MODELING THE EFFECT OF ELEVATED CO2 AND CLIMATE CHANGE ON REFERENCE EVAPOTRANSPIRATION IN THE SEMI-ARID CENTRAL GREAT PLAINS

Changes in evapotranspiration demand due to global warming will have a profound impact on irrigation water demand and agricultural productivity. In this study, the effects of possible future anthropogenic climate change on reference evapotranspiration (ETo) were evaluated using the Penman-Monteith equation. The combined effect of temperature and elevated CO2 concentrations on ETo was the major focus of this study. The ETo under the General Circulation Model (GCM) projected climate change scenarios was estimated for a location in Colorado. Multi-model ensemble climate change scenarios were generated from 112 Bias Corrected and Spatially Disaggregated (BCSD) projections from the World Climate Research Program (WCRP) archive, which cover different levels of greenhouse gas emissions. Results showed a decrease in ETo demand with increases in CO2 levels, which greatly moderated the increase in ETo due to increasing temperature. The effect of increases in CO2 levels up to 450 ppm off set the effect of about 1 degrees C rise in temperature. Simulation results with projected climate change scenarios, without considering the effects of CO2 levels, showed an 8.3%, 14.7% and 21.0% increase in annual ETo during the 2020s, 2050s, and 2080s, respectively, when simulation was carried out using an ensemble of the 112 projections. When the effect of elevated CO2 levels was also considered in combination with projected changes in temperature, changes in annual ETo demand varied from -1.5% to 5.5%, -10.4% to 6.7%, and -19.7% to 6.6% during the 2020s, 2050s, and 2080s, respectively, depending on the different climate change scenarios considered and the relationship or equation used for estimating the effect of elevated CO2 on stomatal resistance term in the Penman-Monteith equation.