Evapotranspiration and soil moisture dynamics on a semiarid ponderosa pinehillslope

Detailed measurements of soil moisture and ET in semiarid forest environmen ts have not been widely reported in the literature. In this study, soil moi sture and water balance components were measured over a four-year period on a semiarid ponderosa pine hillslope, with evapotranspiration (ET) determin ed as the residual of measured precipitation, runoff, and change in soil mo isture storage. ET accounts for approximately 95 percent of the water budge t and has a distinctly bimodal annual pattern, with peaks occurring after s pring snowmelt and during the late summer monsoon season, periods that coin cide with high soil moisture. Weekly growing season ET rates determined by the hillslope water balance are found to be invariably below calculated pot ential rates. Normalized ET rates are linearly correlated (r(2) = 0.62) wit h soil moisture; therefore, a simple linear relation is proposed. Growing s eason soil moisture dynamics were modeled based on this relation. Results a re in fair agreement (r(2) = 0.63) with the observed soil moisture data ove r the four growing seasons; however, for two dry summers with little surfac e runoff, much better results (r(2) > 0.90) were obtained.