Interannual variability of evapotranspiration and energy exchange over an annual grassland in California

Abstract

We report on the interannual variability of evapotranspiration ( E) and energy exchange of an annual grassland in the Mediterranean climate zone of California. They were measured directly with the eddy covariance technique over a 6-year period that spanned between July 2001 and June 2007 and experienced a large range in precipitation ( 376 mm to 888 mm). Despite a two-fold range in precipitation, annual E ranged much less, between 266 mm and 391 mm. We found that pronounced energy-limited and water-limited periods occurred within the same year. In the water-limited period, monthly integrated E scaled negatively with solar radiation and was restrained by precipitation. In the energy-limited period, on the other hand, the majority of E scaled positively with solar radiation (R-g) and was confined by potential E ( E-p). E was most sensitive to the availability of soil moisture during the transition to the senescence period rather than onset of the greenness period, causing annual E to be strongly modulated by growing season length. Bulk surface conductance scaled consistently with Priestley-Taylor alpha coefficient regardless of interannual and seasonal variability of precipitation, E, and solar radiation.