Numerical Ground-Water Change Model of the C Aquifer and Effects of Ground-Water Withdrawals on Stream Depletion in Selected Reaches of Clear Creek, Chevelon Creek, and the Little Colorado River, Northeastern Arizona

The base flow in parts of Chevelon and Clear Creeks and
of the Little Colorado River near Blue Spring in northeastern
Arizona is sustained by discharge from the C aquifer, and in
some reaches supports threatened and endangered fish species.
C aquifer water is proposed as a replacement supply to
relieve pumping from the N aquifer—the current source of water
for a coal slurry pipeline used to transport coal mined from
Black Mesa to Laughlin, Nevada. Locations of the proposed
withdrawals are in the area of Leupp, Arizona, about 25 miles
from a perennial reach of lower Clear Creek. A simulation tool
was needed to determine possible effects of the proposed withdrawals
from the C aquifer, particularly the effects of depletion
of streamflow in Clear Creek, Chevelon Creek, and the
Little Colorado River in the area of Blue Spring. A numerical
ground-water change model was developed for this purpose.
The model uses the U.S. Geological Survey finite-difference
model code MODFLOW-2000 and data sets representing key
features of the C aquifer to simulate change in the system that
would result from withdrawing water at proposed locations.
Aquifer thickness was estimated from a hydrogeologic framework
model, and values of aquifer properties such as hydraulic
conductivity and specific yield were estimated from aquifer-test
data. Two scenarios with differing withdrawal rates were run
for a 101-year period that included 51 years of withdrawals followed
by 50 years of no withdrawals. About 6 percent of the
ultimate volume of depletion occurs in the 101-year period for
either scenario. The maximum streamflow depletion rate for all
reaches in the scenario with the greatest withdrawal rates was
computed to be about 0.6 cubic foot per second. The depletion
rate was highest in lower Clear Creek, the reach that is closest
to the well field. A model that simulates historical conditions
was used to help select the most reasonable parameter sets for a
Monte Carlo analysis of computed stream depletions