The Hydrological Simulation Program-FORTRAN 

(Bicknell and others, 2000), hereafter referred to as HSPF, 

was developed and calibrated for the 303-mi2 Pawcatuck River 

Basin to evaluate the effects of withdrawals and land-use 

change on streamflow. A summary of model development and 

calibration follows, but details of this work are presented in 

appendix 2. 

The basin is represented by hydrologic response units 

(HRUs) composed of 17 pervious areas (PERLNDs) and 2 

impervious areas (IMPLNDs) developed from a combination 

of land-use, surficial-geology, and soil types that are linked 

to 84 stream reaches (RCHRESs) (sites shown in fig. 2–1 and 

described in table A2–4). The area of various HRUs contributing to a reach and the linkage of reaches to one another form 

subbasins in the model. Forested HRUs are the dominant 

land-use type composing about 62 percent of the basin area; 

most forested areas overlie till (about 35 percent of the basin 

area). Wetlands are also an important HRU, composing about 

14 percent of the basin area.

The model was calibrated from January 1, 2000, to 

September 30, 2004, with measured and estimated flows at 

17 continuous-record streamflow-gaging stations and 34 

partial-record stations. The parameter estimation program 

(PEST) was used to refine the empirical model calibration and 

to evaluate parameter sensitivities and uncertainties. Simulated 

and observed hydrographs, flow-duration curves, and scatterplots indicate that the model is generally well calibrated. Statistics computed to assess the model fit, including the coefficient of determination (R2

) and the model-fit efficiency (MFE), 

also indicate that the simulated flows are in good agreement 

with the observed flows. Simulated daily mean flows at the 

17 continuous streamflow-gaging stations had a median R2 of 

0.81 and ranged between 0.66 and 0.87; the median MFE was 

0.79 and ranged between 0.59 and 0.86. Although the model 

is generally well calibrated to observed streamflows, users 

should be aware of the model limitations and uncertainties 

discussed in appendix 2. Scenarios are, therefore, best viewed 

as relative change rather than absolute change; this perspective 

minimizes the effects of model limitations and uncertainties in 

the interpretation of model results. 

Simulated annual water budgets for the 2000–04 

calibration period indicate that the baseflow component of 

streamflow from areas overlying sand and gravel, till, steeprocky areas, and wetlands account for about 98, 71, 22, and 

22 percent of the total average flow, respectively. The 

remaining flow was generated mostly from interflow (fastresponding subsurface flow) except in steep-rocky areas and 

wetlands where flow was about equally split between interflow 

and surface runoff. About 59 percent of the average moisture 

supply to the basin was discharged to the streams, 38 percent 

was lost to evapotranspiration, and the remaining 3 percent 

went into storage. Moisture supply discharged to streams, 

on average, comprises 91 percent subsurface discharge 

(interflow and baseflow) and about 9 percent surface runoff. 

During March 2001, a wet period, streamflow comprised 

about 20 percent surface runoff, 35 percent interflow, and 

47 percent groundwater. During August 2002, a dry period, 

evapotranspiration loss was about twice as great as the 

moisture supply to the basin; thus, streamflow was mostly 

discharge from groundwater storage.

The Pawcatuck River Basin HSPF model was developed as a tool to evaluate the effects of withdrawal practices 

and land-use change on streamflow. Results are intended to 

provide information for making water-resources-management 

decisions. In December 2002, the Pawcatuck Watershed Partnership Water-Use Stakeholders Group (WUSG), in conjunction with the USGS, agreed to the following model scenarios:

• No withdrawals—provides information that approximates unaltered streamflow conditions;

• Current withdrawal practices—provides information 

on long-term streamflow under current (2000–04) 

withdrawals, which in conjunction with simulations of 

no withdrawals, provides the baseline information for 

evaluating other water-management practices;

• Conversion of selected irrigation withdrawals from 

surface-water to groundwater sources—provides 

information on the potential benefits of the lag effect of 

groundwater withdrawals on streamflow;

• Future water-supply withdrawals—provides information on the potential effects of new groundwater withdrawals identified by the RIWRB on streamflow; and

• Buildout—provides information on the effects of 

changes in land use and water demand on streamflow; 

scenarios identify the effects of land-use change only, 

water-demand change only, and the combined effects 

of both types of changes.

Part 2. Simulation of Water-Use and Land-Use Changes on Streamflow 

with a Precipitation-Runoff Model (HSPF)

By Phillip J. Zarriello
46 Part 2. Simulation of Water-Use and Land-Use Changes on Streamflow with a Precipitation-Runoff Model (HSPF)

Each of these scenarios required a new model-run file 

(uci) that altered model input data or structure, or both. Each 

new uci file is uniquely identified by its prefix name; simulation output is uniquely identified with the IDSCEN attribute 

and a unique Data Set Number (DSN) in the Watershed DataManagement (WDM) system. Model-run file names and DSNs 

for the output results associated with the scenario simulations 

are summarized in table 2–1. Model scenarios were simulated 

with an hourly time step for the 1960–2004 period using the 

PROVID station (fig. 1–1) climate data adjusted to the concurrent climate data measured in the basin at the FBWR station 

during the 2000–04 period (appendix 2). 

Effects of Withdrawals on Streamflow

Simulation results can be output at any of the 84 modelreach nodes; however, reporting results from all reaches is 

impractical. The WUSG requested that water-use scenarios 

focus on 13 reaches in the central and eastern part of the basin 

(fig. 2–1). These reaches were selected because of known 

flow alteration, their ecological importance, or both. Model 

results from the upper Meadow Brook subbasin (RCHRES 

47, MEAD1) are not included in the discussion that follows 

because there was little change between scenarios. 

Simulations with average 2000–04 withdrawals and 

without withdrawals indicate the different effects of these two 

sets of conditions on long-term streamflow. These simulations 

also provide a baseline for other simulations. Simulations 

without withdrawals better reflect natural-flow conditions, but 

also reflect current land-use conditions and factors such as 

distributed domestic withdrawals that are incorporated into the 

model-parameter calibration. 

Long-term (1960–2004) simulation of current withdrawals (LT–CDmd) entailed developing long-term withdrawal 

data that represents average 2000–04 withdrawal practices. 

Estimates of long-term irrigation withdrawals were determined 

by logistic-regression equations that relate irrigation needs to 

antecedent conditions and measured use rates as described in 

Part 1. Measured withdrawals, mostly for the 2000–04 period, 

were used where available. Simulations of no withdrawals 

(LT–NoDmd) entailed zeroing out withdrawals specified in the 

External Source block of the uci file by use of the multiplier 

field (MULT). Simulation results for no withdrawals were 

output to DSN 6001 to 6080 and to DSN 6101 to 6180 for 

average 2000–04 withdrawals.

Simulation of selected irrigation withdrawals from 

surface-water to groundwater sources (LT–CDSWR) entailed 

constructing new times-series data of total withdrawals for 

each reach affected by this change. Irrigation withdrawals are 

mostly in the eastern Pawcatuck River or lower Wood River 

subbasins. The new withdrawal time series represent the combined calculated streamflow depletion of individual withdrawals converted from surface-water to groundwater sources, plus 

any continued direct surface-water withdrawals or streamflow 

depletion from existing pumped wells in the reach. In some 

cases, conversion of an irrigation withdrawal from a surfacewater to a groundwater source entailed moving the withdrawal 

to a new reach. Overall, 10 of the 37 individual irrigation 

withdrawals were converted from surface-water to groundwater sources. Of these, 8 withdrawals were moved to a different subbasin mostly from the lower Beaver River subbasin to 

an adjacent subbasin on the Pawcatuck River. Results of this 

simulation were output to DSN 6201 to 6280 in the WDM file.

Table 2–1. Summary of model simulations and target data-set numbers for Hydrologic Simulation Program-FORTRAN (HSPF) of the 

Pawcatuck River Basin, southwestern Rhode Island and southeastern Connecticut.

[uci, model user control input file; DSN and IDSCEN, Data-set number and scenario-identification attribute in the watershed-data-management (WDM) file, 

respectively; Mgal/d, millions of gallons per day]

uci and IDSCEN name Range of output DSNs Description

Withdrawal simulations

LT–NoDmd 6001–6080 No withdrawals (no demands).

LT–CDmd 6101–6180 Average 2000–04 withdrawals (current demands).

LT–CDSWR 6201–6280 Same as LT–CDmd with selected irrigation withdrawals converted from surface-water 

to groundwater sources.

LT–CDWRB 6301–6380 Same as LT–CDSWR with potential water-supply wells pumped at 1 Mgal/d.

Buildout simulations

LT–BldCD 6401–6480 Potential land-use change and 2000–04 water use (current demand).

LT–95LFD 6601–6680 1995 Land use and potential water use change (future demand).

LT–BldFD 6701–6780 Potential land-use and water-use change.
Effects of Withdrawals on Streamflow 47

Focus reaches 

Water withdrawals 

Land-use change 

Worden

Pond

Watchaug

Chapman Pond

Pond

Yawgoo

Pond

Barber

Pond

I

I

*

*

WOOD1

PAWC1

PAWC6

FLAT1

PAWC5

SHUN1

CHIP1

PAWC3

WYAS1

WOOD3

GREE1

PARR1

CHIP3

BREA1

MOSC1

TOMA1

CANO1

PAWC8

PASQ1

WOOD4

BEAV1

PAWC2

POQU1

ASHA1

ROAR1

KELL1

CEDA1

MEAD1

FISH2

FISH1

LOCK1

SHUN2

WOOD2

BEAV2

ASSE1 PAWC4

PEND1

WOOD5

CHIP2

CHIC1

BRUS1

PAWC7

QUFB1

PAWC9

BRUS2

QUEN7

GLEN1

QUEN1

BEAV3

PARM1

PERR1

MEAD2

QUEN5

COON1

GLAD1 CHIC2

TANE1

WOOD6

LWPD1

QUEN6

QUEN3

TRIB1

FISH3

SHER1

QUFB2

QUEN2

PEEP1

QUEN4

LOCK2

RAKE1

4 1

7 5 2

31

17

6 3

8

9

10

11

32

35

33

37

14

18

16

15

20

19

13

34

36

42

41

47

44

46

45

43

58

55

54

52

56

53

57

61

60

62

64

78

63

70 48

59

49 50

65

68

67

69

66

51

73

74

75

76

77

81

82

83

79

84 71

80 72

Basin boundary

State boundary

EXPLANATION

0 1 2 3 4 5 MILES Subbasin boundary

0 1 2 4 KILOMETERS

PAWC4

Continuous streamflow-gaging station

Partial-record streamflow-gaging station

Stream reach number and subbasin 

identification name (IDLOCN)

Eastern

Pawcatuck River 

Region (CHIP2, 

CHIP3, CHIC2, 

PAWC2)

Usquepaug-Queen and 

Beaver Rivers Region 

(QUEN4, QUEN7, BEAV3, 

TANE1)

Lower Wood River 

Region (WOOD5, 

WOOD6, MEAD2, 

PWC4)

50

3 5

71o37' 30" 71o 71 30' o 71 45' o

52' 30"

42o

37'30"

41o

22'30"

41o

30'30"

Base from U.S. Geological Survey digital data

North American Datum of 1983

Rhode Island state plane projection, 1:24,000

Figure 2–1. Focus reaches where the effects of water-withdrawal and land-use changes were examined with the Hydrologic 

Simulation Program-FORTRAN (HSPF) model of the Pawcatuck River Basin, southwestern Rhode Island and southeastern 

Connecticut.
48 Part 2. Simulation of Water-Use and Land-Use Changes on Streamflow with a Precipitation-Runoff Model (HSPF)

Potential new groundwater withdrawals (LT–CDWRB) 

were simulated for 14 sites identified by the RIWRB. Withdrawals from these sites were made through an additional 

specified outflow (exit gate) added to the appropriate reach. 

This modification allows for greater flexibility in future applications of the model; withdrawal rates at these sites can be 

modified easily, or turned on or off as desired without affecting other withdrawals in the model. For the purpose of this 

scenario, each of the 14 potential withdrawals was pumped at 

a constant rate of 1 Mgal/d (1.55 ft3/s) that results in a constant 

rate of streamflow depletion. The LT–CDSWR uci file was 

the base simulation for this scenario and, thus, the simulation results also reflected the conversion of select irrigation 

withdrawals from surface-water to groundwater sources. Water 

withdrawn from new withdrawals was assumed to be lost from 

the basin (water was not returned to the basin through wastewater discharge or other means). Results of this simulation 

were output to DSN 6301 to 6380 in the WDM file. 

Results of the simulations are shown for the 12 focus 

reaches as daily mean flow-duration curves for the 1960–2004 

period and as hydrographs of hourly flow for August 2002, 

the month of lowest flow during the calibration period. The 

August 2002 period was chosen over other historically drier 

periods because actual withdrawal information was used to the 

extent that it was available. The results were divided into three 

general regions of the basin—(1) Usquepaug-Queen and Beaver Rivers, (2) eastern Pawcatuck River, and (3) lower Wood 

River (fig. 2–1). Within each region, results are presented for 

four reaches. Reaches are associated with HSPF subbasins; 

however, the reaches and the model output examined include 

the cumulative effects of the upstream drainage area.

Usquepaug-Queen and Beaver Rivers Region

The Usquepaug-Queen and Beaver Rivers region 

includes the Queen River at Liberty (QUEN4, combined 

flow of RCHRES 9 and 10), the Usquepaug River near its 

mouth to the Pawcatuck River (QUEN7, RCHRES 20), the 

lower Beaver River (BEAV3, RCHRES 43), and Taney Brook 

(TANE1, RCHRES 45), which enters the Pawcatuck River 

just below the confluence with the Beaver River (fig. 2–1). 

QUEN4, QUEN7, and BEAV3 have cumulative drainage 

areas of 20.1, 36.4, and 11.8 mi2, respectively, all of which 

include irrigation withdrawals. TANE1 is a small headwater 

subbasin with a drainage area of 1.61 mi2 and does not include 

municipal or irrigation withdrawals. 

Daily mean flow-duration curves for QUEN4 indicate 

only a slight difference at the lowest flows between no withdrawals and average 2000–04 withdrawals or the other conditions simulated (figs. 2–2A and 2–3A). Irrigation withdrawals 

above this reach were simulated from eight individual sources 

for golf courses, turf farms, and other agricultural operations. 

During the summer months (June through August) irrigation 

withdrawals in and upstream of this reach averaged 0.80 ft3/s. 

One possible reason why the simulations with and without withdrawals show little difference in flow at QUEN4 is 

because most of the withdrawals were from upper Queens Fort 

Brook (QUFB1, RCHRES 7). Most of the active groundwater 

and 80 percent of the interflow from QUFB1 is routed out 

of the subbasin to account for apparent differences between 

the surface and subsurface subbasin divides (see appendix 2 

discussion about the calibration of this reach and Zarriello 

and Bent (2004) for details about the complexity of the flow 

paths in this part of the basin). Because flows are limited by 

diversion of subsurface flow, withdrawals from this subbasin 

have only a minor effect on downstream flows. As a result, the 

effects of withdrawals at QUEN4 are mostly limited to those 

in the upper Queen River and the lower Queens Fort Brook 

subbasins, which collectively accounted for about 12 percent 

of the total withdrawals (about 0.14 ft3/s) above QUEN4 during the 2000–04 period. It should also be noted that all irrigation withdrawals above QUEN4 were estimated. No RIWRB 

potential withdrawal sites were located above this reach; 

therefore, the streamflow response for this simulation (LT–

CDWRB) is the same as for the average 2000–04 withdrawal 

simulation (LT–CDmd). Effects of other potential withdrawals 

on streamflow in the former Ladd School development area 

were previously simulated with an earlier HSPF model and 

described by Zarriello and Bent (2004).

Daily mean flow-duration curves for QUEN7 indicate 

that flows above the 90-percent duration are moderately 

affected by average 2000–04 withdrawals (fig. 2–2B). At 

the 99.8-percent flow duration, daily mean flows under no 

withdrawals were about 30 percent greater than under average 

2000–04 withdrawals. Simulations indicate that converting 

selected irrigation withdrawals (including one that accounts 

for about a fifth of the total irrigation demand from the reach) 

from surface-water to groundwater sources (LT–CDSWR) 

can increase low flows (fig. 2–2B) and appreciably dampen 

the intradaily flow fluctuations compared to average 2000–04 

withdrawals (fig. 2–3B). During August 2002, this conversion 

resulted in streamflow that was slightly above the maximum 

daily flow under average 2000–04 withdrawals, which generally reflects the period of the day when direct stream withdrawals ceased and streamflow was affected only by withdrawals from pumped wells. 

Streamflow in BEAV3 responded similarly in the same 

withdrawal scenarios simulated in QUEN7, but in a somewhat 

more pronounced manner. At the 99.8-percent flow duration, 

streamflow under no withdrawals was about two times greater 

than under average 2000–04 withdrawals (fig. 2–2C). Converting irrigation withdrawals from surface-water to groundwater 

sources increased low flows in the Beaver River to nearly the 

same level as under no demand. This pronounced change in 

low flows was, at least in part, a result of moving about 

70 percent of the total irrigation withdrawals from BEAV3 to 

pumped wells in the PAWC2 (RCHRES 37) subbasin 

(fig. 2–1). The effect of converting from surface-water to 

groundwater sources may be less than indicated because, 

although the pumped wells are in a different model subbasin, 

the contributing area to these wells could continue to affect the 

groundwater discharge to the lower Beaver River and could 
Effects of Withdrawals on Streamflow 49

DISCHARGE, IN CUBIC FEET PER SECOND

0.5 1 2 5 10 20 30 50 70 80 90 95 98 99 99.8 0.5 1 2 5 10 20 30 50 70 80 90 95 98 99 99.8

PERCENTAGE OF THE TIME DISCHARGE WAS EQUALED OR EXCEEDED

LT-NoDmd

LT-CDmd

LT-CDSWR

LT-CDWRB

EXPLANATION

1

10

100

1,000 Queen River (QUEN4, RCHRES 9)

1

10

100

1,000 Usquepaug River (QUEN7, RCHRES 20)

1

10

100

1,000 Beaver River (BEAV3, RCHRES 43)

0.1

1

10

100 Taney Brook (TANE1, RCHRES 45)

A B

C D

Figure 2–2. Flow-duration curves of simulated daily mean streamflow (1960–2004) under no withdrawals (LT–NoDmd), current 

(2000–04) withdrawals (LT–CDmd), current withdrawals with selected irrigation withdrawals converted from surface-water to 

groundwater sources (LT–CDSWR), and current withdrawals with potential new withdrawals (LT–CDWRB) at (A) Queen River 

(QUEN4, RCHRES 9); (B) Usquepaug River (QUEN7, RCHRES 20); (C) Beaver River (BEAV3, RCHRES 43); and (D) Taney Brook 

(TANE1, RCHRES 45), Pawcatuck River Basin, southwestern Rhode Island. (Site locations shown in figure 2–1 and described in 

table A2–4.)
50 Part 2. Simulation of Water-Use and Land-Use Changes on Streamflow with a Precipitation-Runoff Model (HSPF)

0

2

4

6

8

10

12

14

Queen River (QUEN4, RCHRES 9)

0

2

4

6

8 Beaver River (BEAV3, RCHRES 43)

LT-CDmd

LT-NoDmd

LT-CDSWR

LT-CDWRB

0

1

2

3

4 Taney Brook (TANE1, RCHRES 45)

A

C

D

B

EXPLANATION

Usquepaug River (QUEN7, RCHRES 20)

0

4

8

12

16

20

2 4 6 8 10 12 14 16 18 20 22 24 26 28

AUGUST, 2002

30

DISCHARGE, IN CUBIC FEET PER SECOND

Figure 2–3. Simulated hourly streamflow in August 2002 under no withdrawals (LT–NoDmd), current (2000–04) withdrawals (LT–CDmd), 

current withdrawals with selected irrigation withdrawals converted from surface-water to groundwater sources (LT–CDSWR), and 

current withdrawals with potential new withdrawals (LT–CDWRB) at (A) Queen River (QUEN4, RCHRES 9); (B) Usquepaug River (QUEN7, 

RCHRES 20); (C) Beaver River (BEAV3, RCHRES 43); and (D) Taney Brook (TANE1, RCHRES 45), Pawcatuck River Basin, southwestern 

Rhode Island. (Site locations shown in figure 2–1 and described in table A2–4.)
Effects of Withdrawals on Streamflow 51

also affect groundwater discharge to the lower Usquepaug 

River (QUEN7). The simulation of withdrawals from four 

potential supply wells in the Beaver River subbasin (LT–

CDWRB) indicate that any gains in low flows from converting irrigation withdrawals from surface-water to groundwater 

sources are offset by the additional water-supply withdrawals. 

The potential supply wells are also near the subbasin surfacewater divide, which could affect simulated streamflows 

because part of the water to the well may come from areas outside of the subbasin as described above. Under average 2000–

04 withdrawals, the intradaily flow fluctuations from irrigation 

withdrawals varied by about 1 ft3/s, which is about 50 percent 

of the daily peak flow for most days during August 2002 

(fig. 2–3C). Intradaily streamflow fluctuations were damped 

considerably by converting from surface-water to groundwater 

sources, but this damping effect also reflects moving most of 

irrigation withdrawals to the PAWC2 subbasin. 

In about 40 percent (29 of 70) of the subbasins used 

to develop the Pawcatuck River Basin HSPF model, water 

withdrawals are not specified. In some subbasins, such as 

Taney Brook (TANE1), no withdrawals are specified resulting in no change in streamflow (figs. 2–2D and 2–3D) among 

the scenarios simulated. Simulation results are shown for 

Taney Brook because they were requested by the WUSG, 

but results from this subbasin also represent about half of the 

HSPF Pawcatuck River subbasins in that little, if any, change 

in streamflow results from the water management scenarios 

simulated. Results from this subbasin also demonstrate that the 

model can be used to evaluate streamflow at ungaged locations 

under a range of climatic conditions or to evaluate other watermanagement scenarios that may affect a particular subbasin.

Eastern Pawcatuck River Region

The eastern Pawcatuck River region (fig. 2–1) includes 

the lower Chipuxet River (CHIP2, RCHRES 32 and CHIP3, 

RCHRES 33), the upper Pawcatuck River (PAWC1, RCHRES 

34), and the lower Chickasheen Brook (CHIC2, RCHRES 

36). CHIP2 and CHIP 3 are near the headwaters of the eastern 

Pawcatuck River Basin and comprise cumulative drainage areas of 9.69 and 15.4 mi2

, respectively with substantial 

municipal and agricultural withdrawals. PAWC1 has a cumulative drainage area of 25.8 mi2

 that includes the Mink Brook 

drainage area, which is the source area for the largest supply 

withdrawals in the entire Pawcatuck River Basin. CHIC2 is 

a small tributary subbasin to the upper Pawcatuck River with 

a cumulative drainage area of 4.82 mi2 that includes several 

withdrawals for irrigation.

Simulations indicate that streamflows in the eastern 

Pawcatuck River region (RCHRES 31 through 36) were 

the most affected of the reaches examined (figs. 2–4 and 

2–5). Flow-duration curves indicate that average 2000–04 

withdrawals decrease the lowest flows relative no withdrawals 

by about 40 percent in Chickasheen Brook (CHIC2), and 

by about an order of magnitude in the lower Chipuxet River 

(CHIP2) and Pawcatuck River (PAWC1). The daily mean 

flow-duration curves for average 2000–04 withdrawals and no 

withdrawals noticeably deviate from one another at about the 

50-percent duration for each of the eastern Pawcatuck River 

reaches examined; this indicates that withdrawals appreciably 

alter median to low streamflow in these reaches. 

Withdrawals from the eastern Pawcatuck River region 

serve a combination of water needs; withdrawals are predominantly for municipal supply in PAWC1 (nearly 100 percent of 

total withdrawals) and predominantly for irrigation in CHIP2 

and CHIC2 (about 98 and 100 percent of total withdrawals, respectively). Because of these differences in water use, 

the simulated streamflow responses to the alternative watermanagement scenarios vary. In CHIP2, about 70 percent of the 

cumulative irrigation withdrawals were converted from surface-water to groundwater sources in scenario LT–CDSWR. 

This conversion increased low flows slightly above the 

90-percent daily mean flow duration (fig. 2–4A) relative to 

current demands, but the effect on intradaily flow fluctuations 

is more pronounced (fig. 2–5A). The moderating effect of converting source water prevents the intermittent cessation of flow 

in CHIP2 during low-flow periods, such as mid-August 2002 

(fig. 2–5A) and generally maintains flow about equal to the 

daily maximum flow under average 2000–04 withdrawals. The 

sporadic cessation of flow during the day is not reflected in the 

flow-duration curves (fig. 2–4A) because the curves represent 

daily mean flows. Simulations indicate that the addition of a 

well pumped at 1 Mgal/d (LT–CDWRB) in CHIP2 results in 

a daily mean flow of less than 0.1 ft3/s about 1 percent of the 

time (fig. 2–4A); flows were generally equal to the daily minimum flow under average 2000–04 withdrawals (fig. 2–5A). 

Farther downstream in the Chipuxet River (CHIP3), the 

ratio of total cumulative withdrawals to streamflow decreases, 

and although differences in streamflow with and without withdrawals are appreciable (fig. 2–4B), they are less pronounced 

than in CHIP2. Intradaily streamflow fluctuations also 

diminish (fig. 2–5B) in CHIP3 relative to CHIP2 because an 

increasing percentage of the total withdrawals are for municipal-supply purposes (about 2 and 14 percent in CHIP2 and 

CHIP3, respectively), and about 40 percent of the cumulative 

irrigation withdrawals are obtained from groundwater sources 

in the CHIP3 subbasin. Differences between streamflow 

simulated under average 2000–04 demand (LT–CDmd) and 

converted irrigation (LT–CDSWR) are small for the reasons 

above and because only about 17 percent of the existing irrigation withdrawals were affected by this change. The simulation 

of potential supply wells includes two wells in the CHIP3 

subbasin that, together with the potential upstream supply 

well, cumulatively withdraw 3 Mgal/d (LT–CDWRB). This 

simulation indicates that the daily mean flow in CHIP3 drops 

below 1 ft3/s about 1 percent of the time and is about an order 

of magnitude lower than under no withdrawals at the 99-percent flow duration (fig. 2–4B). The decrease in flow caused by 

the additional withdrawals relative to the other simulations is 

also apparent in the August 2002 hydrograph (fig. 2–5B).
52 Part 2. Simulation of Water-Use and Land-Use Changes on Streamflow with a Precipitation-Runoff Model (HSPF)

0.1

1

10

100

1,000

Chipuxet River (CHIP2, RCHRES 32)

0.1

1

10

100

1,000 Chipuxet River (CHIP3, RCHRES 33)

0.1

1

10

100

1,000 Pawcatuck River (PAWC1, RCHRES 34)

LT-NoDmd

LT-CDmd

LT-CDSWR

LT-CDWRB

0.1

1

10

100 Chickasheen Brook (CHIC2, RCHRES 36)

EXPLANATION

DISCHARGE, IN CUBIC FEET PER SECOND

0.5 1 2 5 10 20 30 50 70 80 90 95 98 99 99.8 0.5 1 2 5 10 20 30 50 70 80 90 95 98 99 99.8

PERCENTAGE OF THE TIME DISCHARGE WAS EQUALED OR EXCEEDED

A B

C D

Figure 2–4. Flow-duration curves of simulated daily mean streamflow (1960–2004) under no withdrawals (LT–NoDmd), current 

(2000–04) withdrawals (LT–CDmd), current withdrawals with selected irrigation withdrawals converted from surface-water to 

groundwater sources (LT–CDSWR), and current withdrawals with potential new withdrawals (LT–CDWRB) at (A) Chipuxet River 

(CHIP2, RCHRES 32); (B) Chipuxet River (CHIP3, RCHRES 33); (C) Pawcatuck River (PAWC1, RCHRES 34); and (D) Chickasheen 

Brook (CHIC2, RCHRES 36), Pawcatuck River Basin, southwestern Rhode Island. (Site locations shown in figure 2–1 and described 

in table A2–4.)
Effects of Withdrawals on Streamflow 53

0

2

4

6

8

10

12

14 Chipuxet River (CHIP2, RCHRES 32)

0

2

4

6

8

10

12

14 Chipuxet River (CHIP3, RCHRES 33)

0

2

4

6

8

10

12

14

16

18

20

Pawcatuck River (PAWC1, RCHRES 34)

LT-NoDmd

LT-Cdmd

LT-CDSWR

LT-CDWRB

0

1

2

3

4

5

Chickasheen Brook (CHIC2, RCHRES 36)

EXPLANATION

DISCHARGE, IN CUBIC FEET PER SECOND

2 4 6 8 10 12 14 16 18 20 22 24 26 28

AUGUST, 2002

30

A

C

D

B

Figure 2–5. August 2002 simulated hourly streamflow under no withdrawals (LT–NoDmd), current (2000–04) withdrawals (LT–CDmd), 

current withdrawals with selected irrigation withdrawals converted from surface-water to groundwater sources (LT–CDSWR), and 

current withdrawals with potential new withdrawals (LT–CDWRB) at (A) Chipuxet River (CHIP2, RCHRES 32); (B) Chipuxet River (CHIP3, 

RCHRES 33); (C) Pawcatuck River (PAWC1, RCHRES 34); and (D) Chickasheen Brook (CHIC2, RCHRES 36), Pawcatuck River Basin, 

southwestern Rhode Island. (Site locations shown in figure 2–1 and described in table A2–4.)
54 Part 2. Simulation of Water-Use and Land-Use Changes on Streamflow with a Precipitation-Runoff Model (HSPF)

Simulations at PAWC1 reflect the cumulative affects 

in the Chipuxet River subbasins (CHIP1 through CHIP3; 

RCHRES 31 through 33) and in its own subbasin area, which 

includes the Mink Brook source area for major supply withdrawals. Cumulatively, total withdrawals in and above this 

reach are about 64 percent for municipal supply and 

36 percent for irrigation. The ratio of current cumulative 

withdrawals to streamflow at the outflow of the PAWC1 is the 

largest of the 12 reaches examined; hence, differences between 

streamflow simulated under average 2000–04 withdrawals and 

no withdrawals are among the most pronounced. At extreme 

low flows (greater than the 99-percent flow duration), the daily 

mean streamflow simulated under average 2000–04 withdrawals was about an order of magnitude lower than under no 

withdrawals (fig. 2–4C). Converting select irrigation withdrawals from surface-water to groundwater sources resulted 

in streamflow that differed little from that under average 

2000–04 withdrawals (figs. 2–4C and 2–5C) for reasons similar to those described for CHIP3. Simulations with potential 

supply wells (LT–CDWRB) did not differ appreciably from 

other simulations with withdrawals in PAWC1 as none were 

simulated in the PAWC1 subbasin and any change resulted 

from potential supply wells in upstream subbasins. Simulated 

hourly hydrographs indicate a small diurnal fluctuation in all 

simulations including those with no withdrawals (fig. 2–5C). 

Unlike intradaily streamflow fluctuations caused by direct 

surface-water irrigation withdrawals in other reaches examined, these fluctuations reflect diurnal evaporative losses from 

Worden Pond, the largest open water body in the Pawcatuck 

River Basin, and from expansive wetlands surrounding the 

pond. These diurnal fluctuations were larger in an alternative 

model (described in appendix 2) that simulated the wetlands in 

this part of the basin as a virtual RCHRES.

Extreme low flows in the lower Chickasheen Brook 

(CHIC2) under average 2000–04 withdrawals were about 

41 percent lower than under no withdrawals (fig. 2–4D). These 

differences were less in CHIC2 than in other reaches in the 

eastern Pawcatuck River region examined because the ratio of 

water withdrawal to streamflow was less. Converting irrigation withdrawals from surface-water to groundwater sources 

resulted in only minor differences in streamflow relative to 

current withdrawals (figs. 2–4D, 2–5D). Although the affected 

withdrawal accounted for nearly half the total peak withdrawal in and above CHIC2, the conversion affected only one 

withdrawal in the CHIC1 subbasin, whose affect on streamflow is damped by the storage in Yawgoo and Barber Ponds. 

Simulation of potential water-supply withdrawals included one 

well in CHIC2, which resulted in a sharp decrease in streamflow (about an order of magnitude at extreme low flows); this 

response reflects a large increase in the ratio of water withdrawals to streamflow at low flows.

Lower Wood River Region

The lower Wood River region (fig. 2–1) includes the 

lower Meadow Brook (MEAD2, RCHRES 48), Pawcatuck 

River (PAWC4, RCHRES 50), and Wood River (WOOD5 

and WOOD6, RCHRES 63 and 65, respectively). MEAD2 is 

a small tributary with a cumulative drainage area of 7.37 mi2

that drains into the Pawcatuck River just above the mouth of 

the Wood River and is considered an important ecological 

resource by the environmental community. PAWC4 is centrally located in the Pawcatuck River Basin and represents a 

cumulative drainage area of 205 mi2 below the confluence of 

the Pawcatuck and Wood Rivers. WOOD5 and WOOD6 are 

the two most downstream reaches in the Wood River and have 

cumulative drainage areas of 85.1 and 89.0 mi2, respectively. 

Changes in streamflow that result from average 2000–04 

withdrawals relative to no withdrawals were not appreciably different except in PAWC4 (figs. 2–6 and 2–7). In 

PAWC4, the differences between simulations with average 

2000–04 withdrawals and no withdrawals are attributed to 

water withdrawals in the eastern Pawcatuck River Basin that 

continue to affect streamflow in this reach, even with the 

moderating effect of the Wood River flow contribution. The 

Wood River subbasin composes nearly half the contributing 

area to PAWC4 and is minimally affected by withdrawals. 

Simulations indicate that outflow from PAWC4 under average 

2000–04 withdrawals were about 30 percent less than without 

withdrawals above the 99-percent flow duration. The effects of 

withdrawals in MEAD2, WOOD5, and WOOD6 were negligible because the ratios of cumulative withdrawals to streamflow are small in these reaches.

Simulations indicate that converting selected irrigation 

withdrawals from surface-water to groundwater sources had a 

negligible effect on streamflow in the reaches examined in this 

region. In reaches that already show negligible differences in 

streamflow with and without withdrawals, such as in MEAD2, 

WOOD5, and WOOD6, converting from surface-water to 

groundwater sources will have little effect on streamflow. In 

addition, most withdrawals in this region are from groundwater sources and the ratio of total withdrawal to streamflow in 

these reaches is generally small relative to the other reaches 

examined. Although converting the largest irrigation withdrawal affecting flow in WOOD6 (about 70 percent of the 

total withdrawal volume) from a surface-water to a groundwater source had no appreciable effect on the daily mean flow 

(fig. 2–6D) the intradaily streamflow fluctuations, although 

small, were removed (fig. 2–7D). Similarly, the cumulative 

effect of converting select withdrawals from surface-water to 

groundwater sources had no appreciable effect on the daily 

mean flow at PAWC4 (fig. 2–6B), but damped the small intradaily streamflow fluctuations (fig. 2–7B). These simulations 

underscore that the effects on streamflow of converting withdrawals from surface-water to groundwater sources depend 
Effects of Withdrawals on Streamflow 55

10

100

1,000

10,000 Pawcatuck River (PAWC4, RCHRES 50)

10

100

1,000

10,000 Wood River (WOOD5, RCHRES 63)

10

100

1,000

10,000 Wood River (WOOD6, RCHRES 65)

LT-CDWRB

LT-CDSWR

LT-NoDmd

LT-CDmd

0.1

1

10

100

1,000

Meadow Brook (MEAD2, RCHRES 48)

DISCHARGE, IN CUBIC FEET PER SECOND

0.5 1 2 5 10 20 30 50 70 80 90 95 98 99 99.8 0.5 1 2 5 10 20 30 50 70 80 90 95 98 99 99.8

PERCENTAGE OF THE TIME DISCHARGE WAS EQUALED OR EXCEEDED

EXPLANATION 

A B

C D

Figure 2–6. Flow-duration curves of simulated daily mean streamflow (1960–2004) under no withdrawals (LT–NoDmd), current 

(2000–04) withdrawals (LT–CDmd), current withdrawals with selected irrigation withdrawals converted from surface-water to 

groundwater sources (LT–CDSWR), and current withdrawals with potential new withdrawals (LT–CDWRB) at (A) Meadow Brook 

(MEAD2, RCHRES 48); (B) Pawcatuck River (PAWC4, RCHRES 50); (C) Wood River (WOOD5, RCHRES 63); and (D) Wood River 

(WOOD6, RCHRES 65), Pawcatuck River Basin, southwestern Rhode Island. (Site locations shown in figure 2–1 and described in 

table A2–4.)
56 Part 2. Simulation of Water-Use and Land-Use Changes on Streamflow with a Precipitation-Runoff Model (HSPF)

0

2

4

6

8

10

Meadow Brook (MEAD2, RCHRES 48)

0

10

20

30

40

50 Wood River (WOOD5, RCHRES 63)

Wood River (WOOD6, RCHRES 65)

0

20

40

60

80

100

120

140 Pawcatuck River (PAWC4, RCHRES 50)

60

0

10

20

30

40

50

DISCHARGE, IN CUBIC FEET PER SECOND

2 4 6 8 10 12 14 16 18 20 22 24 26 28

AUGUST, 2002

30

LT-NoDmd

LT-CDmd

LT-CDSWR

LT-CDWRB

EXPLANATION

A

C

D

B

Figure 2–7. August 2002 simulated hourly streamflow under no withdrawals (LT–NoDmd), current (2000–04) withdrawals (LT–CDmd), 

current withdrawals with selected irrigation withdrawals converted from surface-water to groundwater sources (LT–CDSWR), and 

current withdrawals with potential new withdrawals (LT–CDWRB) at selected model reaches at (A) Meadow Brook (MEAD2, RCHRES 

48); (B) Pawcatuck River (PAWC4, RCHRES 50); (C) Wood River (WOOD5, RCHRES 63); and (D) Wood River (WOOD6, RCHRES 65), 

Pawcatuck River Basin, southwestern Rhode Island. (Site locations shown in figure 2–1 and described in table A2–4.)
Effects of Withdrawals on Streamflow 57

on the cumulative ratio of total withdrawals to streamflow, the 

percentage of the total withdrawals that are converted, and 

the extent of the lag effect on streamflow depletion from an 

irregularly pumped well. 

The effects of simulated potential water-supply withdrawals differed among the subbasins examined. In MEAD2, two 

potential water-supply wells, pumped at a combined rate of 

2 Mgal/d, resulted in about a 40 percent decrease in low flows 

compared to streamflow simulated with and without withdrawals (figs. 2–6A and 2–7A). Potential supply withdrawals in or upstream of WOOD5 and WOOD6, have combined 

withdrawal rates of 2 and 3 Mgal/d, respectively, but result 

in no appreciable change in flow relative to average 2000–04 

withdrawals (figs. 2–6C–D, 2–7C–D) because the ratios of 

withdrawal to streamflow in these reaches is small. Similarly, 

although the cumulative withdrawal from potential supply 

wells at PAWC4 is 15 Mgal/d, these withdrawals only have a 

small effect on streamflow even at the lowest flows because 

the ratio of the withdrawals to streamflow is still relatively 

small.

Magnitude of Flow Alteration Relative to 

Streamflow

The effects of flow alteration can be evaluated in a variety of ways including their deviation from target-flow statistics 

(Tennant, 1976; U.S. Fish and Wildlife Service, 1981; Richter 

and others, 1996; Poff and others, 1998) or flow requirements determined from a hydraulic criterion at critical points 

in a reach (Nelsen, 1984; Espegren, 1996; Bovee and others, 

1998). Although these methods provide useful information 

for assessing water-quantity needs, they are not continuous 

measures of flow alteration. One method of evaluating flow 

alteration over the range and frequency of expected flows is to 

represent the magnitude of withdrawal relative to the magnitude of flow at specific flow durations.

The ratios of average summer withdrawals during 

2002–04 to daily mean flow at various flow durations 

simulated without withdrawals for the 1960–2004 period 

were determined at the 12 focus reaches examined. The 

summer withdrawals were used because in many parts of the 

Pawcatuck River Basin withdrawals are seasonal and summer 

withdrawals reflect an upper end member. In addition, the 

2002–04 withdrawals were used because this period reflects 

the best available withdrawal information. Total withdrawals 

include direct surface-water withdrawals combined with 

the computed streamflow depletion from groundwater 

withdrawals. The 1960–2004 simulated streamflow without 

withdrawals is considered an approximation of long-term 

natural streamflow conditions. Streamflow at various flow 

durations were determined from the mean daily flow-duration 

curves at each station (figs. 2–2, 2–4, 2–6). The percent of 

time the withdrawal-to-flow ratio was equaled or exceeded 

(fig. 2–8) is inversely related to the percent of time streamflow 

was equal or exceeded. For example, when the withdrawal-toflow ratio was equaled or exceeded 10 percent of the time, the 

flow was equal or exceeded 90 percent of the time. The graph 

indicates the relative flow alteration in the reaches examined 

and the percentage of time a given withdrawal-to-streamflow 

ratio is exceeded. 

 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

PERCENT OF TIME WITHDRAWAL-TO-FLOW RATIO WAS EQUALED OR EXCEEDED

QUEN4

QUEN7

BEAV3

CHIP2

CHIP3

PAWC1

CHIC2

MEAD2

PAWC4

WOOD5

WOOD6

99 98 95 90 80 70 50 40 30 20 15 10 5 2 1

RATIO OF SUM

MER WITHDRA

W

ALS TO FLOW

Reaches

Figure 2–8. Percentage of time the 

ratio of average summer withdrawals 

(2002–04) to simulated mean daily 

streamflow without withdrawals 

(1960–2004) is equaled or exceeded 

at selected reaches in the Pawcatuck 

River Basin, southwestern Rhode Island.

(Site locations shown in figure 2–1 and 

described in table A2–4.)
58 Part 2. Simulation of Water-Use and Land-Use Changes on Streamflow with a Precipitation-Runoff Model (HSPF)

Of the 12 subbasins examined, PAWC1 stands out as the 

most flow-altered reach. At PAWC1, the cumulative withdrawal extracts 20 percent or more of the daily mean streamflow about 40 percent of the time; withdrawals can exceed 

daily mean streamflow by 50 percent or more about 5 percent 

of the time. The ratios of withdrawals to daily mean streamflow at BEAV3, CHIP2, and CHIP3 generally cluster and represent the next most flow-altered reaches. Withdrawals in and 

above these reaches extract from 10 to 20 percent of the daily 

mean streamflow about 20 to 50 percent of the time; at very 

low flows, withdrawals extract about 40 percent of the daily 

mean streamflow. For CHIC2, PAWC4, QUEN4, and QUEN7 

the withdrawal-to-streamflow ratios also tend to cluster; 

withdrawals extract from 10 to 25 percent of the daily mean 

streamflow about 10 percent of the time or less. The ratio of 

withdrawals to streamflow in MEAD2, WOOD5, and WOOD6 

never exceed 10 percent of the daily mean streamflow and are 

generally considered to be minimally flow altered. Streamflow 

is unaltered by withdrawals in TANE1.

The effects of withdrawals on streamflow become evident 

as the ratio of withdrawals to streamflow increases. This can 

be seen by comparing the plots of streamflow with and without 

withdrawals (figs. 2–2, 2–4, 2–6) to the plot of the withdrawalto-streamflow ratio (fig. 2–8) for a specific reach. In general, 

as the ratio exceeds 0.1, the daily mean flow-duration curves 

with and without withdrawals begin to noticeably separate. 

Once the ratio exceeds 0.2, the flow-duration curves with and 

without withdrawals markedly separate. Examination of the 

withdrawals-to-streamflow ratios over a wide range of flows 

and the corresponding exceedence probabilities can be a useful 

tool for optimizing water resources for habitat protection and 

water-supply needs.

Effects of Potential Future Land Use and 

Water Use on Streamflow

Scenarios that reflect potential future buildout conditions 

were designed to evaluate the effects of (1) land-use change 

only, (2) water-use change only, and (3) combined effects of 

land-use and water-use change. Simulation of land-use change 

entailed modifying the areas of HRUs representing different 

types of land cover in the schematic block of the uci file. Any 

increase in HRU area representing development was offset 

by a corresponding decrease in HRU area representing open 

space, forest, or both. Simulation of water-use change at buildout entailed changing the associated External Source block of 

the uci file accordingly.

Method for Estimating Land-Use Change

The effect of potential land-use change (herein referred to 

as buildout) on streamflow was simulated by changing types 

of HRU areas to reflect changes possible under current zoning 

and land-use plans while maintaining the same total area of 

each subbasin. Buildout was first determined on the basis of 

a statewide buildout land-use map for Rhode Island compiled 

under the provisions of the Rhode Island Comprehensive 

Planning and Regulation Act of 1988 that identified current 

and potential lands for residence, business, industry, municipal 

facilities, public recreation, institutional facilities, mixed use, 

and open space (Paul Jordan, Rhode Island Department of 

Environmental Management, written commun., September 

2006). This map was supplemented in the Connecticut part of 

the basin by town-zoning ordinances from North Stonington 

(Marc Tate, North Stonington IT, written commun., October 

17, 2006) and Stonington (Jason Vincent, Stonington Planning 

Department, written commun., October 17, 2006). The 

remaining townships in the Connecticut part of the basin—

Voluntown and Sterling—compose small percentages of the 

basin area (1.9 and 2.6 percent, respectively) and are mostly 

classified as State forest that is protected from development. 

The small amount of developable area in these townships 

was assumed to be available for low- to moderate-density 

residential development. 

The statewide buildout map did not identify all areas 

designated in other sources as protected or as having limited 

development potential. For this reason, lands with development restrictions were compiled from several sources from 

RIGIS and MAGIC that included: (1) land owned by the 

Audubon Society, (2) land owned or managed by The Nature 

Conservancy or Municipal Land Trust, (3) land owned by 

State and local governments assigned protection status, and 

(4) wildlife-management areas. Combined, these restrictions 

limit development in about a third of the basin. Further development restrictions were added on the basis of proximity to 

water, wetlands, or bedrock outcrops, which are typically steep 

and identified in the state soil surveys (SSURGO) as unsuitable for development. A 50-ft buffer was added to streams, 

lakes, ponds, and wetlands. Within this buffer, and in areas 

with soils mapped as bedrock outcrop, steep (greater then 

15 percent) slopes, or extremely rocky, land use was maintained as classified in the 1995 land cover. Water features and 

buffers surrounding these features restricted development in 

about 18 percent of the basin. Bedrock outcrops and associated steep slopes restricted development in about 4 percent of 

the basin. Areas classified as golf courses on the 1995 land-use 

map (fig. 2–9) were assumed to remain unchanged. 

Areas classified as developed on the 1995 land-use map 

were assumed to be available for redevelopment in the future, 

but were not subject to a change in land-use type. Thus, areas 

suitable for future development were limited to open space, 

forest, turf farms, and other agricultural lands if not otherwise 

restricted from development. The same land-use categories 

developed for the model HRUs (fig. 2–9) that combined many 

different land classifications were used to develop the potential 

buildout map (fig. 2–10). The largest change was from forest 

to low- and medium-density residential development, which 

affected about a third of the basin. Other changes in land use 

by HRU are summarized in table 2–2. Overall, about 

10 percent of the basin was classified as developed in 1995, 

but based on model results about 50 percent of the basin could
Effects of Potential Future Land Use and Water Use on Streamflow 59

be developed within the restrictions described above. Most of 

the development, both current and potential that was simulated 

in the model, is in low- to medium-density residential housing.

The change in land use differed widely among subbasins. 

Potential buildout was estimated to affect about 44 percent 

of the basin, or about a 34-percent increase in developed 

land use. The median developed area by subbasin increased 

from about 8 percent under 1995 land-use conditions to about 

49 percent under potential buildout conditions. Individual 

subbasin development ranged from about 0.2 to 50 percent 

under 1995 land-use conditions and about 2 to 86 percent 

under potential buildout conditions. Estimated effective 

impervious area increased from about 1.6 percent of the basin 

area under 1995 land-use conditions to about 5 percent under 

potential buildout conditions. The change in developed land 

use by subbasin ranged from zero to about 75 percent of the 

subbasin area, with a median change of about 40 percent.

Method for Estimating Future Water Use

Land-use change often affects the demand for water. To 

evaluate the effects of future water demands on streamflow 

in the basin under potential buildout conditions, water-use 

changes associated with land-use change were estimated. 

Water-use change varied by water-use category and the type 

of supply system.

Water-use rates were assigned to the original 

development land-use categories on the 1995 land-use map 

and to the projected buildout land use by subbasin. The 

original data layers were used in this analysis rather than the 

generalized land use represented in the model to preserve 

the detailed land-use information. For example, the original 

land-use data layer has five classes of residential development 

differentiated on the basis of density, whereas the model 

simplified these into two density classes. From the detailed 

Table 2–2. Area of Hydrologic Response Units (HRUs) represented for 1995 land use and potential buildout land use in the Hydrologic 

Simulation Program-FORTRAN of the Pawcatuck River Basin, southwestern Rhode Island and southeastern Connecticut.

[mi2, square mile; S&G, Sand and gravel. Values may be affected by rounding]

HRU Description

1995 land use Buildout land use Land-use change 1995 to buildout

Area 

(mi2)

Percentage 

of basin

Area 

(mi2)

Percentage 

of basin

Difference

(mi2)

Percent 

difference

Percent 

change over 

entire basin

1 Commercial, industrial, and 

transportation over S&G

0.61 0.20 1.10 0.37 0.50 82 0.17

2 Medium-high- to high-density 

residential over S&G

3.08 1.0 3.85 1.3 0.77 25 0.26

3 Low- to medium-low-density 

residential over S&G

8.09 2.7 41.1 14 33.0 408 11

4 Open space over S&G 15.7 5.2 5.41 1.8 -10.3 -65 -3.4

5 Forest over S&G 49.4 16 21.6 7.2 -27.7 -56 -9.2

6 Golf courses 1.81 0.61 1.81 0.61 0 0 0

7 Turf 4.40 1.5 1.51 0.50 -2.89 -66 -0.97

8 Other irrigated 1.04 0.35 0.13 0.04 -0.92 -88 -0.31

11 Commercial, industrial, and 

transportation over till

0.39 0.13 0.88 0.29 0.49 127 0.16

12 Medium-high- to high-density 

residential over till

2.51 0.84 4.10 1.4 1.59 63 0.53

13 Low- to medium-low-density 

residential over till

9.81 3.3 65.4 22 55.6 567 19

14 Open space over till 13.5 4.5 3.16 1.0 -10.3 -77 -3.4

15 Forest over till 107 36 56.3 19 -50.5 -47 -17

16 Steep slope or rocky soils 28.7 9.6 28.7 9.6 0 0 0

17 Water 5.18 1.7 5.18 1.7 0 0 0

18 Nonforested wetland 10.1 3.4 10.1 3.4 0 0 0

19 Forested wetland 33.7 11 33.7 11 0 0 0

20 Impervious residential 1.85 0.62 7.78 2.6 5.93 320 2.0

21 Impervious commercial 3.00 1.0 7.72 2.6 4.71 157 1.6
60 Part 2. Simulation of Water-Use and Land-Use Changes on Streamflow with a Precipitation-Runoff Model (HSPF)

4 1

7

5 2

31

6 3

8

9

10

11

32

35

33 37

14

18 15

20

19

13

34

36

42

41

47

44

46

45

43

58

55

54

52

56

53

57

61

60

62

64

78

63

70 48

59

49

50

65

68

67

69

66

51

73

74

75

76

77

81

82

83

79

84

71

72 80

17

16

71o37' 30" 71o 71 30' o 71 45' o

52'30"

42o

37'30"

41o

30'

41o

22'30"

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

EXETER

NORTH

KINGSTOWN

SOUTH

KINGSTOWN

VOLUNTOWN

NORTH

STONINGTON

STONINGTON

COVENTRY

STERLING

WEST GREENWICH

RICHMOND

CHARLESTOWN

WESTERLY

HOPKINTON

EAST

GREENWICH

50

EXPLANATION

Boundaries and reaches Land Use—Land Cover

Basin boundary

State boundary

Subbasin boundary

Streams 

Stream reach number

Industrial, commercial,

and transportation

Moderate- to high-density

residential

Low- to moderate-density

residential

Open space

Forest

Non-forested wetland

Forested wetland

Golf course

Turf farm

Other irrigated agriculture

Water

Town boundary Base from U.S. Geological Survey, 1995, 1:24,000

Rhode Island state plane projection, NAD83

GRISWOLD

PLAINFIELD

Figure 2–9. Representation of 1995 land use simulated in the Hydrologic Simulation Program-FORTRAN (HSPF) of the Pawcatuck 

River Basin, southwestern Rhode Island and southeastern Connecticut. (Sites described in table A2–4.)
Effects of Potential Future Land Use and Water Use on Streamflow 61

4 1

7

5 2

31

6 3

8

9

10

11

32

35

33 37

14

18 15

20

19

13

34

36

42

41

47

44

46

45

43

58

55

54

52

56

53

57

61

60

62

64

78

63

70 48

59

49

50

65

68

67

69

66

51

73

74

75

76

77

81

82

83

79

84

71

72 80

17

16

71o37' 30" 71o 71 30' o 71 45' o

52'30"

42o

37'30"

41o

30'

41o

22'30"

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

EXETER

NORTH

KINGSTOWN

SOUTH

KINGSTOWN

VOLUNTOWN

NORTH

STONINGTON

STONINGTON

COVENTRY

STERLING

WEST GREENWICH

RICHMOND

CHARLESTOWN

WESTERLY

HOPKINTON

EAST

GREENWICH

EXPLANATION

Note: development limited in buffers surrounding water and wetlands, and areas of bedrock, 

steep slope, or extremely rocky are not shown for clarity

Basin boundary

State boundary

Subbasin boundary

Stream reach number

Industrial, commercial,

and transportation

Moderate- to high-density

residential

Low- to moderate-density

residential

Open space

Forest

Non-forested wetland

Forested wetland

Golf course

Turf farm

Other irrigated agriculture

Water

Base from U.S. Geological Survey, 1995, 1:24,000 Town boundary

Rhode Island state plane projection, NAD83

50

Boundaries and reaches Land Use—Land Cover

Streams 

GRISWOLD

PLAINFIELD

Figure 2–10. Representation of buildout land use simulated in the Hydrologic Simulation Program-FORTRAN (HSPF) of the 

Pawcatuck River Basin, southwestern Rhode Island and southeastern Connecticut. (Sites described in table A2–4.)
62 Part 2. Simulation of Water-Use and Land-Use Changes on Streamflow with a Precipitation-Runoff Model (HSPF)

residential land-use classes, domestic water use was estimated 

by subbasin by multiplying the number of housing units in 

each category (table 2–3) by the median occupancy and a 

usage factor. A household occupancy of 2.5 people is about 

the median reported by the U.S. Census Bureau (2000) for the 

each of the major towns in the basin; the reported occupancy 

ranged from 2.4 to 2.8 people per unit. An average householdusage factor of 70 gal/d/person reported for the 1995–99 

period by Wild and Nimiroski (2004) was used for domestic 

water use in the basin; reported use rates ranged from 67 to 

71 gal/d/person for public and self-supply, respectively. Most 

of the domestic water use in the basin is self-supplied. 

The increase in the self-supply water use at buildout 

was represented in the model as a withdrawal from each reach 

through the External Source block. For buildout areas that are 

self-supplied and have on-site septic, which reflects buildout 

in most subbasins, withdrawals were calculated as 20 percent 

of the difference between the current water use (estimated 

from the 1995 land-use map) and water use at buildout. 

Twenty percent of the total net difference was assumed to 

represent consumptive water use or water that is lost from 

the basin, through practices such as lawn watering, and the 

remaining 80 percent was assumed to be returned within the 

basin through onsite septic systems. The net difference in selfsupply water in the calibrated model and buildout was used 

because the calibrated model incorporates the consumptive 

self-supply in the model-parameter values. 

In areas with municipal water-supply—lower Chipuxet 

River, Mink Brook, a small area around Richmond in the 

Wood River, and lower Pawtucket River around Westerly—the 

change in water use was calculated in one of two ways. For 

systems that service areas completely inside the Pawcatuck 

River Basin (Richmond and Westerly), the change in water use 

was estimated by using the buildout analysis described above. 

For the Richmond area, 20 percent of the net change between 

current and buildout water use (0.261 Mgal/d) was taken 

out of RCHRES 59 because this area is not served by public 

sewer so most of the buildout withdrawals were assumed to 

be returned through onsite septic disposal. If a public-sewer 

system was built, the treated wastewater effluent could be 

returned at a specified rate and RCHRES. For the Westerly 

area, the entire net change between current and buildout water 

use (1.61 Mgal/d) was assumed to be withdrawn from the 

existing production wells (from RCHRES 69 and 71) at a rate 

proportional to the current rate of withdrawals. Increased withdrawal from reach 80 was not considered because withdrawals 

from this reach were assumed to be at or near capacity. The 

entire net change in demand was assumed to discharge near 

the mouth of the Pawcatuck River Basin through the current 

public-sewer system. 

The second method was applied to estimate increased 

water use at buildout in the lower Chipuxet River and Mink 

Brook. Change in water-use from current and buildout 

land-use conditions was estimated by methods previously 

described; however, most of the withdrawals from these 

subbasins are for water demands outside of the basin. Future 

water-use projections for these external demands were 

obtained from estimated 2020 water-supply needs reported 

by the water suppliers; these projected demands were added 

to the change in withdrawals from within the subbasins. The 

reported 2020 water-supply needs may not reflect the full 

potential buildout demands, however. All additional withdrawals in these areas were assumed to leave the basin through 

public sewers that discharge wastewater out of the basin. 

The total domestic water use calculated from 1995 land 

use was estimated to be 5.19 Mgal/d; this is about 10 percent 

more than the total domestic water use (4.72 Mgal/d) reported 

by Wild and Nimiroski (2004), but the good agreement 

indicates that estimating potential water use on the basis of 

land use is a reasonable method. The domestic water use in 

the basin under buildout was estimated at 21.8 Mgal/d for 

the entire basin or about four times the domestic water use 

estimated from 1995 land use and 2000 census information 

(U.S. Census Bureau, 2000). 

Water use for commercial and industrial users was 

estimated on a per unit area basis by applying the 1.22 Mgal/d 

rate reported for 1995–99 by Wild and Nimiroski (2004) for 

this type of use over the 1,368 acres classified as commercial 

and industrial on the 1995 land-use map; this yielded a rate 

of about 8.9 × 10-4 Mgal/d/acre. Multiplying this rate per unit 

area by the area of commercial and industrial development at 

buildout (8,414 acres) yielded a rate of 7.50 Mgal/d over the 

entire basin. This result represents about a six-fold increase 

over the reported 1995–99 commercial and industrial water 

use. Withdrawals for commercial and industrial use were made 

from the appropriate reaches where this type of development 

could occur.

In the analyses of the potential withdrawals at buildout, 

water withdrawals for agricultural purposes were eliminated 

from the model because these areas were assumed to be developable. Irrigation demands for golf courses were maintained 

at the current level (2000–04 use) because these areas were 

assumed to remain unchanged in the future. 

Table 2–3. Domestic water-use rates for residential land-use 

categories used to estimate potential water use under buildout 

conditions in the Pawcatuck River Basin, southwestern Rhode 

Island and southeastern Connecticut.

[gal/acre, gallons per acre; >, greater than; <, less than]

Residential 

housing-density 

classification

Units per acre Domestic 

water use 

(gal/acre) Range

Applied to 

estimate 

water use

High >8 8 1,400

Medium to high 4 to 7.9 4 700

Medium 1 to 3.9 1 175

Medium to low 0.5 to 1 0.5 87.5

Low <0.5 0.25 43.8
Effects of Potential Future Land Use and Water Use on Streamflow 63

Effects on Streamflow

Buildout simulations were evaluated at four reaches—

upper Chipuxet River (CHIP1; RCHRES 31), lower Beaver 

River (BEAV3; RCHRES 43), lower Wood River (WOOD6; 

RCHRES 65) and lower Pawcatuck River (PAWC8; RCHRES 

80) (fig. 2–1). The upper Chipuxet River subbasin in the 

calibrated model includes minor withdrawals, but about 

54 percent of the subbasin is buildable (near the upper quartile 

of buildable area for all subbasins). Thus, potential changes 

in this subbasin reflect large changes in both land use and 

water use. The lower Beaver River reflects potentially large 

land-use changes in BEAV1, BEAV2, and BEAV3 (RCHRES 

41, 42, and 43), but the potential change in water use is less 

pronounced relative to CHIP1. Streamflow in the lower Wood 

River (RCHRES 51 through 66) reflects the overall effects 

of change in a large tributary that flows through one of the 

most pristine watersheds in southeast coastal New England. 

Simulation results for the lower Pawcatuck River reflect the 

overall change in streamflow over the entire basin (only the 

downstream-most subbasin, PAWC9, is not included at this 

model RCHRES).

In general, simulations under buildout indicate that high 

flows and low flows increase slightly as a result of land-use 

change relative to simulations under 1995 land-use conditions (fig. 2–11). Increased high flows are normally expected 

as a basin becomes more urbanized, but increased low flows 

are not. Increased low flows, although slight, result from 

decreased evapotranspiration (ET) losses that are reflected 

in the model parameter values for developed and undeveloped HRU types (see discussion of the model water budget 

in appendix 2 for more information). Changes in ET may, or 

may not, occur because most buildout change in the basin was 

from forested to low-density residential development, but the 

loss of deep-rooted vegetation that drives ET loss may not be 

appreciable enough as a result of this change to cause changes 

in low flows. In localized areas of intense urban development, 

changes in flow can be greater than those simulated at the subbasin scale. Increased effective impervious area can decrease 

low flows because of decreased infiltration and increased surface runoff. The simulated changes in flow could also differ if 

low-impact development (LID) strategies are employed. Thus, 

the extent to which streamflow changes in response to development depends on exactly how the land is developed; this can 

vary widely and produce effects different from those shown in 

figure 2–11, particularly in localized areas. In addition, there 

is no direct calibration of the model to specific land-use types 

and the exact nature of the development is unknown; combined, these factors underscore the uncertainty of hydrologic 

changes that result from land-use change.

In subbasins where agricultural lands become urbanized, the water used for agricultural irrigation can exceed 

the potential water use at buildout, resulting in higher low 

flows after development. The extent of this type of change 

depends on the extent of current irrigation water use and the 

potential and type of future development. In the Beaver River, 

annual irrigation use, largely concentrated in the lower part 

of the subbasin, averaged about 0.30 Mgal/d during 2000–04; 

however, irrigation is concentrated during the summer months 

when flows are typically low and the actual use can exceed 

the annual average by an order of magnitude or more. The 

change in water use associated with land-use change, largely 

to low-density residential, in the Beaver River subbasin averaged about 0.17 Mgal/d. As a result, low flows (above the 

90-percent flow duration) increased about 50 percent under the 

simulated buildout conditions (fig. 2–11B). 

Near the basin outlet of the Pawtucket River at Westerly 

(RCHRES 80), the hydrological effects of land-use change 

increased high flows at the 0.1-percent flow duration by about 

11 percent. Simulations of potential land-use change with 

2000–04 withdrawals (LT–BldCD) indicate that low flows 

(above the 90-percent flow duration) are similar to flows 

with no demands (fig. 2–11D). A possible explanation for 

this similarity is the decrease in evapotranspiration loss from 

forests to predominantly low-density residential development 

is about proportional to the water lost to withdrawals. 

Simulations of water-use change associated with buildout and 

keeping land use constant (LT–95LFD) are similar to flows 

under 2000–04 withdrawals, which reflects that most of the 

additional water use is returned to the basin and that overall 

change in water use is small relative to flow near the mouth of 

the Pawcatuck River.
64 Part 2. Simulation of Water-Use and Land-Use Changes on Streamflow with a Precipitation-Runoff Model (HSPF)

Figure 2–11. Flow-duration curves of simulated daily mean streamflow (1960–2004) under no withdrawals (LT–NoDmd), 

current withdrawals (LT–CDmd), and potential buildout conditions representing effects of change in land-use only 

(LT–BldCD), change in water-use only (LT–95LFD), and the combined effects of land-use and water-use change (LT–BldFD) at 

(A) Chipuxet River (CHIP1, RCHRES 31); (B) Beaver River (BEAV3, RCHRES 43); (C) Wood River (WOOD6, RCHRES 65); and 

(D) Pawcatuck River (PAWC8, RCHRES 80), Pawcatuck River Basin, southwestern Rhode Island and southeastern 

Connecticut. (Site locations shown in figure 2–1 and described in table A2–4.)

Beaver River (BEAV3, RCHRES 43)

10

100

1,000

10,000

Wood River (WOOD6, RCHRES 65)

0.5 1 2 5 10 20 30 50 70 80 90 95 98 99 99.8

Pawcatuck River (PAWC8, RCHRES 80)

0.5 1 2 5 10 20 30 50 70 80 90 95 98 99 99.8

1

10

100

DISCHARGE, IN CUBIC FEET PER SECOND

Chipuxet River (CHIP1, RCHRES 31)

1,000

10

100

1,000

10,000

1

10

100

1,000

A B

C D

LT-NoDmd

LT-CDmd

LT-BldCD

LT-95LFD

LT-BldFD

PERCENTAGE OF THE TIME DISCHARGE WAS EQUALED OR EXCEEDED

EXPLANATION