Modeling the Nonlocal Effects of Summer Growing Season Irrigation in the Great Plains

Many processes that control convective precipitation are modified by the presence of irrigation. Additional surface water alters the thermodynamic budget of the local atmosphere lowering surface temperatures and increasing low-level moisture. These changes lower condensation levels within the boundary layer affecting convective potential and inhibition tendencies. Previous observational and modeling studies concluded minimal increases in precipitation over irrigated regions in the Great Plains with higher intensity and more frequent storms in more convectively favorable environments downwind. However, uncertainty remains in the degree and location of this downstream modification to irrigation-induced precipitation. The purpose of this study is to determine how irrigation can affect convection and its subsequent evolution into precipitation signals over a range of atmospheric scales. Both initiated convection and that advected over irrigated land in Nebraska is examined. Ultimately this work aims to understand the magnitude and frequency at which precipitation can be modified by irrigation well downstream (up to thousands of kilometers) from the irrigation itself.

This work uses an ensemble technique with members of varying soil moisture magnitudes and spatial patterns within the Weather Research and Forecasting (WRF) model. Irrigated simulations over three summer growing seasons where high precipitation events occur in Nebraska are compared to control runs with no added water to understand the breadth of downstream precipitation modification. The three growing seasons represent different degrees of U.S. annual precipitation, allowing the investigation into climate’s influence on irrigation-induced weather modification. Empirical orthogonal function (EOF) analysis and a range of other techniques are used to differentiate realistic precipitation modification from that caused by unrealistic perturbation growth through numerical noise. The implications of these results as to the general, widespread effects of nonlocal irrigation-induced precipitation modification are discussed.