Great Plains irrigation produces enhanced summer precipitation in the Midwest

Over the course of the 20th Century, groundwater pumping for irrigation has depleted water storage in the Ogallala Aquifer of the Great Plains by 333 km³. This has had particularly significant hydrologic impacts over the Texas and Oklahoma panhandles and western Kansas, where groundwater declines have been greatest. The result has been an approximate doubling of the surface water available for evapotranspiration, most of which evaporates rather than runs off or returns to groundwater. Using gridded precipitation observations covering 1900-2003 over North America, we show that this enhanced evapotranspiration has increased precipitation by 25-50% downwind of the Ogallala Aquifer during July. These observations are in contrast to three state-of-the-art climate model simulations of the 20th Century forced with observed sea surface temperatures and changes in atmospheric aerosols and greenhouse gases, which showed no such precipitation changes. Since the 20th Century climate model simulations did not include land-atmosphere interactions related to irrigation, we attribute the discrepancy between the observations and models to the effects of irrigation. These effects must be considered as we study past and future climate change in response to human actions.