SENSITIVITY OF EVAPOTRANSPIRATION AND SOIL-MOISTURE IN WHEAT FIELDS TO CHANGES IN CLIMATE AND DIRECT EFFECTS OF CARBON-DIOXIDE

A physically based model of moisture transfer in the soil-canopy-atmosphere system, fitted to measured data in wheat fields, was applied to calculate the sensitivity of evapotranspiration to the combined effect of possible CO2-induced climate changes and the direct impact of doubling CO2 content in the atmosphere. The impact of carbon dioxide was taken into account by changing the stomatal resistance and the leaf area index. Several climate change scenarios were used. Simulation studies showed that when the changes of a number of climatic and plant factors are considered, evapotranspiration responses can differ greatly from those that consider only temperature change. The temperature effect of the increase in CO2 content in the atmosphere can be compensated by changes in the physiological parameters of vegetation. Thus, under certain conditions, one can expect a decrease in evapotranspiration instead of an increase such as is predicted by a number of models which neglect the direct effects of CO2 on plant behaviour.