| The post-World War II increase in irrigation in the Great Plains represents the largest human-induced hydrologic impact in North America. Drawn primarily from the High Plains Aquifer, water applied as irrigation in the region amounts to billions of cubic meters (2 × 1010 m3 in 1990) annually and is applied to more than 60 000 km 2 of farmland. This perturbation has altered the energy and water balances in the region and is thought to affect precipitation. Following previous studies, empirical orthogonal functions and precipitation magnitude comparisons were employed to examine trends in precipitation over the region. These statistical tools were used to determine if this enormous addition of imgation water to the surface has had a measurable influence on precipitation during the summer months of June, July, and August from 1950–1997. An autocorrelation analysis of hourly rainfall during June, July and August for 1996–1997 was employed to identify the specific location of any precipitation induced by irrigation, and to estimate the magnitude of the irrigation effect. To identify the physical mechanism by which irrigation affected regional precipitation events, a regional atmospheric model (RAMS) was used to explore the impacts of different types of land cover and irrigation levels.;Loading patterns and temporal precipitation trends in monthly rainfall data for 1950–97 show, at best, slight evidence that irrigation induces rainfall. Results from the autocorrelation analysis detected a precipitation anomaly ∼90 km downwind of the irrigated region with a ∼10% elevation in precipitation compared to surrounding areas. Modeling results show that irrigation leads to differential heating of the land surface, thereby altering the balance of latent and sensible heat fluxes. Mesoscale circulations develop from this different partitioning of heat fluxes, entraining irrigation moisture and creating conditions more likely to produce precipitation. If irrigation-induced rainfall exists, its impact is only minor relative to the natural determining factors of plains climate. However, even small levels of weather modification can be very important in arid regions. |
