Evapotranspiration Response to Climate Change

While evapotranspiration rates are known to increase with higher temperature, other factors in addition to rising temperatures also affect evapotranspiration (ET). For example, increasing humidity and higher CO2 concentrations both tend to reduce transpiration and counteract the higher temperature effects on ET. As the oceans and other water bodies warm and evaporate more water into the atmosphere, global humidity is likely to increase. As CO2 concentrations increase, leaf stomata partially close in response to maintain the CO2 concentration inside the stomata. Thus, while climate change is likely to increase air temperature, the effect of higher humidity and CO2 concentration could partially offset the temperature effect on ET. Using the standardized Penman-Monteith (PM) equation for short canopy reference evapotranspiration (ET0) and scenarios for possible changes in temperature, humidity, and CO2, we will report on the likely impact of these changes on ET0. The stomatal resistance of the reference surface (0.12 m tall cool-season grass) was estimated for 550 ppm CO2 by volume from the FACE studies, and the canopy resistance was estimated using the same procedures as those used to determine the PM equation. The minimum temperature was increased by 4 degrees C and the maximum temperature by 2 degrees C over current conditions. Dew point temperature increases of 0, 2, and 4 degrees C were evaluated. It was assumed that aerodynamic resistance will remain unchanged and the grass does not experience water stress. If the canopy resistance increases in response to higher CO2 and the dew point temperature increases at about the same rate as the minimum temperature, the results indicate that little or no change in ET0 is likely due to increasing air temperature. The impact of global warming on ET0 will likely be less in locations with higher wind speeds.