1

Prepared in cooperation with the U.S. Forest Service

Connecticut Highlands Technical Report – Documentation 

of the Regional Rainfall-Runoff Model

Open-File Report 2010–1187

U.S. Department of the Interior

U.S. Geological Survey
2

Cover. Photograph shows Breadloaf Mountain in Sharon, Connecticut. (Photograph by John Mullaney, 

U.S. Geological Survey)
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Prepared in cooperation with the U.S. Forest Service

Connecticut Highlands Technical Report – Documentation 

of the Regional Rainfall-Runoff Model

By Elizabeth A. Ahearn and David M. Bjerklie

Open-File Report 2010–1187

U.S. Department of the Interior

U.S. Geological Survey
4

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

For product and ordering information:

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

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its natural and living resources, natural hazards, and the environment:

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

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Suggested citation:

Ahearn, E.A., and Bjerklie, D.M., 2010, Connecticut Highlands Technical Report – Documentation 

of the Regional Rainfall-Runoff Model: U.S. Geological Survey Open-File Report 2010–1187, 43 p., 

at http://pubs.usgs.gov/of/2010/1187/.

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 material contained within this report.
5

Contents

SECTION 1. WATER RESOURCES ............................................................................................................................ 7

Purpose and Scope.................................................................................................................................................... 7

1.1. Groundwater – Aquifers and Community Wells.................................................................................................. 7

1.2. Groundwater-Level Data .................................................................................................................................... 9

1.3. Surface Water - Streamflow Data and Base-Flow Estimates from Streamflow Records.................................. 11

Base-Flow Estimates from Streamflow Records................................................................................................... 14

Additional Streamflow Data .................................................................................................................................. 15

1.4. Water Withdrawals and Water Use .................................................................................................................. 16

Estimating Domestic Water Use........................................................................................................................... 17

1.5. Water Quality ................................................................................................................................................... 18

1.6. Watershed Budget ........................................................................................................................................... 21

Model Input Data and Methods of Analysis .......................................................................................................... 21

Water-Budget Estimates....................................................................................................................................... 28

Changes in Water-Budget Estimates for High- and Low-Constraint Build-Out Scenarios .................................... 31

Model Verification and Comparison of Results..................................................................................................... 35

Selected References ................................................................................................................................................... 38

Appendix 1. Data for Documenting Information Provided in the Connecticut Highlands Technical Report –

Documentation of the Regional Rainfall-Runoff Model ................................................................................................ 41

Appendix 2. Estimating Effective Impervious Area Using the Connecticut Effective Impervious Area (CT EIA) 

Equation ...................................................................................................................................................................... 42

References .................................................................................................................................................................. 43

Figures

Figure 1. Map showing location of 196 community wells with safe yields of more than 0.001 Mgal/d operating in 

2000 in the Highlands study area .............................................................................................................................. 8

Figure 2. Map showing location of U.S. Geological Survey groundwater wells in the Highlands study area....... 10

Figure 3. Screenshot showing groundwater data for Connecticut (web site) ...................................................... 11

Figure 4. Map showing location of U.S. Geological Survey streamflow-gaging stations in the Highlands study 

area with 5 or more years of continuous record through September 2005 .............................................................. 13

Figure 5. Screenshot showing surface-water data for Connecticut (web site)..................................................... 16

Figure 6. Map showing location of U.S. Geological Survey water-quality stations in the Highlands study area.. 19

Figure 7. Screenshot showing water-quality data for Connecticut (web site)...................................................... 20

Figure 8. Map showing boundaries of the HUC 10 watersheds in the Highlands watershed model ................... 23

Figure 9. Map showing boundaries of HUC 12 subwatersheds in the Highlands watershed model.................... 24

Figure 10. Map showing regional water budget for the Highlands, 2006............................................................... 29

Figure 11. Map showing variations in base flow by subwatersheds ...................................................................... 30

Figure 12. Graph showing changes in water-budget components in relation to percent change in effective 

impervious surface as modeled for the Highlands ................................................................................................... 31

Figure 13. Maps showing percent impervious surface cover for current conditions, high-constraint and lowconstraint build-out scenarios used in the Precipitation Runoff Modeling System (PRMS) of the Highlands study 

area ......................................................................................................................................................... 33

Figure 14. Maps showing groundwater withdrawals for current conditions, high-constraint and low-constraint 

build-out scenarios of the Highlands study area ...................................................................................................... 33
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Figure 15. Maps showing changes in runoff from current conditions to high- and low-constraint build-out scenarios 

in the Highlands study area using the 70-percent effective impervious area ........................................................... 34

Figure 16. Maps showing changes in total streamflow from current conditions to high- and low-constraint build-out 

scenarios in the Highlands study area using the 70-percent effective impervious area........................................... 34

Figure 17. Maps showing changes in base flow from current conditions to high- and low-constraint build-out 

scenarios in the Highlands study area using the 70-percent effective impervious area........................................... 35

Figure 18. Map showing predicted change in runoff and base flow at the HUC 12 watershed scale under the 

low-constraint development scenario....................................................................................................................... 36

Tables

Table 1. U.S. Geological Survey groundwater observation wells, Highlands study area, 2005 ......................... 9

Table 2. List of USGS streamflow-gaging stations in the Highlands study area including information on the 

station’s latitude, longitude, town location, year of record, period of record, and drainage area.............................. 12

Table 3. U.S. Geological Survey water-quality stations with 5 or more years of record in or near the 

Highlands study area, 2005 ..................................................................................................................................... 18

Table 4. Climate stations used in the Highlands Precipitation-Runoff Modeling System (PRMS) model ......... 21

Table 5. Description of HUC 10 subwatersheds in the Highlands study area .................................................. 25

Table 6. Description of HUC 12 subwatersheds in the Highlands study area .................................................. 26

Table 7. Annual mean flows for U.S. Geological Survey gaging stations compard to annual mean flows 

simulated from the PRMS model ............................................................................................................................. 37
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Connecticut Highlands Technical Report – Documentation 

of the Regional Rainfall-Runoff Model

SECTION 1. WATER RESOURCES

Purpose and Scope

This report provides the supporting data and describes the data sources, methodologies, and 

assumptions used in the assessment of existing and potential water resources of the Highlands of 

Connecticut and Pennsylvania (referred to herein as the “Highlands”). Included in this report are 

Highlands groundwater and surface-water use data nd the methods of data compilation. Annual mean

streamflow and annual mean base-flow estimates from selected U.S. Geological Survey (USGS) gaging 

stations were computed using data for the period of record through water year 2005. The methods of 

watershed modeling are discussed and regional and sub-regional water budgets are provided. 

Information on Highlands surface-water-quality trends is presented. USGS web sites are provided as 

sources for additional information on groundwater levels, streamflow records, and ground- and surfacewater-quality data. Interpretation of these data and the findings are summarized in the Highlands study 

report (Connecticut–Pennsylvania Highlands Regional Study, written commun., E.A. Ahearn, 2008).

1.1. Groundwater – Aquifers and Community Wells

Groundwater systems store and transmit water and enable communities and individual well 

owners to obtain water without constructing pipelines or storage facilities. From a regional perspective, 

the groundwater resource appears ample. Throughout the Highlands, however, the availability of 

groundwater varies widely. Understanding the limits of the aquifers (groundwater sources of water) is a 

key aspect of a groundwater assessment. 

A total of 196 community wells are in the Highlands study area. Community wells are 

defined as public drinking-water systems that serve at least 25 residents throughout the year. The 

locations these wells with safe yields of more than 0.001 million gallons per day (Mgal/d) operating in 

2000, and information on the type of aquifer are shown in figure 1 and listed in Appendix 1, 

CT_community_wells. To describe and assess the groundwater resource, information on the 

hydrogeology of the aquifers and information about groundwater use (safe yield and demand) were 

compiled. An aquifer is defined as any geological formation such as sand, soil, gravel, or porous rock 

that can store and supply ground water to wells and springs. High-yielding aquifers composed of coarsegrained stratified sediment such as sand and gravel are limited in areal extent. Low-yielding till aquifers 

cover most of the Highlands area. The term "safe yield" is defined as the maximum dependable quantity 

of water per unit of time that may flow or be pumped continuously from a source of supply during a 

critical dry period without consideration of available water limitations. (State of Connecticut, 2008). 

Groundwater safe yields—as opposed to well yields—were compiled for community wells used for 

municipal supply, industrial, commercial, irrigation, and mining uses. Generally, well yield is the 

withdrawal rate of water from the well (groundwater system) and is a function of the pump capacity. 
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Figure 1. Location of 196 community wells with safe yields of more than 0.001 Mgal/d operating in 2000 in the 

Highlands study area. Areas not shown as underlain by stratified sediment are underlain by till.
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Well yields for community wells are not readily available, and therefore, information on safe 

yields was used to estimate annual groundwater withdrawals from the community wells. In general, 

these withdrawals are about 75 percent of the total safe yield. During summer, groundwater withdrawals

from the smaller community systems can reach 90 to 100 percent of their safe yields (Steve Messer, 

Connecticut Department of Public Health, oral commun., May 9, 2008). 

Forty-six of the 196 community wells are in stratified glacial deposits. The safe yields of wells 

in stratified glacial deposits ranged from 0.06 to 1.14 Mgal/d; the combined safe yield in 2000 was 

15.3 Mgal/d (Connecticut Department of Public Health, Water Supply Division, community well 

database, written commun., 2001). A total of 150 of the 196 wells are in bedrock (both crystalline and 

carbonate). The safe yields of most wells in bedrock ranged from 0.001 to 0.162 Mgal/d; however, one 

bedrock well had a reported safe yield of 0.43 Mgal/d. Although there are substantially more wells in 

bedrock than in stratified glacial deposits in the study area, the combined safe yield of wells in bedrock 

was 3.92 Mgal/d in 2000, considerably less than the combined safe yield of wells in stratified glacial 

deposits. The annual groundwater withdrawals from the community wells in the Highlands were

estimated to be about 14.4 Mgal/d (assumed as 75 percent of the total safe yield).

1.2. Groundwater-Level Data

Groundwater levels from selected USGS network observation wells (fig. 2) provide information 

on how water levels within the Highlands aquifers respond to seasonal changes in climate, changes in 

recharge patterns, and groundwater withdrawals. Groundwater levels typically are highest in winter and 

early spring because of reduced evapotranspiration, low temperatures, snowmelt, and spring rains that 

recharge the aquifers. Typically groundwater levels start to decline as summer begins and continue to 

decline through late fall because more water evaporates from the land surface and transpires from plants 

reducing recharge. Groundwater levels are typically lowest in late fall. A list of the USGS groundwater

observation wells within the Highlands study area is presented in table 1. The locations of the 

groundwater observation wells are shown in figure 2. Each observation well is identified by 

(1) a 15-digit number based on latitude and longitude (USGS groundwater well ID) and (2) a local 

number based on a town code and a sequential number (Local ID). Typically, water-level measurements 

are made monthly in these wells. 

Table 1. U.S. Geological Survey groundwater observation wells, Highlands study area, 2005.

USGS

Groundwater Well ID Local ID Town

Years 

of 

record

Well depth 

from land 

surface 

(feet) Aquifer

413007073250501 BD8 Brookfield 40 53.0 glacial, sand & gravel

414649072574401 BU144 Burlington 10 13.1 glacial, sand & gravel

414704072580501 BU143 Burlington 10 21.3 glacial, sand & gravel

414615072581601 BU2 Burlington 50 42.7 unknown

415649072494801 GR328 Granby 25 22.0 glacial, till 

415643072502201 GR330 Granby 24 22.0 glacial, sand & gravel

415647072495901 GR329 Granby 24 22.0 glacial, till 

415653072501701 GR331 Granby 23 31.5 glacial, sand & gravel

420125073193001 NOC7 North Canaan 48 12.0 glacial, sand & gravel

415925073252001 SY15 Salisbury 40 27.4 glacial, stratified drift

415956073241501 SY24 Salisbury 20 28.7 glacial, stratified drift

415559073253401 SY23 Salisbury 19 51.0 glacial, till 
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Figure 2. Location of U.S. Geological Survey groundwater wells in the Highlands study area. 
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Groundwater-level data collected by the USGS, associated well information, water-level 

hydrographs, and site maps can be accessed from the USGS National Water Information System, 

(NWIS), (U.S. Geological Survey, 2008a) (fig. 3).

Figure 3. Groundwater data for Connecticut (web site) (U.S. Geological Survey, 2008).

1.3. Surface Water - Streamflow Data and Base-Flow Estimates from Streamflow Records

Systematic records on daily streamflow are fundamental to streamflow analyses and forecasting 

water availability at national, regional, and local scales. Streamflow records for 37 streamflow-gaging 

stations in the Highlands study were retrieved from the USGS NWIS. The 37 stations have drainage 

areas ranging from 3 to 996 square miles (mi2 ); the median drainage area is 38 mi2. Each of the stations 

had 5 or more years of daily streamflow record through September 2005. A list of streamflow-gaging 

stations in the Highlands study area including information on the station latitude, longitude, town 

location, years of record, period of record (month and years of operation), and drainage area (area of 

watershed above the stream gage) is provided in table 2. The locations of streamflow-gaging stations are 

shown in figure 4.

The highest and lowest annual mean streamflow for the period of record and the mean annual

streamflow at each gaging station are provided in Appendix 1, CT_streamflow. The highest and lowest 

annual mean streamflows for the period of record were obtained by ranking the individual annual-mean 

streamflows. The mean annual streamflow was obtained by dividing the sum of all the individual (water 

year) annual mean flows by the number of water years. Mean annual streamflow in 
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Table 2. List of USGS streamflow-gaging stations in the Highlands study area including information on the 

station’s latitude, longitude, town location, year of record, period of record, and drainage area.

[ID, identification number; USGS, U.S. Geological Survey; mi2

, square miles; bold italic type indicates station with no or 

minimum flow alteration.]

Map

ID 

(from 

fig. 4)

USGS 

station 

number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees) River name Town

Years 

of 

record Period of record 

Drainage 

area (mi2)

1 01186000 41.9629 -73.0176 West Branch 

Farmington River

Barkhamsted 50 Oct 1955 - Sep 2005 131

2 01186100 41.9309 -73.0818 Mad River Winchester 13 Oct 1956 - Sep 1969 18.5

3 01186400 41.9783 -73.0456 Sandy Brook Colebrook 9 Oct 1967 - Sep 1976 34.9

4 01186500 41.9673 -73.0340 Still River Colebrook 55 Oct 1948 - Jul 1967, 

Jul 1969 -Sep 2005

85.0

5 01187000 41.9537 -73.0137 West Branch 

Farmington River

Barkhamsted 26 Oct 1929 - Sep 1955 217

6 01187300 42.0373 -72.9390 Hubbard River Hartland 66 Jan 1938 - Sep 1955, 

Oct 1956 - Sep 2005

19.9

7 01187400 42.0343 -72.9298 Valley Brook Hartland 32 Oct 1940 - Sep 1972 7.03

8 01187680 41.8723 -72.9062 Cherry Brook Canton 5 Oct 1966 - Sep 1971 8.23

9 01187800 41.8207 -72.9701 Nepaug River New 

Hartford

52 Oct 1921 - Sep 1955, 

Oct 1957 - Sep 1972, 

Oct 1998 - Sep 2001

23.5

10 01187980 41.7993 -72.9254 Farmington River Avon 15 Nov 1962 - Sep 1977 360

11 01188000 41.7862 -72.9648 Burlington Brook Burlington 74 Oct 1931 - Sep 2005 4.10

12 01188090 41.7559 -72.8868 Farmington River Farmington 28 Oct 1977 - Sep 2005 378

13 01189000 41.6732 -72.9007 Pequabuck River Bristol 64 Oct 1941 - Sep 2005 45.8

14 01189200 41.8701 -72.8243 Stratton Brook Simsbury 5 Oct 1966 - Sep 1971 5.13

15 01189210 41.8732 -72.8234 Hop Brook Simsbury 5 Oct 1966 - Sep 1971 10.7

16 01189390 41.9543 -72.7795 East Branch Salmon 

Brook

Granby 13 Oct 1963 - Oct 1976 39.5

17 01189500 41.9373 -72.7762 Salmon Brook East Granby 17 Oct 1946 - Sep 1963 67.4

18 01189995 41.9084 -72.7607 Farmington River Simsbury 34 Oct 1971 - Sep 2005 577

19 01198500 42.0240 -73.3418 Blackberry River North 

Canaan

22 Oct 1949 - Oct 1971 45.9

20 01199000 41.9573 -73.3693 Housatonic River Salisbury 93 Oct 1912 - Sep 2005 634

21 01199050 41.9423 -73.3910 Salmon Creek Salisbury 44 Oct 1961 - Sep 2005 29.4

22 01199200 41.8243 -73.4301 Guinea Brook Sharon 21 Aug 1960 - Oct 1981 3.50

23 01199290 41.7267 -73.4819 Housatonic River Kent 4 May 1985 - Dec 1989 756

24 01200000 41.6589 -73.5289 Tenmile River Wingdale, 

NY

73 Oct 1929 - Sep 1987, 

Oct 1991 - Sep 2005

203

25 01200500 41.6531 -73.4898 Housatonic River New Milford 65 Oct 1940 - Sep 2005 996

26 01201190 41.6079 -73.4246 West Aspetuck River New Milford 10 Oct 1962 - Sep 1972 23.8

27 01201500 41.5201 -73.4182 Still River New Milford 35 Oct 1931 - Sep 1966 67.5

28 01201510 41.5367 -73.4178 Still River New Milford 5 Oct 1966 - Sep 1971 69.6

29 01201930 41.7895 -73.2590 Marshepaug River Goshen 14 Oct 1967 - Oct 1981 9.24

30 01202500 41.7233 -73.2936 Shepaug River Litchfield 6 Oct 1960 - Sep 1966 38.0

31 01202501 41.7192 -73.2933 Shepaug River Litchfield 5 Oct 2000 - Sep 2005 38.1

32 01203000 41.5497 -73.3303 Shepaug River Roxbury 46 Oct 1930 - Sep 1971

Mar 2001 - Sep 2005

132

33 01205600 41.8009 -73.1234 W Brook Naugatuck 

River

Torrington 40 Oct 1956 - Sep 1992, 

Oct 1993 - Apr 1997

33.8

34 01205700 41.8034 -73.1179 East Branch 

Naugatuck River

Torrington 41 Oct 1956 - Apr 1997 13.6

35 01206000 41.7043 -73.0643 Naugatuck River Thomaston 29 Oct 1930 - Sep 1959 71.0

36 01206400 41.7295 -73.0532 Leadmine Brook Harwinton 13 Oct 1960 - Oct 1973 19.6

37 01206500 41.7018 -73.0573 Leadmine Brook Thomaston 29 Oct 1930 - Sep 1959 24.3
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Figure 4. Location of U.S. Geological Survey streamflow-gaging stations in the Highlands study area with 5 or 

more years of continuous record through September 2005.
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cubic feet per second per square mile and mean annual streamflow in inches over the drainage basin 

also are included. These values allow planners and researchers to compare the hydrologic characteristics 

of watersheds regardless of differences in their size. Mean annual streamflow in cubic feet per second 

per square mile is the annual average of the cubic feet of water flowing per second past a gaging station 

for each square mile of area drained above the station, and assumes runoff to be distributed uniformly in 

time and area. Mean annual streamflow in inches over the drainage basin is the depth to which the 

drainage area would be covered if all the annual streamflow were uniformly distributed over it. The 

mean annual streamflow in inches is commonly compared to mean annual precipitation. On an average 

annual basis, the Highlands receives about 49 in. of precipitation. The mean annual streamflow in 

inches over the drainage basin can be thought of as the amount of water that enters the basin as 

precipitation and leaves the basin as streamflow, after being diminished by evapotranspiration and 

ground- or surface-water withdrawals. The mean annual streamflow over the Highlands study area is 

about 25 in. 

Base-Flow Estimates from Streamflow Records

Total streamflow can be separated into two components—runoff and base flow. Runoff consists 

of overland runoff, interflow (water that travels laterally within the unsaturated zone during or directly 

after a precipitation event and discharges to streams without recharging the water table), and 

precipitation that falls directly on surface-water bodies. Base flow is composed of groundwater

discharge. The amount of base flow in a stream is a measure of the infiltration capacity and yield of the 

underlying groundwater system. Base-flow conditions within a watershed can be equated to the amount 

of groundwater recharge in the watershed when averaged over the long term. Runoff and base flow are 

two important components of the water-budget analysis. Changes in runoff or base flow are strong 

indicators of changes in watershed conditions.

Hydrograph separation techniques are used to separate surface (runoff) and groundwater flows 

(base flow). Most hydrograph-separation techniques identify times of negligible runoff where base flow 

(groundwater discharge) equals streamflow; base flow is then interpolated between these periods. The 

automated hydrograph-separation technique called PART used in this report and described by Rutledge 

(1998) uses streamflow partitioning to separate the runoff and groundwater components of streamflow. 

The principal assumption of this method is that base flow is equal to streamflow on days that fit a 

requirement of antecedent recession; that is, streamflow on any one day is greater than or equal to 

streamflow on the following day with a subsequent decline of not more than one-tenth of a logarithmic 

cycle. When days do not fit this requirement, the larger declines or increased streamflow are assumed to 

result from changes in the runoff portion of streamflow, and the groundwater portion of streamflow is 

determined by linear interpolation.

The automated hydrograph-separation technique–PART (Rutledge, 1998)–was used to estimate 

annual mean base flow for each water year at 36 of 37 gaging stations in the Highlands study area. The 

hydrograph-separation technique was not applied to one station (Housatonic River at Gaylordsville, 

USGS 01200500) because of the known large extent of flow alteration. The hydrograph-separation 

technique is intended for analyzing a groundwater-flow system that is considered natural, with 

negligible anthropogenic effects to the hydrologic flow regime. Only 28 percent of the stations (10 of 

36) have natural streamflow (no water withdrawals or point-source releases in the drainage area above 

the gaging station); this data set is not large enough to characterize base-flow conditions throughout the 

Highlands. Subsequently, the hydrograph-separation technique was used to estimate annual mean base 

flows at the remaining 72 percent of the stations (26 of 36) in the Highlands region, many of which are 

affected by water withdrawals, streamflow diversions, regulation, point-source discharges, and (or) 
15

reservoir operations. The results from the hydrograph-separation technique from the altered basins help 

determine if streamflow will be sufficient for water-quality and aquatic-habitat needs. 

Mean annual base-flow estimates for the 36 stations described above are provided in 

Appendix 1, CT_streamflow. Mean annual base flow was obtained by dividing the sum of all the 

individual annual base flows by the number of recorded water years for each gaging station. Calculated 

base flow ranged from about 58 to 86 percent of streamflow; the average was 70 percent for the 36 

stations. For the 10 basins with natural flow, base flows ranged from 58 to 78 percent of streamflow. 

For the 26 basins with altered flows, base flows are slightly higher than the basins with natural flows, 

ranging from 62 to 86 percent of streamflow. To some extent, the higher base-flow estimates from the 

PART program may be explained by regulated releases that artificially increase base flow; the PART 

program interprets most regulated releases as base flow. 

In Connecticut, the highest base-flow percentages and continuous groundwater discharges to 

streams during dry weather and droughts generally are associated with those basins underlain with a 

high percentage of thick deposits of stratified glacial deposits. Not all the basins with natural flow in the 

Highlands appear to follow this pattern. With only 10 stations in the data set, the range of each geologic 

characteristic is very limited. For example, only Bunnell (Burlington) Brook has more than 7 percent 

stratified glacial deposits. Although the percentage of base flow calculated by the PART program at this 

site is not the highest value for the 10 stations analyzed, the stream has never been dry. Another 

explanation for why some of the streams in the Highlands with low percentages of stratified glacial 

deposits have some of the highest base flow in the region could be that most of these basins are very 

small and the groundwater drainage area may not coincide with the surface-water drainage area (defined 

by the topographic drainage divide). Therefore, base flow may be sustained by ground water from 

aquifer areas outside the topographic drainage divide. Lower base-flow percentages generally are 

associated with basins containing high percentages of impervious cover or water withdrawals. 

For basins with altered flows, the highest base-flow percentages generally are associated with 

those basins that have point-source discharges (e.g., wastewater releases from sewage-treatment 

facilities or industries), are underlain with a high percentage of thick deposits of glacial deposits, and 

are waste-receiving streams. 

Additional Streamflow Data

Detailed records of stream stage, discharge, streamflow statistics, peak discharge, long-term 

streamflow hydrographs, and descriptive surface-water site information for gaging stations in the 

Highlands or other areas of Connecticut can be accessed from the USGS NWIS (U.S. Geological 

Survey, 2008b) (fig. 5). 
16

Figure 5. Surface-water data for Connecticut (web site) (U.S. Geological Survey, 2008).

1.4. Water Withdrawals and Water Use

In order to evaluate existing hydrologic conditions within the Highlands study area, an 

assessment of the volume of water withdrawn from aquifers and reservoirs was performed. Site-specific 

and aggregated data on withdrawals of ground water and surface water in 2005 were collected and 

compiled with other reported and estimated water-use data. In Connecticut, three state agencies—

Department of Environmental Protection (DEP), Department of Public Health (DPH), and Department 

of Public Utility Control (DPUC)—are involved in the collection and compilation of water-use 

information. 

The Connecticut DEP has the regulatory authority to allocate groundwater and surface-water 

resources through the Water Division Program. In Connecticut, a diversion permit is required for 

groundwater or surface-water withdrawals greater than or equal to 50,000 gallons per day (gal/d) on any 

given day. There are two types of water diversions – registered diversions (“grandfathered” diversions) 

and permitted diversions. The state has no regulatory control over registered diversions; it does have 

control over permitted diversions. Owners of permitted diversions are required to record water 

withdrawals daily and report total water withdrawals to DEP annually. Individuals with registered 

diversions are requested to maintain daily logs of their water use but are not required to report water 

withdrawals to any state agency (as of January 2008). In the Connecticut Highlands, there are 

162 registered diversions and 67 permitted diversions (permitted diversions represent 127 diversion 
17

locations). Estimates of the registered and permitted water withdrawals in the Highlands can serve as an 

upper limit of water use and withdrawals. A list of the registered and permitted diversions is available 

through the Inland Water Resources Division of the Connecticut DEP (Connecticut Department of 

Environmental Protection, 2008).

The Connecticut DPH has regulatory oversight of the State’s drinking-water systems for 

ensuring safety and adequacy of drinking-water supplies of community water systems. Water suppliers 

of community systems are required to report monthly to the DPH the quantity and quality of the water 

withdrawals. Information on community water-supply systems is available through the Drinking Water 

Section of the Connecticut DPH (Connecticut Department of Public Health, 2008a).

The Connecticut DPUC is charged by statute with the regulation of investor-owned water 

companies. The investor-owned water companies are required to report annually to the DPUC the 

quantities of water delivered to consumers, or lost or unaccounted for, and the system demand. Metered 

sales of water for residential, commercial, industrial, and public authorities are included in the annual 

reports to the DPUC by investor-owned water companies. Information on water production and 

consumption from investor-owned water companies, including quantities of water produced and 

delivered to consumers during the year, can be obtained from the DPUC (Connecticut Department of 

Public Utility Control, 2008).

Estimating Domestic Water Use

Domestic water use was estimated for the 28 towns and cities in the Highlands primarily using 

population data and a per-capita water-use coefficient (Appendix 1, Domestic Use). Data on 

populations served by public supply were obtained from the Connecticut DPH Community Water 

System database (Connecticut Department of Public Health, 2008b). The number of self-supplied 

people was determined by subtracting the population served by community water systems (about 

154,000) from the city or town populations (300,000 total) as reported by the U.S. Bureau of the Census 

(2005). The difference between the population served by community water systems and the total 

population indicated that 47 percent of the population (about 145,000) was self-supplied in 2005. A percapita water-use coefficient, 85 gal/d per person, representing the average amount of water typically 

used for domestic purposes, was multiplied by the populations (self-supplied and public supplied for 

each town and city) to estimate domestic water use. All self-supplied domestic water use was assumed 

to be from ground water.

The self-supplied populations were cross-checked using information from 1990 U.S. Bureau of 

the Census data (1990), which reported about 60,000 private wells in the Highlands. The 1990 U.S. 

Census of Housing provides data on “source of water” (public system or private company, individual 

drilled well, or some other source) by housing unit, as well as the number of total housing units by 

municipality. Data on the number of individual wells and or households served by public systems were 

not compiled for the 2005 U.S. Census. 

The per-capita water-use coefficient was obtained from a report by the USGS on water use 

(Solley, 1998). A detailed study in coastal Connecticut (Fairfield County) has shown per-capita water 

use can vary from as little as 50 to as much as 1,000 gal/d depending on lot size, housing type, 

household income, type of supply (community, dug well, drilled well), and water-use infrastructure 

(pools and landscape-irrigation systems) (Mullaney, 2004). The per-capita water-use coefficient used to 

estimate domestic water use in the Highlands, 85 gal/d per person, is slightly higher than the national 

average per-capita use coefficient, 80 gal/d per person (Solley, 1998). 
18

1.5. Water Quality

The USGS monitors water-quality conditions in Connecticut and maintains a database of waterquality records on a number of different physical properties, chemical constituents, and bacteria. Ten 

surface-water-quality stations, part of a larger network of water-quality stations throughout Connecticut, 

are used to monitor and or assess water-quality conditions of Highlands streams and rivers. A list of the 

10 USGS water-quality stations in or near the Highlands study area is presented in table 3. The 

locations of the 10 water-quality stations are shown in figure 6. 

Table 3. U.S. Geological Survey water-quality stations with 5 or more years of record in or near the Highlands

study area, 2005.

[mi2

, square miles]

U.S. 

Geological 

Survey 

station 

number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees) River name Town

Years 

of 

record 

Period of record 

(Water Years1)

Drainage 

area (mi2)

01188000 41.7862 72.9648 Burlington Brook Burlington 39 1956, 1968-

2005

4.10

01188090 41.7559 72.8868 Farmington River Farmington 26 1977-80, 1984-

2005

378

01189030 41.7167 72.8403 Pequabuck River Bristol 33 1971, 1974-

2005

57.2

01189995 41.9084 72.7607 Farmington River Simsbury 37 1967-68, 1971-

2005

577

01198125 42.0747 73.3339 Housatonic River Ashley Falls, 

MA

15 1991-2005 465

01198990 41.9625 73.3717 Falls Village 

Reservoir

Salisbury 50 1956-2005 633

01200600 41.5931 73.4500 Housatonic River New Milford 31 1962, 1963, 

1974-91, 1995-

2005

1,022

01201487 41.4661 73.4036 Still River Brookfield 22 1984-2005 62.3

01203000 41.5497 73.3303 Shepaug River Roxbury 35 1953, 1954, 

1959, 

1974-2005

132

01208049 41.6153 73.0583 Naugutuck River Waterbury 27 1967, 1980-

2005

136

1

A 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.
19

Figure 6. Location of U.S. Geological Survey water-quality stations in the Highlands study area. 
20

Water-quality data collected by the USGS can be retrieved through the USGS NWIS 

(U.S. Geological Survey, 2008c) (fig. 7).

Figure 7. Water-quality data for Connecticut (web site) (U.S. Geological Survey, 2008).

As part of the National Water-Quality Assessment (NAWQA) Program, the USGS began a 

study to identify, describe, and explain the natural and human factors that affect observed water-quality 

conditions and trends in some of the Nation’s large river basins and aquifer systems. The Housatonic 

River Basin was included in this program. The primary objective of the study was to determine the 

occurrence and distribution of inorganic and organic water-quality constituents in shallow, recently

recharged water beneath areas of undeveloped (forested), agricultural, and urban land use. The second 

objective was to relate variations in groundwater quality to natural and human factors. 

The results of the NAWQA study in the Housatonic River Basin, including descriptions of the 

locations of the monitoring wells and water samples collected, are described by Grady and Mullaney 

(1998). The report indicates that 1) urbanization and agriculture have contributed to excess 

concentrations of nitrate nitrogen and the presence of volatile organic compounds and pesticides, 

2) concentrations of nitrate nitrogen beneath urban and agricultural areas are frequently as much as 

20 times higher than concentrations in forested areas, and 3) pesticides rarely exceed any established 

maximum contaminant levels, but concentrations of solvents may exceed drinking-water standards,

especially in some areas with historical and high-intensity urban development.

Additional sources of surface- and groundwater-quality data can be found in USGS Water 

Resources Inventory of Connecticut —Farmington River Basin (Part 9) (Handman and others, 1986), 

Upper Housatonic River Basin (Part 6) (Cervione and others, 1972), and Lower Housatonic River 

Basin (Part 5) (Wilson and others, 1974). These reports are designed to be useful to planners, public 

officials, water-utility personnel, and others concerned with the development, management, use, 

conservation, and protection of water resources. 
21

1.6. Watershed Budget

Model Input Data and Methods of Analysis

Key features of understanding and protecting the Highlands water-supply infrastructure include 

developing detailed water budgets that include precipitation, evaporation rates, and discharge rates of 

streams, and evaluating factors such as climate, geology, vegetation, and soils that affect the exchange 

between surface and ground water. A generalized water-budget watershed model, PRMS—

Precipitation-Runoff Modeling System—was used to quantify the components of the water budget and 

to investigate potential changes to the water budget caused by various build-out scenarios. 

Documentation for the modeling system is available in the PRMS user’s manual (Leavesley and 

others, 1983). In general, PRMS simulates runoff from a basin. Runoff processes include overland 

flow, groundwater flow, and shallow subsurface flow. Total streamflow comprises the three 

components of the runoff process. Input to the Highlands PRMS model includes climatic data and 

parameters describing the physical characteristics of the basin including slope, aspect, elevation, and 

soil types. Daily total precipitation and maximum and minimum air-temperature data were obtained 

from a Northeast Regional Climate Center, National Oceanic Atmospheric Administration (NOAA)

(written commun., K. Eggleston, 2008). The location names, identification number, latitude and 

longitude, and elevations of the data stations used in the study are shown in table 4. The climate 

stations covered the time period from October 1, 1960 to September 30, 2006. 

Table 4. Climate stations used in the Highlands Precipitation-Runoff Modeling System (PRMS) model.

[NWS, National Weather Service; AP, Airport]

NWS number Name Latitude Longitude

Elevation 

(feet)

060227 BAKERSVILLE 41.84 -73.01 686

060299 BARKHAMSTED 41.92 -72.96 705

060973 BURLINGTON 41.80 -72.93 510

061762 DANBURY 41.40 -73.42 405

062658 FALLS VILLAGE 41.95 -73.37 550

063456 HARTFORD BRADLEY INTL AP 41.94 -72.68 160

065445 NORFOLK 2 SW 41.97 -73.22 1,340

069775 WOODBURY 41.55 -73.23 650

190120 AMHERST 42.38 -72.53 150

191821 DALTON 42.47 -73.15 1,212

194131 LENOX DALE 42.34 -73.25 1,004

196414 PITTSFIELD MUNI AP 42.43 -73.29 1,194

199196 WESTFIELD BARNES MUNI AP 42.16 -72.72 271

199371 WEST OTIS 42.18 -73.22 1,295

301761 COPAKE 42.11 -73.55 550

302286 EAST CHATHAM 42.44 -73.50 924

307035 RHINEBECK 4 SE 41.88 -73.87 301

308304 STORMVILLE 41.53 -73.73 915

The data layers describing the topography, soils, geology, land cover, and forest cover of the 

Highland basins were assembled using a geographic information system (GIS). Topographic 

characteristics such as slope and aspect were derived from the National Elevation Dataset (NED) 

(U.S. Geological Survey, 2001a). The digital elevation dataset has a spatial resolution of 

approximately 30 meters. Soil characteristics were derived from the State Soil Geographic 
22

(STATSGO) dataset (Natural Resources Conservation Service, 1994). Geologic characteristics were 

derived from a 1:1,000,000 scale USGS Surficial Materials Map (Soller, 1998). Land-cover 

characteristics such as impervious cover, land cover, and forest density (canopy cover) were derived 

from the National Land Cover Dataset (NLCD) 2001 database (U.S. Geological Survey, 2001b). The 

NLCD has a spatial resolution of 30 meters. The PRMS model uses 43 parameters to characterize the 

land-surface conditions and processes. Many of the values for these parameters were computed directly 

from the GIS data layers. Values for other parameters were determined by associating the GIS data 

layers for each subwatershed to separate matrix tables for geology, soils, and land use (Leavesley and 

others, 1983). 

Output from the PRMS model is a time-series graph of the rate of runoff from the basin (for the 

period Oct. 1, 1960 through Sep. 30, 2006) and represents the hydrologic response of the basin to 

precipitation input on the basis of the basin’s climatic, hydrologic, and physical characteristics. The 

simulated daily time-series data from 1960 to 2006 were used to estimate the water budget (average 

annual total streamflow, runoff, and base flow), herein referred to as “current conditions.”

The PRMS model also was used to simulate changes to the water budgets under the current 

conditions on the basis of two build-out scenarios referred to as high-constraint and low-constraint 

build-out. The two build-outs are based on current zoning and regulatory constraints that exclude 

certain land from development. Low-constraint build-out is less restrictive on building than the highconstraint build-out. The low-constraint and high-constraint build-out analyses are described in Section 

2-3 Potential Changes and Resources at Risk, Future Change Scenarios—Build out Analysis and 

Econometric Modeling of the CT/PA Highlands Summary Report (Connecticut–Pennsylvania 

Highlands Regional Study, written commun., E.A. Ahearn, 2008). A second objective in the 

development of the PRMS model was to define the groundwater volume for current conditions and 

determine potential changes in groundwater volume based on two future population scenarios. 

The Highlands study area was subdivided into watersheds on the basis of Hydrologic Unit 

Codes (HUCs). The HUCs refer to a numbering system used to identify the boundaries and the 

geographic area of drainage basins for the purpose of water-data management. The largest size 

drainage area is the HUC 8 (identified by an 8-digit number), which corresponds to the entire surfacewater drainage area for major river basins such as the Housatonic and Farmington. These large 

drainage basins have been further subdivided into 17 HUC 10 subwatersheds (identified by a 10-digit 

number) and 64 HUC 12 subwatersheds (identified by a 12-digit number) that drain specific reaches of 

streams and tributaries within the larger basin (figs. 8 and 9). The HUC 10 subwatersheds have an 

average area of approximately 130 mi2 and a maximum area of 284 mi2 (table 5). In contrast, the HUC 

12 subwatersheds have an average area of about 30 mi2 and a maximum area of 62 mi2 (table 6). Ten 

of the 64 HUC 12 subwatersheds are subdivided at stream-gaging stations that are PRMS-modeverification locations. Water budgets were analyzed at HUC 10 and HUC 12 scales. The HUC 12 

subwatersheds are nested within the HUC 10 subwatersheds. The watershed response at the HUC 10 

scale is the areally weighted sum of the responses of all HUC 12 subwatersheds within the HUC 10 

watersheds. In the PRMS model, the subwatersheds are represented by 74 hydrologic response units 

(HRU) through which the physical properties affecting streamflow within the HRU are quantified. For 

this model, the 64 HUC 12 subwatersheds including the 10 subdivided HUC 12 model-verification 

subwatersheds are used to define 74 HRUs. HUCs were included in the model even when only a small 

portion of the HUC is in the defined Highlands region.
23

Figure 8. Boundaries of the HUC 10 watersheds in the Highlands watershed model. 
24

Figure 9. Boundaries of HUC 12 subwatersheds in the Highlands watershed model.
25

Table 5. Description of HUC 10 subwatersheds in the Highlands study area.

[HUC, Hydrologic unit code; mi2

, square miles]

HUC 10 

Drainage 

area 

(mi2) HUC 12 rivers nested within HUC 10 subwatersheds

0108020701 87.0 Mad River, Sandy Brook, Still River mainstem 

0108020702 150 Otis Reservoir, Clam River, W. Branch Farmington River-headwaters to Clam

River and to mouth 

0108020703 66.0 Hubbard River, E. Branch Farmington mainstem

0108020704 153 Nepaug River, Farmington River-headwaters to Burlington, Mine Brook, 

Pequabuck River, Farmington River-Burlington Brook to Thompson Brook

0108020705 72.3 W. Branch Salmon Brook, Salmon Brook

0108020706 48.2 Farmington River - Thompson Brook to Hop Brook and Hop Brook to mouth

0110000501 284 W. Branch Housatonic River, Wahconah Falls Brook, E. Branch Housatonic 

River, Housatonic River-East Branch to Washington Mountain Brook, 

Housatonic River-Washington Mountain Brook to Hop Brook, Hop Brook, 

Housatonic River-Hop Brook to Williams River

0110000502 190 Williams River, Green River, Hubbard Brook, Housatonic mainstem-Williams 

River to Konkapot River

0110000503 231 Konkapot River, Whiting River, Blackberry River, Hollenbeck River, Salmon 

Creek, Housatonic mainstem-Konkapot River to Furnace Brook

0110000504 79.5 Macedonia Brook, Housatonic mainstem-Furnace Brook to Tenmile River

0110000505 211 Webatuck Creek-headwaters to Wassaic River, Mudge Pond Brook, Webatuck 

Creek, Wassaic Creek, Swamp River, Tenmile mainstem

0110000506 131 Candlewood Lake, East and West Aspetuck River, Housatonic mainstemTenmile river to Still River

0110000507 155 Shepaug River, Bantam River 

0110000508 117 Still River, Housatonic mainstem-Still River to Pootatuck River

0110000509 89.0 Nonnewaug River, Weekeepeemee River, Pomperaug River

0110000510 57.4 Pootatuck River, Housatonic mainstem-Pootatuck River to Naugatuck River

0110000511 140 W. Branch and E. Branch Naugatuck River, Leadmine Brook, Branch Brook, 

Naugatuck mainstem-E. Branch to Hancock Brook
26

Table 6. Description of HUC 12 subwatersheds in the Highlands study area.

[mi2

, square miles]

Map ID 

(from 

fig. 9) HUC 12 

Drainage 

area 

(mi2) HUC 12 river name

1 010802070101 31.7 Mad River

2 010802070102 37.4 Sandy Brook

3 010802070103 17.9 Still River mainstem

4 010802070201 15.8 Otis Reservoir

5 010802070202 43.7 West Branch Farmington River-headwaters to Clam River

6 010802070203 31.4 Clam River

7 010802070204 37.8 West Branch Farmington River-Clam River to mouth

8 010802070204 21.0 West Branch Farmington River-Clam River to mouth

9 010802070301 20.9 Hubbard River

10 010802070302 45.2 East Branch Farmington mainstem

11 010802070401 23.8 Nepaug River

12 010802070401 8.2 Nepaug River

13 010802070402 25.9 Farmington River-headwaters to Burlington Brook

14 010802070403 18.6 Mine Brook

15 010802070404 27.0 Pequabuck River

16 010802070404 12.1 Pequabuck River

17 010802070405 16.9 Farmington River-Burlington Brook to Thompson Brook

18 010802070405 20.3 Farmington River-Burlington Brook to Thompson Brook

19 010802070501 26.6 West Branch Salmon Brook

20 010802070502 45.7 Salmon Brook

21 010802070601 25.0 Farmington River-Thompson Brook to Hop Brook

22 010802070602 23.1 Farmington River-Hop Brook to mouth

23 011000050101 60.3 West Branch Housatonic River

24 011000050102 21.5 Wahconah Falls Brook

25 011000050103 49.4 East Branch Housatonic River

26 011000050104 40.9 Housatonic River-East Branch to Washington Mountain Brook

27 011000050105 36.3 Housatonic River-Washington Mountain Brook to Hop Brook

28 011000050106 22.2 Hop Brook

29 011000050107 53.0 Housatonic River-Hop Brook to Williams River

30 011000050201 44.2 Williams River

31 011000050202 53.3 Green River

32 011000050203 50.1 Hubbard Brook

33 011000050204 42.8 Housatonic mainstem-Williams River to Konkapot River

34 011000050301 61.7 Konkapot River

35 011000050302 18.9 Whiting River

36 011000050303 27.1 Blackberry River

37 011000050304 42.9 Hollenbeck River

38 011000050305 29.4 Salmon Creek

39 011000050305 8.6 Salmon Creek

40 011000050306 9.6 Housatonic mainstem-Konkapot River to Furnace Brook

41 011000050306 32.9 Housatonic mainstem-Konkapot River to Furnace Brook

42 011000050401 17.5 Macedonia Brook

43 011000050402 62.0 Housatonic mainstem-Furnace Brook to Tenmile River
27

Table 6. Description of HUC 12 subwatersheds in the Highlands study area.—Continued

HUC 12 

Drainage 

area (mi2) HUC 12 river name

44 011000050501 34.4 Webatuck Creek-headwaters to Wassaic Creek

45 011000050502 18.9 Mudge Pond Brook

46 011000050503 30.3 Webatuck Creek

47 011000050504 37.4 Wassaic Creek

48 011000050505 47.5 Swamp River 

49 011000050506 42.2 Tenmile mainstem

50 011000050601 40.5 Candlewood Lake

51 011000050602 25.3 East Aspetuck River

52 011000050603 25.5 West Aspetuck River

53 011000050604 40.1 Housatonic mainstem-Tenmile River to Still River

54 011000050701 38.4 Shepaug River-headwaters to Bantam River

55 011000050701 2.2 Shepaug River-headwaters to Bantam River

56 011000050702 54.8 Bantam River

57 011000050703 60.0 Shepaug River-Bantam River to mouth

58 011000050801 39.6 Still River-headwaters to Limekiln Brook

59 011000050802 22.8 Still River-Limekiln Brook to mouth

60 011000050802 9.0 Still River-Limekiln Brook to mouth

61 011000050803 45.7 Housatonic mainstem-Still River to Pootatuck River

62 011000050901 17.7 Nonewaug River

63 011000050901 9.4 Nonewaug River

64 011000050902 27.1 Weekeepeemee River

65 011000050903 21.1 Pomperaug River

66 011000050903 13.7 Pomperaug River

67 011000051001 26.1 Pootatuck River

68 011000051003 31.2 Housatonic mainstem-Pootatuck River to Naugatuck River

69 011000051101 34.0 West Branch Naugatuck River

70 011000051102 14.1 East Branch Naugatuck River

71 011000051103 24.7 Leadmine Brook

72 011000051104 22.6 Branch Brook

73 011000051105 27.8 Naugatuck mainstem-East Branch to Hancock Brook

74 011000051105 17.0 Naugatuck mainstem-East Branch to Hancock Brook

The PRMS model simulations are based on the following assumptions: 1) precipitation does 

not vary from existing conditions to future build-out scenarios; 2) future domestic water-supply 

needs are met from ground water; 3) water withdrawn from domestic wells is returned near the point 

of withdrawal by a septic system; and 4) not all the impervious area within a watershed is effective; 

some portions function as pervious. The effective impervious area (EIA) of a basin is an important 

parameter in the rainfall to runoff process because it directly affects the volume of runoff and the 

amount of infiltration recharging water-bearing aquifers. Impervious surfaces such as rooftops, 

asphalt, and concrete prevent the infiltration of water into the ground. However, these surfaces 

commonly drain to other areas where the water can infiltrate. Hydrologic modeling of watersheds in 

the New York–New Jersey and Pennsylvania Highlands Regional Studies assumed 70 percent of the 

total impervious area is effectively acting as impervious area (Hoppe, 2003).
28

Recent watershed modeling of the Pomperaug River Basin in western Connecticut included 

an equation to improve the overall accuracy of the EIA (Appendix 2). In the current study, three 

additional PRMS model simulations were performed using the Pomperaug EIA equation, herein 

referred to as CT EIA, from the Pomperaug River watershed model. The three additional model 

simulations using the CT EIA equation include current conditions, low-constraint build-out analysis, 

and high-constraint build-out analysis. 

Water-Budget Estimates

Estimates of the water-budget components derived from the PRMS model—evapotranspiration, total streamflow, runoff, and base flow—are summarized in Appendix 1, 

CT_HUC10_70EIA and CT_HUC12_70EIA provide the 70 percent EIA data for the HUC 10 and 

HUC 12 subwatersheds, respectively; Appendix 1, CT_HUC10_CTEIA and CT_HUC12_CTEIA

provide the data from the CT EIA equation for the HUC 10 and HUC 12 subwatersheds, 

respectively. The effects of water-budget changes can be evaluated by comparing the modelsimulation results for the current conditions (average annual for the period 1960-2006) with 

simulation results from (1) the low-constraint build-out scenario, and (2) the high-constraint buildout scenario across the HUC 10 and HUC 12 subwatersheds.

The following data are provided for each HUC 10 and HUC 12 subwatershed:

 Map ID ( fig. 9, HUC 12 only)

 Watershed/subwatershed name (based on stream name or stream segment draining the basin)

 Area, in square miles

 Hydrologic unit codes (HUC 8, HUC 10, AND HUC 12 digits)

 Average annual precipitation, in inches per year

For the current conditions, high-constraint scenario, and low-constraint scenario model simulations, 

the following information is provided for each subwatershed: 

 Evapotranspiration, in inches per year

 Total streamflow, in inches per year

 Runoff, in inches per year

 Base flow, in inches per year

 Percent total impervious area

 Percent effective impervious area

 Groundwater withdrawals from domestic wells, in inches per year

These data were specifically used to develop the regional water budget for the Highlands

(fig. 10) and determine the percentage of total streamflow that is base flow (fig. 11). The PRMS 

model divides the flow regime into three components: surface runoff, subsurface flow, and ground 

water. Base flow is considered to be the groundwater component of the flow regime plus a portion of 

the subsurface flow. Model results indicate that, on average, base flow made up approximately 

65 percent of total streamflow over the Highlands region (74 HUC 12 subwatersheds) from

October 1960 to September 2006. The percentage of base flow ranged from 23 to 86 for individual 

subwatersheds, and the range of base-flow contribution for the individual subwatersheds was 1.2 to 

62 Mgal/d, depending in large part on the watershed area.
29

Figure 10. Regional water budget for the Highlands, 2006. On an average annual basis, the Highlands receive 

about 49 inches of precipitation, which is the equivalent of 5,296 Mgal/d. Water that leaves the Highlands

includes evapotranspiration (2,642 Mgal/d), streamflow (2,569 Mgal/d), transfers (77.3 Mgal/d), and 

consumptive use (7.5 Mgal/d).
30

Figure 11. Variations in base flow by subwatersheds. The percentage of total streamflow that is base flow was 

calculated from model-generated water budgets for each HUC 12 subwatershed using the 1960-2006 

average climate and impervious-surface data.
31

Changes in Water-Budget Estimates for High- and Low-Constraint Build-Out Scenarios

Information from the two build-out analyses (low-constraint and high-constraint) conducted 

by the Regional Plan Association (J. Cox, Regional Plan Association, written commun., 2008) was 

incorporated into the PRMS model to simulate the effects of increased impervious area and increased 

groundwater withdrawals on the various water-budget components—streamflow, runoff, base flow, 

and evapotranspiration. The low-constraint scenario represents a greater increase in impervious area 

and consequently a larger impact on the water budget than the high-constraint scenario. Using 

impervious-surface information derived from the low-constraint build-out scenario, the simulated 

total impervious area increased from 2.6 to 6.7 percent. For current land-cover conditions, five 

subwatersheds have greater than 10 percent total impervious area. For the low-constraint build-out, 

13 subwatersheds have greater than 10 percent total impervious area. The maximum increase in total 

impervious area for the low-constraint build-out condition for any HUC 12 subwatershed is 

8.9 percent; about 40 percent (22 of 53) of the subwatersheds had a greater than 5-percent increase. 

For the high-constraint build-out, eight subwatersheds have greater than 10 percent total impervious 

area. The maximum increase in total impervious area for the high-constraint build-out condition for 

any HUC 12 subwatershed is 6.8 percent; about 20 percent (11 of 53) of the subwatersheds had a 

greater than 5-percent increase. The percent increase in total impervious area is based on impervious 

areas for 53 subwatersheds that lie within the Highlands boundary.

As the percentage of impervious area in a subwatershed increases, runoff increases, base flow 

decreases, and total streamflow increases. In general, as impervious area increases, runoff increases 

more than base flow decreases because of a reduction in evapotranspiration. The change in runoff, 

base flow, and total streamflow resulting from increasing amounts of change in the effective 

impervious area in the model HUC 12 subwatersheds is shown in figure 12. The linear relations 

imply changes in the water-budget components are directly proportional to the changes in the 

effective impervious surface up to the maximum change simulated, which is approximately 6 percent.

Figure 12. Changes in water-budget components in relation to percent change in effective impervious surface 

as modeled for the Highlands. 
32

Research from the University of Connecticut Cooperative Extension System suggests that 

where greater than 10 percent of a watershed is covered with impervious area, streams are likely 

impacted and overall water quality is degraded (University of Connecticut, 1994). The effects from 

the distribution of the impervious area within a watershed have not been fully evaluated. 

Under the high-constraint and low-constraint build-out scenarios, the population was 

estimated to be 560,000 and 670,000 people, respectively. The projected groundwater withdrawals 

for the two build-out scenarios were calculated by multiplying the population by an average rate of 

water use of 85 gal/d per person. Under the low-constraint scenario, groundwater withdrawals 

increased from 11.7 to about 44 Mgal/d (new withdrawals of 32 Mgal/d) from current conditions 

assuming a population increase of 370,000 people. Under the high-constraint scenario, groundwater

withdrawals increased from 11.7 to about 34 Mgal/d (new withdrawals of 22 Mgal/d) from current 

conditions assuming a population increase of 270,000 people. Groundwater withdrawals were based 

on population increases for the 28 towns and cities within the Highlands boundary and were not 

estimated for upstream subwatersheds in Massachusetts. Groundwater withdrawals were not 

subtracted from the base flows in the water budget because it was assumed that water withdrawn 

from domestic wells would be returned to the ground water by a nearby septic system. The 

impervious areas (in percent) and the groundwater withdrawals (in inches per year) for the 

subwatersheds for the current conditions, high- and low-constraint build-out analysis of the

Highlands study area are shown in figure 13 and figure 14, respectively. The water-budget changes

simulated from the PRMS model show increases in runoff (fig. 15) and total streamflow (fig. 16) and 

decreases in base flows (fig. 17). For the low-constraint build-out scenario based on the 70-percent 

EIA methodology, runoff increased by 76 Mgal/d, total streamflow increased by 45 Mgal/d, and base 

flow decreased by 31 Mgal/d. For the high-constraint build-out, runoff increased by 55 Mgal/d, total 

streamflow increased by 32 Mgal/d, and base flow decreased by 23 Mgal/d. The predicted change in 

runoff and base flow at the HUC 12 watershed scale under the low-constraint development scenario

is shown in figure 18. The areas of moderate and greatest change are directly related to the projected 

increase in impervious-surface area and groundwater withdrawals. The predicted change is expressed 

as the sum of the absolute values of the runoff and base flow.

Additional model simulations based on the CT EIA equation for the two build-out scenarios 

(high- and low-constraint) yielded smaller changes to the water budget than the model simulations 

based on the 70-percent EIA methodology. For the low-constraint scenario, runoff increased by 

21 Mgal/d; total streamflow increased by 12 Mgal/d; and base flow decreased by 8 Mgal/d. For the 

high-constraint build-out scenario, runoff increased by 16 Mgal/d; total streamflow increased by 

9 Mgal/d; and base flow decreased by 6 Mgal/d. The results from the PRMS model simulations 

based on the EIA methodology in the New York-New Jersey Highlands study and the CT EIA 

equation provide an upper and lower range of potential changes to the water budgets. For example, 

increases in runoff ranged from 21 to 76 Mgal/d; increases in total streamflow ranged from 12 to 

45 Mgal/d; and decreases in base flow ranged from 8 to 31 Mgal/d for the low-constraint build-out 

scenario based on the CT EIA equation and 70-percent EIA, respectively. 
33

Figure 13. Percent impervious surface cover for current conditions, high-constraint and low-constraint build-out

scenarios used in the Precipitation Runoff Modeling System (PRMS) of the Highlands study area.

Figure 14. Groundwater withdrawals for current conditions, high-constraint and low-constraint build-out

scenarios of the Highlands study area
34

Figure 15. Changes in runoff from current conditions to high- and low-constraint build-out scenarios in the 

Highlands study area using the 70-percent effective impervious area. 

Figure 16. Changes in total streamflow from current conditions to high- and low-constraint build-out scenarios 

in the Highlands study area using the 70-percent effective impervious area.
35

Figure 17. Changes in base flow from current conditions to high- and low-constraint build-out scenarios in the 

Highlands study area using the 70-percent effective impervious area. 

Although the water-budget estimates did not account for increases in impervious areas or 

groundwater withdrawals in the subwatersheds outside Connecticut, the predicted changes—

increased runoff, decreased base flow, and an overall increase in streamflow—can be expected in 

those subwatersheds as well. It is important to note that the quantity and quality of water in the 

Highlands headwater streams draining from Massachusetts are of considerable importance to waterresources planning and management in the Highlands study area. Additional increases in 

groundwater withdrawals and impervious areas in the upstream drainage basins would most likely

change the water-budget estimates and put additional stresses on the resource by reducing water 

availability and degrading the water quality of the streams. 

Model Verification and Comparison of Results

Simulation results from the PRMS model were examined graphically and statistically and 

comparisons of simulated and observed streamflow hydrographs for the gaged watersheds were 

made. Statistical summaries of the simulated and observed streamflow and base flow for gaged 

watersheds are shown in table 7.
36

Figure 18. Predicted change in runoff and base flow at the HUC 12 watershed scale under the low-constraint 

development scenario. 
37

Table 7. Annual mean flows for U.S. Geological Survey gaging stations compared to annual mean flows simulated from the PRMS model.

[mi2

, square miles; ft3/s, cubic feet per second; (ft3/s)/mi2, cubic feet per second per square mile; unk, unknown; stdev, standard deviation]

River name

Subwatershed ID

(from fig. 18)

Drainage 

Area (mi2)

Years of 

Record 

(water 

years1

)

DiversionsRegulation

Annual 

Mean 

Streamflow 

Observed 

(ft3/s)

Annual 

Mean 

Streamflow 

Simulated 

from 

PRMS 

(ft3/s)

Annual 

Mean 

Streamflow 

Error 

Fraction 

Percent

Baseflow 

Fraction 

Percent 

(PART 

program)

Baseflow 

Fraction 

Percent 

(PRMS 

model)

Streamflow per 

unit area 

Observed 

((ft3/s)/mi2)

Streamflow 

per unit 

area 

Simulated 

from PRMS 

((ft3/s)/mi2)

Least Disturbed

East Branch Salmon Brook 20 39.5 1964-1976 minor minor 65 64 -0.02 77% 74% 1.65 1.62

Leadmine Brook 71 19.6 1961-1973 minor minor 37 30 -0.19 69% 59% 1.89 1.53

Pomperaug River 62,63,64,65 75.1 1961-2006 minor minor 132 118 -0.11 70% 73% 1.76 1.57

Sandy Brook 2 34.9 1968-1976 minor minor 83 74 -0.10 62% 50% 2.37 2.13

Tenmile River 44,45,46,47,48,49 203 1961-2006 minor minor 318 332 0.04 78% 78% 1.57 1.64

Nepaug Brook 11 23.5 1961-1972 none none 38 34 -0.11 66% 76% 1.62 1.45

West Aspetuck River 52 23.8 1963-1972 minor minor 34 33 -0.03 77% 59% 1.43 1.39

Hubbard Brook 9 19.9 1961-2006 none none 42 44 0.05 58% 57% 2.11 2.21

Salmon River 38 29.4 1962-2006 minor minor 50 55 0.10 78% 60% 1.70 1.87

Most Disturbed

Blackberry River 35,36 45.9 1961-1971 unk flood 60 70 0.17 66% 82% 1.31 1.53

W Br. Farmington River 4,5,6,7 131 1961-2006

supply 

reservoir

reservoir, 

power 

plant 255 268 0.05 77% 63% 1.95 2.05

W Br Naugatuck River 69 33.8 1961-1996

supply 

diversion flood 60 65 0.08 64% 57% 1.78 1.92

E Br. Naugatuck River 70 13.6 1961-1996 unk flood 25 24 -0.04 69% 56% 1.84 1.76

Still River 1,2,3 85 1961-2006 unk

reservoir, 

power 

plant 171 164 -0.04 65% 54% 2.01 1.93

Mad River 1 18.5 1961-1967

supply 

reservoir flood 19 26 0.38 65% 52% 1.02 1.41

Pequabuck River 15,16 45.8 1961-2006

supply 

reservoir

reservoir 

outlet 82 78 -0.05 70% 67% 1.80 1.70

Shepaug River at 

Woodville 54 38.1

1961-

1966,2001,

2006

supply 

reservoir

reservoir 

outlet 46 60 0.29 68% 46% 1.21 1.56

Shepaug River at Roxbury 54,55,56,57 132

1961-

1971,2002-

2006

upstrea

m supply 

reservoir

upstream 

reservoir 

outlet 212 188 -0.11 76% 55% 1.60 1.42

Summary Statsitics

All Rivers Mean 96.05 95.94 0.02 70% 62% 1.70 1.71

All Rivers Stdev 86.76 87.52 0.14 6% 10% 0.33 0.26

Least Disturbed Rivers Mean 88.73 87.17 -0.04 71% 65% 1.79 1.71

Least Disturbed Rivers Stdev 91.49 95.86 0.09 7% 10% 0.29 0.29

Most Disturbed Rivers Mean 103.37 104.72 0.08 69% 59% 1.61 1.70

Most Disturbed Rivers Stdev 86.64 83.15 0.17 5% 10% 0.35 0.23 1

A 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.
38

Annual mean streamflow and annual mean base-flow estimates simulated from the PRMS 

model were compared to statistical analysis and streamflow partitioning of streamflow data from 

USGS gages with at least 5 years of continuous-flow record within the Highlands study area. Both 

relatively natural rivers and disturbed rivers were compared (streamflow is considered artificially 

increased or decreased by water withdrawals, out-of-basin transfers, point-source discharges, 

regulation, and (or) reservoir operations). 

Overall, the estimates of annual mean streamflow simulated from the PRMS model are 

within 2 percent of the observed annual mean streamflow from USGS gaging stations. For rivers 

with disturbed flow conditions, PRMS overestimated the annual mean streamflow by an average of 

8 percent, which may be an indication of the out-of-basin diversions not accounted for in the model. 

For the rivers with natural-flow conditions, PRMS underestimated the annual mean streamflow by an 

average of 4 percent, indicating that the model slightly under predicts annual mean streamflow. 

The base-flow fraction of total streamflow simulated by the PRMS model was comparable to 

the base-flow estimates derived from the PART program, which uses streamflow partitioning to 

estimate base flow from the daily streamflow record (described in section “Base-flow determination 

at selected gaging stations”). Overall, the estimates from PRMS are slightly lower than the estimates 

from the PART program. For the rivers with relatively natural rivers flow conditions, the base-flow 

fraction of total streamflow is 65 and 71 percent from PRMS and PART, respectively. For the 

disturbed rivers, the base-flow fraction of total streamflow is 59 percent and 69 percent from PRMS 

and PART, respectively. To some extent, the higher base-flow estimates from the PART program 

may be explained by regulated releases that artificially increase base flow; the PART program 

interprets most regulated releases as base flow. 

Numerical watershed models simplify the complex processes and physical characteristics of a 

basin. It should be noted that in this study, the PRMS model of the Connecticut Highlands did not 

include (1) major groundwater and surface-water withdrawals from municipal water supply and 

commercial/industrial uses, (2) wastewater-return flows, (3) out-of-basin flows, (4) in-stream flow 

releases from reservoirs, and (4) groundwater withdrawals from private wells. Subsequently, the 

PRMS model parameters and coefficient values for the gaged watersheds were not calibrated to 

closely simulate runoff, streamflow, and base flow of the gaged watersheds. Model simulation results 

are subject to various sources of uncertainty. Some uncertainties are inherent in the model structure 

and some uncertainties are dependent on model development and input data. Consequently, there are 

uncertainties to the water-budget estimates that should be considered when using its results for 

water-resources management decisions. 

Selected References

Alley, W.M., and Veenhuis, J.E., 1983, Effective impervious area in urban runoff modeling: Journal 

of Hydraulic Engineering, v. 109, no. 2, p. 313-319.

Cervione, M.A., Jr., Mazzafero, D.L., and Melvin, R.L., 1972, Water resources inventory of 

Connecticut; Part 6, Upper Housatonic River basin, Connecticut: Connecticut Water Resources 

Bulletin 21, 84 p.

Connecticut Department of Environmental Protection, 2008, Lists of registered and permitted 

diversions: Hartford, Conn., accessed December 29, 2008, at 

http://www.ct.gov/dep/cwp/view.asp?a=2720&q=404934&depNav_GID=1654

Connecticut Department of Public Health, 2008a, Drinking water section: Hartford, Conn., 

accessed December 29, 2008, at 

http://www.ct.gov/dph/cwp/view.asp?a=3139&q=387304&dphNav_GID=1824
39

Connecticut Department of Public Health, 2008b, Public water system classifications and inverntory: 

Hartford, Conn., accessed December 29, 2008, at 

http://www.ct.gov/dph/cwp/view.asp?a=3139&q=387346

Connecticut Department of Public Utility Control, 2008, Water unit: New Britain, Conn., accessed 

December 29, 2008, at 

http://www.ct.gov/dpuc/cwp/view.asp?a=3361&q=415120&dpucNav=|

Connecticut General Statute 25-32d-4, 2008, 

Grady, S.J., and Mullaney, J.R., 1998, Natural and human factors affecting shallow water quality in 

surficial aquifers in the Connecticut, Housatonic, and Thames River Basins: U.S. Geological 

Survey Water-Resources Investigations Report 98-4042, 81 p.

http://www.cga.ct.gov/asp/menu/statutes.asp

Handman, E.H., Haeni, P.F., and Thomas, M.P., 1986, Water resources inventory of Connecticut; 

Part 9, Farmington River basin, Connecticut: Connecticut Water Resources Bulletin 29, 91 p.

Healy, D.F., 1990, Connecticut water supply and use, with sections on Public-supply water use, by 

H.D. Sternberg, and Water management, by C.J. Hughes, in Carr, J.E., Chase, E.B., Paulson, R.W., 

and Moody, D.W., comps., National water summary 1987—Hydrologic events and water supply 

and use: U.S. Geological Survey Water-Supply Paper 2350, p. 193-200.

Hoppe, M.C., compiler, 2003, New York – New Jersey Highlands Regional Study, Technical Report: 

U.S. Forest Service, NA-TP-04-03, April 2003.

Hoppe, M.C., and Phelps, M.G., 2002, New York – New Jersey Highlands Regional Study, 2002 

Update: U.S. Forest Service, NA-TP-02-03, December 2002.

Leavesley, G.H., Lichty, R.W., Troutman, B.M., and Saindon, L.G., 1983, Precipitation-runoff 

modeling system, user’s manual: U.S. Geological Survey Water-Resources Investigations Report 

83-4238, 207 p.

Melvin, R.L., Grady, S.J., and Healy, D.F., 1988, Connecticut groundwater quality, with a section on 

Groundwater quality management, by Fred Banach, in Moody, D.W., Carr, Jerry, Chase, E.B., and 

Paulson, R.W., comps., National water summary 1986—Hydrologic events and groundwater

quality: U.S. Geological Survey Water-Supply Paper 2325, p. 191-198.

Miller, D.R., Warner, G.S., Ogden, F.L., and DeGaetano, A.T., 2002, Precipitation in Connecticut: 

Storrs, Conn., University of Connecticut, Connecticut Institute of Water Resources, 65 p.

Mullaney, J.R., 2004, Water use, groundwater recharge and availability, and quality of water in the 

Greenwich area, Fairfield County, Connecticut and Westchester County, New York, 2000-2002: 

U.S. Geological Survey Water-Resources Investigations Report 03-4300, 64 p. Online at 

http://water.usgs.gov/pubs/wri/wri034300/

Natural Resources Conservation Service, National Soil Survey Center, 1991, STATe Soil 

GeOgraphic (STATSGO) database: Miscellaneous Publication 1492, 110 p. (revised July, 1994).

University of Connecticut, Cooperative Extension System, NEMO Fact Sheet #3, “Impacts of 

Development on Waterways”; The Center for Watershed Protection’s “Watershed Protection 

Techniques,” Vol. 1, No.3, Fall 1994. 

Rutledge, A.T., 1993, Computer programs for describing the recession of groundwater discharge and 

for estimating mean groundwater recharge and discharge from streamflow record: U.S. Geological 

Survey Water-Resources Investigations Report 93-4121, 45 p.

Rutledge, A.T., 1998, Computer programs for describing the recession of groundwater discharge and 

for estimating mean groundwater recharge and discharge from streamflow records--Update: U.S. 

Geological Survey Water-Resources Investigations Report 98-4148, 43 p.
40

Soller, D.R., and Packard, P.H., 1998, Digital representation of a map showing the thickness and 

character of quaternary sediments in the glaciated United States east of the Rocky Mountains: 

U.S. Geological Survey Digital Data Series 38. Online at http://pubs.usgs.gov/dds/dds38/.

Solley, W.B., 1998, Estimates of water use in the western United States in 1990, and water-use 

trends, 1960–90: U.S. Geological Survey Open-File Report 97–176, 15 p.

State of Connecticut, 2008, Source water protection measures: Regulation of Department of Public 

Health, M-39 Rev. 1/77, accessed December 29, 2008, at

http://www.cwwa.org/pdf/regulation_swp.pdf

Sutherland, R.C., 2000. Methods for estimating the effective impervious area of urban watersheds, 

Article 32, in Scheler, T.R., and Holland, H.K., eds., The Practice of watershed protection: Ellicott 

City, Md., Center for Watershed Protection, p.193-195.

University of Connecticut, 1994, Cooperative Extension System, NEMO Fact Sheet #3, “Impacts of 

Development on Waterways”; The Center for Watershed Protection’s “Watershed Protection 

Techniques,” Vol. 1, No.3, Fall 1994. 

U.S. Bureau of the Census, 1990, American FactFinder: accessed February 8, 2008, at 

http://factfinder.census.gov/servlet/DatasetMainPageServlet?_program=DEC&_tabId=DEC2&_

submenuId=datasets_1&_lang=en&_ts=268737827513.

U.S. Bureau of the Census, 2005, American FactFinder: accessed February 8, 2008, at 

http://factfinder.census.gov/servlet/GCTTable?_bm=y&-geo_id=04000US09&-

_box_head_nbr=GCT-T1&-ds_name=PEP_2008_EST&-_lang=en&-format=ST-9&-_sse=on

U.S. Department of Agriculture, Natural Resources Conservation Service, Watershed Boundary 

Dataset: accessed July 11, 2007 at http://datagateway.nrcs.usda.gov/.

U.S. Geological Survey, 1997, STATSGO, Soil characteristics for the conterminous United States: 

U.S. Geological Survey Open-File Report 656, accessed January 15, 2003, at 

http//water.usgs.gov/GIS/metadata/usgswrd/muid.html

U.S. Geological Survey, 2001a, National elevation dataset: U.S. Geological Survey, accessed 

October 20, 2003, at http://gisdata.usgs.gov/NED/.

U.S. Geological Survey, 2001b, National land cover dataset: U.S. Geological Survey Fact Sheet 

108-00, accessed February 18, 2004, at http://mac.usgs.gov/mac/isb/pubs/ factsheets/.

U.S. Geological Survey, 2008a, USGS groundwater data for Connecticut: National Water 

Information System—Web interface, accessed December 29, 2008, at 

http://waterdata.usgs.gov/ct/nwis/gw?

U.S. Geological Survey, 2008b, USGS surface-water data for Connecticut: National Water 

Information System—Web interface, accessed December 29, 2008, at 

http://waterdata.usgs.gov/ct/nwis/sw?

U.S. Geological Survey, 2008c, USGS water-quality data for Connecticut: National Water 

Information System—Web interface, accessed December 29, 2008, at 

http://waterdata.usgs.gov/ct/nwis/qw?

Wilson, W.E., Burke, E.L., and Thomas, C.E., 1974, Water resources inventory of Connecticut; 

Part 5, Lower Housatonic River basin, Connecticut: Connecticut Water Resources Bulletin 19, 

79 p.
41

Appendix 1. Data for Documenting Information Provided in the Connecticut 

Highlands Technical Report – Documentation of the Regional Rainfall-Runoff 

Model

(Excel file CT_Highlands_appendix1.xls linked to Web page)

This appendix contains an Excel file with seven worksheets documenting information provided in the 

Connecticut Highlands Technical Report – Documentation of the Regional Rainfall-Runoff Model. 

CT_community_wells contains estimates of groundwater safe yields for community wells used for 

municipal supply, industrial, commercial, irrigation, and mining uses.

CT_streamflow contains estimates of mean-annual streamflow and mean-annual base flow 

(groundwater discharge) derived using PART program.

Domestic_use contains estimates of domestic water use in the Highlands by Town in 2005.

Estimates of the water-budget components—evapotranspiration, total streamflow, runoff, and base 

flow—derived from the U.S. Geological Survey Precipitation Runoff Model System (PRMS) are 

summarized in the following Excel tables: CT_HUC10_70EIA and CT_HUC12_70EIA provide the 

data for the HUC 10 and HUC 12 subwatersheds on the basis of the 70 percent EIA, respectively; 

CT_HUC10_CTEIA and CT_HUC12_CTEIA provide the data for the HUC 10 and HUC 12 

subwatersheds on the basis of the CT EIA equation, respectively. 
42

Appendix 2. Estimating Effective Impervious Area Using the Connecticut 

Effective Impervious Area (CT EIA) Equation

The effective impervious area (EIA) is the portion of the total impervious area (TIA) within a 

basin that includes rooftops, paved driveways, and parking lots connected to storm-sewer systems 

and results in direct runoff to a stream. Runoff generated on impervious surfaces not directly 

connected to drainage systems is routed to grassed or wooded areas where some or all of it can 

infiltrate into the ground. Impervious area that does not contribute directly to runoff is subtracted 

from the TIA to obtain the EIA. 

Empirical equations for determining the EIA have been developed as part of several studies 

(Alley and Veenhuis, 1983; Sutherland, 2000). In a Connecticut study (D.M. Bjerklie, U.S. 

Geological Survey, written commun., 2009), the EIA was determined as a function of the TIA 

computed by an assumed connectedness relation with the following general form (Alley and 

Veenhuis, 1983):

EIA = k1*(TIA)exp (1)

where,

EIA is the effective impervious area, in percent,

k1 is a constant, 

TIA is the total impervious area, in percent, and

exp is an exponent.

Alley and Veenhuis (1983) assumed the values for k1 and exp are 0.15 and 1.41, respectively, 

for a “typical” region in the United States with storm sewers in place. The Alley and Veenhuis 

equation was modified for Connecticut using streamflow and precipitation data for the state. Because 

the streamflow and precipitation data were analyzed with a model that had a daily time step, the

storm-runoff analysis was limited to single-day storms. The analysis assumed that the storm 

hydrograph rose and fell within a single day and that the runoff from the impervious surfaces entered 

the stream on the same day. Ice and snow cover or saturated ground also can prevent precipitation 

from infiltrating the ground. In order to limit the analysis to impervious surfaces, the storm-runoff 

analysis considered only storms from May through October with precipitation amounts of 0.5 in. or 

less (Zarriello and Barlow, 2002; Zarriello and Ries, 2000).

The runoff analysis described used 18 watersheds in Connecticut. The selected watersheds 

represented the range of land use and impervious-area percentages in Connecticut. The area of the 

watersheds was comparable to, or smaller than, the basins used in the Pomperaug River model, such 

that relative attenuation due to scale effects is limited. Two years of recent streamflow record were

analyzed for each of the basins. Suitable runoff events were isolated from the records and then 

correlated with precipitation records from nearby National Oceanic Atmospheric Administration 

weather stations. 

The weather stations used for the analysis included 12 with a continuous record of 

precipitation. From one to three stations were used to represent the precipitation for each of the 

watersheds, depending on proximity of the stations to the watershed of interest. Where more than one 
43

station was used, the values from each station were averaged. Thus, the analysis did not attempt to 

evaluate the spatial distribution of precipitation as it related to the watersheds of interest. Instead, the 

large number of events winnowed from the data are assumed to average out the errors associated 

with the use of potentially non-representative stations.

The results of the Pomperaug River study indicate the percent impervious area for the 

Connecticut watersheds was low compared to the data from Alley and Veenhuis (1983). This reflects 

the rural nature of the study basins, such as the Pomperaug River Basin. The EIA estimation model 

for Connecticut is given as:

EIA = 0.00013*(TIA)3 + 0.005*(TIA)2 + 0.2287*(TIA) (2)

where,

EIA is the effective impervious area, in percent, and

TIA is the total impervious area, in percent.

This equation, referred to as the CT EIA in the Highlands Study, is considered appropriate for 

rural watersheds in the western region of Connecticut. The CT EIA equation provides estimates of 

EIA that are very comparable to an equation proposed by Sutherland (2000) for somewhat 

disconnected basins (where 50 percent of the urban areas are serviced by ditches or swales) with 

some infiltration measures (swales, permeable payments, and or storm-water detention) over the 

range of impervious surface percentages in the Pomperaug watershed. 

References

Alley, W.M., and Veenhuis, J.E., 1983, Effective impervious area in urban runoff modeling: Journal 

of Hydraulic Engineering, v. 109, no. 2, p. 313-319.

Sutherland, R.C., 2000. Methods for estimating the effective impervious area of urban watersheds, 

Article 32, in Scheler, T.R., and Holland, H.K., eds., The Practice of watershed protection: Ellicott 

City, Md., Center for Watershed Protection, p.193-195.

Zarriello, P. J., and Barlow, L.K., 2002, Measured and simulated runoff to the lower Charles River, 

Massachusetts, October 1999-September 2000: U.S. Geological Survey Water-Resources 

Investigation Report 2002-4129, 89 p.

Zarriello, P. J., Ries, K.G., III, 2000, A precipitation-runoff model for analysis of the effects of water 

withdrawals on streamflow, Ipswich River basin, Massachusetts: U.S. Geological Survey WaterResources Investigation Report 2000-4029, 99 p.
Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

Director

U.S. Geological Survey

Connecticut Water Science Center

101 Pitkin Street

East Hartford, CT 06108

dc_ct@usgs.gov

or visit our Web site at:

http://ct.water.usgs.gov