HYDROLOGIC PROCESSES, SCALE, CLIMATE VARIABILITY, AND WATER RESOURCES FOR SEMIARID WATERSHED MANAGEMENT
Location: Southwest Watershed Research
Title: Estimating evapotranspiration under warmer climates: Insights from a semiarid riparian system
Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 20, 2010
Publication Date: February 17, 2011
Citation: Serrat-Capdevila, A., Scott, R.L., Shuttleworth, W.J., Valdez, J.B. 2011. Estimating evapotranspiration under warmer climates: Insights from a semiarid riparian system. Journal of Hydrology. 399: 1-11.
Interpretive Summary: The quantification of climate change impacts on hydrology has focused on how changes in precipitation and temperature can affect runoff, with less emphasis on how evapotranspiration (ET)-- the dominant water loss from many watersheds—will change. This study focuses on estimating climate induced changes in the ET. We analyzed ET and meteorological data from three riparian sites located in a semiarid watershed in southern Arizona and found that a relatively simple model could be developed that would allow us to estimate future ET rates given climate model projections. Climate predictions for this region indicate that atmospheric evaporative demand will be greater, but actual ET rates at the studied field sites will remain largely unchanged due to plant mechanisms that will constrain the loss of water from their leaves. However, the length of the growing season is projected to increase due to warmer temperatures and this will result in a greater annual riparian water use. These findings of increased riparian water use will likely lead to greater groundwater deficits and decreased streamflow and have important implications for water management in semiarid regions.
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.