Identifying the effect of irrigation on evapotranspiration variability over the High Plains

Abstract:

Irrigation is widely adopted as a measure to maintain crop yield when precipitation is limited and stabilize crop yield to buffer climatic fluctuation. Irrigation has considerably interfered with hydrological processes in many areas with extensive and intensive irrigation requirement; with the increasing demand for food and weather variability related to climate change, irrigation application is expected to increase, which would aggravate the interferences to hydrologic processes. Current studies focus on the impact of irrigation on the mean value of ET at either local or regional scale, however, how irrigation changes the variability of ET has not been well understood. This study analyzes the impact of extensive irrigation on ET variability in the High Plains. We apply an ET variance decomposition framework (Zeng and Cai 2015) to quantify the effects of both climate and irrigation on ET variance in in the High Plains watersheds. Based on climatic and groundwater storage data, we assess the monthly ET variance and its components for both pre-development (1930s-1960s) and development periods (1970-2010s). It is found that irrigation not only causes the well-known groundwater drawdown and stream depletion problems in the area associated with, but also changes ET variance, which further affects land surface processes. With complementary water supply from irrigation, ET approaches to potential ET, and ET variance is more attributed to climatic variables such as temperature, while causing significant seasonal fluctuations to groundwater storage. For sustainable water resources management in the High Plains, we argue that both the mean value and the variance of ET should be considered together for the regulation of irrigation in this region.