Large-scale water cycle perturbation due to irrigation pumping in the US High Plains: A synthesis of observed streamflow changes

We explore the influence of long-term, large-scale irrigational pumping on spatial and seasonal patterns of streamflow regimes in the High Plains aquifer using extensive observational data to elucidate the effects of regional-scale human alterations on the hydrological cycle. Streamflow, groundwater and precipitation time series spanning all or part of the period of intensive irrigation development (1940–1980) in the region were analyzed for trend and step changes using the non-parametric Mann–Kendall test and the parametric Student’s t-test, respectively. Based on several indicators to evaluate the streamflow–groundwater connection degree over the High Plains aquifer, we found a systematic decrease in the hydraulic connection between groundwater and streamflow from the Northern High Plains to Southern High Plains. Trends and step changes are consistent with this regional pattern. Decreasing trends in annual and dry-season (mean July–August) streamflow and increasing trends in the number of low-flow days are prevalent in the Northern High Plains with a gradual decrease in trend detection towards the south. Additionally, field significance of trends was assessed by the Regional Kendall’s S test over the period of most intensive irrigation development (1940–1980). The step-change results imply that the observed decreases in streamflow are likely attributable to the significant declines in groundwater levels and unlikely related to changes in precipitation because the majority of precipitation data over the region did not reveal any significant changes. Thus, it is very likely that extensive irrigational pumping have caused streamflow depletion, more severely, in the Northern High Plains, and to a lesser extent in the Southern High Plains over the period of study.