Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 2, 2012
Publication Date: N/A
Interpretive Summary: Climate scientists predict significant increases in global surface temperatures (between 1.1 to 6OC) by the turn of the current century owing to enhanced green house gas (CO2, methane, Nitrous oxides, chlorofluorocarbons, etc.) emissions from human activities over the earth. Increased warming of the earth can significantly change various components of the water (hydrological) cycle in the land-ocean-atmosphere system. Evapotranspiration (ET) is the major component of the hydrological cycle and an important factor affecting irrigation water demand, availability and management. If a region becomes warmer and leads to increased evaporative demand, more irrigation water will be required to maintain the yields. ET represents the simultaneous loss of water to the atmosphere by transpiration and evaporation from a cropping system. In this study, possible future CO2 increases and global warming effects on ET in a semiarid climate at Greeley, Colorado in the Great Plains of USA has been evaluated. The Penman-Monteith equation for reference crop ET (ETo) equation was used for studying the general sensitivity of ET to changes in climatic variables and CO2 concentrations. Climate change scenarios were generated by averaging 112 projections of the World Climate Research Programme (WCRP) Coupled Model Inter-comparison Project phase 3 (CMIP3) results, covering different possible current-future levels of greenhouse gas emissions scenarios. Decrease in ETo demand with increase in CO2 concentration (due to decrease in transpiration caused by stomata closure with increase in CO2) can be offset by increasing ET due to global warming. The study showed that changes in annual ETo demand due the global change at Greeley, Colorado can vary between -1.5 and 5.5%, and -19.7 and 6.60% during 2020 and 2080, respectively, under various CO2 emission and global warming scenarios.
Technical Abstract: Changes in evapotranspiration demand due to global warming will have profound impact on irrigation water demand and agricultural productivity. In this study, effects of possible future anthropogenic climate change on reference evapotranspiration (ETo) was evaluated. The Penman-Monteith equation was used for studying the general sensitivity of ETo to changes in climatic variables and CO2 concentrations, and for estimation of ETo under GCM projected climate change scenarios for a location in Colorado. Multi-model ensemble climate change scenarios were generated from 112 projections of the World Climate Research Programme (WCRP) Coupled Model Inter-comparison Project phase 3 (CMIP3), covering different levels of greenhouse gas emissions. Results of sensitivity analysis showed decrease in ETo demand with increase in CO2 concentration but this decrease in ETo was offset by increasing temperature. The effect of increase in CO2 level up to 450 ppm is offset by about 1 °C rise in temperature. Simulation results with projected climate change scenarios, without considering effect of CO2, showed 8.3, 14.7 and 21.0% increase in annual ETo during 2020, 2050, and 2080, respectively, when simulation was carried out using ensembles of 112 projections. When the effect of elevated CO2 was also considered in combination with projected changes in temperature, changes in annual ETo demand varied from -1.5 to 5.5% and -19.7 to 6.60% during 2020 and 2080, respectively, depending upon the climate change scenarios considered and the relationship/equation used for estimating stomatal resistance term in the Penman-Monteith equation.