EVAPOTRANSPIRATION FROM DOUGLAS-FIR STANDS EXPOSED TO SOIL-WATER DEFICITS

The rate of evapotranspiration from thinned and unthinned stands of Douglas fir was measured using energy and water balance methods. At high values of soil water storage in the root zone the evapotranspiration rate was approximately 80% of the equilibrium evaporation rate. Below a critical value of soil water storage the ratio of the evapotranspiration rate to the equilibrium evaporation rate (E/Eeq) tended to decrease linearly with decreasing soil water storage. The critical values of soil water storage in the root zone were 11.8 and 8.3 cm for the thinned and unthinned stand, respectively. Below these critical storage values, there was approximately 3.5 cm of water remaining in both root zones that was extractable by the trees. The relationship between E/Eeq and the fraction of extractable water in the root zone for both stands was very similar for sunny days. In this relationship, E/Eeq began to decrease when there was approximately 40% of the extractable water remaining in the root zones of both stands. Copyright 1979 by the American Geophysical Union.