EVALUATING THE IMPACT OF IRRIGATION ON STREAMFLOW IN THE LOWER REPUBLICAN RIVER BASIN, KANSAS THROUGH INTEGRATED WATERSHED MODELING

We developed an integrated watershed model capable of simulating the surface water, groundwater, and stream-aquifer interactions on a continuous basis, and applied it to the Lower Republican River basin in Kansas. The model code links two well-known computer programs: MODFLOW, which simulates ground water flow and stream-aquifer interaction, and SWAT, a soil water budget simulator for an agricultural watershed. SWAT represents a basin as a collection of subbasins in terms of soil, land use, and weather data, and simulates each subbasin on a daily basis to determine the various hydrologic-budget components. Because SWAT applies a lumped hydrologic model to each subbasin, spatial heterogeneities with respect to factors such as soil type and land use are represented statistically using the hydrologic response unit (HRU) concept. For the Lower Republican River basin model, each combination of six soil types and three land uses was simulated with separate executions of SWAT. A spatially weighted average was then taken over these results for each hydrologic flux and time step by a component program, SWBAVG. We also wrote a separate package for MODFLOW to associate each subbasin with a subset of aquifer grid cells and stream reaches; and to distribute the hydrologic fluxes given for each subbasin by SWAT and SWBAVG over MODFLOW's stream-aquifer grid to represent tributary flow, surface- and ground-water diversions, ground water recharge, and evapotranspiration from ground water. The various model components are coordinated as functional parts of the linkage between SWAT and MODFLOW. The model of the Lower Republican River basin from Concordia to Clay Center, Kansas was calibrated with respect to measured ground water levels, streamflow, and reported irrigation water use. The model was used to examine the relative contributions of stream yield components, and the impact on stream yield and baseflow of administrative measures to restrict irrigation water use during droughts. Model results indicate that tributary flow is the dominant component of stream yield, and that reduction of irrigation water use produces a corresponding increase in baseflow and stream yield. However, the increase in stream yield resulting from reduced water use does not appear to be of sufficient magnitude to restore minimum desirable streamflows.