114,020 9,285 41,939 51,224 

Figure 80. SWAT model channel simulation processes 

107 
 

 

 

 

 

 

	 Sediment routing. The transport of sediment in the 

channel is controlled by the simultaneous operation of 

two processes, deposition and degradation. A stream 

power function was used to determine degradation as a 

function of channel slope and velocity. The maximum 

amount of sediment that can be transported from a reach 

segment is a function of the peak channel velocity. 

Available stream power is used to re-entrain loose and 

deposited material until all of the material is removed. 

Excess stream power causes bed degradation. Bed 

degradation is adjusted for streambed erodibility and 

cover. 

	 Nutrient routing. Nutrient transformations in the stream 

are controlled by the instream water quality component of 

the model. The model tracks nutrients dissolved in the 

stream and nutrients adsorbed to the sediment. Dissolved 

nutrients are transported with the water, while those 

sorbed to sediments are deposited with the sediment on 

the bed of the channel. 

	 Pesticide routing. As with nutrients, the total pesticide 

load in the channel is partitioned into dissolved and 

sediment-attached components. While the dissolved 

pesticide is transported with water, the pesticide attached 

to sediment is affected by sediment transport and 

deposition processes. Pesticide transformations in the 

dissolved and sorbed phases are governed by first-order 

decay relationships. The major in-stream processes 

simulated by the model are settling, burial, resuspension, 

volatilization, diffusion, and transformation. 

Reservoirs alter the dynamics of a free-flowing river, resulting 

in different rates of sediment deposition and chemical 

transformations. SWAT includes routines for reservoirs that 

account for the hydrological aspects of reservoirs. Basic 

reservoir data such as storage capacity and surface area were 

obtained from the dams database. 

	 Reservoir outflow. A simple reservoir simulation 

approach was used in this study. It is a monthly target 

release-storage approach based on the storage capacity 

and flood seasons. 

	 Sediment routing. The concentration of sediment in the 

reservoir is estimated using a simple continuity equation 

based on volume and concentration of inflow, outflow, 

and water retained in the reservoir. Settling of sediment in 

the reservoir is governed by an equilibrium sediment 

concentration and the median sediment particle size. The 

amount of sediment in the reservoir outflow is the product 

of the volume of water flowing out of the reservoir and 

the suspended sediment concentration in the reservoir at 

the time of release. 

	 Reservoir nutrients. The model assumes that (1) the 

reservoir is completely mixed, (2) phosphorus is the 

limiting nutrient, and (3) total phosphorus is a measure of 

the trophic status. The phosphorus mass balance equation 

includes the concentration in the reservoir, inflow, 

outflow, and overall loss rate. 

	 Reservoir pesticides. The model partitions the system 

into a well-mixed surface water layer underlain by a wellmixed sediment layer for simulating the fate of pesticides. 

The pesticide is partitioned into dissolved and particulate 

phases in both the water and sediment layers. The major 

processes simulated by the model are loading, outflow, 

transformation, volatilization, settling, diffusion, 

resuspension, and burial. 

Calibration 

Delivery of surface water and subsurface water from upland 

processes (HRUs and CEAP sample points) was spatially 

calibrated for each watershed to ensure that water inputs to the 

SWAT model were in balance with long-term data on 

streamflow for the region. Water yields from HRUs and 

sample points were compared to long-term water yields 

estimated by USGS. Hydrologic parameters in APEX 

(cultivated cropland) and SWAT (HRUs) were adjusted 

separately for each 8-digit watershed until differences in the 

long-term water yield were minimized.21 

The “background” scenario 

An additional scenario was conducted to represent loadings 

that would be expected if no acres in the watershed were 

cultivated. These estimates were derived by simulating with 

APEX a grass-and-tree mix cover without any tillage or 

addition of nutrients or pesticides for all cultivated cropland 

acres in the watershed.22 All SWAT modeling remained the 

same for this scenario. Thus, “background” loads include 

loads from all other land uses—hayland, pastureland, 

forestland, and urban land—as well as point sources. The 

results of this scenario are labeled “background” in tables and 

figures. 

21 For a complete documentation of calibration procedures and results for the 

Chesapeake Bay region, see “Calibration and Validation for CEAP HUMUS” 

at http://www.nrcs.usda.gov/technical/nri/ceap. 

22 In a natural ecosystem, the vegetative cover would include a mix of species, 

which would continually change until a stable ecosystem was established. 

APEX allows for multiple species and simulates plant competition over time 

according to plant growth, canopy cover, vegetative form, and relative 

maturity or growth stage. The initial mix of species at the beginning of the 47-

year simulation was similar to the mix of grasses and trees used to establish 

long-term conserving cover. Mixes included at least one grass and one 

legume. Over the 47-year simulation, the mix of grasses and trees shifted due 

to plant competition. The grass species typically dominate in the simulation 

until shaded out by tree cover. For further details on how the background 

simulation was conducted, see “Assumptions and Procedures for Simulating 

the Natural Vegetation Background Scenario for Cropland” at 

http://www.nrcs.usda.gov/technical/nri/ceap. 

108 
 

 

 

 

 

 

 

 

 

Source Loads and Instream Loads 

All source loads are introduced into SWAT at the outlet of 

each watershed (8-digit hydrologic unit code [HUC]). Flows 

and source loads from upstream watersheds are routed through 

each downstream watershed, including reservoirs when 

present. 23 

A sediment delivery ratio was used to account for deposition 

in ditches, floodplains, and tributary stream channels during 

transit from the edge of the field to the outlet. The sediment 

delivery ratio used in this study is a function of the ratio of the 

time of concentration for the HRU (land uses other than 

cultivated cropland) or field (cultivated cropland) to the time 

of concentration for the watershed (8-digit HUC). The time of 

concentration for the watershed is the time from when a 

surface water runoff event occurs at the most distant point in 

the watershed to the time the surface water runoff reaches the 

outlet of the watershed. It is calculated by summing the 

overland flow time (the time it takes for flow from the 

remotest point in the watershed to reach the channel) and the 

channel flow time (the time it takes for flow in the upstream 

channels to reach the outlet). The time of concentration for the 

field is derived from APEX. The time of concentration for the 

HRU is derived from characteristics of the watershed, the 

HRU, and the proportion of total acres represented by the 

HRU. Consequently, each cultivated cropland sample point 

has a unique delivery ratio within each watershed, as does 

each HRU.24 

The sediment delivery ratio in addition to an enrichment ratio 

was used to simulate organic nitrogen, organic phosphorus, 

and sediment-attached pesticide transport in ditches, 

floodplains, and tributary stream channels during transit from 

the edge of the field to the outlet. The enrichment ratio was 

defined as the organic nitrogen, organic phosphorus, and 

sediment attached pesticide concentration from the edge of 

field divided by the concentration at the watershed outlet. As 

sediment is transported from the edge of field to the watershed 

outlet, coarse sediments are deposited first while more of the 

fine sediment that hold organic particles remain in suspension, 

thus enriching the organic concentrations delivered to the 

watershed outlet. 

A separate delivery ratio is used to simulate the transport of 

nitrate nitrogen, soluble phosphorus, and soluble pesticides. In 

general, the proportion of soluble nutrients and pesticides 

delivered to rivers and streams is higher than the proportion 

attached to sediments because they are not subject to sediment 

deposition. 

23 For a complete documentation of HUMUS/SWAT as it was used in this 

study, see “The HUMUS/SWAT National Water Quality Modeling System” 

at http://www.nrcs.usda.gov/technical/nri/ceap. 

24 For a complete documentation of delivery ratios used for the Chesapeake 

Bay region, see “Delivery Ratios Used in CEAP Cropland Modeling” at 

http://www.nrcs.usda.gov/technical/nri/ceap. 

For reporting, edge-of-field loads and source loads were 

aggregated over the 8-digit HUCs to the four subregions in the 

region (4-digit HUCs). Figure 81 shows the location of each 

subregion and the 8-digit HUCs included in each. For the 

Susquehanna River and the Potomac River (8-digit HUC 

groups I and III), instream loads represent the loads at the 

outlet of the subregion. For the Upper Chesapeake (8-digit 

HUC group II), the instream loads represent sum of the loads 

at the outlets of 8-digit HUCs draining to into Bay in 

subregion 0206. For the Lower Chesapeake (8-digit HUC 

groups IV), instream loads represent the sum of the loads at 

the outlets of Rappahannock, York and James Rivers in 

subregion, 0208. For the Lower Chesapeake (8-digit HUC 

group V), instream loads represent the load at the outlet of the 

Lower Eastern 8-digit HUC (0208109). 

There are four points in the modeling process at which source 

loads or instream loads are assessed, shown in the schematic 

in figure 82 for sediment. 

1.	 Edge-of-field loads from cultivated cropland—aggregated 

APEX model output as reported in the previous chapter. 

(Edge-of-field loads for the Chesapeake Bay watershed 

differ slightly from those reported in the previous chapter 

because two 8-digit HUCS that drain to the Atlantic 

Ocean were excluded and loads from land in long-term 

conserving cover were included.) 

2.	 Delivery to the watershed outlet from cultivated 

cropland—aggregated edge-of-field loads after 

application of delivery ratios. Loadings delivered to 

streams and rivers differ from the amount leaving the 

field because of losses during transport from the field to 

the stream. Delivery ratios are used to make this 

adjustment. 

3.	 Delivery to the watershed outlet from land uses other than 

cultivated cropland as simulated by SWAT, after 

application of delivery ratios. Point sources are included. 

4.	 Loadings in the stream or river at a given point. Instream 

loads include loadings delivered to the watershed outlet 

from all sources as well as loads delivered from upstream 

watersheds, after accounting for channel and reservoir 

processes. 

109 
 

Figure 81. Subregions and 8-digit HUC groups used for reporting of source loads and instream loads for the Chesapeake Bay 

watershed 

Terminology Used in this Report: Chesapeake Bay Watershed versus Chesapeake Bay 

Region 

Estimates presented in this chapter exclude two 8-digit watersheds in the Upper Chesapeake Bay 

subregion that drain to the Atlantic Ocean (8-digit HUCs 02060010 and 02080110). The area 

excluding these two subregions is referred to as the Chesapeake Bay watershed. However,tables 

and figures elsewhere in the report include the cropped acres in these two 8-digit HUCs; the area 

that includes these two watersheds is referred to as the Chesapeake Bay region. 

110 
 

 

Figure 82. Schematic of sediment sources and delivery as modeled with HUMUS/SWAT for the Chesapeake Bay watershed 

Erosion – Detachment,
 

Sheet and Rill
 

Hillslope 

Deposition 

Terraces 

Buffers 

Cultivated 

cropland 

(APEX) 

MUSLE 

Sediment Leaving edge of field Leaving HRU Yield 

Other land uses 

(SWAT) 

Ditches, 

Channels, HUC Outlet 

and Flood Sediment Yields 

Plains 

Sediment delivery ratio 

(Edge of field HUC Outlet) 

Channel & 

Floodplain 

Deposition 

HUC River
 

Routing (SWAT)
 

Point Sources 

Sediment Transported 

from River 

111 
 

 

 

 

 

 

 

 

Conservation Practice Effects on Water 

Quality

HUMUS/SWAT accounts for the transport of water, sediment, 

pesticides, and nutrients from the land to receiving streams 

and routes the flow downstream to the next watershed and 

ultimately to estuaries and oceans. Not all of the sediment, 

nutrients, and pesticides that leave farm fields is delivered to 

streams and rivers. Some material is bound up permanently in 

various parts of the landscape during transport. In addition, 

instream degradation processes and streambed deposition and 

accumulation remove or trap a portion of the sediment, 

nutrients, and pesticides after delivery to streams and rivers. 

The results from the onsite APEX model simulations for 

cultivated cropland, including land in long-term conserving 

cover, were integrated into HUMUS/SWAT to assess the 

effects of conservation practices on instream loads of 

sediment, nitrogen, phosphorus, and atrazine. The effects of 

conservation practices on water quality were assessed by 

comparing HUMUS/SWAT model simulation results for the 

baseline conservation condition to simulation results for the 

no-practice scenario. For each scenario, only the conditions 

for cultivated cropland were changed, as described previously. 

All other aspects of the simulations—including sediment and 

Table 38. Land use in the Chesapeake Bay watershed 

nutrient loads from point sources and land uses other than 

cultivated cropland—remained the same. 

Land Use in the Chesapeake Bay Watershed 

The USGS National Land-Cover Database for 2001 (Homer et 

al. 2007) was the principle source of acreage for 

HUMUS/SWAT modeling. The 2003 National Resources 

Inventory (USDA-NRCS 2007) was used to adjust NLCD 

cropland acreage estimates to include acres in Conservation 

Reserve Program General Signups, used here to represent 

cropland currently in long-term conserving cover. 

Consequently, cultivated cropland acres used to simulate the 

water quality effects of conservation practices differ slightly 

from the cropped acres reported in the previous chapters, 

which were estimated on the basis of the CEAP Cropland 

sample. In addition, estimates presented in this chapter on offsite water quality exclude two 8-digit watersheds in the Upper 

Chesapeake Bay subregion that drain to the Atlantic Ocean (8-

digit HUCs 02060010 and 02080110) and include land in 

long-term conserving cover with cultivated cropland. 

Estimates of the acreage by land use used to estimate the 

effects of conservation practices in this chapter are presented 

in table 38 and figure 83. 

Pasture and 

grazing land 

Sub- Cultivated Hay land not in not in rotation Forest and 

region cropland rotation with crops with crops Urban land other Total land 

code Subregion name (acres)* (acres) (acres) (acres) (acres)** (acres)*** 

0205 Susquehanna River 2,007,380 1,314,114 1,519,448 1,314,783 11,230,468 17,386,193 

0206 Upper Chesapeake Bay 1,218,106 49,817 812,045 526,715 2,310,880 4,917,564 

0207 Potomac River 611,355 670,212 1,565,170 1,021,360 5,385,808 9,253,905 

0208 Lower Chesapeake Bay 553,641 451,427 1,381,713 734,820 7,307,893 10,429,494 

Total 4,390,482 2,485,571 5,278,375 3,597,679 26,235,048 41,987,155 

* Acres of cultivated cropland include land in long-term conserving cover as well as hay land and pastureland in rotation with crops. 

** Includes forests (all types), wetlands, range brush, horticulture, and barren land.
 

*** Exclusive of water.
 

Note: Estimates in this table differ from estimates for the Chesapeake Bay Region by excluding the two 8-digit HUCs draining into the Atlantic Ocean. Estimates were
 

obtained from HUMUS databases on land use, and thus cultivated cropland estimates do not exactly match the acreage estimates obtained from the NRI-CEAP sample. 

Figure 83. Percent acres for land use/cover types in the Chesapeake Bay watershed (exclusive of water) 

10.5% 

5.9% 

12.6% 

8.6% 

62.5% 

Cultivated cropland 

Hayland 

Pasture and grazing land 

Urban land 

Forest and other 

112 
 

 

 

 

 

 

 

 

 

Sediment 

Model simulation results show that of the 5.5 million tons of 

sediment exported from farm fields in the Chesapeake Bay 

watershed (table 39), about 2 million tons are delivered to 

rivers and streams each year (table 40), on average, under 

conditions represented by the baseline conservation condition, 

which includes farming activities and conservation practices in 

use during the period 2003 to 2006. Most (about 73 percent) 

of the sediment from cultivated cropland originates in the 

Susquehanna River subregion. Sediment delivered to rivers 

and streams from cultivated cropland represents about 22 

percent of the total sediment load delivered from all sources 

(table 41, figure 84). Runoff from urban nonpoint sources 

represents about 40 percent of the total load delivered to 

streams and rivers. 

Instream loads—the amount of sediment delivered from all 

sources to the Bay after accounting for instream deposition 

and transport processes—totals about 6.8 million tons, 

averaged over the 47 years of weather as simulated in the 

model (table 42, figure 85). Overall, about 7 percent of these 

instream loads delivered to the Bay are attributable to 

cultivated cropland sources. 

Loads from all sources delivered to the Bay were greatest for 

the Potomac River and the Lower Chesapeake Bay (table 42, 

figure 85), in part because of their close proximity to the Bay, 

which reduces opportunities for deposition during transport. 

Reservoirs in the Susquehanna River basin trap much of the 

sediment from cultivated cropland in that subregion, 

preventing it from being transported to the Bay. Although the 

Susquehanna River subregion delivers more sediment to rivers 

and streams compared to the Potomac River (tables 40 and 

41), the instream sediment load reported at the outlet of 

Susquehanna River (subregion 0205) is less than the instream 

load for the Potomac River (subregion 0206) because the 

Conowingo Reservoir, located just above the outlet of the 

Susquehanna River, traps a significant portion of the sediment 

from this subregion (table 42). 

Sediment loads delivered to streams and rivers would have 

been much larger if soil erosion control practices were not in 

use. Model simulations indicate that conservation practices 

have reduced the delivery of sediment from fields to rivers and 

streams by about 57 percent (table 40), on average, and 

reduced delivery of sediment to the Bay by about 10 percent 

(table 42, figure 85. Without conservation practices, the total 

sediment delivered to the Bay would be larger by 747,000 tons 

(table 42) per year. The Upper Chesapeake subregion has the 

highest percent reduction in instream loads delivered to the 

Bay due to conservation practices—24 percent. This subregion 

also has the highest proportion of instream sediment loads 

attributed to cultivated cropland sources (15 percent). 

Figure 84. Percentage by source of average annual sediment loads delivered to rivers and streams in the Chesapeake Bay watershed 

21.5% 

10.1% 

5.9% 

39.5% 

23.0% 

Cultivated cropland 

Hayland 

Pasture and grazing land 

Urban point and non-point 

Forest and other 

113 
Table 39. Average annual sediment loads delivered to edge of field (APEX model output) from cultivated cropland for the four subregions in 

Baseline 

conservation condition 

8-digit HUC 

group* 

Subregion 

code Subregion name 

Amount 

(1,000 tons) 

Percent of 

basin total 

Tons delivered 

per cropland 

acre 

No-practice

 Scenario 

(1,000 tons) 

I 0205 Susquehanna River 4,065 74 2.03 8,558 

II 0206 Upper Chesapeake** 561 10 0.46 1,741 

III 0207 Potomac River 535 10 0.87 1,518 

IV + V 0208 Lower Chesapeake** 318 6 0.57 837 

Total 5,479 100 1.25 12,653 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Loads represent both cropped acres and land in long-term contotals because of rounding.*See figure 81.**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).Table 40 Average annual sediment loads delivered to watershed outlets (8-digit HUCs) from cultivated cropland for the four subregions in th

Baseline 

conservation condition 

8-digit HUC 

group* 

Subregion 

code Subregion name 

Amount 

(1,000 tons) 

Percent of 

basin total 

Tons delivered 

per cropland 

acre 

No-practice

 Scenario 

(1,000 tons) 

I 0205 Susquehanna River 1,429 73 0.71 3,042 

II 0206 Upper Chesapeake** 218 11 0.18 685 

III 0207 Potomac River 196 10 0.32 571 

IV + V 0208 Lower Chesapeake** 127 6 0.23 336 

Total 1,970 100 0.45 4,634 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. The differences between loadings in this table and table 39 arwhich were used to simulate delivery of sediment from the edge of the field to the watershed outlet (8-digit HUC). Some columns do not add to totals because of rounding. 

*See figure 81.**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).
Table 41. Average annual sediment loads delivered to watershed outlets (8-digit HUCs) from all sources for the four subregions in the Chesape

Urba

Sub- 

8-digit HUC region Cultivated Pasture and Non-point 

group code Subregion name All sources cropland* Hayland grazing land sources** 

Amount (1,000 tons) 

I 0205 Susquehanna River 4,246 1,429 708 139 1,274 

II 0206 Upper Chesapeake  1,119 218 7 79 473 

III 0207 Potomac River 2,010 196 139 147 1,083 

IV + V 0208 Lower Chesapeake** ** 1,780 127 69 178 787 

Total 9,155 1,970 924 543 3,617 

Percent of all sources 

I 0205 Susquehanna River 100 34 17 3 30 

II 0206 Upper Chesapeake  100 19 1 7 42 

III 0207 Potomac River 100 10 7 7 54 

IV + V 0208 Lower Chesapeake** ** 100 7 4 10 44 

Total 100 22 10 6 40 

* Includes land in long-term conserving cover, excludes horticulture. ** Includes construction sources and urban land runoff.*** Includes forests (all types), wetlands, range brush, horticulture, and barren land. Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).Table 42. Average annual instream sediment loads (all sources) delivered to the Chesapeake Bay 

Baseline 

conservation condition 

Percent of load 

Sub- Background attributed to No-prac

region 8-digit HUC Load from all sources**  cultivated cropland scen

Subregion name code group* sources (1,000 tons) (1,000 tons) sources (1,000 t

Upper Chesapeake Bay 

Susquehanna River 0205 I 1,427 1,295 9 1

Upper Chesapeake 0206 II 934 795 15 1Potomac River 0207 III 2,364 2,256 5 2

Sub-total 4,725 4,346 8 5

Lower Chesapeake Bay 

Rappahannock, York, and James Rivers 0208 IV 2,023 1,962 3 2

Eastern Shore 0208 V 35 31 13 

Sub-total 2,058 1,993 3 2

Total 6,783 6,339 7 7

*See figure 81.** “Background sources” represent loadings that would be expected if no acres in the watershed were cultivated. These estimates were derived by running an additional scenario tha

without any tillage or addition of nutrients or pesticides for all cultivated cropland acres in the watershed. Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Some columns do not add to totals because of rounding.
 

 

Figure 85. Estimates of average annual instream sediment loads for the baseline conservation condition compared to the no-practice 

scenario for subregions in the Chesapeake Bay watershed* 

* Instream sediment loads delivered to the Chesapeake Bay (all sources) are shown for each of the four subregions, corresponding to estimates presented in table 42. 

The total sediment load delivered to the Chesapeake Bay from all areas is shown in the bar chart in the lower right hand corner, labeled “Sediment load to Bay.” 

Note: “Background sources” represent loads that would be expected if no acres in the watershed were cultivated. These estimates were derived by running an additional 

scenario that simulated a grass and tree mix cover without any tillage or addition of nutrients or pesticides for all cultivated cropland acres in the watershed. 

“Background” loads include loads from all other land uses—hayland, pastureland, forestland, and urban land—as well as point sources. 

116 
 

 

 

 

 

 

 

 

 

Total Nitrogen 

Proportionally, total nitrogen loads (all forms) from cultivated 

cropland are higher than sediment or phosphorus loads. Model 

simulation results show that about 148 million pounds of 

nitrogen are lost from farm fields (edge-of-field) through 

pathways that result in delivery to streams and rivers within 

the Chesapeake Bay watershed (table 43). Of this, about 102 

million pounds are delivered into rivers and streams each year, 

on average, under conditions represented by the baseline 

conservation condition (table 44), which include farming 

activities and conservation practices in use during the period 

2003 to 2006. The majority (about 58 percent) of the nitrogen 

from cultivated cropland originates in the Susquehanna River 

subregion, as is the case for sediment and phosphorus. 

Nitrogen delivered to rivers and streams from cultivated 

cropland represents about 31 percent of the total nitrogen load 

delivered from all sources (table 45, fig. 86). Runoff from 

urban land, including point sources, delivers somewhat more 

nitrogen—about 37.7 percent of the total load delivered to 

streams and rivers. 

Instream loads—the amount of nitrogen delivered to the Bay 

after accounting for denitrification, deposition, and other 

instream transport processes—totals about 308 million pounds 

from all sources, averaged over the 47 years of weather as 

simulated in the model (table 46, fig. 87). Overall, about 29 

percent of these instream loads delivered to the Bay are 

attributable to cultivated cropland sources. 

Instream loads from all sources delivered to the Bay were 

greatest for the Susquehanna River subregion (table 46, fig. 

87). The Susquehanna River also has the highest proportion of 

instream loads attributable to cultivated cropland—41 percent. 

Conservation practices in use throughout the watershed have 

reduced nitrogen loads, but not as dramatically as sediment 

loads, as discussed in the previous chapter. Model simulations 

indicate that conservation practices have reduced the delivery 

of nitrogen from fields to rivers and streams by about 36 

percent (table 44), on average, and reduced delivery of 

nitrogen to the Bay by about 14 percent (table 46, fig. 87). 

Nitrogen loads delivered to the Bay would have been larger by 

about 52 million pounds per year if conservation practices 

were not in use (table 46). Over half of this reduction is in the 

Susquehanna River subregion, where total nitrogen instream 

loads have been reduced by 19 percent due to the use of 

conservation practices. 

Figure 86. Percentage by source of average annual nitrogen loads delivered to rivers and streams in the Chesapeake Bay watershed 

30.8% 

13.3% 

6.3% 

37.7% 

11.8% 

Cultivated cropland 

Hayland 

Pasture and grazing land 

Urban point and non-point 

Forest and other 

117 
Table 43. Average annual nitrogen source loads delivered to edge of field (APEX model output) from cultivated cropland for the four subregi

watershed 

Baseline 

conservation condition 

Sub- Pounds No-practice

8-digit HUC region Amount Percent of delivered per Scenario 

group* code Subregion name (1,000 pounds) basin total cropland acre (1,000 pounds) 

I 0205 Susquehanna River 87,691 59 43.68 138,060 

II 0206 Upper Chesapeake** 31,214 21 25.63 48,338 

III 0207 Potomac River 18,417 12 30.12 29,799 

IV + V 0208 Lower Chesapeake** 10,411 7 18.80 16,461 

Total 147,733 100 33.65 232,658 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Loads represent both cropped acres and land in long-term contotals because of rounding.*See figure 81.**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).Table 44. Average annual nitrogen source loads delivered to watershed outlets (8-digit HUCs) from cultivated cropland for the four subregio

watershed 

Baseline 

conservation condition 

8-digit HUC 

group* 

Subregion 

code Subregion name 

Amount 

(1,000 pounds) 

Percent of 

basin total 

Pounds 

delivered per 

cropland acre 

No-practice

 Scenario 

(1,000 pounds) 

I 0205 Susquehanna River 58,939 58 29.4 92,134 

II 0206 Upper Chesapeake** 22,592 22 18.5 34,731 

III 0207 Potomac River 12,761 13 20.9 20,523 

IV + V 0208 Lower Chesapeake** 7,319 7 13.2 11,765 

Total 101,611 100 23.1 159,153 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. The differences between loadings in this table and table 43 arwhich were used to simulate delivery of nitrogen from the edge of the field to the watershed outlet (8-digit HUC). Some columns do not add to totals because of rounding. 

*See figure 81.**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).
Table 45. Average annual nitrogen loads delivered to watershed outlets (8-digit HUCs) from all sources for the four subregions in the Chesape

Urba

Sub- 

8-digit HUC region Cultivated Pasture and Non-point 

group code Subregion name All sources cropland* Hayland grazing land sources** 

Amount (1,000 pounds) 

I 0205 Susquehanna River 140,802 58,939 13,891 15,822 9,335 

II 0206 Upper Chesapeake  53,112 22,592 543 4,111 5,047 

III 0207 Potomac River 78,256 12,761 4,457 12,601 9,743 

IV + V 0208 Lower Chesapeake** ** 57,326 7,319 1,856 6,302 6,840 

Total 329,496 101,611 20,747 38,836 30,965 

Percent of all sources 

I 0205 Susquehanna River 100 42 10 11 7 

II 0206 Upper Chesapeake  100 43 1 8 10 

III 0207 Potomac River 100 16 6 16 12 

IV + V 0208 Lower Chesapeake** ** 100 13 3 11 12 

Total 100 31 6 12 9 

* Includes land in long-term conserving cover, excludes horticulture. ** Includes construction sources and urban land runoff.*** Includes forests (all types), wetlands, range brush, horticulture, and barren land. Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).Table 46. Average annual instream total nitrogen loads (all sources) delivered to the Chesapeake Bay 

Baseline 

conservation condition 

Percent of load 

Sub- Load from all Background attributed to No-prac

region 8-digit HUC sources (1,000 sources**  cultivated cropland scen

Subregion name code group* pounds) (1,000 pounds) sources (1,000 pou

Upper Chesapeake Bay 

Susquehanna River 0205 I 125,260 73,613 41 155

Upper Chesapeake 0206 II 46,634 29,343 37 56Potomac River 0207 III 80,365 67,454 16 88

Sub-total 252,259 170,410 32 300

Lower Chesapeake Bay 

Rappahannock, York, and James Rivers 0208 IV 54,605 48,505 11 58

Eastern Shore 0208 V 1,372 858 37 1

Sub-total 55,977 49,363 12 60

Total 308,236 219,773 29 360

*See figure 81.** “Background sources” represent loadings that would be expected if no acres in the watershed were cultivated. These estimates were derived by running an additional scenario tha

without any tillage or addition of nutrients or pesticides for all cultivated cropland acres in the watershed. Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Some columns do not add to totals because of rounding.
Figure 87. Estimates of average annual instream nitrogen loads for the baseline conservation condition compared to the no-practice 

scenario for subregions in the Chesapeake Bay watershed* 

* Instream nitrogen loads delivered to the Chesapeake Bay (all sources) are shown for each of the four subregions, corresponding to estimates presented in table 46. The 

total sediment load delivered to the Chesapeake Bay from all areas is shown in the bar chart in the lower right hand corner, labeled “Nitrogen load to Bay.” 

Note: “Background sources” represent loads that would be expected if no acres in the watershed were cultivated. These estimates were derived by running an additional 

scenario that simulated a grass and tree mix cover without any tillage or addition of nutrients or pesticides for all cultivated cropland acres in the watershed. 

“Background” loads include loads from all other land uses—hayland, pastureland, forestland, and urban land—as well as point sources. 

120 
 

 

 

 

 

 

 

 

Total Phosphorus 

Model simulation results show that about 16 million pounds of 

phosphorus are lost from farm fields (edge-of-field) through 

pathways that result in delivery to streams and rivers within 

the Chesapeake Bay watershed (table 47). Of this, about 6.4 

million pounds is delivered into rivers and streams each year, 

on average, under conditions represented by the baseline 

conservation condition (table 48), which include farming 

activities and conservation practices in use during the period 

2003 to 2006. The majority of the phosphorus from cultivated 

cropland originates in the Susquehanna River subregion. 

Phosphorus delivered to rivers and streams from cultivated 

cropland represents about 28 percent of the total phosphorus 

load delivered from all sources (table 49, fig. 88). The 

dominant source of phosphorus delivered into streams and 

rivers is runoff from urban land and point sources—about 50 

percent of the total load delivered to streams and rivers. 

Instream loads—the amount of phosphorus delivered from all 

sources to the Bay after accounting for deposition and other 

instream transport processes—totals about 14.8 million 

pounds, averaged over the 47 years of weather as simulated in 

the model (table 50, fig. 89). Overall, about 23 percent of 

these instream loads delivered to the Bay are attributable to 

cultivated cropland sources. 

Instream loads from all sources delivered to the Bay were 

greatest for the Lower Chesapeake subregion (table 50, fig. 

89), mostly from sources other than cultivated cropland. The 

Susquehanna River and the Upper Chesapeake subregions 

have the highest proportion of instream loads attributable to 

cultivated cropland—34 and 35 percent, respectively (table 

50). 

Phosphorus loads delivered to streams and rivers would have 

been much larger if soil erosion control practices were not in 

use. Model simulations indicate that conservation practices 

have reduced the delivery of phosphorus from fields to rivers 

and streams by about 39 percent (table 48), on average, and 

reduced delivery of phosphorus to the Bay by about 14 percent 

(table 50, fig. 89). Phosphorus loads delivered to the Bay 

would have been larger by about 2.4 million pounds per year 

if conservation practices were not in use (table 50). The Upper 

Chesapeake subregion has the highest percent reduction in 

instream loads delivered to the Bay due to conservation 

practices—31 percent. 

Figure 88. Percentage by source of average annual phosphorus loads delivered to rivers and streams in the Chesapeake Bay watershed 

6.8% 

28.3% 

50.1% 7.4% 

7.4% 

Cultivated cropland 

Hayland 

Pasture and grazing land 

Urban point and non-point 

Forest and other 

121 
Table 47. Average annual phosphorus source loads delivered to edge of field (APEX model output) from cultivated cropland for the four subr

watershed 

Baseline 

conservation condition 

Sub- Pounds No-practice

8-digit HUC region Amount Percent of delivered per Scenario 

group* code Subregion name (1,000 pounds) basin total cropland acre (1,000 pounds) 

I 0205 Susquehanna River 9,798 61 4.88 16,188 

II 0206 Upper Chesapeake** 2,615 16 2.15 5,737 

III 0207 Potomac River 2,612 16 4.27 4,106 

IV + V 0208 Lower Chesapeake** 1,157 7 2.09 1,805 

Total 16,183 100 3.69 27,836 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Loads represent both cropped acres and land in long-term contotals because of rounding.*See figure 81.**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).Table 48. Average annual phosphorus source loads delivered to watershed outlets (8-digit HUCs) from cultivated cropland for the four subre

watershed 

Baseline 

conservation condition 

8-digit HUC 

group* 

Subregion 

code Subregion name 

Amount 

(1,000 pounds) 

Percent of 

basin total 

Pounds 

delivered per 

cropland acre 

No-practice

 Scenario 

(1,000 pounds) 

I 0205 Susquehanna River 3,702 58 1.84 5,822 

II 0206 Upper Chesapeake** 1,152 18 0.95 2,474 

III 0207 Potomac River 1,077 17 1.76 1,558 

IV + V 0208 Lower Chesapeake** 499 8 0.90 753 

Total 6,430 100 1.46 10,607 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. The differences between loadings in this table and table 47 arwhich were used to simulate delivery of phosphorus from the edge of the field to the watershed outlet (8-digit HUC). Some columns do not add to totals because of rounding.

*See figure 81.**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).
Table 49. Average annual phosphorus loads delivered to watershed outlets (8-digit HUCs) from all sources for the four subregions in the Chesa

Urba

Sub- 

8-digit HUC region Cultivated Pasture and Non-point 

group code Subregion name All sources cropland* Hayland grazing land sources** 

Amount (1,000 pounds) 

I 0205 Susquehanna River 10,599 3,702 1,316 554 580 

II 0206 Upper Chesapeake  2,726 1,152 15 132 198 

III 0207 Potomac River 4,717 1,077 270 602 531 

IV + V 0208 Lower Chesapeake** ** 4,714 499 87 406 417 

Total 22,756 6,430 1,689 1,693 1,726 

Percent of all sources 

I 0205 Susquehanna River 100 35 12 5 5 

II 0206 Upper Chesapeake  100 42 1 5 7 

III 0207 Potomac River 100 23 6 13 11 

IV + V 0208 Lower Chesapeake** ** 100 11 2 9 9 

Total 100 28 7 7 8 

* Includes land in long-term conserving cover, excludes horticulture. ** Includes construction sources and urban land runoff.*** Includes forests (all types), wetlands, range brush, horticulture, and barren land. Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).Table 50. Average annual instream total phosphorus loads (all sources) delivered to the Chesapeake Bay 

Baseline 

conservation condition 

Percent of load 

Sub- Load from all Background attributed to No-prac

region 8-digit HUC sources (1,000 sources**  cultivated cropland scen

Subregion name code group* pounds) (1,000 pounds) sources (1,000 pou

Upper Chesapeake Bay 

Susquehanna River 0205 I 3,815 2,522 34 4

Upper Chesapeake 0206 II 2,362 1,525 35 3

Potomac River 0207 III 4,000 3,086 23 4

Sub-total 10,177 7,133 30 12

Lower Chesapeake Bay 

Rappahannock, York, and James Rivers 0208 IV 4,544 4,135 9 4

Eastern Shore 0208 V 92 75 19 

Sub-total 4,636 4,210 9 4

Total 14,813 11,342 23 17

*See figure 81.** “Background sources” represent loadings that would be expected if no acres in the watershed were cultivated. These estimates were derived by running an additional scenario tha

without any tillage or addition of nutrients or pesticides for all cultivated cropland acres in the watershed. Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Some columns do not add to totals because of rounding.
 

Figure 89. Estimates of average annual instream phosphorus loads for the baseline conservation condition compared to the no-practice 

scenario for subregions in the Chesapeake Bay watershed* 

* Instream phosphorus loads delivered to the Chesapeake Bay (all sources) are shown for each of the four subregions, corresponding to estimates presented in table 50. 

The total sediment load delivered to the Chesapeake Bay from all areas is shown in the bar chart in the lower right hand corner, labeled “Phosphorus load to Bay.” 

Note: “Background sources” represent loads that would be expected if no acres in the watershed were cultivated. These estimates were derived by running an additional 

scenario that simulated a grass and tree mix cover without any tillage or addition of nutrients or pesticides for all cultivated cropland acres in the watershed. 

“Background” loads include loads from all other land uses—hayland, pastureland, forestland, and urban land—as well as point sources. 

124 
 

 

 

 

 

 

Atrazine 

Although the full suite of pesticides were modeled for edgeof-field losses, atrazine was the only pesticide for which instream loads were assessed because it was the dominant 

contributor to mass loss of pesticide residues from farm fields 

and the primary contributor to environmental risk from 

pesticides in the region. First registered in the United States in 

1959, atrazine is used to control broadleaf and grassy weeds. 

Cultivated cropland (primarily corn acres) was the only source 

for atrazine in the model simulations. 

Model simulation results show that nearly 18,000 pounds of 

atrazine are lost from farm fields (edge-of-field) through 

pathways that result in delivery to streams and rivers within 

the Chesapeake Bay watershed (table 51). Of this, about 

16,000 pounds is delivered into rivers and streams each year, 

on average, under conditions represented by the baseline 

conservation condition (table 52). About half of the atrazine 

from cultivated cropland originates in the Susquehanna River 

subregion. Instream loads—the amount of atrazine delivered 

to the Bay after accounting for degradation and other instream 

transport processes—totals about 11,590 pounds (table 53, fig. 

90). 

Conservation practices—including Integrated Pest 

Management (IPM) techniques and practices—have reduced 

the delivery of atrazine from fields to rivers and streams by 

about 21 percent (table 52), on average, and reduced delivery 

of atrazine to the Bay by about 18 percent (table 53, figure 

90). Atrazine loads delivered to the Bay would have been 

larger by about 2,500 pounds per year if conservation 

practices were not in use (table 53). 

125 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 51. Average annual atrazine source loads delivered to edge of field (APEX model output) from cultivated cropland for the 4 

subregions in the Chesapeake Bay watershed 

Reductions in loads 

Baseline due to conservation 

conservation condition practices 

Pounds 

8-digit 

HUC 

group* 

Subregion 

code Subregion name 

Amount 

(1,000 pounds) 

Percent 

of basin 

total 

delivered 

per 

cropland 

acre 

No-practice

 Scenario 

(1,000 

pounds) 

Reduction 

(1,000 

pounds) Percent 

I 0205 Susquehanna River 8.55 47 0.0043 11.05 2.50 23 

II 0206 Upper Chesapeake** 4.68 26 0.0038 6.54 1.86 28 

III 0207 Potomac River 3.15 17 0.0052 3.56 0.41 11 

IV + V 0208 Lower Chesapeake** 1.98 11 0.0036 2.13 0.16 7 

 Total 18.35 100 0.0042 23.28 4.93 21 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Loads represent both cropped acres and land in 

long-term conserving cover. Some columns do not add to totals because of rounding. 

*See figure 81.
 

**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).
 

Table 52. Average annual atrazine source loads delivered to watershed outlets (8-digit HUCs) from cultivated cropland for the 4 

subregions in the Chesapeake Bay watershed 

Reductions in loads 

Baseline due to conservation 

conservation condition practices 

Pounds 

8-digit 

HUC 

group* 

Subregion 

code Subregion name 

Amount 

(1,000 pounds) 

Percent 

of basin 

total 

delivered 

per 

cropland 

acre 

No-practice

 Scenario 

(1,000 

pounds) 

Reduction 

(1,000 

pounds) Percent 

I 0205 Susquehanna River 7.40 46 0.0037 9.55 2.14 22 

II 0206 Upper Chesapeake** 4.26 26 0.0035 5.98 1.73 29 

III 0207 Potomac River 2.80 17 0.0046 3.12 0.32 10 

IV + V 0208 Lower Chesapeake** 1.72 11 0.0031 1.88 0.16 8 

 Total 16.18 100 0.0037 20.53 4.35 21 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. The differences between loadings in this table 

and table 51 are due to the application of delivery ratios, which were used to simulate delivery of atrazine from the edge of the field to the watershed outlet (8-digit 

HUC). Some columns do not add to totals because of rounding.
 

*See figure 81.
 

**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).
 

Table 53. Average annual instream atrazine loads delivered to the Chesapeake Bay 

Reductions in loads due to 

conservation practices 

Baseline 

Subregion name 

Subregion 

code 

8-digit HUC 

group* 

conservation 

condition 

(1,000 pounds) 

No-practice 

scenario 

(1,000 pounds) 

Reduction 

(1,000 pounds) Percent 

Upper Chesapeake Bay 

Susquehanna River 0205 I 4.63 5.96 1.33 22 

Upper Chesapeake 0206 II 3.05 3.78 0.74 19 

Potomac River 0207 III 2.43 2.68 0.26 10 

Sub-total 10.10 12.42 2.33 19 

Lower Chesapeake Bay 

Rappahannock, York, and James Rivers 0208 IV 1.44 1.59 0.14 9 

Eastern Shore 0208 V 0.05 0.08 0.03 33 

Sub-total 1.50 1.66 0.17 10 

Total 11.59 14.09 2.49 18 

*See figure 81.
 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Some columns do not add to totals because of
 

rounding.
 

126 
Figure 90. Estimates of average annual instream atrazine loads for the baseline conservation condition compared to the no-practice 

scenario for subregions in the Chesapeake Bay watershed* 

* Instream atrazine loads delivered to the Chesapeake Bay (all sources) are shown for each of the four subregions, corresponding to estimates presented in table 53. The 

total sediment load delivered to the Chesapeake Bay from all areas is shown in the bar chart in the lower right hand corner, labeled “Atrazine load to Bay.” 

127 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Assessment of Potential Water Quality 

Gains from Further Conservation Treatment 

The field-level model results for the scenarios with additional 

erosion control practices and nutrient management (chapter 6) 

were used with the HUMUS/SWAT model to determine the 

potential for further reductions in loads delivered from 

cultivated cropland to rivers and streams within the watershed 

and total loads delivered to the Bay (instream loads) with 

additional conservation treatment. 

Percent reductions relative to the current conservation 

condition were estimated for each of two scenarios (tables 54 

through 65)— 

1.	 treatment of the 0.8 million critical under-treated acres 

(acres with a high need for additional treatment), and 

2.	 treatment of all 3.4 million under-treated acres, including 

the 0.8 million critical under-treated acres). 

The distribution of under-treated acres within the Chesapeake 

Bay watershed is shown in chapter 5, table 30. 

The model simulations not only demonstrate the relative gains 

that can be expected from different levels of conservation 

effort but also provide insight into which subregions are the 

most important in terms of reducing overall loads exported to 

the Bay. Comparisons of the baseline condition with the 

estimates of reductions in total loads (all sources) achievable 

under the two scenarios are shown in figures 91 through 94. 

Model simulations showed that if the 0.8 million under-treated 

acres were fully treated with the appropriate soil erosion 

control and/or nutrient management practices, loads from 

cultivated cropland delivered to rivers and streams in the 

watershed would be reduced by, relative to the baseline 

conservation condition (tables 55, 58, 61, and 64)— 

	 39 percent for sediment, 

	 22 percent for nitrogen, 

	 24 percent for phosphorus, and 

	 7 percent for atrazine. 

The largest reductions for all four resource concerns would 

occur in the Susquehanna River subregion. 

Model simulations further showed that if all of the undertreated acres (an additional 2.6 million acres) were fully 

treated with the appropriate soil erosion control and/or nutrient 

management practices, loads from cultivated cropland 

delivered to rivers and streams in the watershed would be 

reduced, relative to the baseline conservation condition (tables 

(tables 55, 58, 61, and 64)— 

	 84 percent for sediment, 

	 52 percent for nitrogen, 

	 51 percent for phosphorus, and 

	 11 percent for atrazine. 

These reductions in loads delivered to rivers and streams 

would also have a significant impact on the total loads from all 

sources delivered to the Bay. If all the critical under-treated 

acres (0.8 million acres) were fully treated with the 

appropriate soil erosion control and/or nutrient management 

practices, total loads delivered to the Bay would be reduced, 

relative to the baseline conservation condition (tables 56, 59, 

62, and 65 and figs. 91 through 94)— 

	 2 percent for sediment, 

	 7 percent for nitrogen, 

	 5 percent for phosphorus, and 

	 6 percent for atrazine. 

If all the under-treated acres (2.6 million additional acres) 

were fully treated with the appropriate soil erosion control 

and/or nutrient management practices, total loads delivered to 

the Bay would be reduced, relative to the baseline 

conservation condition (tables 56, 59, 62, and 65 and 

through91-94)— 

	 6 percent for sediment, 

	 15 percent for nitrogen, 

	 12 percent for phosphorus, and 

	 9 percent for atrazine. 

At this level of conservation treatment, sediment loads 

delivered to the Bay would be very close to the background 

level, indicating that contributions from cultivated cropland 

would be negligible. The background scenario represents 

loads that would be expected if no acres in the watershed were 

cultivated. For sediment, background loads would total 6.339 

million tons (table 56). Total loads delivered from all sources 

after treating all under-treated acres with appropriate erosion 

control and nutrient management practices would total 6.378 

million tons (table 56), leaving only 39,000 tons originating 

from cultivated cropland. 

Similarly, if all under-treated acres were fully treated, loads 

delivered to the Bay originating from cultivated cropland 

would be only about 42 million pounds for nitrogen and 1.6 

million pounds for phosphorus (tables 59 and 62). 

128 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 54. Effects of additional conservation treatment with erosion control practices and nutrient management practices on average 

annual sediment source loads delivered to edge of field (APEX model output) from cultivated cropland for the 4 subregions in the 

Chesapeake Bay watershed 

Baseline Treatment of 0.8 million critical under- Treatment of all 3.4 million 

conservation condition treated acres under-treated acres 

8-digit 

HUC 

group* 

Subregion 

code Subregion name 

Average 

annual load 

(1,000 tons) 

Average annual 

load 

(1,000 tons) Percent reduction 

Average annual 

load 

(1,000 tons) 

Percent 

reduction 

I 0205 Susquehanna River 4,065 2,141 47 441 89 

II 0206 Upper Chesapeake** 561 488 13 175 69 

III 0207 Potomac River 535 424 21 66 88 

IV + V 0208 Lower Chesapeake** 318 228 28 59 82 

Total 5,479 3,281 40 742 86 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Loads represent both cropped acres and land in 

long-term conserving cover. Some columns do not add to totals because of rounding. 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

*See figure 81.
 

**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).
 

Table 55. Effects of additional conservation treatment with erosion control practices and nutrient management practices on average 

annual sediment source loads delivered to watershed outlets (8-digit HUCs) from cultivated cropland for the 4 subregions in the 

Chesapeake Bay watershed 

Baseline Treatment of 0.8 million critical under- Treatment of all 3.4 million 

conservation condition treated acres under-treated acres 

8-digit Sub- Average Average annual Average annual 

HUC region annual load load load Percent 

group* code Subregion name (1,000 tons) (1,000 tons) Percent reduction (1,000 tons) reduction 

I 0205 Susquehanna River 1,429 762 47 181 87 

II 0206 Upper Chesapeake** 218 192 12 75 66 

III 0207 Potomac River 196 157 20 28 86 

IV + V 0208 Lower Chesapeake** 127 93 27 26 79 

Total 1,970 1,204 39 310 84

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. The differences between loadings in this table 

and table 54 are due to the application of delivery ratios, which were used to simulate delivery of sediment from the edge of the field to the watershed outlet (8-digit 

HUC). Some columns do not add to totals because of rounding.
 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

*See figure 81.
 

**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).
 

Table 56. Effects of additional conservation treatment with erosion control practices and nutrient management practices on average 

annual instream sediment loads from all sources delivered to the Chesapeake Bay 

Treatment of 0.8 million critical Treatment of all 3.4 million 

Baseline conservation condition under-treated acres under-treated acres 

Average Average annual load 

Sub- 8-digit annual load from from background Average annual Average annual 

region HUC all sources sources** load Percent load Percent 

Subregion name code group* (1,000 tons) (1,000 tons) (1,000 tons) reduction (1,000 tons) reduction 

Upper Chesapeake Bay 

Susquehanna River 0205 I 1,427 1,295 1,369 4 1,294 9 

Upper Chesapeake 0206 II 934 795 914 2 836 11 

Potomac River 0207 III 2,364 2,256 2,337 1 2,249 5 

Sub-total 4,725 4,346 4,620 2 4,379 7 

Lower Chesapeake Bay 

Rappahannock, York, 

and James Rivers 0208 IV 2,023 1,962 2,004 1 1,966 3 

Eastern Shore 0208 V 35 31 35 <1 33 7 

Sub-total 2,058 1,993 2,039 1 1,999 3 

Total 6,783 6,339 6,659 2 6,378 6 

*See figure 81.
 

** “Background sources” represent loads that would be expected if no acres in the watershed were cultivated. These estimates were derived by running an additional 

scenario that simulated a grass and tree mix cover without any tillage or addition of nutrients or pesticides for all cultivated cropland acres in the watershed.
 

“Background” loads include loads from all other land uses—hayland, pastureland, forestland, and urban land—as well as point sources.
 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Some columns do not add to totals because of
 

rounding.
 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

129 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 57. Effects of additional conservation treatment with erosion control practices and nutrient management practices on average 

annual nitrogen source loads delivered to edge of field (APEX model output) from cultivated cropland for the 4 subregions in the 

Chesapeake Bay watershed 

Baseline Treatment of 0.8 million critical under- Treatment of all 3.4 million 

conservation condition treated acres under-treated acres 

8-digit 

HUC 

group* 

Subregion 

code Subregion name 

Average 

annual load 

(1,000 pounds) 

Average annual 

load 

(1,000 pounds) Percent reduction 

Average annual 

load 

(1,000 pounds) 

Percent 

reduction 

I 0205 Susquehanna River 87,691 58,756 33 36,811 58 

II 0206 Upper Chesapeake** 31,214 28,703 8 15,302 51 

III 0207 Potomac River 18,417 15,996 13 8,183 56 

IV + V 0208 Lower Chesapeake** 10,411 9,203 12 6,646 36 

Total 147,733 112,658 24 66,942 55 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Loads represent both cropped acres and land in 

long-term conserving cover. Some columns do not add to totals because of rounding. 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

*See figure 81.
 

**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).
 

Table 58. Effects of additional conservation treatment with erosion control practices and nutrient management practices on average 

annual nitrogen source loads delivered to watershed outlets (8-digit HUCs) from cultivated cropland for the 4 subregions in the 

Chesapeake Bay watershed 

Baseline Treatment of 0.8 million critical under- Treatment of all 3.4 million 

conservation condition treated acres under-treated acres 

8-digit 

HUC 

group* 

Subregion 

code Subregion name 

Average 

annual load 

(1,000 pounds) 

Average annual 

load 

(1,000 pounds) Percent reduction 

Average annual 

load 

(1,000 pounds) 

Percent 

reduction 

I 0205 Susquehanna River 58,939 40,430 31 26,842 54 

II 0206 Upper Chesapeake** 22,592 20,810 8 11,279 50 

III 0207 Potomac River 12,761 11,204 12 6,050 53 

IV + V 0208 Lower Chesapeake** 7,319 6,621 10 4,954 32 

Total 101,611 79,065 22 49,124 52 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. The differences between loadings in this table 

and table 57 are due to the application of delivery ratios, which were used to simulate delivery of nitrogen from the edge of the field to the watershed outlet (8-digit 

HUC). Some columns do not add to totals because of rounding.
 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

*See figure 81.
 

**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).
 

Table 59. Effects of additional conservation treatment with erosion control practices and nutrient management practices on average 

annual instream nitrogen loads from all sources delivered to the Chesapeake Bay 

Treatment of 0.8 million critical Treatment of all 3.4 million 

Baseline conservation condition under-treated acres under-treated acres 

Average Average annual load 

Sub- 8-digit annual load from from background Average annual Average annual 

region HUC all sources sources** load Percent load Percent 

Subregion name code group* (1,000 pounds) (1,000 pounds) (1,000 pounds) reduction (1,000 pounds) reduction 

Upper Chesapeake Bay 

Susquehanna River 0205 I 125,260 73,613 108,590 13 96,393 23 

Upper Chesapeake 0206 II 46,634 29,343 45,375 3 38,215 18 

Potomac River 0207 III 80,365 67,454 78,777 2 73,475 9 

Sub-total 252,259 170,410 232,742 8 208,083 18 

Lower Chesapeake Bay 

Rappahannock, York, 

and James Rivers 0208 IV 54,605 48,505 53,870 1 52,495 4 

Eastern Shore 0208 V 1,372 858 1,372 0 1,264 8 

Sub-total 55,977 49,363 55,242 1 53,759 4 

Total 308,236 219,773 287,984 7 261,842 15 

*See figure 81.
 

** “Background sources” represent loads that would be expected if no acres in the watershed were cultivated. These estimates were derived by running an additional 

scenario that simulated a grass and tree mix cover without any tillage or addition of nutrients or pesticides for all cultivated cropland acres in the watershed.
 

“Background” loads include loads from all other land uses—hayland, pastureland, forestland, and urban land—as well as point sources.
 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Some columns do not add to totals because of
 

rounding.
 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

130 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 60. Effects of additional conservation treatment with erosion control practices and nutrient management practices on average 

annual phosphorus source loads delivered to edge of field (APEX model output) from cultivated cropland for the 4 subregions in the 

Chesapeake Bay watershed 

Baseline Treatment of 0.8 million critical under- Treatment of all 3.4 million 

conservation condition treated acres under-treated acres 

8-digit 

HUC 

group* 

Subregion 

code Subregion name 

Average 

annual load 

(1,000 pounds) 

Average annual 

load 

(1,000 pounds) Percent reduction 

Average annual 

load 

(1,000 pounds) 

Percent 

reduction 

I 0205 Susquehanna River 9,798 6,162 37 3,630 63 

II 0206 Upper Chesapeake** 2,615 2,464 6 1,703 35 

III 0207 Potomac River 2,612 2,142 18 1,012 61 

IV + V 0208 Lower Chesapeake** 1,157 889 23 415 64 

Total 16,183 11,657 28 6,760 58 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Loads represent both cropped acres and land in 

long-term conserving cover. Some columns do not add to totals because of rounding. 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

*See figure 81.
 

**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).
 

Table 61. Effects of additional conservation treatment with erosion control practices and nutrient management practices on average 

annual phosphorus source loads delivered to watershed outlets (8-digit HUCs) from cultivated cropland for the 4 subregions in the 

Chesapeake Bay watershed 

Baseline Treatment of 0.8 million critical under- Treatment of all 3.4 million 

conservation condition treated acres under-treated acres 

8-digit 

HUC 

group* 

Subregion 

code Subregion name 

Average 

annual load 

(1,000 pounds) 

Average annual 

load 

(1,000 pounds) Percent reduction 

Average annual 

load 

(1,000 pounds) 

Percent 

reduction 

I 0205 Susquehanna River 3,702 2,490 33 1,660 55 

II 0206 Upper Chesapeake** 1,152 1,099 5 805 30 

III 0207 Potomac River 1,077 904 16 476 56 

IV + V 0208 Lower Chesapeake** 499 395 21 191 62 

Total 6,430 4,888 24 3,131 51 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. The differences between loadings in this table 

and table 60 are due to the application of delivery ratios, which were used to simulate delivery of phosphorus from the edge of the field to the watershed outlet (8-digit 

HUC). Some columns do not add to totals because of rounding.
 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

*See figure 81.
 

**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).
 

Table 62. Effects of additional conservation treatment with erosion control practices and nutrient management practices on average 

annual instream phosphorus loads from all sources delivered to the Chesapeake Bay 

Treatment of 0.8 million critical Treatment of all 3.4 million 

Baseline conservation condition under-treated acres under-treated acres 

Average Average annual load 

Sub- 8-digit annual load from from background Average annual Average annual 

region HUC all sources sources** load Percent load Percent 

Subregion name code group* (1,000 pounds) (1,000 pounds) (1,000 pounds) reduction (1,000 pounds) reduction 

Upper Chesapeake Bay 

Susquehanna River 0205 I 3,815 2,522 3,380 11 3,075 19 

Upper Chesapeake 0206 II 2,362 1,525 2,311 2 2,086 12 

Potomac River 0207 III 4,000 3,086 3,854 4 3,479 13 

Sub-total 10,177 7,133 9,544 6 8,639 15 

Lower Chesapeake Bay 

Rappahannock, York, 

and James Rivers 0208 IV 4,544 4,135 4,448 2 4,257 6 

Eastern Shore 0208 V 92 75 92 0 92 1 

Sub-total 4,636 4,210 4,540 2 4,349 6 

Total 14,813 11,342 14,084 5 12,988 12 

*See figure 81.
 

** “Background sources” represent loads that would be expected if no acres in the watershed were cultivated. These estimates were derived by running an additional 

scenario that simulated a grass and tree mix cover without any tillage or addition of nutrients or pesticides for all cultivated cropland acres in the watershed.
 

“Background” loads include loads from all other land uses—hayland, pastureland, forestland, and urban land—as well as point sources.
 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Some columns do not add to totals because of
 

rounding.
 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment.
 

131 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 63. Effects of additional conservation treatment with erosion control practices and nutrient management practices on average 

annual atrazine source loads delivered to edge of field (APEX model output) from cultivated cropland for the 4 subregions in the 

Chesapeake Bay watershed 

Baseline Treatment of 0.8 million critical under- Treatment of all 3.4 million 

conservation condition treated acres under-treated acres 

8-digit 

HUC 

group* 

Subregion 

code Subregion name 

Average 

annual load 

(1,000 pounds) 

Average annual 

load 

(1,000 pounds) Percent reduction 

Average annual 

load 

(1,000 pounds) 

Percent 

reduction 

I 0205 Susquehanna River 8.55 7.32 14 6.73 21 

II 0206 Upper Chesapeake** 4.68 4.64 1 4.46 5 

III 0207 Potomac River 3.15 3.04 3 2.86 9 

IV + V 0208 Lower Chesapeake** 1.98 1.87 5 1.77 10 

Total 18.35 16.87 8 15.81 14 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Loads represent both cropped acres and land in 

long-term conserving cover. Some columns do not add to totals because of rounding. 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

*See figure 81.
 

**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).
 

Table 64. Effects of additional conservation treatment with erosion control practices and nutrient management practices on average 

annual atrazine source loads delivered to watershed outlets (8-digit HUCs) from cultivated cropland for the 4 subregions in the 

Chesapeake Bay watershed 

Baseline Treatment of 0.8 million critical under- Treatment of all 3.4 million 

conservation condition treated acres under-treated acres 

8-digit 

HUC 

group* 

Subregion 

code Subregion name 

Average 

annual load 

(1,000 pounds) 

Average annual 

load 

(1,000 pounds) Percent reduction 

Average annual 

load 

(1,000 pounds) 

Percent 

reduction 

I 0205 Susquehanna River 7.40 6.49 12 6.13 17 

II 0206 Upper Chesapeake** 4.26 4.23 1 4.09 4 

III 0207 Potomac River 2.80 2.73 3 2.63 6 

IV + V 0208 Lower Chesapeake** 1.72 1.65 4 1.56 9 

Total 16.18 15.09 7 14.42 11 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. The differences between loadings in this table 

and table 63 are due to the application of delivery ratios, which were used to simulate delivery of atrazine from the edge of the field to the watershed outlet (8-digit 

HUC). Some columns do not add to totals because of rounding.
 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

*See figure 81.
 

**Excludes watersheds that drain into the Atlantic Ocean (8-digit HUCs 02060010 and 02080110).
 

Table 65. Effects of additional conservation treatment with erosion control practices and nutrient management practices on average 

annual instream atrazine loads delivered to the Chesapeake Bay 

Baseline conservation Treatment of 0.8 million critical Treatment of all 3.4 million 

condition under-treated acres under-treated acres 

Average 

Sub- 8-digit annual load from all Average annual Average annual 

region HUC sources load Percent load Percent 

Subregion name code group* (1,000 pounds) (1,000 pounds) reduction (1,000 pounds) reduction 

Upper Chesapeake Bay 

Susquehanna River 0205 I 4.6 4.1 11 3.9 15 

Upper Chesapeake 0206 II 3.0 3.0 1 2.9 4 

Potomac River 0207 III 2.4 2.4 2 2.4 3 

Sub-total 10.10 9.50 6 9.20 9 

Lower Chesapeake Bay 

Rappahannock, York, and 

James Rivers 0208 IV 1.4 1.4 4 1.3 8 

Eastern Shore 0208 V 0.1 0.1 0 0.0 12 

Sub-total 1.50 1.44 4 1.37 8 

Total 11.59 10.93 6 10.57 9 

*See figure 81.
 

Note: Percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. Some columns do not add to totals because of
 

rounding.
 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

132 
 

Figure 91. Estimates of average annual instream sediment loads* for the baseline conservation condition compared to two scenarios 

simulating additional water erosion control and nutrient management practices for subregions in the Chesapeake Bay watershed 

* Instream sediment loads delivered to the Chesapeake Bay (all sources) are shown for each of the four subregions, corresponding to estimates presented in table 56. 

The total sediment load delivered to the Chesapeake Bay from all areas is shown in the bar chart in the lower right hand corner, labeled “Sediment load to Bay.” 

Note: “Background sources” represent loads that would be expected if no acres in the watershed were cultivated. These estimates were derived by running an additional 

scenario that simulated a grass and tree mix cover without any tillage or addition of nutrients or pesticides for all cultivated cropland acres in the watershed. 

“Background” loads include loads from all other land uses—hayland, pastureland, forestland, and urban land—as well as point sources. 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

133 
 

 

Figure 92. Estimates of average annual instream nitrogen loads* for the baseline conservation condition compared to two scenarios 

simulating additional water erosion control and nutrient management practices for subregions in the Chesapeake Bay watershed 

* Instream nitrogen loads delivered to the Chesapeake Bay (all sources) are shown for each of the four subregions, corresponding to estimates presented in table 59. The 

total nitrogen load delivered to the Chesapeake Bay from all areas is shown in the bar chart in the lower right hand corner, labeled “Nitrogen load to Bay.” 

Note: “Background sources” represent loads that would be expected if no acres in the watershed were cultivated. These estimates were derived by running an additional 

scenario that simulated a grass and tree mix cover without any tillage or addition of nutrients or pesticides for all cultivated cropland acres in the watershed. 

“Background” loads include loads from all other land uses—hayland, pastureland, forestland, and urban land—as well as point sources. 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

134 
Figure 93. Estimates of average annual instream phosphorus loads* for the baseline conservation condition compared to two scenarios 

simulating additional water erosion control and nutrient management practices for subregions in the Chesapeake Bay watershed 

* Instream phosphorus loads delivered to the Chesapeake Bay (all sources) are shown for each of the four subregions, corresponding to estimates presented in table 62. 

The total phosphorus load delivered to the Chesapeake Bay from all areas is shown in the bar chart in the lower right hand corner, labeled “Phosphorus load to Bay.” 

Note: “Background sources” represent loads that would be expected if no acres in the watershed were cultivated. These estimates were derived by running an additional 

scenario that simulated a grass and tree mix cover without any tillage or addition of nutrients or pesticides for all cultivated cropland acres in the watershed. 

“Background” loads include loads from all other land uses—hayland, pastureland, forestland, and urban land—as well as point sources. 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

135 
Figure 94. Estimates of average annual instream atrazine loads* for the baseline conservation condition compared to two scenarios 

simulating additional water erosion control and nutrient management practices for subregions in the Chesapeake Bay watershed 

* Instream atrazine loads delivered to the Chesapeake Bay (all sources) are shown for each of the four subregions, corresponding to estimates presented in table 65. The 

total atrazine load delivered to the Chesapeake Bay from all areas is shown in the bar chart in the lower right hand corner, labeled “Atrazine load to Bay.” 

Note: Critical under-treated acres have a high need for additional treatment. Under-treated acres have either a high or moderate need for additional treatment. 

136 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Summary of Conservation Practice Effects 

on Water Quality in the Chesapeake Bay 

Watershed 

Reductions in field-level losses due to conservation practices, 

including land in long-term conserving cover, translate into 

improvements in water quality in streams and rivers in the 

region. Transport of sediment, nutrients, and pesticides from 

farm fields to streams and rivers involves a variety of 

processes and time-lags, and not all of the potential pollutants 

leaving fields contribute to instream loads. 

Cultivated cropland represents only about 10 percent of the 

land base in the Chesapeake Bay watershed. At the 2003–06 

level of conservation practice use, cultivated cropland 

delivered a disproportionate amount of sediment and nutrients 

to rivers and streams and ultimately to the Bay. Of the total 

loads delivered to rivers and streams from all sources, 

cultivated cropland is the source for 22 percent of the 

sediment, 28 percent of the phosphorus, and 31 percent of the 

nitrogen. 

Figures 95, 96, and 97 summarize the extent to which 

conservation practices on cultivated cropland acres have 

reduced, and can further reduce, sediment, nitrogen, and 

phosphorus loads in the Chesapeake Bay watershed, on the 

basis on model simulations. 

In each figure, the top map shows delivery from cultivated 

cropland to rivers and streams and the bottom map shows 

delivery from all sources to the Bay itself. The effects of 

practices in use during 2003–06 are seen by contrasting loads 

for the baseline conservation condition to loads for the nopractice scenario. The effects of additional conservation 

treatment on loads are seen by contrasting the loads for the 

baseline condition to either— 

1.	 loads for treatment of acres with a high level of 

treatment need (0.8 million critical under-treated 

acres), or 

2.	 loads for treatment of all under-treated acres (3.4 

million acres with either a high or moderate level of 

treatment need). 

Background levels, representing loads that would be expected 

if no acres in the watershed were cultivated, are also shown in 

the bar charts. These estimates simulate a grass and tree mix 

cover without any tillage or addition of nutrients or pesticides 

for all cultivated cropland acres in the watershed. Background 

loads also include loads from all other land uses—hayland, 

pastureland, forestland, and urban land—as well as point 

sources. 

Sediment loss 

In figure 95, the top map shows that the use of conservation 

practices has reduced sediment loads delivered from cropland 

to rivers and streams in the watershed by 57 percent from 

conditions that would be expected without conservation 

practices. It also shows that the application of additional 

conservation practices could reduce baseline sediment loads 

delivered to rivers and streams by 39 percent by treating acres 

with a high level of treatment need and by 84 percent by 

treating all under-treated acres. 

The bottom map shows that the use of conservation practices 

on cropland has reduced sediment loads delivered to 

Chesapeake Bay from all sources by 10 percent from 

conditions that would be expected without conservation 

practices. It also shows that the application of additional 

conservation practices could reduce baseline sediment loads 

delivered to the Bay by 2 percent by treating acres with a high 

level of treatment need and by 6 percent by treating all undertreated acres. 

Total nitrogen loss 

In figure 96, the top map shows that the use of conservation 

practices has reduced total nitrogent loads delivered from 

cropland to rivers and streams in the watershed by 36 percent 

from conditions that would be expected without conservation 

practices. It also shows that the application of additional 

conservation practices could reduce baseline total nitrogen 

loads delivered to rivers and streams by 22 percent by treating 

acres with a high level of treatment need and by 52 percent by 

treating all under-treated acres. 

The bottom map shows that the use of conservation practices 

on cropland has reduced total nitrogen loads delivered to 

Chesapeake Bay from all sources by 14 percent from 

conditions that would be expected without conservation 

practices. It also shows that the application of additional 

conservation practices could reduce baseline total nitrogen 

loads delivered to the Bay by 7 percent by treating acres with 

a high level of treatment need and by 15 percent by treating all 

under-treated acres. 

Total phosphorus loss 

In figure 97, the top map shows that the use of conservation 

practices has reduced total phosphorus loads delivered from 

cropland to rivers and streams in the watershed by 39 percent 

from conditions that would be expected without conservation 

practices. It also shows that the application of additional 

conservation practices could reduce baseline total phosphorus 

loads delivered to rivers and streams by 24 percent by treating 

acres with a high level of treatment need and by 51 percent by 

treating all under-treated acres. 

The bottom map shows that the use of conservation practices 

on cropland has reduced total phosphorus loads delivered to 

Chesapeake Bay from all sources by 14 percent from 

conditions that would be expected without conservation 

practices. It also shows that the application of additional 

conservation practices could reduce baseline total phosphorus 

loads delivered to the Bay by 5 percent by treating acres with 

a high level of treatment need and by 12 percent by treating all 

under-treated acres. 

137 
Figure 95. Summary of the effects of conservation practices on sediment loads in the Chesapeake Bay watershed 

Critical under-treated acres are acres with a high need for 

additional treatment. Under-treated acres have either a high or 

moderate need for additional treatment. 

Critical under-treated acres are acres with a high need for 

additional treatment. Under-treated acres have either a high or 

moderate need for additional treatment. 

138 
Figure 96. Summary of the effects of conservation practices on total nitrogen loads in the Chesapeake Bay watershed 

Critical under-treated acres are acres with a high need for 

additional treatment. Under-treated acres have either a high or 

moderate need for additional treatment. 

Critical under-treated acres are acres with a high need for 

additional treatment. Under-treated acres have either a high or 

moderate need for additional treatment. 

139 
 

 

Figure 97. Summary of the effects of conservation practices on total phosphorus loads in the Chesapeake Bay watershed 

Critical under-treated acres are acres with a high need for 

additional treatment. Under-treated acres have either a high or 

moderate need for additional treatment. 

Critical under-treated acres are acres with a high need for 

additional treatment. Under-treated acres have either a high or 

moderate need for additional treatment. 

140 
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Margins of error are provided in table A1 for selected acres Appendix A: Estimates of Margins of estimates found elsewhere in the report. The margin of error is 

Error for Selected Acres Estimates a commonly used measure of statistical uncertainty and can be 

used to construct a 95-percent confidence interval for an 

The CEAP cultivated cropland sample is a subset of NRI 

sample points from the 2003 NRI (USDA/NRCS 2007). The 

2001, 2002, and 2003 Annual NRI surveys were used to draw 

the sample. (Information about the CEAP sample design is in 

“NRI-CEAP Cropland Survey Design and Statistical 

Documentation,” available at 

http://www.nrcs.usda.gov/technical/nri/ceap.) 

The sample for cropped acres consists of 771 sample points in 

the Chesapeake Bay region. Acres reported using the CEAP 

sample are “estimated” acres because of the uncertainty 

associated with statistical sampling. 

Statistics derived from the CEAP database are based upon data 

collected at sample sites located across all parts of the region. 

This means that estimates of acreage are statistical estimates 

and contain some amount of statistical uncertainty. Since the 

NRI employs recognized statistical methodology, it is possible 

to quantify this statistical uncertainty. 

estimate. The lower bound of the confidence interval is 

obtained by subtracting the margin of error from the estimate; 

adding the margin of error to the estimate forms the upper 

bound. Measures of uncertainty (e.g., margins of error, 

standard errors, confidence intervals, coefficients of variation) 

should be taken into consideration when using CEAP acreage 

estimates. The margin of error is calculated by multiplying the 

standard error by the factor 1.96; a coefficient of variation is 

the relative standard for an estimate, usually in terms of 

percentages, and is calculated by taking 100 times the standard 

error and then dividing by the estimate. 

The precision of CEAP estimates depends upon the number of 

samples within the region of interest, the distribution of the 

resource characteristics across the region, the sampling 

procedure, and the estimation procedure. Characteristics that 

are common and spread fairly uniformly over an area can be 

estimated more precisely than characteristics that are rare or 

unevenly distributed. 

Table A1. Margins of error for acres estimates based on the CEAP sample 

Estimated acres Margin of error 

Cropped Acres 

Susquehanna River (subregion 0205) 1,734,800 186,358 

Upper Chesapeake Bay (subregion 0206) 1,187,900 100,029 

Potomac River (subregion 0207) 684,000 102,780 

Lower Chesapeake Bay (subregion 0208) 673,200 96,868 

Total for Chesapeake Bay region 4,279,900 285,254 

Highly erodible land (HEL) 

Susquehanna River (subregion 0205) 1,099,996 129,653 

Upper Chesapeake Bay (subregion 0206) 236,129 70,853 

Potomac River (subregion 0207) 388,458 86,377 

Lower Chesapeake Bay (subregion 0208) 153,532 65,151 

Chesapeake Bay region 1,878,115 197,003 

Irrigated acres 

Susquehanna River (subregion 0205) 19,734 29,332 

Upper Chesapeake Bay (subregion 0206) 144,246 52,336 

Potomac River (subregion 0207) 4,779 10,011 

Lower Chesapeake Bay (subregion 0208) 40,207 35,788 

Chesapeake Bay region 208,966 67,197 

Acres receiving manure 

Susquehanna River (subregion 0205) 913,625 247,935 

Upper Chesapeake Bay (subregion 0206) 401,765 85,069 

Potomac River (subregion 0207) 293,993 96,762 

Lower Chesapeake Bay (subregion 0208) 7,805 8,812 

Chesapeake Bay region 1,617,188 307,753 

143 
 

 

 

 

 

 

Table A1—continued. 

Estimated acres Margin of error 

Cropping Systems (table 5) 

Corn-soybean only 1,174,736 175,174 

Corn-soybean with close grown crops 830,308 134,871 

Corn only 690,403 140,309 

Soybean only 161,087 76,210 

Soybean-wheat only 124,649 73,605 

Corn and close grown crops 295,685 90,258 

Vegetable or tobacco with or without other crops 139,064 89,212 

Hay-crop mix 688,255 154,400 

Remaining mix of crops 175,713 75,284 

Use of structural practices (table 6) 

Overland flow control practices 1,439,296 251,808 

Concentrated flow control practices 736,132 175,173 

Edge-of-field buffering and filtering practices 446,780 112,200 

One or more water erosion control practices 1,984,945 284,974 

Wind erosion control practices 279,491 87,013 

Use of cover crops 177,833 89,132 

Use of residue and tillage management (table 7) 2,063,175 

Average annual tillage intensity for crop rotation meets criteria for no-till 1,704,087 232,726 

Average annual tillage intensity for crop rotation meets criteria for mulch till 217,776 81,729 

Reduced tillage on some crops in rotation but average annual tillage 

intensity greater than criteria for mulch till 294,862 115,854 

Continuous conventional tillage in every year of crop rotation 2,063,175 251,622 

Use of structural practices and/or residue and tillage management (table 8) 

No-till or mulch till with carbon gain, no structural practices 887,102 139,950 

No-till or mulch till with carbon loss, no structural practices 1,131,758 161,413 

Some crops with reduced tillage, no structural practices 102,999 56,947 

Structural practices and no-till or mulch till with carbon gain 833,385 195,619 

Structural practices and no-till or mulch till with carbon loss 915,018 220,180 

Structural practices and some crops with reduced tillage 114,777 68,957 

Structural practices only 121,766 70,460 

No water erosion control treatment 173,096 90,688 

Conservation treatment levels for structural practices (fig. 7) 

High level of treatment 220,173 80,248 

Moderately high level of treatment 580,243 124,035 

Moderate level of treatment 1,184,529 258,234 

Low level of treatment 2,294,955 231,431 

Conservation treatment levels for residue and tillage management (fig. 8) 

High level of treatment 1,496,831 207,528 

Moderately high level of treatment 237,248 87,882 

Moderate level of treatment 2,321,320 273,439 

Low level of treatment 224,501 96,499 

Conservation treatment levels for nitrogen management (fig. 9) 

High level of treatment 459,334 105,938 

Moderately high level of treatment 1,167,435 205,512 

Moderate level of treatment 1,698,549 254,086 

Low level of treatment 954,583 212,129 

144 
 

 

 

 

 

 

 

 

Table A1—continued. 

Estimated acres Margin of error 

Conservation treatment levels for phosphorus management (fig. 10) 

High level of treatment 765,841 154,429 

Moderately high level of treatment 854,467 175,852 

Moderate level of treatment 596,832 147,210 

Low level of treatment 2,062,760 274,489 

Conservation treatment levels for IPM (fig. 11) 

High level of treatment 402,008 147,771 

Moderate level of treatment 1,632,206 198,019 

Low level of treatment 2,245,686 249,316 

Conservation treatment levels for water erosion control practices (fig. 52) 

High level of treatment 672,167 94,822 

Moderately high level of treatment 680,848 192,428 

Moderate level of treatment 2,145,424 253,327 

Low level of treatment 781,462 126,956 

Conservation treatment levels for nitrogen runoff control (fig. 53) 

High level of treatment 30,182 32,450 

Moderately high level of treatment 985,946 137,331 

Moderate level of treatment 2,508,040 240,887 

Low level of treatment 755,732 163,926 

Conservation treatment levels for phosphorus runoff control (fig. 54) 

High level of treatment 89,838 35,753 

Moderately high level of treatment 942,710 151,782 

Moderate level of treatment 1,948,614 243,610 

Low level of treatment 1,298,738 207,490 

Soil runoff potential (fig. 55) 

High 979,325 170,751 

Moderately high 812,140 151,116 

Moderate 496,021 184,386 

Low 1,992,414 238,387 

Soil leaching potential (fig. 57) 

High 717,434 109,658 

Moderately high 1,249,166 206,858 

Moderate 2,038,260 207,679 

Low 275,040 85,229 

Level of conservation treatment need by resource concern 

Sediment loss (table 23) 

High (critical under-treated) 202,678 97,623 

Moderate (non-critical under-treated) 837,465 202,408 

Low (adequately treated) 3,239,757 282,145 

Nitrogen loss with surface runoff (sediment attached and soluble) (table 24) 

High (critical under-treated) 158,399 70,567 

Moderate (non-critical under-treated) 853,062 213,029 

Low (adequately treated) 3,268,439 260,176 

Nitrogen loss in subsurface flows (table 25) 

High (critical under-treated) 425,019 165,626 

Moderate (non-critical under-treated) 2,228,113 252,230 

Low (adequately treated) 1,626,768 236,866 

Phosphorus lost to surface water (table 26) 

High (critical under-treated) 386,555 110,788 

Moderate (non-critical under-treated) 1,789,656 216,815 

Low (adequately treated) 2,103,689 170,245 

145 
 

 

 

Table A1—continued. 

Margin of 

Estimated acres error 

Level of conservation treatment need for one or more resource concerns 

Chesapeake Bay region (table 27) 

High (critical under-treated) 809,997 173,113 

Moderate (non-critical under-treated) 2,597,927 176,929 

Low (adequately treated) 871,976 159,339 

Susquehanna River (subregion 0205) (table 29) 

High (critical under-treated) 558,625 137,577 

Moderate (non-critical under-treated) 905,625 145,439 

Low (adequately treated) 270,549 142,794 

Upper Chesapeake Bay (subregion 0206) (table 29) 

High (critical under-treated) 92,499 40,360 

Moderate (non-critical under-treated) 770,379 85,598 

Low (adequately treated) 325,022 78,352 

Potomac River (subregion 0207) (table 29) 

High (critical under-treated) 120,517 44,224 

Moderate (non-critical under-treated) 469,746 105,285 

Low (adequately treated) 93,737 40,513 

Lower Chesapeake Bay (subregion 0208) (table 29) 

High (critical under-treated) 38,356 32,205 

Moderate (non-critical under-treated) 452,177 99,913 

Low (adequately treated) 182,667 70,619 

146 
 

 

 

 

 

 

 

 

 

 

 

The column headings refer to the subregion code. The names Appendix B: Model Simulation of the subregions are shown below: 

Results for the Baseline 

Conservation Condition for the Four Subregion 

code Subregion name 

Subregions in the Chesapeake Bay 

Region 

0205 

0206 

0207 

Susquehanna River 

Upper Chesapeake 

Potomac River 

0208 Lower Chesapeake 

Model simulation results presented in Chapter 4 for the 

baseline conservation condition are presented in tables B-1 

and B-2 for the four subregions in the Chesapeake Bay region. 

Table B-1. Average annual estimates of water flow, erosion, and soil organic carbon for the baseline conservation condition for 

cropped acres, by subregion, in the Chesapeake Bay region 

Chesapeake 

Model simulated outcome Bay region 0205 0206 0207 0208 

Cropped acres (million acres) 4,279,900 1,734,800 1,187,900 684,000 673,200 

Percent of acres in region 100% 41% 28% 16% 16% 

Percent of acres highly erodible 44% 63% 20% 57% 23% 

Percent of acres irrigated 5% 1% 12% 1% 6% 

Percent of acres receiving manure 38% 53% 34% 43% 1% 

Water sources (average annual inches) 

Non-irrigated acres 

Precipitation 42 41 44 41 44 

Irrigated acres 

Precipitation 43 38 44 40 41 

Irrigation water applied 12 10 14 10 11 

Water loss pathways (average annual inches) 

Evapotranspiration 27.7 26.7 28.2 28.0 29.4 

Surface water runoff 5.1 5.2 5.9 3.9 4.5 

Subsurface water flow 10.7 9.6 12.3 8.9 12.6 

Erosion and sediment loss (average annual tons/acre) 

Wind erosion 0.03 0.02 0.04 0.01 0.05 

Sheet and rill erosion 0.99 1.54 0.50 0.99 0.44 

Sediment loss at edge of field due to water erosion 1.18 2.05 0.43 1.02 0.43 

Soil organic carbon (average annual pounds/acre) 

Loss of soil organic carbon with wind and water erosion 152 191 126 139 110 

Change in soil organic carbon, including loss of carbon with wind 

and water erosion -27 -68 -14 27 3 

147 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table B-2. Average annual estimates of nitrogen loss, phosphorus loss, and pesticide loss for the baseline conservation condition for 

cropped acres, by subregion, in the Chesapeake Bay region 

Chesapeake 

Model simulated outcome Bay region 0205 0206 0207 0208 

Nitrogen (average annual pounds/acre) 

Nitrogen sources 

Atmospheric deposition 8.7 9.7 7.5 8.5 8.6 

Bio-fixation by legumes 26.4 18.4 34.6 22.6 36.3 

Nitrogen applied as commercial fertilizer and manure 95.3 102.1 87.5 105.5 81.2 

All nitrogen sources 130.4 130.2 129.6 136.6 126.0 

Nitrogen in crop yield removed at harvest 84.1 73.7 92.2 85.0 95.6 

Nitrogen loss pathways 

Nitrogen loss by volatilization 6.9 5.8 7.4 8.2 7.6 

Nitrogen loss through denitrification processes 1.6 2.0 1.0 1.9 1.5 

Nitrogen lost with windborne sediment 0.2 0.1 0.3 0.1 0.2 

Nitrogen loss with surface runoff , including waterborne 

sediment 8.8 12.9 5.4 8.3 4.9 

Nitrogen loss in subsurface flow pathways 32.7 43.2 25.7 32.5 18.2 

Total nitrogen loss for all pathways 50.2 63.9 39.8 51.0 32.4 

Change in soil nitrogen -5.2 -9.3 -3.4 -0.6 -2.8 

Phosphorus (average annual pounds/acre) 

Phosphorus applied as commercial fertilizer and manure 25.2 29.5 19.1 31.4 18.8 

Phosphorus in crop yield removed at harvest 13.3 11.9 14.2 13.6 14.9 

Phosphorus loss pathways 

Phosphorus lost with windborne sediment 0.03 0.02 0.05 0.03 0.04 

Phosphorus lost to surface runoff, including waterborne 

sediment and soluble phosphorus in surface water runoff and 

lateral flow into drainage ditches 3.67 5.25 2.07 4.39 1.67 

Soluble phosphorus loss to groundwater 0.07 0.08 0.06 0.07 0.06 

Total phosphorus loss for all pathways 3.77 5.35 2.18 4.49 1.77 

Change in soil phosphorus 8.1 12.1 2.6 13.3 2.2 

Pesticides 

Average annual amount of pesticides applied (grams of active 

ingredient/hectare) 2,000 1,733 2,061 2,178 2,397 

Pesticide loss 

Average annual mass loss of pesticides for all pathways (grams 

of active ingredient/hectare) 15 15 14 17 12 

Edge-of-field pesticide risk indicator 

Average annual surface water pesticide risk indicator for 

aquatic ecosystem 1.8 1.9 1.5 2.1 1.6 

Average annual surface water pesticide risk indicator for 

humans 0.4 0.4 0.4 0.6 0.4 

Average annual groundwater pesticide risk indicator for 

humans 0.4 0.3 0.4 0.5 0.2 

148 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Appendix C: Simulation of Full 

Adoption of Cover Crops in the 

Chesapeake Bay Region 

Use of cover crops in the Chesapeake Bay region is 

increasing, particularly where state incentive programs have 

been implemented. Cover crops in the Bay Area are planted in 

the fall and then tilled or killed with herbicides (or by some 

other means) prior to the spring planting. When used properly, 

cover crops protect the soil from erosion during the winter 

months, take up nutrients remaining in the soil, and release 

plant available nutrients slowly over the subsequent cropping 

period, thereby reducing nutrient leaching and runoff during 

the non-growing season. 

The 2003–06 NRI-CEAP survey found that only about 4 

percent of the cropped acres in the Chesapeake Bay region had 

cover crops during the time period when the survey was 

conducted. The baseline conservation condition for 2003–06 

included only 31 sample points where a cover crop was used 

in the rotation, representing only about 200,000 acres, and 

these samples had on average a cover crop about every other 

year. Consequently, the benefits of cover crop adoption in the 

region at the current rate could not be estimated from this 

study. 

To demonstrate the potential for cover crops to reduce 

sediment and nutrient loss from fields in this region, a special 

“what if” scenario was conducted to simulate the use of cover 

crops on all cropped acres. Cover crops were inserted into the 

crop rotations of the baseline according to the following rules: 

1.	 For every sample, in every crop year, the crop rotation 

was examined and if no crop was growing during the 

traditional winter period, a cover crop was “planted” the 

day after harvest, or the day after the last major fall tillage 

operation. The crop was allowed to grow until the time of 

the first spring tillage operation, or 1 week before the 

planting operation, in the case of a no-till spring planting. 

2.	 Rye was used as the cover crop. 

3.	 There was no other change to the baseline other than the 

addition of a broadcast seeder to plant the cover crop in 

the fall. 

After the model and data setup were completed, it was found 

that 14 samples had neither an original nor a newly added 

cover crop, representing about 64,000 acres. These samples 

had, in each year of the rotation, either a winter small grain 

crop that was harvested in the spring or a hay crop present, so 

that there was no opportunity to add a cover crop. 

The field-level results of the cover crop simulation are 

contrasted to the original baseline scenario in table C1 for the 

Chesapeake Bay region as a whole and separately for each of 

the 4 subregions in tables C2-C5. 

Precipitation and nutrient inputs were the same for both 

scenarios. For irrigated acres, irrigation water use in the cover 

crop scenario increased slightly—about 6 percent on 

average—because of depetition of soil moisture by the cover 

crop and a somewhat higher demand for water by the crops 

that followed the cover crop.25 Biofixation of nitrogen by 

legumes decreased by an average of about 1 pound per acre 

due to the presence of cover crops (table C1). Crop yields 

were very slightly affected by the presence of the cover crop; 

the uptake and removal of nitrogen and phosphorus at 

harvest—indicators of crop yield—decreased by an average of 

only about 1 percent for nitrogen and 2 percent for phosphorus 

(table C1). 

Water loss pathways would only be slightly altered by the 

addition of cover crops (table C1). Evapotranspiration would 

increase only about 6 percent overall, and surface water runoff 

would decrease slightly (3 percent). However, subsurface 

flows would decrease about 1.4 inches per year (13 percent), 

on average, due primarily to water uptake by the cover crop. 

Erosion and Sediment Loss 

Erosion and sediment loss would be dramatically reduced for 

the region as a whole by the addition of cover crops on all 

acres (table C1). Wind erosion would be reduced 76 percent, 

on average, compared to the 2003–06 baseline condition. 

Sediment loss would be reduced by an average of 0.7 tons per 

acre per year, a 59 percent reduction, bringing the average 

annual sediment loss for the region to less than 0.5 tons per 

acre per year. The potential for reducing sediment loss using 

cover crops is greatest in the Potomac River and the Lower 

Chesapeake subregions, where reductions in sediment loss 

would be expected to exceed 70 percent (tables C4-C5) 

compared to the 2003–06 baseline condition. 

However, figures C1 and C2 show that the effects of cover 

crops on reducing sediment loss is modest for the majority of 

cropped acres—generally those with the lowest losses in the 

2003–06 baseline condition. The median sediment loss 

reduction would be only about 0.2 tons per acre per year. 

About 60 percent of the acres would have a reduction in 

sediment loss of 0.35 tons per acre or less (fig. C2). In 

contrast, 18 percent of the acres would have average annual 

reductions of 1 ton per acre or more. Nonetheless, figure 1 

shows that full adoption of cover crops would bring average 

annual sediment loss to below 2 tons per acre for 93 percent of 

the acres in the region, compared to 83 percent for the 2003– 

06 baseline condition. 

Soil Organic Carbon 

Soil organic carbon levels would increase on more cropped 

acres and decrease more slowly on other cropped acres if 

cover crops were used and the biomass retained on the field. 

The annual change in soil organic carbon would average about 

97 pounds per acre higher (table C1). A portion of this gain 

would be due to a reduction in the loss of soil organic carbon 

from the field due to wind and water erosion (26 percent 

reduction, on average). Figure C3 shows that if cover crops 

were added to all cropped acres that 27 percent of cropped 

acres would move from losing soil organic carbon to gaining 

25 In the model, irrigation water is applied automatically when the plant stress 

level exceeds a threshold; for some irrigated acres in some years, the presence 

of the cover crop reduced soil moisture enough to trigger additional 

application of irrigation water during the crop growing season. 

149 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

soil organic carbon, bringing acres that are gaining soil 

organic carbon to 70 percent of cropped acres in the region. 

The potential for gains in soil organic carbon due to cover 

crop use are greatest in the Potomac River and Susquehanna 

River subregions (tables C2 and C4). 

Nitrogen loss 

Model simulations show that use of cover crops has a 

significant impact on nitrogen loss from fields. All of the 

nitrogen loss pathways are affected by the addition of cover 

crops. For the region as a whole— 

 Nitrogen loss by volatilization would increase by an 

average of 3.8 pounds per acre per year (from 6.9 to 10.7 

pounds per acre), representing a 55-percent increase 

compared to the 2003–06 baseline condition; 

	 Nitrogen loss through denitrification would increase by an 

average of 0.7 pound per acre per year (from 1.6 to 2.3 

pounds per acre), representing a 44-percent increase 

compared to the 2003–06 baseline condition; 

	 Nitrogen lost with windborne sediment would decrease a 

very small amount; 

	 Nitrogen loss with surface runoff, including waterborne 

sediment, would decrease an average of 4.0 pounds per 

acre per year (from 8.8 to 4.8 pounds per acre), 

representing a 46-percent decrease; and 

	 Nitrogen loss in subsurface flow pathways would 

decrease an average of 10 pounds per acre per year (from 

33 to 23 pounds per acre), representing a 31-percent 

decrease. 

Total nitrogen loss through all loss pathways would be 

reduced from an average of 50.2 pounds per acre per year to 

40.5 pounds per acre per year, representing a 19-percent 

decrease overall. As shown in figures C4 and C5, the effects 

of cover crops on total nitrogen loss vary; one-third of the 

acres would be expected to have loss reductions less than 5 

pounds per acre, about 19 percent of the acres would be 

expected to have loss reductions greater than 15 pounds per 

acre (ranging to as high as 30 or more), and about half of the 

acres would be expected to have loss reductions between 5 

and 15 pounds per acre. 

For water quality concerns, the most important loss pathway 

affected by cover crops is nitrogen loss in subsurface flow 

pathways (figs. C6 and C7), followed by nitrogen lost with 

surface runoff (figs. C8 and C9). While these reductions in 

nitrogen loss due to cover crop use are significant, they are not 

uniform across all the acres. For nitrogen loss in subsurface 

flows, 30 percent of the acres would be expected to have loss 

reductions less than 5 pounds per acre, while about 10 percent 

of the acres would be expected to have loss reductions greater 

than 20 pounds per acre. For nitrogen lost with surface runoff, 

half of the acres would be expected to have loss reductions 

less than 2 pounds per acre, while about 10 percent of the 

acres would be expected to have loss reductions greater than 

10 pounds per acre. 

The other significant loss pathway affected by cover crops is 

nitrogen loss by volatilization (fig. C10). The increase in 

nitrogen volatilization is due to decaying vegetation and the 

re-routing of nitrogen among the loss pathways. For example, 

nitrogen that had been lost with runoff and leaching without 

cover crops present is now being physically held in place or 

chemically retained in plant biomass and thus more available 

to degradation processes, resulting in more nitrogen lost in 

gaseous forms. While losses of nitrogen through volatilization 

are no longer available to water loss pathways, a portion of 

these nitrogen compounds may return to waterways through 

atmospheric deposition, depending on the kind of nitrogen 

compounds volatilizing. The median volatilization loss for the 

region is about 8.6 pounds per acre with cover crops used on 

all acres, compared to 5.5 pounds per acre for the 2003–06 

baseline condition (fig. C10). 

Cover crops would be equally effective in reducing total 

nitrogen loss in all four subregions in the region (tables C2-

C5). For nitrogen loss in subsurface flows, the largest per-acre 

reduction—12.6 pounds per acre—would occur in the 

Potomac River subregion (table C4), while the greatest percent 

reduction—54 percent—would occur in the Lower 

Chesapeake (table C5). 

Phosphorus loss

Cover crops would also be effective in reducing total 

phosphorus loss, although the effects are more pronounced on 

some acres than others. Total phosphorus loss through all loss 

pathways would be reduced from an average of 3.8 pounds per 

acre per year to 2.6 pounds per acre per year, representing a 

32-percent decrease overall. Only a small amount of 

phosphorus is lost with windborne sediment or to 

groundwater. The bulk is lost to surface water, which includes 

waterborne sediment and soluble phosphorus in surface water 

runoff as well as lateral flow into drainage ditches, tile drains, 

and natural seeps. The reduction in total phosphorus loss due 

to cover crop use is shown in figures C11 and C12. About half 

of the acres would be expected to have loss reductions less 

than 0.5 pound per acre, while about 15 percent of the acres 

would be expected to have loss reductions greater than 2 

pounds per acre. While the potential for cover crops to reduce 

phosphorus loss is about the same in the four regions in terms 

of the percent reductions, per-acre reductions would be 

highest—averaging over 1.5 pounds per acre—in the Potomac 

River and Susquehanna River subregions (tables C2 and C4). 

150 
8 

10 

12 

14 

16 

18 

20 

Figure C1. Estimates of average annual sediment loss for Figure C3. Estimates of average annual change in soil organic 

cropped acres, comparing the baseline scenario to a “what if” carbon for cropped acres, comparing the baseline scenario to a 

scenario with cover crops added to all cropped acres, “what if” scenario with cover crops added to all cropped acres, 

Chesapeake Bay region Chesapeake Bay region 

600
 

500
 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual change in soil organic 

Average annual sediment loss (tons/acre)

6
 

4
 

2
 

0
 

400
 

-100 

-200 

-300 

-400 carbon (pounds/acre)300 

200 

100 

0 

-500
 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres Cumulative percent acres 

Baseline
 

Baseline
 Baseline with cover crops added Baseline with cover crops added 

Figure C2. Estimates of the average annual potential Figure C4. Estimates of average annual total nitrogen loss for 

reduction in sediment loss if cover crops were used on all cropped acres, comparing the baseline scenario to a “what if” 

cropped acres in the Chesapeake Bay region scenario with cover crops added to all cropped acres, 

Chesapeake Bay region 16 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual reduction in sediment 

loss (tons/acre) 

15 240 

14
 

13
 

12
 

11
 

10
 

9
 

8
 

7
 

6
 

5
 

4
 

3
 

2
 

1
 

0
 

Average annual total nitrogen loss 

(pounds/acre) 

220
 

200
 

180
 

160
 

140
 

120
 

100
 

80
 

60
 

40
 

20
 

0 Cumulative percent acres 0 10 20 30 40 50 60 70 80 90 100 

Cumulative percent acres 

Baseline Baseline with cover crops added 

151 
 

Figure C5. Estimates of the average annual potential Figure C7. Estimates of the average annual potential 

reduction in total nitrogen loss if cover crops were used on all reduction in nitrogen loss in subsurface flows if cover crops 

cropped acres in the Chesapeake Bay region were used on all cropped acres in the Chesapeake Bay region 

50
 50

45
 45

40
 40 nitrogen loss in subsurface flows 

Average annual reduction in 

total nitrogen loss (pounds/acre) 35
 

30
 

25
 

20
 

15
 

10
 

5
 

0
 

Average annual reduction in 

35
 

(pounds/acre) 30
 

25
 

20
 

15
 

10
 

5
 

0
 

0 10 20 30 40 50 60 70 80 90 100 
 0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres 

Figure C6. Estimates of average annual nitrogen loss in 

subsurface flows for cropped acres, comparing the baseline 

scenario to a “what if” scenario with cover crops added to all 

cropped acres, Chesapeake Bay region 

200
 

Cumulative percent acres 

Figure C8. Estimates of average annual nitrogen lost with 

surface runoff (including waterborne sediment) for cropped 

acres, comparing the baseline scenario to a “what if” scenario 

with cover crops added to all cropped acres, Chesapeake Bay 

region 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual nitrogen loss in subsurface 

75

Average annual nitrogen lost with 

surface runoff (pounds/acre) 

175
 

150
 

70
 65
 60
 55
 50
 45
 40
 35
 30
 25
 20
 15
 10
 

5
 

0
 

125
 

100
 

75
 

50
 

flows (pounds/acre) 

25

0
 

0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres Cumulative percent acres 

Baseline 

Baseline with cover crops added Baseline Baseline with cover crops added 

152 
Figure C9. Estimates of the average annual potential Figure C11. Estimates of average annual total phosphorus 

reduction in nitrogen lost with surface runoff (including loss for cropped acres, comparing the baseline scenario to a 

waterborne sediment) if cover crops were used on all cropped “what if” scenario with cover crops added to all cropped acres, 

acres in the Chesapeake Bay region Chesapeake Bay region 

26
 20
 

24
 

22
 18

Average annual reduction in 

nitrogen lost with surface runoff 

(pounds/acre) 

20
 

18
 

16
 

14
 

12
 

10
 

8
 

6
 

4
 

2
 

0
 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual total phosphorus loss 

(pounds/acre) 

16
 

14
 

12
 

10
 

8
 

6
 

4
 

2
 

0
 

Cumulative percent acres 0 10 20 30 40 50 60 70 80 90 100 
 

Cumulative percent acres 

Baseline Baseline with cover crops added 

Figure C10. Estimates of average annual nitrogen loss by Figure C12. Estimates of the average annual potential 

volatilization for cropped acres, comparing the baseline reduction in total phosphorus loss if cover crops were used on 

scenario to a “what if” scenario with cover crops added to all all cropped acres in the Chesapeake Bay region 

cropped acres, Chesapeake Bay region 15
 45
 

0 10 20 30 40 50 60 70 80 90 100 
 

Average annual reduction in total 

phosphorus loss (pounds/acre) 14 

13 

12 

11 

10 

9 

8 

7 

6 

5 

4 

3 

2
 

1
 

0

Average annual nitrogen loss by 

volatilization (pounds/acre) 

40
 

35
 

30
 

25
 

20
 

15
 

10
 

5 0 10 20 30 40 50 60 70 80 90 100 

Cumulative percent acres 0 

Cumulative percent acres 

Baseline Baseline with cover crops added 

153 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table C1. Average annual estimates of water flow, erosion, soil organic carbon, nitrogen loss, and phosphorus loss for the baseline 

conservation condition compared to a “what if” scenario with cover crops on all acres, Chesapeake Bay region (4.280 million acres) 

Baseline Percent 

condition Change due change due 

Baseline with cover to cover to cover 

Model simulated outcome condition crops added* crops** crops 

Water sources (average annual inches) 

Non-irrigated acres 

Precipitation 42.2 42.2 0.0 0% 

Irrigated acres 

Precipitation 42.8 42.8 0.0 0% 

Irrigation water applied 12.3 13.0 0.8 6% 

Water loss pathways (average annual inches) 

Evapotranspiration 27.7 29.4 1.7 6% 

Surface water runoff 5.1 4.9 -0.2 -3% 

Subsurface water flow 10.7 9.3 -1.4 -13% 

Erosion and sediment loss (average annual tons/acre) 

Wind erosion 0.027 0.006 -0.021 -76% 

Sheet and rill erosion 0.99 0.56 -0.43 -44% 

Sediment loss at edge of field due to water erosion 1.18 0.48 -0.70 -59% 

Soil organic carbon (average annual pounds/acre) 

Loss of soil organic carbon with wind and water erosion 152 113 -39 -26% 

Change in soil organic carbon, including loss of carbon with wind and water 

erosion -27 71 97 --

Nitrogen (average annual pounds/acre) 

Nitrogen sources 

Atmospheric deposition 8.7 8.7 0.0 0% 

Bio-fixation by legumes 26.4 25.3 -1.1 -4% 

Nitrogen applied as commercial fertilizer and manure 95.3 95.3 0.0 0% 

All nitrogen sources 130.4 129.3 -1.1 -1% 

Nitrogen in crop yield removed at harvest 84.1 83.5 -0.6 -1% 

Nitrogen loss pathways

 Nitrogen loss by volatilization 6.9 10.7 3.8 55%

 Nitrogen loss through denitrification processes 1.6 2.3 0.7 44%

 Nitrogen lost with windborne sediment 0.2 0.0 -0.1 -77% 

 Nitrogen loss with surface runoff , including waterborne sediment 8.8 4.8 -4.0 -46% 

 Nitrogen loss in subsurface flow pathways 32.7 22.7 -10.1 -31% 

 Total nitrogen loss for all pathways 50.2 40.5 -9.7 -19% 

Change in soil nitrogen -5.2 4.1 9.3 --

Phosphorus (average annual pounds/acre) 

Phosphorus applied as commercial fertilizer and manure 25.2 25.2 0.0 0% 

Phosphorus in crop yield removed at harvest 13.28 13.00 -0.28 -2% 

Phosphorus loss pathways 

Phosphorus lost with windborne sediment 0.03 0.01 -0.03 -79% 

Phosphorus lost to surface runoff, including waterborne sediment and soluble 

phosphorus in surface water runoff and lateral flow into drainage ditches 3.67 2.51 -1.16 -32% 

Soluble phosphorus loss to groundwater 0.07 0.05 -0.01 -20% 

Total phosphorus loss for all pathways 3.77 2.57 -1.20 -32% 

Change in soil phosphorus 8.11 9.63 1.52 --

* If no crop was already growing, or newly planted after the fall (or late summer) harvest, a cover crop was planted the day after harvest, or the day after the last major 

fall tillage operation if there were any. The crop was allowed to grow until one week before the first spring tillage operation, or 1 week before the planting operation if 

a no-till planting. The simulated cover crop was rye.
 

** Baseline with cover crops added minus original baseline.
 

Note: Reductions and percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. 

154 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table C2. Average annual estimates of water flow, erosion, soil organic carbon, nitrogen loss and phosphorus loss for the baseline 

conservation condition compared to a “what if” scenario with cover crops on all acres, Susquehanna River (Subregion code 0205, 

1.735 million acres) 

Baseline Percent 

condition Change due change due 

Baseline with cover to cover to cover 

Model simulated outcome condition crops added* crops** crops 

Water sources (average annual inches) 

Non-irrigated acres 

Precipitation 41.3 41.3 0.0 0% 

Irrigated acres 

Precipitation 38.2 38.2 0.0 0% 

Irrigation water applied 9.1 9.6 0.4 5% 

Water loss pathways (average annual inches) 

Evapotranspiration 26.7 27.6 0.9 3% 

Surface water runoff 5.2 5.1 -0.1 -2% 

Subsurface water flow 9.6 8.8 -0.7 -8% 

Erosion and sediment loss (average annual tons/acre) 

Wind erosion 0.017 0.005 -0.012 -72% 

Sheet and rill erosion 1.54 0.96 -0.58 -38% 

Sediment loss at edge of field due to water erosion 2.05 0.93 -1.12 -55% 

Soil organic carbon (average annual pounds/acre) 

Loss of soil organic carbon with wind and water erosion 191 129 -62 -33% 

Change in soil organic carbon, including loss of carbon with wind and water 

erosion -68 41 110 --

Nitrogen (average annual pounds/acre) 

Nitrogen sources 

Atmospheric deposition 9.7 9.7 0.0 0% 

Bio-fixation by legumes 18.4 17.8 -0.7 -4% 

Nitrogen applied as commercial fertilizer and manure 102.1 102.1 0.0 0% 

All nitrogen sources 130.2 129.6 -0.6 0% 

Nitrogen in crop yield removed at harvest 73.7 74.1 0.4 1% 

Nitrogen loss pathways

 Nitrogen loss by volatilization 5.8 8.0 2.3 39%

 Nitrogen loss through denitrification processes 2.0 2.7 0.7 37%

 Nitrogen lost with windborne sediment 0.1 0.0 -0.1 -73% 

 Nitrogen loss with surface runoff , including waterborne sediment 12.9 7.4 -5.5 -43% 

 Nitrogen loss in subsurface flow pathways 43.2 34.6 -8.6 -20% 

 Total nitrogen loss for all pathways 63.9 52.7 -11.2 -18% 

Change in soil nitrogen -9.3 1.1 10.4 --

Phosphorus (average annual pounds/acre) 

Phosphorus applied as commercial fertilizer and manure 29.5 29.5 0.0 0% 

Phosphorus in crop yield removed at harvest 11.88 11.71 -0.17 -1% 

Phosphorus loss pathways 

Phosphorus lost with windborne sediment 0.02 0.01 -0.02 -78% 

Phosphorus lost to surface runoff, including waterborne sediment and soluble 

phosphorus in surface water runoff and lateral flow into drainage ditches 5.25 3.60 -1.66 -32% 

Soluble phosphorus loss to groundwater 0.08 0.07 -0.01 -12% 

Total phosphorus loss for all pathways 5.35 3.67 -1.68 -31% 

Change in soil phosphorus 12.14 14.04 1.90 --

* If no crop was already growing, or newly planted after the fall (or late summer) harvest, a cover crop was planted the day after harvest, or the day after the last major 

fall tillage operation if there were any. The crop was allowed to grow until one week before the first spring tillage operation, or 1 week before the planting operation if 

a no-till planting. The simulated cover crop was rye.
 

** Baseline with cover crops added minus original baseline.
 

Note: Reductions and percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. 

155 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table C3. Average annual estimates of water flow, erosion, soil organic carbon, nitrogen loss and phosphorus loss for the baseline 

conservation condition compared to a “what if” scenario with cover crops on all acres, Upper Chesapeake (Subregion code 0206, 

1.188 million acres) 

Baseline Percent 

condition Change due change due 

Baseline with cover to cover to cover 

Model simulated outcome condition crops added* crops** crops 

Water sources (average annual inches) 

Non-irrigated acres 

Precipitation 43.8 43.8 0.0 0% 

Irrigated acres 

Precipitation 43.9 43.9 0.0 0% 

Irrigation water applied 13.5 14.3 0.8 6% 

Water loss pathways (average annual inches) 

Evapotranspiration 28.2 30.5 2.3 8% 

Surface water runoff 5.9 5.7 -0.2 -4% 

Subsurface water flow 12.3 10.3 -2.0 -16% 

Erosion and sediment loss (average annual tons/acre) 

Wind erosion 0.036 0.006 -0.029 -82% 

Sheet and rill erosion 0.50 0.24 -0.26 -51% 

Sediment loss at edge of field due to water erosion 0.43 0.15 -0.29 -66% 

Soil organic carbon (average annual pounds/acre) 

Loss of soil organic carbon with wind and water erosion 126 113 -13 -10% 

Change in soil organic carbon, including loss of carbon with wind and water 

erosion -14 69 83 --

Nitrogen (average annual pounds/acre) 

Nitrogen sources 

Atmospheric deposition 7.5 7.5 0.0 0% 

Bio-fixation by legumes 34.6 33.1 -1.5 -4% 

Nitrogen applied as commercial fertilizer and manure 87.5 87.4 0.0 0% 

All nitrogen sources 129.6 128.0 -1.5 -1% 

Nitrogen in crop yield removed at harvest 92.2 90.8 -1.4 -1% 

Nitrogen loss pathways

 Nitrogen loss by volatilization 7.4 12.3 4.9 66%

 Nitrogen loss through denitrification processes 1.0 1.5 0.6 57%

 Nitrogen lost with windborne sediment 0.3 0.0 -0.2 -83% 

 Nitrogen loss with surface runoff , including waterborne sediment 5.4 2.8 -2.6 -48% 

 Nitrogen loss in subsurface flow pathways 25.7 14.9 -10.8 -42% 

 Total nitrogen loss for all pathways 39.8 31.6 -8.2 -21% 

Change in soil nitrogen -3.4 4.8 8.1 --

Phosphorus (average annual pounds/acre) 

Phosphorus applied as commercial fertilizer and manure 19.1 19.1 0.0 0% 

Phosphorus in crop yield removed at harvest 14.24 13.88 -0.36 -3% 

Phosphorus loss pathways 

Phosphorus lost with windborne sediment 0.05 0.01 -0.04 -82% 

Phosphorus lost to surface runoff, including waterborne sediment and soluble 

phosphorus in surface water runoff and lateral flow into drainage ditches 2.07 1.47 -0.60 -29% 

Soluble phosphorus loss to groundwater 0.06 0.04 -0.02 -28% 

Total phosphorus loss for all pathways 2.18 1.53 -0.65 -30% 

Change in soil phosphorus 2.64 3.67 1.03 --

* If no crop was already growing, or newly planted after the fall (or late summer) harvest, a cover crop was planted the day after harvest, or the day after the last major 

fall tillage operation if there were any. The crop was allowed to grow until one week before the first spring tillage operation, or 1 week before the planting operation if 

a no-till planting. The simulated cover crop was rye.
 

** Baseline with cover crops added minus original baseline.
 

Note: Reductions and percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. 

156 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table C4. Average annual estimates of water flow, erosion, soil organic carbon, nitrogen loss and phosphorus loss for the baseline 

conservation condition compared to a “what if” scenario with cover crops on all acres, Potomac River (Subregion code 0207, 0.684 

million acres) 

Baseline Percent 

condition Change due change due 

Baseline with cover to cover to cover 

Model simulated outcome condition crops added* crops** crops 

Water sources (average annual inches) 

Non-irrigated acres 

Precipitation 40.6 40.6 0.0 0% 

Irrigated acres 

Precipitation 39.7 39.7 0.0 0% 

Irrigation water applied 9.3 11.2 1.9 20% 

Water loss pathways (average annual inches) 

Evapotranspiration 28.0 29.6 1.6 6% 

Surface water runoff 3.9 3.7 -0.2 -5% 

Subsurface water flow 8.9 7.5 -1.4 -16% 

Erosion and sediment loss (average annual tons/acre) 

Wind erosion 0.013 0.002 -0.011 -86% 

Sheet and rill erosion 0.99 0.44 -0.55 -56% 

Sediment loss at edge of field due to water erosion 1.02 0.27 -0.75 -74% 

Soil organic carbon (average annual pounds/acre) 

Loss of soil organic carbon with wind and water erosion 139 94 -46 -33% 

Change in soil organic carbon, including loss of carbon with wind and water 

erosion 27 150 123 --

Nitrogen (average annual pounds/acre) 

Nitrogen sources 

Atmospheric deposition 8.5 8.5 0.0 0% 

Bio-fixation by legumes 22.6 21.6 -1.0 -4% 

Nitrogen applied as commercial fertilizer and manure 105.5 105.5 0.0 0% 

All nitrogen sources 136.6 135.6 -1.0 -1% 

Nitrogen in crop yield removed at harvest 85.0 83.5 -1.5 -2% 

Nitrogen loss pathways

 Nitrogen loss by volatilization 8.2 13.0 4.8 59%

 Nitrogen loss through denitrification processes 1.9 3.1 1.2 60%

 Nitrogen lost with windborne sediment 0.1 0.0 -0.1 -79% 

 Nitrogen loss with surface runoff , including waterborne sediment 8.3 3.7 -4.6 -55% 

 Nitrogen loss in subsurface flow pathways 32.5 19.9 -12.6 -39% 

 Total nitrogen loss for all pathways 50.9 39.7 -11.2 -22% 

Change in soil nitrogen -0.6 11.3 11.9 --

Phosphorus (average annual pounds/acre) 

Phosphorus applied as commercial fertilizer and manure 31.4 31.4 0.0 0% 

Phosphorus in crop yield removed at harvest 13.61 13.16 -0.45 -3% 

Phosphorus loss pathways 

Phosphorus lost with windborne sediment 0.03 0.00 -0.02 -83% 

Phosphorus lost to surface runoff, including waterborne sediment and soluble 

phosphorus in surface water runoff and lateral flow into drainage ditches 4.39 2.88 -1.51 -34% 

Soluble phosphorus loss to groundwater 0.07 0.05 -0.01 -23% 

Total phosphorus loss for all pathways 4.49 2.93 -1.55 -35% 

Change in soil phosphorus 13.25 15.34 2.09 --

* If no crop was already growing, or newly planted after the fall (or late summer) harvest, a cover crop was planted the day after harvest, or the day after the last major 

fall tillage operation if there were any. The crop was allowed to grow until one week before the first spring tillage operation, or 1 week before the planting operation if 

a no-till planting. The simulated cover crop was rye.
 

** Baseline with cover crops added minus original baseline.
 

Note: Reductions and percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. 

157 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table C5. Average annual estimates of water flow, erosion, soil organic carbon, nitrogen loss and phosphorus loss for the baseline 

conservation condition compared to a “what if” scenario with cover crops on all acres, Lower Chesapeake (Subregion code 0208, 

0.673 million acres) 

Baseline Percent 

condition Change due change due 

Baseline with cover to cover to cover 

Model simulated outcome condition crops added* crops** crops 

Water sources (average annual inches) 

Non-irrigated acres 

Precipitation 43.7 43.7 0.0 0% 

Irrigated acres 

Precipitation 41.3 41.3 0.0 0% 

Irrigation water applied 10.0 10.5 0.5 5% 

Water loss pathways (average annual inches) 

Evapotranspiration 29.3 31.9 2.6 9% 

Surface water runoff 4.5 4.3 -0.3 -6% 

Subsurface water flow 12.6 10.5 -2.2 -17% 

Erosion and sediment loss (average annual tons/acre) 

Wind erosion 0.051 0.015 -0.036 -71% 

Sheet and rill erosion 0.44 0.20 -0.25 -55% 

Sediment loss at edge of field due to water erosion 0.43 0.12 -0.31 -71% 

Soil organic carbon (average annual pounds/acre) 

Loss of soil organic carbon with wind and water erosion 110 93 -17 -15% 

Change in soil organic carbon, including loss of carbon with wind and water 

erosion 3 67 64 --

Nitrogen (average annual pounds/acre) 

Nitrogen sources 

Atmospheric deposition 8.6 8.6 0.0 0% 

Bio-fixation by legumes 36.3 34.8 -1.5 -4% 

Nitrogen applied as commercial fertilizer and manure 81.2 81.2 0.0 0% 

All nitrogen sources 126.0 124.5 -1.5 -1% 

Nitrogen in crop yield removed at harvest 95.6 95.0 -0.6 -1% 

Nitrogen loss pathways

 Nitrogen loss by volatilization 7.6 12.5 4.9 64%

 Nitrogen loss through denitrification processes 1.4 1.9 0.4 29%

 Nitrogen lost with windborne sediment 0.2 0.1 -0.2 -68% 

 Nitrogen loss with surface runoff , including waterborne sediment 4.9 2.6 -2.3 -48% 

 Nitrogen loss in subsurface flow pathways 18.2 8.5 -9.8 -54% 

 Total nitrogen loss for all pathways 32.4 25.5 -7.0 -21% 

Change in soil nitrogen -2.8 3.4 6.2 --

Phosphorus (average annual pounds/acre) 

Phosphorus applied as commercial fertilizer and manure 18.8 18.8 0.0 0% 

Phosphorus in crop yield removed at harvest 14.86 14.61 -0.25 -2% 

Phosphorus loss pathways 

Phosphorus lost with windborne sediment 0.04 0.01 -0.03 -72% 

Phosphorus lost to surface runoff, including waterborne sediment and soluble 

phosphorus in surface water runoff and lateral flow into drainage ditches 1.67 1.16 -0.51 -31% 

Soluble phosphorus loss to groundwater 0.06 0.04 -0.02 -33% 

Total phosphorus loss for all pathways 1.77 1.21 -0.56 -32% 

Change in soil phosphorus 2.15 2.97 0.81 --

* If no crop was already growing, or newly planted after the fall (or late summer) harvest, a cover crop was planted the day after harvest, or the day after the last major 

fall tillage operation if there were any. The crop was allowed to grow until one week before the first spring tillage operation, or 1 week before the planting operation if 

a no-till planting. The simulated cover crop was rye.
 

** Baseline with cover crops added minus original baseline.
 

Note: Reductions and percent reductions were calculated prior to rounding the values for reporting in the table and the associated text. 

158