Evaluation of the impact of groundwater irrigation on streamflow in Nebraska

Nonparametric techniques were applied to the analysis of streamflow depletion and trends in precipitation and temperature in Nebraska and northwestern Kansas. Fifty years of streamflow data from 110 gauging stations in eight major river basins were examined. Temporal trends of streamflow in Nebraska showed a spatial tendency of decreasing streamflow mostly in the west but was insignificant in the east. This spatial pattern in streamflow depletion is unlikely to be due to a long-term change in precipitation over the entire state because precipitation, based on the records of 28 weather stations from 1948 to 2003, did not indicate a spatial trend. For the Republican River basin, 20 of the 28 gauging stations showed decreasing streamflow. To evaluate the trend of baseflow, the Local Weighted Regression method was used to generate precipitation-adjusted stream discharge. Additional analyses suggested that the local precipitation-adjusted discharge from 17 of the 22 stations decreased since the 1950s in the Republican River basin. This decrease plausibly matches a pattern of an increasing number of irrigation wells and the declines of the groundwater level. Because there was no decreasing trend in precipitation, it is most likely that groundwater withdrawal in this basin was a primary factor in streamflow depletion. Besides Nebraska, where a significant amount of groundwater was withdrawn from the High Plains regional aquifer, irrigators in Kansas and Colorado were the other likely contributors to streamflow depletion in the Republican River.