Irrigation-induced rainfall and the great plains

The post-World War II increase in irrigation in the Great Plains represents the largest human-induced hydrologic impact in North America. Drawn primar ily from the High Plains aquifer, water applied as irrigation in the region amounts to billions of cubic meters (2 x 10(10) m(3) in 1990) annually and is applied to more than 60 000 km(2) of farmland. Following studies by Sch ickedanz and by Barnston and Schickedanz, empirical orthogonal functions an d precipitation magnitude comparisons were employed to examine trends in pr ecipitation over the region and to determine if this enormous addition of i rrigation water to the surface has had a measurable influence on precipitat ion during the summer months of June, July, and August. The Barnston and Schickedanz study observed a transition from unirrigated t o heavily irrigated conditions; in contrast, this examination focused on a more recent period during which irrigation took place throughout the time o f interest. Loading patterns and temporal precipitation trends for 1950-97 show, at best, slight evidence that irrigation induces rainfall. The most p rominent evidence of an irrigation effect is found in the Texas Panhandle f or 1950-82. If irrigation-induced rainfall exists, its impact is only minor relative to the natural determining factors of plains climate. It also is possible that the chief influence of irrigation on rainfall may take place at some threshold magnitude of irrigation (not explored in this study) that already had been exceeded by 1950.