Two major factors have influenced the climate of California in the past century. As elsewhere on the globe the first is global climate change. The second is regional land use changes, associated with the conversion of large blocks of natural vegetation to irrigated agriculture and urban areas. We explore these influences by comparing the output of three regional climate model runs. In the first we prescribe modern land cover conditions, apply realistic water amounts to irrigated agricultural regions, and force the model with observed 1995-96 meteorological conditions. In the second we prescribe an historical vegetated land cover and force the model with conditions for 1900-01, derived from the United Kingdom Meteorological Office global coupled climate model. In the third we prescribe the historical vegetated land cover, but force with the 1995-96 boundary conditions.

The precipitation and maximum surface temperature difference between the 1996 and 1901 results generally agree with the observed winter and summer trends over the last century. The results also show in summer that irrigation has a strong effect on the differences between 1996 and 1901 in maximum temperature, surface latent and sensible heat fluxes, surface moisture and equivalent snow water.