Abstract
A set of watershed models for four basins (Crab Creek, Rocky Ford Creek,
Rocky Coulee, and Lind Coulee), draining into Potholes Reservoir in
east-central Washington, was developed as part of a decision support
system to aid the U.S. Department of the Interior, Bureau of Reclamation,
in managing water resources in east-central Washington State. The project
is part of the U.S. Geological Survey and Bureau of Reclamation collaborative
Watershed and River Systems Management Program. A conceptual model of
hydrology is outlined for the study area that highlights the significant
processes that are important to accurately simulate discharge under
a wide range of conditions. The conceptual model identified the following
factors as significant for accurate discharge simulations: (1) influence
of frozen ground on peak discharge, (2) evaporation and ground-water
flow as major pathways in the system, (3) channel losses, and (4) influence
of irrigation practices on reducing or increasing discharge.
The Modular Modeling System was used to create a watershed model for
the four study basins by combining standard Precipitation Runoff Modeling
System modules with modified modules from a previous study and newly
modified modules. The model proved unreliable in simulating peak-flow
discharge because the index used to track frozen ground conditions was
not reliable. Mean monthly and mean annual discharges were more reliable
when simulated. Data from seven USGS streamflow-gaging stations were
used to compare with simulated discharge for model calibration and evaluation.
Mean annual differences between simulated and observed discharge varied
from 1.2 to 13.8 percent for all stations used in the comparisons except
one station on a regional ground-water discharge stream. Two thirds
of the mean monthly percent differences between the simulated mean and
the observed mean discharge for these six stations were between -20
and 240 percent, or in absolute terms, between -0.8 and 11 cubic feet
per second.
A graphical user interface was developed for the user to easily run
the model, make runoff forecasts, and evaluate the results. The models;
however, are not reliable for managing short-term operations because
of their demonstrated inability to match individual storm peaks and
individual monthly discharge values. Short-term forecasting may be improved
with real-time monitoring of the extent of frozen ground and the snow-water
equivalent in the basin. Despite the models unreliability for short-term
runoff forecasts, they are useful in providing long-term, time-series
discharge data where no observed data exist.
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