Irrigation impacts on California's climate with the variable-resolution CESM

The variable-resolution capability within the Community Earth System Model (VR-CESM) is applied to understand the impact of irrigation on the regional climate of California. Irrigation is an important contributor to the regional climate of heavily irrigated regions, and within the U.S. there are few regions that are as heavily irrigated as California's Central Valley, responsible for 25% of domestic agricultural products. A flexible irrigation scheme with relatively realistic estimates of agricultural water use is employed. The impact of irrigation on mean climatology and heat extremes is investigated over the 26 year period 1980-2005 using a relatively fine grid resolution of 0.25 degrees (approximate to 28 km). Three simulations are performed, including an unirrigated control run and two irrigation-enabled runs, with results compared to gridded observations and weather station data sets. During the summer months (when irrigation peaks), irrigation leads to cooling of the daily maximum near-surface temperature field (Tmax) by approximately 1.1 K. Under irrigation, latent heat flux increased by approximate to 61% during the daytime as a result of increased surface evaporation; specific humidity increased by about 12%; heat stress was reduced by 22% and the average soil moisture exhibited a small (approximate to 4.4%) but statistically significant increase. Compared with observations, irrigation improved the frequency distribution of Tmax, and both length and frequency of hot spells were better represented with irrigation enabled. Consequently, we argue that high-resolution simulations of regional climate in CESM, particularly over heavily irrigated regions, should likely enable the irrigation parameterization to better represent local temperature statistics.