Alter, Ross Evan. Evidence for the influence of irrigation on precipitation intensity and totals in the midwestern United States. Retrieved from https://doi.org/doi:10.7282/T3707ZW5
DescriptionSignificant increases in summer precipitation occurred in the Midwestern United States over the last century for reasons that remain unclear. It is postulated that the expansion of irrigation and cropland in the central US over the past sixty years has been a major contributor to these observed increases in precipitation. As a first step toward attribution of these regional precipitation changes, a detailed analysis of observed daily summer precipitation frequency and intensity is conducted for the contiguous United States over multiple spatial scales and time periods from 1895 to 2011. Robust increases in precipitation frequency, total precipitation, and moderate to heavy precipitation intensity are identified during July and August in the Midwestern US. Analysis of changes in mean monthly precipitation from the early- to late-20th century initially points to increasing frequency as the source of increasing monthly precipitation in the Midwestern US during the summer, especially during August; however, comparable increases in precipitation frequency occur during other times of the year. On the other hand, changes in precipitation intensity and total precipitation are both greatest during July and August and coincide spatially in the Midwestern US. Additionally, the greatest intensity change occurs downwind of the most heavily irrigated regions, especially for the period between 1950 and 1980 when irrigation rapidly intensified. A 15-day simulation using the WRF regional climate model with a simplified irrigation scheme over Nebraska confirmed the postulated increase in moisture, decrease in temperature, and subsequent increases in both convective inhibition and convective available potential energy over Nebraska, which led to weakened convection over the irrigated areas. Wind anomalies produced by irrigation seem to be instrumental in enhancing precipitation intensity and totals downwind of Nebraska in general, and in the eastern Midwest region for one particular heavy precipitation event. The increases in Midwestern precipitation in both analyses – one based on observation and rooted in reality and one based on model experiments and controlled for irrigation – support the hypothesis that irrigation in Nebraska has led to an increase in the intensity and total of precipitation downwind of the irrigated regions.