Technical Abstract: This paper presents an approach to quantify evapotranspiration under changing climates, using field observations, theoretical evaporation models and meteorological predictions from global climate models. We analyzed evaporation and meteorological data from three riparian sites located in a semiarid watershed in southern Arizona USA and found that the surface resistance to water vapor transport was closely related to the vapor pressure deficit. From this, we developed a relatively simple daily conductance model and included a growing season index to accurately replicate the onset and the end of the growing season. After the model was calibrated with observations from January 2003 to December 2007, it was used to predict daily evapotranspiration rates from 2000 to 2100 using Penman-Monteith equation and meteorological projections from the IPCC fourth assessment report climate model runs. Results indicate that atmospheric demand will be greater and lead to increased reference crop evaporation, but evapotranspiration rates at the studied field sites will remain largely unchanged due to stomatal regulation. However, the length of the growing season will increase leading to a greater annual riparian water use. These findings of increased riparian water use and atmospheric demand, likely affecting recharge processes, will lead to greater groundwater deficits and decreased streamflow and have important implications for water management in semiarid regions.