U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 116–03

February 2004

The Value of Long-Term Monitoring in the Development 

of Ground-Water-Flow Models

By Daniel T. Feinstein1, David J. Hart 2, and James T. Krohelski 1

Printed on recycled paper

500 feet

0 (NGVD 29)

-500 feet

-1,000 feet

-1,500 feet

WASH

DOD

INGTON

GE

OZAUKEE

MILWAUKEE

WAUKESH

JEFF

A

ERSON

RACINE

KENOSHA

N

25 MILES

Lake M ich i g a n

Unlithified (shallow)

Dolomite (shallow)

Shale

Dolomite (deep)

Sandstone (deep)

Silty sandstone (deep)

Crystalline basement

Lake Michigan

EXPLANATION

Study area MICHIGAN

WISCONSIN

ILLINOIS INDIANA

Figure 1. Block diagram of hydrogeologic 

framework in southeastern Wisconsin. Diagram 

shows shallow formations above shale and deep 

formations below shale and above crystalline 

basement.

1

 U.S. Geological Survey, Water Resources Discipline, Wisconsin District Office

2

 Wisconsin Geological and Natural History Survey, University of Wisconsin–

Extension

As environmental issues have come to the 

forefront of public concern, so has the awareness of the importance of ground water in 

the overall water cycle and as a source of the 

Nation’s drinking water. Heightened interest 

has spawned a host of scientific enterprises 

(Taylor and Alley, 2001). Some activities 

are directed toward collection of water-level 

data and related information to monitor the 

physical and chemical state of the resource. 

Other activities are directed at interpretive 

studies undertaken, for example, to optimize 

the location of new water-supply wells or to 

protect rivers and lakes fed by ground water. 

An important type of interpretive study is the 

computer ground-water-flow model that integrates field data in a mathematical framework. 

Long-term, systematic collection of hydrologic data is crucial to the construction and 

testing of ground-water models so that they 

can reproduce the evolution of flow systems 

and forecast future conditions. 

This Fact Sheet provides an example of the 

incorporation of historical hydrologic data in a 

regional ground-water model for southeastern 

Wisconsin. The example demonstrates how 

the collection of three categories of long-term 

records—water levels from observation wells, 

streamflow measurements at gaging stations, 

and water-use records from high-capacity 

wells—allows the model to reproduce past 

conditions and simulate future trends. It is the 

availability of these long-term data, dating 

back to measurements from the earliest watersupply wells in the 19th century, that couple 

the computer model to real-world conditions, 

and, thus, give it value as a scientific and 

water-resources management tool. 

The model discussed in this Fact Sheet was 

a collaborative effort among the U.S. Geological Survey, the Wisconsin Geological and 

Natural History Survey, and the Southeastern 

Wisconsin Regional Planning Commission. A 

complete description of the hydrologic study 

of southeastern Wisconsin including groundwater flow modeling is described in Feinstein 

and others, 2004. Additional information and 

documentation are available from the Wisconsin Geological and Natural History and from 

the website http://wi.water.usgs.gov/glpf/.

Study Area and Modeling 

Objectives

The computer model focused on a 10-

county region in southeastern Wisconsin. The 

four easternmost counties border Lake Michigan. The model simulated the evolution of 

ground-water levels and flows between 1864 

and 2000 for the entire flow system extending 

from shallow deposits consisting of near-surface sand, clay and dolomite to deeply buried 

units consisting mostly of sandstone (fig. 1). 

Intensive ground-water use has affected the 

ground-water flow system 

throughout the region. 

Along with producing a 

large cone of depression, 

pumping over time has 

reduced flow from 

ground water to surface 

water (including Lake 

Michigan), shifted groundwater divides, increased 

leakage from the shallow 

to the deep parts of the 

ground-water-flow system, 

and changed the direction of 

ground-water flow. The primary purpose of the model 

was to quantify each of these 

changes in order to arrive 

at a better understanding of 

the state of the resource. A 

secondary purpose was to 

simulate trends using different assumptions about 

future pumping, climatic 

change, and land use. 

Model Construction and 

Calibration

Ground-water models require inputs that 

define the horizontal and vertical distribution 

of rock units, the water-bearing properties of 

the rocks, and the stresses that affect groundwater movement, notably recharge to the 

water table and pumping from wells open to 

shallow and (or) deep deposits. The groundwater-flow model for southeastern Wisconsin 

required more complicated input than usual 

because it integrated shallow ground-water 

flow, circulating mostly through fast, smallscale local systems where ground water discharges to streams and lakes, with slow, deep 

flow that extends over miles and typically 

discharges to Lake Michigan or high-capacity 

wells completed in the sandstones. Over the 

eastern half of the study area, a thick 

shale unit acts as a confining layer 

that severely limits circulation 

between the unlithified and 

dolomite deposits which constitute the shallow part of the flow 

system and the sandstones which 

constitute most of the deep part 

of the flow system. In the western 

half of the study area, the confining 

layer is absent and it is easier for 

vertical flow to replenish 

the deeper units. 
Shallow well, steady water level

Shallow well, declining water level

Deep well, steady water level

Deep well, declining water level

Shallow/deep well, steady water level

Shallow/deep well, declining water level

EXPLANATION

WASHINGTON OZAUKEE

DODGE

JEFFERSON WAUKESHA

MILWAUKEE

RACINE

KENOSHA

WALWORTH ROCK

89°

43°30'

42°30'

88°

0 5 10 MILES

0 5 10 KILOMETERS

Lake

M

i c

hig

a n

W I S CONSI N

ILL INO I S

WAUKESHA

MILWAUKEE

RACINE

KENOSHA

300

500

700

1940 1960 1980 2000

WATER LEVEL, IN FEET

YEAR

300

500

700

1940 1960 1980 2000

WATER LEVEL, IN FEET

YEAR

300

500

700

1940 1960 1980 2000

WATER LEVEL, IN FEET

YEAR

300

500

700

1940 1960 1980 2000

WATER LEVEL, IN FEET

YEAR

300

500

700

1940 1960 1980 2000

WATER LEVEL, IN FEET

YEAR

300

500

700

1940 1960 1980 2000

WATER LEVEL, IN FEET

YEAR

300

500

700

1940 1960 1980 2000

WATER LEVEL, IN FEET

YEAR

300

500

700

1940 1960 1980 2000

WATER LEVEL, IN FEET

YEAR

300

500

700

1940 1960 1980 2000

WATER LEVEL, IN FEET

YEAR

300

500

700

1940 1960 1980 2000

WATER LEVEL, IN FEET

YEAR

EXPLANATION

Simulated (uppermost model layer)

Simulated (lowest model layer)

Observed

Water level

0 5 10 MILES

0 5 10 KILOMETERS

Figure 3. Observed and simulated water levels in deep observation wells that show appreciable drawdown 

since 1940. The red points are the observed water levels over time. The blue lines are the simulated water 

levels in the uppermost model layer open to the well; the black lines are simulated water levels in the lowest 

layer open to the well.

To simulate the entire flow system, 18 model 

layers associated with distinct rock types were 

defined. In addition to knowledge of shallow 

and deep ground-water levels, the different 

spatial and temporal scales of the shallow 

and deep components of flow required that 

the database used for the model input include 

both a dense network of surface-water bodies 

to accommodate the shallow flow and a good 

accounting of pumping wells at all depths to 

properly quantify the fraction of water that 

follows deep regional flow paths. 

Ground-water-flow systems change with 

time, especially in the presence of pumping. 

The model was used to study the changes 

produced by increased pumping in southeastern Wisconsin from the late 1800s to 2000. 

Increased well withdrawals continue to affect 

the flow system as pumping centers shift and 

pumping depths change. Because of the timedependent nature of flow in the ground-water 

system, it is not possible to simulate current 

conditions without first reproducing the evolution of ground-water flow starting from the 

natural, relatively stable conditions present 

before the first water-supply wells were dug. 

The model simulates this evolution with 16 

sequential sets of pumping conditions averaging about 10 years in length.

To increase confidence in model simulation of changing water levels and flows, it 

was subjected to a testing procedure called 

calibration. In the case of the southeastern 

Wisconsin model, calibration took many 

forms. The availability of early water-level 

measurements in deep sandstone wells 

allowed the model results to be compared to 

conditions prior to large-scale ground-water 

development. Water levels from subsequent 

decades guided the adjustment of model 

inputs so that they matched the gradual 

response of the flow system to pumping. 

Streamflow measurements guided the choice 

of model inputs that control the balance 

between the amount of recharge entering 

the ground-water system at any location and 

the amount of ground water that discharges 

to nearby streams or other surface-water 

bodies. 

Both the construction and calibration of 

the model depended largely on the availability of long-term data. The following sections 

describe these data in more detail.

Water Levels from 

Observation Wells

Historical water levels are available from 

1940 to the present from about 50 wells in 

the Wisconsin Ground-Water Observation 

Well Network (http://wi.water.usgs.gov/gw). 

These wells provide the data needed to 

adjust inputs to the model so that the 

simulated water levels are close to those 

observed. Some wells in the calibration data 

set show stable water levels over their period 

of record, while others show declining levels 

in response to pumping (fig. 2). The model 

was adjusted to reproduce these trends. 

Within the observation-well network for 

southeastern Wisconsin, long-term records 

are available for 10 wells that are open to 

the deep ground-water system over one or 

more rock layers. The simulated water levels 

for these wells were calculated for the top 

and bottom model layers intersected by the 

well and then compared to the measured 

Figure 2. Observation well network, southeastern 

Wisconsin. Data for each well are available over 

part of the period between 1940 and 2000.
8

8

4

1

2 3

5

6

89°

43°30'

42°30'

88°

L a k

e Mi

chigan

0 5 10 MILES

0 5 10 KILOMETERS

Stream

Gaged basin boundary

Gaging station and identifier

EXPLANATION

7 OZAUKEE WASHINGTON

DODGE

WAUKESHA

RACINE

JEFFERSON

WALWORTH

OZAUKEE WASHINGTON

KENOSHA

W ISCONSI N

JEFFERSON

WALWORTH ROCK

OZAUKEE WASHINGTON

DODGE

WAUKESHA

KENOSHA

W ISCONS I N

I L L I N O I S

MILWAUKEE

ROCK

Figure 4. U.S. Geological Survey streamgaging network, southeastern 

Wisconsin.

12345678

Low estimate

Model simulated

High estimate

STREAMFLOW, IN CUBIC FEET PER SECOND

0

20

40

60

80

100

GAGING STATION IDENTIFIER

Figure 5. Comparison of model-simulated baseflow 

to estimated baseflow at streamgaging stations, 

southeastern Wisconsin.

EXPLANATION

Total

Deep pumping

Shallow pumping

Pumping rate,

in million gallons per day

0

10

1860 2000

Washington

0

40

1860 2000

Waukesha

0

40

1860 2000

Rock

0

10

1860 2000

Dodge

0

10

1860 2000

Ozaukee

0

10

1860 2000

Walworth 0

10

1860 2000

Racine

0

10

1860 2000

Kenosha

0

10

20

1860 2000

Milwaukee

0

20

1860 2000

Jefferson

0 5 10 MILES

0 5 10 KILOMETERS

water levels (fig. 3). In general, the measured 

water-level trend for each well is similar to 

simulated trends. Throughout the study area, 

appreciable decreases in water levels over time 

were measured in the deep part of the flow 

system due to pumping. The model reflects 

the rate and magnitude of these decreases.

Streamflow Measurements at 

Gaging Stations

In addition to water levels, streamflows 

were used for model calibration. Streamflows 

measured at gaging stations are useful to 

calibrate a model because they provide a way 

to determine if the amount of ground water 

circulating in different parts of the simulated 

flow system is reasonable. 

The streamflow measurements that serve 

as calibration targets for the regional model 

were derived from selected stations belonging 

to the USGS streamgaging network in southeastern Wisconsin (fig. 4, also see website: 

http://wi.water.usgs.gov/nwis/sw). These 

stations were selected because their historical records were long enough that a statistical analysis could be conducted to estimate 

the part of streamflow that is attributable to 

ground-water discharge across the streambed, 

a quantity referred to as base flow. In practice, 

it is difficult to calculate a single value for 

ground-water discharge to a stream, but a 

range of the long-term average base flow can 

be identified. The modelsimulated base flows should 

fall within this range. In 

the case of the southeastern 

Wisconsin model, the baseflow calibration targets were 

met at most of the available gaging stations (fig. 

5). The ability to reproduce 

base-flow estimates derived 

from analysis of long-term 

streamgaging data suggests 

that the model reasonably 

simulates the magnitude and 

direction of ground water 

that circulates through shallow flow systems to streams. 

Water-Use Records

Ground-water use was 

quantified by compiling 

pumping rates for individual 

high-capacity wells from 

1864 to 2000. The compilation includes pumping 

rates for 794 high-capacity 

industrial, agricultural, and 

municipal wells for southeastern Wisconsin 

and 508 high-capacity wells representing 

pumping centers in surrounding areas in Wisconsin and 

Illinois. The water-use data 

were compiled from a number 

of published sources as well 

as from long-term records 

maintained by the Wisconsin Department of Natural 

Resources (WDNR) and by 

the Illinois State Water Survey. 

The data sources include two 

categories of pumping wells. 

The first consists of municipal high-capacity wells with 

generally complete withdrawal 

records. The second consists 

of industrial high-capacity 

wells with generally incomplete withdrawal records. 

Withdrawal rates from these 

wells must be estimated from 

pump-capacity data from 

individual well-construction reports on file with State 

agencies. Long-term records 

for domestic wells generally 

are not available. These wells 

number in the thousands over 

the study area, but typically 

they serve single households 

and discharge small volumes. Because most 

domestic wells pump from the shallow 

deposits, and then return the discharge to the 

subsurface through septic systems, their net 

withdrawal is not appreciable and they were 

not included in the model simulation.

The pumping history in the study area varies by county (fig. 6). Municipal and industrial 

pumping has decreased in some counties that 

have shifted from ground water to Lake Michigan water (for example, Milwaukee County), 

but has increased in other counties farther 

from the lake that have growing populations 

Figure 6. Shallow and deep pumping rates by county, southeastern Wisconsin, 1860–2000. Shallow refers to aquifers above shale, deep refers to 

aquifers below shale.
WASHINGTON OZAUKEE

DODGE

JEFFERSON

WAUKESHA

MILWAUKEE

RACINE

KENOSHA

WALWORTH ROCK

89°

43°30'

42°30'

88°

0 5 10 MILES

0 5 10 KILOMETERS

L a k e Mic h iga n

Center of 2000

pumping

(33.5 million

gallons per day)

Center of 1950

pumping

(16.9 million

gallons per day)

WIS C O N SIN

ILL INO I S

Deep ground-water divide (2002)

Deep ground-water divide (1950)

Deep ground-water divide (predevelopment)

Drawdown,

in feet

5

20

35

50

65

80

95

110

125

Lake Michigan

WASHINGTON OZAUKEE

DODGE

JEFFERSON WAUKESHA

MILWAUKEE

RACINE

KENOSHA

WALWORTH ROCK

89°

43°30'

42°30'

88°

0 5 10 MILES

0 5 10 KILOMETERS

WIS C O N S I N

I L L I N OIS

Figure 7. Effect of shifting the pumping center on deep ground-water flow, southeastern 

Wisconsin. Ground-water divides mark estimated western extent of contributing area to deep 

wells and Lake Michigan. 

Figure 8. Simulated drawdown in deep part of flow system in southeastern Wisconsin, from 2000 

to 2020. Predicted conditions assume a 30–40 percent increase in pumping from supply wells during 

simulated period.

(for example, Waukesha County). Some counties rely 

heavily on shallow wells in sandy deposits near rivers 

(for example, Rock County) or fractured dolomite 

(for example, Ozaukee County), whereas withdrawals 

in others are focused on the deep sandstone units (for 

example, Jefferson County). 

Key features of the pumping history are: 1) the 

geographic shift in the main pumping center from 

Milwaukee County westward to Waukesha County; 

and, 2) the increase in total withdrawals over time. 

The pumping history is reflected in the displacement 

and deepening of the regional cone of depression 

over time and in the migration of the deep groundwater divide marking the western boundary of the 

regional flow system in southeastern Wisconsin. 

Model results show that the area of contribution for 

deep wells has moved about 10 miles west of the 

natural divide as wells have diverted more water from 

shallow systems located far from the regional pumping centers (fig. 7). 

Model Application

The capacity of the model to track ground-water 

conditions over time depends on the availability of 

long-term data used in its construction and calibration. Once the model is calibrated to historical conditions, it can be applied to predict future conditions. In 

southeastern Wisconsin, the model has been used to 

simulate the future water-level response in the deep 

part of the flow system under the assumption that 

the distribution of all wells remains the same as in 

2000, but that the withdrawals at each well will grow 

between 2000 and 2020 at a rate that matches recent 

overall trends. The additional simulated drawdown, 

shown in figure 8, is large enough to raise economic 

concerns (for example, increased cost of pumping 

with drawdown) as well as water-quality concerns 

(for example, adverse chemical reactions triggered by 

dewatering at the top of the deep part of the flow system). To reduce these potential adverse effects, part 

of deep pumping could be redistributed on a regional 

basis, replaced by shallow pumping, or replaced by 

diversions from surface water. The model currently 

is being used to study these management scenarios to 

guarantee an adequate water supply for southeastern 

Wisconsin.

References

Feinstein, D.T., Hart, D.J., Eaton, T.T., Krohelski, 

J.T., and Bradbury, K.R., 2004, Simulation of 

regional groundwater flow in southeastern Wisconsin: Wisconsin Geological and Natural History 

Survey Open-File Report 2004-01, 134 p.

Taylor, C.J. and Alley, W.M., 2001, Ground-waterlevel monitoring and the importance of long-term 

water-level data: U.S. Geological Survey Circular 

1217, 68 p.