Cover.  Digital Elevation Model (DEM) of the Delaware Inland Bays watershed developed at the University of Delaware Spatial Analysis

Lab, 2000.
Simulation of Nutrient and Sediment 

Concentrations and Loads in the Delaware 

Inland Bays Watershed: Extension of the 

Hydrologic and Water-Quality Model to 

Ungaged Segments

By Angélica L. Gutiérrez-Magness

In cooperation with the

Delaware Department of Natural Resources and Environmental Control 

and the Delaware Geological Survey

Scientific Investigations Report 2006–5038

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

P. Lynn Scarlett, Acting Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

For more information on the USGS--the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Gutiérrez-Magness, A.L., 2006, Simulation of nutrient and sediment concentrations and loads in the 

Delaware Inland Bays Watershed: Extension of the hydrologic and water-quality model to ungaged segments: 

U.S. Geological Survey Scientific Investigations Report 2006–5038, 26 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................2

Previous Investigations........................................................................................................................2

Description of Study Area ............................................................................................................................3

Watershed Segmentation....................................................................................................................5

Land Use.................................................................................................................................................5

Development of Water-Quality Database for Model Calibration..................................................5

Parameter Transference Approach............................................................................................................5

Simulation of Nutrient and Sediment Concentrations and Loads .........................................................8

Annual and Monthly Water Balance .................................................................................................9

Predicted Sediment Concentrations ...............................................................................................10

Predicted Total Nitrogen Concentrations .......................................................................................10

Predicted Total Phosphorus Concentrations .................................................................................16

Predicted Sediment and Nutrient Loads.........................................................................................16

Summary and Conclusions.........................................................................................................................20

Acknowledgments.......................................................................................................................................21

Selected References...................................................................................................................................21

Appendix A. Recalculated and Updated Yields by Model Segment

 A-1. Simulated Total Suspended Solids Yield During Water Year 1999 by Model Segment for 

the Delaware Inland Bays Watershed Model .......................................................................24

 A-2. Simulated Total Nitrogen Yield .......................................................................................................25

 A-3. Simulated Total Phosphorus Yield .................................................................................................26

Figures

1–4. Maps showing—

1. Hydrogeomorphic regions of the Delmarva Peninsula and location of the 

Delaware Inland Bays watershed, Delaware, Maryland, and Virginia ......................3

2. Detailed view of the Delaware Inland Bays watershed, Sussex County, 

Delaware...............................................................................................................................4

3. Hydrological Simulation Program - FORTRAN (HSPF) Inland Bays model 

segmentation and location of U.S. Geological Survey gaging stations in the 

Inland Bays watershed, hourly and daily precipitation stations in the study 

area, and major nutrient point source from the municipal facility located in 

Georgetown, Delaware.......................................................................................................6

4. Aggregated land-use categories in the Delaware Inland Bays watershed..............7

5–9. Graphs showing—

5. Annual measured and predicted runoff for the source and recipient segments 

for the Delaware Inland Bays watershed during 1999 ................................................10

6. Monthly measured runoff at calibrated stream segments (010, 030, 130, 310, 

 and 450) and monthly predicted runoff averaged over the recipient segments 

for the Delaware Inland Bays watershed during 1999 ................................................11
iv

7. Simulated hourly total-suspended-solids concentrations and measured 

instantaneous values for the nearest calibrated basin for the Delaware 

Inland Bays watershed (segment source of parameter values), and 

U.S. Environmental Protection Agency’s (USEPA) STORET database 

during 1999..........................................................................................................................12

8. Simulated hourly total nitrogen concentrations...........................................................14

9. Simulated hourly total phosphorus concentrations ....................................................17

Tables

1. Ungaged model segment and Total Maximum Daily Load stations for the Delaware 

Inland Bays watershed from which measured data were used for the validation of 

model predictions .........................................................................................................................8

2. Groups of segments defined for the transference of parameter values for source 

and recipient model segments for the Delaware Inland Bays watershed .........................9

3. Predicted edge-of-stream loads and yields of total nitrogen (TN), total phosphorus 

(TP), and total suspended sediment (TSS) by model segment during water year 

1999 ...............................................................................................................................................19
Conversion Factors and Vertical Datum

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2) 0.0002471 acre 

hectare (ha) 2.471 acre

square kilometer (km2) 247.1 acre

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 0.2642 gallon (gal)

Flow rate

liter per day (L/d) 0.2642 gallon per day (gal/d) 

Mass

kilogram (kg) 2.205 pound, avoirdupois (lb)

kilogram (kg) 0.001102 ton (T)

Application rate

kilograms per hectare per year 

[(kg/ha)/yr]

0.8921 pounds, per acre per year 

[(lb/acre)/yr]

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Concentrations of chemical constituents in water are given in milligrams per liter (mg/L).

Elevations are referenced to the North American Vertical Datum of 1983 (NAVD 83).

Water year is the 12-month period October 1 through September 30. The water year is 

designated by the calendar year in which it ends, and includes 9 of the 12 months. Thus, the 

year ending September 30, 1999 is called the “1999 water year.”

Simulation of Nutrient and Sediment Concentrations 

and Loads in the Delaware Inland Bays Watershed: 

Extension of the Hydrologic and Water-Quality Model to 

Ungaged Segments

By Angélica L. Gutiérrez-Magness

Abstract

Rapid population increases, agriculture, and industrial 

practices have been identified as important sources of excessive nutrients and sediments in the Delaware Inland Bays 

watershed. The amount and effect of excessive nutrients 

and sediments in the Inland Bays watershed have been well 

documented by the Delaware Geological Survey, the Delaware 

Department of Natural Resources and Environmental Control, 

the U.S. Environmental Protection Agency’s National Estuary 

Program, the Delaware Center for Inland Bays, the University 

of Delaware, and other agencies. This documentation and data 

previously were used to develop a hydrologic and waterquality model of the Delaware Inland Bays watershed to 

simulate nutrients and sediment concentrations and loads, 

and to calibrate the model by comparing concentrations and 

streamflow data at six stations in the watershed over a limited 

period of time (October 1998 through April 2000). Although 

the model predictions of nutrient and sediment concentrations 

for the calibrated segments were fairly accurate, the predictions for the 28 ungaged segments located near tidal areas, 

where stream data were not available, were above the range of 

values measured in the area.

The cooperative study established in 2000 by the 

Delaware Department of Natural Resources and Environmental Control, the Delaware Geological Survey, and the 

U.S. Geological Survey was extended to evaluate the model 

predictions in ungaged segments and to ensure that the model, 

developed as a planning and management tool, could accurately predict nutrient and sediment concentrations within the 

measured range of values in the area. The evaluation of the 

predictions was limited to the period of calibration (1999) of 

the 2003 model.

To develop estimates on ungaged watersheds, parameter 

values from calibrated segments are transferred to the ungaged 

segments; however, accurate predictions are unlikely where 

parameter transference is subject to error. The unexpected 

nutrient and sediment concentrations simulated with the 2003 

model were likely the result of inappropriate criteria for the 

transference of parameter values. From a model-simulation 

perspective, it is a common practice to transfer parameter 

values based on the similarity of soils or the similarity of landuse proportions between segments. For the Inland Bays model, 

the similarity of soils between segments was used as the basis 

to transfer parameter values. An alternative approach, which 

is documented in this report, is based on the similarity of the 

spatial distribution of the land use between segments and the 

similarity of land-use proportions, as these can be important 

factors for the transference of parameter values in lumped 

models. Previous work determined that the difference in the 

variation of runoff due to various spatial distributions of land 

use within a watershed can cause substantial loss of accuracy 

in the model predictions.

The incorporation of the spatial distribution of land use 

to transfer parameter values from calibrated to uncalibrated 

segments provided more consistent and rational predictions 

of flow, especially during the summer, and consequently, 

predictions of lower nutrient concentrations during the same 

period. For the segments where the similarity of spatial 

distribution of land use was not clearly established with 

a calibrated segment, the similarity of the location of the 

most impervious areas was also used as a criterion for the 

transference of parameter values.

The model predictions from the 28 ungaged segments 

were verified through comparison with measured in-stream 

concentrations from local and nearby streams provided by 

the Delaware Department of Natural Resources and Environmental Control. Model results indicated that the predicted 

edge-of-stream total suspended solids loads in the Inland Bays 

watershed were low in comparison to loads reported for the 

Eastern Shore of Maryland from the Chesapeake Bay watershed model. The flatness of the terrain and the low annual 

surface runoff are important factors in determining the 

amount of detached sediment from the land that is delivered 

to streams. The highest predicted total suspended solids loads 

were found in the southern part of the watershed, where the 

values are associated with high total streamflow and a high 

surface-runoff component, and related to soil and aquifer 
permeability and land use. Nutrient loads from model segments in the southern part of the Inland Bays watershed were 

also higher than those measured in the northern part of the 

basin, due to relatively high runoff and the substantial amount 

of available organic fertilizer (animal waste) that results in 

over-application of organic fertilizer to crops.

Time series of simulated hourly concentrations indicated 

a seasonal pattern in the simulated base flow for total nitrogen, 

with the lowest values occurring during the summer and the 

highest values during the winter months. Total phosphorus and 

total-suspended-solids concentrations were less seasonal and 

were more storm-dependent; in general, base-flow concentrations of total phosphorus and total suspended solids were low. 

During storm events, the total nitrogen concentrations tended 

to be diluted and total phosphorus concentrations tended to 

rise sharply. Nitrogen was transported mainly in the aqueous 

phase and largely through ground water, whereas phosphorus 

was strongly associated with sediment, which washes off during rainfall events.

Introduction

Rapid population increases, agriculture, and industrial 

practices have been identified as important sources of excessive nutrients and sediments in the Delaware Inland Bays 

watershed (fig. 1). In order to better understand nutrient and 

sediment concentration loads in the watershed, a published 

hydrologic and water-quality model of the Delaware Inland 

Bays watershed (Gutiérrez-Magness and Raffensperger, 2003) 

was calibrated through comparison of concentration and 

streamflow data at six stations in the watershed over a limited 

time period (October 1998 through April 2000). Model-simulation results indicated that soil and aquifer permeability, 

ditching, dominant land-use class, and land-use practices 

affected the amount of runoff, the mechanism or flow path 

(surface flow, interflow, or base flow), and the concentrations 

of sediment and nutrients. Although the spatial distribution of 

nutrient and sediment concentrations averaged over watershed 

land uses were represented reasonably in the model simulation, unexpected concentrations were sometimes predicted 

for certain land uses of model segments located near the tidal 

areas where stream data were not available to calibrate model 

parameters. Because the Hydrological Simulation ProgramFORTRAN (HSPF) model predictions were used not only to 

address contamination problems in local streams, but also in 

connection with a hydrodynamic model to develop the Total 

Maximum Daily Loads (TMDLs) for the Bays, it became 

necessary to address the unexpected model predictions.

The cooperative study established in 2000 by 

the Delaware Department of Natural Resources and 

Environmental Control (DNREC), the Delaware Geological 

Survey (DGS), and the U.S. Geological Survey (USGS) 

was extended to evaluate the model predictions in ungaged 

watersheds and to ensure that the model, developed as a 

planning and management tool, could accurately predict 

nutrient and sediment concentrations within the measured 

range of values in the area. The simulation period included 

the calendar years 1998 and 1999; however, the simulation 

for 1998 was used only to account for the unknown initial 

conditions of the watershed system.

Purpose and Scope

The purpose of this report is to (1) document the model 

predictions for the 28 ungaged segments where streamflow 

and water-quality data were not available for calibration, (2) 

document the approach used to transfer parameter values from 

calibrated to uncalibrated model segments, and (3) provide 

findings, analysis, and implications of the predictions from 

model simulation for the understanding of hydrologic and 

nutrient processing functions in the uncalibrated segments of 

the Inland Bays watershed. Water-quality data from the U.S. 

Environmental Protection Agency’s (USEPA) STORET database, the DNREC’s General Assessment Monitoring Network 

(GAMN), and the DNREC’s Total Maximum Daily Load 

(TMDL) monitoring network, were used to compare in-stream 

measured and edge-of-stream predicted model concentrations. 

Although the 719-square kilometer (km2) watershed was 

divided into 45 model segments, only 28 of these segments 

are located near tidal areas. The predictions of concentrations 

and loads for these 28 ungaged segments are documented in 

this report. Model parameters were adjusted systematically so 

that the discrepancies between the simulated values and the 

observations from nearby streams were minimized.

Previous Investigations

Nutrient contamination of ground water and surface 

water in the Inland Bays watershed has been studied for 

decades. Nitrate contamination in the surficial sediments has 

been documented in various previous studies (Miller, 1972; 

Robertson, 1977; Denver, 1989; Andres, 1991; Hamilton 

and others, 1993). Elevated concentrations of nitrate (above 

background levels) are found in surface water, especially 

in well-drained watersheds associated with agricultural 

land use (Phillips and Bachman, 1996). Base-flow nitrate 

concentrations in surface water have been attributed to the 

discharge of shallow ground water (Shedlock and others, 

1999). Concentrations of nitrate in ground water were highest 

in sandy soils underlying well-drained agricultural fields 

where poultry manure and inorganic fertilizer were applied, 

and lowest where soils were poorly drained and dissolved 

oxygen was absent. Concentrations in surface water follow 

similar patterns, although other processes (such as biological 

uptake and denitrification) in stream channels can also affect 

nutrient concentrations in surface water. Nitrate concentrations 

in ground water and surface water have been attributed in part 

to on-site wastewater-disposal systems and confined animal 

feeding operations.

 Extension of Hydrologic and Water-Quality Model of the Delaware Inland Bays Watershed
Phosphorus is not commonly present in ground 

water at elevated concentrations, but when it is attached 

to soil particles, it is present in high concentrations (above 

background levels), especially in areas where poultry manure 

has been applied to agricultural fields (Sims and Wolf, 1993). 

Phosphorus transport is primarily attributed to overland flow 

that transports sediment with attached phosphorus ions.

A regional assessment of ground-water quality in the 

Delmarva Peninsula (that includes the Delaware Inland Bays) 

was conducted under the USGS National Water-Quality 

Assessment (NAWQA) Program (Hamilton and others, 1993; 

Shedlock and others, 1999), using data collected through 

1991. Various wells sampled during the study in 1999 were 

in the Inland Bays watershed. The results indicated that the 

chemical character of water in the surficial sediments is 

affected by agricultural activities over most of the Delmarva 

Peninsula. Elevated concentrations of nitrate were found in 

the surficial sediments in nearly all areas of the peninsula. 

Hydrogeomorphic Regions (HGMRs) that describe different 

physical settings with characteristic patterns of ground-water 

flow and water quality were delineated for the Delmarva 

Peninsula on the basis of geologic and geomorphic features, 

drainage patterns, soil types, and land-use patterns (Hamilton 

and others, 1993). The Inland Bays watershed includes four of 

these regions: (1) a poorly drained upland unit at the western 

edge of the watershed; (2) a well-drained upland unit covering 

most of the northern and central parts of the watershed; (3) a 

surficial confined region in the southern part; and (4) a unit 

described as “others” (beaches, tidal marshes, lagoons, and 

barrier islands) that covers the part of the watershed closest 

to the Atlantic Coast and immediately surrounding the Inland 

Bays themselves. The potential effect on nutrient transport and 

transformation in each HGMR is related to differences in soil 

characteristics, slope, drainage properties, and aquifer configurations that determine the movement of water and the potential 

for oxidation and reduction in the aquifer and streams.

Description of Study Area

The Inland Bays watershed encompasses approximately 

803 km2 in southeastern Sussex County, Delaware (figs. 1 

and 2). The Bays themselves cover 84 km2, and the remaining 

719 km2 are land. Various major and many smaller streams 

drain to the three bays (Rehoboth, Indian River, and Little 

Assawoman; (fig. 2). Land use/cover in the watershed, based 

on 1997 estimates, is 32 percent agriculture (including crops, 

orchard, and pasture), 21.5 percent forest (including brush), 

30 percent water (including the Bays, wetlands, and barren 

areas), 14.3 percent residential (including low-, medium-, and 

high-density), and 2.2 percent urban (institutional/government, 

industrial, and commercial land uses) (Delaware Office of 

State Planning Coordination, 2002).

Topography in the Inland Bays watershed is flat, typical 

of the Atlantic Coastal Plain Physiographic Province. The 

Figure 1. Hydrogeomorphic regions of the Delmarva Peninsula 

and location of the Delaware Inland Bays watershed, Delaware, 

Maryland, and Virginia (modified from Shedlock and others, 1999).

Description of Study Area 
Figure 2. Detailed view of the Delaware Inland Bays watershed, Sussex County, Delaware.

 Extension of Hydrologic and Water-Quality Model of the Delaware Inland Bays Watershed
Development of Water-Quality Database for 

Model Calibration

All water-quality data used for the validation of the 

model predictions in the segments located near tidal areas 

were provided by DNREC. Delaware maintains two networks, 

the GAMN, and the TMDL monitoring network. Whereas 

GAMN stations are considered long-term stations, TMDL 

monitoring stations are generally in place for 1 to 2 years 

to support data needs for TMDL model development and 

calibration. The data from these two networks are stored in the 

USEPA’s STORET database. DNREC’s ambient water-qualitymonitoring-program data from 1998 through 2000 were 

retrieved from USEPA’s STORET database to supplement the 

Inland Bays TMDL database, version 6.62 (Andres and others, 

2002). All methods used for sample collection, processing, 

preservation, and analysis are described in Ullman and others 

(2002). For the validation of the model predictions, 14 of the 

ungaged segments were paired to a TMDL or GAMN station 

with water-quality data during 1999 (table 1). Because of the 

brevity of the observation record, however, the predictions 

were also compared to the water-quality data from the nearest 

calibrated model segment. The constituents analyzed and 

specified concentration units are described in GutiérrezMagness and Raffensperger (2003).

Parameter Transference Approach

The transfer of parameter values is an important 

consideration in making accurate predictions for ungaged 

watersheds. The lack of measured streamflow and waterquality data makes it difficult to validate the model predictions 

in ungaged watersheds. When using lumped models, such 

as HSPF, criteria based on the similarity of soils or landuse proportions between gaged and ungaged segments are 

commonly used to transfer parameter values. Whereas the 

similarity of soils assumes that the accuracy of all parameters 

is the same after calibration, the similarity of land-use 

proportions recognizes that the accuracy of the parameters 

varies according to the importance of the land-use class on the 

simulated processes; parameters for larger land-use classes 

may be the most accurately calibrated as they may drive the 

accuracy of the predictions. Although the criteria take into 

account the similarities of the physical system, the criteria 

ignore fundamental simplifications of lumped models specific 

to the spatial variation of the data.

Lumped models do not explicitly take into account the 

spatial variability of inputs, outputs, or parameters. Lumped 

models are usually structured to utilize average values of the 

watershed characteristics that affect runoff volumes. Thus, 

averaging a certain parameter also implicitly averages the 

elevations in the watershed range from 0 to 22.9 (m) meters, 

based on 30-m Digital Elevation Model (DEM) data, North 

American Datum 1983, (University of Delaware Spatial 

Analysis Lab, 2003), with a mean of 6.9 m. Slopes in the 

watershed are generally very small (less than 1 percent). 

Streams in the uplands may be incised, especially in the 

well-drained upland region. The land surface in the surficial 

confined region is flat.

Watershed Segmentation

The watershed-model segmentation was delineated on 

the basis of various factors including the location of impaired 

streams in the Inland Bays watershed listed in the 303(d) list 

for 1998 from the State of Delaware (Delaware Department 

of Natural Resources and Environmental Control, 1998), the 

location of flow gages and water-quality monitoring stations, 

and the location of point sources. On the basis of these factors, 

the watershed was divided into 45 segments, with segment 

areas ranging from 1.3 to 53 km2 (fig. 3). Twenty-eight of the 

45 segments are located in tidal areas and were modeled to 

drain directly to the Bay. Since flow through a reach in the 

HSPF model is assumed to be unidirectional, streams were 

not simulated in these 28 ungaged segments. To validate the 

accuracy of the model-predicted concentrations in the ungaged 

segments, the simulated concentrations from the land, referred 

to as the “edge-of-stream” concentrations, were compared to 

the measured in-stream concentrations from streams located 

near the individual segments.

Land Use

Land-use information was derived from 1997 landuse/land-cover data (Delaware Office of State Planning 

Coordination, 2002). The hectares of each land use within 

each model segment are described in Gutiérrez-Magness 

and Raffensperger (2003). The categories were aggregated 

into a broader classification as follows: (1) forest, wetland, 

barren, and brush categories were aggregated into a “forest” 

classification; (2) crop and orchard categories were aggregated 

into “crops;” (3) the pasture category was termed “pasture;” 

and (4) low-, medium-, and high-density residential as well 

as institutional, industrial, and commercial categories were 

aggregated into an “urban” classification. The wetlands 

category was simulated as forest because of HSPF limitations 

in simulating chemical processes in wetlands. These 

aggregated categories were used to determine the source of 

parameter values for the 28 ungaged segments located in tidal 

areas. A generalized land-use map with the forest and pasture 

categories aggregated as vegetation is shown in figure 4.

Parameter Transference Approach 
Figure 3. Hydrological Simulation Program - FORTRAN (HSPF) Inland Bays model segmentation and location of U.S. Geological 

Survey gaging stations in the Inland Bays watershed, hourly and daily precipitation stations in the study area, and major nutrient 

point source from the municipal facility located in Georgetown, Delaware. [Red shaded areas represent the six calibrated 

watersheds that were used as the source of parameter values for the uncalibrated watersheds.]

 Extension of Hydrologic and Water-Quality Model of the Delaware Inland Bays Watershed
Figure 4. Aggregated land-use categories in the Delaware Inland Bays watershed.

Parameter Transference Approach 
The similarity of the spatial distribution of land use 

between the uncalibrated and the calibrated segments was 

determined through a visual examination of the generalized 

land-use map shown in fig. 4. The similarity of the land-use 

proportions between the segments was determined from the 

land-use classes (Gutiérrez-Magness and Raffensperger, 

2003). The similarity of the spatial distribution of land use, 

followed by the similarity of land-use proportions between 

uncalibrated and calibrated segments, were the criteria for the 

transference of model parameters. For the ungaged segments, 

where the similarity of spatial distribution of land use was 

not clearly established with a calibrated segment, the similarity 

of the location of the most impervious areas was used as 

an alternative.

Five groups were defined for the transference of the 

parameter values, with each group consisting of a calibrated segment and a set of ungaged segments as shown in 

table 2. The calibrated segments in table 2 are referred to 

as the “source” segments, whereas the ungaged segments 

are referred to as the “recipient” segments. The calibrated 

parameters of segments 010, 030, 130, 310, and 450, discussed 

in the report by (Gutiérrez-Magness and Raffensperger, 

(2003), were the values transferred to the recipient segments. 

Parameters from the calibrated segment at Beaverdam Ditch 

(segment 330) were not transferred to ungaged segments, 

as the land-use proportions and the spatial distribution of 

land-use criteria were difficult to meet. The transferred 

values corresponded to the inland parameters that control 

the hydrologic processes and to the parameters that control 

the edge-of-stream concentrations of nitrogen, phosphorus, 

and sediment. In the case of ungaged segments, where a 

stream was simulated (segments 260, 270, 290, and 360), 

the parameters that control the in-stream processes were also 

transferred. The names and descriptions of the parameters 

controlling these processes are documented in the user’s 

manual of the HSPF model (Bicknell and others, 1997).

Simulation of Nutrient and Sediment 

Concentrations and Loads

The accuracy of the simulation for the prediction of 

nutrient and sediment concentrations and loads was evaluated 

using comparisons of water balances and comparisons of 

predicted and measured nutrient and sediment concentrations. 

Although the similarity of water balances between the 

measured and predicted flows was important, priority was 

given to the accuracy of the predicted concentrations in the 

individual segments. The water mass balance within land areas 

was simulated as a series of storages with flows between the 

storages determined by empirical (constitutive) relations.

The annual and monthly water balances were calculated 

using the measured streamflow at the source segments, and the 

edge-of-stream flows at the corresponding recipient segments. 

For segments 260, 270, 290, and 360, where a stream was 

process being represented—because of nonlinearity and 

threshold values, this condition can lead to significant errors. 

In the case of the HSPF model, all processes are computed for 

a spatial unit of 1 acre 1 for each of the simulated land uses; to 

obtain the total edge-of-stream load for the land part of a given 

model segment, the fluxes and loads computed for the spatial 

unit are multiplied by the number of acres of each land use.

When the land use in a watershed is spatially nonhomogeneous, the calibration of a spatially lumped model may 

distort the calibrated parameters. Ultimately, this would 

introduce error variation, both systematic and nonsystematic, 

into the predictions. Calibrated parameter values for a watershed with flashy runoff will likely indicate low infiltration 

and low values of retention storage. Conversely, runoff from 

a watershed that is forested at the outlet would be considerably different than runoff from a watershed where impervious 

land use is near the outlet. If the parameters from a watershed 

with an impervious area near the outlet were to be transferred 

for predictions in a watershed with similar soils, but with a 

forested area located at the outlet of the watershed, predicted 

runoff rates would likely be erroneous. Thus, when transferring parameters between watersheds, it is important to apply 

criteria that account for both the physical characteristics of the 

watershed system and the model simplifications.

1

 Although the simulations were set to operate on a per-acre basis, hectares 

(ha) will be the unit for area used in this report. One hectare is equal to 104m2

(square meters) or 2.471 acres.

Table 1. Ungaged model segment and Total Maximum Daily 

Load stations for the Delaware Inland Bays watershed from 

which measured data were used for the validation of model 

predictions.

[TMDL, total maximum daily load; GAMN, General Assessment Monitoring 

Network; locations of segments shown in figure 3; –––––, data not available]

Model segment TMDL GAMN

020 LR305011 –––––

040 IR308291 –––––

050 ––––– IR308301

100 RB308051 –––––-

110 ––––– IR306111

120 ––––– IR306321

250 ––––– IR308301

260 ––––– IR309021

270 ––––– IR309041

290 ––––– IR308091

350 LA312041 –––––

370 ––––– LA310031

410 LA310101 –––––

420 LR305011 –––––

 Extension of Hydrologic and Water-Quality Model of the Delaware Inland Bays Watershed
simulated, the water balances were calculated using the predicted streamflow. The predicted annual and monthly water 

balances were calculated as the average of the predicted values 

for the individual recipient segments; these averages were 

compared to the water balances of the measured streamflow. 

Differences between the measured and the predicted water 

balances were expected, because in some cases the precipitation data used in the source and recipient segments were 

different, or because during the evaluation of the predicted 

nutrient or sediment edge-of-stream concentrations, modifications were applied to the hydrologic parameters that control 

the amount of water lost to deep percolation and the amount 

of evaporation from base flow and ground water.

The validation of the water-quality model predictions 

for the ungaged segments was performed using DNREC’s 

ambient water-quality data collected in the area during 1999. 

The predicted edge-of-stream concentrations were compared 

to the monitored in-stream concentrations from streams within 

or near the assessed model segment. The primary objective 

when comparing the predicted edge-of-stream concentrations 

in ungaged segments (020, 040, 050, 110, 120, 290, and 420) 

to the in-stream concentrations from the calibrated segments 

was to guarantee that the model predictions were within the 

range of the water-quality measurements, rather than a perfect 

match between the individual predicted and measured values. 

A similar rationale was applied when the predicted concentrations of all the remaining ungaged segments were compared to 

measured water-quality data from streams within or near the 

assessed model segment.

Although it is a common practice to validate model 

predictions from ungaged watersheds using the edge-of-stream 

average annual loads by land-use class, the predictions in the 

ungaged segments were validated using the predicted concentrations as the climatic conditions prior to and during the calibration period were not the long-term average. During 1998, 

the recorded precipitation for the northern and southern parts 

of the watershed was below average by -53.3 mm (millimeters) and -198.1 mm, respectively; during 1999, the recorded 

precipitation was the long-term average for the northern area, 

whereas the precipitation was still below average (-55.88 mm) 

for the southern area (Lewes, Delaware) (National Oceanic 

and Atmospheric Administration, 2001).

The predicted edge-of-stream concentrations for total 

nitrogen (TN) were expected to be lower than or similar 

to the in-stream concentrations measured in the monitored 

ambient data. The rationale for the expectation was a dilution 

effect recognized during the calibration of the 2003 model 

with reduced in-stream concentrations during storm events. 

In contrast, the predicted edge-of-stream concentrations of 

total phosphorus (TP) were expected to be a function of the 

delivered sediments and within the range of values measured 

in the monitored in-stream concentrations. The hourly 

predictions of nutrients and sediment concentrations were 

plotted for visual examination and the total annual loads were 

computed for each of the ungaged segments.

Annual and Monthly Water Balance

The amount of runoff from a watershed is a function of 

climate, topographic relief, water retention, soil properties, 

and the geologic structure of the basin. Approximate values 

representing these factors are included in the model simulation 

of the watershed and the values are calibrated against monitored discharge data. In the case of ungaged basins, however, 

the amount of runoff will also be a function of the criteria 

applied for the transference of parameter values and the 

accuracy of the calibrated parameter values.

The predicted annual water balances (fig. 5) of the 

recipient segments using parameters from the calibrated 

segments 010, 030, 310, and 450 were within 25 percent of 

the measured values. The large difference when using parameters from segment 130 between the measured and predicted 

average annual streamflow for the recipient segments is in 

part explained by the reduction of the parameters DEEPFR, 

BASETP, and AGWETP (the fraction of ground-water inflow 

lost from the watershed system as deep percolation, the fraction of potential evapotranspiration from base flow, and the 

Table 2. Groups of segments defined for the transference of parameter values for source and recipient 

model segments for the Delaware Inland Bays watershed.

[Locations of segments shown in figure 3]

Source segments

Station number Recipient segments

(segment number) Station name

01484668 (010) Munchy Branch near Rehoboth Beach, DE 020, 100, 380, 390, 400, 420, 440

01484654 (030) Bundicks Branch at Robinsonville, DE 040, 050, 070, 080, 250, 260

01484534 (130) Swan Creek near Millsboro, DE 060, 090, 110, 270, 350, 410

01484600 (310) Blackwater Creek near Clarkesville, DE 290, 360

01484525 (450) Millsboro Pond Outlet at Millsboro, DE 120, 280, 300, 320, 340, 370, 470

Simulation of Nutrient and Sediment Concentrations and Loads 
some overprediction resulted during the summer season. 

The overprediction was also observed for the streamflow of 

the calibrated segments (Gutiérrez-Magness and Raffensperger, 2003).

Predicted Sediment Concentrations

The validation of model predictions through the analyses 

of predicted concentrations rather than through the evaluation 

of monthly or annual predicted loads is important to ensure 

that the model properly represents the dynamics of the 

simulated processes. The significance of this analysis increases 

when the climatological conditions under which the model 

has been calibrated are different from the long-term average. 

The accuracy of the model was evaluated through visual 

examination of time series of the hourly total suspendedsediment concentrations at the edge-of-stream. Time series of 

predicted hourly edge-of-stream and measured in-stream totalsuspended-solids (TSS) concentrations (fig. 7) were used to 

assess the capacity of the model to represent dynamic features 

of the sediment runoff response in each of the ungaged 

segments. The primary objective of these comparisons was to 

guarantee that the model predictions were within the range of 

the water-quality measurements rather than to have a perfect 

match between the individual predicted and measured values.

Stormflow concentrations were appreciably higher than 

base-flow concentrations for TSS in all segments. Differences 

in concentrations among ungaged segments were most likely 

related to the differences in land use and the storage capacity 

of the aquifer material, with relatively lower values in the 

southern part of the watershed. Higher concentrations were 

predicted in segments located in the southern part of the 

watershed, with concentrations up to 500 mg/L (milligrams 

per liter) (segment 290). The predictions closely resemble 

the dynamics of the surface runoff, with higher values 

occurring during storm events. The predictions of sediment 

concentrations for all segments in the northern area were 

below 100 mg/L.

Predicted Total Nitrogen Concentrations

Concentrations in mg/L are the mass of a constituent 

present in a given volume of water sample. Seasonality was 

reflected in the measured and simulated TN concentrations, 

with the lowest values during the summer and the highest 

values during the winter months. Nitrogen was transported 

mainly in the aqueous phase and largely through ground water 

in the form of nitrate/nitrite. Stormflow concentrations were 

appreciably higher than base-flow concentrations for organic 

nitrogen at all tributaries and for ammonia in segments with 

a high application of organic fertilizer, and in segments with 

large urban areas. During storm events, TN concentrations 

(fig. 8) tended to be diluted as the concentrations of nitrate and 

nitrite in the surface runoff were appreciably lower than the 

concentrations observed during base flow. The dilution effect 

fraction of potential evapotranspiration from ground water, 

respectively) from 0.055, 0.001, and 0.01, respectively, to a 

zero value for all the recipient segments. High concentrations 

of ammonia, nitrate, and phosphate were caused by the almost 

zero flows during the summer season. Once these parameter 

values were reduced, the amount of water lost to evaporation or deep percolation was added to the edge-of-stream 

flow throughout the year, and the predicted concentrations of 

TN and TP were within the range of the measured values. In 

addition, the precipitation used for the predictions in segments 

270, 350, and 410 was different than the precipitation measured near the segment source.

Streamflow was lower throughout the watershed in 1999, 

with Swan Creek (segment 130) only discharging 46 mm of 

streamflow, while receiving more than 1,000 mm of precipitation. Higher streamflow was measured in the southern part of 

the watershed (segments 310 and 450) than in the northern 

part (segments 010, 030, and 130). This result was anticipated 

because the sub-watersheds in the southern area are located 

in the surficial confined aquifer region, where the storage 

capacity of the aquifer material is expected to be relatively low 

compared to the storage capacity of watersheds located in the 

northern area.

The monthly water balances for the groups using 

parameters from segments 010, 030, 310, and 450 show good 

agreement, with most predicted values within 35 percent of 

the measured values (fig. 6). In the group using parameters 

from segment 130, the amount of water is significantly larger 

because of the reduction of evapotranspiration from base flow 

and ground water (BASETP and AGWETP, respectively) 

and the elimination of deep percolation (DEEPR) from the 

watershed system. Seasonality in the edge-of-stream flow and 

streamflow was properly simulated by the model, although 

Figure 5. Annual measured and predicted runoff for the 

source and recipient segments for the Delaware Inland Bays 

watershed during 1999.

10 Extension of Hydrologic and Water-Quality Model of the Delaware Inland Bays Watershed
Figure 6. Monthly measured runoff at calibrated stream segments (010, 030, 130, 310, and 450) and monthly predicted runoff 

averaged over the recipient segments for the Delaware Inland Bays watershed during 1999. (See figure 3 for location of stream 

segments.)

Simulation of Nutrient and Sediment Concentrations and Loads 11
Figure 7. Simulated hourly total-suspended-solids concentrations and measured instantaneous values for the 

nearest calibrated basin for the Delaware Inland Bays watershed (segment source of parameter values), and 

U.S. Environmental Protection Agency’s (USEPA) STORET database during 1999. [Delaware Department of Natural 

Resources and Environmental Control (DNREC)]. (See figure 3 for location of stream segments.)

12  Extension of Hydrologic and Water-Quality Model of the Delaware Inland Bays Watershed
Figure 7. Simulated hourly total-suspended-solids concentrations and measured instantaneous values for the 

nearest calibrated basin for the Delaware Inland Bays watershed (segment source of parameter values), and 

U.S. Environmental Protection Agency’s (USEPA) STORET database during 1999. [Delaware Department of Natural 

Resources and Environmental Control (DNREC)]. (See figure 3 for location of stream segments.)—Continued

Simulation of Nutrient and Sediment Concentrations and Loads 13
Figure 8. Simulated hourly total nitrogen concentrations and measured instantaneous values for the nearest calibrated basin for the Delaware Inland Bays watershed (segment source of parameter values), and U.S. Environmental 

Protection Agency’s (USEPA) STORET database during 1999. [Delaware Department of Natural Resources and 

Environmental Control (DNREC)]. (See figure 3 for location of stream segments.)

14  Extension of Hydrologic and Water-Quality Model of the Delaware Inland Bays Watershed
Figure 8. Simulated hourly total nitrogen concentrations and measured instantaneous values for the nearest calibrated basin for the Delaware Inland Bays watershed (segment source of parameter values), and U.S. Environmental 

Protection Agency’s (USEPA) STORET database during 1999. [Delaware Department of Natural Resources and 

Environmental Control (DNREC)]. (See figure 3 for location of stream segments.)—Continued

Simulation of Nutrient and Sediment Concentrations and Loads 15
exhibited seasonal variability, with stronger dilution rates 

during the spring, fall, and winter months, and lower dilution 

rates during the summer months. The dilution effect was also 

stronger for predictions in ungaged segments located in the 

southern part of the watershed than for those in the northern 

part. The contribution to TN concentrations by constituent 

was as follows: between 1 percent and 5 percent ammonia, 

between 90 percent and 95 percent nitrate, and between 5 

percent and 10 percent organic nitrogen. The contribution of 

nitrate was higher at segments located in the lower part of the 

watershed and in segments where the application of organic 

fertilizer was higher than the recommended crop rates (U.S. 

Department of Agriculture, Natural Resources Conservation 

Service, 2000).

Nutrient applications varied by type (organic or mineral 

fertilizer), amount, and timing of application. The type of 

fertilizer applied was determined by the proximity of manure 

production throughout the watershed. For segments 020, 

060, 070, and 110, only mineral fertilizer was applied, and 

the amount applied was assumed to be 110 percent of the 

amount necessary to meet the crop goal yield. The timing 

of the applications was prior to the growing season. The 

effect of these assumed conditions can be seen mainly in the 

concentrations of TN, which are generally lower in these 

segments than in segment 290, for example, where the nutrient 

application rates more noticeably exceeded the recommended 

amounts (U.S. Department of Agriculture, 2000).

Predicted Total Phosphorus Concentrations

Soil amendment with animal waste is a common practice in the Delaware Inland Bays watershed that introduces a 

substantial amount of phosphorus (P) that is present in both 

organic and inorganic forms. In general, 30 percent of the 

total P from manure applied to soils in solid or liquid forms 

is inorganic, mostly as orthophosphate (Hansen and others, 

2004). Phosphates are strongly sorbed to clays and oxyhydroxides of iron in soils, and consequently, phosphates are resistant 

to leaching. Some fraction of the remaining total P in soils is 

available for plant uptake. Off-site transport of phosphates to 

surface water is therefore strongly associated with sediment 

that washes off during precipitation events. When the fertilizer is applied above the recommended plant uptake rates, the 

amount of phosphates stored in the soil will be excessive, and 

may be carried into surface water with storm runoff.

Time series of simulated hourly TP concentrations 

and measured instantaneous values are shown in (fig. 9). 

The contribution of inorganic P to the concentration of TP 

was about 35 percent in the northern segments and about 

50 percent for segments located in the southern part of the 

watershed. During storm events, total P concentrations tended 

to rise sharply with average predictions of about 0.4 mg/L 

(segment 110). No national or state guidelines have been 

established for concentrations of TP in water; however, the 

USEPA recommends that concentrations should not exceed 

0.05 mg/L in a stream at a point where it enters a lake or 

reservoir, and should not exceed 0.1 mg/L in streams that 

do not discharge directly into lakes or reservoirs (Mueller 

and Helsel, 1999). During storm events, these values were 

exceeded throughout the watershed as shown in the measured 

and predicted data.

Predicted Sediment and Nutrient Loads

The hydrologic conditions under which the model was 

calibrated (below-average precipitation) made it difficult 

to evaluate the predicted edge-of-stream loads by land-use 

class in the ungaged segments. The edge-of-stream loads and 

yields (table 3) for total nitrogen, total phosphorus, and total 

suspended sediment were computed using the predicted edgeof-stream concentrations, the predicted edge-of-stream flow 

yielded by model segment, and the surface area of the model 

segments. Due to the drought during 1998 and 1999, and to 

Hurricane Floyd in September 1999, the predicted loads and 

yields during water year 1999 may not be representative of the 

loads during an average water year. These conditions should 

be taken into consideration when using the model results 

to formulate load reductions or establishing programs for 

pollution control.

Predicted edge-of-stream loads of TSS are strongly 

linked to annual streamflow, and higher edge-of-stream loads 

of TSS were calculated in the southern watersheds than in the 

northern watersheds. This result is a consequence of the low 

permeability in the southern watersheds as well as the intense 

ditching, a common practice in the region to increase drainage 

and maintain optimum soil conditions for the crops. In the 

northern watersheds, higher permeability reduces the annual 

streamflow and thus, the potential for TSS transport.

Under the applied parameter transference criteria 

discussed earlier, the predicted edge-of-stream loads at the 

ungaged segments were affected by the annual streamflow 

and the application of organic fertilizer (animal waste). In 

water year 1999, the highest edge-of-stream loads of TN 

were predicted in the southern and western segments (such 

as segments 300 and 360), largely because of the application 

of organic fertilizer exceeding the recommended rates. In the 

case of the predicted edge-of-stream loads of TP, the pattern 

of higher values in the southern watersheds was not as evident 

as the pattern found for TSS or TN predictions. The uncertainty of these predictions is linked to the uncertainty of the 

calibrated parameters using extreme hydrologic conditions (in 

this case during drought) and the short period of calibration. It 

is important to emphasize that the concentrations of TP shown 

in figure 9, however, indicate that the predicted model concentrations of TP were within the range of measured values at 

nearby gaged stations.

Using the new criteria for the transference of parameter 

values discussed earlier, the edge-of-stream yields for TSS, 

TN, and TP were lower than the edge-of-stream yields 

calculated using the criteria based on the similarity of soils 

16  Extension of Hydrologic and Water-Quality Model of the Delaware Inland Bays Watershed
Figure 9. Simulated hourly total phosphorus concentrations and measured instantaneous values for the nearest calibrated basin for the Delaware Inland Bays watershed (segment source of parameter values), and U.S. Environmental 

Protection Agency’s (USEPA) STORET database during 1999. [Delaware Department of Natural Resources and 

Environmental Control (DNREC)]. (See figure 3 for location of stream segments.)

Simulation of Nutrient and Sediment Concentrations and Loads 17
Figure 9. Simulated hourly total phosphorus concentrations and measured instantaneous values for the nearest calibrated basin for the Delaware Inland Bays watershed (segment source of parameter values), and U.S. Environmental 

Protection Agency’s (USEPA) STORET database during 1999. [Delaware Department of Natural Resources and 

Environmental Control (DNREC)]. (See figure 3 for location of stream segments.)—Continued

18  Extension of Hydrologic and Water-Quality Model of the Delaware Inland Bays Watershed
Table 3. Predicted edge-of-stream loads and yields of total nitrogen (TN), total phosphorus (TP), and total 

suspended sediment (TSS) by model segment during water year 1999.

[All load values are in kilograms per year; all yield values are in kilograms per hectare per year–location of segments shown in 

figure 3; *, = calibrated segments]

Segment Loads Yields

TN TP TSS TN TP TSS

010* 318 20.8 11,800 2.39 0.157 88.7

020 4,330 316 40,600 2.35 0.172 22.0

 030* 6,300 86.4 104,000 3.89 0.053 64.4

040 3,790 156 14,900 1.61 0.066 6.31

050 2,940 139 13,600 1.32 0.062 6.10

060 2,450 194 23,400 1.19 0.094 11.4

070 3,860 61 3,980 2.87 0.045 2.96

080 1,940 37 2,530 2.63 0.050 3.41

090 814 55 5,630 2.00 0.135 13.8

100 3,540 184 16,100 1.82 0.095 8.30

110 746 49 5,940 1.48 0.097 11.8

120 13,200 490 42,200 4.15 0.157 13.2

 130* 699 23.7 7,460 0.595 0.020 6.36

140 28,300 353 64,200 13.5 0.169 30.7

150 10,500 199 40,400 9.68 0.183 37.2

160 11,600 169 41,700 11.6 0.169 41.7

170 9,680 226 54,900 7.77 0.182 44.0

180 11,600 246 37,600 10.0 0.211 32.3

190 7,680 234 47,100 8.92 0.272 54.8

200 28,700 388 61,600 14.1 0.191 30.3

210 14,700 280 43,000 8.33 0.159 24.4

220 12,600 287 40,700 9.06 0.206 29.2

230 8,630 149 29,200 11.3 0.194 38.1

240 5,480 124 17,000 11.9 0.270 37.0

250 2,910 75 5,200 1.95 0.050 3.48

260 39,800 244 132,000 20.1 0.123 66.7

270 40,700 178 68,900 28.0 0.122 47.3

280 14,600 208 130,000 6.48 0.092 57.4

290 41,200 229 174,000 19.7 0.110 83.3

300 32,900 325 406,000 6.25 0.062 77.1

 310* 14,200 263 33,100 15.8 0.293 36.8

320 8,310 223 19,800 4.28 0.115 10.2

 330* 11,000 605 124,000 17.3 0.948 194

340 19,100 297 31,800 8.08 0.125 13.5

350 4,170 89 9,340 4.73 0.101 10.6

360 37,200 1,140 354,000 21.1 0.649 201

370 4,944 257 16,800 1.83 0.095 6.22

380 797 46 5,480 2.62 0.151 18.0

390 4,190 188 23,600 3.51 0.157 19.7

400 696 37 4,490 0.89 0.047 5.72

410 1,660 103 7,360 1.61 0.100 7.12

420 5,380 489 226,000 1.25 0.114 52.6

440 1,430 143 16,500 2.81 0.281 32.5

 450* 13,900 520 89,000 7.76 0.290 49.6

470 2,800 111 51,100 2.33 0.092 42.6

Simulation of Nutrient and Sediment Concentrations and Loads 19
from calibrated segments 010, 030, 310, and 450 were within 

25 percent of the measured values. When using parameters 

from segment 130, considerable difference between the 

measured and predicted average annual streamflow for the 

recipient segments may be explained by the reduction of the 

parameter controlling deep percolation and by the parameters 

controlling evapotranspiration. This change to the parameter 

values was in response to the high predicted concentrations 

of ammonia and phosphate during the summer season, and 

due to zero predicted flow in the ungaged segments. The 

monthly water balances for the groups using parameters from 

segments 010, 030, 310, and 450 showed good agreement, 

with most predicted values within 35 percent of the measured 

values. Seasonality in the predicted edge-of-stream flow 

and the predicted streamflow was well represented in the 

model, although some overpredictions resulted during the 

summer season. This overprediction was also observed for the 

streamflow of the calibrated segments.

The highest total-suspended-solids concentrations were 

predicted in segments 300, 420, and 360, which coincide 

with areas of low permeability and the highest annual runoff: 

507 millimeters per year, 341 millimeters per year, and 328 

millimeters per year, respectively; all had high sediment 

concentrations associated with edge-of-stream storm runoff.

Temporal and spatial patterns in nutrient concentrations 

throughout the watershed indicate that the amount of animal 

waste in kilograms per hectare per year applied in the 

agricultural land use within each of the ungaged segments 

exerts a dominant control on the variations in concentrations 

and loads. For segments where the nutrient application for 

particular crops was above the recommended rate (120, 280, 

300, 320, and 340), the predicted nutrient loads were higher 

than in segments where the recommended application in 

kilograms per hectare per year was not exceeded. Except 

for segment 120, located in the northern part of the Inland 

Bays watershed, the higher streamflow predicted in segments 

located in the southern area was also a factor in the higher 

predicted loads.

Time series of simulated hourly total nitrogen and 

phosphorus concentrations and measured instantaneous 

values indicate seasonality in the measured and simulated 

concentrations for total nitrogen, with the lowest values during 

the summer and the highest values during the winter months. 

Phosphorus concentrations were less seasonal than nitrogen 

concentrations. In general, total phosphorus concentrations 

during base flow were low, typically less than 0.05 milligrams 

per liter. During storm events, total nitrogen concentrations 

tend to be diluted and total phosphorus concentrations tend 

to rise sharply. Nitrogen is transported mainly in the aqueous 

phase and largely through ground water, whereas phosphorus 

is strongly associated with sediment, which washes off during 

precipitation events.

between gaged and ungaged segments (Gutiérrez-Magness 

and Raffensperger, 2003). Although it is difficult to state that 

the new yields are better estimates of the actual values, the 

verification of the predicted model concentrations of TSS, 

TN, and TP with monitoring data from nearby sites (fig. 

7, fig. 9), indicate that these estimates may provide better 

approximations of the annual yields (kilograms per hectare) 

under drought conditions. The recalculated and updated values 

by model segment are shown in the maps in Appendix A.

Summary and Conclusions

Rapid population increases, agriculture, and industrial 

practices have been identified as important sources of 

excessive nutrients and sediments in the Delaware Inland 

Bays watershed. A cooperative study was established in 

2000 by the Delaware Department of Natural Resources and 

Environmental Control, the Delaware Geological Survey, 

and the U.S. Geological Survey for the development of the 

Delaware Inland Bays watershed model in order to estimate 

nutrient and sediment concentrations and loads in the 

watershed. The study was extended to evaluate the model 

predictions in ungaged segments and to ensure that the model 

developed in 2003 as a planning and management tool could 

accurately predict nutrient and sediment concentrations 

within the measured range of values in ungaged segments. 

The evaluation required analyses on the method of 

transferring parameter values from calibrated to uncalibrated 

model segments. The U.S. Geological Survey used a welldocumented model, Hydrological Simulation Program–

FORTRAN (HSPF) for the analyses. Water-quality data for 

streams within the Inland Bays watershed were retrieved 

from the U.S. Environmental Protection Agency’s STORET 

database to supplement the Inland Bays Total Maximum 

Daily Loads database and used for the validation of model 

predictions in ungaged segments. The primary objective when 

comparing the predicted edge-of-stream concentrations at 

ungaged segments to the measured in-stream concentrations 

at the calibrated segments, or at streams within or near the 

ungaged model segments, was to guarantee that the model 

predictions were within the range of the observations, rather 

than to have a perfect match between the individual predicted 

and measured values.

The ungaged segments were grouped into five sets for the 

transfer of parameter values. For purposes of model validation, 

the monthly and annual water balances at the source segments 

were compared with the average of the monthly and annual 

water balances for the corresponding recipient segments. 

Higher streamflow was measured in the southern part of the 

watershed than in the northern part. The predicted annual 

water balances of the recipient segments using parameters 

20 Extension of Hydrologic and Water-Quality Model of the Delaware Inland Bays Watershed
Acknowledgments

The author gratefully acknowledges the assistance of 

Richard Greene and Hassan Mirsajadi (Delaware Department 

of Natural Resources and Environmental Control), who 

provided feedback on the model results and analyses, and 

thanks the peer technical reviewers Dr. Jeff Raffensperger 

and Dr. Cherie Miller (U.S. Geological Survey) for their 

contributions. Also acknowledged are U.S. Geological 

Survey employees Timothy Auer for illustrations, Ann Marie 

Squillacci for report layout, and Valerie Gaine for the editorial 

review.

Selected References

Andres, A.S., 1991, Results of the coastal Sussex County, 

Delaware ground-water quality survey: Delaware Geological Survey Report of Investigations No. 49, 28 p.

Andres, A.S., Savidge, K.B., Scudlark, J.R., and Ullman, W.J., 

2002, Delaware Inland Bays tributary Total Maximum Daily 

Load water-quality database: Delaware Geological Survey 

Digital Product No. 02-02, version 6.62.

Bicknell, B.R., Imhoff, J.C., Kittle, J.L., Jr., Donigian, A.S., 

and Johanson, R.C., 1997, Hydrologic Simulation Program

–FORTRAN. User’s Manual for Release 11: Athens, GA, 

U.S. Environmental Protection Agency, National Research 

Laboratory, EPA/600/R-97/080, 755 p.

Delaware Department of Natural Resources and Environmental Control, 1998, Final determination for the State of Delaware 1998 Clean Water Act Section 303(d) list of waters 

needing TMDLs, accessed May 6, 2003, at http://www.

dnrec.state.de.us/DNREC2000/Library/Water/303(d)list.pdf

Delaware Office of State Planning Coordination, 2002, 1997 

Land use/land cover data—GIS Data, accessed April 21, 

2003, at http://www.state.de.us/planning/info/lulcdata/lulc.

htm

Denver, J.M., 1989, Effects of agricultural practices and septic-system effluent on the quality of water in the unconfined 

aquifer in parts of eastern Sussex County, Delaware: University of Delaware Report of Investigations No. 45, 66 p.

Donigian, A.S., Jr., Bicknell, B.R., and Imhoff, J.C., 1995, 

Hydrological Simulation Program—FORTRAN (HSPF), in

Singh, V.P., ed., Computer models of watershed hydrology: 

Water Resources Publications, p. 395–442.

Gutiérrez-Magness, A.L., and Raffensperger, J.P., 2003, 

Development, calibration, and analysis of a hydrologic and 

water-quality model of the Delaware Inland Bays watershed: U.S. Geological Survey Water-Resources Investigations Report 03–4124, 42 p.

Hamilton, P.A., Denver, J.M., Phillips, P.J., and Shedlock, R.J., 

1993, Water-quality assessment of the Delmarva Peninsula, 

Delaware, Maryland, and Virginia—Effects of agricultural 

activities on, and distribution of, nitrate and other inorganic 

constituents in the surficial aquifer: U.S. Geological Survey 

Open-File Report 93–40, 87 p.

Hansen, J.C., Cade-Menun, B.J., and Strawn, D.G., 2004, 

Phosphorus speciation in manure-amended alkaline soils: 

Journal of Environmental Quality, v. 33, p. 1,521–1,527.

Miller, J.C., 1972, Nitrate contamination of the water-table 

aquifer in Delaware: Delaware Geological Survey Report 

of Investigations No. 20, 36 p.

Mueller, D.K., and Helsel, D.R., 1999, Nutrients in the 

Nation’s waters—too much of a Good Thing?: U.S. Geological Survey Circular 1136, accessed June 15, 2005, at 

http://water.usgs.gov/nawqa/circ-1136.html

National Oceanic and Atmospheric Administration, 2001, 

National Climatic Data Center, accessed May 8, 2003, at 

http://www.ncdc.noaa.gov/oa/ncdc.html

Phillips, P.J., and Bachman, L.J., 1996, Hydrologic landscapes 

on the Delmarva Peninsula—Part 1: Drainage basin type 

and base flow chemistry: Journal of the American Water 

Resources Association, v. 32, no. 4, p. 767–778.

Robertson, F.N., 1977, The quality and potential problems of 

the ground water in coastal Sussex County, Delaware: 

University of Delaware Water Resources Center, 58 p.

Shedlock, R.J., Denver, J.M., Hayes, M.A., Hamilton, P.A., 

Koterba, M.T., Bachman, L.J., Phillips, P.J., and Banks, 

W.S.L., 1999, Water-quality assessment of the Delmarva 

Peninsula, Delaware, Maryland, and Virginia: Results of 

investigations, 1987–91: U.S. Geological Survey WaterSupply Paper 2355–A, 41 p.

Sims, J.T., and Wolf, D.C., 1993, Poultry waste management: Agricultural and environmental issues: Advances in 

Agronomy, v. 52, p. 1–81.

Ullman, W.J., Andres, A.S., Scudlark, J.R., and Savidge, K.B., 

2002, Stormwater and base flow sampling and analysis in 

the Delaware Inland Bays: Preliminary report of findings, 

1998–2000: Delaware Geological Survey Open-File Report 

No. 44, 40 p.

University of Delaware Spatial Analysis Lab, 2003, 30-meter 

resolution Digital Elevation Models for Delaware and 

Chester County, PA, accessed May 7, 2003, at 

http://www.udel.edu/FREC/spatlab/dems/dem_intro.html

U.S. Department of Agriculture, Natural Resources Conservation Service, 2000, Part 402. Nutrient management, 

accessed May 7, 2003, at http://www.nrcs.usda.gov/

technical/ECS/nutrient/gm-190.html

Acknowledgments  21

Appendix A. Recalculated and Updated Yields 

by Model Segment
Figure A–1. Simulated total suspended solids yield during water year 1999 by model segment for the Delaware Inland Bays watershed model (modified from Gutiérrez-Magness and Raffensperger, 2003).

24  Extension of Hydrologic and Water-Quality Model of the Delaware Inland Bays Watershed
Figure A–2. Simulated total nitrogen yield during water year 1999 by model segment for the Delaware Inland Bays watershed 

model (modified from Gutiérrez-Magness and Raffensperger, 2003).

Appendix A. Recalculated and Updated Yields by Model Segment  25
Figure A–3. Simulated total phosphorus yield during water year 1999 by model segment for the Delaware Inland Bays watershed 

model (modified from Gutiérrez-Magness and Raffensperger, 2003).

26  Extension of Hydrologic and Water-Quality Model of the Delaware Inland Bays Watershed
Prepared by the Maryland-Delaware-District of Columbia

Water Science Center’s Publications Unit

For additional information, contact:

Director, MD-DE-DC Water Science Center

U.S. Geological Survey

8987 Yellow Brick Road

Baltimore, MD 21237

or visit our Web site at:

http://md.water.usgs.gov