Analyzing streamflow changes: irrigation-enhanced interaction between aquifer and streamflow in the Republican River basin
Groundwater-fed irrigation has altered surface and groundwater interactions in the Republic River basin (RRB) in the midwestern United States, where agriculture heavily depends on irrigation. The decreasing flow trend recorded at the RRB gauging station since the 1950s reflects the synthetical effect of dynamic interactions between surface water and groundwater systems, which has been enhanced by groundwater pumping and irrigation return flow. This study uses a systematic modeling approach to analyze the conjunctive effects of pumping and return flow on streamflow. A watershed management model, the Soil and Water Assessment Tool (SWAT), is modified and established for the Frenchman Creek basin (FCB), a subbasin of RRB, to examine the causes of streamflow changes. The baseflow component in SWAT is linked to aquifer storage so that the model can simulate the combined effects of groundwater pumping and irrigation return flow on natural streamflow. Results show that irrigation has not only depleted streamflow but also changed the flow pattern and seasonal variability. The changes can be decomposed into decrease in the slow component (baseflow) and increase in the fast components (surface and subsurface flow). Since the fast components are subject to higher variability than the slow component, the annual streamflow variability is amplified. Agricultural water use in this region also has changed the groundwater storage seasonal regime from the pattern of "summer recharge and winter discharge" in the past to "summer discharge and winter recharge" at present. This challenges the existing groundwater modeling, which usually assumes fixed recharge pattern and rates.