Computer program for simulation of variable recharge with the U. S. Geological Survey modular finite-difference ground-water flow model (MODFLOW)

The Variable-Recharge Package is a
computerized method designed for use with the
U.S. Geological Survey three-dimensional finitedifference
ground-water flow model
(MODFLOW-88) to simulate areal recharge to an
aquifer. It is suitable for simulations of aquifers in
which the relation between ground-water levels
and land surface can affect the amount and
distribution of recharge. The method is based on
the premise that recharge to an aquifer cannot
occur where the water level is at or above land
surface. Consequently, recharge will vary
spatially in simulations in which the Variable-
Recharge Package is applied, if the water levels
are sufficiently high. The input data required by
the program for each model cell that can
potentially receive recharge includes the average
land-surface elevation and a quantity termed
?water available for recharge,? which is equal to
precipitation minus evapotranspiration.
The Variable-Recharge Package also can
be used to simulate recharge to a valley-fill
aquifer in which the valley fill and the adjoining
uplands are explicitly simulated. Valley-fill
aquifers, which are the most common type of
aquifer in the glaciated northeastern United
States, receive much of their recharge from
upland sources as channeled and(or) unchanneled
surface runoff and as lateral ground-water flow.
Surface runoff in the uplands is generated in the
model when the applied water available for
recharge is rejected because simulated water
levels are at or above land surface. The surface
runoff can be distributed to other parts of the
model by (1) applying the amount of the surface
runoff that flows to upland streams (channeled
runoff) to explicitly simulated streams that flow
onto the valley floor, and(or) (2) applying the
amount that flows downslope toward the valley-
fill aquifer (unchanneled runoff) to specified
model cells, typically those near the valley wall.
An example model of an idealized valley-
fill aquifer is presented to demonstrate application
of the method and the type of information that can
be derived from its use. Documentation of the
Variable-Recharge Package is provided in the
appendixes and includes listings of model code
and of program variables. Comment statements in
the program listings provide a narrative of the
code. Input-data instructions and printed model
output for the package are included.