Measurement of Monthly Evapotranspiration from Grass and Estimates of Potential Evapotranspiration

Evapotranspiration in a sand dune area (I)

Abstract

Measurement of monthly evapotranspiration from grass by a nonweighing lysimeter in the Tottori sand dunes, 1972 and 1973, showed maximum daily mean values of 6.1 mm/day in June, 1972 and 6.7 mm/day in August, 1973 and 3.5 mm/day and 3.6 mm/day in annual average, respectively. The relationship between evapotranspiration and solar radiation represented a “loop effect” due to thermal lag behind radiation.The liner relationship was obtained by use of solar radiation weighted according to air temperature.The potential evapotranspiration was estimated by the routine meteorological data (1952-1975) based on the air temperature method, the solar radiation method, the combination method and the pan evaporation method.The prediction methods were calibrated against the measured evapotranspiration, 1972 and 1973.The results obtained from the solar radiation method, the combination method and the pan evaporation method represented similar seasonal patterns of the maximum values in August and of the dip in June due to the rainy season, while, with the air temperature method, the maximum values occurred in July and the values in June were larger than in May.It is felt that the combination method and the solax radiation method offered the best results for predicting potential evapotranspiration.The pan evaporation method and the air temperature method may be graded next, since the accuracy of estimates by the former could be influenced by siting, and since the latter includes only two factors such as air temperature and maximum bright sunshine hours.