High Resolution Regional Climate Modeling of the Irrigation Cooling Effect in California

Recent research on the impact of irrigation on climate using regional climate models (RCMs) has revealed the presence of an irrigation cooling effect (ICE). By introducing large amounts of water to the land surface via irrigation there is a substantial decrease in daytime surface air temperatures during the dry season in California. Previous studies of the ICE in California utilized RCMs with horizontal resolution 30 km x 30 km. We have completed new experiments at 10 km x 10 km resolution for an 11-year time period from January 1 1979 to January 1 1990. The higher resolution allows for an improved representation of topography, which plays an important role in influencing the regional and local climate of California. An initial comparison of the 30 km and 10 km experiments reveals interesting similarities and differences in surface air temperature, humidity, and the surface energy budget. We also examined the changes in the winds due to irrigation between the 30 km and 10 km experiments. The cooling induced by irrigation leads to an increase in surface air pressure over the irrigated areas in California's Central Valley, which leads to a decrease in near surface onshore flow during the dry season. This onshore flow plays an important role for many crops, particularly wine grapes. By comparing the 30 km and 10 km experiments we are able to better quantify the uncertainty of the magnitude of changes in onshore flow.