conditions and the amount and distribution of annual precipitation. The sand and gravel aquifers in
glacial deposits contribute an estimated 17 percent of the total surface-water flow in the LIRB. This
estimate is derived from the amount of water entering and leaving the basin and subtracting out the
return flows during the driest month of 1992–93. During the driest month of the year, it is assumed
that most surface-water flow is from return flows of industry and public works and ground-water
discharge.
Water budgets are hard to estimate, especially the ground-water component, which is often
the residue. Schicht and Walton (1961) estimated the water budget of two small headwater basins
in the LIRB, Goose and Panther Creeks, based on the assumption that water entering the basin is
equal to water leaving the basin, plus or minus changes in basin storage. These two basins have
similar geology, land use, soils, temperature, and depth to the water table. The water budgetsof
these two basins were determined as
=R +ET 
±DSg,
P is the precipitation,
Ris the surface-water and ground-waterdischarge,
ETis evapotranspiration, and
DS is the change in ground-water storage.
Schicht and Walton (1961) assumed bothconstant soil and subsurface interbasin flow.
Ground-water flow is 40 and 73 percent of the total outflowfor Goose Creek and Panther Creek,
respectively. An important factor of the ground-water component in the water budget is the change
in ground-water storage, which depends on the porosity of deposits. Three to five days after pre-
cipitation ceases in these basins, there is no surface-water runoff, and streamflow is derived entirely
from ground-water discharge. Areas where thick sequences of sand and gravel deposits are inbur-
ied bedrock valleys that traverse a tributary have large contributions from ground water.
A similar approach used for estimating the water budget of the LIRB was based on precipi-
tation,surface-water discharge, and evaporation datapreviously referenced. The entire inflow and
outflow budget for the LIRB is difficult to estimate because many factors that affect the flow of
water in and outof the basin are affected by intrabasin characteristics and changes among basins.
The input of water fromthe upper Illinois River Basin to the LIRB is about20 percent of the total
water input to the basin (fig.
24). The output of water is mostly evaporation, but surface-water
flow
is a substantial output of water. There is very little ground-water flow from outside the basin
because the shallow aquifers follow the surface topography; thus, ground-water divides are
assumed to followsurface-water drainage divides. Deep ground-water flow does not discharge in
the basin but moves to the southeast toward the center of the structural Illinois Basin. Recent work
indicates that some deep buried bedrock valley aquifers may be recharged locallyfrom adjacent
bedrock aquifers (Panno and others, 1994). The hydrologic budget for a small stream or small
watershed may have a substantial input from ground-water flow from adjacent basins because
small watershed drainage divides are less likely tofollow local ground-water divides.
Determining the hydrologic budget for anaquifer under unconfined conditions, such as in
some areas in the LIRB, may require even more detailed information, such as recharge from pre-
