Water users rely on surface water and ground
water to irrigate crops and to maintain lakes on wildlife
refuges in the 2,200-square-mile Mud Lake study area.
Ground-water development between the late 1970's
and 1989 increased withdrawals from about 240,000
acre-feet in 1983 to about 370,000 acre-feet in 1990.
Concurrent with ground-water development, change
from subirrigation to sprinkler irrigation was predicted
to reduce recharge by 95,000 acre-feet, according to an
independent study. Of the 660,000 acre-feet total
estimated recharge from precipitation and irrigation in
the study area in 1980, half was in the area in which
irrigation methods were changed. Water managers need
the ability to evaluate the effects of water-use changes
on the future supply of surface water and ground water.
Basalt and rhyolite predominate on the surface and
in the subsurface of the study area. Total basalt
thickness is less than 4,000 feet; total sediment thickness
(clay, silt, sand, and gravel) is less than 1,000 feet.
Basalt and sediment interbeds contribute to confined
ground-water conditions and affect movement and
supply of water in parts of the aquifer system.
Estimated losses from and gains to perennial
streams and lakes in 1980 were each about 110,000
acre-feet. Water-table altitudes ranged from about
4,500 to 6,200 feet above sea level, and water-table
gradients were 3 to 120 feet per mile. Underflow from
basins tributary to the study area was estimated to be
about 450,000 acre-feet in 1980; measured discharge
from flowing wells was about 10,000 acre-feet.
A five-layer, three-dimensional, finite-difference,
numerical ground-water flow model was calibrated by
trial-and-error to assumed 1980 steady-state hydrologic
conditions to obtain a better understanding of the
geohydrology and provide a tool to evaluate water-use
alternatives. Water-level gradients simulated by the
model were similar to gradients measured in 1980.
Simulated underflow across model boundaries for 1980
was 932,000 acre-feet. Simulated losses from and gains
to most streams and lakes were within 2 percent of
estimated values. Simulated discharge from flowing
wells matched measurements for 1980. An attempt to
calibrate the numerical model to transient hydrologic
conditions in monthly increments from 1981 to 1990
was discontinued because available data did not justify
changes that were indicated by model simulations.
Additional publication details
USGS Numbered Series
Geohydrology and simulation of flow and water levels in the aquifer system in the Mud Lake area of the eastern Snake River plain, eastern Idaho