Abstract
This project studies the sensitivity of evapotranspiration of alfalfa in California’s Sacramento-San Joaquin Delta (the Delta) to changes in climate variables. A sensitivity analysis was carried out using climate data measured at alfalfa crops on Twitchell Island, CA during the drought years 2014 and 2015. The analysis involved applying the data to compute daily historical evapotranspiration for the collected data years using the Penman-Monteith equation for evapotranspiration of tall reference grass, ETr. Sensitivity coefficients were then calculated for windspeed U2, minimum temperature Tmin, maximum temperature Tmax, solar radiation Rs, and vapor pressure deficit VPD by measuring the response of ETr to incremental changes in each respective parameter and developing a sensitivity coefficient for each variable. Results and conclusions were then discussed in the context of Twitchell Island climate and the potential effects of climate change on rates of evapotranspiration among alfalfa in the Delta. Daily surface meteorological data was collected from the Daymet public database hosted by the Oak Ridge National Laboratory and from the AmeriFlux public database hosted by the Lawrence Berkeley National Laboratory. The results suggested that evapotranspiration rates in the area of Twitchell Island exhibit the sensitivity trends that are typical of arid and semi-arid regions. Evapotranspiration rates exhibited the highest responses to changes in vapor pressure deficit. Changes to windspeed and maximum daily temperature also resulted in considerable changes in evapotranspiration rates, primarily during summer months. Changes to minimum daily temperature and to solar radiation resulted in minimal response in evapotranspiration, due to the climate environment of Twitchell Island. In the context of these results, evapotranspiration rates of Delta alfalfa crops will be sensitive to higher maximum temperatures. The sensitivity analysis results were subsequently applied to downscaled climate change models for the Twitchell Island area to gauge the effects of increasing maximum temperature on evapotranspiration rates. It was found that ETR sensitivity to maximum temperature can potentially result in an increase of daily water consumption on Twitchell Island of 2.22-2.99 acre-feet of water consumption per day.