Is Potential Evapotranspiration and Its Relationship with Actual Evapotranspiration Sensitive to Elevated Atmospheric CO2 Levels

Abstract

The possibility is examined that potential evapotranspiration values may be sensitive to changes in atmospheric carbon dioxide content. Enhanced levels of atmospheric CO2 increase water use efficiency of vegetation by improving growth rates and suppressing transpiration per unit leaf area. Highly cultivated crops without water or nutrient constraints are able to show the greatest growth improvements. In many natural or semi-natural ecosystems, under enhanced atmospheric CO2 concentrations, limits on the availability of soil nutrients severely constrains the possibility of improvements in growth and significant increases in leaf area index that could compensate for a decrease in transpiration per unit leaf area. Thus, in many natural or semi-natural ecosystems, which often form water gathering grounds in river basins, enhanced levels of CO2 will suppress transpiration and perhaps increase the proporation of precipitation that forms runoff or ground water. In low vegetation covers, such as grassland, the rates of transpiration and also evaporation from canopies that are wet after rainfall (interception loss) are very similar. In these canopies, evapotranspiration is unlikely to be significantly increased by small increases in leaf area index. It is suggested that the suppression of potential evapotranspiration by enhanced CO2 levels will be small, but that actual transpiration from tall, slow growing vegetation covers may be significantly suppressed. Thus for some vegetation covers the relationship between actual and potential evapotranspiration may be sensitive to CO2 levels. If this is so, it could be of importance to many water balance calculations. The suppression of evapotranspiration by enhanced CO2 levels will be most noticeable in dry climates where interception loss is insignificant and largely masked in very wet climates where a large proportion of evapotranspiration consists of interception loss.