California Institute for Energy and Environment (CIEE)

This paper discusses the effects of irrigation and albedo change due to agriculture on California climate and air pollution and the effects of black carbon and soildust impurities within snow on California climate and snowmelt. High-resolution irrigation, landuse, soil, albedo, and emissions data were used with the nested global-through-urban GATOR-GCMOM model to examine these issues. With respect to agriculture, results suggest that irrigation alone increased nighttime temperatures but decreased daytime temperatures to a greater extent causing a net decrease in near-surface air temperatures averaged over irrigated and nonirrigated land in California and parts of Nevada by about 0.03 K during August 2006. The conversion of native land in the 1800s to agriculture was found to increase the surface albedo of the northern and middle San Joaquin Valley and to decrease the albedo of the southernmost valley, resulting in a net cooling over land dueto irrigation plus agriculture of 0.04 K. Maximum local decreases in August-averaged temperatures were about 0.7 K, occurring in the San Joaquin Valley. Since agriculture alone caused a net summer cooling of the San Joaquin Valley, observed historic warmingof the valley may be due to other factors, such as anthropogenic greenhouse gas andparticle buildup. Irrigation from agriculture also increased land-averaged soil moisture byabout 5%, the relative humidity by about 0.4%, cloud optical depth by about 5.5%, cloudfraction by about 2.9%, drizzle by about 5.5%, and downward thermal-infrared radiation by about 0.16%. Irrigation reduced primary pollutant concentrations when precipitation or drizzle was imminent or present but increased such pollution when precipitation was absent. Thus, in polluted, irrigated regions with low rainfall, such as eastern Los Angeles, daytime and spray irrigation should be replaced with nighttime and subterranean or drip irrigation to the extent that this is not already done, to minimize evaporation and the positive feedback of evaporative cooling to air pollution. Reducing evaporation has the collateral benefit of increasing water available for a growing population. Absorption of solar radiation by black carbon and soildust in snow during February reduced land (snowplus nonsnow) albedo by about 0.3%, reduced land-averaged snow depth by about 0.5%,increased ground temperatures over land by about 0.11 K, increased soil moisture by about 0.07%, and increased the relative humidity by about 0.04% over land. These results imply that impurities in snow decrease water supply by increasing sublimation and hasten meltwater release. Although the effects of impurities on snow melting should be greater in April than in February due to warmer temperatures and greater solar radiation in April, snow area is much lower in April than in February, so newly-deposited impurities fromAsia may have a lesser aggregate impact on snowmelt by the time they arrive in April, than local impurities, which are deposited over a larger snow area and over a longer period. This study implies that efforts to reduce both local and Asian particle emissionswill slightly alleviate the effect of climate change on early release of meltwater. Such particle reductions will have the additional benefit of improving human health