Evapotranspiration

The principal goal of evapotranspiration (ET) research is to find methods for calculating the value of ET under any given conditions. Progress in the United States since 1970 has centered on understanding the water relations of plants, particularly the limitation of transpiration by variable stomatal aperture. Readers who compare this report with the preceding one [ Ekern , 1971] will notice a change in emphasis from ET as a physically controlled process to ET as a physiologically controlled process. This is not due solely to the bias of the author but represents a real shift in ET research toward the stomata and their operation.