Modeling the impact of iIrrigation on precipitation over the Great Plains.

Abstract

Since World War II, the rapid expansion of irrigation throughout the Great Plains has threatened the sustainability of the Ogallala Aquifer. Irrigation has been shown to modify the surface energy and water budgets over the Great Plains by altering the partitioning of latent and sensible heating. An increase in latent heating from irrigation contributes to a cooler and more humid surface, which has competing impacts on convection. In this study, the Weather Research and Forecasting model was modified to simulate the effects of irrigation at sub-grid scales. Nine April-October simulations were completed for different hydrologic conditions over the Great Plains. Data from these simulations was assimilated into a back-trajectory analysis to identify where evapotranspired moisture from irrigated fields predominantly falls out as precipitation. May through September precipitation increased on average over the Great Plains by 4.97 mm (0.91%), with the largest increases during wet years (6.14 mm; 0.98%) and the smallest increases during drought years (2.85 mm; 0.63%). Large precipitation increases occurred over irrigated areas during normal and wet years, with decreases during drought years. On average, only 15.8% of evapotranspired moisture from irrigated fields fell out as precipitation over the Great Plains, resulting in 5.11 mm of May-September irrigation-induced precipitation. The heaviest irrigation-induced precipitation occurred over north-central Nebraska, coincident with simulated and observed precipitation increases. While irrigation resulted in localized and region-wide increases in precipitation, large evapotranspiration increases suggest that irrigation contributes to a net loss of water in the Great Plains.