Irrigation Effects on Land-Atmosphere Coupling Strength in the United States

Land-atmosphere coupling strength describes the degree to which the atmosphere responds (e. g., via changes in precipitation) to changes in the land surface state (e. g., soil moisture). The Midwest and Great Plains of the United States have been shown to be "hot spots'' of coupling by many climate models and some observations. However, very few of the modeling studies have reported whether the climate models applied irrigation in the Midwest and Great Plains, where 24%-27% of farmland is irrigated, leaving open the question of whether irrigation affects current estimates of coupling strength. This study used a regional climate model that incorporated dynamic crop growth and precision irrigation (WRF3.3-CLM4crop) to investigate irrigation effects on land-atmosphere coupling strength. Coupling strength was quantified using multiple indices and the irrigated land-induced precipitation was tracked using a back trajectory method. The indices showed a consistent and significant decline in local coupling strength with irrigation in the Midwest and northern Great Plains. These reductions were due to increased soil moisture but decreased local precipitation and lower sensitivity of latent heat flux to soil moisture over irrigated regions. The back trajectories of water vapor transport confirmed that irrigation largely did not contribute to local precipitation. Water vapor from irrigated land was transported to the Midwest and U.S. Northeast where it fell as precipitation, suggesting that irrigation has a broader spatial impact on soil moisture-precipitation coupling than simply through local soil moisture-evapotranspiration coupling. The present study suggests that climate models without irrigation schemes may overestimate the land-atmosphere coupling strength over irrigated agricultural regions but underestimate coupling strength over neighboring nonirrigated regions.