Regional Climate Impacts of Irrigation during the 2012 Central Plains Drought

In the summer of 2012, the Central Plains of the United States experienced one of its most severe droughts on record. This study examines the meteorological impacts of irrigation during this drought through observations and model simulations using the Community Land Model (CLM) coupled to the Weather Research and Forecasting (WRF) model. A simple parameterization of irrigation processes is added into the WRF model. In addition to keeping soil moisture in irrigated areas at a minimum of 50% of soil moisture hold capacity, this irrigation scheme has the following new features: (1) accurate representation of the spatial distribution of irrigation area in the study domain by using MODIS-based 250-m pixel size land surface classification; and (2) improved representation of the time series of leaf area index (LAI) values derived from crop modeling and satellite observations in both irrigated and non-irrigated areas. Several numerical sensitivity experiments are conducted. The WRF-simulated temperature field when including soil moisture and LAI modification within the model is shown to be most consistent with ground and satellite observations, all indicating a 2-3 K decrease of temperature in irrigated areas compared to the control run. Modification of LAI in irrigated and dryland areas led to smaller changes, with a 0.2 K temperature decrease in irrigated areas and up to a 0.5 K temperature increase in dryland areas. Furthermore, the increased soil moisture and modified LAI is shown to lead to statistically-significant increases in surface divergence and surface pressure, and decreases in planetary boundary layer height over irrigated areas.