Private Forests, Housing Growth, and 

America’s Water Supply

A Report From the Forests on the Edge and 

Forests to Faucets Projects

Miranda H. Mockrin, Rebecca L. Lilja, 

Emily Weidner, Susan M. Stein, Mary A. Carr

A Forests on the Edge Report

Rocky Mountain General Technical

Forest Service Research Station Report RMRS-GTR-327 September 2014

United States Department of Agriculture
Photos on front cover, from top then left to right: Robert Gauthier ©2012 

Los Angeles Times. Reprinted with permission; fireground / Flickr; KevinCass / 

istockphoto.com; Ben Margot / AP Photo.

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Front cover credits: Autumn Lake, istockphoto.com; Under construction, istockphoto.com; Kiddie drinking, istockphoto.com; No dumping, Cindy 

Cornett Seigle.

Private forests provide a vast array of public goods and services, including abundant, clean surface water. Boys fishing, thinkstock.com.
ABSTRACT

America’s private forests provide a vast array of public goods 

and services, including abundant, clean surface water. Forest 

loss and development can affect water quality and quantity when 

forests are removed and impervious surfaces, such as paved 

roads, spread across the landscape. We rank watersheds across 

the conterminous United States according to the contributions 

of private forest land to surface drinking water and by threats to 

surface water from increased housing density. Private forest land 

contributions to drinking water are greatest in the East but are 

also important in Western watersheds. Development pressures on 

these contributions are concentrated in the Eastern United States 

but are also found in the North-Central region, parts of the West 

and Southwest, and the Pacific Northwest; nationwide, more 

than 55 million acres of rural private forest land are projected to 

experience a substantial increase in housing density from 2000 to 

2030. Planners, communities, and private landowners can use a 

range of strategies to maintain freshwater ecosystems, including 

designing housing and roads to minimize impacts on water 

quality, managing home sites to protect water resources, and 

using payment schemes and management partnerships to invest in 

forest stewardship on public and private lands.

Key words: forest, drinking water, development, stewardship, 

community

AUTHORS

Miranda H. Mockrin is a research biological scientist, U.S. 

Department of Agriculture, Forest Service, Rocky Mountain 

Research Station, 240 W. Prospect Road, Fort Collins, CO 

80526. Rebecca L. Lilja is a GIS Specialist, Forest Service, 

Northeastern Area State and Private Forestry, 271 Mast Road, 

Durham, NH 03867. Emily Weidner is an ecosystem services 

specialist and Susan M. Stein is director of the U.S. Department 

of Agriculture’s National Agroforestry Center, and was formerly 

coordinator of private forestland studies for the Forest Service, 

sstein@fs.fed.us. Mary A. Carr is a technical publications editor, 

Forest Service, Publishing Arts, 1835 Black Lake Boulevard SW, 

Olympia, WA 98512. 

Private Forests, Housing Growth, and America’s 

Water Supply

A Report From the Forests on the Edge and 

Forests to Faucets Projects

Miranda H. Mockrin, Rebecca L. Lilja, 

Emily Weidner, Susan M. Stein, Mary A. Carr

September 2014

CITATION

Mockrin, M.H.; Lilja, R.L.; Weidner, E.; Stein, S.M.; Carr, M.A. 2014. Private forests, housing growth, and America’s water 

supply: A report from the Forests on the Edge and Forests to Faucets Projects. Gen. Tech. Rep. RMRS-GTR-327. Fort Collins, 

CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 29 p.
CONTENTS

INTRODUCTION.......................................................................................................................................1

About the Forests on the Edge and Forests to Faucets Projects .................................................... 2

FORESTS, WATER, AND HOUSING—A COMPLEX RELATIONSHIP................................................... 2

Forested Ecosystems and Water .................................................................................................... 3

Importance of Private Forests..........................................................................................................3

Who Are U.S. Private Forest Owners? ............................................................................................ 4

Housing Development Effects On Water ......................................................................................... 5

The Economic Downturn and Housing Growth ............................................................................... 5

METHODS: ESTIMATING THE IMPORTANCE OF PRIVATE FORESTS TO DRINKING WATER ........ 7

What is a Watershed? .....................................................................................................................8

Estimating the Presence of Private Forests..................................................................................... 8

Measuring a Watershed’s Contribution to Drinking Water ............................................................... 8

Determining the Role of Private Forests in Supplying Drinking Water............................................. 8

Assessing the Impacts of Housing Development............................................................................. 9

The Bottom Line: Combining Data Layers....................................................................................... 9

FINDINGS .................................................................................................................................................9

Private Forest Contributions to Drinking Water ............................................................................... 9

Future Housing Increases on Private Forests ............................................................................... 10

Future of Private Forests and Water Quality.................................................................................. 11

STRATEGIES TO CONSERVE WATER QUALITY ................................................................................ 11

Designing Development To Preserve Water Quality ..................................................................... 15

Best Management Practices For Homeowners To Preserve Water Quality................................... 17

Innovative Payment Schemes and Forest Management Partnerships.......................................... 18

SUMMARY AND CONCLUSIONS.......................................................................................................... 20

ACKNOWLEDGEMENTS.......................................................................................................................22

LITERATURE CITED ..............................................................................................................................22

APPENDIX: Methodology......................................................................................................................26

ii
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327. 1

INTRODUCTION

F

orested lands in the United States provide multiple 

goods and services to the American public, including: 

forest products, fish and wildlife habitats, and 

opportunities for outdoor recreation and education. One 

of the most vital benefits provided by forested ecosystems 

is clean and abundant supplies of water for drinking and 

agricultural and manufacturing uses, as well as for aquatic 

habitats and numerous other ecological, social, and 

economic purposes. 

Both public and private forests contribute to maintaining 

the quality of the water supply, but private forests are 

uniquely vulnerable to being converted to or affected by 

housing development. This report focuses on the role 

of privately owned forests in providing clean drinking 

water, and how increasing housing density may alter these 

forests and water quality. 

This report is one of several produced by the U.S. 

Department of Agriculture (USDA), Forest Service as 

part of the ongoing Forests on the Edge project; it is 

also produced in conjunction with the Forests to Faucet 

project (see box). We display and describe information 

at a national level that can improve understanding of the 

connection between forest land development and water. 

The report draws on the scientific literature to describe 

how increased housing density in private forests affects 

water quality. We then combine national datasets on water 

quality and housing density to answer the questions:

• Where, nationwide, do private forests make substantial 

contributions to clean water?

• Where are projected increases in housing density 

expected to negatively affect private forests and water? 

We present several examples of how land use planners, 

natural resource managers, and communities are 

responding to these threats and taking action to maintain 

water quality in the face of forest development. Clean water flowing from private forests supports a myriad 

of recreational opportunities.

Larry Jernigan, volunteer, US Fish and Wildlife Service Southeast Region

Fed by forested 

streams, 

California’s Lake 

Shasta is a critical 

source of water 

for irrigation, 

drinking water, 

and power supply.

istockphoto.com

Forested ecosystems provide high-quality water for drinking 

and other human uses.

istockphoto.com
2 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327.

For these analyses, we focused on water quality and 

housing characteristics for which nationally consistent 

data were available. Similar to other Forest on the Edge 

reports, the data are analyzed by watershed—an area 

of land that drains into a river, stream, or other body of 

water. We chose watersheds as the unit of analysis to 

emphasize the vital connection between private forests 

and clean water. Analysis at the watershed scale also 

provides information useful for States, counties, and 

national forests. However, as with previous Forests on 

the Edge reports and other national assessments, the final 

results are not intended to assess individual watersheds 

of interest, but rather to give a wider understanding of the 

trends and patterns across the landscape. 

FORESTS, WATER, AND HOUSING—A COMPLEX 

RELATIONSHIP

F

orests are critical to America’s clean water supplies, 

but they are also very desirable places to live. As the 

U.S. population increases, in forests and elsewhere, 

so does our need for reliable sources of clean water. 

However, housing growth in forested areas is associated 

with negative impacts on water supply.

About the Forests on the Edge and Forests to Faucets 

Projects

Sponsored by the State and Private Forestry, Cooperative 

Forestry staff of the U.S. Forest Service, in cooperation 

with Forest Service Research and Development and other 

partners, the Forests on the Edge project (http://www.

fs.fed.us/openspace/fote/) uses data prepared and analyzed 

by scientists across the country to increase understanding 

of the many public benefits derived from private and 

public forests, and of the pressures that might affect these 

benefits. The Forests on the Edge project is one of several 

efforts to assess the status, condition, and trends of forests 

across the United States.

The Forests to Faucets project, sponsored by the 

Cooperative Forestry Ecosystem Services staff, uses 

geographic information systems (GIS) to model and map 

the continental United States land areas most important to 

surface drinking water; the role forests play in protecting 

these areas; and the extent to which these forests are 

threatened by development, insects and disease, and 

wildland fire. The spatial dataset can be used to identify 

areas of interest for protecting surface drinking water 

quality, can be incorporated into broad-scale planning, and 

can help identify areas for further local analysis. For more 

information visit http://www.fs.fed.us/ecosystemservices/

FS_Efforts/forests2faucets.shtml.

NRCS / Jeff Vanuga

Handline sprinkler irrigation germinating crops in Yuma, AZ.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327. 3

Forested Ecosystems and Water 

Water supply begins as precipitation. Water that seeps 

underground into pores between sand, clay, and rock 

formations is termed groundwater. In this report we focus 

on surface water—water derived from precipitation that 

enters streams and rivers, eventually flowing downstream 

to oceans. Approximately two-thirds of the U.S. 

population relies on surface water for drinking, with the 

rest using groundwater (EPA 2009). 

The health of the lands through which freshwater passes 

play an important role in the supply and quality of surface 

water resources. Forests are known to play a particularly 

important role in water quality, reliably producing the 

highest quality stream water (Neary and others 2009). 

Forests are highly efficient at capturing precipitation 

(owing in part to organic matter, such as leaves, on the 

forest floor) and at maintaining water quality (owing 

to the filtration functions played by high quality soils) 

(Neary and others 2009). Forests also maintain effective 

nutrient cycles and prevent erosion and sediment 

runoff (Neary and others 2009, Vose and others 2011, 

Wickham and others 2011). These water resources in 

turn support vital riparian and wetland habitat for aquatic 

and terrestrial animals as well as high-quality water for 

drinking and other human uses.

In total, an estimated 53 percent of the water supply in the 

conterminous 48 States originates on forests (Brown and 

others 2008)1. Consequently, the U.S. population relies 

heavily on healthy forested watersheds to produce a stable 

and high quality supply of water. For example, urban 

areas throughout Arizona, California, and Nevada derive 

most of their water from the Colorado and Green Rivers, 

which originate in high-elevation forested headwaters in 

Colorado and Wyoming (Neary and others 2009). Not 

all water resources originate on public lands: most of 

New York City’s water supply comes from New York’s 

Catskills area, a region that is 75 percent privately owned 

(Postel and Thompson 2005).

Maintaining forest health can minimize the cost of 

additional treatment by local governments. In fact, some 

U.S. cities—such as Seattle (WA), Boston (MA), Portland 

(OR), and New York (NY)—have chosen to invest in 

land conservation and watershed protection rather than 

in additional water treatment facilities to maintain water 

quality (Carpe Diem West 2013, Postel and Thompson 

2005). In other cities (such as Denver, CO, Santa Fe, NM, 

and Flagstaff, AZ), the drinking water supplier or city 

residents have invested money in forest restoration and 

wildfire risk reduction in watersheds that supply drinking 

water (see Innovative Payment Schemes, later in this 

document).

Importance of Private Forests

More than half of America’s forests (56 percent, or 423 

million acres) are privately owned (Fig. 1)—that is, 

they are owned and managed by individuals, families, 

corporations, tribes, and the forest industry (Butler 2008). 

1 Brown and others (2008) derived vegetation cover from the 1992 

National Land Cover Database, with forest cover composed of cover 

classes 41 (deciduous forest), 42 (evergreen forest), and 43 (mixed 

forest).

More than half of the water supply in the conterminous United 

States originates on forests.

USFS / Susan Stein

istockphoto.com

North-South Lake and the Hudson Valley in the Catskills 

Mountains, upstate New York.
4 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327.

Private forests are not distributed equally across the U.S.: 

three-quarters are in the Eastern United States, although 

private lands in the Western United States are also 

ecologically important. 

Private forests make valuable contributions to our water 

supplies: more than a quarter of our fresh water flows 

from and is filtered by private forest lands (Brown 

and others 2008). Public lands—those areas owned by 

Federal, State, or local governments—were not included 

in the analysis presented here because these lands are 

typically not at risk of increased housing density.

Private forests come in all sizes, from smaller than 

10 acres to larger than 10,000 acres. Most private forest 

acreage is owned by “family forest” owners—individuals 

and families—while others are owned by corporations; 

these owners have diverse management goals, styles, 

and resources (Butler 2008). However, this is a time of 

uncertainty and change, both for corporate and family 

forest owners. Among corporations, traditional forest 

industry owners have been increasingly replaced by 

institutional investors, leading to a higher frequency of 

land turnover and subdivision of large land holdings 

(Clutter and others 2005). 

Figure 1. Location of private and public forest, nonforest, and urban areas. Private Forest includes 

private conservation land and easements. SOURCE: CBI (2012), Fry and others (2011).

Who Are U.S. Private Forest Owners?

Data from the Forest Service’s National Woodland Owner 

Survey (Butler 2008) show that most of the 11 million 

diverse owners of private forests have relatively small 

properties—close to 8 million landowners have fairly small 

holdings of fewer than 50 acres each. Although there are 

fewer large properties, their bigger size means that those 

owners with more than 100 acres hold some two-thirds of 

the private forest acreage (Fig. 2).

Figure 2. Percentage of private forest acres by parcel size. 

SOURCE: Butler (2008).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327. 5

Many family forest owners are older than 55 years, and 22 

percent of all family forest land may be available for sale 

or transfer to new owners in the near future (Butler 2008). 

In combination, the rising costs of owning family forests, 

demand for housing in many forested areas, and shifting 

economic realities for timber production will likely 

increase land subdivision and housing development on 

private forests. These trends are expected to continue into 

the future, despite the economic downturn and slowdown 

in the housing market that started in 2007.

Housing Development Effects On Water

When forests are developed for housing, wide-ranging 

environmental impacts can ensue. During development, 

forest vegetation is cleared and slopes and soils are 

graded for construction, which can lead to erosion and 

sedimentation. After construction, impervious surfaces 

(those that do not allow water to seep through) such as 

roads, parking lots, and rooftops are introduced; and new 

infrastructure (such as gutters and storm drains) enables 

fast removal of surface water from the landscape (Hansen 

and others 2005, Liu and others 2003). 

The Economic Downturn and Housing Growth 

The economic downturn that started in 2007 had wideranging effects on the American economy, lowering rates 

of home ownership and slowing the record housing 

development that had been seen in the previous 2 decades 

(Congressional Budget Office 2008, Jacobsen and Mather 

2011, Yen 2011). The 30-year period used for our housing 

projections (from 2000 to 2030) does not factor in these 

economic changes and their impacts on the housing market 

(Theobald 2005). However, economists and housing experts 

expect that housing growth will recover with the economy 

(Williams 2012), so that the rate of land conversion for 

residential development is expected to rise again in the 

future. 

The slowdown in housing expansion may offer a 

valuable opportunity to plan for the future of residential 

development and land conservation. For example, as land 

has become more affordable and development has slowed, 

land trusts have been able to expand conservation efforts, 

often through easements that allow continued forestry, 

timber, or agriculture (Christensen and others 2011). The 

housing downturn is also allowing time for communities 

to address planning concerns; for example, county 

governments and local groups are working to reshape 

unfinished housing and resort developments that are 

common across the West (Best 2012). 

Private forest landowners have diverse 

objectives for their forests, ranging from 

sheer enjoyment to forest products to 

conservation and family legacy.
6 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327.

New housing development also leads to expanded 

infrastructure far beyond the site of the house, including 

roads and transportation networks. (See Building Roads 

and Infrastructure, later in this document.) 

Thus, housing development not only removes existing 

forest vegetation that had been protecting the water 

supply, but also significantly alters freshwater systems 

by increasing the amount of impervious surface, mostly 

roads (Jacobson 2011, Schuler and Ince 2005). As 

impervious surface expands, less precipitation is able 

to recharge the groundwater supply, and more is lost to 

surface runoff, thus altering the timing of stream flows 

(Fig. 3). Additional impacts of housing and associated 

development include more pollutants in the water, more 

variable water flow due to increased impervious surface 

(which may cause in-stream sedimentation), and increased 

water temperature due to loss of vegetation and expansion 

of impervious surface (Allan 2004, Brabec and others 

2002).

These alterations in water quality and quantity have 

far-reaching effects on people and wildlife. As surface 

water runoff is altered, riparian zones experience reduced 

abundance and diversity of many organisms, including 

algae, invertebrates, amphibians, and fishes (Paul and 

Meyer 2001, Price and others 2006, Riley and others 

2005) (Fig. 3). A review of scientific studies (Brabec 

and others 2002) found a decline in the diversity and 

abundance of fish, invertebrates, and other aquatic 

organisms when impervious surface ranged from 3 to 

15 percent of the area, as a result of habitat degradation 

(turbidity, altered flow regimes, increased temperatures). 

As water quality declines, challenges for human use 

increase, including rising costs for filtration and treatment 

(Davies and Mazumder 2003, Dearmont and others 1998, 

Grolleau and McCann 2012). And while such treatment 

might clean up certain aspects of water pollution for direct 

human consumption, water treatment facilities operate in 

only one location and do not address water quality issues 

across the entire landscape, as a healthy forest can do. 

These impacts have become more prevalent across the 

United States as housing development has intensified, 

with a notable increase in housing in rural areas with 

high natural amenities, such as mountains, forest cover, 

or lakes (Hammer and others 2004, McGranahan 1999, 

More impervious surfaces means less water is absorbed into 

the ground to recharge groundwater supplies.

istockphoto.com

The addition of homes and other structures to the landscape 

can cause higher streamflow rates, and greater stream bank 

erosion during rains.

USFS / Susan Stein
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327. 7

Theobald 2005). Nationally, housing and infrastructure 

are expanding at higher rates than are human populations, 

as a result of decreasing household size, more widespread 

home ownership, and multiple home ownership (Hammer 

and others 2004). By 2000, urban and exurban settlement 

in the United States had expanded to cover four to five 

times the area they had covered in 1950 (Brown and 

others 2005). 

Development in rural or exurban areas is of particular 

concern because the average housing unit in these areas 

causes more development impacts than the average urban 

housing unit, largely because of the additional roads and 

infrastructure needed for each household (Brabec and 

others 2002, Brown and others 2005).

METHODS: ESTIMATING THE IMPORTANCE OF PRIVATE 

FORESTS TO DRINKING WATER 

T

o determine the relationship between private forests, 

housing growth, and water quality and supply, we 

first determined where watersheds with private forests 

are located. We then created a measure of how important 

each watershed is for surface drinking water, and we 

estimated the relative contribution of private forests to 

the supply of surface drinking water. We combined this 

information about where private forests are important to 

surface drinking water with projections of where housing 

densities are expected to increase, to identify those areas 

where future housing development may threaten water 

supplies. 

Figure 3. Impacts of impervious surface on water quality and biodiversity. Used with permission from the 

Maryland Department of Natural Resources.

New housing development leads to expanded infrastructure, 

such as roads, which can affect water quality.

istockphoto.com
8 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327.

The analyses summarized and presented here originated 

in the Forests to Faucets project (Weidner and Todd 

2011) and the Forests on the Edge project conducted 

by the Forest Service (e.g., Stein and others 2009). The 

methods used here in combining layers and ranking 

watersheds, including the housing density categories used, 

were first developed by the Forests on the Edge project. 

Weidner and Todd (2011) developed the surface drinking 

water importance index and worked at the finer 12-digit 

watershed level (see box, What is a Watershed?). See the 

appendix for further methodology details.

Estimating the Presence of Private Forests

Where do private forests make substantial contributions to 

clean water, and where might future increases in housing 

density affect private forests vital to our drinking water 

supplies? 

To answer these questions, we made a map identifying the 

percentage of private forest by watershed, based on data 

from the National Land Cover Dataset (NLCD) (Fry and 

others 2011), data on National Forest System (NFS) land 

locations (USDA Forest Service 2009), and the Protected 

Areas Database (CBI 2012). The watersheds used are 

referred to as 6th-level or 12-digit hydrologic units (HUCs). 

The definition we used for “forest land” is the same as that 

used by the U.S. Geological Survey (USGS) when creating 

the NLCD: land with at least 25 percent tree crown cover 

from trees that are greater than 20 feet tall (Fry and others 

2011). 

Measuring a Watershed’s Contribution to Drinking Water

To determine what areas are most important for surface 

drinking water, we used a data layer created by Weidner 

and Todd (2011), which is an index of the relative 

importance of surface drinking water in each watershed 

across the United States. Weidner and Todd created 

this index by combining information on the volume of 

water available (water supply), the landscape surface 

flow patterns and the natural processes that affect water 

quality, and the need for drinking water downstream 

(water demand). Extra importance was given to 

watersheds that generate more water supply.

We focus on human drinking water needs, at the 

12-digit HUC level, although we recognize that research 

studies at smaller scales have focused on additional 

aspects of water quality, such as water temperature or 

concentrations of chemical pollutants and their effects 

on larger ecological systems.

Determining the Role of Private Forests in Supplying 

Drinking Water

To understand the relative contribution of private forests 

to surface drinking water, we multiplied the surface 

drinking water importance index by the percentage of 

each watershed identified as private forest. This new 

value represented the importance of private forest 

What is a Watershed? 

A watershed is an area of land that catches rainfall and 

other precipitation and funnels it into a network of 

marshes, streams, rivers, lakes, soils, or groundwater. The 

U.S. Geological Survey (USGS) has classified all U.S. lands 

according to a series of nested watersheds, referred to 

hydrological units. For this analysis, the 6th-level or 12-digit 

hydrologic unit (HUC) was used (USDA/NRCS). Within the 

hierarchy of hydrological units, these 12-digit units are 

termed subwatersheds, but we refer to them as watersheds 

throughout this report for simplicity. There are more than 

90,000 of these watersheds in the United States, with a 

mean size of 35 square miles, and ranging in size from 0.62 

to 986 square miles.

Much of the Potomac River is fed by surrounding private and is 

affected by the use of these lands.

USFS / Susan Stein
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327. 9

land to surface drinking water. Watersheds with small 

amounts of private forests or low importance to surface 

drinking water had a small value for private forest 

contribution to surface drinking water. 

Assessing the Impacts of Housing Development

To assess the relative impact of future housing increases 

on private forests we relied on a dataset showing the 

percentage of a watershed predicted to experience a 

substantial increase in housing density, based on a 

method used by Stein and others (2009). Projected 

increases in housing density were derived from a 

spatially explicit model of housing growth, which 

simulates future patterns of development based on 

historical growth and accessibility to urban areas and 

protected lands (Theobald 2005). Housing densities were 

divided into three categories: rural 1 (more than 40 acres 

per housing unit), rural 2 (10–40 acres per housing unit), 

or exurban/urban (fewer than 10 acres per housing unit). 

Any areas projected to change from a rural category to 

a higher category were considered areas of substantial 

increase in housing density. We then determined the 

percentage of each watershed expected to experience 

an increase in housing density in private forested areas 

between 2000 and 2030.

The Bottom Line: Combining Data Layers

Finally, for each watershed, we multiplied the percentage 

of the watershed projected to experience increased 

housing density in forested areas by the index of 

contribution of private forest land to surface drinking 

water (the index, as described above, includes information 

on water supply, water demand, and percentage of private 

forests). In this way, we identified those privately owned 

forested areas that are important for surface drinking 

water and are likely to be affected by future increases in 

housing density. 

We ranked the top 100 watersheds according to the 

importance of private forests to surface drinking 

water supplies, 50 each from both East and West of 

the Mississippi River. Examining critical watersheds 

by region allowed us to highlight areas and States of 

importance across the U.S.

FINDINGS

Private Forest Contributions to Drinking Water 

H

ighest ranked watersheds, shown in dark green on the 

map (Fig. 4), are those where private forests are of 

greatest importance to surface drinking water. 

Lake Fanny Hooe, Upper Peninsula, MI, an area that is an important resource for drinking water, fishing, and recreation.

thinkstockphotos.com
10 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327.

Watersheds where private forests make the greatest 

contributions to surface drinking water are found across 

much of the Eastern United States. This finding makes 

sense as, relative to other parts of the country, many 

Eastern States have high population densities, high 

percentages of private forest, and a greater reliance on 

surface water than on groundwater. Notable areas include 

southern New Hampshire and eastern Massachusetts; 

eastern New York, the New Jersey Highlands and eastern 

Pennsylvania; western Pennsylvania; and the southern 

Appalachian highlands. In the West, higher values occur 

along the northern coast and high-elevation areas, such 

as the Pacific Coast ranges, the Sierra Nevada, and 

Colorado’s Front Range. Values are lower for much of the 

arid West, owing to both lower population density and a 

much greater reliance on deep groundwater systems than 

in other areas of the country.

This model does not explicitly consider water scarcity; as 

a result, much of the arid West, although facing challenges 

with water supply issues, does not get high values in the 

surface water drinking importance index. Many other 

States that also rely on groundwater sources for drinking 

water (such as Wisconsin, Florida, and other southeastern 

coastal States) also have lower values of surface drinking 

water importance in this analysis.

Future Housing Increases on Private Forests 

More than 150 million acres of the conterminous United 

States, including 55 million acres of private forest lands, 

are predicted to experience a substantial increase in 

housing density between 2000 and 2030 [(Stein and 

others 2009)]. Among private forests, the 55 million acres 

expected to experience a substantial increase in housing 

density represents about 17 percent of rural private forests 

(rural 1 and 2). By 2030 we predict that the percentage 

of the United States with housing densities in the urban/

exurban category will have doubled (Table 1). Nearly half 

the new acreage in urban/exurban areas will have come 

from rural 2 areas, with a similar amount going from rural 

1 to rural 2 (Fig. 5).

Watersheds where private forest lands are predicted to 

experience a substantial increase in housing density 

Figure 4. Private forest importance to surface drinking water (see Step 2 in the appendix) identifies those watersheds 

where private forest lands are most important in protecting surface drinking water. SOURCE: CBI 2012, Fry and others 

2011, NRCS 2009b, Weidner and Todd 2011.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327. 11

are located throughout the East, concentrated along the 

Boston–New York–Washington DC corridor as well as 

in the Southeast, particularly in Florida (Fig. 6). Other 

areas where a substantial increase in housing density is 

expected in private forests include pockets in the upper 

Midwest and the Gulf Coast States, as well as watersheds 

along Colorado’s Front Range, the northern California 

coast and the Sierra Nevada, and the Cascade Mountains 

in Washington State.

Future of Private Forests and Water Quality

Watersheds where water quality is most likely to change 

due to increased housing density on private forests are 

most prevalent in the East. Eastern watersheds in the 

90th percentile include many in New England and in 

the highland areas of the Southeast, well as the upper 

Midwest and southern Missouri (Fig. 7). In addition, 

the Colorado Front Range and high-elevation areas in 

Oregon, Washington, and California contain watersheds 

where private forest land development is likely to affect 

drinking water. 

Looking at the top 50 watersheds where development 

is likely to affect surface drinking water in the East, we 

find high-ranking watersheds throughout the region: in 

the Mid-Atlantic and New England areas, but also in the 

Southeastern United States (Table 2, Fig. 8). At the state 

level, New Hampshire and Georgia combined have more 

than half of these high ranking watersheds. In the West, 

the top 50 watersheds are found in only 7 States, with 

more than half in California.

STRATEGIES TO CONSERVE WATER QUALITY

G

iven the vital role that private forests play in 

preserving water quality and supply, and the forecasts 

for continued and widespread housing development, 

planners and communities across the country are using 

Table 1. Acreage and percentage of private forest in housing density classes from 2000 to 2030 

Percent of total Percent of total

Housing density class Acres (2000) land (2000) Acres (2030) land (2030)

Rural 1 244,025,619 70 213,124,773 61

Rural 2 76,820,759 22 78,728,937 23

Exurban/urban 26,960,774 8 55,953,441 16

TOTAL 347,807,151 347,807,151

0

5,000

10,000

15,000

20,000

25,000

30,000

Rural II from Rural I Exurban/urban from Rural I Exurban/urban from Rural II

Acres of land (in thousands)

Figure 5. Acres of land predicted to 

transition between housing density 

classes, 2000–2030 (in thousands).
12 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327.

Figure 6. Percentage of each watershed expected to experience an increase in housing density in private 

forested areas between 2000 and 2030. SOURCE: CBI 2012, Fry and others 2011, NRCS 2009b, Stein and 

others 2009.

Figure 7. Watersheds ranked by importance of forests for both drinking water importance and future 

housing density increases combined (see Step 3 in the appendix. SOURCE: CBI 2012, Fry and others 

2011, NRCS 2009b, Stein and others 2009, Weidner and Todd 2011.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327. 13

Table 2. Number of top-ranked watersheds at risk, by State, west and 

east of the Mississippi River

State

Number of 

top 50 Western 

watersheds

Arkansas 3

California 27

Colorado 5

Missouri 7

Texas 5

Utah 2

Washington 1

State

Number of 

top 50 Eastern 

watersheds 

Alabama 1

Connecticut* 1

Georgia 11

Maine 4

New Hampshire 16

New York 3

North Carolina 1

Pennsylvania 4

Rhode Island* 2

South Carolina 1

Tennessee 5

Virginia 1

West Virginia 1

*Watershed crosses State boundaries. See Fig. 8 for a visual distribution of 

high-ranking watersheds by State.

Figure 8. One hundred highest ranking 

watersheds where water quality on private 

forests is threatened by increased housing 

density. In some States the dots on the 

map overlap or are too close to distinguish 

individually because of multiple high-ranking 

watersheds; see Table 2 for the number of 

watersheds in each State.
14 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327.

thinkstockphotos.com thinkstockphotos.com

McIntosh Lake, Rocky Mountains. Colorado contains five watersheds where private forest land development is likely to 

affect surface drinking water.

At-risk watersheds are found throughout the southeastern United States.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327. 15

a number of strategies to maintain water quality before, 

during, and after residential development (Carpe Diem 

West 2013). Practices reviewed here include working 

with developers to design infrastructure, roads, and homes 

wisely before housing is built, as well as working with 

homeowners to ensure that their management practices 

around the home preserve water quality over the long 

term. 

Although the focus of this section is on preserving the 

quality of water flowing from private forest lands, we also 

recognize that private individuals and municipalities have 

powerful incentives to protect both public and private 

watersheds, in order to preserve drinking water. This 

section also highlights several examples of how unique 

payments for ecosystem service arrangements can help 

protect water quality on both private and public land.

Designing Development To Preserve Water Quality 

Low-Impact Development (LID)

As environmental impacts of residential development 

on water quality have become better understood, urban 

planners have responded by improving the design of 

residential neighborhoods to preserve water quality 

(Dietz 2007). The most commonly discussed strategy is 

low-impact development (LID), a set of techniques for 

land development (or re-development) that uses natural 

processes to manage stormwater (water originating during 

precipitation events), as close to its source as possible 

(Ahiablame and others 2012, Dietz 2007). LID is often 

implemented in concert with other planning strategies 

such as conservation developments (see next section) to 

reduce the environmental impact of housing development. 

The main goals of LID are to reduce runoff, increase 

water infiltration back into the soil and eventually 

groundwater, maintain the natural flow of water after 

storm events in local streams and rivers, enhance water 

quality, and remove pollution (Ahiablame and others 

2012, Dietz 2007). LID minimizes changes to water 

flow and quality by retaining and infiltrating runoff 

through plants and soils and minimizing impervious 

service, instead of conventional processing that gathers 

stormwater through pipes and deposits the water in large 

detention basins and outfalls. 

Many practices to improve stormwater management can 

be considered LID, including rain gardens (specially 

selected plants in shallow depressions that capture runoff), 

vegetated rooftops, rain barrels, and permeable pavements 

(Ahiablame and others 2012, Dietz 2007). 

By implementing LID principles and practices, water 

can be managed in ways that reduce the impact of built 

areas and promote the natural movement of water within 

an ecosystem or watershed. Applied on a broad scale, 

Louis Cook / Philadelphia Water Department thinkstockphotos.com

Roof-top gardens, permeable pavements, and rain 

gardens can help reduce negative impacts of heavy rains 

in the built environment.
16 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327.

LID can maintain or restore a watershed’s hydrologic 

and ecological functions. As climate change leads to 

more variable precipitation, including both more frequent 

intense storm events and droughts, LID can be a costeffective method to retain stormwater locally and reduce 

flooding (Pyke and others 2011).

Conservation Developments

Conservation developments, sometimes termed openspace or clustered developments, are residential 

developments that purposefully restrict the footprint of 

housing while providing functional protection for natural 

resources (Pejchar and others 2007). By clustering or 

concentrating homes and infrastructure into one portion 

of a residential area while leaving open space elsewhere, 

conservation developments provide an alternative to the 

dispersed homes and infrastructure seen in many rural or 

exurban settings. 

Because conservation developments are designed and 

managed to protect natural resources, they may include 

environmentally beneficial design and management, such 

as LID practices for stormwater management. Modeling 

simulations show that development with this higher 

density but smaller footprint can lead to better water 

quality than traditional development (Jacob and Lopez 

2009, Williams and Wise 2006). Strategically siting the 

open space portion of a conservation development to 

protect water resources, such as stream corridors, can 

preserve watersheds and help maintain water quality 

(Carter 2009).

Building Roads and Infrastructure To Maintain Water Quality

Proper planning for and design of roads and infrastructure 

(such as bridges and culverts) not only can save money 

and construction time but also will protect and minimize 

the impact to streams, lakes, and wetlands (Daniels and 

others 2004). Such cost-effective and water-protective 

road planning considers not only construction site 

specifics (soil, vegetation, geology, and the like) but also 

how the road will be used in the future; coordination with 

neighboring landowners is also important. Temporary 

roads can be a good option if the situation is appropriate 

because temporary roads can lessen the impact on local 

water quality compared to a permanent road (Daniels and 

others 2004).

Conservation developments are designed to protect natural resources.

Metropolitan Design Center Image Bank © Regents of the University of Minnesota.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327. 17

To help minimize the impact of road construction on 

water quality, the Wyoming Department of Environmental 

Quality (WDEQ 2012) offers some general guidelines 

when developing road construction plans; examples 

include:

• Construct roads a safe distance from a water body. 

• Minimize the number of stream crossings.

• Carefully design stream crossings with bridges, culverts, 

rip-rap, and other measures.

• Construct stream crossings during periods of low 

streamflow.

• Fit upland roads to the topography using culverts, 

waterbars, stabilized slopes, and the like.

• Route road runoff to a filter strip and not directly into a 

water body.

• Provide energy dissipaters (such as bales, rocks, or logs) 

to reduce the erosive force of runoff.

• Carefully locate, and properly reclaim, borrow pits and 

gravel sources.

• Seed or otherwise stabilize disturbed areas as soon as 

possible.

• Complete road construction prior to the main runoff 

season to minimize exposure of unfinished roads to 

heavy runoff.

Best Management Practices For Homeowners To Preserve 

Water Quality

Homeowners can make positive contributions to the 

protection of water quality by taking a number of steps to 

reduce negative impacts of stormwater runoff, such as the 

following (adapted from EPA 2003):

• Use porous pavement materials for driveways and 

sidewalks.

• Use native vegetation and mulch to replace high 

maintenance (and high water-use) grass lawns.

• Keep litter, pet wastes, leaves, and debris out of street 

gutters and storm drains, because these outlets 

typically drain directly to bays, lakes, streams, rivers, 

and wetlands. Instead of disposing of yard waste, use 

the materials to start a compost pile. 

• Apply lawn and garden chemicals sparingly and 

according to directions. Consider using Integrated Pest 

Management, which promotes multiple pest control 

strategies (biological, cultural, and chemical).

• Clean up spilled brake fluid, oil, and grease, and dispose 

of used oil, antifreeze, paints, and other household 

chemicals properly; don’t dump or hose such products 

into storm sewers or drains. Many communities have 

programs for collecting household hazardous wastes.

• Use “green” car wash facilities that do not generate 

runoff, to avoid washing detergents and grease into 

local streams and lakes.

• Control soil erosion by planting ground cover and 

stabilizing erosion-prone areas.

• Have septic systems inspected and pumped at least 

every 3 to 5 years to ensure they operate properly. The provision of passageways for water flow reduces the 

amount of erosion associated with road construction.

istockphoto.com
18 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327.

Practice water conservation measures to extend the life 

of septic systems.

• Purchase household detergents and cleaners that are 

low in phosphorous to reduce the amount of nutrients 

discharged into lakes, streams, and coastal waters.

Innovative Payment Schemes and Forest Management 

Partnerships

Many of the management actions discussed here can be 

taken by private landowners and developers. However, 

private and public lands must often be considered together 

to deliver the biggest conservation benefit. A growing 

number of projects are accomplishing water quality 

goals through collaborative restoration and management 

on public (Federal, State, or local) and private lands, 

sometimes sharing payment for these ecosystem services 

with local consumers and municipalities. Working to 

identify the needs of local users and conduct forest 

management through partnerships can be a cost-effective 

way to deliver better water conservation outcomes over 

landscapes (Carpe Diem West 2013, Weidner and others 

2013). Some examples of effective forest management 

partnerships are described in this section.

Denver, CO: Conducting Forest Restoration with the Forest 

Service To Protect Water Quality

In 2010, the Denver Water Authority and the U.S. Forest 

Service conducted a partnership called From Forests to 

Faucets, to jointly fund forest restoration and watershed 

protection. Denver Water serves 1.3 million people in 

the metropolitan region—one-quarter of the State’s 

population—with 2 percent of the State’s water (Denver 

Water 2011a). The water comes from rivers and streams 

fed by mountain snowmelt that is stored in reservoirs. 

Severe forest fires in the late 1990s and early 2000s led to 

forest degradation and substantial sedimentation in some 

of these reservoirs. Denver Water spent more than $26 

million on water quality treatment, sediment and debris 

removal, and reclamation after these fires. 

In response to these costly expenditures, and with concern 

about the risk of additional forest fires and declines in 

forest health due to bark beetle outbreaks, Denver Water 

and the Forest Service each agreed to contribute $16.5 

million toward fuel treatments designed to reduce the 

risk of future catastrophic wildfire (Denver Water 2011b). 

Denver Water’s funds for the restoration partnership are 

included in customer water fees, with each household 

paying $27 on average over the course of the 5-year 

project. Denver Water has been working on its own lands 

to restore forest and reduce wildfire risks since the late 

1990s and is beginning to collaborate with agencies to 

complete more work on private lands.

Washington County, OR: Funding Restoration on Public and 

Private Land To Protect Endangered Species

Serving more than 500,000 residents west of Portland in 

Washington County, OR, the Tualatin River is intensively 

used for agriculture and urban development; it also must 

sustain high water quality to serve human needs and to 

protect federally endangered salmon species. New State 

regulations in 2001 meant the local water utility had 

Instead of disposing yard waste, use the materials to start 

a compost pile.

Signs like these help to make people more mindful of the 

connection between ground surfaces, water flow, and 

the quality of water in our waterways.

thinkstockphotos.com

Cindy Cornett Seigle
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327. 19

Forests in South Platte, CO, are critical to protecting Denver’s water supply.

thinkstockphotos.com

Downtown Denver.

istockphoto.com
20 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327.

to find a way to minimize the impacts of wastewater 

discharge on stream temperatures. Managers concluded 

that restoring riparian habitat and shade, along with 

building two new reservoirs to temporarily hold treated 

water, would be the most cost-effective way to restore 

water quality (Cochran and Logue 2011).

By 2007, the riparian restoration had cost $4.3 million, a 

95 percent cost savings in comparison to more traditional 

options such as installing cooling/refrigeration equipment 

or building discharge pipelines to larger water bodies 

(Cochran and Logue 2011). From 2004 to 2008, more 

than 1.6 million native trees and shrubs were planted 

throughout the watershed on both public and private 

lands, an effort so large that the utility established its 

own nursery for plants. A variety of groups led replanting 

efforts: in urban areas volunteers with community 

organizations and professional restoration crews managed 

by the public water utility conducted restoration work, 

while in rural areas local committees including farmers 

and private forest owners managed the restoration 

(Cochran and Logue 2011). A diversity of funding 

sources and mechanisms also helped pay for restoration, 

including Federal grants, a modification of the USDA’s 

Conservation Reserve Program, and resources from 

municipalities and the water utility.

Bethlehem, PA: Managing a Watershed with The Nature Conservancy To Protect Water Quality And Generate Revenue

The city of Bethlehem, PA, recently agreed to manage 

their 22,000-acre watershed in the southern Pocono 

Mountains through a long-term conservation easement 

with The Nature Conservancy (TNC), a non-profit 

conservation organization. The Bethlehem Water 

Authority provides water for approximately 115,000 

customers in 10 municipalities. The Bethlehem 

Authority’s water comes entirely from surface sources in 

the Pocono Mountains. 

Through TNC’s Working Woodlands Program, 

Bethlehem’s watershed will be protected as a working 

forest throughout the 60 years of the agreement, through 

a combination of easements, Forest Stewardship Council 

forest management certification, and forest carbon 

payments (Sadowski 2011, TNC 2011). There will be no 

commercial development on the land, but recreation will 

continue. 

The Bethlehem water authority benefits from the 

sustainable management of timber and preservation 

of their watershed, as well as the additional revenue 

streams from certified timber sales and carbon credits. 

Water managers had incurred substantial debt through 

infrastructure upgrades and considered the outright 

sale of land (City of Bethlehem 2008), but the Working 

Woodland program will allow the water authority to 

retain and manage their watershed, while also providing 

additional revenue streams.

SUMMARY AND CONCLUSIONS

A

mericans rely on forested ecosystems for clean and 

abundant supplies of water for drinking as well as 

for agricultural and manufacturing inputs and other 

values. Both public and private forests play an important 

role in preserving natural hydrological systems and water 

Riparian restoration has helped to conserve habitat for 

federally endangered Chinook salmon (Oncorhynchus 

tshawytscha). Autumn in the Poconos, Pennsylvania. USDA APHIS

istockphoto.com
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327. 21

quality, but private forests are uniquely vulnerable to 

environmental threats from residential development. 

Over the next 30 years, we expect to see widespread 

increases of housing in private forests that currently 

have low housing density (rural 1 or rural 2); by 2030 

we predict that the percentage of the conterminous 

United States in urban/exurban housing densities will 

have doubled. Using national data sets on land cover, 

housing development, and water quality and demand, we 

examined where housing density increases are expected to 

have the most substantial impact on water resources.

Those watersheds where private forests make the 

highest contributions to surface drinking water are 

found predominantly across the Eastern United States, 

reflecting both the high human population densities and 

water demand in that region relative to other parts of the 

country, and a greater reliance on surface water than on 

groundwater. Watersheds where water quality is likely 

to change owing to increased housing density on private 

lands are also concentrated in the East, but there are other 

regions where private forest land development is likely 

to affect drinking water, including the Colorado Front 

Range and mountain regions of Oregon, Washington, and 

California. 

Of the States located East of the Mississippi River, 

Georgia and New Hampshire contain the greatest 

number of watersheds with private forests that are 

important to drinking water and are threatened by high 

levels of development. More than half of the West’s 

top 50 threatened watersheds are located in California. 

Analysis at this scale provides information useful for 

States, counties, and national forests; however, similar 

to other Forests on the Edge reports and other national 

assessments, the final map results are not intended 

to assess individual watersheds of interest. Our work 

also does not explicitly consider water scarcity or 

groundwater; as a result, much of the arid West and 

other areas that rely on groundwater sources for drinking 

water, though they may face challenges with water supply 

issues, do not have high levels of surface drinking water 

importance reflected in this report.

Private forests play a critical role in 

the health of the Mississippi River, 

flowing from Minnesota (top) to 

Louisana (bottom) and the Gulf of 

Mexcio.

thinkstockphotos.com
22 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327.

In response to development threats to water quality, 

planners, communities, and private landowners are 

using a range of strategies to preserve water quality and 

healthy freshwater ecosystems. Developers can design 

housing and roads to minimize impacts on water quality, 

homeowners can manage their properties to protect 

water resources, and innovative payment schemes and 

management partnerships can allow municipalities and 

water users to invest in forest stewardship on public 

and private lands. Further investing and refining these 

strategies to protect water resources in the face of housing 

development will be essential to conserving private forest 

lands and the high-quality water resources they provide 

nationwide.

ACKNOWLEDGEMENTS

W

e wish to thank the following people for their 

helpful reviews: Tom Brown, Forest Service 

Rocky Mountain Research Station; Polly Hays and 

Claire Harper, Forest Service Rocky Mountain Region; 

and Steve McNulty, Forest Service Southern Research 

Station. We gratefully acknowledge David M. Theobald 

of Colorado State University for generously providing 

data on housing density projections. Stacy Chesney and 

Don Kennedy at Denver Water supplied information 

about water partnerships. Al Todd with the Forest Service 

helped lay the framework for this report as an author of 

the Forests to Faucets project.

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2009. Private forests, public benefits: increased 

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26 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327.

APPENDIX: Methodology

T

he analyses presented in this report are the same as 

those conducted by Weidner and Todd (2011) when 

they examined freshwater provided by the Nation’s 

forests, both private and public, at the 12-digit hydrologic 

unit code (HUC) sub-watershed2 scale. Several of the 

datasets have been updated since the original Forests to 

Faucets analysis (Weidner and Todd 2011). Newer data 

versions were used for this publication and therefore 

results vary slightly from the original analysis. Below we 

provide more information on their methods and updated 

data layers, and we refer readers interested in obtaining 

more information to the original report (Weidner and 

Todd 2011).

Updated Data

We updated watershed boundaries from USDA NRCS 

(2009a) to USDA NRCS (2009b). We updated the results 

of the original step 1 analysis into the new watershed 

boundaries by first converting the new watershed 

boundaries (NRCS 2009b) to a point file. Next, we ran a 

spatial join between the new points and the original step 1 

forests to faucets data. This gave the attributes of step 1 

forests to faucets data to the new watersheds. Steps 2 and 

3 were rerun according to the original methods below 

for this publication using updated data layers for forest 

ownership and housing density datasets (CBI 2010; Fry 

and others 2011; NRCS 2009b; Theobald 2008.) 

To focus this updated analysis on watersheds with private 

forests, we analyzed only watersheds with at least 10 

percent forested land and more than 50 acres of private 

forest. All other watersheds were labeled as “insufficient 

private forest for this analysis.”

Methods

Weidner and Todd used a three-step approach to produce 

three unique but interrelated sets of model outputs, as 

shown in Figure A-1. These data were combined to 

produce a measure of the importance of private forest to 

drinking water:

PriFIMPn = (Qn) * PRn * (PriFORn) ,

Where PriFIMPn (the measure of private forest 

contribution to surface drinking water) is equal to the 

combination of Qn, a measure of water supply; PRn, a 

measure of drinking water demand over the landscape; 

and PriFORn, the proportion of a watershed that is 

privately owned forestland. Below, we explain each step 

in the model in succession.

Step 1: Important Areas for Surface Drinking Water

To calculate an index of surface drinking water 

importance by sub-watershed, Weidner and Todd 

considered the land’s contribution to water supply 

(volume), the landscape surface flow patterns and the 

natural processes that affect water quality, as well as 

downstream drinking water demand (consumption). In 

its most basic form, the index of importance to surface 

drinking water (IMP) model [was] broken down into two 

parts:

IMPn = (PRn) * (Qn), 

where IMPn is the index of importance to surface 

drinking water for watershed n; PRn is the risk-based 

drinking water protection model for each watershed n; and 

Qn is the mean annual water supply for each watershed 

n. For IMP, the final non-zero outputs were split into 100 

quantiles, or 100 groups with approximately 1 percent 

of the data each. This ranks the relative importance from 

least, 0, to most, 100.

The risk-based drinking water protection model, PRn, 

models the magnitude of demand and the flow patterns 

of water to sites of withdrawal for use, while the mean 

annual water supply, Qn, represents the supply of water 

and weights a sub-watershed based on how much water 

supply is generated on that land. The model therefore 

shows which areas have the highest potential to affect 

water quality through the input of sediments and 

contaminants from the land, because it includes areas 

important for providing water (water supply), areas where 

drinking water is removed for use (water demand), and 

lands that connect the water supply and demand. 

Locations and number of people served by each intake 

were provided by the U.S. EPA Safe Drinking Water 

Information System (SDWIS) (EPA 2009). In these 

analyses only the surface water springs, surface water 

Weidner and Todd generally used the term sub-watershed to describe 

the 12-digit HUC but they sometimes also used “watersheds.” 

With the exception of this appendix, elsewhere in this report we 

have simplified the terminology to refer to these 12-digit HUCs 

as watersheds. In this appendix we retain Weidner and Todd’s 

terminology.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327. 27

intakes, surface water reservoirs, and surface water 

infiltration galleries were used. Wells were included only 

where the SDWIS database specified that groundwater 

was directly influenced by surface water. In general, 

groundwater wells were not included. Consecutive 

connections, treatment plants, sampling stations, and 

non-piped drinking water were not included in the 

analyses. All drinking water facilities are referred to as 

“surface drinking water intakes” or simply “intakes.” The 

population served at each intake was derived by dividing 

the number of people served in a drinking water system 

by the number of intakes in the system. Intake locations 

are sited at the point of water extraction, not at point of 

use.

The model component representing the critically 

important areas close to intakes and the upstream areas 

from where the water flows is represented by this drinking 

water protection model (PR),

PRn = Σ (Wi * Pi),

where PRn is the drinking water protection model for each 

sub-watershed n; Pi is the population served by intakes in 

the ith downstream sub-watershed from sub-watershed n; 

and Wi is the proportional weight for ith downstream subwatershed from sub-watershed n.

As contaminants move through streams and rivers, 

they are affected by many processes including dilution, 

dispersion, decay, and deposition. Weidner and Todd 

represented these processes in a generalized way for 

multiple drinking water contaminants including sediment, 

using an exponential decay relationship to represent 

the relationship of distance from intake and relative 

importance to the surface drinking water. The proportional 

weights, Wi, for the ith sub-watershed away from an 

intake in watershed n are based on the equation,

W = (1 – 0.01) ^ (d), 

where W is the proportional weight and d is the distance 

from the intake, with each sub-watershed assumed to 

be 25 km (15 mi) in stream length distance to the next 

sub-watershed. Therefore, for every sub-watershed on 

the map, the surface drinking water protection model 

value equals the number of people served by intakes 

in that sub-watershed plus a fraction of the population 

served by downstream intakes. In this way it considers 

that risk to an intake declines with distance, and that peak 

concentrations decline moving downstream.

Because of the national focus of this assessment, and 

recognition of the heterogeneity of water yield across the 

[United States], we used Brown and others’ (2008) water 

balance based modeling of mean annual water supply to 

create a weighting of geography for water supply. Brown 

and others estimated water supply across the United 

States as precipitation minus evapotranspiration across the 

period 1953 to 1994. The final model of surface drinking 

water importance (IMP, Fig. A-2) combines the surface 

drinking water protection model (PRn),capturing the 

flow of water and water demand, with Brown and others’ 

(2008) model of mean annual water supply (Qn). 

Private	
  Forests	
  

All	
  Forests	
  

STEP	
  3	
  

Forest	
  Threat:	
  

Expected	
  Increase	
  in	
  

Housing	
  Density	
  

Land	
  Cover/Ownership	
  

Surface	
  drinking	
  water	
  	
  

intake	
  locaFons	
  &	
  pop

Mean	
  annual	
  water	
  

supply

STEP	
  1	
  

STEP	
  2	
  

Forests

Private	
  Forests	
  

MODEL	
  INPUTS	
  

Threatened	
  forests'

importance	
  to	
  surface	
  

drinking	
  water

Surface	
  drinking	
  water	
  	
  

importance	
  areas

Forest	
  importance	
  to	
  

surface	
  drinking	
  water

MODEL	
  OUTPUTS	
  
28 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327.

Step 2: Forest Importance to Surface Drinking Water

The surface drinking water importance index (IMP) 

described in the previous section shows the relative 

importance of geographic areas across the country for 

surface drinking water, but it does not distinguish between 

land cover types. To determine the extent to which private 

forests in particular are currently protecting these areas, 

Weidner and Todd created an index of forest importance 

to surface drinking water by weighting the surface 

drinking water importance index, IMP, by the percentage 

of forest in each watershed. This is represented by:

FIMPn = (IMP n) * (FOR n) / 100, 

where FIMPn is the index of forest importance to surface 

drinking water, IMPn is the surface drinking water 

importance index, and FORn is the percentage of forest 

land in each sub-watershed n. Both IMP and FOR range 

from 0 to 100, so the final FIMP values also fall between 

0 and 100. In this way areas with small amounts of forests 

will have a small FIMP value no matter what IMP value 

the sub-watershed had. Similarly, areas with small IMP 

values will have small FIMP values no matter what FOR 

values they had. Only areas with both high IMP and high 

FOR values will also have high FIMP values.

The data used to distinguish between non-forest, protected 

forest, and private forest were derived from the National 

Land Cover Dataset (Fry and others 2011), data on 

National Forest System land locations (USDA Forest 

Service 2009), and the Protected Areas Database (PAD) 

(CBI 2012). Classes 41, 42, 43, and 95 from the National 

Land Cover Dataset were considered forest. All remaining 

areas were labeled as non-forest. We used NFS data for 

National Forest system holdings, and the land owner 

description in PAD to determine other protected lands 

(other Federal Land, Native American Land, State Land, 

Local Land, Private Conservation Land). The index of 

forest importance to surface drinking water was repeated 

for each forest type; thus, for private forest [the model is]:

PriFIMPn = (IMPn) * (PriFORn).

Step 3: Threats to Private Forests Important for Surface 

Drinking Water

In this third and final step of the analysis, Weidner and 

Todd identify private forest areas important for surface 

drinking water that are likely to be affected by future 

increases in housing density, insects and diseases, and 

wildland fire. The procedures used for this step were 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-327. 29

tiered from similar analyses undertaken by the Forests on 

the Edge project (Stein et al. 2009). 

To begin, we multiplied the index of forest importance 

to surface drinking water (the output from step 2) by a 

value 0–100 that represents the percentage of a watershed 

that is highly threatened by housing development. This is 

expressed as,

(PriFIMPn) *(THRn) / 100,

where THRn is the percentage of watershed n that is 

“highly threatened” by development. To measure the 

threat of development, Weidner and Todd focused on 

increases in housing density, which reflects a change 

in the landscape more accurately than does population 

density alone. Projections of future housing density 

increases on rural forest lands (as defined below) were 

used to quantify the threat of development across all U.S. 

forests.

The Forests to Faucets analysis used data layers produced 

by Stein and others (2009) and Theobald (2008), who 

condensed 12 categories of housing density produced by 

Dave Theobald’s SERGoM v3 housing density model 

for 2000 and 2030 (Theobald 2005) into 3 categories. 

We subtracted the 2000 values from values expected for 

2030 to highlight areas with expected increase in housing 

development over this 30-year period. 

The three categories (Stein and others 2009) were based 

on a review of the literature on impacts of development on 

benefits provided by forests. The categories were: rural 1 

(fewer than 40 acres per housing unit), rural 2 (10–40 

acres per housing unit), or exurban/urban (fewer than 10 

acres per housing unit). Any change from rural 1 to rural 

2, rural 2 to exurban/urban, or rural 1 to exurban/urban 

between 2000 and 2030 were considered as areas “highly 

threatened” by development.

Forests on the Edge is a project of the U.S. Department 

of Agriculture, Forest Service, State and Private Forestry, 

Cooperative Forestry staff, in conjunction with Forest Service 

Research and Development, National Forest System staff, 

universities, and other partners. The project aims to increase 

public understanding of the contributions of and pressures on 

America’s forests, and to create new tools for strategic planning. 

The first report (Stein and others 2005) identified private 

forested watersheds in the conterminous United States most 

likely to experience increased housing density. Subsequent 

reports have provided more in-depth discussion and data on 

related topics, including: development pressures on America’s 

national forests and grasslands (Stein and others 2007), impacts 

of increased housing density and other pressures on private 

forest benefits (Stein and others 2009), threats to at-risk species 

(Stein and others 2010), sustaining America’s urban trees and 

forests (Nowak and others 2010), understanding and preparing 

for wildfire in the wildland-urban interface (Stein and others 

2013), and threats to forests on U.S. Pacific and Caribbean 

islands (Stein and others 2014). This report presents an 

overview of the role of forests in providing clean drinking water 

and the impacts of increasing housing density on private forests 

and water quality.

For further information on Forests on the Edge, contact:

Coordinator, Open Space Initiative, U.S. Forest Service, 

Cooperative Forestry staff, 1400 Independence Avenue, SW, 

Mailstop 1123, Washington, DC 20250-1123. 

(202) 205-1389 http://www.fs.fed.us/openspace/fote/.

FORESTS ON THE EDGE

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