The groundwater-flow models of the Wood River and 

eastern Pawcatuck River subbasins (fig. I–1) were developed 

for this investigation by using MODFLOW (Harbaugh 

and others, 2000) to analyze the effects of groundwater 

withdrawals on streamflows, aquifer levels, and pond levels. 

Simulations were made to compare and contrast the effects 

of changing hydrologic stresses caused by groundwater 

withdrawals on rivers with low and high baseflows to 

determine the responses of these rivers and on the surrounding 

aquifer. These simulations also included analyses of the effects 

of constant and varying pumping and recharge conditions and 

of different well distances from rivers on streamflow.

Streamflows simulated in the MODFLOW models were 

average monthly flows, except during May to September 2002 

when they were average weekly flows. Recharge rates used in 

MODFLOW models were based on estimates determined from 

the HSPF modeling effort described in Part 2. Information on 

the development and calibration of the models as well as the 

limitations of this analysis are described in Appendix Part 3, 

“Development of Groundwater-Flow Models.” 

Effects of Pumping under Constant and Varying 

Recharge Conditions

Nearly all water that enters the lower Wood River model 

area (fig. 3–1) as recharge leaves the aquifer as groundwater 

discharge to streams. The amount of groundwater discharge 

to streams is controlled primarily by the difference between 

stream stage and water levels in the surrounding aquifer. As a 

result, seasonal changes in aquifer recharge result in changes 

in aquifer levels and therefore changes in the amount of 

groundwater that discharges to streams. During dry periods, 

the amount of groundwater discharge to the streams from an 

aquifer is much lower than during wet periods, when aquifer levels are generally higher relative to stream stage. An 

example is Meadow Brook (fig. 3–1), which went dry during 

summer 2002, an extremely dry period. (fig. 3–2).

Groundwater discharge to streams also can be affected 

by pumping. A pumping well can alter groundwater discharge 

to downgradient streams (fig. 3–3A) by capturing water that 

would otherwise discharge to streams (fig. 3–3B); and if the 

pumping rate is great enough, by reversing the flow direction 

so that the stream contributes water directly to the pumping 

well (fig. 3–3C). Two proposed municipal supply sites near 

rivers in the lower Wood River model area (RIW–481A and 

RIW–550) (fig. 3–1) were used in this analysis to compare and 

contrast the effects of pumping on baseflows and water levels 

in the aquifer under conditions of constant (steady-state) and 

varying (transient) recharge. These sites were selected to illustrate the contrasting effects of pumping on the stream-aquifer 

system when streams flow and when they go dry. Modelcalculated baseflows and water levels are shown as differences 

from their respective values under nonpumping conditions. 

Simulated pumping of 1.0 Mgal/d (1.5 ft3/s) from proposed site RIW–481A, about 200 ft from Meadow Brook, 

resulted in a reduction of baseflow of 1.4 ft3/s at the HSPF 

subbasin MEAD2 outlet (RCHRES 48) (sites described in 

table A2–4) under steady-state conditions (stress period 1 of 

model simulations shown as “SS” on figures 3–2 and 3–4 

through 3–9). About 90 percent of the pumping demand 

(1.4 ft3/s) is satisfied by captured baseflow (fig. 3–3B) and 

induced infiltration from the stream (fig. 3–3C) between the 

pumping well and MEAD2. The remaining 10 percent of 

pumped water was derived from captured baseflow captured 

farther downstream of MEAD2. 

In addition to a reduction in baseflow, pumping from 

RIW–481A also affected water levels in the surrounding 

aquifer. During the steady-state condition when both pumping and recharge rates were constant, the resulting simulated 

drawdown was 1.2 ft about 400 ft west of the pumping well at 

observation well MW–9 (figs. 3–1, 3–5). 

The steady-state simulations are based on the assumption that pumping and recharge rates remain constant over 

time; however, in reality, recharge rates change in response 

to changes in precipitation and evapotranspiration. Therefore, 

simulations of the effects of constant pumping with timevarying recharge were made to determine the response of this 

stream-aquifer system under intermittent-flow conditions in 

Meadow Brook. The simulation period of this analysis was 

from January 1, 2000, to September 30, 2004, as discussed in 

Part 2, “Simulation of Water-Use and Land-Use Changes on 

Streamflow with a Precipitation-Runoff Model (HSPF).”

Simulation results showed that as baseflow varied in 

response to the changes in recharge, the amount of baseflow 

reduction from pumping also varied. During wet periods 

when simulated baseflow in Meadow Brook was at or above 

average, the amount of baseflow reduction was similar to the 

Part 3. Simulated Effects of Withdrawals on Groundwater Flow 

(MODFLOW Models)

By John P. Masterson
66 Part 3. Simulated Effects of Withdrawals on Groundwater Flow (MODFLOW Models)

Pawcatuck River

Wood River

Meadow Brook

Diamond Brook

Wood River

Pawcatuck River

Pawcatuck River

Wood River

Meadow Brook

Diamond Brook

Wood River

Pawcatuck River

0

1 KILOMETER

1 MILE

0

Base from U.S. Geological Survey digital data, 1995, 1:24,000

1983 North American datum of 1983

Rhode Island state plane projection 

RIW–458B

RIW–458A

RIW–458

RIW–550

RIW–481A

MW–49

RIW–785

RIW–458

RIW–458A

RIW–550

RIW–481A

MW–49

RIW–785

Diamond Bog

WOOD5

WOOD6

PAWC4

MEAD2

Chariho

71°44' 71°42'

41°28'

41°26'

EXPLANATION

Surficial geologic deposits

Simulated model-boundary conditions

Till deposits

Sand and gravel deposits

Pond

Wetland

Specified flow

Specified flux

Inactive area of model

Head-dependent flux

Observation well and identifier

Proposed pumping well 

 and identifier

HSPF subbasin outlet

RIW–550

RIW–785

WOOD6

Figure 3–1. Geographic extent, surficial geology, observation wells, proposed withdrawal sites, simulated model 

boundary conditions, and outflow points from HSPF subbasins for the lower Wood River model in the Pawcatuck River 

Basin, southwestern Rhode Island.
Effects of Pumping under Constant and Varying Recharge Conditions 67

0

5

10

15

20

25

30

35

MODEL-CALCULATED BASEFLOW, IN CUBIC FEET PER SECOND

Flow at MEAD2 under current (no

pumping) conditions 

Flow at MEAD2 with well RIW-481A pumping 

at a rate of 1.0 million gallons per day

2000 2001 2002 2003 2004

SS J M M J S N J M M J S N J M M J S N J M M J S N J M M J S 

Figure 3–2. Effects of simulated pumping at well RIW–481A on baseflow in Meadow Brook in the lower Wood River 

model area (MEAD2, RCHRES 48) in the Pawcatuck River Basin, southwestern Rhode Island. Months on x-axis are 

January, March, May, July, September, and November for years shown. Steady state (SS) represents average steadystate conditions for simulation period. (Site locations shown in figure 3–1.)
68 Part 3. Simulated Effects of Withdrawals on Groundwater Flow (MODFLOW Models)

baseflow reduction predicted by the steady-state simulation in 

which pumping and recharge rates were constant (about 

1.4 ft3/s, fig. 3–4). During the summer and early fall, however, 

when simulated baseflow was at or near zero (fig. 3–2), the 

reduction in baseflow from pumping became progressively 

less than the 1.4 ft3/s reduction determined for steady-state 

conditions. The reason for this decrease in baseflow reduction 

is that, as the stream went dry, it could no longer be a source 

of water to the pumped well. When the pumped well could no 

longer satisfy its demand by depleting streamflow, the demand 

was met from another source, which is usually aquifer storage.

The shift in the source of water from streamflow to 

aquifer storage results in much greater drawdowns in the 

nearby aquifer than if there had been water in the stream. This 

effect is illustrated in the hydrograph for observation well 

MW–9 (fig. 3–5) where drawdowns greatly increased during 

dry periods compared to the drawdowns observed during the 

steady-state simulation. During the dry period of late summer 

and early fall 2002, the calculated drawdown at MW–9 was 

about 2.5 times larger than that calculated in the steady-state 

simulation (fig. 3–5). 

During the first few months that followed extended 

dry periods, such as the period of November 2000 through 

February 2001 (fig. 3–4), the simulated reduction in baseflow 

was about 2.1 ft3/s, about 50 percent greater than the 1.4 ft3/s 

reduction calculated for steady-state conditions. This increase 

was the result of aquifer recharge replenishing depleted aquifer storage rather than contributing groundwater discharge to 

the stream. It was not until early spring 2001 that water levels 

recovered; baseflow reduction decreased to about 1.4 ft3/s, and 

the calculated drawdown at MW–9 decreased to about 1.2 ft 

(fig. 3–5), which are similar to the amounts calculated for the 

steady-state condition. This pattern of changes in water levels 

and baseflow reduction rates was repeated during the successive dry and wet periods throughout the simulation period. 

A comparison of drawdowns calculated for observation 

well MW–9 near the proposed RIW–481A well (about 

400 ft away) and that of the Chariho observation well 

(fig. 3–5), about 2,500 ft from the RIW–481A well, was 

made to determine the effect of distance from the pumping 

well on changes in aquifer response. This analysis showed 

that changes in water levels became increasingly delayed and 

dampened with increasing distance from the pumping well. 

The model-calculated time lag between changes in water 

levels for the two observation wells appeared to be about 

two months. The largest drawdowns at these wells in 2002 

occurred at observation well MW–9 in September 2002 and 

at the Chariho observation well in November 2002 (fig. 3–5). 

These results also indicate that, because of the distance of the 

Chariho observation well from the pumping well, the water 

level at the Chariho observation well did not fully recover 

from the effects of pumping during summer 2001 before the 

effects of pumping in summer 2002 became evident (fig. 3–5).

The time-lag effect of pumping observed in the analysis 

of changing water levels at two observation wells is directly 

related to distance from the pumping well; as distance 

from the pumping well increases, the lag effect of pumping 

increases. This lag effect is the basis of conjunctive-management modeling, which optimizes pumping rate and well 

location to minimize the effects of pumping on streamflow. 

Conjunctive-management simulations are discussed in 

Part 4, “Conjunctive-Management Models as Tools for 

Water-Resources Planning.”

An assessment of changes in baseflow reduction also was 

made for the Wood River at HSPF subbasin WOOD5 outlet 

(RCHRES 63) where baseflow was never lower than 16.0 ft3/s 

during the simulation period (fig. 3–6). For the steady-state 

simulation, the amount of baseflow reduction was equal to the 

1.5 ft3/s (1.0 Mgal/d) of water pumped from RIW–550 about 

500 ft from the Wood River (fig. 3–1). During the time-varying recharge simulations, this baseflow reduction ranged from 

about 1.3 to 2.1 ft3/s (fig. 3–7), similar to the rates calculated 

Schematic diagram, not to scale

Unconfined aquifer

Unconfined aquifer

Q1

Q2

Recharge area

Land surface

Water table

C

B

A

Land surface

Land surface

Surface-water

body

Surface-water

body

Surface-water

body

Unconfined aquifer

Water table

Water table

Bedrock

Bedrock

Bedrock

Recharge area

Recharge area

Figure 3–3. Groundwater discharge from a hypothetical aquifer 

to a stream with (A) no pumping; (B) pumping at a such a rate (Q1) 

that the well captures some water that would otherwise discharge 

to the surface-water body; and (C) pumping at a higher rate (Q2) 

that results in the reversal of groundwater flow direction and in 

the flow of water out of the surface-water body into the aquifer 

(induced infiltration).
Effects of Pumping under Constant and Varying Recharge Conditions 69

0

1

2

3

4

5

6

7

8

9

10

RECHARGE, IN INCHES PER MONTH

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0

CHANGE IN MODEL-CALCULATED BASEFLOW, IN CUBIC FEET PER SECOND

2000

J M M J S N J M M J S N J M M J S N J M M J S N J M M J S 

2001 2002 2003 2004

SS

Change in baseflow at 

Meadow Brook 

Monthly recharge

Weekly recharge

Less water available

Less water available

Less water available

Figure 3–4. Change in baseflow in the Meadow Brook at HSPF subbasin MEAD2 outlet (RCHRES 

48) in response to simulated pumping at well RIW–481A in the lower Wood River model area in the 

Pawcatuck River Basin, southwestern Rhode Island. Months on x-axis are January, March, May, 

July, September, and November for years shown. Steady state (SS) represents average steady-state 

conditions for simulation period. (Site locations shown in figure 3–1.)

2000 2001 2002 2003 2004

SSJ M M J S N J M M J S N J M M J S N J M M J S N J M M J S 

Monthly recharge

Weekly recharge

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0

0

1

2

3

4

5

6

7

8

9

10

CHANGE IN WATER LEVEL, IN FEET

Change in water level at MW–9

Change in water level at Chariho

RECHARGE, IN INCHES PER MONTH

Drawdown (taken from storage)

Offsets in drawdowns

Drawdown

(taken from storage)

Drawdown (taken from storage)

Figure 3–5. Comparison of the changes in water levels at observation wells MW–9 and Chariho 

in response to simulated pumping at well RIW–481A in the lower Wood River model area in the 

Pawcatuck River Basin, southwestern Rhode Island. Months on x-axis are January, March, May, 

July, September, and November for years shown. Steady state (SS) represents average steady-state 

conditions for simulation period. (Site locations shown in figure 3–1.)
70 Part 3. Simulated Effects of Withdrawals on Groundwater Flow (MODFLOW Models)

for MEAD2 in Meadow Brook (fig. 3–4). Unlike Meadow 

Brook, however, the Wood River did not go dry during the 

simulation period, and therefore the amount of baseflow 

reduction did not vary with changes in aquifer recharge 

(Barlow and Dickerman, 2001). 

The simulated response of the Wood River at WOOD5 to 

pumping under varying recharge conditions can be attributed 

to seasonal changes in baseflow at Diamond Brook, a small 

tributary to the Wood River between RIW–550 and Diamond 

Bog (fig. 3–1). This tributary discharges to the Wood River 

upstream of WOOD5 and, for current average annual conditions, contributes about 0.5 ft3

/s of flow to the Wood River 

(fig. 3–8). The times during which the reduction in baseflow 

at WOOD5 was at a minimum of 1.3 ft3/s coincided with 

the times during which Diamond Brook was dry under the 

pumped condition (the red line on fig. 3–8) and therefore 

unable to contribute water to pumping well RIW–550 

(fig. 3–8). As a result, the reduction in baseflow calculated for 

the Wood River downstream of the confluence with Diamond 

Brook was derived only from the main branch of the Wood 

River and aquifer storage rather than from Diamond Brook. 

Changes in water levels beneath Diamond Brook in 

response to simulated pumping at RIW–550 under varying 

recharge rates can potentially affect the hydroperiod in nearby 

vernal pools such as Diamond Bog, one of the State’s most 

ecologically sensitive critical habitats (Colin Apse, The Nature 

Conservancy, written commun., 2005) (figs. 3–1 and 3–9). 

The drawdown calculated for this area was about 0.2 ft under 

steady-state conditions (shown as “ss” on fig. 3–9). Water levels in vernal pools are ecologically important during the spring 

0

50

100

150

200

250

300

350

400

450

500

550

600

2000 2001 2002 2003 2004

J M M J S N J M M J S N J M M J S N J M M J S N J M M J S 

MODEL-CALCULATED BASEFLOW, IN CUBIC FEET PER SECOND

Flow at WOOD5 under current (no pumping) conditions

Flow at WOOD5 with well RIW–550 pumping at 

a rate of 1.0 million gallons per day

SS

Figure 3–6. Effects of simulated pumping at well RIW–550 on baseflow in the Wood River at HSPF subbasin WOOD5 

outlet (RCHRES 63) in the lower Wood River model area in the Pawcatuck River Basin, southwestern Rhode Island. 

Months on x-axis are January, March, May, July, September, and November for years shown. Steady state (SS) 

represents average steady-state conditions for simulation period. (Site locations shown in figure 3–1.)
Effects of Pumping under Constant and Varying Recharge Conditions 71

season, however. The drawdown associated with pumping at 

RIW–550 for March 2000 was assumed to be representative of 

average spring conditions, and at 0.2 ft, was nearly the same 

as that calculated for the steady-state condition. In March 

2002, representative of dry spring conditions. the drawdown 

increased to 0.7 ft (fig. 3–9). Thus, a 14 percent decrease in 

recharge between March 2000 and March 2002 resulted in 

a more than threefold decline in water level in the bog; this 

comparison illustrates the sensitivity of the water level in the 

bog to changes in recharge.

During March of 2000, the baseflow of 1.0 ft3/s at the 

mouth of Diamond Brook was reduced to 0.4 ft3/s in response 

to the proposed pumping at well RIW–550. In contrast, under 

the same pumping conditions, the baseflow in Diamond 

Brook decreased from 0.3 to 0.0 ft3/s in March 2002 

(fig. 3–8). Because the reduction of 0.3 ft3/s is only half of 

the total reduction calculated for March 2000 and because 

the brook went dry, the remainder of the pumped water was 

derived from aquifer storage and increased the drawdown 

calculated for Diamond Bog during March 2002 (fig. 3–9). 

A map-view comparison of the drawdown calculated for 

these two times illustrates the influence of Diamond Brook 

on the extent of drawdown associated with the simulated 

pumping at well RIW–550 (figs. 3–10A,B). In March 2000, 

when there was flow in Diamond Brook, drawdown contours 

terminated (were zero) at the brook (fig. 3–10A). In March 

2002, when Diamond Brook was dry, the drawdown contours 

extended beyond the brook; this result indicated that the brook 

was no longer contributing flow to the pumping well and that 

aquifer storage was being depleted (fig. 3–10B). 

The depletion of aquifer storage was much greater during the months with little or no summer recharge because of 

the effects of streams going dry. This aquifer response was 

recorded in the simulated change in water levels at observation wells MW–9 and Chariho (fig. 3–5), and at Diamond Bog 

(fig. 3–9). The greatest reductions in baseflow at Meadow 

Brook and Wood River occurred after the greatest drawdowns 

in the aquifer because the increase in aquifer recharge initially 

replenished the depleted aquifer storage before contributing 

groundwater discharge to streams. 

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0

0

1

2

3

4

5

6

7

8

9

10

CHANGE IN MODEL-CALCULATED BASEFLOW, IN 

CUBIC FEET PER SECOND

Change in baseflow at Wood River

RECHARGE, IN INCHES PER MON

T

H

2000 2001 2002 2003 2004

SS J M M J S N J M M J S N J M M J S N J M M J S N J M M J S 

Monthly recharge

Weekly recharge

Figure 3–7. Change in baseflow in the Wood River at HSPF subbasin WOOD5 outlet (RCHRES 63) in response to 

simulated pumping at well RIW–550 in the lower Wood River model area in the Pawcatuck River Basin, southwestern 

Rhode Island. Months on x-axis are January, March, May, July, September, and November for years shown. Steady 

state (SS) represents average steady-state conditions for simulation period. (Site locations shown in figure 3–1.)
72 Part 3. Simulated Effects of Withdrawals on Groundwater Flow (MODFLOW Models)

Once the aquifer storage was replenished and water levels 

rose, the simulated reduction in baseflows were comparable 

to the streamflows under steady-state rates–about 1.5 ft3/s for 

both the Meadow Brook and the Wood River simulations. 

Nearly all of the pumped water from wells RIW–481A and 

RIW–550 was derived from baseflow reductions when water 

was available in the streams. During extended dry periods, 

however, the magnitudes of baseflow reductions can differ 

depending on whether the streams or their tributaries go dry.

Effects of Varying Pumping-Well Distance with 

Constant Pumping Rates

The effects of pumping were simulated for several 

alternative well locations on streamflow at the HSPF model 

subbasin WOOD6 and MEAD2 outlets (RCHRES 65 and 48, 

respectively) in the Wood River and Meadow Brook (fig. 3–1). 

The proposed well location for RIW–458 was changed in two 

simulations to two additional sites (Wells RIW–458A and 

RIW–458B) (fig. 3–1) to determine the effects of pumpingwell location on baseflow. 

Simulated pumping from well site of RIW–458, about 

1,800 ft from the Wood River and 3,500 ft from Meadow 

Brook, resulted in about 68 percent of the simulated 

1.0 Mgal/d of pumping being derived from a reduction of 

groundwater flow to the Wood River, the remaining 32 percent 

was from a reduction of groundwater flow to Meadow Brook 

(fig. 3–11). In the case of the Wood River, a reduction in baseflow of about 1.05 ft3

/s [68 percent of 1.55 ft3/s (1.0 Mgal/d) 

of pumping] represents a less than 1-percent decrease in the 

total average streamflow of 180 ft3/s. In contrast, a reduction 

in baseflow of about 0.5 ft3/s [32 percent of 1.55 ft3/s 

(1.0 Mgal/d) of pumping] in the Meadow Brook represents 

about a 7-percent decrease in the total average streamflow of 

7 ft3/s. 

As the pumping well is moved 500 and 1,600 ft closer to 

the Wood River (Wells RIW–458A and RIW–458B, respectively), the effect of pumping on baseflow changes such that 

the amount of pumping derived from captured baseflow in the 

Figure 3–8. Comparison of the changes in baseflows in Diamond Brook and the Wood River at HSPF subbasin WOOD5 outlet 

(RCHRES 63) in response to simulated pumping at well RIW–550 in the lower Wood River model area in the Pawcatuck River 

Basin, southwestern Rhode Island. Months on x-axis are January, March, May, July, September, and November for years 

shown. Steady state (SS) represents average steady-state conditions for simulation period. (Site locations shown in 

figure 3–1.)

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

CHANGE IN MODEL-CALCULATED BASEFLOW, IN

CUBIC FEET PER SECOND

MODEL-CALCULATED BASEFLOW, IN CUBIC

FEET PER SECOND

2000 2001 2002 2003 2004

J M M J S N J M M J S N J M M J S N J M M J S N J M M J S 

 Current (no pumping) model-calculated baseflow at Diamond Brook

Change in model-calculated baseflow at Wood River (RCHRES 63)

Model-calculated baseflow at Diamond Brook with well 

RIW-550 pumping at a rate of 1.0 million gallons per day

SS
Effects of Varying Pumping-Well Distance with Varying Pumping Rates 73

Figure 3–9. Change in water levels at Diamond Bog in response to simulated pumping at well RIW–550 in the lower 

Wood River model area in the Pawcatuck River Basin, southwestern Rhode Island. Months on x-axis are January, March, 

May, July, September, and November for years shown. Steady state (SS) represents average steady-state conditions for 

simulation period. (Site locations shown in figure 3–1.)

Wood River increases from 68 percent at the RIW–458 site to 

77 and 88 percent from alternate sites Wells RIW–458A and 

RIW–458B, respectively (fig. 3–11). With the increase in captured baseflow from the Wood River, the amount of captured 

baseflow from Meadow Brook is reduced from about 32 to 

23 percent to about 12 percent of the total amount of captured 

baseflow (fig. 3–11); the pumping well is capturing successively less water that otherwise would have discharged to the 

Meadow Brook as the well is moved closer to the Wood River. 

These simulations indicate that the effect of pumping 

on baseflow depends on well location and that the effect 

can be managed so streams with higher average flow like 

the Wood River are affected by pumping more than streams 

with lower average flow like Meadow Brook. In the case of 

the RIW–458 simulations, as the well location was moved 

closer to the Wood River, the baseflow depletion in the 

Wood River remained less than 1 percent of the total average 

streamflow while the baseflow depletion in Meadow Brook 

diminished from about 7 percent to about 2 percent of the total 

streamflow. 

Effects of Varying Pumping-Well Distance with 

Varying Pumping Rates

The previous examples have focused on the effects of 

constant pumping rates for municipal supply; however, a vital 

water use in the Pawcatuck River Basin is that of irrigation 

for turf farms where pumping rates are highly variable and 

depend upon ambient weather conditions. Currently, most of 

the irrigation water used for turf farms in the modeled areas is 

obtained from withdrawal directly from surface-water bodies. 

To reduce the effects on surface-water bodies during low-flow 

periods, which generally coincide with the largest withdrawals 

for irrigation, the NRCS is considering the potential benefits 

of utilizing the inherent lag and dampening effect of 

groundwater withdrawals on surface waters by replacing 

instream withdrawals with a groundwater well some distance 

away from surface waters. 

The groundwater models developed for this investigation 

were based on the understanding that groundwater models 

have certain limitations regarding the simulation of highly 

-2.0

-1.5

-1.0

-0.5

0

0

1

2

3

4

5

6

7

8

9

10

 RECHARGE, IN INCHES PER MONTH

CHANGE IN WATER LEVEL, IN FEET

Change in water level at Diamond Bog

2000 2001 2002 2003 2004

SS J M M J S N J M M J S N J M M J S N J M M J S N J M M J S 

Monthly recharge

Weekly recharge
74 Part 3. Simulated Effects of Withdrawals on Groundwater Flow (MODFLOW Models)

Diamond Bog

Diamond Brook

Wood River

RIW-550

A

March 2000

March 2002

B

1

2

0.2

0.7

3

4 5

6–15

Diamond Bog

Diamond Brook

Wood River

RIW-550

6–15

1

2

3

4

5

EXPLANATION

Surface-water body

Diamond Bog area

Contour of change (drawdown) 

in water level–interval is 1 foot

1

71°43' 71°42'

41°28'30”

41°29'30”

41°28'30”

41°29'30”

0

0.5 KILOMETER

0.5 MILE

0

0.25

0.25

Figure 3–10. Changes in water levels 

at Diamond Bog in (A) March 2000 and 

(B) March 2002 in response to simulated 

pumping at well RIW–550 in the lower Wood 

River model area in the Pawcatuck River 

Basin, southwestern Rhode Island.
Effects of Varying Pumping-Well Distance with Varying Pumping Rates 75

0 200 400 600 800

Wood River

Meadow Brook

1,000 1,200 1,400 1,600 1,800

0

10

20

30

40

50

60

70

80

90

100

CONTRIBUTION OF MODEL-CALCULATED 

BASEFLOW, IN PERCENT

RIW–458

RIW–458ARIW–458B

DISTANCE FROM ORIGINAL WELL SITE, IN FEET

Figure 3–11. Effects of simulated pumping at different well locations RIW–458, RIW–458A, and RIW–458B on 

baseflow in the Wood River at HSPF subbasin WOOD5 outlet (RCHRES 63) and the Meadow Brook at HSPF subbasin 

MEAD2 outlet (RCHRES 48) in the lower Wood River model area in the Pawcatuck River Basin, southwestern Rhode 

Island. (Site locations shown in figure 3–1.)
76 Part 3. Simulated Effects of Withdrawals on Groundwater Flow (MODFLOW Models)

variable short-term withdrawals that are typical of actual 

irrigation-water demands. Because of the computational 

limitations on the number of stress periods that would 

be required to represent extremely short time periods, 

hourly and daily pumping rates were lumped into weekly 

and monthly averages, resulting in an underestimate of 

the short-term hourly or daily effects on the rivers from 

irrigation withdrawals. As a result, the effects on baseflow of 

highly variable withdrawals made directly from, or in close 

proximity to, the stream are not well represented. When 

highly variable irrigation withdrawals are moved from the 

stream, however, the response of the stream to this pumping is 

dampened sufficiently because of the time lag resulting from 

the increased distance from the stream. As a result, the time 

discretization of a groundwater model can adequately simulate 

the effects of groundwater withdrawals on baseflow. 

One area for which the relocation of instream 

withdrawals is under consideration is the lower Beaver and 

Usquepaug Rivers in the eastern Pawcatuck River subbasin 

(fig. 3–12). Currently, there are six surface-water withdrawal 

points along these rivers (five in the Beaver River and one in 

the Usquepaug River). The NRCS proposed three scenarios 

in which these surface-water withdrawals are moved to six 

well locations (scenario A), two well locations (two groups 

of three, scenario B), and one centralized location between 

the two rivers (scenario C) (fig. 3–13A–C). The effects of 

pumping changes on streamflow in the Beaver and Usquepaug 

Rivers at the HSPF subbasin BEAV3 and QUEN7 outlets 

(RCHRES 43 and 20, respectively) (fig. 3–12) were assessed 

in these three scenarios. 

Results for simulations of the three aforementioned 

scenarios indicate that relocating the irrigation withdrawals, 

which are currently in the rivers, to wells away from the rivers 

had little effect on baseflow for steady-state conditions (shown 

as “SS” on figs. 3–14A–C, 3–15A–C), but had a large effect 

on baseflow under varying pumping and recharge conditions. 

These results indicate that baseflows increased relative to 

current baseflows during the summer months and decreased 

during the fall through early spring months as the distance 

between the pumping wells and both rivers was increased 

(figs. 3–14B, 3–15C). The lag in the response of the streams 

to the pumping stress delayed the effects of summer irrigation 

pumping on baseflow until later in the fall. This pattern was 

repeated at BEAV3 and QUEN7 (figs. 3–14B,C, 3–15B,C) 

throughout the study period.

Although the absolute changes in baseflow in the Beaver 

and Usquepaug Rivers were similar (for example, about 

1.3 ft3/s for scenario B during summer 2002) (figs. 3–14B, 

3–15B), the changes as percentages of the total flow were 

different because of the large differences in simulated 

baseflows between the two rivers (figs. 3–14C, 3–15C). In 

the Beaver River, the simulated baseflow in August 2002 was 

about 3.0 ft3/s (fig. 3–14A), whereas the simulated baseflow 

in the Usquepaug River for the same time was about 14.0 ft3/s 

(fig. 3–15A). Therefore, a change in flow of about +1.3 ft3/s 

in the Beaver River represents about a 43 percent increase in 

baseflow for that month, whereas a +1.3 ft3/s change in flow in 

the Usquepaug River represented only a 9 percent increase in 

baseflow (figs. 3–14C, 3–15C). Conversely, as the effects of 

pumping reached the rivers in September and the streamflow 

depletion was the greatest (about 0.7 ft3/s in each river), the 

percent changes in flow were small (about -8 percent in the 

Beaver and almost -3 percent in the Usquepaug) (figs. 3–14C, 

3–15C) because of the increase in baseflow in these rivers in 

response to increased aquifer recharge. 

These results indicate that relocating instream irrigation 

withdrawals to groundwater withdrawals away from the rivers resulted in gains in baseflow relative to conditions during 

summer low-flow periods and losses in baseflow relative to 

conditions in the early fall when baseflow is typically higher. 

The benefit of this shift in the effects of pumping on baseflow 

from the summer to the fall is that, by the fall, streamflows are 

higher from increased recharge; therefore, the baseflow depletion represents a smaller percentage of the total streamflow 

than in the summer.
Effects of Varying Pumping-Well Distance with Varying Pumping Rates 77

71°38' 71°32'

41°28'

41°36'

EXPLANATION

Surficial geologic deposits

Simulated boundary conditions

Till deposits

Sand and gravel deposits

Moraine deposits

Pond

Wetland

Specified flow

Inactive area of model

Head dependent flux

Constant head boundary

Observation well and identifier,

 not all wells numbered

HSPF subbasin outlet

 and identifier

BEAV3

Chipuxet River

Beaver River

Usquepaug River

Queen River

BEAV3

QUEN7

Worden Pond

SNW–006

EXW–554

RIW–417

Pawcatuck River

Chipuxet River

Beaver River

Usquepaug River

Queen River

Pasquiset Brook

BEAV3

QUEN7

Worden Pond

SNW–006

EXW–554

RIW–417

Pawcatuck River

SNW–006

0

0 1

1 2 MILES

2 KILOMETERS

Base from U.S. Geological Survey digital data, 1995, 1:24,000

North American datum of 1983

Rhode Island state plane projection

Figure 3–12. Model extent and boundary conditions used in the eastern Pawcatuck River model area in the 

Pawcatuck River Basin, southwestern Rhode Island.
78 Part 3. Simulated Effects of Withdrawals on Groundwater Flow (MODFLOW Models)

Figure 3–13. Locations of potential groundwater irrigation wells between the Beaver and Usquepaug Rivers in the eastern Pawcatuck River model area, HSPF subbasin outlets BEAV3 (RCHRES 43) and QUEN7 (RCHRES 20), in the Pawcatuck River Basin, southwestern Rhode Island for (A) six irrigation wells; (B) two irrigation wells; and (C) one irrigation well.

71°38 71°36'

41°30'41°28'

71°38 71°36' 71°38 71°36'

EXPLANATION

Proposed irrigation well site and identifierInstream withdrawal

Simulated stream 

Pawcatuck River

Beaver River

Usquepaug River

Pawcatuck River

Beaver River

Usquepaug River

Pawcatuck River

Beaver River

Usquepaug River

MODFLOW Model Scenario A: six wells MODFLOW Model Scenario B: two wells MODFLOW Model Scenario C: one well

HSPF subbasin outletPonds and wetlands

QUEN7

QUEN7

BEAV3

QUEN7

BEAV3

QUEN7

BEAV3

0

1 KILOMETER

1 MILE

0

A

B

C
Effects of Varying Pumping-Well Distance with Varying Pumping Rates 79

0

10

20

30

40

50

Instream withdrawals

A: six wells

Scenario 

B: two wells

C: one well

2000 2001 2002 2003 2004

J M M J S N J M M J S N J M M J S N J M M J S N J M M J S 

MODEL-CALCULATED BASEFLOW, IN CUBIC FEET PER SECOND

A

SS

EXPLANATION

Figure 3–14. Model-calculated baseflow (A), changes in model-calculated baseflow (B), and percent change in modelcalculated baseflow (C), in three proposed groundwater irrigation withdrawal scenarios on baseflow in the Beaver River 

at HSPF subbasin BEAV3 outlet (RCHRES 43) in the eastern Pawcatuck River model area in the Pawcatuck River Basin, 

southwestern Rhode Island. Months on x-axis are January, March, May, July, September, and November for years shown. 

Steady state (SS) represents average steady-state conditions for simulation period. (Site locations shown in figure 3–13.)
80 Part 3. Simulated Effects of Withdrawals on Groundwater Flow (MODFLOW Models)

2000 2001 2002 2003 2004

J M M J S N J M M J S N J M M J S N J M M J S N J M M J S 

CHANGE IN MODEL-CALCULATED BASEFLOW, IN CUBIC FEET PER SECOND

A: six wells

Scenario 

EXPLANATION

B: two wells

C: one well

B

SS

2.0

0.5

1.0

1.5

-0.5

-1.0

0.0

Figure 3–14. —Continued
Effects of Varying Pumping-Well Distance with Varying Pumping Rates 81

-10

0

10

20

30

40

50

2000 2001 2002 2003 2004

J M M J S N J M M J S N J M M J S N J M M J S N J M M J S 

CHANGE IN MODEL-CALCULATED BASEFLOW, IN PERCENT

C

SS

A: six wells

Scenario 

EXPLANATION

B: two wells

C: one well

Figure 3–14. —Continued
82 Part 3. Simulated Effects of Withdrawals on Groundwater Flow (MODFLOW Models)

Figure 3–15. Model-calculated baseflow (A), changes in model-calculated baseflow (B), and percent change 

in model-calculated baseflow in three proposed groundwater irrigation withdrawal scenarios on baseflow (C), in 

the Beaver River at HSPF subbasin QUEN7 outlet (RCHRES 20) in the eastern Pawcatuck River model area in the 

Pawcatuck River Basin, southwestern Rhode Island. Months on x-axis are January, March, May, July, September, and 

November for years shown. Steady state (SS) represents average steady-state conditions for simulation period. 

(Site locations shown in figure 3–13.)

2000 2001 2002 2003 2004

J M M J S N J M M J S N J M M J S N J M M J S N J M M J S 

MODEL-CALCULATED BASEFLOW, IN CUBIC FEET PER SECOND

Instream withdrawals

A: six wells

Scenario 

B: two wells

C: one well

A

SS

120

110

100

90

80

70

60

50

40

30

20

0

10

EXPLANATION
Effects of Varying Pumping-Well Distance with Varying Pumping Rates 83

-1.0

-0.5

0.5

0.0

1.0

1.5

2.0

2000 2001 2002 2003 2004

J M M J S N J M M J S N J M M J S N J M M J S N J M M J S 

CHANGE IN MODEL-CALCULATED BASEFLOW, IN CUBIC FEET PER SECOND

A: six wells

Scenario 

B: two wells

C: one well

B

SS

EXPLANATION

Figure 3–15. —Continued
84 Part 3. Simulated Effects of Withdrawals on Groundwater Flow (MODFLOW Models)

SS

Scenario 

EXPLANATION

A: six wells

B: two wells

C: one well

2000 2001 2002 2003 2004

J M M J S N J M M J S N J M M J S N J M M J S N J M M J S 

CHANGE IN MODEL-CALCULATED BASEFLOW, IN PERCENT

-3

-2

-1

0

1

2

3

4

5

6

7

8

C

Figure 3–15. —Continued