U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5114

Prepared in cooperation with the 

Pomperaug River Watershed Coalition and the Town of Woodbury, Connecticut

Estimation of the Effects of Land Use and 

Groundwater Withdrawals on Streamflow 

for the Pomperaug River, Connecticut
Cover. Photographs show views of the Pomperaug River in Southbury, Connecticut. (Photographs by Edward Edelson, Pomperaug 

River Watershed Coalition)
Estimation of the Effects of Land Use and 

Groundwater Withdrawals on Streamflow 

for the Pomperaug River, Connecticut

By David M. Bjerklie, J. Jeffrey Starn, and Claudia Tamayo

Prepared in cooperation with the 

Pomperaug River Watershed Coalition and the Town of Woodbury, Connecticut

Scientific Investigations Report 2010–5114

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

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

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Suggested citation:

Bjerklie, D.M., Starn, J.J., and Tamayo, Claudia, 2010, Estimation of the effects of land use and groundwater 

withdrawals on streamflow for the Pomperaug River, Connecticut: U.S. Geological Survey Scientific Investigations 

Report 2010–5114, 81 p., at http://pubs.usgs.gov/sir/2010/5114.
iii

Contents

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

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

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

Description of the Watershed......................................................................................................................3

Climate....................................................................................................................................................5

Streamflow.............................................................................................................................................5

Groundwater..........................................................................................................................................5

Land Use and Land Cover....................................................................................................................6

Water Use ..............................................................................................................................................6

The Precipitation-Runoff Model..................................................................................................................6

General Description of the Precipitation Runoff Modeling System (PRMS) ..............................9

Model Sensitivity and Uncertainty...................................................................................................10

Representation of the Watershed—Hydrologic Response Units (HRUs) .................................10

Precipitation, Temperature, and Geographic Input Data.............................................................12

Parameterization and Calibration of the Model.............................................................................13

Water-Balance Considerations...............................................................................................20

Climatic Variables—Precipitation, Temperature, and Evapotranspiration......................21

Interception.................................................................................................................................22

Soil-Moisture Balance..............................................................................................................23

Runoff .........................................................................................................................................23

Surface Runoff ..................................................................................................................23

Surface Runoff Generated from Variable-Source Areas ..................................23

Surface Runoff Generated from Impervious Surfaces......................................24

Subsurface Runoff............................................................................................................25

Groundwater Flow ............................................................................................................28

Channel Routing.........................................................................................................................29

Diversions....................................................................................................................................33

Calibration, Objective Functions, and Optimization ............................................................34

Model Results......................................................................................................................................35

Streamflow and Water Balance..............................................................................................35

Flow Components and Groundwater Recharge ...................................................................47

The Groundwater-Runoff Model (MODFLOW)........................................................................................58

General Description of MODFLOW Developed for the Watershed............................................58

Representation of the Watershed–Hydrogeology.........................................................................58

Streams, Transient Recharge, Pumping, and Diversions.............................................................59

Calibration and MODFLOW Simulation Results.............................................................................59

Base Flow to Streams ...............................................................................................................59

Losing Reaches..........................................................................................................................62

Management Scenario Applications........................................................................................................62

Effects of Land Use and Groundwater Withdrawal on Streamflow...........................................62

Effects of Land Use and Groundwater Withdrawal on Recharge and Base Flow...................69

Limitations.....................................................................................................................................................74

Summary and Conclusions.........................................................................................................................74

Acknowledgments.......................................................................................................................................75

References....................................................................................................................................................76
iv

Figures

1. Map showing location of the Pomperaug River watershed, Connecticut, showing 

weather stations and U.S. Geological Survey streamgages used in the study .................3

2. Map showing the Pomperaug River watershed, Connecticut, showing town 

boundaries, watershed boundaries of the two main tributaries (Weekeepeemee and 

Nonnewaug Rivers), streamgages, and location of coarse glacial stratified drift 

deposits in the watershed...........................................................................................................4

3. Map showing the Pomperaug River watershed, Connecticut, showing hydrographic 

features and hydrologic response unit boundaries..............................................................11

4. Bar graph showing annual precipitation totals for water years 1976 to 2005 spatially 

averaged from 12 surrounding National Weather Service coop stations ........................12

5. Maps showing (A) topographic relief (slope) of the watershed and (B) the mean 

topographic slope for each hydrologic response unit used in the Precipitation Runoff 

Modeling System model of the Pomperaug River watershed, Connecticut.....................14

6. Maps showing (A) distribution of coarse glacial stratified deposits in the watershed 

and (B) the fraction of coarse glacial stratified deposits in each hydrologic response 

unit used in the Precipitation Runoff Modeling System model of the Pomperaug River 

watershed, Connecticut ............................................................................................................15

7. Maps showing (A) the drainage network (stream and rivers) in the watershed and 

(B) the drainage density of each hydrologic response unit used in the Precipitation 

Runoff Modeling System model of the Pomperaug River watershed, Connecticut........16

8. Maps showing (A) distribution of Class D soils in the watershed and (B) the fraction of 

Class D soils in each hydrologic response unit used in the Precipitation Runoff 

Modeling System model of the Pomperaug River watershed, Connecticut.....................17

9. Maps showing (A) land use and land cover in the watershed and (B) the percent 

impervious surface in each hydrologic response unit in the Pomperaug River 

watershed, Connecticut ............................................................................................................18

10. Maps showing land-cover density (tree canopy) class for each hydrologic response 

unit in the (A) summer and (B) winter as used in the Precipitation Runoff Modeling 

System model of the Pomperaug River watershed, Connecticut.......................................19

11. Hydrograph showing relation between potential evapotranspiration (PET) estimated 

from the Precipitation Runoff Modeling System and by the Northeast Regional Climate 

Center and averaged for four weather stations reporting hourly for the period January 

1, 1999, to April 30, 2003, near the Pomperaug River watershed, Connecticut ................22

12. Graph showing relation between total impervious area in the watershed [as mapped 

by Prisloe and others (2002)] and effective impervious area as used in the Precipitation 

Runoff Modeling System model for three different data sets, Pomperaug River 

watershed, Connecticut ............................................................................................................27

13. Hydrograph showing the continuous response (in minutes) of flow events in 

the Nonnewaug, Weekeepeemee, and Pomperaug Rivers streamgages, Connecticut, 

March 1 to 7, 2004.......................................................................................................................29

14. Hydrograph of stage for the Pomperaug, Weekeepeemee, and Nonnewaug Rivers, 

Connecticut, during the flood event of March 29, 2005, showing the effect of storage 

at the Pomperaug streamgage.................................................................................................30

15. Aerial map showing location of three abandoned quarry ponds and their proximity to 

the Pomperaug River, Connecticut ..........................................................................................31

16. Photograph looking south (downstream) of the high-flow event of March 29, 2005, 

showing the Pomperaug River, Connecticut, on the extreme left, and overflow into the 

quarry pond connector stream on the right...........................................................................32
v

17. Bar graph showing simulated and observed annual mean streamflow for the 

Pomperaug River at Southbury, Connecticut, for water years 1999 through 2005 ..........36

18. Hydrograph showing simulated and observed monthly mean streamflows for the 

Pomperaug River at Southbury, Connecticut, for calendar years 1998 through 2005.....37

19. Hydrographs showing simulated and observed daily streamflow for the (A) Pomperaug 

(B) Nonnewaug and (C) Weekeepeemee Rivers, Connecticut...........................................41

20. Scatterplots showing log residual probability and quartile distribution plots, showing 

the fit line if the residuals are normally distributed, for the daily residuals (A and B)

and the monthly residuals (C and D), Pomperaug, Weekeepeemee, and Nonnewaug 

Rivers, Connecticut ....................................................................................................................43

21. Graphs showing simulated and observed flow-duration curves for the (A) Pomperaug 

(B) Nonnewaug and (C) Weekeepeemee Rivers, Connecticut...........................................45

22. Scatterplot showing relation between the percentage of coarse glacial stratified 

deposits and the simulated average annual recharge in each hydrologic response 

unit, Pomperaug River watershed, Connecticut....................................................................51

23. Maps showing simulated spatially distributed monthly mean recharge (inches) for 

each hydrologic response unit in the Pomperaug River watershed, Connecticut..........52

24. Maps showing simulated spatially distributed monthly mean subsurface runoff 

(inches) for each hydrologic response unit in the Pomperaug River watershed, 

Connecticut..................................................................................................................................54

25. Maps showing simulated spatially distributed monthly mean surface runoff (inches) 

for each hydrologic response unit in the Pomperaug River watershed, Connecticut....56

26. Daily hydrographs of the streamflow in the Pomperaug River at the Southbury 

streamgage for the period 1999 to 2005 derived by combining the MODFLOW-derived 

base flow with PRMS-derived subsurface and surface-runoff components showing 

(A) the PRMS + MODFLOW hydrograph and the observed flow, and (B) the PRMS + 

MODFLOW hydrograph and the PRMS only hydrograph.....................................................61

27. Daily base-flow hydrograph for South Brook near its confluence with the 

Pomperaug River, Connecticut, showing several periods of no flow in the stream 

(dry streambed)...........................................................................................................................63

28. Maps showing impervious surface in the Pomperaug River watershed, Connecticut, 

as a percentage of each hydrologic response unit determined on the basis of 

land-use type for (A) current (2002) conditions, and (B) projected full build-out 

impervious surface.....................................................................................................................64

29. Bar graphs showing simulated daily average runoff in the Pomperaug River, 

Connecticut, as a function of hypothetical land-use change, (A) total streamflow, in 

inches; (B) percentage of total runoff that is groundwater and surface runoff; and 

(C) runoff by source (groundwater, subsurface, and surface), in inches .........................65

30. Graph showing probability of daily flow rate exceeded in the Pomperaug River at 

Southbury on any given day as a function of hypothetical land-use scenario in the 

watershed ....................................................................................................................................67

31. Scatterplots showing change in groundwater runoff, subsurface runoff, and surface 

runoff as a function of change in effective impervious surface for the two full build-out 

scenarios with (A) maximum and (B) minimum stormwater collection relative to existing conditions for the Pomperaug River at Southbury, Connecticut..................................68

32. Bar graphs showing average percentage change in (A) recharge and (B) surface 

runoff relative to existing conditions by hydrologic response unit, Pomperaug River 

watershed, Connecticut, for the full build-out scenarios, with maximum and minimum 

stormwater collection................................................................................................................69
vi

Tables

1. Groundwater-use data for the Pomperaug River watershed, Connecticut........................7

2. Registered and permitted water diversions in the Pomperaug River watershed, 

Connecticut....................................................................................................................................8

3. Data and parameter requirements for the Precipitation Runoff Modeling System 

(PRMS) model................................................................................................................................9

4. Impervious-surface data used for Connecticut runoff analysis.........................................26

5. Simulated and observed annual mean streamflow for the Pomperaug River at 

Southbury, Connecticut, for water years 1999 through 2005...............................................36

6. Daily and monthly streamflow statistics for the Pomperaug River at Southbury, 

Connecticut..................................................................................................................................38

7. Daily and monthly streamflow statistics for the Weekeepeemee and Nonnewaug 

Rivers, Connecticut ....................................................................................................................40

8. Simulated and observed streamflow percentiles for the Pomperaug, Weekeepeemee, 

and Nonnewaug Rivers, Connecticut, October 1, 1998, to September 30, 2005...............46

9. Tributary flow-ratio statistics for the Pomperaug, Weekeepeemee, and Nonnewaug 

Rivers, Connecticut ....................................................................................................................47

10. Simulated flow component contribution to the Pomperaug, Weekeepeemee, and 

Nonnewaug Rivers, Connecticut, at their respective gages...............................................48

11. Simulated and observed annual mean base flow estimated using PART.........................49

12. Daily and monthly base-flow statistics and recession characteristics for the 

Pomperaug River, Connecticut.................................................................................................50

13. Estimated average annual recharge in the Pomperaug River watershed, 

Connecticut..................................................................................................................................50

14. Simulated runoff data for South Brook (three hydrologic response units), 

Connecticut, July 1–11, 2004.....................................................................................................50

15. Wells and pumping rates used in the MODFLOW-2000 model in the Pomperaug River 

watershed, Connecticut ............................................................................................................60

16. Simulated streamflow percentiles for different impervious scenarios in the 

Pomperaug, Weekeepeemee, and Nonnewaug Rivers, Connecticut ...............................66

17. Regression statistics for multiple linear regression of physical attributes to predict 

recharge for the Pomperaug River watershed, Connecticut..............................................71

18. Percentage of stream reaches in the Pomperaug River watershed, Connecticut, that 

can lose water to the groundwater, gain water from the groundwater, or can go dry 

for the full build-out scenarios compared to the existing scenario...................................73

A1. GIS data and supporting parameter information for each HRU .........................................78

33. Scatterplots showing percentage change in streamflow as a function of simulated 

streamflow for full build-out scenarios with (A) maximum and (B) minimum stormwater 

collection relative to existing conditions for the Pomperaug River at Southbury, 

Connecticut..................................................................................................................................70

34. Boxplots showing average monthly groundwater recharge for uplands and valleys for 

(A) all-forest and (B) all-grass scenarios, Pomperaug River watershed, Connecticut ..72

35. Hydrograph showing effect of projected increases in pumping on base flow assuming 

full build-out land use, Pomperaug River watershed, Connecticut....................................73
vii

Conversion Factors and Datum

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 929.0 square centimeter (cm2)

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2)

Volume

gallon (gal) 3.785 liter (L)

cubic foot (ft3) 0.02832 cubic meter (m3)

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

Transmissivity*

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d)

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

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

 °C=(°F–32)/1.8

Vertical coordinate information is referred to as elevation (above a standard vertical datum) and 

referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot 

times foot of aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, foot 

squared per day (ft2/d), is used for convenience.
 THIS PAGE INTENTIONALLY LEFT BLANK
Abstract

A precipitation runoff model for the Pomperaug River 

watershed, Connecticut was developed to address issues of 

concern including the effect of development on streamflow 

and groundwater recharge, and the implications of water 

withdrawals on streamflow. The model was parameterized 

using a strategy that requires a minimum of calibration and 

optimization by establishing basic relations between the 

parameter value and physical characteristics of individual 

hydrologic response units (HRUs) that comprise the model. 

The strategy was devised so that the information needed can 

be obtained from Geographic Information System and other 

general databases for Connecticut. Simulation of groundwater 

recharge enabled evaluation of the temporal and spatial 

mapping of recharge variation across the watershed and the 

spatial effects of changes in land cover on base flow and 

surface runoff. 

The modeling indicated that over the course of a year, 

groundwater provides between 60 and 70 percent of flow in 

the Pomperaug River; the remainder is generated by more 

rapid flow through the shallow subsurface and runoff from 

impermeable surfaces and saturated ground. Groundwater is 

recharged primarily during periods of low evapotranspiration 

in the winter, spring, and fall. The largest amount of recharge 

occurs in the spring in response to snowmelt. During floods, 

the Weekeepeemee and Nonnewaug Rivers (tributaries that 

form the Pomperaug River) respond rapidly with little flood 

peak attenuation due to flood-plain storage. In the Pomperaug 

River, flood-plain storage is more important in attenuating 

floods; abandoned quarry ponds (O&G ponds) adjacent to the 

river provide substantial flood storage above specific river 

stages when flow from the river spills over the banks and fills 

the ponds. Discharge from the ponds also helps to sustain low 

flows in the Pomperaug River. Similarly, releases from the 

Bronson-Lockwood reservoir sustain flow in the Nonnewaug 

River and tend to offset the effect of groundwater withdrawals 

from a well field adjacent to the river during periods of natural 

low flow.

The model indicated that under the current zoning, future 

development could reduce low flows by as much as 10 percent 

at the 99 percent exceedance level (99 percent of flows are 

greater than or equal to this flow), but would not substantially 

increase the highest flows. Simulation of projected and hypothetical development in the watershed shows, depending on 

how stormwater is managed, that between 10 and 20 percent 

effective impervious area in an HRU results in streamflow 

becoming dominated by the surface-runoff component. This 

shift from a groundwater-dominated system would likely 

result in substantial changes in water quality and instream 

habitat characteristics of the river. 

Base flow to streams in the Pomperaug River watershed is reduced by both increased impervious surface and 

increased groundwater withdrawals. For the watershed as a 

whole, increasing groundwater withdrawals have the potential 

for causing greater overall reductions in flow compared to 

increased development and impervious surfaces. Additionally, on the basis of groundwater-modeling simulations, the 

projected increase in development across the watershed and, to 

a lesser extent the increase in groundwater withdrawals, will 

increase the number of local losing reaches experiencing dry 

conditions and the duration of these dry periods. The location 

of the losing reaches tends to be in areas near the transition 

from the uplands to the valley bottoms that are filled with 

coarse glacial stratified deposits. The simulated increase in the 

duration and extent of localized dry stream reaches is most 

sensitive to local increase in impervious surface.

Conversion of land from forest or developed land cover 

to pasture or agricultural land increases groundwater recharge 

and discharge to streams, while at the same time increasing 

overall streamflow (the opposite effect as increased impervious surface). These results indicate that preservation of agricultural land and pasture can provide hydrologic benefit with 

regard to recharge potential. If all registered diversions are 

used during periods of low flow, there is a risk that portions 

of the river system will experience extreme low flows below 

tolerable levels or possibly dry streambeds. 

Estimation of the Effects of Land Use and 

Groundwater Withdrawals on Streamflow 

for the Pomperaug River, Connecticut

By David M. Bjerklie, J. Jeffrey Starn, and Claudia Tamayo
2 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Introduction

In recent years, increased use of surface-water and 

groundwater supplies in the Pomperaug River watershed in 

Connecticut has created concern among resource managers 

that future development will result in streamflows insufficient 

for aquatic habitat and recreational use. In response to the concern, the Pomperaug River Watershed Coalition (PRWC) was 

formed, and stakeholders in the watershed identified a need 

to determine the amount of water available for future allocation while protecting the ecological integrity of the watershed. 

Although the watershed is rural, the population increased by 

15 percent between 1990 and 2000 as a result of suburban 

sprawl (Pomperaug River Watershed Coalition, Inc., 2001). 

Increases in population and development can lead to changes 

in land use and land cover that cause changes in the distribution of runoff and groundwater recharge. Unless these effects 

are considered when allocating the water resources, instream 

flow losses and degradation of water quality and ecosystems 

can occur. Because of uncertainty about the quantity of water 

available for future use, the effects of new applications for 

water diversion are poorly understood.

The future health and economic welfare of the communities within the watershed depend on a continuing supply of 

uncontaminated freshwater. Many existing sources of water 

are being stressed by increasing withdrawals, diversions, and 

demands for instream flow. More comprehensive water-use 

data and analysis of water-use information are needed to 

quantify the stress on existing supplies and to better model and 

evaluate possible demand-management options to supplement 

the traditional supply approaches. Improved watershed characterization and flow-system definition and simulation also are 

needed to manage aquifers and streams that serve as important 

local or regional sources of water supply and to manage and 

support watershed ecosystems. Because aquifers and streams 

commonly are highly interdependent, improved tools for simulating interactions between groundwater and surface water that 

account quantitatively for effects of withdrawals and climate 

variations also are needed so that watersheds can be managed 

more readily as systems. Hydrologic systems models capable 

of showing the consequences of various decisions over a wide 

range of hydrologic and climatic conditions will be helpful to 

local water managers. 

The U.S. Geological Survey (USGS), in cooperation 

with the Town of Woodbury and the PRWC, developed a 

watershed model for the Pomperaug River as part of a study to 

evaluate the potential effects of land-use changes and watermanagement strategies on streamflow, instream habitat, and 

groundwater availability. The study goals are stated below:

• Characterize and develop an understanding of natural 

and anthropogenic influences on the groundwater and 

surface-water processes in the watershed.

• Develop a model parameter framework representative 

of hydrogeologic conditions in Connecticut and transferable to other watersheds in the State.

• Evaluate the effects of specific land-use and waterresource management scenarios on streamflow in the 

Pomperaug River through the development and application of a simulation model that is based on available 

data and could be transferable to other watersheds.

In addition, the instream-flow information from the model 

was used to assess habitat suitability (Parasiewicz and others, 

2007) and the recharge simulated by the model was used as 

input to a groundwater-flow model that was developed for 

the watershed (Starn and Brown, 2007) in order to evaluate 

groundwater/surface-water interactions in the watershed and 

the effects of land use and water management on the availability of groundwater. 

This study will benefit the municipalities and state 

agencies charged with managing water resources to maximize 

the economic benefits of a sustainable water supply while 

minimizing the environmental consequences of withdrawing 

water from the watershed. Data collected as part of this study 

could have other uses, for example to develop statistical 

methods for estimating flow characteristics for streams 

in Connecticut. Additionally, the information and model 

results presented in this report will assist the PRWC and the 

Connecticut Department of Environmental Protection (DEP) 

in understanding the effects of land-use and water-use change 

on habitat stress in the Pomperaug River (Parasiewicz and 

others, 2007).

Purpose and Scope

This report presents the results of the study to assess and 

model the naturally occurring surface-water and groundwater 

resources and an analysis of the current and possible future 

cumulative effects of land-use changes and withdrawals on 

the available streamflow in the Pomperaug River. The report 

describes the Precipitation Runoff Modeling System (PRMS) 

watershed model (Leavesley and others, 1996) used for the 

study and presents the strategy developed to determine objectively the appropriate parameters for the model. This strategy 

relies on readily available data (in a Geographic Information 

System (GIS)) for physical parameters such as geology, land 

cover, soil characteristics, and precipitation. The study area 

discussed in this report is the Pomperaug River basin. This 

report complements another report discussing a groundwaterflow model of a subarea of the basin (Starn and Brown, 2007). 

Recharge estimates derived from the PRMS model were used 

in the groundwater model and base flows calculated in the 

groundwater model were used to calibrate the PRMS model. 
Description of the Watershed 3

Description of the Watershed

The Pomperaug River watershed is in western 

Connecticut (fig. 1) and covers an area of about 75 mi2 above 

the streamgage at Southbury (USGS station 01204000, 

fig. 2). The watershed covers 91.7 mi2 above its mouth 

where it flows into the Housatonic River (Pomperaug River 

Watershed Coalition, 2001). Major subwatersheds include 

the Weekeepeemee and Nonnewaug Rivers. (Nonnewaug is 

the spelling preferred by the PRWC, and used in this report; 

however, Nonewaug is the spelling on USGS topographic 

maps and in the USGS National Water Information System.) 

The Weekeepeemee and Nonnewaug Rivers combine to form 

the Pomperaug River. The Weekeepeemee watershed at its 

gage (USGS station 01203805 located at the confluence with 

the Nonnewaug River) has a watershed area of 26.8 mi2. The 

Nonnewaug River at its streamgage (USGS station 01203600 

located upstream of the confluence with the Weekeepeemee 

River) has a watershed area of 17.7 mi2 and a watershed area 

of 27 mi2 at the confluence with the Weekeepeemee River. 

The Pomperaug River watershed also supports major areas of 

groundwater withdrawal from production wells completed in 

glacial stratified deposits. Analysis of the available streamflow, 

groundwater, and climatological data and review of the 

literature (Meinzer and Stearns, 1929; Wilson and others, 

1974; Mazzaferro, 1986a, 1986b; Grady and Weaver, 1988; 

Starn and others, 2000; Pomperaug River Watershed Coalition, 

2001; Lyford and others, 2007) pertaining to the hydrology 

of the Pomperaug River watershed provide a general 

description and quantification of the hydrologic processes in 

the watershed. Portions of eight towns including Southbury, 

Woodbury, Bethlehem, Roxbury, Morris, Washington, 

Watertown, and Middlebury are located within the watershed 

boundaries, and water is exported out of the watershed to 

portions of Watertown and Oxford that are not within the 

watershed boundaries (fig. 2).

Figure 1. Location of the Pomperaug River watershed, Connecticut, showing weather stations and U.S. Geological Survey 

streamgages used in the study.

CONNECTICUT

RHODE ISLAND

NEW YORK

MASSACHUSETTS

Housatonic River

Connecticut River

Thames River

Long Island Sound

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

##

#

Danbury

Ansonia

Meriden

Danbury Airport

Woodbury

Hartford

Stevenson

Torrington Burlington

Shepaug Dam

Bakersville

Rocky River

Bulls Bridge

Mount Carmel

Wigwam Reservoir

Oxford

Nonnewaug River

Pomperaug River

Weekeepeemee River

EXPLANATION

Pomperaug Basin

Weather station and name

Streamgage and associated

watershed

X

#

Woodbury

Pomperaug River

73°30' 73°00' 72°30' 72°00'

42°00'

41°30'

41°00'

0 5 10 15 MILES

0 5 10 15 KILOMETERS

Base from U.S. Geological Survey digital data 1:100,000, 2000

Connecticut State Plane projection
4 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Figure 2. The Pomperaug River watershed, Connecticut, showing town boundaries, watershed boundaries of the two 

main tributaries (Weekeepeemee and Nonnewaug Rivers), streamgages, and location of coarse glacial stratified drift 

deposits in the watershed.

Morris Litchfield

Washington

Thomaston

Watertown

Bethlehem

Waterbury

Woodbury

Roxbury

Middlebury

Bridgewater

Naugatuck

Southbury

Oxford

Newtown

Beacon

Falls

Base from U.S. Geological Survey, 1:24,000, 1969 to 1984

Connecticut State Plane projection

0 1 2 MILES

0 1 2 KILOMETERS

01203600

01203805

01204000

Woodbury

EXPLANATION

Coarse glacial stratified deposits

Study watershed

Nonnewaug River

Pomperaug River

Weekeepeemee River

Weather station and name

01204000 Streamgage and number

Woodbury

River

Pomperaug

Weekeepeemee River

Nonnewaug River

73°18' 73°15' 73°12' 73°09' 73°06'

41°39'

41°36'

41°33'

41°30'

41°27'
Description of the Watershed 5

Climate

Precipitation in the watershed is generated from regional 

frontal systems, more localized thunderstorms, and cyclonic 

systems that can produce large events resulting in floods. 

Precipitation in the area varies with location and generally 

increases with increasing elevations. 

Mean precipitation in the Pomperaug River watershed, 

on the basis of 38 years of record, was reported by Meinzer 

and Stearns (1929) to be 48.8 in. per year; for 1931–1960, the 

mean annual precipitation was reported as 47 in. (Wilson and 

others, 1974); and more recent data (1966–1999) from the 

National Oceanic and Atmospheric National Weather Service 

(NOAA_NWS) Woodbury coop weather station indicates an 

average annual precipitation of 50.6 in. (Pomperaug River 

Watershed Coalition, 2001). The precipitation totals are a 

function of the location and period of record, but mean annual 

precipitation for the watershed is in the range 47 to 50 in. The 

annual totals have ranged historically from less than 40 in. to 

greater than 60 in. The amount of snow varies considerably 

from year to year and averages around 30 to 40 in. per year 

(3 to 4 in. of water equivalent depending on density of the 

snow) (Miller and others, 2002). Precipitation is generally 

evenly distributed throughout the year and varies spatially 

across the watershed depending on elevation and local 

topography as well as in response to the more or less random 

distribution of isolated storm cells. 

Mean annual air temperature for the region varies from 

45 to 50°F depending on elevation and location. Generally, 

temperature decreases with increasing elevation. The highest temperatures typically occur in July and August, and 

the lowest temperatures in January and February. Potential 

evapotranspiration (PET) is driven by cloud cover and solar 

radiation, wind speed, humidity, and air temperature. PET is 

highest in the summer months because of the high solar radiation and temperature, and may often be negligible during the 

winter months. Actual evapotranspiration (AET) is a function 

of PET, ground cover/vegetation, and soil moisture; therefore, 

even when PET is high (for example in July), AET may be 

small if soil moisture is low. Because of its dependence on 

PET, ground cover, and soil moisture, AET varies topographically and spatially throughout the watershed.

On the basis of an estimated annual precipitation of 47 to 

50 in. and an annual runoff of 23 to 24 in. (Wilson and others, 

1974), the long-term average annual AET for the watershed 

is estimated to range between 23 and 27 in., or approximately 

49 to 54 percent of the annual precipitation; the remainder is 

mostly runoff. Wilson and others (1974) estimated AET for the 

Pomperaug River watershed to be on the order of 50 percent 

of the mean annual precipitation. 

Streamflow

The Pomperaug River is formed at the confluence of its 

two major tributaries, the Weekeepeemee and the Nonnewaug 

Rivers (fig. 2). Streamflow in the Pomperaug River at 

Southbury (USGS Station 01204000, fig. 2) averaged 130 ft3/s 

for the period 1933 to 2002. This flow was equivalent to 

23.4 in. of runoff annually from the watershed upstream from 

the streamgage. The highest monthly streamflow typically 

occurs in March, and the lowest in July, August, or September. 

The annual peak flow can occur during any month and is 

generally associated with rain and snowmelt events or floods 

associated with hurricane systems. 

Groundwater

Several estimates of the groundwater contribution to 

streamflow have been made for the watershed. Meinzer and 

Stearns (1929) estimated that 40 to 50 percent of the total 

annual streamflow in the Pomperaug River at the Southbury 

streamgage originates as groundwater base flow. This estimate 

was based on a subjective base-flow separation technique 

applied by hand to the streamflow hydrograph. At the time 

this estimate was made, most of the watershed was cleared for 

agriculture, which likely affected the amount of groundwater 

recharge and discharge in the watershed. More recently, base 

flow was estimated to be between 60 and 70 percent of the 

annual flow (depending on storage assumptions) using an 

automated base-flow separation program (Rutledge, 1993; 

Rutledge, 1998). 

The watershed contains two general aquifer types, 

defined on the basis of the underlying geologic materials as 

they relate to well yield. These two types of aquifers include 

an aquifer comprised of coarse glacial stratified deposits and 

one comprised of till and bedrock. The till and bedrock aquifer 

system is complex and may include a range of hydraulic 

conditions; however, in general, it includes at least an order of 

magnitude lower hydraulic conductivity and storativity than 

the coarse glacial stratified deposits (Lyford and others, 2007). 

The coarse glacial stratified aquifer supplies groundwater to 

domestic and production wells of varying capacity. The till 

and bedrock aquifer supplies groundwater primarily to individual domestic wells. 

The coarse glacial stratified deposits are located in the 

valley bottom of the Pomperaug River and its two main 

tributaries, principally in the Pomperaug and Nonnewaug 

River Valleys (fig. 2) and range in thickness from several feet 

to as much as 150 ft (Lyford and others, 2007). These deposits 

comprise approximately 13 percent of the watershed area. 

Those areas of the watershed not classified as coarse glacial 

stratified deposits are assumed to contain the till and bedrock 

aquifer. The thickness of this aquifer is defined by the depth 

to which groundwater yield is sufficient for an individual 

domestic well and is assumed be on the order of 350 ft (Lyford 

and others, 2007).
6 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Land Use and Land Cover

Land use in the watershed has changed over the last 

century, from being primarily agricultural to primarily 

forested, and the percentage of urban and residential land use 

has increased over the last 30 years. The current land-use and 

land-cover data used in this study are based on the 1997 Land 

Cover Map of Connecticut (Prisloe and others, 2002). The data 

are derived from 30-m pixel LandSat imagery (see following 

section). Land cover and land use in the watershed currently 

are dominated by deciduous and mixed deciduous and 

coniferous forest, with smaller areas of agriculture (primarily 

hay and corn), urban, and residential land use. Approximately 

1.3 percent of the watershed is comprised of open-water 

surfaces, including lakes, small ponds, streams, and reservoirs.

Water Use

Water use in the watershed is primarily domestic, supplied from groundwater. The majority of withdrawals from 

private wells are for domestic water supply. Several public 

water supply systems are located in the watershed, all of 

which use groundwater as the source. Groundwater withdrawals for the watershed, on the basis of recent information 

from the suppliers, are compiled on table 1. The three large 

public supply systems with documented pumping rates and 

diversions are Watertown Fire District, Heritage Village, and 

United Water. There are also nine smaller systems located 

within the watershed with substantial estimated withdrawals 

provided on table 1. Groundwater withdrawals from individual 

home wells are estimated to be 1.46 Mgal/d on the basis of an 

assumed water-use rate that is the same as that for customers 

of United Water (74 gal/d per person), and a population served 

by individual home wells of approximately 19,700 people. The 

United Water rate was used because this system supplies rural 

areas that are similar to areas with private wells. The population estimate is calculated as the difference between the total 

population in the watershed, estimated to be 34,000 people 

(Pomperaug River Watershed Coalition, 2001) and the population served by public systems shown on table 1 (not including 

the population served by the Watertown Fire District, because 

these people are located outside of the watershed). The total 

groundwater withdrawal from the watershed for the year 

2001, assumed to be representative for this study, is therefore 

3.76 Mgal/d. Much of this water is returned to the watershed 

through septic systems and sewer outfalls.

Water diversions that are registered (in operation before 

1983) or permitted with the Connecticut DEP for surfacewater and groundwater sources are listed on table 2. Some of 

the surface-water diversions listed on table 2 are not currently 

used but remain potentially active. Some of the diversions that 

are used, notably the Watertown Fire District wells and part 

of the pumpage from the Heritage Village water system, are 

diverted outside of the watershed, and therefore are entirely 

lost to the watershed. The out-of-watershed diversions are in 

effect consumptive uses of water. Additionally, surface-water 

diversions are used for irrigation in some areas; however, 

the magnitudes of these diversions are not reported and are 

considered to be sporadic compared to the more continuous 

groundwater withdrawals.

The Bronson-Lockwood Reservoir augments water 

supply to the Watertown Fire District well field. The amount 

of water released specifically for flow augmentation is not 

fully documented, and the accuracy of release quantity 

estimates is unknown. However, during periods of low flow, 

the reservoir releases are channeled into recharge ponds 

within the well field and are used to maintain low flows in 

the Nonnewaug River. A review of reservoir releases for the 

6-month period from January 1, 2004, through July 11, 2004, 

indicates water is stored within the reservoir during periods 

of high flow and released during periods of low flow (Ernie 

Coppock, Watertown Fire District, written commun., 2004). 

Additionally, overflow from the Pomperaug River into a series 

of three ponds, formed in abandoned gravel pits adjacent to 

the river on the O&G mining property, provides considerable 

storage of water during flood events that overtop the river 

banks and allows water to enter the pond system. This water is 

returned to the river through an outlet structure (weir).

The Precipitation-Runoff Model

The PRMS, developed by the USGS (Leavesley and 

others, 1996) simulates the hydrologic-system response to 

different land-use, water-use, and climatic conditions and will 

provide water-quantity information required for assessment 

of water allocation and instream flow. PRMS has been used to 

evaluate the effects of changes in land use and land cover on 

streamflow and hydrologic regimes in watersheds 

(Risley, 1994). 

The PRMS modeling strategy developed for the 

Pomperaug River watershed is based on dividing the 

watershed into subunits within which a daily water balance 

is calculated from climatic input variables and hydrologic 

processes that can be characterized. The watershed subunits 

are called hydrologic response units (HRUs) and form the 

basic structure of the model. The modeling strategy included 

the following specific objectives:

• Develop a parameterization strategy that is objective 

and transferable to other watersheds in Connecticut.

• Distribute the HRUs to address management issues and 

hydrologic complexity in the watershed.

• Develop a method for determining effective impervious 

surface from LandSat-derived impervious surface data 

from Connecticut.
The Precipitation-Runoff Model 7

Table 1. Groundwater-use data for the Pomperaug River watershed, Connecticut.

[Q, discharge; Mgal/d, million gallons per day; Stdev, standard deviation; gal/d, gallons per day; est., estimated; data from individual water companies]

Large public systems

Month

Watertown Fire District

(7,200 people served)

Heritage Village 

(7,300 people served)

United Water

(2,260 people served)

Years Mean Q

(Mgal/d) Stdev gal/d/person

Year 2001

(Mgal/d)

Year 2001

(gal/d/

person)

Year 2001

(Mgal/d)

Year 2001

(gal/d/

person)

Year 2001

(Mgal/d)

Year 2001

(gal/d/

person)

Jan 97–02 0.60 0.122 83 0.54 75 0.75 102 0.12 52

Feb 97–02 0.62 0.157 86 0.54 74 0.78 107 0.12 51 est.

Mar 97–02 0.56 0.097 78 0.52 72 0.76 104 0.11 49 est.

Apr 97–02 0.62 0.076 86 0.59 81 0.83 113 0.13 56 est.

May 97–02 0.70 0.100 98 0.86 120 1.13 155 0.19 83 est.

Jun 97–02 0.79 0.153 109 0.96 133 1.1 151 0.19 85

Jul 97–01 0.85 0.179 117 1.11 154 1.22 167 0.21 91

Aug 97–01 0.80 0.186 110 1.07 148 1.28 175 0.21 91

Sep 97–00 0.63 0.047 88 0.86 119 est. 0.98 134 0.19 84

Oct 97–01 0.64 0.136 89 0.87 121 0.88 120 0.19 83

Nov 97–01 0.58 0.098 81 0.75 105 0.8 109 0.18 79

Dec 97–01 0.57 0.078 79 0.71 98 0.74 101 0.18 81

Annual mean 0.66 92 0.78 108 0.94 128 0.17 74

Small public systems

Name Population served Mgal/d1

, 

est.

Heritage Hill Condominiums 120 0.01

Romatic Manufacturing 120 0.01

Quassuk Heights Condominiums 108 0.01

Southbury Training School 1,730 0.16

Town in Country Condominiums 240 0.02

Woodbury Knolls Condominiums 258 0.03

Woodbury Place Condominiums 72 0.01

Nonnewaug High School 850 0.09

Woodlake Tax District 1,200 0.07

Total 4,698 0.41

1

Estimated annual use based on 92 gal/d/person (average Watertown Fire District use).
8 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Table 2. Registered and permitted water diversions in the Pomperaug River watershed, Connecticut.

[Mgal/d, million gallons per day; ft3/s, cubic feet per second; DMR, Connecticut Department of Mental Retardation]

Name Status Quantity Source Use Subwatershed

(see fig. 2) Town

Mgal/d ft3/s

Heritage Inn #1 Permitted 0.377 0.58 Groundwater Domestic Pomperaug Southbury

O&G Industries Permitted 0.65 1.01 Groundwater/

surface water

Industrial Pomperaug Southbury

Triangulum Associates Permitted 0.25 0.39 Pond/surface water Golf course 

irrigation

Pomperaug Southbury

DMR-Southbury Training School #1 Registered 0.144 0.22 Groundwater Domestic Pomperaug Southbury

DMR-Southbury Training School #2 Registered 0.288 0.45 Groundwater Domestic Pomperaug Southbury

DMR-Southbury Training School #3 Registered 0.36 0.56 Groundwater Domestic Pomperaug Southbury

Heritage Village Water Company #1 Registered 0.331 0.51 Groundwater Domestic Pomperaug Southbury

Heritage Village Water Company #2 Registered 0.389 0.6 Groundwater Domestic Pomperaug Southbury

Heritage Village Water Company #3 Registered 0.504 0.78 Groundwater Domestic Pomperaug Southbury

Heritage Village Water Company #4 Registered 0.504 0.78 Groundwater Domestic Pomperaug Southbury

Heritage Village Water Company #5 Registered 0.324 0.5 Groundwater Domestic Pomperaug Southbury

Town of Southbury-Berry Farm Registered 2 3.09 Pond/surface water Irrigation Pomperaug Woodbury

Town of Southbury-Berry Farm Registered 2 3.09 River/surface water Irrigation Pomperaug Woodbury

Town of Southbury-Berry Farm Registered 2.3 3.56 River/surface water Irrigation Pomperaug Woodbury

Town of Southbury-Berry Farm Registered 3.3 5.11 Pond/surface water Irrigation Pomperaug Woodbury

United Water (Aquarion) #1 Registered 0.144 0.22 Groundwater Domestic Nonnewaug Woodbury

United Water (Aquarion) #2 Registered 0.216 0.33 Groundwater Domestic Pomperaug Woodbury

Watertown Fire District–

Bronson-Lockwood Reservoir

Registered 0.87 1.35 Pond/surface water Domestic Nonnewaug Woodbury

Watertown Fire District #1 Registered 0.115 0.18 Groundwater Domestic Nonnewaug Woodbury

Watertown Fire District #2 Registered 0.194 0.3 Groundwater Domestic Nonnewaug Woodbury

Watertown Fire District #3 Registered 0.274 0.42 Groundwater Domestic Nonnewaug Woodbury

Watertown Fire District #4 Registered 0.31 0.48 Groundwater Domestic Nonnewaug Woodbury

Watertown Fire District #5 Registered 0.187 0.29 Groundwater Domestic Nonnewaug Woodbury

Watertown Fire District #6 Registered 0.396 0.61 Groundwater Domestic Nonnewaug Woodbury

Watertown Fire District #7 Registered 0.167 0.26 Groundwater Domestic Nonnewaug Woodbury

Watertown Fire District #8 Registered 0.176 0.27 Groundwater Domestic Nonnewaug Woodbury

Watertown Fire District #9 Registered 1.008 1.56 Groundwater Domestic Nonnewaug Woodbury

Watertown Fire District Registered 0.4 0.62 River/surface water Domestic Nonnewaug Bethlehem

Woodlake Tax District #1 Registered 0.18 0.28 Groundwater Domestic Pomperaug Woodbury

Woodlake Tax District #3 Registered 0.104 0.16 Groundwater Domestic Pomperaug Woodbury

Total 18.462 28.56
The Precipitation-Runoff Model 9

• Develop surface-runoff, soil, subsurface-runoff, and 

groundwater-runoff parameters that are based on geology, land cover, and soil characteristics specific to 

each HRU.

• Evaluate model calibration by comparing the simulated 

streamflow and the observed streamflow using general objective functions including mean and standard 

deviation of streamflow, flow-duration curves, residual 

statistics, and flow relations among streamgages.

The primary objective of the watershed modeling study 

was to simulate streamflow response so that the statistical 

characteristics of the daily hydrograph are reproduced with 

sufficient accuracy to assess water availability, groundwater 

and surface-water interaction, instream-habitat conditions, and 

changes in streamflow and groundwater recharge resulting 

from changes in land use and water use. As such, accurately 

simulating the magnitude and timing of streamflow for a 

specific day is of secondary importance. For this reason, the 

primary calibration objective functions for the model are longer-term statistical measures of runoff, including flow-duration 

curves, the mean and standard deviation of component runoff 

volume, and general characteristics of the hydrograph shape, 

especially base-flow recessions and groundwater contribution 

to streamflow. 

General Description of the Precipitation Runoff 

Modeling System (PRMS)

The PRMS model is comprised of three computational 

layers: the climate/soil interaction, which computes the 

surface-water balance, and two subsurface reservoirs, which 

route excess precipitation through shallow (subsurface) and 

deep (groundwater) reservoir layers. In this study, the subsurface runoff is conceptualized to be similar to interflow 

and consists of shallow and rapid groundwater flow, originating from ephemeral water tables perched above the deeper 

groundwater reservoir (aquifer), which may flow through 

the large secondary porosity in the subsurface just below the 

root zone. The climate/soil layer is where water input from 

precipitation, water loss from evapotranspiration, and direct 

surface runoff from impervious surfaces and variable-source 

contributing areas is generated. The variable-source contributing areas consist of soils adjacent to streams and wetlands that 

become saturated for short periods generating direct runoff by 

preventing infiltration. The model accounts for snow interception and estimates snow accumulation and snowmelt on the 

basis of temperature inputs, computed solar radiation, and 

various snowmelt process functions driven by elevation and 

other geographic factors. As such, the model parameterization process is described most easily from the standpoint of 

climate/soil-water balance and runoff functions. 

PRMS computes the water balance and runoff from a 

series of independent HRUs defined for the watershed. Runoff 

from each HRU, in the daily time step, is simply the addition 

of the three runoff components (surface runoff, subsurface 

runoff, and groundwater runoff) computed for each HRU. The 

basic model processes and input data requirements needed 

for each HRU are summarized on table 3. The model has the 

option to account for the timing of runoff, including overland 

flow and channel routing, on less than daily time steps. This 

study employed the daily time step without routing on the 

basis of an analysis of the effects of routing on the downstream accumulation of discharge. 

Table 3. Data and parameter requirements for the Precipitation 

Runoff Modeling System (PRMS) model.

[HRU, hydrologic response unit]

Model input data

Type Description

Domain Discretized land areas of similar hydrologic 

response, based on elevation, latitude, and 

slope of each HRU, including vegetation 

cover of each HRU. May or may not be based 

on subwatershed delineation.

Flow Discharge as a function of time step at outflow 

(at Pomperaug River streamgage)

Climate/

meteorology Air temperature

Precipitation

Saturation vapor pressure

Solar radiation (calculated or input)

Cloud cover

Potential evapotranspiration (calculated or input)

Modeled physical processes

Type Description

Hydrology Interception capacity of vegetation (pervious)

Retention storage (impervious)

Overland flow routing parameters

Groundwater and subsurface recession 

parameters

Nominal storage in soil zone

Parameters regulating evapotranspiration 

potential of soil and groundwater zones

Parameters regulating infiltration

Snowpack Lapse rate correcting air temperature at gages to 

elevation of HRU

Threshold temperature for precipitation as snow

Density of new snow

Water-holding capacity of snowpack
10 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Model Sensitivity and Uncertainty

The estimated streamflow volume simulated by the model 

is sensitive to key parameters and physical characteristics of 

each HRU that control the climatic water balance, including 

the method for distributing station-based precipitation and 

temperature input data to each HRU (the xyz module of latitude, longitude, and elevation), the method for estimating PET 

(Jensen-Haise module), the soil water capacity and rooting 

depth, direct soil recharge to the groundwater reservoir, vegetation type and ground cover (including percent impervious 

surface), and the snow accumulation and melt parameters. The 

PET and snow parameters are determined primarily by general 

climatic characteristics of the region and are estimated from 

general functions that are not typically changed. Exceptions to 

this are temperature thresholds set for snow accumulation and 

melt. Land-cover characteristics and soil properties are determined directly from GIS information. The xyz distribution 

module uses a 3-dimensional distribution function estimated 

from the station data to assign temperature and precipitation 

on the basis of its xyz coordinates. The method preserves the 

mean values of the input precipitation and temperature records 

derived from the station data.

The timing of streamflow, the shape of the hydrograph, 

and the amount of water assigned to the surface, subsurface, 

and groundwater reservoirs are controlled by those parameters 

that control water storage and routing through the watershed. 

These include surface storage parameters (surface runoff is not 

routed, instead it is all assumed to enter the stream within the 

1-day time step), subsurface storage parameters and routing 

coefficients, and groundwater storage parameters and routing 

coefficients. All these parameters are subject to calibration and 

alter the rate at which excess precipitation enters the stream 

and the source of streamflow. In this study, these parameters 

were derived from physical data to the extent possible and as 

such were not calibrated. The methods used to determine these 

values are described in later sections of the report.

Sources of model uncertainty include (1) those associated 

with the input data such as nonrepresentative station data and 

errors in the data itself, (2) those associated with incomplete 

model representation of watershed processes, (3) those associated with unknown factors not accounted for in the model 

including changes in land use and water use during the 

simulation period, unknown water diversions, and changes in 

off-channel flood-plain storage during the simulation period, 

(4) those associated with differences in scale of the subbasins that comprise the HRUs, and (5) those associated with 

misclassification of land-cover data. Generally, systematic 

errors are indicative of an incomplete model, missing data or 

unknown factors, or a consistent bias in input data, and random errors are associated with random error in the input data. 

In some cases, particularly with regard to extreme input 

events, a single large input error can cause large errors in mean 

statistics, especially considering the effect of persistence in 

the error resulting from storage effects. For example, during a 

month with a mean streamflow of 120 ft3/s, if one event during 

that month was simulated to be 2,000 ft3/s rather than 200 ft3/s, 

the monthly mean would be increased by 50 percent. A flow 

rate of 2,000 ft3/s could be generated by an excess rainfall of 

2 in. over the watershed, indicating that a 2-in. error in a daily 

precipitation record would readily result in large errors in the 

monthly mean. Additionally, a large precipitation event can 

also cause a high-flow memory for several days or weeks after 

the event such that one erroneously large precipitation event 

can lead to several days or weeks of simulated streamflow that 

are too high. 

Representation of the Watershed—Hydrologic 

Response Units (HRUs)

PRMS computes the water balance for each HRU 

in the model. The HRU can be a small watershed or any 

delineated area that represents a region from which the 

desired hydrologic abstraction (variable) is estimated. For 

this study, the variable of interest is runoff, including surface 

runoff, subsurface runoff, and groundwater runoff. As such, 

the HRUs are delineated for all areas where an estimate of 

runoff is desired or needed. The hydrologic homogeneity 

of an HRU is also an important consideration in order to 

reduce variability within the HRU and thus facilitate more 

accurate water-balance computations. However, because the 

scale and extent defining hydrologic homogeneity varies 

depending on the process considered, too many HRUs can 

introduce unnecessary and extraneous complexity into the 

model. Therefore, the HRU delineation balances the need 

for homogeneity across all processes and scales with the 

spatial complexity needed for the desired data output and 

management application. 

The HRU delineation, therefore, is based on the need to 

be able to model streamflows and watershed processes at locations where specific management questions will be addressed, 

including those associated with groundwater recharge, surfacewater management, land-use and land-cover change (such as 

urban development), and instream-flow requirements for the 

Pomperaug River and its tributaries. An inherent assumption 

in the HRU boundaries is that they follow a flow boundary 

represented by a watershed (or subwatershed) divide and (or) 

a river or stream (flow line) and that the groundwater flow 

boundary coincides with the surface-water flow boundary.

The HRU delineation is designed to sufficiently represent the diversity and complexity of geologic, water-use, and 

land-use environments in the watershed so that these can be 

explicitly evaluated. This allows for an assessment of watershed response within the different HRU environments defined 

in the model. Additionally, model output must be suitable to 

provide a distributed streamflow network within the watershed 

with enough detail so that water-management and habitat 

questions that relate to the quantity, location, and timing of 

daily streamflows can be addressed. The HRU boundaries are 

provided on figure 3. 
The Precipitation-Runoff Model 11

Figure 3. Pomperaug River watershed, Connecticut, showing hydrographic features and hydrologic response unit 

boundaries. 

Morris Litchfield

Washington

Thomaston

Watertown

Bethlehem

Waterbury

Woodbury

Roxbury

Middlebury

Bridgewater

Naugatuck

Southbury

Oxford

Newtown

Beacon

Falls

Base from U.S. Geological Survey, 1:24,000, 1969 to 1984

Connecticut State Plane projection

0 1 2 MILES

0 1 2 KILOMETERS

46

53

54

52

51

50

55

34

32 31

41

40

36

35

38 39 37

45 44

16

15

3

23

19

17

29

30

18

20 21

22

28

27 26 25

42

56

57

58

59

60 61

62

1

64

10

11

12

13

14

2

4

5

24 6

7

8

9

33

43

49

47

48

63

01203600

01203805

01204000

Bronson E. Lockwood

Reservoir

Watertown Fire District

well field

O & G Quarry ponds

EXPLANATION

Hydrologic response unit

and number

Study basin

Nonnewaug

Pomperaug

Weekeepeemee

01204000 Streamgage and number

7

73°18' 73°15' 73°12' 73°09' 73°06'

41°39'

41°36'

41°33'

41°30'

41°27'

River

Pomperaug

Weekeepeemee River

Nonnewaug River
12 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

The following specific criteria were used for HRU selection and delineation:

• Retain general subwatershed basis for delineation 

so that model output can be directly translated to a 

streamflow at desired locations. 

• Provide streamflow for specific habitat reaches (for use 

in a separate study by Parasiewicz and others, 2007).

• Provide a diversity of HRUs dominated by developed 

and undeveloped areas (for both the existing and 

future conditions).

• Provide a diversity of HRUs primarily underlain by 

coarse glacial stratified deposits and relatively 

shallow slopes and underlain by till and bedrock with 

steeper slopes.

• Explicitly delineate HRUs to the outlet of those reservoirs where specific management questions will be 

asked and evaluated.

Precipitation, Temperature, and Geographic 

Input Data

In order to use a daily time step, PRMS requires daily 

precipitation and daily maximum and minimum air temperature as input variables. For this study, two data sets were compiled from weather stations in the vicinity of the Pomperaug 

River watershed. The first data set consisted of daily precipitation and daily maximum and minimum air temperatures from 

12 National Climatic Data Center, National Weather Service 

(2002) coop stations including the Woodbury station, which is 

the only weather station in the watershed (fig. 1). The period 

of record for this data set begins October 10, 1975, and ends 

September 30, 1999. Unfortunately, a number of these stations 

were discontinued on or before September 30, 1999. Because 

of this, and to provide the potential for hourly simulations in 

future studies, a recent data set consisting of daily precipitation and daily maximum and minimum air temperature was 

compiled for the period January 1, 1998, to September 30, 

2005, from the Woodbury station and three additional stations 

with hourly recording maintained by NOAA and distributed 

by the Northeast Regional Climate Center (NRCC) (Keith 

Eggleston, Northeast Regional Climate Center, written commun., 2006).The four weather stations are Hartford, Danbury, 

Meriden, and Woodbury (fig. 1). The annual precipitation 

totals (by water year, October 1 to September 30) derived from 

the model input data for 1976 to 2005 are shown on figure 4.

The comparability of the two weather data sets was 

evaluated on the basis of a statistical comparison of the 

457 days of overlapping information (July 1, 1998, to 

September 30, 1999). The total precipitation from the two 

data sets for the period of comparison was nearly equal 

(within 1 percent of each other); however, the AET simulated 

from the model was higher using the input data of the four 

stations. The higher AET is probably because of somewhat 

higher minimum temperatures. The PRMS model simulations 

using the two input data sets showed differences in mean 

simulated streamflows of 3 percent and simulated recharge 

less than 7 percent. The mean simulated streamflow from 

the input data of the 4 stations was closer to the observed 

mean than the simulation from the data set of the 12 stations. 

This comparison indicates that the two input data sets do not 

affect the resulting simulated runoff to a large degree, and it 

is assumed that they are compatible for application in longterm simulations. However, for comparative and calibration 

purposes, the two data sets are treated separately throughout 

this report, except where a long-term record is desired. 

In addition to the required weather input data, observed 

streamflow for the Pomperaug River at Southbury (USGS 

station 01204000) was input to the model. These data were 

used for model optimization and provide the comparative data 

necessary to evaluate the accuracy of the model-simulated 

streamflow. In addition to the Pomperaug River streamflow 

data, data for the two primary tributaries to the Pomperaug, 

the Weekeepeemee River (USGS station 01203805) and 

Nonnewaug River (USGS station 01203600), also were 

compared. The observed Pomperaug River streamflow data 

were available for the entire period of the simulation (1975–

2005), whereas the Weekeepeemee and the Nonnewaug data 

were available from October 1, 2000, through June 30, 2005. 

One of the key objectives of this study was to use 

available GIS data (Viger and others, 1998) as the basis for 

developing the spatial distribution patterns of the input data 

and process-function parameters. The GIS data used in the 

parameterization process included the following physical 

information obtained from statewide digital coverages and 

tabulated for each HRU:

• Percent coarse glacial stratified deposits and 

till and bedrock, and aquifer properties.

• Land-cover characteristics.

• Percent impervious surface.

• Soil information and percent class D soils 

(hydrologic soil class D—low permeability clay and 

silt dominated soils).

• Topography and slope.

• Drainage density.

The latitude and longitude (and albers coordinates), 

elevation, and aspect of each HRU were used to distribute the 

air temperature and precipitation data from the weather-station 

network to each HRU in the watershed. Elevation and aspect 

of each HRU also were used to determine snowmelt characteristics. The soil and ground-cover type were used to determine 

the interception and storage characteristics of the soil and 

vegetative cover, and the evapotranspiration capacity. 

The pertinent GIS data for the watershed and its 

translation to each HRU are shown on figures 5 through 10. 
The Precipitation-Runoff Model 13

The methods for translating the GIS information into the 

model parameters is summarized for selected parameters in 

the following sections.

Parameterization and Calibration of the Model

The initial model parameters were determined on the 

basis of techniques developed by Viger and others (1998) 

and climate-data distribution techniques developed by Hay 

and others (2000) and Hay and McCabe (2002). This section 

describes the parameters and parameter-estimation methods developed specifically for this study. This section also 

describes the model calibration and calibration results. 

Two fundamental model parameterization and calibration 

targets are: (1) The long-term water balance and streamflow 

(mean total runoff), which includes the climate and soil 

water balance; and (2) the magnitude of the flux of the three 

runoff components and the hydrograph shape (variability 

of runoff), which includes the surface, subsurface, and 

groundwater-runoff components. The annual water balance 

is calibrated first in order to achieve the proper balance of 

climatic variables and thus the proper runoff magnitude. 

Given an acceptable long-term seasonal water balance, the 

mean surface fluxes are assumed to be sufficiently modeled. 

The runoff partitioning (routing) parameters are calibrated 

after the mean water balance. These parameters include 

those associated with surface-runoff, storage, and infiltration 

processes; subsurface-runoff, storage, and infiltration 

processes; and groundwater-runoff and storage processes.

The following describes the approach to calibration of the 

model parameters in order of priority. Geomorphic, land-use, 

land-cover, and soil-type parameters for each HRU are considered fixed and not subject to calibration. The parameterization 

proceeds from those elements of the water balance for which 

the greatest knowledge is available.

The sensitivity of the model results to various input 

parameters that control the apportionment of streamflow was 

assessed during the model calibration. The model response to 

precipitation inputs and simulated spatial distribution of these 

Figure 4. Annual precipitation totals for water years 1976 to 2005 spatially averaged from 12 surrounding National 

Weather Service coop stations.

1976

1977

1978

19791980

19811982

1983

19841985

1986

1987

1988

1989

19901991

1992

1993

19941995

1996

19971998

1999

2000200120022003

2004

2005

YEAR

0

10

20

30

40

50

60

70

ANNUAL PRECIPITATION, IN INCHES
14 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut Figure 5. (A) Topographic relief (slope) of the watershed and (B) the mean topographic slope for each hydrologic response unit used in the Precipitation Runoff Modeling System model of the Pomperaug River watershed, Connecticut. Woodbury Southbury Oxford Watertown Roxbury Bethlehem Washington Middlebury Morris Newtown ThomastonWaterbury Naugatuck Litchfield Beacon Falls Bridgewater Seymour Woodbury Southbury Oxford Watertown Roxbury Bethlehem Washington Middlebury Morris Newtown ThomastonWaterbury Naugatuck Litchfield Beacon Falls Bridgewater Seymour

0 1 2 MILES

0 1 2 KILOMETERS

A

B

Base from U.S. Geological Survey, 1:24,000, 1969 to 1984Connecticut State Plane projection

EXPLANATIO

N

Mean slope, in percent6 to 88.1 to 1010.1 to 1212.1 to 1414.1 to 1616.1 to 18

EXPLANATIO

N

Slope, in percent0 to 88.1 to 1010.1 to 1212.1 to 1414.1 to 1616.1 to 18

73°06' 73°09' 73°12' 73°15' 73°18' 73°06' 73°09' 73°12' 73°15' 73°18'

41°39'41°36'41°33'41°30'41°27'
The Precipitation-Runoff Model 15 Figure 6. (A) Distribution of coarse glacial stratified deposits in the watershed and (B) the fraction of coarse glacial stratified deposits in each hydrologic response unit used in the Precipitation Runoff Modeling System model of the Pomperaug River watershed, Connecticut. 

Woodbury

Southbury Oxford

Watertown

Roxbury

Bethlehem

Washington

Middlebury

Morris

Newtown

Thomaston

Waterbury

Naugatuck

Litchfield

Beacon Falls

Bridgewater

Seymour

Woodbury

Southbury Oxford

Watertown

Roxbury

Bethlehem

Washington

Middlebury

Morris

Newtown

Thomaston

Waterbury

Naugatuck

Litchfield

Beacon Falls

Bridgewater

Seymour

0 1 2 MILES

0 1 2 KILOMETERS

A B

Base from U.S. Geological Survey, 1:24,000, 1969 to 1984Connecticut State Plane projection

EXPLANATION

Fraction coarse glacialstratified deposits0 to 0.050.06 to 0.150.16 to 0.300.31 to 0.500.51 to 1.00

EXPLANATION

Coarse glacialstratified deposits

73°06' 73°09' 73°12' 73°15' 73°18' 73°06' 73°09' 73°12' 73°15' 73°18'

41°39'41°36'41°33'41°30'41°27'
16 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut Figure 7. (A) The drainage network (stream and rivers) in the watershed and (B) the drainage density of each hydrologic response unit used in the Precipitation Runoff Modeling System model of the Pomperaug River watershed, Connecticut. Woodbury Southbury Oxford Watertown Roxbury Bethlehem Washington Middlebury Morris Newtown ThomastonWaterbury Naugatuck Litchfield Beacon Falls Bridgewater Seymour Woodbury Southbury Oxford Watertown Roxbury Bethlehem Washington Middlebury Morris Newtown ThomastonWaterbury Naugatuck Litchfield Beacon Falls Bridgewater Seymour

0 1 2 MILES

0 1 2 KILOMETERS

A

B

Base from U.S. Geological Survey, 1:24,000, 1969 to 1984Connecticut State Plane projection

EXPLANATIO

N

Drainage density, inmiles per square mile0.77 to 2.002.01 to 4.004.01 to 6.006.01 to 8.00

EXPLANATIO

N

Hydrologic streamnetwork

73°06' 73°09' 73°12' 73°15' 73°18' 73°06' 73°09' 73°12' 73°15' 73°18'

41°39'41°36'41°33'41°30'41°27'
The Precipitation-Runoff Model 17 Figure 8. (A) Distribution of Class D soils in the watershed and (B) the fraction of Class D soils in each hydrologic response unit used in the Precipitation Runoff Modeling System model of the Pomperaug River watershed, Connecticut. 

Woodbury

Southbury Oxford

Watertown

Roxbury

Bethlehem

Washington

Middlebury

Morris

Newtown

Thomaston

Waterbury

Naugatuck

Litchfield

Beacon Falls

Bridgewater

Seymour

Woodbury

Southbury Oxford

Watertown

Roxbury

Bethlehem

Washington

Middlebury

Morris

Newtown

Thomaston

Waterbury

Naugatuck

Litchfield

Beacon Falls

Bridgewater

Seymour

0 1 2 MILES

0 1 2 KILOMETERS

A B

Base from U.S. Geological Survey, 1:24,000, 1969 to 1984Connecticut State Plane projection

EXPLANATION

Fraction Class D soils0 to 0.020.03 to 0.050.06 to 0.150.16 to 0.300.31 to 0.54

EXPLANATION

Class D soils

73°06' 73°09' 73°12' 73°15' 73°18' 73°06' 73°09' 73°12' 73°15' 73°18'

41°39'41°36'41°33'41°30'41°27'
18 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut Figure 9. (A) Land use and land cover in the watershed and (B) the percent impervious surface in each hydrologic response unit in the Pomperaug River watershed, Connecticut. [Prisloe and others (2002)]. Woodbury Southbury Oxford Watertown Roxbury Bethlehem Washington Middlebury Morris Newtown ThomastonWaterbury Naugatuck Litchfield Beacon Falls Bridgewater Seymour Woodbury Southbury Oxford Watertown Roxbury Bethlehem Washington Middlebury Morris Newtown ThomastonWaterbury Naugatuck Litchfield Beacon Falls Bridgewater Seymour

0 1 2 MILES

0 1 2 KILOMETERS

A

B

Base from U.S. Geological Survey, 1:24,000, 1969 to 1984Connecticut State Plane projection

EXPLANATIO

N

Impervious area, inpercent2.12 to 4.004.01 to 8.008.01 to 12.0012.01 to 16.0016.01 to 20.00

EXPLANATIO

N

Land coverAgriculturalConiferous forestForestRural residentialTransportation/otherUrbanWaterWetland

73°06' 73°09' 73°12' 73°15' 73°18' 73°06' 73°09' 73°12' 73°15' 73°18'

41°39'41°36'41°33'41°30'41°27'
The Precipitation-Runoff Model 19 Figure 10. Land-cover density (tree canopy) class for each hydrologic response unit in the (A) summer and (B) winter as used in the Precipitation Runoff Modeling System model of the Pomperaug River watershed, Connecticut.

Woodbury

Southbury Oxford

Watertown

Roxbury

Bethlehem

Washington

Middlebury

Morris

Newtown

Thomaston

Waterbury

Naugatuck

Litchfield

Beacon Falls

Bridgewater

Seymour

Woodbury

Southbury Oxford

Watertown

Roxbury

Bethlehem

Washington

Middlebury

Morris

Newtown

Thomaston

Waterbury

Naugatuck

Litchfield

Beacon Falls

Bridgewater

Seymour

0 1 2 MILES

0 1 2 KILOMETERS

A B

Base from U.S. Geological Survey, 1:24,000, 1969 to 1984Connecticut State Plane projection

EXPLANATION

Winter cover density0 to 0.250.26 to 0.500.51 to 0.750.76 to 1.00

EXPLANATION

Summer cover density0 to 0.250.26 to 0.500.51 to 0.750.76 to 1.00

73°06' 73°09' 73°12' 73°15' 73°18' 73°06' 73°09' 73°12' 73°15' 73°18'

41°39'41°36'41°33'41°30'41°27'
20 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

inputs was not specifically assessed and was considered to be 

independent of calibration. These inputs and the associated 

distribution were assumed to be fixed and as such serve as 

a limitation to the model. The parameters tested included 

the groundwater-runoff coefficient, the subsurface-runoff 

coefficients (horizontal and vertical), and the soil storage 

coefficients. On the basis of the analysis, the groundwater 

coefficient and the subsurface coefficients were important in 

controlling the shape of the hydrograph recession, and the 

soil storage and subsurface-runoff coefficients were important 

in controlling the amount of groundwater recharge. An 

optimization method was employed to determine reasonable 

values for the watershed mean groundwater-runoff coefficient 

and the subsurface-runoff coefficients, and physically 

based methods were used to assign unique values for these 

parameters to each HRU.

A method was derived to assign a value to the 

groundwater coefficient on the basis of the general knowledge 

of the hydrogeology of the watershed. The two subsurface 

routing parameters that were optimized were isolated and 

calibrated. Even though the general value of the parameters 

was not determined directly from physical characteristics 

of the watershed, the parameter values were distributed 

and assigned to each HRU on the basis of the physical 

characteristics of each HRU. The soil storage values were 

based on a conceptual model of soil depth and type. 

Water-Balance Considerations

The general water-balance equation describing the 

streamflow hydrograph is as written:

Qt = P – AET – Ql – δS, (1)

where

Qt is the streamflow for the river reach of 

interest;

P is the precipitation falling on the watershed 

upstream of the reach of interest;

AET is the actual evapotranspiration loss from 

the watershed upstream of the reach of 

interest;

Ql is flow loss caused by diversions or sinks 

upstream of the reach of interest; and

δS is the change in water stored within the 

watershed upstream of the point of interest.

The flow source components of Qt (surface runoff, 

subsurface runoff, and groundwater runoff) are simulated by 

PRMS, and thus the total streamflow is defined as:

Qt = Qsr + Qss + Qgw, (2)

where

Qsr is the surface-runoff component,

Qss is the subsurface-runoff component, and

Qgw is the groundwater-runoff component.

The components of the change in storage (δS) are simulated 

by PRMS and are defined as:

δS = δSsr + δSss + δSsoil + δSSgw, (3)

where

δSsr is the change in surface-runoff storage,

δSss is the change in subsurface-runoff storage,

δSsoil is the change in soil water storage, and

δSgw is the change in groundwater-runoff storage.

Assuming that δSsr and Ql are negligible, equation (1) can be 

written to define any one of the three runoff components, for 

example:

 Qss = P – AET – (δSss + δSsoil + δSgw) – (Qsr + Qgw). (4)

Over a long enough period of time there is no change in storage (δS = 0), the simulated water balance should agree closely 

with the values reported earlier, with the ratio Qt/P averaging 

48 to 49 percent. 

Qsr is determined by the model as a function of the 

impervious area assigned for each HRU and the contributingarea function. In this report, the effective impervious area was 

determined from the stream response during short-duration 

summer rainfall events and a priori assumptions regarding the 

contributing-area function (srunoff_smidx.f). If Qsr and Qgw 

can be defined, and the total runoff is known, the value for 

Qss can be calculated. Thus, defining the expected values for 

Qsr and Qgw provides the critical objective functions for the 

hydrograph component separation. Over a sufficient length of 

time, it can be assumed that δS = 0. Thus, given an expected 

value for Qgw and reliable estimates of the effective impervious area and contributing-area function, Qt at each observation point in the watershed can serve as the model objective 

function for the watershed. Expressing all terms in inches, for 

a sufficient length of time, δt, equation 4 becomes:

Qss/δt = P/δt – ET/δt – Qsr/δt – Qgw/δt (5)

Thus, the overall objective function for the water balance and 

hydrograph components is a function of the δt and the water 

balance is thus evaluated for monthly and yearly values of δt.

Over the long term, the groundwater component of the 

streamflow hydrograph can be assumed equal to the total 

groundwater recharge within a defined watershed. In the 

Pomperaug River watershed, the boundaries of the groundwater watershed are essentially equivalent to the watershed 

boundaries (Meinzer and Stearns, 1929), thus the long-term 

contribution of groundwater to streamflow in the Pomperaug 

River can be considered equal to the long-term recharge to 

the groundwater aquifers in the watershed. The definition of 
The Precipitation-Runoff Model 21

long-term from the standpoint of groundwater flow in the 

watershed is assumed to be on the order of several years and 

would depend on the specific history of recharge and discharge 

in response to annual weather patterns. In any case, annual 

changes in groundwater storage are likely to be much less than 

the total groundwater discharge over a typical period of 1 to 

3 years. Meinzer and Stearns (1929) estimated that changes in 

groundwater storage over a 3-year period (1913–1916) were 

less than 7 percent of the total groundwater discharge. 

A major consideration in assigning the groundwater component and evaluating groundwater recharge is defining what 

is meant by groundwater. For example, Meinzer and Stearns 

(1929) assumed that groundwater includes any precipitation 

that infiltrates the soil and reaches a zone of saturation, and 

that this water flows laterally downgradient towards streams 

and other discharge points within the geologic formation 

where it is subject to evapotranspiration. 

From the PRMS modeling standpoint, however, two 

groundwater reservoirs—the subsurface and the groundwater 

reservoirs—are used to account for the movement of water 

that infiltrates past the root zone. Because they are below 

the root zone, neither of these reservoirs is subject to 

evapotranspiration. The distinction between the two runoff 

components is based on the characteristics of transport, 

which in turn are a function of the shallow surficial geologic 

materials and the interaction with surface waters. Shallow 

subsurface runoff is rapid groundwater runoff often occurring 

where shallow perched groundwater zones exist, where there 

is substantial bank storage, and where there is substantial 

secondary porosity in the shallow subsurface. Thus, the 

subsurface reservoir would be expected to occur in shallow till 

and weathered bedrock zones overlying till/bedrock aquifers. 

Lateral flow out of the subsurface reservoir is generally 

more rapid than groundwater runoff but much slower than 

surface runoff; thus, the base flow can be considered derived 

from groundwater and some portion of the subsurface runoff 

that is sustained over longer timeframes than surface runoff. 

On the basis of Meinzer and Stearns (1929), Mazzaferro 

(1986b), and the automated base-flow separation methods 

(Rutledge, 1998), the component of total streamflow delivered 

by the groundwater reservoir on an annual basis would be 

expected to be in the range of 45 to 70 percent. 

The automated base-flow separation methods provided 

estimates of groundwater runoff ranging from 60 to 

70 percent. Considering the definition of groundwater runoff 

applied to the model assumptions, which assumes that rapid 

subsurface runoff is not groundwater runoff, it is anticipated 

that the simulated groundwater-runoff component would be 

less than the maximum value of 70 percent and more in the 

range of 60 percent, which is also closer to the base-flow 

separation estimates developed by Meinzer and Stearns (1929) 

and Mazzaferro (1986b) using manual methods.

Climatic Variables—Precipitation, Temperature, 

and Evapotranspiration

Six PRMS modules control the climatic inputs and 

outputs in the long-term water balance. These modules 

include the magnitude and distribution of precipitation 

and temperature (xyz module), magnitude and distribution 

of snowmelt (snow module), magnitude and distribution 

of evapotranspiration (potet module), magnitude of solar 

radiation (solrad module), distribution of solar radiation 

(soltab module), and the interception and evaporation of 

precipitation from vegetative surfaces (intcp module). All of 

these modules use basic assumptions regarding the relation 

between latitude, longitude, and elevation to define lapse rates 

and spatial patterns of temperature, precipitation, and 

solar radiation. 

The values for the various parameters associated with 

the xyz, snowmelt, evapotranspiration, and solar-radiation 

modules are determined from the input climate data and GIS 

information for the watershed and are generally not subject 

to calibration and optimization. The methods and procedures 

for determining their values are discussed by Hay and 

others (2000) and Hay and McCabe (2002). Initial modeling 

indicated the climate parameterization provided good 

overall water-balance results. Precipitation, as snow or rain, 

temperature thresholds have been defined specifically for the 

Pomperaug River watershed and are assumed to be consistent 

for Connecticut as a whole.

The accuracy and appropriateness of the climate parameterization are based on comparison of the two fundamental 

climatic driving variables, the amount of precipitation and 

the potential evapotranspiration. The distributed average 

annual watershed-wide precipitation, calculated from the 12 

stations in the vicinity of the study, was 48 in. for the period 

1976–1999 and 46 in. for the period 1998–2005. These values 

are similar to the estimate of 47 in. made by Wilson and others 

(1974) for the watershed as whole. These data indicate the 

precipitation input and simulated distribution of precipitation 

in the watershed is appropriate. The PRMS model distributes 

the observed precipitation from the input station records to 

each HRU on the basis of the spatial and vertical relative position of the HRU. This distribution is done by first establishing 

a weighting function that is based on the spatial coordinates 

and elevations of the input station records. PRMS preserves 

the weighted precipitation time-series statistics from the input 

record. In this study, it is assumed that the input weather data 

are not subject to modification and are taken at face value after 

an initial level of quality control to remove erroneous data. In 

some cases, precipitation amounts and temperatures appear 

to be out of range relative to adjacent stations and knowledge 

of the observed response. For example, if the weather record 

for a station reports 10 in. of rainfall and surrounding stations 

report no rain or minor rain and there is no attendant observed 

change in the streamflow, then the 10-in. rainfall is considered 

a reporting error in the data. This level of quality control was 
22 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

done for obvious errors. For cases where a potential reporting 

error was not obvious, the data were left in the record. 

The PRMS model simulates the PET from the input 

temperature data and computed incoming global solar 

radiation for the latitude and longitude of each HRU using 

the Jensen-Haise method (Leavesley and others, 1996). For 

1999–2003, for which independently estimated PET data are 

available from the Hartford, Danbury, Meriden, and Oxford1 

weather stations (Northeast Regional Climate Center, 2006), 

the estimated PET is 29 in. annually, which is comparable to 

the PRMS-simulated PET averaged for the watershed of 

30 in. annually. The mean monthly error between the 

simulated and NRCC-estimated PET is 4 percent, and the 

Nash-Sutcliffe goodness-of-fit coefficient (Nash and 

Sutcliffe, 1970) for the daily record is 0.75 and for the 

1

Precipitation data from the Oxford station were not used in PRMS model 

because they contained numerous errors.

monthly record is 0.88, both indicating relatively good timeseries simulations fit. The simulated PET is higher during the 

summer months than the estimates made by the NRCC 

(fig. 11). This discrepancy is likely because of a combination 

of the differences in the method of estimation and the fact 

that the PRMS distribution of PET is averaged over the entire 

watershed, whereas the NRCC estimate is not distributed.

Interception

The amount of precipitation intercepted is a function of 

the ground-cover characteristics (vegetative cover) for summer 

and winter conditions and the PET. Interception of precipitation is simulated by assuming a specific maximum quantity 

of precipitation is stored on vegetative surfaces during the 

summer (leaf on) and winter (leaf off) months. The assumed 

quantity is a function of the cover type and cover density, both 

Figure 11. Relation between potential evapotranspiration (PET) estimated from the Precipitation Runoff Modeling 

System and by the Northeast Regional Climate Center and averaged for four weather stations reporting hourly for the 

period January 1, 1999, to April 30, 2003, near the Pomperaug River watershed, Connecticut. 

1999 2000 2001 2002 2003

JAN

APR

JUL

OCT

JAN

APR

JUL

OCT

JAN

APR

JUL

OCT

JAN

APR

JUL

OCT

JAN

APR

 0

0.05

0.10

0.15

0.20

0.25

POTENTIAL EVAPOTRANSPIRATION, IN INCH PER DAY

Simulated

Calculated—Northeast Regional Climate Center
The Precipitation-Runoff Model 23

of which are defined by general relations developed by Viger 

and others (1998). PRMS assumes that all the intercepted precipitation is eventually evaporated. The cover type and density 

associated with each HRU for summer and winter conditions 

are shown on figure 10.

Soil-Moisture Balance

The soil-moisture balance module in PRMS controls the 

rate of AET and recharge to the subsurface and groundwater 

reservoirs. The available soil moisture is controlled by the 

soil type, precipitation, and PET, with the option to directly 

route available water to the groundwater reservoir. The 

soil type determines the field capacity and maximum water 

content of the soil, which is determined from National 

Resource Conservation Service (NRCS) soil coverages for the 

watershed (Viger and others, 1998). The module operation is 

summarized below: 

• Water is removed from the soil reservoir by evapotranspiration.

• Any remaining water is routed either to the subsurface 

reservoir or the groundwater reservoir depending on 

the local geology. In areas of coarse glacial stratified 

deposits, excess soil water is routed directly to the 

groundwater reservoir; thus, in these areas there is no 

intermediate subsurface reservoir. In areas of till and 

bedrock, excess soil water is routed to the subsurface 

reservoir.

The simulated long-term AET is approximately 65 percent of 

the simulated annual PET, which is consistent with the 

64 percent estimated by Meinzer and Stearns (1929).

Runoff

Three PRMS modules control the magnitude and 

distribution of water flow from the land surface, subsurface, 

and groundwater reservoirs. These include surface runoff, 

subsurface storage and runoff, and groundwater storage and 

runoff. As previously discussed, the climate and soil modules 

control the long-term water balance. The runoff modules 

control the rate and timing of runoff and, therefore, control the 

discharge magnitude and shape of the daily hydrograph. The 

parameter descriptions and method of determination of their 

values for the three runoff/flow modules are described below.

Surface Runoff

The percent surface-runoff contribution to total flow 

cannot be estimated from the hydrograph because of the 

difficulty in separating this runoff component from rapid 

subsurface runoff. However, the physical basis for this process 

can be defined within reasonable limits so that the PRMS 

surface-runoff function can be parameterized on the basis 

of measurable watershed quantities. PRMS determines the 

surface runoff by assuming direct contribution from open 

water and wetlands, defined variable-source contributing 

areas, and impervious areas. Therefore, surface runoff 

is controlled by the rate of runoff from variable-source 

contributing areas and the amount of impervious surface in 

each HRU. The variable-source contributing areas are those 

areas near streams and wetlands that are considered to behave 

as impervious surfaces once they become saturated during 

precipitation events.

Surface Runoff Generated from Variable-Source Areas

Variable-source areas are assumed to be present near 

streams and wetlands, and the maximum extent is equivalent 

to the area of Hydrologic Class D soils in each HRU (see 

fig. 7). The hydrologic soil class is defined in NRCS Soil 

Survey reports as those soils that have a high runoffproducing potential due to poor drainage either because of low 

permeability, high water table, or shallow impermeable layer 

such as hard-pan or bedrock (U.S. Department of Agriculture, 

Natural Resources Conservation Service, 2004).

The rate of growth of the variable-source area is defined 

by:

ca_percent = smdix_coef*10[smidx_exp*smdix], (6)

where

ca_percent is the percentage of an HRU that comprises 

the variable-source contributing area,

smdix_coef is the area of surface water,

smdix_exp is determined from drainage density, and

smdix is the soil-moisture index calculated by 

PRMS.

A maximum ca_percent is defined (carea_max), thus limiting the directly contributing area for each HRU. As an initial 

assumption, carea_max is assumed to be the area of water 

bodies plus the area of hydrologic soil class D in each HRU. 

The smdix_coeff surface-runoff routing coefficient represents the minimum value (intercept) of the logarithmic relation 

between percent contributing area and available moisture 

(values range from 0.0001 to 1.0). The value of this parameter 

was set equal to the area of surface water in each HRU; when 

smdix is zero or very low, the contributing area would consist, 

at a minimum, of the surface-water bodies themselves.

The smdix_exp surface-runoff routing coefficient represents the rate at which the contributing area increases towards 

its maximum value (carea_max) (values range from 0.2 to 

0.8). The value reflects the HRU slope and surface conditions—low values on low slopes with higher storage potential 

and high values on high slopes with less storage potential. The 

initial value was assumed to be directly related to the drainage 

density of the HRU (fig. 6). This assumption is based on the 

consideration that higher drainage densities typically occur in 

steeper headwater areas and that the drainage density reflects 

the surface conditions most favorable to runoff generation.
24 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Because the range of smidx_exp is small (0.2 to 0.8) 

and the potential range of drainage density is logarithmic, 

the general relation with the drainage density is developed 

assuming that smidx_exp is a logarithmic function of drainage 

density so that higher values of drainage density are correlated 

to higher smidx_exp values, and vice versa. The relation is 

developed on the basis of a “typical” drainage density (defined 

as Dd = ΣL/A) function defined from the total drainage length 

(L, in miles) to the watershed area (A, in square miles) 

(Leopold and others, 1964):

L = 1.4*A0.60, (7)

Re-arranging equation 7 in terms of drainage density gives:

Dd = 1.75/L0.67, (8)

And relating Dd directly to smidx_exp gives:

smidx_exp = c*(Dd)X. (9)

where c and X are model-defined fitting parameters.

Using the range of stream lengths in each HRU, the 

equation can be solved for c in the range of smidx_exp from 

0.8 to 0.2. For the HRU distribution in the Pomperaug River 

watershed, an approximate value for c is 0.5 and X is 0.4. 

Values of stream length greater than 100 mi would result in 

smidx_exp = 0.2 and values less than 1 mi would result in 

smidx_exp = 0.8. (This relation is designed to give reasonable 

initial values for this parameter.)

Surface Runoff Generated from Impervious Surfaces

Impervious surfaces are those areas that generate direct 

precipitation runoff. The fraction of observed impervious surfaces associated with pavement, rooftops, and other nonporous 

surfaces is a function of how well the direct runoff is collected 

and transported to the stream. As such, the degree of stormwater control and retention is an important element in estimating the effective impervious area that the model uses to route 

precipitation directly to the stream. 

The direct surface runoff from impervious surfaces is 

computed as the precipitation minus depression storage over 

these surfaces. On the basis of the watershed conditions, the 

impervious-surface depression storage is set at 0.001. The 

fraction of effective impervious area in an HRU ranges from 

0.0 to 1.0 and is assumed to be a log-function of the percent 

urbanization within the HRU. This assumes that as urbanization increases, the connectedness of the impervious areas and 

the likelihood for drainage features (storm sewers, ditches, 

etc.) are also increased. 

The PRMS model, in the daily time step computational 

mode, assumes that runoff from impervious surfaces is routed 

directly to streamflow after accounting for impervious-surface 

depression storage. Thus, the fraction of impervious surface assigned to each HRU, times the rainfall and snowmelt 

computed for each HRU, contributes directly to the volume 

of surface runoff to the stream for the day. In urban areas 

with stormwater-conveyance systems, the total impervious 

area represented by pavement and rooftops would provide the 

necessary data to define the percent impervious area for each 

HRU in PRMS. However, in less-developed areas, the total 

impervious surface would not necessarily represent the total 

area contributing directly to runoff. This is because runoff 

generated on impervious surfaces may not be routed directly 

to streams but would instead be routed to grassed or wooded 

areas where some or all of it would infiltrate. Because of this, 

the total impervious area in an HRU is converted to an effective impervious surface for different categories of land use. 

The actual percent impervious surface area provides the 

basis from which the parameter for effective impervious area 

can be estimated. The relation between effective impervious 

area and land-use category is developed from land-use/runoff 

data available for the Pomperaug River and its tributaries as 

well as other available data. The total impervious area does 

not include areas of water (lake, pond, and stream surfaces). 

The direct precipitation on water surfaces is taken into account 

as part of the srunoff_smidx module previously discussed. The 

relation between land use and impervious surface would take 

the following form:

(Total impervious area, in percent) = 

Σ (ci*(land use, in percent)i), (10)

where

c is the assumed percent impervious for each 

land-use category, and

i is summed within each HRU.

This relation has been developed for the state of Connecticut 

(Prisloe and others, 2002) on the basis of LANDSAT satellite 

imagery (fig. 9).

The total estimated effective impervious area (EIA) 

for each HRU (as a fraction) would then be determined 

as a function of the estimated total impervious area (TIA) 

computed by an assumed connectedness relation with the 

following general form:

(EIA) = k1*(TIA)N, (11)

where

k1 is the intercept of the log regression, and

N is the slope of the log regression.

The values for k1 and N from the literature (Alley and 

Veenhuis, 1983) are often assumed to be 0.15 and 1.41, 

respectively, for a “typical” region in the United States with 

storm sewers in place.

Equation 11 is modified for Connecticut using streamflow 

and precipitation data for the State. Assuming that the runoff 

from impervious surfaces enters the stream directly within the 

daily time step of the model, direct runoff from impervious 
The Precipitation-Runoff Model 25

surfaces is assumed to be occurring during periods where the 

hydrograph rises and falls within a single day in response to 

a single storm event. Additionally, the potential effect of ice 

and snow accumulation were avoided by analyzing runoff 

only during the late spring, summer, and early fall. The storm 

runoff analysis is thus constrained to include single-day storms 

with concurrent runoff occurring from April through October 

during a period of streamflow recession. An additional constraint limits the storms analyzed to 0.5 in. of rainfall or less in 

order to avoid potential issues associated with runoff generation from increasing ground saturation (Zarriello and Barlow, 

2002; Zarriello and Ries, 2000).

The runoff analysis described above used 18 watersheds 

in Connecticut representing the range of land-use and impervious-surface percentages in the State. The selected watersheds 

included only those with areas comparable to or smaller than 

the Pomperaug River (ranging in area from 4 to 75 mi2) so 

that attenuation of surface runoff due to scale effects would 

be of the same order of magnitude as would be expected for 

watersheds of the size of the Pomperaug, Weekeepeemee, and 

Nonnewaug Rivers. Within the range of watersheds used in 

the analysis, no relation was found between the ratio of EIA 

to TIA and the size of the watershed. This indicates that at 

the scale of interest, the estimated EIA is independent of the 

watershed size.

Two years of recent streamflow record were analyzed for 

each of the watersheds. Suitable runoff events were isolated 

from the records and correlated with precipitation records 

from nearby NOAA weather stations. 

The pool of weather stations used for the analysis 

included 12 stations with continuous record. 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 multiple stations were used, the 

values from each station were averaged. Thus, the analysis 

does not attempt to evaluate the spatial distribution of precipitation as it relates to the watersheds of interest. Instead, the 

large number of events extracted from the data is assumed to 

average out the errors associated with the use of potentially 

nonrepresentative weather stations.

The results of the runoff analysis are provided on 

table 4 with a general description of the development status 

of the watershed. In general, the percent total impervious area 

for the Connecticut watersheds was low compared to the data 

from Alley and Veenhuis (1983). For this reason, the Alley 

and Veenhuis data were combined with the Connecticut data 

in order to provide a data set appropriate for use in areas that 

are partly urban and partly rural (herein referred to as the 

Connecticut modified data set). In addition, an assumed value 

for EIA when EIA is 100 percent was included. It was found 

that the data shown on table 4 fit a polynomial model better 

than a power function, indicated by equation 11. Because there 

does not appear to be any specific physical reasoning behind 

the form of the model, the polynomial form is assumed to be 

similarly appropriate. The modified EIA estimation model for 

Connecticut determined from the data on table 4 is given as:

(EIA) = 0.008*(TIA)2 + 0.191*(TIA), (12)

for the Connecticut and Alley and Veenhuis data (Connecticut 

modified), and

(EIA) = 0.00013*(TIA)3 + 0.005*(TIA)2 + 0.2287*(TIA), (13)

for the Connecticut data only. This latter equation is considered the most appropriate for the Pomperaug River watershed 

because it is rural.

The increase in EIA with increasing percent impervious 

surface for three different data sets is shown on figure 12. 

The curve for the Alley and Veenhuis (1983) data (green 

line) is representative of urban areas in and around Denver, 

Colo. These data show a steeper rate of increase of EIA in 

response to TIA than the Connecticut data (blue line). This 

difference is likely because of the larger and more rural nature 

of the watersheds used to develop the Connecticut data. 

The Connecticut modified curve (red line) is a composite of 

the two data sets and could be used in areas of Connecticut 

that are transitioning from rural to urban settings. The blue 

curve is the most appropriate for use in the Pomperaug River 

watershed because of the generally low density of impervious 

area in the watershed and for all the HRUs in the model. 

The data on figure 12 indicate that a family of curves 

describing rural to urban settings (reflective of imperviousarea density) may be an appropriate way to characterize 

the TIA/EIA relation for different hydrologic conditions. 

However, these curves also may be modified on the basis of 

a specific knowledge of the existence and effectiveness of 

storm drains and impervious connectedness to convey rainfall 

runoff directly to water bodies. Estimates of EIA derived 

from equation 13 are comparable to an equation proposed by 

Sutherland (2000) for somewhat disconnected watersheds 

with some infiltration measures over the range of impervioussurface percentages found in the Pomperaug River watershed. 

Subsurface Runoff

The subsurface-runoff component of the total streamflow 

is not easily determined because the mechanisms by which 

it occurs may be highly variable and not necessarily a function of general watershed characteristics. For this reason, the 

subsurface-runoff contribution is calculated from equation 4 

once groundwater and surface runoff are estimated. Because 

groundwater discharge equals groundwater recharge, the longterm recharge is fixed by the estimate of long-term groundwater runoff. The values of the subsurface parameters are 

optimized once the parameters for groundwater and surface 

runoff are estimated. 

The Rosenbrock optimization routine in PRMS 

(Rosenbrock, 1960; Leavesley and others, 1996) was used to 

determine the two key subsurface-runoff parameters that route 

water vertically (recharge to groundwater) and horizontally 

(rapid subsurface runoff). The optimization was accomplished 

for the watershed as a whole, assuming a single subsurface 
26 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Table 4. Impervious-surface data used for Connecticut runoff analysis.

[USGS, U.S. Geological Survey; mi2, square miles; na, not applicable]

River/streamgage name

USGS 

streamgage 

identification 

number

Actual 

impervious 

surface

(percent)

Percent 

water

Effective 

impervious 

surface 

(percent)

Watershed 

area

(mi2)

Development status

Whitford Brook near Old Mystic 01118668 3.64 2.30 0.9 12.0 Rural no controls

Mount Hope River near Warrenville 01121000 2.82 1.98 1.4 28.6 Minor regulation

Stony Brook near Suffield 01184100 4.24 0.85 0.8 10.4 Rural no controls

Broad Brook at Broad Brook 01184490 4.95 0.82 1.7 15.5 Minor regulation

Burlington Brook near Burlington 01188000 3.41 0.71 1.8 4.10 Minor regulation

Hockanum River near East Hartford 01192500 12.4 2.16 2.2 73.4 Urban stormwater 

collection and 

minor regulation

Coquinchaug River at Middlefield 01192983 4.33 1.07 0.7 29.8 Rural no controls

E.B. EightMile River near N. Lyme 01194500 3.44 0.77 0.7 22.3 Rural no controls

Indian River near Clinton 01195100 4.30 0.37 0.1 5.70 Rural no controls

Quinnipiac River at Southington 01195490 18.0 1.14 2.6 17.4 Minor stormwater

Mill River near Hamden 01196620 8.73 0.55 1.1 24.5 Rural no controls

Still River at Brookfield 01201487 12.8 2.72 1.7 62.3 Minor stormwater 

and regulation

Pomperaug River at Southbury 01204000 3.84 1.29 1.1 75.1 Minor regulation

Rooster River at Fairfield 01208873 27.7 1.06 5.6 10.6 Urban stormwater 

collection

Mill River near Fairfield 01208925 6.97 5.78 1.9 28.6 Flow regulation

Sasco Brook near Southport 01208950 6.02 0.57 1.1 7.40 Rural no controls

Saugatuck River near Redding 01208990 3.12 1.54 1.4 21.0 Rural no controls

Norwalk River at South Wilton 01209700 6.75 1.52 1.8 30.0 High flow regulation

Data from Alley and Veenhuis (1983) na 39 0 23 na

Data from Alley and Veenhuis (1983) na 26 0 15 na

Data from Alley and Veenhuis (1983) na 15 0 8.5 na

Data from Alley and Veenhuis (1983) na 60 0 42 na

Data from Alley and Veenhuis (1983) na 88 0 83 na

Assumed na 100 0 95 na

Average for Connecticut watersheds 7.63 1.43 1.6
The Precipitation-Runoff Model 27

reservoir, and then distributed to each HRU on the basis of the 

specific physical characteristics of each HRU. 

The subsurface reservoir represents areas of rapid nontraditional groundwater flow (not an extensive aquifer) often 

occurring as shallow perched groundwater zones:

• overlying the interface between till and bedrock, 

• existing as sand lenses, and 

• existing as secondary porosity below and within the 

root zone. 

The reservoir is assumed to behave in a linear manner with 

vertical and horizontal outflow components driven by the total 

water stored within it. Thus, the vertical and horizontal routing 

coefficients are defined as

Vertical flow = ssr2gw_rate(ssres_stor) (14)

Horizontal flow = ssrcoef_lin(ssres_stor) (15)

where

ssr2gw_rate

and ssrcoef_lin are the vertical and horizontal routing 

coefficients, respectively, and

ssres_stor is the total water volume stored in the 

reservoir (in inches).

The initial magnitude of the calibrated parameters ssr2gw_rate 

and ssrcoef_lin is determined by optimization using the 

Rosenbrock module (Leavesley and others, 1996) and distributed to each HRU on the basis of percent coarse glacial 

stratified deposits, assuming that this is an indication of the 

presence of non-traditional groundwater flow paths.

The ssr2gw_rate routing coefficient ranges from 0 to 1 

and its spatial variability is assumed to be a function of the 

subsurface conditions in each HRU. As an initial assumption, 

this value is approximated by the percent coarse glacial stratified deposits in each HRU, with a nominal value even if there 

is no occurrence of coarse glacial stratified deposits. Thus, 

vertical infiltration will generally only occur in areas of coarse 

glacial stratified deposits. 

The ssrcoef_lin routing coefficient also ranges from 0 to 

1 and is assumed to be a function of surficial geology, so that 

the horizontal to vertical flow preference is determined as a 

function of the percent glacial stratified deposits in the HRU. 

Where there are more coarse glacial stratified deposits, vertical 

movement is favored and vice versa. Additionally, the surface 

slope will influence the rate of horizontal flow out of the 

reservoir and, therefore, the value of the parameter. Because 

this parameter sets the rate of horizontal flow from the 

Figure 12. Relation between total impervious 

area in the watershed [as mapped by Prisloe and 

others (2002)] and effective impervious area as 

used in the Precipitation Runoff Modeling System 

model for three different data sets, Pomperaug 

River watershed, Connecticut. 

0 10 20 30 40 50 60 70 80 90 100

TOTAL IMPERVIOUS AREA, IN PERCENT

0

10

20

30

40

50

60

70

80

90

100

EFFECTIVE IMPERVIOUS AREA, IN PERCENT

Runoff data, by source

Alley and Veenhuis (1983)

Connecticut data—U.S. Geological Survey

Connecticut modified—U.S. Geological Survey

and Alley and Veenhuis (1983)

Connecticut data—U.S. Geological Survey
28 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

subsurface reservoir, it is assumed to be equal to the inverse 

of the percent coarse glacial stratified deposits (allowing for a 

nominal value even where there is 100 percent coarse glacial 

stratified deposits) and the relative slope of the HRU (HRU 

slope relative to the mean slope of all the HRUs). 

Groundwater Flow

In the Pomperaug River watershed, previous studies 

on groundwater contribution to streamflow have generally 

concluded that the long-term contribution from groundwater 

is approximately 60 percent (see previous sections). Thus, 

the long-term simulated groundwater recharge should 

be approximately 60 percent of the observed total runoff 

(streamflow) at the streamgage. The two generalized aquifer 

types, one comprised of till and bedrock (identified by the 

subscript till), and the other coarse glacial stratified deposits 

(identified by the subscript cs) are characterized as having the 

following values for transmissivity (T) and storativity (S) on 

the basis of work by Starn (J.J. Starn, U.S. Geological Survey, 

oral commun., 2004): 

Tcs = 10,000 ft2/d for coarse glacial stratified 

deposits,

Ttill = 200 ft2/d for till/bedrock,

Scs = 0.2 for coarse glacial stratified deposits, and

Still = 0.01 for till/bedrock.

Each HRU has a percentage of both aquifer types, and the 

representative value for the hydraulic variables for each HRU 

is computed as the area-weighted harmonic mean. 

The groundwater runoff is simulated as a linear reservoir. 

Recharge to the reservoir occurs from the subsurface reservoir 

controlled by the vertical subsurface-runoff coefficient and by 

direct recharge from the soil zone. Physically based estimates 

of the groundwater-reservoir routing coefficient for each HRU 

are developed in order to provide the spatial and temporal distribution of groundwater runoff in the watershed. The following linear reservoir equation controls groundwater runoff from 

the groundwater reservoir in PRMS:

gwflow = (gwflow_coef)*(gwres_stor). (16)

The gwflow_coef coefficient sets the linear groundwaterrunoff rate from the groundwater reservoir (gwres_stor) for 

each HRU. The coefficient ranges from 0 to 1; a high value 

represents highly conductive reservoir materials, and a low 

value represents poorly conductive materials. Equation 16 is 

updated each time step of the model and can thus be formulated as a linear differential equation:

gwflow = δh/δt = k*(H), (17)

where

h is the flow depth per day from the reservoir 

with total storage H, 

k is a routing coefficient,

δh is the change in head, and

δt is the change in time.

The solution to this equation is

k*t = ln(H) (18)

where t is elapsed time.

Equation 18 indicates that gwflow_coef is a linear reservoir 

decay coefficient. The value of gwflow_coef can therefore 

be conceptualized as the base-flow recession coefficient for a 

stream hydrograph, where the source reservoir is the PRMS 

groundwater reservoir. As developed by Halford and Mayer 

(2000), the base-flow decay over time period t is given as:

Q = Q0*exp 

2

2

4 L S

− ∗ Τ ∗  t

∗ ∗

 

    , (19)

where

Q is the flow after time t has elapsed,

Q0 is the initial flow,

T is the representative transmissivity of the 

HRU,

S is the representative storativity of the HRU, 

and

L is the groundwater flow length.

Assuming Q and Q0 to be equivalent to reservoir storage, h, 

and t (the time step) = 1 day, equation 19 can be considered 

as a corollary to equation 17 where the decay coefficient k is 

equal to 

exp 

2

2

4 L S

− ∗ Τ ∗  t

∗ ∗

 

    .

Because equation 17 is updated for each time step and 

computes the outflow from the reservoir depth rather than 

the reservoir depth after the elapsed time, the value for 

gwflow_coef is equivalent to 1–k. This is shown by assuming 

that the reservoir head, H, is equal to the head during the 

previous time step times k. Thus, the updated reservoir head is 

given by:

Hi+1 = k*Hi . (20)

The updated reservoir head is also calculated as the difference between the initial reservoir head minus the outflow 

over the time step (hi), thus: 

Hi+1 = Hi – hi (21)

Equating these two relations and re-arranging gives:

hi = Hi*(1–k) (22)
The Precipitation-Runoff Model 29

which shows that over each time step, i, the outflow rate (h) is 

determined from the beginning reservoir head (H) times (1–k).

The flow length L in equation 19 is computed as half of 

the average distance to a discharge point (stream channel), 

which is determined as the inverse of the drainage density for 

each HRU. The values of T and S are determined as the areaweighted average values for each HRU.

Although a finite difference groundwater model 

(MODFLOW-2000) (Harbaugh and others, 2000) is used to 

determine the simulated groundwater flow (Starn and Brown, 

2007), parameterizing it effectively in PRMS is important 

so that the recharge estimates generated by PRMS reflect the 

expected groundwater conditions.

Channel Routing

The ratio of the daily mean streamflow in the Pomperaug 

River at Southbury and tributary streamflow measured at the 

continuous streamgages on the Weekeepeemee River and the 

Nonnewaug River is variable and indicates that spatial and 

temporal timing and quantity of tributary flow results from 

variability of hydrologic conditions within the watershed. 

However, some of the variability observed could be caused by 

the time lag between flows at upstream locations and those at 

the downstream location. For example, the lag time between 

a high flow at the Nonnewaug streamgage and a high flow 

at the Pomperaug streamgage for a given storm event may 

be sufficiently long that the daily peaks would not occur on 

the same day. If this happened, the accumulation of flow in 

a downstream direction would have to consider flow timing 

(channel routing), which currently is not part of the model 

simulation. To evaluate this possibility, the instream traveltime 

for water to move through the stream network was assessed 

from 15-minute hydrographs from the Weekeepeemee, 

Nonnewaug, and Pomperaug streamgages for numerous 

short-duration high-flow events. The 15-minute hydrographs 

for a short period of record during which a rapid rise and fall 

of streamflow was observed at each streamgage are shown on 

figure 13. The analysis shows that the lag time is typically less 

than 4 hours and indicates that the use of a daily time step for 

the model simulations should not introduce major errors due 

to flow timing and channel-routing effects. It should be noted, 

however, that this lag time could happen over 2 days.

Figure 13. The continuous response (in minutes) of flow events in the Nonnewaug, 

Weekeepeemee, and Pomperaug Rivers streamgages, Connecticut, March 1 to 7, 2004. 

1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000

MINUTES, MARCH 1–7, 2004

0

0

200

400

600

STREAMFLOW, IN CUBIC FEET PER SECOND

Nonnewaug River

Pomperaug River

Weekeepeemee River
30 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

To verify these observations, the Muskingum (Maidment, 

1993) and Convex channel-routing methods (Dingman, 2002) 

were used to route streamflow from two 24-hour, 15-minute 

time step hydrographs at the Nonnewaug and Weekeepeemee 

streamgages to the Pomperaug streamgage. The analysis found 

that the traveltime between these stations and the Pomperaug 

streamgage varied from 2 to 4 hours, similar to the analysis of 

the hydrographs. 

With the exception of the Bronson-Lockwood Reservoir 

and several abandoned quarry ponds near the Pomperaug 

River in the lower portion of Woodbury, surface-storage reservoirs are not considered to have a major effect on streamflows. 

The Bronson-Lockwood Reservoir is important only in that 

the Watertown Fire District periodically releases water from 

the reservoir above the normal throughflow (natural flow 

going into and out of the reservoir) to augment flow in the 

Nonnewaug River when it is needed (Michael Taunis, Operations Manager Watertown Fire District, oral commun., 2005, 

see following section). 

The quarry ponds affect peak flows in the Pomperaug 

River at Southbury by diverting high flows out of the stream 

into the pond system, thus attenuating flood peaks. This effect 

is shown on the stage hydrographs (fig. 14) for the flood of 

March 29, 2005, at the Weekeepeemee, Nonnewaug, and 

Pomperaug River streamgages. The figure shows that the peak 

observed at the two upstream streamgages is cutoff by the 

time the flow reaches the downstream Pomperaug streamgage. 

The observations at the Pomperaug streamgage clearly show 

the effect of storage in the quarry ponds on flow when the 

river overtops a section of its bank adjacent to the ponds 

(figs. 15 and 16). The low berm that separates the overflow 

channel connecting the ponds from the Pomperaug River is 

only a few feet across and appears to be continuing to erode, 

raising the possibility that the river may flow into the ponds 

more frequently or at lower stages in the future. It is not clear 

when this phenomenon became important hydraulically, but it 

appears to have been occurring since the mid-1990s. 

Figure 14. Stage for the Pomperaug, Weekeepeemee, and Nonnewaug Rivers, Connecticut, during the 

flood event of March 29, 2005, showing the effect of storage at the Pomperaug streamgage.

500 1,000 1,500 2,000 2,500 3,000 3,500

MINUTES, MARCH 28–30, 2005

0

2

1

3

4

5

6

7

STAGE, IN FEET

Nonnewaug River

Pomperaug River

Weekeepeemee River
The Precipitation-Runoff Model 31

Figure 15. Location of three abandoned quarry ponds and their proximity to the Pomperaug River, Connecticut.

Pomperaug River

43

44

45

42

40

47

49

48

39

38

35

34

43

44

45

42

40

47

49

48

39

38

35

34

Abandoned

quarry ponds

Overflow

channel

48

Hydrologic response unit boundary

and number

EXPLANATION

41°30'30"

73°13' 73°12'30" 73°12'

41°31'

Base from U.S. Geological Survey, 1:24,000, 1969 to 1984

Connecticut State Plane projection 0 600 1,200 FEET

0 150 300 METERS
32 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

The overflow phenomenon has been observed by operators at the quarry as well as by USGS personnel, and there 

is abundant post-flood evidence that the river overflows into 

the pond system at the location of the low berm. In the past, 

attempts have been made to shore up the berm with rip-rap; 

however, the phenomenon continues to occur. Observations 

at the streamgage show that the overflow occurs at a stage of 

approximately 6.5 ft (fig. 14), which corresponds to a flow at 

the streamgage of approximately 1,400 ft3/s. Estimates of the 

hydraulic capacity of the Pomperaug River channel adjacent 

to the low berm indicate that it would overtop the berm at a 

streamflow of 800 to 1,000 ft3/s; this is verified by the PRMS 

model. Additionally, there is a designed overflow at the 

upstream end of the reach adjacent to the quarry ponds that 

will overtop at extreme high flows (estimated to be at or near 

the 20-year flood event).

To account for the overflow and storage phenomenon, the 

PRMS-generated streamflow tabulated for the reach adjacent 

to the ponds was adjusted by routing streamflow above 

800 ft3/s through a linear reservoir, similar to the reservoir 

routine used in PRMS. After the flow was routed, it was added 

back to the PRMS-generated hydrograph, thus adjusting the 

hydrograph for the effect of storage in the quarry ponds. 

The linear reservoir coefficient was adjusted in the model in 

order to match the reduction in peak flow between the PRMS 

simulation and the observed streamflow record. 

The stage at which the stream overflows, however, has 

been variable, which may reflect natural and anthropogenic 

changes in the condition of the berm. A review of the stage 

hydrographs over time at the Pomperaug streamgage shows 

that the condition of the bank separating the river and the 

overflow channel is dynamic and that the frequent overtopping 

may be a relatively recent phenomenon. The overflow above 

800 ft3/s is likely not constant in time and cannot be verified as appropriate under all conditions. For this reason, the 

adjustment of flood peaks to account for the overflow has been 

applied only to the recent record (1998–2005) and is considFigure 16. Looking south (downstream) of the high-flow event of March 29, 2005, showing the Pomperaug River, Connecticut, on the 

extreme left, and overflow into the quarry pond connector stream on the right. 
The Precipitation-Runoff Model 33

ered to be only an approximate adjustment that illustrates a 

potential effect.

The return flow from the ponds to the river is through an 

outlet structure that consists of a concrete broad-crested weir 

and rip-rap spillway channel. Field observations have confirmed that the outlet structure becomes submerged as the river 

rises and the ponds fill. In addition, other areas, notably a large 

wetland downstream from the ponds, become filled during 

high flows, providing further off-channel storage. The return 

flow from these other areas may be relatively slow, because 

there is no observed outlet channel. In summary, a complex 

hydraulic situation exists in the reach adjacent to the quarry 

ponds that facilitates diversion of river flow into large offchannel storage areas, notably the abandoned quarry ponds, 

and this water is returned more slowly to the river through 

an outlet structure and bank seepage. The effect of the offchannel storage is important and, therefore, has been explicitly 

modeled and incorporated into the simulation results.

The simple approach to routing flow into and out of the 

quarry ponds and their associated wetlands, described above, 

was developed on the basis of a physical representation of 

the river/pond interaction and outlet structure. Given the true 

complexity of the situation, however, the routing approach 

is considered only an approximation designed to provide an 

appropriate level of peak-flow attenuation. The fillable storage 

area of the ponds was estimated from topographic maps, and 

the outlet structure was represented by an 8-ft wide broadcrested weir. The diversion threshold was assumed to be 

800 ft3/s in the river (as determined by PRMS adjacent to the 

ponds) above which water is diverted out of the river into the 

pond/wetland storage. The fraction of water that flows into 

the ponds and the weir coefficient were treated as arbitrary 

variables and were determined by modeling judgment.

The routing procedure was accomplished on the daily 

time step by determining a total inflow to the storage that was 

translated into a height of stored water. This height was then 

averaged with the height from the previous time step to obtain 

an average pond head from which an average outflow rate was 

estimated. The total daily outflow was then computed, and this 

volume was subtracted from the storage volume to obtain a 

new pond height. This procedure was repeated each day until 

the fillable storage area of the ponds was emptied. PRMS 

provided the input data to the ponds, and the outflow rate was 

added back to the hydrograph and combined with downstream 

runoff also generated by PRMS to reconstruct the hydrograph. 

The pond/wetland storage has no effect on river flows below 

the 800 ft3/s threshold for the reach adjacent to the ponds. 

Diversions

The total water used and derived from the watershed, 

estimated from public water system records of per-capita 

water use (table 1), including water from individual domestic 

wells, is estimated to be 6.4 ft3/s during the spring and summer 

months (May, June, July, August, and September) and 5.3 ft3/s 

during the fall and winter (October, November, December, 

January, February, March, and April). All the water used and 

derived from within the watershed comes from groundwater 

sources, except for some local irrigation water pumped from 

surface-water sources. This source of water use is not documented and is not reported here. 

All of the groundwater withdrawals from Watertown 

Fire District and some of the groundwater withdrawals from 

Heritage Village are diverted out of the watershed. Tabulation 

of water use by the three largest public-supply systems shows 

that during the fall, winter, and early spring months (October 

through April) approximately 1.4 ft3/s from the aquifer is 

diverted out of the watershed on a daily basis (1 ft3/s from the 

Watertown Fire District and 0.4 ft3/s from Heritage Village), 

and during the late spring and summer months (May through 

September), the average increases to approximately 2.1 ft3/s

(1.5 ft3/s from the Watertown Fire District and 0.6 ft3/s from 

Heritage Village). During periods of low flow, the Watertown 

Fire District releases water from the Bronson-Lockwood 

Reservoir upstream of the well field to maintain flows in the 

Nonnewaug River and to recharge the well field. On the basis 

of a review of 6 months of documented releases (January to 

July 2004; Ernie Coppock, Watertown Fire District, written 

commun., 2004) and reservoir inflows estimated by adjusting 

the Nonnewaug streamgage record for watershed drainage 

area to the reservoir, there appears to be a general balance 

between reservoir release and reservoir inflow during the 

period analyzed, and the difference between reservoir release 

and inflow is greatest during periods of low inflow. Thus, 

the reservoir operation acts to redistribute flow from high 

flow to low flow, consistent with the operational goal of the 

Watertown Fire District. Over the course of time, there is 

no net gain or loss of water from the reservoir relative to 

natural inflow; however, the residence time of water in the 

reservoir has not been determined. Consequently, the yearto-year change in storage within the reservoir and the aquifer 

around the well field is not documented and is assumed to be 

sufficient to maintain the out-of-watershed diversion from the 

well field while maintaining flow-through volumes.

Approximately 1/3 of the water pumped from the 

Heritage Village well field also is diverted out of the 

watershed (Pomperaug River Watershed Coalition, 2001). 

This water is not made up through storage and, therefore, 

is considered a net loss to the watershed as a whole. Part of 

the water used by individual homes and public supplies in 

the watershed is consumptive, defined as that part of water 

withdrawn that is evaporated, transpired, incorporated into 

products or crops, consumed by humans or livestock, or 

otherwise removed from the immediate water environment. 

Consumptive use is assumed to be 20 percent of water used 

during the late spring and summer months (Starn and Brown, 

2007; Solley and others, 1998) and 15 percent during the fall, 

winter, and early spring. The PRMS model does not account 

for the diversions or consumptive use, and therefore, these 

water losses were accounted for by direct subtraction from 

the simulated hydrograph, similar to the method of addressing 

high-flow routing through the quarry ponds.
34 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Calibration, Objective Functions, and 

Optimization

The objective functions for the model provide a measure 

of the goodness of fit between the simulated and observed 

streamflow. As previously mentioned, the weather input data 

(temperature and precipitation) and the GIS data were considered to be true measures after initial quality control and 

were not subject to calibration or optimization. Similarly, the 

streamflow data recorded at the Pomperaug, Weekeepeemee, 

and Nonnewaug River streamgages are considered to be 

true and are not subject to calibration or optimization. Final 

calibration was achieved by adjusting parameters that were 

not previously determined from the water-balance analysis to 

achieve an acceptable result for the streamflow hydrograph as 

a function of the total flow volume and the variability of flow.

It is recognized that errors are associated with all the 

input data sources, and the model results calibrated and optimized to these data are considered to be applicable within the 

constraints of the input data errors. As a result, and to meet 

the objective that the model be transferable in time and space, 

the philosophical approach taken in this study was to optimize 

and calibrate over the entire record and not over specific time 

periods or regions of the model to achieve a better fit with the 

streamflow record. With this in mind, the model parameters 

are consistent in time and space and are based on the input 

data so that the model is reasonably transferable to regions and 

times where similar input data are available. 

The Pomperaug River watershed study was concerned 

with the spatial and temporal (primarily seasonal) accuracy 

of each simulated component of the hydrograph—including 

surface runoff, subsurface runoff, and groundwater runoff 

(base flow)—thus, the objective functions must be suitable for 

use with each of the components as well as the total streamflow hydrograph at different locations in the watershed. The 

objective functions include the observed daily flow for the 

Pomperaug, Nonnewaug, and Weekeepeemee Rivers and the 

runoff components as a percentage of the mean daily flow. 

Additionally, the groundwater base flow can serve as an objective function given a consistent analysis that can be used for 

both the observed and simulated records. The accuracy of the 

simulated streamflow for the period and statistical distribution 

of interest is expected to be within the range of accuracy for 

the same measures as the observed streamflow. In this study, 

the time frame of interest is the entire period of record and not 

necessarily any given day, month, or year. The accuracy of the 

observed streamflow is generally considered to be between 5 

and 10 percent of the actual streamflow; thus, over the period 

and distribution of interest, the simulated streamflow should 

be within 5 and 10 percent of the observed record with no 

unexplained bias.

Two input data sets were used to evaluate the model 

calibration—daily precipitation and maximum/minimum 

temperature for 1975–1999 developed from 12 NOAA _NWS 

coop stations in the region (including 1 in the watershed—

Woodbury) (see fig. 1) and daily precipitation and maximum/

minimum temperature for 1998–2005 developed for 3 automated NOAA climate stations that report hourly data and the 

Woodbury coop station. The evaluation was focused on statistical similarities between the simulated streamflow data and 

the observed streamflow from the Pomperaug, Weekeepeemee, 

and Nonnewaug River streamgages and similarities between 

the simulated groundwater base flow and the groundwater base 

flow estimated from the streamgage data. The comparisons 

were made against the whole simulated record, and parameter 

calibration was accomplished in a consistent manner for the 

entire simulated record with less emphasis on specific time 

periods or individual daily events. This approach minimizes 

over-calibration to periods of time that may be subject to 

greater data input uncertainty and provides greater consistency 

in simulations assuming different land-use conditions. The 

evaluation was based on five objective functions derived from 

the observed streamflow at the three streamgages. These 

functions are:

1. Comparison of the total flow statistics for the 

calibration periods including the mean and standard 

deviation of the observed and simulated streamflows.

2. Goodness of fit between the observed and simulated 

streamflow evaluated using the log-residual or 

Nash-Sutcliffe statistic.

3. Comparison of the observed and simulated flowduration curves and frequency statistics.

4. Comparison of the magnitude and trends between 

the observed and simulated ratios of the tributary 

streamflow (Weekeepeemee and Nonnewaug Rivers) 

and the streamflow at the Pomperaug River.

5. Comparison of the observed and simulated 

groundwater contribution to streamflow and baseflow recession characteristics using a consistent 

base-flow separation program, PART (Rutledge, 

1993), and the overall percent groundwater contribution to streamflow.

The model parameterization approach is based on physical reasoning; thus all the parameters are assigned on the basis 

of the physical characteristics of the watershed or as a function 

of the input data. For example, the groundwater flow coefficient is determined on the basis of the hydraulic characteristics 

of the two dominant aquifer system types in the watershed—

coarse glacial stratified deposits and till/bedrock materials. 

Characteristic values for the transmissivity, storativity, and 

flow-path length are assigned to each HRU on the basis of the 

percent coarse glacial stratified deposits in the HRU. Similarly, the precipitation and temperature values assigned to each 

HRU are determined by the xyz spatial relations statistically 

determined from the available weather-station records.

The final calibration of PRMS was through optimization 

of the subsurface-runoff routing parameters: ssr2gw_rate, 

which controls the rate that water leaves the subsurface reservoir vertically to recharge the groundwater reservoir, and 
The Precipitation-Runoff Model 35

ssrcoef_lin, which controls the rate at which water from the 

subsurface reservoir leaves horizontally directly to streams. 

The subsurface reservoir consists of a potentially complex 

assemblage of unconsolidated materials between the bottom 

of the soil layer and the top of the groundwater reservoir. The 

rate of vertical and horizontal leakage out of the subsurface 

reservoir is inherently difficult to measure and is generally 

unknown for watersheds in Connecticut. The range of values 

for these parameters was difficult to determine from the GIS 

data in the watershed; therefore, the average values of these 

two parameters were determined from optimization using 

the PRMS Rosenbrock (1960) optimization routine with the 

Pomperaug River flow at the Southbury streamgage as the 

objective function.

Multiple measures of goodness of fit are available in the 

optimization schemes, including error computed from the 

untransformed or log-transformed flow data and the absolute 

or square error. In order to minimize the weight of the largest flows on the optimization (and thus give equal weight to 

the lowest flows), the log-transformation option was used for 

this application. The squared error and absolute error options 

provided similar results; however, the squared error was 

considered the most appropriate. This is in keeping with the 

objectives of the study. 

The two subsurface parameters, horizontal and vertical, 

were optimized for 1998–2005, assuming a single subsurface 

reservoir. Thus, the resulting parameter values represented a 

watershed average. A parameter value was assigned to each 

HRU by distributing the values on the basis of the percent 

coarse glacial stratified deposits and the average slope of 

each HRU, so that the area-weighted average of all the values 

equaled the watershed-wide calibrated value. 

The distribution functions are defined as:

ssrcoef_lin = (1–c)*(S/Sa)*f, (23)

where

c is the fraction coarse glacial stratified deposits 

in each HRU;

S is the HRU slope;

 Sa is the average HRU slope for the watershed; 

and

f is an adjustment factor determined by 

matching the mean value for each HRU 

with the optimal value for the entire 

watershed.

and

ssr2gw_rate = c + R – c*R, (24)

where R is a nominal recharge adjustment factor.

Both f (in equation 23) and R (in equation 24) were determined by matching the mean value for each HRU with the 

optimal value for the entire watershed. In addition, including 

R in the calculation of the ssr2gw_rate prevents any HRU 

from having zero groundwater recharge.

Equation 23 determined that the horizontal flow from 

the subsurface reservoir was greatest where there is the least 

coarse glacial stratified deposits (because in these areas it is 

assumed that vertical drainage is impeded by till and bedrock) 

and that high relative slope also increased the horizontal outflow from this reservoir. Equation 24 determined that, if there 

is no nominal recharge, the rate was simply a function of the 

fraction coarse glacial stratified deposits. Thus, where there is 

100 percent coarse glacial stratified deposits, this rate coefficient is at its maximum. If a nominal value is assumed (to 

meet the expected mean value from the Rosenbrock single-reservoir optimization), the equation ensures the coefficient value 

cannot exceed its maximum value of 1 by subtracting c*R.

Model Results

The simulated long-term, seasonal, and spatial 

distribution of runoff was compared with available streamflow 

records for the Pomperaug River and its two major tributaries, 

the Weekeepeemee and Nonnewaug Rivers; the simulated 

and estimated groundwater-runoff components were also 

compared. Two input data sets were used to generate the 

simulated daily streamflow time-series records. The first data 

set included daily precipitation and maximum and minimum 

air temperature from 12 coop weather stations operated 

from 1975 to 1999 as part of the daily climatological data 

series (Northeast Regional Climate Center, 2006) for New 

England. A second data set of more recent weather data also 

was compiled using currently operating stations that report 

precipitation, temperature, wind speed, relative humidity, 

estimated PET, and other meteorological data. Although both 

data sets are used for model simulation, the more-recent data 

set is considered the most important for evaluating model 

performance and will serve as the baseline data set that can be 

updated as part of future modeling efforts in the watershed.

Streamflow and Water Balance

For the period simulated by the model, the ratio of total 

streamflow to precipitation (Qt/P) equals 49 percent. This is 

comparable to the expected value, reported by Meinzer and 

Stearns (1929) and Wilson and others (1974), suggesting 

that the long-term simulation of runoff quantity is relatively 

accurate. The simulated annual runoff totals and the observed 

streamflow at the Pomperaug River streamgage for water 

years 1999 through 2005 are shown on table 5 and figure 17. 

The data show that, over the simulation period, the relative 

annual error (Qsim–Qobs)/Qobs where Qsim is the simulated 

streamflow and Qobs is the streamflow observed at the 

streamgage) is less than 5 percent. This result indicates the 

PRMS model simulates the overall water balance well and 

also indicates, over a sufficiently long period of time, the 

input precipitation totals and estimated evapotranspiration 
36 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

for the watershed as a whole are reasonably well represented. 

However, for any given year, the total runoff may be in 

error by as much as 14 percent. This finding indicates 

the precipitation inputs are not fully representative of the 

watershed average for specific years. The absolute error 

Qsim–Qobs shows an average over-prediction of streamflow 

by 3.9 ft3/s. 

The monthly mean simulated and observed streamflows 

at the Pomperaug River streamgage for 1998 to 2005 are 

shown on figure 18. The goodness of fit of the simulation, as 

indicated by a Nash-Sutcliffe model-efficiency coefficient 

(Nash and Sutcliffe, 1970) of 0.76, is relatively good and 

acceptable for PRMS model results (Hay and McCabe, 2002; 

Coon and Johnson, 2005). The coefficient of determination is 

0.76, which indicates about 76 percent of the variation in the 

observed record can be predicted from the simulated record. 

The simulated mean monthly streamflow for the entire 

simulation period is 126 ft3

/s, and the observed mean monthly 

streamflow for the same period is 122 ft3/s, a difference of 

less than 4 percent. The standard deviation of the simulated 

monthly flows is 81 ft3/s, whereas the standard deviation of the 

observed flows is 89 ft3/s. This finding indicates the simulated monthly means exhibit similar variability from month to 

month, with about 9 percent less variability in the simulated 

record. The mean of the monthly log residuals log(Qsim)–

log(Qobs) between the simulated and observed monthly 

streamflow is 0.04, which indicates the mean error between 

Table 5. Simulated and observed annual mean streamflow for 

the Pomperaug River at Southbury, Connecticut, for water years 

1999 through 2005.

[ft3/s, cubic feet per second]

Water 

year

Annual mean 

daily flow 

simulated 

(ft3/s)

Annual mean 

daily flow 

observed 

(ft3/s)

Relative 

error

(dimensionless)

Absolute 

error

(ft3/s)

1999 136.8 120.3 0.14 16.5

2000 126.4 136.5 -0.07 -10.1

2001 101.9 115.4 -0.12 -13.5

2002 72.8 63.5 0.15 9.2

2003 164.4 147.8 0.11 16.5

2004 155.4 152.2 0.02 3.1

2005 122.1 116.6 0.05 5.8

Mean 125.7 121.8 0.04 3.9

Figure 17. Simulated and 

observed annual mean streamflow 

for the Pomperaug River at 

Southbury, Connecticut, for water 

years 1999 through 2005.

1999 2000 2001 2002 2003 2004 2005

WATER YEAR

0

20

40

60

80

100

120

140

160

180

ANNUAL MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

Simulated

Observed
The Precipitation-Runoff Model 37

any given monthly mean estimate and the observed monthly 

mean is less than 10 percent. The log residual is used where 

the range of estimated streamflow values is greater than an 

order of magnitude. The standard deviation of the log residual 

for the monthly means is 0.17 (48 percent), which indicates 

there is greater error for any given monthly estimate compared 

to the long-term mean error. 

The relative error for each month ranges from 1 to greater 

than 100 percent, and 20 percent of the months show errors 

greater than 50 percent. Eighty percent of the months with 

errors greater than 50 percent are during the late summer and 

fall (July through October). These months exhibit the highest 

variation in day-to-day and year-to-year precipitation between 

stations and at individual stations because of thunderstorms 

and large cyclonic systems (nor’easters and hurricanes) that 

can develop heavy and generally localized precipitation. 

Within the daily precipitation record, there appear to be anomalies in total precipitation associated with most of the months 

with high errors. These anomalies consist of apparently excessive amounts of daily precipitation at one station, usually the 

Woodbury coop station. Although there is no means by which 

to say that these values are in error, particularly because they 

may reflect localized heavy rain, it is important to recognize 

Figure 18. Simulated and observed monthly mean streamflows for the Pomperaug River at Southbury, 

Connecticut, for calendar years 1998 through 2005.

that they might not be representative of the watershed and 

could certainly contribute to the monthly mean being too 

high. The precipitation input record shows that most months 

with a relative error greater than 50 percent contained a single 

anomalously high precipitation event at one of the stations 

and subsequent memory of this event in subsequent simulated 

daily streamflows. 

The statistics of the daily simulated streamflows at the 

Pomperaug River at Southbury over the period of record 

1999–2005 are provided on table 6. The overall mean error 

between the observed and simulated daily streamflow record 

is shown on table 6 as the mean log residual, which is equivalent to an error of less than 4 percent, which is similar to the 

monthly means. However, there is greater variability in the 

simulated daily record than in the observed record, which is 

not the case for the monthly means. If adjustments to the simulated record to account for the effect of the O&G ponds and 

groundwater diversions are not included, the simulated record 

shows much greater variability than the observed record and 

overall lower goodness-of-fit statistics. This finding indicates 

that groundwater diversions and particularly the off-channel 

flood storage in the O&G ponds substantially affect the 

streamflow regime of the Pomperaug River at Southbury. 

1998 1999 2000 2001 2002 2003 2004 2005

CALENDAR YEAR

0

50

100

150

200

250

300

350

400

450

STREAMFLOW, IN CUBIC FEET PER SECOND 

Simulated

Observed
38 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Table 6. Daily and monthly streamflow statistics for the Pomperaug River at Southbury, Connecticut.

[na, not applicable]

Flow statistics

Streamflow (cubic feet per second) Log residual 

without routing 

and diversions

Log residual 

with routing and 

diversions Observed

Simulated without 

routing and 

diversions

Simulated with 

routing and 

diversions

Recent data representing current land-use conditions, 1999–2005

Daily

Mean 122 128 126 0.032 0.019

Standard deviation 148 184 159 0.239 0.239

Minimum 3.6 7.3 4.9 -1.066 -1.082

Maximum 1,670 2,737 1,677 1.115 1.034

Nash-Sutcliffe coefficient na 0.31 0.52 na na

Coefficient of determination R2 na 0.55 0.61 na na

Monthly

Mean 122 128 126 0.054 0.040

Standard deviation 89 82 81 0.177 0.173

Minimum 5.9 12.2 9.1 -0.324 -0.354

Maximum 391 333 318 0.671 0.664

Nash-Sutcliffe coefficient na 0.75 0.76 na na

Coefficient of determination R2 na 0.75 0.76 na na

Historical data, 1976–1999

Daily

Mean 139 139 138 0.010 -0.005

Standard deviation 216 204 194 0.221 0.228

Minimum 3.3 3.6 2.6 -1.353 -1.423

Maximum 5,500 4,382 3,678 0.933 0.931

Nash-Sutcliffe coefficient na 0.62 0.64 na na

Coefficient of determination R2 na 0.64 0.65 na na

Monthly

Mean 139 139 138 0.019 0.009

Standard deviation 118 105 106 0.163 0.165

Minimum 5.8 8.4 6.4 -0.581 -0.639

Maximum 671 525 524 0.463 0.452

Nash-Sutcliffe coefficient na 0.81 0.81 na na

Coefficient of determination R2 na 0.81 0.81 na na
The Precipitation-Runoff Model 39

The simulated daily streamflow record for the historical period, 1976–1999, also shows good overall mean daily 

streamflow estimates (table 6); the mean log residual is less 

than 5 percent. However, the standard deviation of the simulated daily streamflow for this period of record was less than 

that for the observed record, suggesting that the attenuating 

effect of the O&G ponds is not as important during this simulation period. This may be because of the gradual erosion of 

the streambank adjacent to the ponds such that the more recent 

record is subject to more frequent overbank flow into the pond 

system or the effect of using different data sets. 

The simulated daily streamflows for the tributaries, the 

Weekeepeemee and Nonnewaug Rivers, for the recent data 

(2001–2005) also show good overall mean accuracy; mean log 

residual between the observed and simulated daily streamflows is less than 4 percent (table 7). Similar to the unadjusted 

Pomperaug River simulations, the standard deviations of 

the simulated streamflows are greater than for the observed 

records for both tributaries. The Nonnewaug simulated record 

included estimated out-of-watershed diversions from the 

Watertown Fire District and was adjusted by subtracting them 

directly from the time series of simulated streamflow. However, the unadjusted record more closely approximated the 

observed record at the Nonnewaug streamgage at low flow. 

This finding indicates the low-flow augmentation from the 

Bronson-Lockwood Reservoir is effective in compensating for 

the diversions.

The Weekeepeemee River has no documented diversions; 

however, there is a relatively large departure from the linear 

probability distribution for extreme low flows. This departure is not captured by the simulated record and may indicate 

an unknown low-flow diversion from the Weekeepeemee. 

Evidence of undocumented diversions from the river is based 

on anecdotal observations (Piotr Parasiewicz, Rushing Rivers 

Institute, written commun., 2007). Clearly, some of the discrepancies between the simulated and observed records for the 

Nonnewaug and Weekeepeemee streamgages may result from 

unknown effects of diversion, documented and undocumented, 

as well as unknown off-channel storage effects. 

The simulated and observed daily streamflows for the 

Pomperaug, Weekeepeemee, and Nonnewaug Rivers are 

shown on figure 19. Some major flow events were not simulated by the model, and the model simulated major events that 

did not occur in the observed record. These results indicate 

the input data set does not fully represent the overall conditions in the watershed probably because of the localized 

nature of many events, as well as the xyz distribution assumptions inherent in the model (Hay and McCabe, 2002). The 

1976–1999 data set for the Pomperaug River exhibits the same 

input-data issues indicating that even with a larger number of 

weather stations, accurately representing the precipitation and 

temperature distribution for the entire watershed is a problem.

The analysis of the residual statistics for the simulated 

and observed daily and monthly flow indicates the simulation predicts the long-term streamflow accurately, although 

there may be considerable error for any given day. The normal 

probability distribution and normal quartile distribution of the 

log residuals for all three streamgages for daily and monthly 

mean streamflow estimates are shown on figure 20. The plots 

indicate a tendency to show non-normal error response at the 

high and low extremes of the hydrograph for the daily residuals (figs. 20A and 20B) and at the high end for the monthly 

residuals (figs. 20C and 20D). This tendency is evident at all 

three streamgages. The majority of the estimated streamflow 

values at all the streamgages were normally distributed, however, showing random error.

The random error over most of the streamflow range 

indicates the model errors primarily result from error in the 

input-data distribution, rather than a model bias that would 

result if a major hydrogeologic process was not represented 

in the model. Also, the model does not show any trend 

in time, indicating the simulated water balance and storage fluxes generally are unbiased and representative of the 

long-term hydrologic processes in the watershed. The greater 

non-random error at the high end of the distribution suggests 

either a bias in the input data, with a tendency towards nonrepresentative precipitation over the watershed for the largest 

events, or indicates the model over-predicts runoff at the high 

end, possibly because of over-representation of the size of 

the maximum contributing area during large runoff events 

(smidx module). Another issue is the potential for error in the 

observed streamflow values themselves, which is unknown at 

the extreme ends of the hydrograph, and issues of unknown 

withdrawals from streams and inaccurate representation of 

overbank storage (such as in the O&G ponds). Taken together, 

the residual distribution patterns indicate the model estimates 

of the highest flows should be used with caution and may 

not be entirely representative of runoff processes that generate these large events. However, the majority of streamflow 

estimates, including the monthly mean estimates of the lowest 

flows, are unbiased and representative of the groundwater and 

surface-water processes in the watershed. 

The exact timing of peak flows is of secondary 

importance compared with preserving the range of flows 

over the simulation record. This is because, from the habitat 

and land/water-use management perspective of the study, it 

is more important to simulate the statistical characteristics of 

the flow record than to predict a specific event. Thus, flowduration curves provide the most appropriate comparative 

result. The simulated and observed flow-duration curves for 

the Pomperaug, Nonnewaug, and Weekeepeemee Rivers 

using the recent data set (1999–2005) are shown on 

figure 21. The patterns are similar to those observed on 

the residual plots. Streamflow statistics for specific flow 

percentiles are shown on table 8. Comparison between 

the observed and simulated flows for the Pomperaug, 

Weekeepeemee, and unadjusted Nonnewaug Rivers shows 

that most estimates of the specific flow frequencies are within 

10 percent, and that the extreme high and low percentiles 

are within 30 percent. This result indicates relatively good 

simulation (Coon and Johnson, 2005). 
40 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Table 7. Daily and monthly streamflow statistics for the Weekeepeemee and Nonnewaug Rivers, Connecticut.

[na, not applicable]

Flow statistics

Streamflow (cubic feet per second) Log residual 

without routing 

and diversions

Log residual 

with routing and 

diversions Observed

Simulated without 

routing and 

diversions

Simulated with 

routing and 

diversions

Recent data representing current land-use conditions, 2000–2005

Weekeepeemee River

Daily

Mean 47 45 na 0.003 na

Standard deviation 60 75 na 0.292 na

Minimum 0.1 1.7 na -1.133 na

Maximum 680 984 na 1.614 na

Nash-Sutcliffe coefficient na 0.38 na na na

Coefficient of determination R2 na 0.60 na na na

Monthly

Mean 47 45 na 0.018 na

Standard deviation 37 34 na 0.222 na

Minimum 0.9 2.9 na -0.497 na

Maximum 141 136 na 0.660 na

Nash-Sutcliffe coefficient na 0.80 na na na

Coefficient of determination R2 na 0.80 na na na

Nonnewaug River

Daily

Mean 31 30 30 0.003 -0.098

Standard deviation 42 44 44 0.293 0.462

Minimum 0.5 0.28 0.01 -1.054 -2.771

Maximum 442 613 613 0.973 0.970

Nash-Sutcliffe coefficient na 0.58 0.58 na na

Coefficient of determination R2 na 0.64 0.64 na na

Monthly

Mean 31 30 30 0.008 -0.030

Standard deviation 25 23 23 0.209 0.230

Minimum 1.4 1.1 0.4 -0.362 -0.552

Maximum 101 89 89 0.751 0.743

Nash-Sutcliffe coefficient na 0.78 0.77 na na

Coefficient of determination R2 na 0.78 0.78 na na
The Precipitation-Runoff Model 41

Figure 19. Simulated and observed daily streamflow for the (A) Pomperaug (B) Nonnewaug and (C) Weekeepeemee 

Rivers, Connecticut.

1998 1999 2000 2001 2002 2003 2004 2005 OCT JAN APR JUL OCT JAN APR JUL OCT JAN APR JUL OCT JAN APR JUL OCT JAN APR JUL OCT JAN APR JUL OCT JAN APR JUL

SEPT

STREAMFLOW, IN CUBIC FEET PER SECOND

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

Simulated

Observed

A

POMPERAUG RIVER
42 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Figure 19. Simulated and observed daily streamflow for the (A) Pomperaug (B) Nonnewaug 

and (C) Weekeepeemee Rivers, Connecticut.—Continued

2000 2001 2002 2003 2004 2005 OCT JAN APR JUL OCT JAN APR JUL OCT JAN APR JUL OCT JAN APR JUL OCT JAN APR JUL

SEPT

STREAMFLOW, IN CUBIC FEET PER SECOND

0

200

400

600

800

0

200

400

600

800

1,000

1,200

Simulated

Observed

Simulated

Observed

B

NONNEWAUG RIVER

C

WEEKEEPEEMEE RIVER
The Precipitation-Runoff Model 43

Figure 20. Log residual probability and quartile distribution plots, showing the fit line if the residuals are normally distributed, for the 

daily residuals (A and B) and the monthly residuals (C and D), Pomperaug, Weekeepeemee, and Nonnewaug Rivers, Connecticut.

0 0.2 0.4 0.6 0.8 1.0 1.1

NORMAL PROBABILITY

-5 -4 -2 0 2 4

NORMAL DISTRIBUTION

-1.2

-1.4

-1.0

-0.6

-0.4

-0.8

-0.2

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

LOG RESIDUAL

-1.2

-1.4

-1.0

-0.6

-0.4

-0.8

-0.2

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

LOG RESIDUAL

5

Line of normal probability

Nonnewaug River

Pomperaug River

Weekeepeemee River

Daily residual

Nonnewaug River

Pomperaug River

Weekeepeemee River

Line of normal distribution

Nonnewaug River

Pomperaug River

Weekeepeemee River

Daily residual

Nonnewaug River

Pomperaug River

Weekeepeemee River

A

B
44 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Figure 20. Log residual probability and quartile distribution plots, showing the fit line if the residuals are normally distributed, for the daily 

residuals (A and B) and the monthly residuals (C and D), Pomperaug, Weekeepeemee, and Nonnewaug Rivers, Connecticut.—Continued

0 0.2 0.4 0.6 0.8 1.0 1.1

NORMAL PROBABILITY

-5 -4 -2 0 2 4

NORMAL DISTRIBUTION

-1.2

-1.4

-1.0

-0.6

-0.4

-0.8

-0.2

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

LOG RESIDUAL LOG RESIDUAL

-1.2

-1.4

-1.0

-0.6

-0.4

-0.8

-0.2

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

C

D

Line of normal probability

Nonnewaug River

Pomperaug River

Weekeepeemee River

Monthly residual

Nonnewaug River

Pomperaug River

Weekeepeemee River

Line of normal distribution

Nonnewaug River

Pomperaug River

Weekeepeemee River

Monthly residual

Nonnewaug River

Pomperaug River

Weekeepeemee River

5
The Precipitation-Runoff Model 45

Figure 21. Simulated and observed flow-duration 

curves for the (A) Pomperaug (B) Nonnewaug and 

(C) Weekeepeemee Rivers, Connecticut.

0 20 40 60 80 100

PERCENT EXCEEDANCE

 0.1

 1

 10

 100

1,000

Simulated

Observed

C

WEEKEEPEEMEE RIVER

B

A

NONNEWAUG RIVER

POMPERAUG RIVER

0.001

0.01

 0.1

 1

10

100

1,000

STREAMFLOW, IN CUBIC FEET PER SECOND

Simulated with no diversions

Simulated with diversions

Observed

 1

 10

 100

1,000

10,000

Simulated with no routing

and diversions

Simulated with routing and

diversions

Observed
46 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Deviations from the observed low flows for the 

Pomperaug River indicate that groundwater withdrawals and 

diversions may not be fully accounted for in the streamflowadjustment strategy. On the basis of pumping records from the 

three major groundwater systems in the watershed, out-ofwatershed groundwater diversions may account for 2 to 3 ft3

/s 

(which is less than 3 percent of the observed mean annual 

daily flow for the period 1998 through 2005, and nearly 

100 percent of the lowest observed flows for the Pomperaug 

River at Southbury for the same time period). Additionally, if 

a base-flow reduction function that accounts for the flattening 

of the hydraulic gradient by a drop in water level (with a 

subsequent reduction in flow rate) during low-flow periods is 

assumed, the low-flow deviation observed on the simulated 

flow-duration curve can be adjusted to match the observed 

flow duration. These groundwater complexities cannot be fully 

accounted for within the groundwater module of the 

PRMS model. 

The flow-duration curves and table 8 show that the 

PRMS model provides good representation of the overall flow 

statistics of the Pomperaug River considering flow diversions 

and high-flow routing through the abandoned quarry ponds. 

The simulated extreme low flows for the Nonnewaug River, 

in accounting for diversions from the Watertown Fire District 

wells, indicate the river runs nearly dry at times, which is not 

observed at the streamgage. This finding apparently is because 

of management practices implemented by the Watertown 

Fire District, which augments low flows by releasing stored 

water from the Bronson-Lockwood Reservoir upstream, thus 

maintaining flow levels in the river and filling a series of 

infiltration ponds at the well field. This management practice, 

if assumed to be sufficient to compensate for flow reductions 

caused by pumping and out-of-watershed diversions at the 

well field, would indicate the simulated flow-duration curve, 

assuming no diversions, is more representative of the observed 

flow conditions. 

Table 8. Simulated and observed streamflow percentiles for the Pomperaug, Weekeepeemee, 

and Nonnewaug Rivers, Connecticut, October 1, 1998, to September 30, 2005.

[%, percent]

Flow percentile

Flow value (cubic feet per second)

Observed Simulated without 

routing and diversions

Simulated with 

routing and diversions

Pomperaug River

0.01 - Exceeded 99% of the time 5.6 9.4 7.3

0.10 - Exceeded 90% of the time 16 19 17

0.25 - Exceeded 75% of the time 36 43 41

0.50 - Exceeded 50% of the time 82 83 82

0.75 - Exceeded 25% of the time 147 141 144

0.90 - Exceeded 10% of the time 257 256 262

0.99 - Exceeded 1% of the time 746 945 917

Weekeepeemee River

0.01 - Exceeded 99% of the time 0.6 2.4 2.4

0.10 - Exceeded 90% of the time 4.5 5.0 5.0

0.25 - Exceeded 75% of the time 10 12 12

0.50 - Exceeded 50% of the time 31 26 26

0.75 - Exceeded 25% of the time 60 47 47

0.90 - Exceeded 10% of the time 102 94 94

0.99 - Exceeded 1% of the time 299 349 349

Nonnewaug River

0.01 - Exceeded 99% of the time 0.7 0.7 0.01

0.10 - Exceeded 90% of the time 2.7 2.4 1.6

0.25 - Exceeded 75% of the time 6 7.9 7.1

0.50 - Exceeded 50% of the time 19 19 18

0.75 - Exceeded 25% of the time 38 35 34

0.90 - Exceeded 10% of the time 69 65 64

0.99 - Exceeded 1% of the time 211 245 245
The Precipitation-Runoff Model 47

As previously discussed, the simulated flow-duration 

curve for the Weekeepeemee River shows deviations from 

the observed record at the high and low end. There may be 

undocumented diversions from the river, which could cause 

lower observed flows that would account for some of the 

observed discrepancies with the simulated record. Despite 

these issues, the Nonnewaug simulation and to a large extent 

the Weekeepeemee simulation provide good representation of 

the natural flow statistics for the tributaries. Thus, changes to 

the streamflow regime as a result of physical changes in the 

watershed (changes to land use and land cover, for example) 

are expected to be appropriately quantified and distributed 

within the watershed over most of the natural range of flows. 

Further analysis of the simulated and observed streamflows for the tributaries was accomplished by evaluating the 

dynamics of the ratio of the flows in the tributaries (Nonnewaug and Weekeepeemee Rivers) to the Pomperaug River 

flow. The statistical characteristics of the observed and simulated flow ratios for the two tributary streams are presented 

on table 9. Most of the simulated streamflow ratios reproduce 

the mean, maximum, minimum, and standard deviation of the 

range of values that occur in the observations. This finding 

indicates that on average, the simulated streamflows are properly quantified and distributed across the watershed. 

Exceptions to the general agreement between observed 

and simulated streamflow statistics are the standard 

deviation and maximum of the ratio of the Nonnewaug to 

Weekeepeemee River. The observed record shows much 

lower flow in the Weekeepeemee River in the summer of 

2005 than was observed during the previous 5 years or than 

was simulated by the model. During this period, the amount 

of precipitation during the summer was not particularly low 

relative to previous years, indicating the extreme low flows 

in the Weekeepeemee River may be caused by undocumented 

diversions, but may also be due in part to errors of low-flow 

extrapolation of the rating curve for the streamgage. This 

latter possibility, however, is not considered to be responsible 

for a large percentage of the apparent change in the flow 

relation. Another exception is the minimum of the ratio of 

the Weekeepeemee to Pomperaug streamflow. The observed 

record shows much lower extreme values for this ratio relative 

to the simulated record, further indicating the possibility of 

undocumented diversions in the Weekeepeemee River and (or) 

an indication that the model simulation over-represents storage 

in the Weekeepeemee tributary watershed. 

Flow Components and Groundwater Recharge

The component breakdown of the hydrograph simulated 

by PRMS provides an opportunity to evaluate the model representation of the runoff and flow processes. It was assumed 

that the surface-runoff component of the hydrograph was 

essentially fixed by the assumptions regarding runoff response 

to impervious area and variable-source contributing areas. The 

model parameters that control surface runoff were developed 

from observed information; however, the surface-runoff 

volume cannot be independently evaluated or verified. The 

subsurface-reservoir parameterization was based on physical 

characteristics of the watershed, although the physical characteristics were optimized for the watershed. Because of this, 

the simulated volume of subsurface runoff cannot be independently evaluated or verified. In contrast, the simulated groundwater-runoff component of the hydrograph can be evaluated 

and verified on the basis of independent estimates. 

Table 9. Tributary flow-ratio statistics for the Pomperaug, Weekeepeemee, and Nonnewaug Rivers, Connecticut.

[USGS, U.S. Geological Survey; Pomperaug River at USGS streamgage 01204000, Weekeepeemee River at USGS streamgage 012203805, and 

Nonnewaug River at USGS streamgage 01203600]

Tributary flow-ratio 

statistics 2001–2005

Ratio of Nonnewaug to 

Pomperaug daily 

flow–observed

Ratio of Nonnewaug to 

Pomperaug daily 

flow–simulated

Ratio of Weekeepeemee 

to Pomperaug daily 

flow–observed

Ratio of Weekeepeemee 

to Pomperaug daily 

flow–simulated

Mean 0.23 0.21 0.37 0.34

Standard deviation 0.08 0.07 0.10 0.06

Minimum 0.04 Less than 0.01 0.02 0.14

Maximum 0.81 0.61 1.28 0.86

Tributary flow-ratio 

statistics 2000–2005

Ratio of Nonnewaug to 

Weekeepeemee daily 

flow–observed

Ratio of Nonnewaug to 

Weekeepeemee daily 

flow–simulated

Mean 0.68 0.62

Standard deviation 0.38 0.25

Minimum 0.14 Less than 0.01

Maximum 6.45 2.77
48 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

The groundwater-runoff (base-flow) component of 

the simulated streamflow over the long term is equal to 

recharge of the groundwater reservoir. Thus, estimates of 

groundwater recharge serve as a point of verification for the 

groundwater runoff as well as recharge estimates simulated 

by the model. Additionally, estimates of groundwater runoff 

to the Pomperaug River derived from hydrograph-separation 

techniques can provide a point of verification for the simulated 

groundwater runoff.

The simulated hydrograph-component runoff quantities, 

as a percentage of the total flow, for the Pomperaug, 

Weekeepeemee, and Nonnewaug Rivers are shown on 

table 10. On the basis of manual hydrograph-separation 

techniques, groundwater flow has been estimated as 

approximately 45 percent of the total flow (Meinzer and 

Stearns, 1929). Similarly, a recharge estimation equation 

developed by Mazzaferro (1986b) for Connecticut predicts 

that groundwater flow/recharge would be approximately 

43 percent of total flow. However, during the Meinzer and 

Stearns study (1929), the majority of the watershed was under 

agriculture that used large water withdrawals from local 

surface-water bodies for irrigation (J.L. Pond, Pomperaug 

River Watershed Coalition, oral commun., 2005). Although 

the effects of these withdrawals are unknown (and cannot be 

quantified), it is likely that they reduced low flows and would 

be reflected in decreased base flows.

More recently, automated hydrograph-separation 

techniques have been developed to estimate the groundwaterrunoff component of streamflow (Rutledge, 1998). These 

methods estimate that groundwater runoff accounts for 

approximately 70 percent of the total flow for the Pomperaug 

River. The hydrograph-separation techniques do not 

distinguish between groundwater and subsurface runoff and 

inherently include some of the subsurface runoff in the baseflow estimates. If it is assumed that half of the subsurfacerunoff component is included in the base-flow estimates, the 

actual groundwater runoff would be more on the order of 

70 percent, comparable to estimates derived using base-flow 

separation techniques (Rutledge, 1998).

The manual hydrograph-separation techniques employed 

by Meinzer and Stearns (1929) and Mazzaferro (1986b) 

involve a certain degree of subjectivity and may underestimate 

the more rapid groundwater response that occurs after large 

recharge events. Therefore, these estimates may not be as reliable as the method developed by Rutledge, 1993; Rutledge, 

1998, which identifies logarithmic decay functions consistent with the mechanics of groundwater flow. However, the 

Rutledge methods may include elements of runoff generated 

from shallow groundwater reservoirs that are included as part 

of the subsurface-runoff component in the PRMS model, or 

even surface-water storage reservoirs that exhibit logarithmic 

declines in flow rates. On balance, it is expected that the “true” 

groundwater-runoff component would be somewhere between 

45 and 70 percent of the total flow. The PRMS groundwaterrunoff estimate for the Pomperaug River is approximately 

60 percent of the total flow, which is in the expected range. 

The groundwater contribution from each HRU varies in large 

part as a function of the percentage of coarse glacial stratified deposits, and there is a strong linear relation between the 

percentage of groundwater contribution and the percentage of 

Table 10. Simulated flow component contribution to the Pomperaug, Weekeepeemee, and Nonnewaug Rivers, 

Connecticut, at their respective gages.

Statistic Groundwater runoff Surface runoff Subsurface runoff

Pomperaug River source-water contribution 1976–1998

(12.7 percent coarse glacial stratified deposits)

Mean daily runoff (cubic feet per second) 81.3 20.3 38.6

Fraction of total 58 15 27

Pomperaug River source-water contribution 1999–2005

(12.7 percent coarse glacial stratified deposits)

Mean daily runoff (cubic feet per second) 72.6 20.5 34.8

Fraction of total 57 16 27

Weekeepeemee River source-water contribution 1999–2005

(7.0 percent coarse glacial stratified deposits)

Mean daily runoff (cubic feet per second) 24.3 6.2 15.9

Fraction of total 53 13 34

Nonnewaug River source-water contribution 1999–2005

(9.4 percent coarse glacial stratified deposits)

Mean daily runoff (cubic feet per second) 17.0 3.4 10.5

Fraction of total 55 11 34
The Precipitation-Runoff Model 49

coarse glacial stratified deposits in each watershed similar to 

the findings of Mazzaferro (1986b).

The PART base-flow separation program (Rutledge, 

1993; Rutledge, 1998) was used to characterize and compare 

the daily observed and simulated base-flow contribution to the 

Pomperaug River at the Southbury streamgage. The estimated 

base-flow contribution also is used to compare the observed 

and simulated estimates of long-term groundwater recharge 

assuming that the long-term base flow is equivalent to the 

long-term recharge. As discussed previously in the description of the watershed, the estimates of groundwater base flow 

and recharge determined by PART are typically high relative 

to other estimation methods and include a portion of rapid 

subsurface runoff. Considering this, the PART program is used 

to provide a consistent estimation methodology for daily measurements as a means to compare the observed and simulated 

records. The comparative annual mean base-flow statistics 

derived from PART are provided on table 11. A comparison of 

information on tables 5 and 11 indicates that PRMS simulates 

base flow better than total streamflow. The model simulation 

provides good estimates of both the short-term and long-term 

recharge and base flow.

Another approach to assessing how well PRMS simulates 

the base flow is to compare the slope of the log-log plot of the 

observed and simulated base-flow recession time-series data 

for a number of time periods. This slope is independent of the 

absolute magnitude of the flow values and is a characteristic 

of the base-flow decay. The statistic provides a direct comparison of how well PRMS simulates the base-flow decay process 

whether or not the individual recession being compared has 

the correct magnitude. Thirty-nine recession periods were analyzed, ranging from 6 to 24 days in length, during which the 

daily flow consisted entirely of base flow (as determined by 

PART). The recession periods selected spanned all seasons of 

the year. The comparative statistics of this analysis are shown 

on table 12 and indicate that PRMS generally duplicated the 

measured recessions relatively accurately. This result supports 

the previous indication that the PRMS recharge and base-flow 

simulation can be reasonably expected to duplicate watershed 

base flow and recharge processes with an accuracy sufficient 

for understanding how land-use change would affect recharge 

and base flow.

The estimated mean annual recharge to till and coarse 

glacial stratified aquifers in the Pomperaug River watershed 

is shown on table 13. This information is based on the general 

relation derived from PRMS that indicates 77 percent of the 

variance associated with recharge is due to the percentage of 

coarse glacial deposits in an HRU (red line on fig. 22). The 

PRMS-estimated recharge is similar to the previous estimates 

of recharge by Starn and others (2000) and Mazzaferro 

(1986b). These comparisons indicate the recharge and 

groundwater-discharge estimates derived from the PRMS 

simulation are reasonably represented and can be considered 

to meet the objectives of the modeling. 

The simulated recharge variability indicates that the 

larger the percentage of coarse glacial stratified deposits in an 

HRU, the more variable the recharge. Large recharge events, 

which likely cause the variability, are more likely to occur in 

HRUs dominated by coarse glacial stratified deposits. Also, 

there are higher percentages of class D soils in upland areas. 

During periods of low flow when there is no surface runoff 

and little or no subsurface runoff, the streamflow is maintained 

by groundwater runoff. During these periods, the percent 

coarse glacial stratified deposits in the HRU indicates the contribution that the HRU has in maintaining flow in the river.

The change in percent contribution of the three runoff 

components for South Brook, a small tributary to the 

Pomperaug, is shown on table 14. The HRUs that comprise the 

South Brook watershed include an upland HRU with 1 percent 

coarse glacial stratified deposits, a transitional HRU with 

4 percent coarse glacial stratified deposits, and a valley bottom HRU with 100 percent coarse glacial stratified deposits. 

The change in runoff-component quantity for South Brook 

illustrates the effect that watershed position and percent coarse 

glacial stratified deposits in the HRU had on the simulated 

flow, with a dramatic shift in the dominant flow-generating 

mechanisms depending on these factors.

The spatially distributed recharge by month for each 

HRU, over the period of simulated record, is shown on 

figure 23. Recharge is typically greatest in March. Additionally, the greatest amount of recharge occurs in the valley 

bottoms (fig. 23), although in some cases, upland HRUs can 

show relatively large recharge. Similarly, the spatial distribution of mean subsurface runoff and surface runoff are shown 

on figures 24 and 25, respectively.

Table 11. Simulated and observed annual mean base flow 

estimated using PART.

[ft3/s, cubic feet per second]

Water 

year

Annual 

mean flow 

observed

(ft3/s)

Annual 

mean flow 

simulated 

(ft3/s)

Relative 

error 

(dimensionless)

Absolute 

error 

(ft3/s)

1999 69.5 75.1 0.08 5.6

2000 94.4 80.9 -0.14 -13.5

2001 76.7 64.4 -0.16 -12.3

2002 43.3 44.1 0.02 0.8

2003 91.3 95.9 0.05 4.6

2004 100.2 92.0 -0.08 -8.2

2005 78.0 79.7 0.02 1.7

Mean 79.1 76.0 -0.03 -3.0
50 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Table 12. Daily and monthly base-flow statistics and recession characteristics for the Pomperaug River, Connecticut.

[na, not applicable]

Pomperaug River 

base-flow statistic

PART base-flow 

estimate–

observed

PART base-flow 

estimate–

simulated

Log residual 

simulated–

observed

Recession slope 

estimates–

observed

Recession slope 

estimates–

simulated

Daily

Mean 79 76 0.006 -0.22 -0.19

Standard deviation 63 54 0.196 0.11 0.08

Minimum 3.6 4.9 -0.708 -0.04 -0.07

Maximum 458 536 1.373 -0.56 -0.44

Nash-Sutcliffe coefficient na 0.72 na na na

Coefficient of determination R2 na 0.72 na na na

Monthly

Mean 79 76 0.013 na na

Standard deviation 56 46 0.181 na na

Minimum 4.7 6.4 -0.588 na na

Maximum 230 179 0.776 na na

Nash-Sutcliffe coefficient na 0.80 na na na

Coefficient of determination R2 na 0.81 na na na

Table 13. Estimated average annual recharge in the Pomperaug River watershed, Connecticut.

Simulated assuming a 1 percent coarse 

glacial stratified deposits 1999–2005 

(inches)

Mazzaferro 

(1986) 

(inches)

Starn and others 

(2000) 

(inches)

Till 8 8 9

Coarse stratified drift 24 22 24

Table 14. Simulated runoff data for South Brook (three hydrologic response units), Connecticut, July 1–11, 2004.

[HRU, hydrologic response unit]

Watershed position

Annual groundwater 

runoff contribution

(inches)

Annual subsurface 

runoff contribution

(inches)

Annual surface 

runoff contribution

(inches)

Annual total 

runoff

(inches)

Upland–Till dominated (HRU 37) 5.9 8.1 7.1 21.1

Transition–Till and coarse stratified drift (HRU 38) 10.7 8.6 2.5 21.8

Valley–Coarse stratified drift (HRU 39) 21.6 0.0 1.9 23.5
The Precipitation-Runoff Model 51

10 30 50 70 90

COARSE GLACIAL STRATIFIED DEPOSITS, IN PERCENT

0

5

10

15

20

25

RECHARGE, IN INCHES PER YEAR

General relation

Recharge

Figure 22. Relation between the percentage of coarse glacial stratified deposits and the simulated average annual recharge 

in each hydrologic response unit, Pomperaug River watershed, Connecticut. 
52 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, ConnecticutFigure 23. Simulated spatially distributed monthly mean recharge (inches) for each hydrologic response unit in the Pomperaug River watershed, Connecticut. Groundwater runoff, in inches 0 to 1 1.01 to 2 2.01 to 3 3.01 to 4 EXPLANATION

0 2 4 MILES0 2 4 KILOMETERS

Base from U. S. Geological Survey, 1:24,000, 1969 to 1984Connecticut State Plane projection

73°10' 73°15' 73°20'

41°40'41°35'41°30'

73°10' 73°15' 73°20' 73°10' 73°15' 73°20'

41°40'41°35'41°30'

June May April

March February January
The Precipitation-Runoff Model 53 Figure 23. Simulated spatially distributed monthly mean recharge (inches) for each hydrologic response unit in the Pomperaug River watershed, Connecticut.—Continued

Groundwater runoff,in inches0 to 11.01 to 22.01 to 33.01 to 4

EXPLANATION

0 2 4 MILES0 2 4 KILOMETERS

Base from U. S. Geological Survey, 1:24,000, 1969 to 1984Connecticut State Plane projection

73°10' 73°15' 73°20'

41°40'41°35'41°30'

73°10' 73°15' 73°20' 73°10' 73°15' 73°20'

41°40'41°35'41°30'

July August

October November

September

December
54 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, ConnecticutFigure 24. Simulated spatially distributed monthly mean subsurface runoff (inches) for each hydrologic response unit in the Pomperaug River watershed, Connecticut. Subsurface runoff, in inches 0 to 1 1.01 to 2 2.01 to 3 EXPLANATION

0 2 4 MILES0 2 4 KILOMETERS

Base from U. S. Geological Survey, 1:24,000, 1969 to 1984Connecticut State Plane projection

73°10' 73°15' 73°20'

41°40'41°35'41°30'

73°10' 73°15' 73°20' 73°10' 73°15' 73°20'

41°40'41°35'41°30'

June May April

March February January
The Precipitation-Runoff Model 55 Figure 24. Simulated spatially distributed monthly mean subsurface runoff (inches) for each hydrologic response unit in the Pomperaug River watershed, Connecticut.—Continued

Subsurface runoff,in inches0 to 11.01 to 22.01 to 3

EXPLANATION

0 2 4 MILES0 2 4 KILOMETERS

Base from U. S. Geological Survey, 1:24,000, 1969 to 1984Connecticut State Plane projection

73°10' 73°15' 73°20'

41°40'41°35'41°30'

73°10' 73°15' 73°20' 73°10' 73°15' 73°20'

41°40'41°35'41°30'

July August

October November

September

December
56 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, ConnecticutFigure 25. Simulated spatially distributed monthly mean surface runoff (inches) for each hydrologic response unit in the Pomperaug River watershed, Connecticut. Surface runoff, in inches 0 to 1 1.01 to 2 2.01 to 3 EXPLANATION

0 2 4 MILES0 2 4 KILOMETERS

Base from U. S. Geological Survey, 1:24,000, 1969 to 1984Connecticut State Plane projection

73°10' 73°15' 73°20'

41°40'41°35'41°30'

73°10' 73°15' 73°20' 73°10' 73°15' 73°20'

41°40'41°35'41°30'

June May April

March February January
The Precipitation-Runoff Model 57 Figure 25. Simulated spatially distributed monthly mean surface runoff (inches) for each hydrologic response unit in the Pomperaug River watershed, Connecticut.—Continued

Surface runoff,in inches0 to 11.01 to 22.01 to 3

EXPLANATION

0 2 4 MILES0 2 4 KILOMETERS

Base from U. S. Geological Survey, 1:24,000, 1969 to 1984Connecticut State Plane projection

73°10' 73°15' 73°20'

41°40'41°35'41°30'

73°10' 73°15' 73°20' 73°10' 73°15' 73°20'

41°40'41°35'41°30'

July August

October November

September

December
58 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

The Groundwater-Runoff Model 

(MODFLOW)

A transient model of groundwater flow in the study area 

was developed using the modular groundwater flow model 

to help evaluate the effects of groundwater pumping from 

production wells and land-use change on the spatial variability of base flow to streams. Groundwater flow was simulated 

using the computer program MODFLOW-2000 (Harbaugh 

and others, 2000; Hill and others, 2000), which uses a finitedifference method to simulate 3-dimensional groundwater 

flow through a porous medium. MODFLOW-2000 uses a 

non-linear regression technique to estimate parameter values 

that result in the best match between observed and simulated 

values. This model, when combined with boundary and initial 

conditions, describes 3-dimensional groundwater flow in a 

heterogeneous and anisotropic medium, provided that the 

principal axes of hydraulic conductivity are aligned with the 

coordinate directions.

General Description of MODFLOW Developed 

for the Watershed

The MODFLOW model was initially developed as a 

steady-state groundwater flow representation for the 

Pomperaug River watershed as part of the USGS National 

Water Quality Assessment (NAWQA) Program (Lyford and 

others, 2007; Starn and Brown, 2007). The modeled area 

included most of the Pomperaug River watershed (approximately 50 mi2

) except the Weekeepeemee River watershed 

(Lyford and others, 2007). The model represents general 

hydraulic conditions that existed from 1999 to 2005. Model 

input includes boundary conditions, model stresses, and 

hydraulic properties. The model discretized the study area 

into 500 × 500 ft cells. The model used the Stream Package 

(Prudic, 1989) to simulate base flow to the streams and leakage through the streambed and the evapotranspiration package 

(Harbaugh and others, 2000) to simulate evapotranspiration 

losses from the groundwater zone (this was not simulated by 

PRMS). Detailed discussion of the model and model applications can be found in Lyford and others (2007) and Starn and 

Brown (2007). For this study, the model was run in transient 

mode to derive daily values of groundwater flow, groundwater levels, and discharge to wells and streams throughout the 

modeled area. The model was driven by daily recharge input 

derived from the PRMS model output. 

Representation of the Watershed–Hydrogeology

Detailed discussion of geologic and hydrogeologic 

conditions in the watershed is found in Lyford and others 

(2007), Starn and Brown (2007), Mazzaferro (1986a, 

1986b), and Meinzer and Stearns (1929). Lyford and others 

(2007) developed a groundwater-flow model (herein called 

the original model). Starn and Brown (2007) modified the 

layer thickness, recharge, and parameter estimates from the 

original model in order to incorporate information that became 

available after the model was completed and to simulate 

transient conditions. 

The original model simulated an area surrounded by a 

no-flow (zero-flux) boundary, which assumes that groundwater 

divides are coincident with topographic divides, inflow to the 

modeled area as underflow from three tributary watersheds 

is negligible, and underflow at the mouth of the Pomperaug 

River is negligible. Two layers that parallel the land surface 

were selected to represent the vertical dimension. Layer 1 

is approximately 150 ft thick and represents till and shallow 

bedrock in upland areas and stratified, glacially derived course 

glacial stratified deposits and shallow bedrock in the valleys. 

Layer 2 represents a section of bedrock approximately 

350 ft thick. It is assumed that most groundwater flows 

through a total thickness of about 500 ft. 

Hydraulic-conductivity zones for each layer of the 

original model were defined on the basis of the mapped 

distribution of geologic units and saturated thicknesses 

presented by Grady and Weaver (1988). Layer-1 zones 

included surface till and thick till zones in uplands and four 

zones in glacial coarse glacial stratified deposits that fill the 

valley. The zones for valley-fill materials are defined largely 

on the basis of saturated thickness of sand and gravel and are 

best visualized as transmissivity zones rather than hydraulicconductivity zones because layer-1 thickness generally is 

greater than actual geologic-unit thickness. These zones 

included (1) areas along the valley wall where the saturated 

thickness is less than 10 ft, (2) areas where the saturated 

thickness is 10 to 50 ft, (3) areas where the saturated thickness 

is greater than 50 ft, and (4) a fairly extensive area near North 

Woodbury where coarse materials overlie fine materials 

(fig. 6.8 in Lyford and others, 2007). The hydraulicconductivity zones for layer 2 included one for Paleozoic 

crystalline rocks and a second for the more transmissive 

Mesozoic rocks.

Changes to layer thickness in the original model were 

necessary because that model represented a combination of 

glacial stratified deposits and bedrock in a single layer. In the 

current modified model, the upper layer was subdivided into 

three layers, and the lower layer, which represents only bedrock, was left as one layer. The model also was modified by 

using a more detailed representation of groundwater recharge 

derived from daily, spatially distributed values for each HRU, 

discussed previously. Other minor changes were made in the 

modified model, such as using a more accurate estimate of 

bedrock elevation. The different layering changed the geohydrologic-unit definition, and an accurate estimate of groundwater flow could be obtained only by re-estimating model 

parameters. The re-estimation was done using the parameterestimation capability available in MODFLOW-2000 and the 

same set of calibration data used in the original calibration. 

These modifications resulted in slightly different aquiferproperty estimates.
The Groundwater-Runoff Model (MODFLOW) 59

Streams, Transient Recharge, Pumping, and 

Diversions

Model stresses include streams, recharge, and 

extraction wells. Perennial streams were simulated using 

the MODFLOW stream package (Prudic, 1989). The stream 

package accounts for gains and losses in the simulated 

streams and routes flow from upstream reaches to downstream 

reaches. The ends of stream segments were placed at mapped 

stream origins in headwater areas, at stream intersections, 

and at major changes in stream-channel slope. The stream 

elevations were interpolated linearly within a segment. This 

approach closely matched actual stream elevations at stream 

reaches for low-gradient, uniformly sloped main stems, but 

was less accurate for tributary streams with high-gradient, 

non-uniform slopes. The Nonnewaug, Weekeepeemee, and 

Pomperaug Rivers were assumed to be 50 ft wide except near 

the Watertown Fire District wells where a 100-ft width was 

assigned to account for diversions from the Nonnewaug River 

through the recharge ponds. All other streams were assumed to 

be 10 ft wide. A streambed hydraulic conductivity of 1.0 ft/d 

was assumed for all streams on the basis of literature-reported 

values (Kontis and others, 2004). 

Recharge was applied as daily input values distributed by 

HRU across the watershed. Each model cell (500 × 500 ft) was 

assigned a recharge value depending on the HRU where it was 

located. Discharge wells were placed at locations of publicsupply wells completed in sand and gravel and bedrock. 

The locations of these wells are shown in Lyford and others 

(2007). Current monthly pumping rates for the wells used in 

the MODFLOW model are shown on table 15. The extraction 

rates used were set to observed or estimated average rates for 

the year (annual column of table 15).

Calibration and MODFLOW Simulation Results

The MODFLOW model was initially calibrated by 

Lyford and others (2007) and modified by Starn and Brown 

(2007). The current model relied on these previous calibrations, with the addition of time-variant recharge in transient 

mode. The best low-flow results were achieved using the 

evapotranspiration package turned on in the model. This package allows for additional water losses from the groundwater 

reservoir from areas where the water table approaches the land 

surface. The model assumes that groundwater is available for 

evapotranspiration when the water table comes within 4 ft of 

the ground surface in the model. The simulation assumes that 

evapotranspiration begins in mid-April and ends in midOctober, similar to the PRMS simulation. When evapotranspiration is active, a maximum rate of 0.005 ft/d was specified, 

based on an approximation of the net precipitation minus 

streamflow in the Pomperaug River watershed. This rate was 

decreased linearly from land surface (maximum) to 4 ft (the 

extinction depth), where the rate was zero. Therefore, the position of the water table with respect to land surface determined 

the rate of evapotranspiration used in the model. The water 

table in most places is deeper than 4 ft, and simulated evapotranspiration generally occurs near groundwater discharge 

locations, particularly streams and wetlands. 

Inclusion of the evapotranspiration process in 

MODFLOW was necessary in order to simulate steep 

recessions observed in the streamflow record; no reasonable 

combination of hydraulic conductivity or storage changes 

could simulate the observed phenomenon. The extinction 

depth was based on an estimate of the average rooting depth 

in the Pomperaug River watershed. For comparison, in a 

groundwater model in Massachusetts, DeSimone (2004) used 

a maximum rate of 0.007 ft/d and an extinction depth between 

2 and 6 ft. DeSimone (2004) found the model to be moderately 

sensitive to the maximum evapotranspiration rate.

The MODFLOW-generated evapotranspiration is an 

additional loss not accounted for by PRMS. These results indicate the PRMS representation of evapotranspiration, although 

providing good results across the basin, may not adequately 

account for localized evapotranspiration from saturated 

ground in riparian zones near streams and other groundwater 

discharge locations. This finding suggests the need for more 

localized spatial representation of evapotranspiration in HRUs 

when trying to model groundwater/surface-water interactions and is also evidence for the need to couple watershed 

and groundwater processes at comparable scales. Because 

the PRMS water balance is not preserved in MODFLOW, 

the quantitative results of MODFLOW can only be considered comparatively accurate. However, the relative effects 

of changes in pumping and land use on base flow to streams, 

as determined by MODFLOW, can be appropriately used for 

interpretive purposes. 

Base Flow to Streams

The PRMS simulations were coupled with MODFLOW 

by replacing the PRMS groundwater-runoff component with 

that derived from MODFLOW using the PRMS-generated 

recharge. The hydrograph was reconstructed by combining 

the subsurface-runoff and surface-runoff components of 

PRMS with the groundwater-runoff component simulated 

by MODFLOW. The reconstructed hydrograph for the 

Pomperaug River is shown on figures 26A and 26B. At the 

watershed scale (measured at the Pomperaug streamgage), 

the PRMS groundwater and subsurface simulations provide 

base-flow quantity and recession characteristics that are 

comparable to MODFLOW (fig. 26) because the localized 

spatial and temporal effects of pumping wells and localized 

evapotranspiration on groundwater flow are not important. 

However, because PRMS does not represent groundwater 

elevations, water-table conditions, and the local effects of 

groundwater withdrawals and pumping of production wells, it 

cannot be used to evaluate the spatial and temporal effects of 

well pumping, stream depletion, and groundwater mounding. 

This finding indicates that the PRMS groundwater simulation 

for the Weekeepeemee is a reasonable representation of base 
60 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut Table 15. Wells and pumping rates used in the MODFLOW-2000 model in the Pomperaug River watershed, Connecticut. [%, percent; na, not available] Name Time frame Pumping rate (million gallons per day) Out-of- watershed diversion Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual Watertown Fire District Current 0.6 0.6 0.6 0.6 0.7 0.8 0.8 0.8 0.6 0.6 0.6 0.6 0.65 100% 2050 0.6 0.6 0.6 0.6 0.7 0.8 0.8 0.8 0.6 0.6 0.6 0.6 0.65 100% Maximum 2.83 2.83 2.83 2.83 2.83 2.83 2.83 2.83 2.83 2.83 2.83 2.83 2.83 100% Heritage Village Current 0.7 0.8 0.8 0.8 1.1 1.1 1.2 1.3 1 0.9 0.8 0.7 0.93 43% 2050 1.1 1.2 1.2 1.2 1.7 1.7 1.9 2 1.5 1.4 1.2 1.1 1.45 56% Maximum 2.04 2.04 2.04 2.04 2.04 2.04 2.04 2.04 2.04 2.04 2.04 2.04 2.04 Unknown United Water Current 0.1 0.1 0.1 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.17 0% 2050 0.15 0.15 0.15 0.15 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.25 0% Maximum 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0% Woodlake Tax District Current na na na na na na na na na na na na 0.07 0% 2050 na na na na na na na na na na na na 0.08 0% Maximum 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0% Woodbury Place Current na na na na na na na na na na na na 0.01 0% 2050 na na na na na na na na na na na na 0.01 0% Maximum 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0% Woodbury Knolls Current na na na na na na na na na na na na 0.03 0% 2050 na na na na na na na na na na na na 0.03 0% Maximum 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0% Town in Country Current na na na na na na na na na na na na 0.02 0% 2050 na na na na na na na na na na na na 0.03 0% Maximum 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0% Heritage Hill Current na na na na na na na na na na na na 0.01 0% 2050 na na na na na na na na na na na na 0.01 0% Maximum 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0% Quassuk Heights Current na na na na na na na na na na na na 0.01 0% 2050 na na na na na na na na na na na na 0.01 0% Maximum 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0% Romatic Current na na na na na na na na na na na na 0.01 0% 2050 na na na na na na na na na na na na 0.01 0% Maximum 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0% Nonnewaug High Current na na na na na na na na na na na na 0.09 0% 2050 na na na na na na na na na na na na 0.09 0% Maximum na na na na na na na na na na na na na 0%
The Groundwater-Runoff Model (MODFLOW) 61

Figure 26. Daily hydrographs of the streamflow in the Pomperaug River at the Southbury streamgage for the 

period 1999 to 2005 derived by combining the MODFLOW-derived base flow with PRMS-derived subsurface and 

surface-runoff components showing (A) the PRMS + MODFLOW hydrograph and the observed flow, and (B) the 

PRMS + MODFLOW hydrograph and the PRMS only hydrograph.

1998 1999 2000 2001 2002 2003 2004 2005 OCT JAN APR JUL OCT JAN APR JUL OCT JAN APR JUL OCT JAN APR JUL OCT JAN APR JUL OCT JAN APR JUL OCT JAN APR JUL

SEPT

STREAMFLOW, IN CUBIC FEET PER SECOND

0

500

1,000

1,500

2,000

2,500

3,000

STREAMFLOW, IN CUBIC FEET PER SECOND

0

500

1,000

1,500

2,000

2,500

3,000

Simulated—Precipitation runoff modeling

system plus modular finite-difference

groundwater flow model

Observed

Simulated—Precipitation runoff modeling

system plus modular finite-difference

groundwater flow model

Simulated—Precipitation runoff modeling

system

A

B
62 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

flow to the stream and can be added to the MODFLOW output 

to estimate base flows in the Pomperaug downstream from the 

confluence with the Nonnewaug. 

MODFLOW assumes pumped groundwater is not 

returned to the groundwater system and, therefore, represents 

a consumptive water use. In fact, pumped groundwater used 

within the watershed will return eventually to the groundwater 

system where it is discharged to septic systems or to 

wastewater-treatment plants within the watershed. Typically, 

return flows from domestic water use lose approximately 

20 percent of the volume to evaporative losses (Solley and 

others, 1998). However, return flow through septic systems 

will take a relatively long time to return to the stream. Thus, 

the assumption that pumped groundwater is 100 percent 

consumptive is probably a reasonable assumption for 

consideration of pumping effects during periods of drought.

Losing Reaches

The MODFLOW model predicted that 22 percent of the 

stream reaches in the modeled area were losing water through 

the streambed to the groundwater system at least sometime 

during the simulation period. This loss generally occurs in 

smaller tributaries to the Pomperaug River just downstream 

from the transition from upland areas to the valley bottom. 

However, some of the losing reaches are short reaches in 

the mainstem of the Nonnewaug River near its confluence 

with the Weekeepeemee River and short reaches of the 

Pomperaug River near Heritage Village and at the dam on 

South Pomperaug Avenue in Woodbury (Starn and Brown, 

2007; Lyford and others, 2007). As an example, the base-flow 

hydrograph for South Brook simulated by MODFLOW for 

1999 through 2005 is shown on figure 27. The hydrograph 

shows frequent periods when the flow is zero. Observations 

have shown that South Brook does go dry in the location 

of the hydrograph simulated on figure 27, although it is not 

known whether the reach goes dry as often as indicated by 

the model.

Management Scenario Applications

The calibrated PRMS model was used to generate simulation data sets of streamflow and component runoff for each 

HRU on the basis of a set of land and water-use management 

scenarios outlined by the PRWC. The PRWC and local town 

governments will use the management-scenario simulation 

runs to evaluate the temporal and spatial hydrologic effects of 

different water-use and land-use options so that they can make 

informed watershed management decisions. The management 

issues include water availability (groundwater and instream), 

source-water variability and runoff rates (as it pertains to 

water quality), and effects of decisions on extreme high-flow 

and low-flow conditions.

Effects of Land Use and Groundwater 

Withdrawal on Streamflow

To evaluate the effects of land use and land cover on 

streamflow, six land-use scenarios were run including current 

(2002) conditions, pre-development conditions (no human 

influence), maximum (full build-out) development that is 

based on zoning information for the towns within the watershed, all grass (to represent agricultural use and pasture), an 

arbitrary 10 percent EIA, and an arbitrary 20 percent EIA. The 

current-condition scenario forms the body of this discussion. 

An additional pristine-condition scenario assumed a uniform 

forest land cover for the entire watershed and no reservoirs 

and assumed a thicker humus layer on the forest floor, which 

was simulated with a slight increase to the soil storage volume. This scenario provides a reference or baseline streamflow 

condition for evaluating instream habitat. 

The full build-out scenario was based on a compilation 

of the zoning regulations completed by the Council of 

Governments of the Central Naugatuck Valley for each 

town within the watershed (Virginia Mason, Council of 

Governments of the Central Naugatuck Valley, written 

commun., 2006) and assumed a representative percentage of 

impervious surface associated with the maximum type and 

areal extent of developable land allowed by the regulations. 

The current impervious surface in the watershed as a 

percentage of each HRU and the projected full build-out 

impervious surface as a percentage of each HRU are shown on 

figure 28. Two runoff management options were considered 

with the full build-out scenario. The first uses the Connecticut 

EIA, as shown on figure 12, assuming generally rural 

conditions with relatively low connectedness of impervious 

surfaces and minimal drainage control (minimum stormwater 

collection). The second runoff management option assumes 

a developed setting with stormwater collection and discharge 

to streams and is represented by the Alley and Veenhuis data 

(1983) shown on figure 12 (maximum stormwater collection). 

The all-grass and arbitrary 10 and 20 percent EIA scenarios 

were run for comparative purposes.

In general terms, land-use changes (for example, 

increasing the amount of impervious surface in the watershed 

and conversion of agriculture and forested lands) affect the 

amount of recharge to the groundwater reservoirs and the 

timing and total amount of streamflow. These effects were 

quantified for the watershed by comparing hypothetical 

changes of land use in the watershed to the existing condition 

(approximately 1 percent EIA), assuming approximately 

2 percent impervious surface (full build-out with minimum 

stormwater collection) to 5 percent (full build-out assuming 

extensive or maximum stormwater collection), 10 and 

20 percent EIA distributed evenly across the watershed, 

all-forest (pristine) conditions, and all-grass (pasture and 

agricultural use) conditions. Note that the all-grass scenario 

does not represent lawns and manicured grass surfaces, 

because these tend to be more compacted with short grass 

relative to pasture and agricultural uses. The PRMS-generated 
Management Scenario Applications 63

effect of these hypothetical land-use/land-cover changes on 

streamflow at the Pomperaug River streamgage at Southbury 

are shown on figure 29. 

The effect of increasing impervious surface on 

streamflow statistics is provided on table 16. The simulations 

indicate that increasing impervious surface up to the maximum 

allowable by current zoning regulations within the watershed, 

with EIA estimated using either the relation representing 

maximum stormwater collection described by Alley and 

Veenhuis (1983) or by the relation developed in this study 

representing minimum stormwater collection, would decrease 

the lowest flows at the Pomperaug streamgage (exceeded 

99 percent of the time) by 10 to 14 percent. The effect on 

higher flows is less. There is little effect on the simulations 

for the Weekeepeemee and Nonnewaug Rivers, because the 

potential for development in these watersheds is less. In the 

future, the portion of the watershed downstream from the 

confluence of the Weekeepeemee and Nonnewaug Rivers will 

likely experience the greatest reductions in base flow resulting 

from development.

The forested watershed has the least amount of total runoff (fig. 29A), because of higher amounts of evapotranspiration, and streamflow increases proportionately to the increase 

in impervious surface because of reduced evapotranspiration 

losses. In addition, the timing of runoff is altered because the 

source of the water in the stream, whether it comes from direct 

surface runoff, groundwater runoff, or from shallow subsurface runoff, also is altered in response to land-use change. 

The transition from a groundwater-runoff to surfacerunoff regime is illustrated on figure 29B, which shows the 

total percentage of flow that is either groundwater or surface 

runoff. Under natural scenarios, including the forest and grass 

land-cover conditions, groundwater is the dominant source of 

water in the stream, indicating that mixed land cover (agriculture, meadow grass, and forest) would maintain a river 

dominated by groundwater, whereas increased development 

will steadily drive the flow regime to be dominated by surface 

runoff. Another interesting result is that the all-forest scenario, 

although showing lower overall runoff (fig. 29A), has the 

second highest percentage contribution from groundwater; 

1998 1999 2000 2001 2002 2003 2004 2005 2006

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STREAMFLOW, IN CUBIC FEET PER SECOND

0

1

2

3

4

Dry streambed, South Brook

Figure 27. Daily base-flow hydrograph for South Brook near its confluence with the Pomperaug River, Connecticut, showing several 

periods of no flow in the stream (dry streambed).
64 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut Woodbury Southbury Oxford Watertown Roxbury Bethlehem Washington Middlebury Morris Newtown ThomastonWaterbury Naugatuck Litchfield Beacon Falls Bridgewater Seymour Woodbury Southbury Oxford Watertown Roxbury Bethlehem Washington Middlebury Morris Newtown ThomastonWaterbury Naugatuck Litchfield Beacon Falls Bridgewater Seymour

0 1 2 MILES

0 1 2 KILOMETERS

A

B

Base from U.S. Geological Survey, 1:24,000, 1969 to 1984Connecticut State Plane projection

EXPLANATIO

N

Projected full build-outimpervious area, inpercent2.12 to 4.004.01 to 8.008.01 to 12.0012.01 to 16.0016.01 to 20.0020.01 to 40.00

EXPLANATIO

N

Existing imperviousarea, in percent2.12 to 4.004.01 to 8.008.01 to 12.0012.01 to 16.0016.01 to 20.0020.01 to 40.00

73°06' 73°09' 73°12' 73°15' 73°18' 73°06' 73°09' 73°12' 73°15' 73°18'

41°39'41°36'41°33'41°30'41°27'

Figure 28. Impervious surface in the Pomperaug River watershed, Connecticut, as a percentage of each hydrologic response unit determined on the basis of land-use type for (A) current (2002) conditions, and (B) projected full build-out impervious surface.
Management Scenario Applications 65

FOREST GRASS 1 2 5 10 20

 LAND COVER PERCENT IMPERVIOUS AREA

0.060

0.065

0.070

0.075

0.080

RUNOFF, IN INCHES

0

0.2

0.4

0.6

0.7

 FRACTION

0

0.01

0.02

0.03

0.04

0.05

SOURCE RUNOFF, IN INCHES

Groundwater runoff

Surface runoff 

Groundwater runoff

Surface runoff

Subsurface runoff

A

B

C

Figure 29. Simulated daily average 

runoff in the Pomperaug River, 

Connecticut, as a function of hypothetical 

land-use change, (A) total streamflow, 

in inches; (B) percentage of total runoff 

that is groundwater and surface runoff; 

and (C) runoff by source (groundwater, 

subsurface, and surface), in inches.
66 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Table 16. Simulated streamflow percentiles for different impervious scenarios in the Pomperaug, 

Weekeepeemee, and Nonnewaug Rivers, Connecticut.

[%, percent]

Flow percentile

Simulated flow value 

(cubic feet per second)

Existing condition Full build-out 

Connecticut impervious

Full build-out Alley and 

Veenhuis (1983) impervious

Pomperaug River

0.01 - Exceeded 99% of the time 7.3 6.5 6.3

0.10 - Exceeded 90% of the time 17 16 16

0.25 - Exceeded 75% of the time 41 40 41

0.50 - Exceeded 50% of the time 82 82 82

0.75 - Exceeded 25% of the time 144 144 146

0.90 - Exceeded 10% of the time 262 265 272

0.99 - Exceeded 1% of the time 917 917 937

Weekeepeemee River

0.01 - Exceeded 99% of the time 2.4 2.4 2.3

0.10 - Exceeded 90% of the time 5.0 5.0 5.1

0.25 - Exceeded 75% of the time 12 13 13

0.50 - Exceeded 50% of the time 26 27 27

0.75 - Exceeded 25% of the time 47 48 48

0.90 - Exceeded 10% of the time 94 94 96

0.99 - Exceeded 1% of the time 348 356 371

Nonnewaug River

0.01 - Exceeded 99% of the time 0.7 0.7 0.7

0.10 - Exceeded 90% of the time 2.4 2.5 2.6

0.25 - Exceeded 75% of the time 7.9 8.2 8.6

0.50 - Exceeded 50% of the time 19 19 19

0.75 - Exceeded 25% of the time 35 35 35

0.90 - Exceeded 10% of the time 65 66 66

0.99 - Exceeded 1% of the time 245 245 246
Management Scenario Applications 67

the highest being the all-grass scenario. The all-grass scenario 

maximizes the total recharge, and the all-forest scenario maximizes the percentage of groundwater contribution compared to 

developed land cover, indicating that maintaining mixed forest 

and pasture lands may be important to the sustainability of 

base flows in a developing watershed. 

The quantity of each source component comprising the 

streamflow for the different land-use conditions is shown 

on figure 29C. On average for most land-use conditions, 

groundwater is the dominant source of water in the stream 

followed by subsurface runoff. However, beginning at 

10 percent EIA, surface runoff begins to dominate over 

subsurface runoff; at 20 percent, surface runoff is the 

dominant source over both groundwater and subsurface runoff. 

This transition from a groundwater-dominated flow regime 

to a surface-runoff flow regime is important for habitat and 

water quality because of the different chemical and physical 

characteristics of each water source.

The effect that land-use change has on reducing base 

flows is shown on figure 30. As evapotranspiration decreases 

in response to fewer trees, the amount of groundwater 

recharge and runoff increases (the all-grass scenario). As 

evapotranspiration and recharge both decrease because of 

increasing impervious surfaces, the lowest flows get lower 

as indicated by the streamflow percentiles on table 16. The 

increase in groundwater base flow assuming the all-grass scenario is based on natural infiltration capacity of uncompacted 

soil and, therefore, is not equivalent to lawns that tend to be 

more compacted and will shed water more quickly because of 

the short grass. Conversely, under the full build-out scenario, 

percent changes in high flows are not substantial.

The relative change in runoff from the three simulated 

sources as a function of change in EIA is shown on figure 31. 

Surface runoff increases at a greater rate than the rate at which 

groundwater runoff and subsurface runoff decrease. 

Figure 32 shows the average percentage change in recharge 

 20 40 60 80 100

PERCENT EXCEEDANCE

0

10

100

1,000

10,000

STREAMFLOW, IN CUBIC FEET PER SECOND

Existing

Full build-out scenario with maximum stormwater collection

Full build-out scenario with minimum stormwater collection

All forest scenario

All grass scenario

Figure 30. Probability of daily flow rate exceeded in the Pomperaug River at Southbury on any given day as a 

function of hypothetical land-use scenario in the watershed. 
68 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

by HRU over the simulation period for the two full buildout scenarios, one using minimum stormwater collection 

(Connecticut EIA) and the second using maximum stormwater 

collection (Alley and Veenhuis EIA). 

For the full build-out scenarios the percentage change 

in streamflow during moderate-flow and high-flow events 

is relatively small, whereas during low-flow periods, the 

percentage change is much more substantial (fig. 33). This 

pattern of change is because recharge to the groundwater was 

reduced and therefore surface runoff increased. This result 

suggests the potential for a compounding effect on water 

quality during low-flow periods because surface runoff is 

generally of lower quality than groundwater. Wastewater 

return flows, such as those from Heritage Village in 

Southbury (averaging approximately 1.02 ft3/s under existing 

conditions), may comprise as much as 20 percent of the flow 

in the Pomperaug River at the point of wastewater discharge 

during extreme low flows under the full build-out scenario 

and maximum diversions. Therefore, the combined effect 

of a lower percentage of groundwater runoff and increased 

wastewater return flows could have a large effect on instream 

water quality, such as increased temperatures and increased 

nitrogen concentrations. Additionally, population increases in 

non-sewered areas of the watershed will increase the number 

of septic systems, which could contribute to increases in 

nitrate loading and other water-quality changes to groundwater 

beneath and downgradient from the septic system (Starn and 

Brown, 2007). 

0 5 10 15 20 25

0 1 2 3 4 5

EFFECTIVE IMPERVIOUS PERCENT

-8

-6

-4

-2

0

2

4

6

8

10

12

CHANGE IN ANNUAL RUNOFF, IN INCHES

-2

-1

0

1

2

3

CHANGE IN ANNUAL RUNOFF, IN INCHES

Subsurface runoff

Surface runoff

Groundwater runoff

B

A

Figure 31. Change in groundwater runoff, 

subsurface runoff, and surface runoff as a function 

of change in effective impervious surface for the 

two full build-out scenarios with (A) maximum 

and (B) minimum stormwater collection relative 

to existing conditions for the Pomperaug River at 

Southbury, Connecticut. 
Management Scenario Applications 69

Effects of Land Use and Groundwater 

Withdrawal on Recharge and Base Flow

In the PRMS model, the key physical attributes of the 

land surface in each HRU that control runoff quantity and 

source are the (1) surficial geology, (2) soil type, (3) land 

cover (and in particular the amount of impervious surface), 

and (4) drainage density. The effect of these physical factors 

on groundwater recharge (and conversely surface runoff) was 

assessed in the model by multiple linear regression analysis. The simulated recharge was regressed against the above 

physical attributes for each HRU, and a simplified predictive equation was derived. Because the existing watershed 

has relatively low amounts of EIA, the hypothetical models 

developed for the management scenarios were used for this 

analysis because they offer a wider range of percent impervious surfaces across the watershed. The coefficients of the 

derived regression equation and their statistical significance 

are shown on table 17.

The coefficients can be used to directly compare the magnitude of the effect of each attribute on the recharge estimate, 

and the t-statistic indicates how significant the attribute was 

in the prediction outcome. The most important attributes for 

predicting discharge are the coarse glacial stratified deposits, 

class D soils, the impervious surface, and, to a lesser degree, 

the drainage density. For the Pomperaug River watershed, the 

presence of glacial stratified deposits and high drainage density indicates high recharge and the presence of class D soils 

and impervious surfaces indicates reduced recharge. 

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 6364

HYDROLOGIC RESPONSE UNIT

-5

0

-10

-15

-20

-25

CHANGE, IN PERCENT

0

100

200

300

400

500

600

CHANGE, IN PERCENT

Minimum stormwater collection

Maximum stormwater collection

A

B

Figure 32. Average percentage change in (A) recharge and (B) surface runoff relative to existing conditions by hydrologic 

response unit, Pomperaug River watershed, Connecticut, for the full build-out scenarios, with maximum and minimum 

stormwater collection.
70 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Figure 33. Percentage change in streamflow as a function of simulated streamflow for full build-out 

scenarios with (A) maximum and (B) minimum stormwater collection relative to existing conditions for the 

Pomperaug River at Southbury, Connecticut. 

0 1,000 2,000 3,000 4,000

STREAMFLOW, IN CUBIC FEET PER SECOND

 -20

 0

 20

 40

 60

 80

 100

PERCENT CHANGE

 -20

 0

 20

 40

 60

 80

 100

PERCENT CHANGE

A

B
Management Scenario Applications 71

As previously discussed, vegetative land-cover type is 

also important in the amount of recharge. The monthly pattern 

and quantity of recharge to the groundwater as a function of all 

forest (undeveloped) and all grass (pasture and agriculture) by 

land-cover type are shown on figure 34. The all-grass scenario 

shows an increase in recharge compared to the other scenarios, 

particularly during the summer months. 

Two MODFLOW simulations were run to assess the 

effect of increased pumping on base flows. The first simulated 

the projected pumping rates to the year 2050, which were 

based on information provided from the water companies or 

estimated on the basis of expected population growth. The 

second simulated the pumping rate at the maximum registered 

diversion rate (including groundwater pumping and surfacewater withdrawals). The simulations were run assuming 

recharge from the full build-out land-use scenarios given the 

two different stormwater-collection scenarios. The pumping rates used in the simulations are shown on table 15. The 

maximum pumping rate is assumed to be reached for each 

month of the year for the maximum pumping scenario, and 

smaller systems that do not require a permit are assumed to 

pump year round at the maximum rate allowable without a 

permit—50,000 gal/d. 

MODFLOW-derived base-flow estimates for the 

Pomperaug River at the Southbury streamgage for full buildout scenarios that include (1) maximum stormwater collection 

and the estimated maximum pumping rates, and (2) maximum 

stormwater collection with projected pumping rates in 2050 

are shown on figure 35 for the simulation period 1998 to 

2006. In the Pomperaug River, the projected increases in 

groundwater pumping throughout the watershed from public 

supply systems results in about a 2 to 3 ft3/s reduction in flow 

during the summer. This quantity is a substantial part of the 

overall streamflow during the low-flow period; however, it is 

somewhat less than the total groundwater withdrawals from 

the watershed. This indicates that storage within the aquifer 

attenuates the effects of pumping during periods of low flow, 

even though over the course of time, mass balance dictates 

that the water withdrawn by pumping must reduce discharge 

to streams by an equivalent amount. The effect of pumping 

throughout the watershed is cumulative in a downstream 

direction, and thus the magnitude of the effect would change 

depending on the ratio of cumulative flow to pumping in the 

downstream direction. The cumulative effect of individual 

home wells would also contribute to a reduction in base flow.

The simulation of the maximum full build-out scenario 

indicates that the duration of low-flow periods increases and 

low flows become lower with development. The change in the 

percentage of reaches that gain and lose water from (to) the 

groundwater and the percentage of reaches that are subject 

to going dry within the entire modeled areas over the course 

of the simulation period (1998–2006) is shown on table 18. 

Overall, the percentage of losing reaches does not change 

substantially; however, the number that go dry increases by 

33 percent, assuming minimum stormwater collection; and 

by 67 percent, assuming maximum stormwater collection. In 

addition, the change in the number of days that South Brook is 

subject to going dry over the simulation period increases from 

119 days for existing conditions to 123 days for the maximum full build-out scenario with increased pumping. These 

increases occur where development is projected to increase 

and where production wells are located. The length of time 

that the reaches are dry also increases with projected increases 

in impervious surface, exemplified by South Brook. These 

effects are associated primarily with change in land cover as 

opposed to groundwater withdrawals, because of the more 

widespread nature of the land-cover change.

Table 17. Regression statistics for multiple linear regression of physical attributes to predict recharge for the Pomperaug River 

watershed, Connecticut.

[Estimated recharge in inches per year; %, percent]

Regression statistics

Multiple R 0.813615

Coefficient of determination R2 0.66197

Adjusted R2 0.639053

Standard error 0.009251

Observations 64

Coefficient Standard error t Stat P-value Lower 95% Upper 95%

Intercept 0.032953 0.003666 8.989751 1.19E-12 0.025618 0.040288

Coarse stratified drift percent 0.032147 0.005286 6.081957 9.45E-08 0.021571 0.042724

Class D soil percent -0.03792 0.010748 -3.52784 0.000818 -0.05942 -0.01641

Drainage density 0.002036 0.001031 1.975862 0.052853 -2.6E-05 0.004099

Percent impervious -0.09292 0.027984 -3.32032 0.001546 -0.14891 -0.03692
72 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Figure 34. Average monthly groundwater recharge for uplands and valleys for (A) all-forest and (B) all-grass scenarios, Pomperaug 

River watershed, Connecticut.

JAN FEB MAR APR MAY JUN JULY AUG SEPT OCT NOV DEC

0

0.002

0.004

0.006

0.008

0.010

0.012

RECHARGE, IN FEET PER DAY

0

0.002

0.004

0.006

0.008

0.010

0.012

RECHARGE, IN FEET PER DAY

EXPLANATION

Number of samples

Data value greater than 3.0 times the

interquartile range above the box

Data value 1.5 to 3.0 times the interquartile

range above the box

Largest data value within 1.5 times the

interquartile range above the box

75th percentile

Median (50th percentile)

25th percentile

Smallest data value within 1.5 times the

interquartile range below the box

Data value 1.5 to 3.0 times the interquartile

range below the box

Interquartile range{

64

x

x

o

x x

xx

xx

ox x

x

xx

x x

x

A

B

64 64 64 64 64 64 64 64 64 64 64 64

64 64 64 64 64 64 64 64 64 64 64 64
Management Scenario Applications 73

1998 1999 2000 2001 2002 2003 2004 2005 2006

JAN

APR

JUL

OCT

JAN

APR

JUL

OCT

JAN

APR

JUL

OCT

JAN

APR

JUL

OCT

JAN

APR

JUL

OCT

JAN

APR

JUL

OCT

JAN

APR

JUL

OCT

JAN

APR

JUL

OCT

JAN

FEB

10

1

100

1,000

STREAMFLOW, IN CUBIC FEET PER SECOND

Full build-out scenario with diversions

Full build-out scenario with year 2050 diversions

Full build-out scenario with maximum diversions 

Figure 35. Effect of projected increases in pumping on base flow assuming full build-out land use, Pomperaug River watershed, 

Connecticut.

Table 18. Percentage of stream reaches in the Pomperaug River watershed, Connecticut, that can lose water to the groundwater, 

gain water from the groundwater, or can go dry for the full build-out scenarios compared to the existing scenario.

Streamflow

Percent of total stream reaches for all times in each scenario

Existing conditions Full build-out with maximum 

stormwater collection

Full build-out with maximum stormwater 

collection and projected pumping in the year 2050

Uplands Valley Uplands Valley Uplands Valley

Losing 12 10 11 10 11 11

Gaining 39 25 38 26 39 26

Dry 11 3 10 4 10 5
74 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Limitations

The PRMS model is based on physical relations between 

hydrologic variables and, as such, represents physical 

processes that control the water balance, streamflow, and 

hydrograph characteristics in time and in space. The model 

performs best for estimating monthly mean values and also 

performs well on the long-term daily mean values. However, 

simulation estimates of streamflow for any specific day may 

have considerable error and should be used with caution. 

The long-term daily streamflow statistics and the long-term 

representation of streamflow dynamics and seasonal fluxes are 

considered to be sufficiently accurate for understanding spatial 

distribution of hydrologic variables within the watershed, for 

assessing spatial and seasonal flow-regime characteristics of 

the watershed, and for assessing watershed and streamflow 

response to changes in land-cover conditions. 

The base flow and recharge characteristics of the basin 

are considered to be accurately simulated by the model and 

can be used for assessing the effects of land-cover change on 

the temporal and spatial distribution of recharge and base flow 

to streams. Because the recharge and base-flow component 

of the hydrograph are accurately represented, the overall 

assessment of the source contributions to streamflow are 

considered sufficiently accurate to understand implications 

associated with changes in the source-water contribution 

to the streams within the watershed. The groundwater 

simulations using MODFLOW are considered representative 

of the spatial effects of recharge on groundwater discharge to 

streams and the effect of pumping on discharge to streams. 

The MODFLOW results are limited; they are not fully 

compatible quantitatively with PRMS because of differences 

in scale. Consequently, the MODLFOW results are considered 

sufficient for comparing relative temporal and spatial effects 

of land-cover and pumping changes on base flow to streams, 

but not necessarily the quantitative effects. 

Water diversions are represented in the model in a 

simplified way and do not capture the full monthly and daily 

dynamics of the processes. In addition, flow augmentation 

from the Bronson-Lockwood Reservoir and artificial 

recharge to the Watertown Fire District well field are not 

explicitly modeled. Flow routing through the O&G ponds 

is more dynamic than represented in the model, because the 

streambank that constrains the Pomperaug River is subject to 

erosion and reconstruction that alter the overflow conditions. 

Additionally, other channel and off-channel storage effects 

are not specifically incorporated into the model. As such, 

the routing of daily flows and accounting for groundwater 

diversions provides a general magnitude of the effect of these 

processes, but the data do not allow specific day-to-day or 

even month-to-month assessment of their effect on streamflow 

quantity and variability. 

Summary and Conclusions

The Precipitation Runoff Modeling System (PRMS) 

parameterization strategy developed for the Pomperaug River 

watershed employs a minimum of calibration and optimization by establishing basic relations between the parameter 

value and physical characteristics of the individual hydrologic 

response units (HRUs) that can be obtained from general 

databases for Connecticut and quantified using geographic 

information system. This approach is important to successful 

application of PRMS because it provides a method to objectively optimize and calibrate the parameters for each HRU that 

would otherwise be unknown or unmeasurable. Additionally, 

it is important to have confidence that the parameterization 

process can be objectively repeated in other watersheds and 

would be relevant to past and future scenarios with changed or 

different physical watershed conditions. This latter condition 

is the basis for applying the model to simulate and evaluate 

the possible effects of various land-use and water-management 

strategies on streamflow and hydrology in the watershed.

Anthropogenic influences on streamflow in the watershed are important in maintaining low flows and reducing 

peak flows in the lower reach of the Pomperaug River. These 

influences include overflow into the O&G quarry ponds and 

releases from the Bronson-Lockwood Reservoir. The absence 

of the reservoir releases could substantially reduce low flows 

in the river, and changes in the overflow and release characteristics into and out of the O&G ponds would change the highflow and low-flow characteristics of the Pomperaug River 

downstream of the ponds. 

The PRMS simulations, for the 22-year record from 1976 

to 1998 and the 7-year record from 1999 to 2005, exhibit 

accurate long-term water balance and runoff characteristics. 

Additionally, the spatial and temporal variability of runoff is 

accurately simulated. This result indicates that the xyz precipitation parameterization method can successfully distribute 

measured precipitation from different input data sets across 

the watershed and that evapotranspiration is also accurately 

simulated. Although the day-to-day runoff simulations are 

acceptable from the standpoint of the modeling objective, the 

simulated daily runoff may have considerable error compared 

to the observed daily runoff. These conclusions hold for each 

of the comparisons made at the three streamgages in the watershed, indicating that the model simulations provide reasonably 

accurate results across the watershed. Thus, the model simulations can provide accurate representations of the hydrologic 

conditions and flow regime, both temporally and spatially 

distributed, but must be used with caution when considering 

the simulated runoff for a particular day. The mean annual 

streamflow estimates over a 7-year period show an error of 

less than 5 percent; however, the mean streamflow estimate for 

any given year may have an error up to about 14 percent. 

Model-fit statistics indicate that the long-term water 

balance and runoff simulations are sufficiently accurate for 

assessing the effects of land-use and water-use changes on 
Acknowledgments 75

streamflow and groundwater recharge. The dynamics of the 

groundwater system can generally be considered to operate 

on much longer time frames than the surface dynamics; thus 

daily error in the simulated recharge values are less important 

when evaluating the effects of watershed changes on the 

groundwater. Overall, groundwater discharge supplies 

60 percent or more of the flow to the stream system and is 

the principle source of water to the river. 

The groundwater simulations made using MODFLOW 

are comparable to the simpler representation of groundwater 

runoff in PRMS considering the daily and longer term 

groundwater base-flow quantity and timing. However, 

MODFLOW provided much greater spatial detail in the 

simulation of base-flow conditions, including the occurrence 

of dry reaches at many locations within the stream network. 

To achieve greater comparability to groundwater recession 

and extreme low flows, MODFLOW required the inclusion 

of additional localized evapotranspiration losses from the 

aquifer riparian zones. Because of this, MODFLOW did not 

preserve the water balance as simulated by PRMS; therefore, 

the quantitative results of the MODFLOW simulation must 

be used with caution. This result indicates the need for greater 

spatial detail in the recharge and evapotranspiration elements 

of PRMS so that localized hydrologic processes occurring in 

riparian zones that affect groundwater discharge to streams are 

more accurately simulated. 

Base-flow simulations using MODFLOW show that 

many stream reaches lose water to the subsurface and act 

to recharge local aquifers in the watershed. Some of these 

reaches can go dry for short periods of time. The location of 

the losing reaches tends to be in areas near the transition from 

the uplands to the valley bottoms that are filled with coarse 

glacial stratified deposits. 

Simulation of projected and hypothetical development 

in the watershed shows that streamflow changes from being 

dominated by groundwater to being dominated by surface 

runoff when the effective impervious area in an HRU is 

between 10 and 20 percent, depending on the manner in 

which stormwater is managed. This shift would likely result 

in substantial changes in water quality and instream habitat 

characteristics of the river. Increasing impervious surface area 

generally increases streamflow and at the same time reduces 

groundwater and subsurface-runoff components, thus causing 

a compounding effect on the percentage of the flow that is 

comprised of groundwater. 

Base flow to streams is reduced by increased impervious 

surface and increased groundwater withdrawals. In the 

Pomperaug River watershed, considering current zoning 

regulations, increased groundwater withdrawals have the 

potential for causing greater overall reductions in flow 

compared to increased development and impervious surfaces. 

Additionally, on the basis of the simulations, the projected 

increase in groundwater withdrawals and development across 

the watershed will increase the number of stream reaches 

experiencing dry conditions and the duration of these dry 

periods. The simulated increase in the duration and extent of 

dry stream reaches, however, appears to be related more to 

the general increase in impervious surface than to increased 

pumping, primarily because of the limited extent and number 

of large groundwater withdrawals in the watershed.

Conversion of land from forest or developed land cover 

to pasture or agricultural land (the all-grass scenario) increases 

groundwater recharge and discharge to streams, while at the 

same time increasing overall streamflow (the opposite effect 

as increased impervious surface). This increase in streamflow 

occurs because of a reduction in evapotranspiration. The allgrass scenario assumes no reduction in infiltration of precipitation, which may be representative of agricultural land but cannot be considered to be representative of lawns, which would 

be expected to be more compacted. These results indicate 

that preservation of agricultural land and pasture can provide 

hydrologic benefit with regard to recharge potential. As water 

use increases along with development and increasing population, the risk to instream habitat conditions that are dependent 

on flow volume is compounded. Additionally, there is a higher 

risk to flow reduction if registered diversions are used during 

these periods. The results indicate that if all registered diversions are used during periods of low flow, there is an increased 

risk that parts of the river system will experience extreme low 

flows below tolerable levels or possibly dry streambeds. 

Because groundwater is the principle source of water to 

the river system, increased groundwater pumping and factors affecting recharge quantity and quality are important to 

the river health. Groundwater pumping will become a greater 

issue in the future, and development projections that are based 

on current zoning indicate that land-use change in the lower 

Pomperaug River watershed will reduce recharge and base 

flow to the lower reach of the river. 

Acknowledgments

The authors would like to thank the members of the 

PRWC, especially Marc Taylor, Lawrence Pond, Laurie 

Giannotti, Cris Schaefer, Edward Edelson, and Donna Lesch, 

for their input and assistance with data collection and analysis. 

The authors would also like to thank Ken Ferroni of O&G 

Industries for his cooperation with the study and Virginia 

Mason and Glenda Prentiss of the Naugatuck Valley Council 

of Governments for their input and development of projected 

full build-out land use and impervious surface area in the 

watershed. The authors appreciate the expertise and input 

provided by Phil Zarriello, Ed Koerkle, Steve Garabedian, 

and Paul Barlow of the U.S. Geological Survey for their 

technical reviews of the study. Also, the authors would like to 

thank George Leavesley, Lauren Hay, Steve Markstrom, and 

Roland Viger for their invaluable assistance with the PRMS 

model application. 
76 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

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78 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Table A1. GIS data and supporting parameter information for each HRU.—Continued

[HRU, hydrologic response unit; N1, HRU in Nonnewaug watershed; W1, HRU in Weekeepeemee watershed; PU1, HRU in Pomperaug watershed downstream of the 

HRU

HRU 

reference 

number

HRU 

area 

(acres)

Mean 

HRU slope 

(percent)

Median 

HRU slope 

(percent)

Median 

HRU 

elevation 

(meters)

Root depth 

(inches)

Cover 

density, 

summer 

(fraction)

Cover 

density, 

winter 

(fraction)

X coordinate, 

Albers

Y coordinate, 

Albers

Fraction 

coarse glacial 

stratified 

deposits 

in HRU

N1 1 877 6.9 6 322 23.1 0.46 0.21 1867107 2299038 0.000 

N2 2 1,858 7.8 7 262 27.2 0.61 0.27 1867807 2295318 0.023 

N3 3 452 9.3 9 274 23.0 0.41 0.25 1868787 2296118 0.024 

N4 4 2,675 9.1 8 260 26.4 0.57 0.26 1870207 2295898 0.059 

N5 5 284 9.9 9 218 30.6 0.75 0.31 1869207 2293218 0.076 

N6 6 3,911 8.2 8 204 27.8 0.64 0.28 1872027 2291378 0.050 

N7 7 1,231 10.1 9 173 30.0 0.72 0.30 1869767 2291358 0.081 

N8 8 1,592 9.4 9 189 29.7 0.72 0.34 1868527 2290058 0.212 

N9 9 763 9.4 8 160 33.0 0.87 0.41 1869607 2289638 0.036 

N10 10 1,188 8.9 8 155 31.4 0.81 0.35 1871547 2287798 0.184 

N11 11 1,382 11.2 11 163 30.4 0.81 0.41 1871327 2285978 0.152 

N12 12 741 10.2 10 152 32.7 0.87 0.35 1868447 2287998 0.188 

N13 13 76 6.4 6 78 18.5 0.26 0.11 1869347 2286678 0.803 

N14 14 315 8.6 8 88 23.3 0.53 0.30 1869487 2285958 0.245 

W1 15 1,053 9.0 8 303 28.1 0.71 0.30 1866147 2297998 0.014 

W2 16 693 10.1 10 241 29.2 0.70 0.33 1865907 2295278 0.102 

W3 17 1,107 8.9 8 215 29.7 0.73 0.35 1866847 2293078 0.084 

W4 18 3,325 11.6 11 279 32.2 0.83 0.36 1864067 2295738 0.042 

W5 19 794 9.8 9 206 29.3 0.69 0.28 1865387 2292138 0.060 

W6 20 1,599 10.0 9 260 30.1 0.73 0.31 1864287 2291698 0.012 

W7 21 1,875 13.1 12 236 32.6 0.85 0.36 1863027 2292298 0.036 

W8 22 3,860 12.8 11 250 32.0 0.81 0.34 1862347 2289958 0.026 

W9 23 540 11.5 12 181 33.3 0.90 0.46 1866947 2289998 0.100 

W10 24 407 8.6 8 139 26.4 0.55 0.23 1865927 2290258 0.203 

W11 25 276 14.2 13 145 30.7 0.79 0.34 1867007 2288518 0.247 

W12 26 161 12.1 12 124 28.6 0.67 0.30 1866507 2288218 0.217 

W13 27 284 12.6 12 121 31.5 0.80 0.34 1865907 2288118 0.417 

W14 28 992 12.5 12 177 33.0 0.87 0.37 1865587 2287278 0.113 

W15 29 186 11.5 11 97 31.7 0.81 0.43 1867747 2286938 0.426 

W16 30 172 13.2 12 90 29.9 0.72 0.30 1868007 2287318 0.622 

PU1 31 1,228 11.1 10 242 32.6 0.84 0.37 1865607 2285118 0.021 

PU2 32 821 9.9 9 129 29.3 0.72 0.34 1867227 2285018 0.310 

PU3 33 501 10.4 10 125 33.0 0.88 0.37 1867707 2285918 0.303 

PU4 34 541 7.9 7 80 23.0 0.49 0.27 1869087 2284898 0.222 

PU5 35 428 8.6 8 86 26.5 0.67 0.43 1870067 2283918 0.717 

PU6 36 412 12.0 12 149 31.5 0.83 0.44 1870987 2284138 0.024 

PU7 37 1,065 9.9 9 203 32.6 0.87 0.38 1872267 2283098 0.014 

PU8 38 216 11.2 10 157 35.0 0.96 0.67 1871107 2282638 0.036 
Table A1 79

confluence of the Nonnewaug and Weekeepeemee and upstream of the streamgage; PD1, HRU in Pomperaug watershed downstream of the streamgage]

Percent 

impervious 

surface

Decimal percent effective 

impervious 

surface in 

HRU

Dominant 

HRU 

aspect 

(degrees) 

Fraction 

of open 

water in 

HRU

Total length 

of all 

streams in 

HRU 

(miles)

Characteristic 

available soil 

water capacity 

for each HRU 

(inch/inch)

Fraction 

type D 

soils in 

HRU

Characteristic 

summer rain 

interception 

for HRU 

(inches)

Characteristic 

winter rain 

interception 

for HRU 

(inches)

Characteristic 

snow interception for 

HRU 

(inches)

3.47 0.007 90 0.079 2.400 0.1139 0.03 0.041 0.019 0.011

4.02 0.008 90 0.003 11.885 0.1074 0.02 0.047 0.021 0.015

3.62 0.008 270 0.004 3.554 0.1109 0.01 0.043 0.024 0.017

3.46 0.007 225 0.014 13.607 0.1080 0.03 0.046 0.021 0.014

3.02 0.006 45 0.008 1.957 0.1119 0.00 0.048 0.02 0.015

3.83 0.008 270 0.006 19.496 0.1076 0.08 0.046 0.021 0.015

3.38 0.007 90 0.000 7.152 0.0971 0.08 0.048 0.021 0.016

3.18 0.007 90 0.007 6.808 0.1093 0.07 0.047 0.022 0.02

3.23 0.007 90 0.001 2.731 0.1074 0.10 0.048 0.023 0.025

3.1 0.007 270 0.001 7.869 0.1022 0.26 0.049 0.022 0.02

3.61 0.008 270 0.027 7.490 0.1022 0.21 0.045 0.023 0.028

3.28 0.007 225 0.008 3.774 0.1044 0.17 0.048 0.021 0.018

9.03 0.017 180 0.000 0.722 0.1017 0.00 0.032 0.015 0.007

10.68 0.020 315 0.000 1.314 0.1098 0.07 0.037 0.022 0.026

3.12 0.007 270 0.097 3.179 0.1118 0.05 0.043 0.019 0.016

3.11 0.007 270 0.011 5.870 0.1183 0.01 0.047 0.022 0.019

3.91 0.008 270 0.006 4.640 0.1130 0.02 0.047 0.023 0.022

2.64 0.006 90 0.005 13.425 0.1217 0.03 0.047 0.021 0.019

3 0.006 90 0.004 5.137 0.1130 0.01 0.048 0.02 0.013

3.11 0.007 90 0.004 6.886 0.1204 0.04 0.049 0.021 0.016

2.71 0.006 270 0.003 6.957 0.1171 0.03 0.049 0.021 0.019

2.71 0.006 90 0.003 17.408 0.1193 0.11 0.049 0.021 0.017

2.98 0.006 270 0.002 2.149 0.1103 0.01 0.048 0.025 0.032

3.24 0.007 135 0.003 2.206 0.1072 0.04 0.047 0.021 0.011

3.57 0.008 270 0.005 1.353 0.1128 0.04 0.044 0.021 0.019

3.12 0.007 45 0.017 1.630 0.1190 0.02 0.049 0.021 0.016

3.63 0.008 225 0.000 2.137 0.1144 0.01 0.049 0.021 0.018

2.69 0.006 45 0.000 3.333 0.1162 0.05 0.049 0.022 0.02

2.48 0.005 90 0.002 1.819 0.0890 0.28 0.048 0.028 0.03

4.78 0.010 225 0.000 1.347 0.1107 0.17 0.047 0.021 0.017

2.68 0.006 90 0.000 5.767 0.1117 0.03 0.049 0.022 0.021

3.66 0.008 90 0.005 5.236 0.1130 0.03 0.047 0.023 0.021

2.83 0.006 90 0.004 3.369 0.1085 0.22 0.047 0.022 0.02

8.29 0.016 270 0.031 3.776 0.1071 0.03 0.042 0.022 0.021

7.89 0.015 270 0.008 1.972 0.0965 0.29 0.041 0.025 0.037

4.68 0.010 270 0.000 1.020 0.1131 0.07 0.047 0.024 0.031

3.27 0.007 225 0.013 3.972 0.1236 0.19 0.048 0.022 0.021

2.29 0.005 270 0.000 1.021 0.1040 0.03 0.049 0.035 0.058
80 Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut

Table A1. GIS data and supporting parameter information for each HRU.—Continued

[HRU, hydrologic response unit; N1, HRU in Nonnewaug watershed; W1, HRU in Weekeepeemee watershed; PU1, HRU in Pomperaug watershed downstream of the 

HRU

HRU 

reference 

number

HRU 

area 

(acres)

Mean 

HRU slope 

(percent)

Median 

HRU slope 

(percent)

Median 

HRU 

elevation 

(meters)

Root depth 

(inches)

Cover 

density, 

summer 

(fraction)

Cover 

density, 

winter 

(fraction)

X coordinate, 

Albers

Y coordinate, 

Albers

Fraction 

coarse glacial 

stratified 

deposits 

in HRU

PU9 39 60 8.4 8 76 22.1 0.46 0.25 1870327 2282898 0.998 

PU10 40 443 11.3 10 107 31.8 0.85 0.59 1869067 2283338 0.335 

PU11 41 1,933 11.8 11 141 30.4 0.79 0.35 1867407 2282318 0.144 

PU12 42 650 12.0 11 110 28.5 0.78 0.49 1868687 2280618 0.000 

PU13 43 197 10.3 10 75 21.0 0.56 0.45 1869727 2281918 0.450 

PU14 44 164 7.6 7 70 26.8 0.64 0.31 1870247 2281858 0.978 

PU15 45 230 11.4 11 122 32.8 0.88 0.53 1870807 2281858 0.019 

PU16 46 852 9.3 9 180 32.8 0.88 0.37 1871667 2280898 0.036 

PU17 47 396 8.9 9 106 32.0 0.84 0.42 1870767 2279678 0.001 

PU18 48 488 6.2 6 63 23.4 0.50 0.21 1869867 2279618 0.734 

PU19 49 516 7.5 7 68 20.6 0.44 0.21 1869167 2279698 0.676 

PU20 50 135 7.9 7 80 20.7 0.40 0.17 1869167 2278038 0.645 

PU21 51 116 6.3 6 68 17.2 0.30 0.14 1869927 2277638 0.898 

PU22 52 129 6.4 6 68 20.1 0.41 0.17 1870207 2278138 1.000 

PU23 53 884 8.1 8 185 33.2 0.88 0.38 1872187 2279318 0.000 

PU24 54 203 9.8 9 90 18.8 0.48 0.21 1871067 2278078 0.898 

PU25 55 1,102 9.9 9 145 31.0 0.81 0.33 1872227 2277498 0.424 

PD1 56 1,164 11.7 11 220 34.1 0.91 0.42 1865605 2281943 0.043 

PD2 57 1,325 13.2 12 187 33.1 0.87 0.38 1865205 2280043 0.046 

PD3 58 883 10.5 10 117 30.0 0.78 0.36 1867165 2278723 0.111 

PD4 59 1,244 12.5 11 172 25.7 0.53 0.25 1865605 2277863 0.052 

PD5 60 349 12.4 10 79 29.2 0.72 0.42 1867665 2276743 0.000 

PD6 61 1,053 11.1 10 107 28.8 0.72 0.49 1868585 2277363 0.106 

PD7 62 1,442 13.2 12 105 27.4 0.70 0.35 1869945 2275723 0.142 

PD8 63 586 17.5 16 92 33.7 0.91 0.56 1867225 2274863 0.095 

PD9 64 537 11.2 11 72 32.5 0.87 0.55 1867985 2274783 0.159 
Table A1 81

confluence of the Nonnewaug and Weekeepeemee and upstream of the streamgage; PD1, HRU in Pomperaug watershed downstream of the streamgage]

Percent 

impervious 

surface

Decimal percent effective 

impervious 

surface in 

HRU

Dominant 

HRU 

aspect 

(degrees) 

Fraction 

of open 

water in 

HRU

Total length 

of all 

streams in 

HRU 

(miles)

Characteristic 

available soil 

water capacity 

for each HRU 

(inch/inch)

Fraction 

type D 

soils in 

HRU

Characteristic 

summer rain 

interception 

for HRU 

(inches)

Characteristic 

winter rain 

interception 

for HRU 

(inches)

Characteristic 

snow interception for 

HRU 

(inches)

12.31 0.022 270 0.000 0.601 0.0919 0.01 0.036 0.019 0.023

2.73 0.006 90 0.015 2.628 0.0978 0.34 0.048 0.032 0.05

3.34 0.007 90 0.058 8.587 0.1175 0.17 0.045 0.021 0.021

6.99 0.014 135 0.009 0.786 0.1015 0.53 0.041 0.027 0.041

3.78 0.008 90 0.245 1.075 0.0985 0.39 0.03 0.023 0.042

6.7 0.013 270 0.004 1.756 0.1129 0.00 0.048 0.022 0.02

3.18 0.007 270 0.003 0.489 0.1082 0.07 0.049 0.029 0.041

4.02 0.008 270 0.001 3.824 0.1178 0.03 0.049 0.021 0.021

3.46 0.007 270 0.003 1.325 0.1165 0.03 0.049 0.024 0.028

9.53 0.018 270 0.009 4.605 0.1186 0.00 0.042 0.018 0.012

14.59 0.026 90 0.007 5.559 0.1046 0.02 0.038 0.018 0.017

12.99 0.023 90 0.000 0.454 0.1168 0.00 0.044 0.02 0.012

19.47 0.033 315 0.000 0.454 0.1016 0.00 0.036 0.018 0.015

18.17 0.031 225 0.000 0.856 0.1072 0.00 0.036 0.018 0.015

3.85 0.008 270 0.002 2.880 0.1165 0.08 0.048 0.021 0.022

17.13 0.030 315 0.000 2.128 0.1063 0.00 0.029 0.013 0.02

5.46 0.011 225 0.000 3.645 0.1124 0.01 0.047 0.02 0.019

2.52 0.005 225 0.0000 4.050 0.1166 0.03 0.050 0.023 0.025

2.71 0.006 90 0.0029 4.380 0.1130 0.14 0.049 0.023 0.022

7.88 0.015 270 0.0086 3.563 0.1153 0.02 0.045 0.022 0.024

3.56 0.008 90 0.0043 6.623 0.1089 0.11 0.048 0.022 0.014

7.85 0.015 270 0.0026 1.243 0.1172 0.21 0.046 0.027 0.034

8.61 0.017 90 0.0046 4.465 0.1148 0.27 0.045 0.030 0.044

9.22 0.018 315 0.0011 5.550 0.1151 0.18 0.041 0.021 0.027

2.12 0.005 90 0.0114 3.812 0.1119 0.22 0.049 0.030 0.044

3.23 0.007 270 0.0137 3.408 0.1073 0.00 0.047 0.030 0.044
Prepared by the Pembroke, West Trenton, and 

Lafayette Publishing Service Centers.

For more information concerning this report, contact:

Director

U.S. Geological Survey

New Hampshire-Vermont Water Science Center

331 Commerce Way, Suite 2

Pembroke, NH 03275

dc_nh@usgs.gov

or visit our Web site at:

http://nh.water.usgs.gov
Bjerklie and others—Estimation of the Effects of Land Use and Groundwater Withdrawals on Streamflow for the Pomperaug River, Connecticut—SIR 2010–5114

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