USE OF A GIS TO PREPARE INPUT DATA
FOR AND EVALUATE RESULTS FROM A FINITE-DIFFERENCE
GROUND-WATER FLOW MODEL FOR NORTHEASTERN WISCONSIN
T.D. Conlon
U.S. Geological Survey
Madison, Wisconsin USA
ABSTRACT
Ground-water data for a finite-difference ground-water flow model in northeastern Wisconsin
were managed using a geographic information system (GIS). Spatial data, such as aquifer thickness,
river extent, well location and pumpage, and ground-water levels were compiled and analyzed using
the GIS. The GIS provided an efficient method to compile hydrogeologic information in a spatial
data base, graphically check the data for completeness and accuracy, and create data sets in a suitable
format for input to a ground-water flow model. Changing input to the model was facilitated by the
ability to edit data in the GIS graphically and to automate the transfer of data from the GIS to model
data sets. Model-generated aquifer water levels were easily transferred to the GIS, where they were
compared with measured water levels to check the accuracy of the model.
1.0 BACKGROUND
Most large communities along the Fox
River between Lake Winnebago and Green
Bay in northeastern Wisconsin obtain their
water supply from a rock aquifer consisting of
sandstones and dolomites, known as the
sandstone aquifer. Withdrawal of water from
the aquifer began in the late 1880’s. Ground
water naturally flowed from early deep wells
indicating that the aquifer is overlain by a
relatively impermeable rock or soil layer and
artesian conditions existed in the aquifer. The
low-permeability layer confines the aquifer and
separates the sandstone aquifer from the
overlying aquifer which consists of glacial
deposits. As the population increased and
industrial development occurred, withdrawal
of ground water from the sandstone aquifer
increased. By 1990, water levels in the
sandstone aquifer had declined by
approximately 200 feet from 1880’s levels. The
rate of decline of water levels in some wells is
approximately 3 feet per year. The U.S.
Geological Survey (USGS), in cooperation
with the major municipal ground-water users
in the area, used a GIS to develop a ground-
water flow model to 1) improve the
understanding of the hydrogeology of the
sandstone aquifer, 2) evaluate the effects of
68
present-day pumping on water levels and
ground-water flows, 3) predict the effect of
future withdrawals from the aquifer on ground-
water levels.
The ground-water flow model used in this
study is the USGS’s modular finite-difference
ground-water flow model, commonly referred
to as MODFLOW (McDonald and Harbaugh,
1988). The model requires that the study area
be divided into a grid of discrete cells of finite
area. The smaller the cell area the better the
resolution of the model assuming measured
properties of the aquifer systems are available
to support this resolution. Hydraulic properties
are assigned to each cell. By solving the
equation governing ground-water flow in each
cell, MODFLOW can provide water levels and
flows for each cell. The model is considered
calibrated when model-generated water levels
and flows compare acceptably with measured
water levels and flows. Because input values of
the hydraulic properties of the aquifer system
are seldom well known, the calibration process
requires numerous simulations of the model
with different input-data sets. With the advent
of GIS, the ability to prepare input-data sets for
models of many cells has been greatly
improved. This paper describes how a GIS,
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