Department of Earth System Science

 We measured the evaporative fraction (EF) across a semiarid elevation gradient in the San Jacinto and Santa Rosa Mountains of southern California using the Regional Evaporative Fraction Energy Balance platform and four eddy covariance towers. We compared our measurements to precipitation estimates and satellite observations of vegetation indices to assess the seasonal and interannual controls of precipitation and vegetation on surface energy exchanges. Precipitation amount and timing had the largest effect on evaporative fraction, with vegetation having a relatively greater importance at higher elevations than lower elevations. Vegetation cover was linearly related to mean annual EF, but did not predict seasonal variation in EF in most of the study area's ecosystems. Multiyear vegetation observations show that vegetation density increases in a stepwise pattern with precipitation, probably due to shifts in dominant plant communities. Precipitation is a more important factor in controlling EF than temperature. Possible future climate change, including decreases in precipitation amount and increases in variability, could decrease vegetation cover, thus reducing EF and increasing albedo.