Temporal and Spatial Pattern Change in Evapotranspiration Over the High Plains: The Impact of and Guide on Extensive Groundwater-Fed Irrigation

Aquifer depletion due to extensive and intensive irrigation in the High Plains has threatened the environmental sustainability of the region. The change of crop evapotranspiration (ET), the major form of agriculture water consumption, presents a critical signature of hydrologic cycle change. This study evaluates the relative contributions of climate and groundwater-fed irrigation to ET temporal and spatial pattern change over the High Plains Aquifer, one of the most severely depleted aquifers in the US. We developed a framework to extend the Budyko hypothesis to assess the impact of catchment storage change on long-term ET. It is found that irrigation from groundwater pumping contributes more than half of the increase in ET (74.1 mm) from the period of 1940–1975 to 1976–2010, despite an increase of precipitation (35.0 mm) in the region. ET seasonal variance is decreased by the decline in precipitation variability (at −20.7 mm) and increase in irrigation (at +14.2 mm). As expected, irrigation decreases ET coefficient of variability (i.e., the ratio of standard deviation to mean). Spatially, we find that the human-induced ET heterogeneity post-1975 superimposes over the natural east-to-west gradient (in precipitation and ET) of this region. A correlation between the statistics (mean vs. coefficient of variation) on ET and crop yield provides promising signatures for understanding the coupled natural and human system of High Plains agriculture. Guides are discussed regarding how to handle the tradeoffs between agricultural development and natural resource sustainability under climate variability in the High Plains and other regions with similar conditions.