Daily estimation of local evapotranspiration using energy and water balance approaches

Abstract

Meteorological and environmental (i.e. soil water content) data measured from semiarid watersheds (Lucky Hills and Kendall) during the summer rainy and winter periods were used to study the interrelationships between variables, and to evaluate the effects of variables on the daily estimation of actual evapotranspiration (AET). The relationship between AET and potential evapotranspiration (PET) as a function of an environmental factor was the major consideration of this research. The relationship between AET and PET as a function of soil water content as suggested by Thornthwaite-Mather, Morton and Priestley-Taylor was studied to determine its applicability to the study area. Furthermore, multiple linear regression (MLR) analysis was employed to evaluate the order of importance of the meteorological and soil water factors involved. Finally, the information gained was used for MLR model development. The results of MLR analysis showed that the combined effects of available energy, soil water content and wind speed were responsible for 77 % of the observed variations in AET at Lucky Hills watershed and 70 % at Kendall watershed during the summer rainy period. The analyses also indicated that the combined effects of available energy, vapor pressure deficit and wind speed were responsible for 70 % of the observed variations in AET at Lucky Hills watershed and 72 % at Kendall watershed during the winter period. However, the test results of three different approaches, using the relationships between AET and PET as a function of soil water content indicated some inadequacy. The low correlation between PET, AET, and soil moisture conditions raised some doubt concerning the validity of methods developed elsewhere, and indicated the effects of energy availability on the relationship between PET, AET, and soil water content regardless of the soil water condition. In contrast, agreement between observed AET and estimated AET from MLR models during the summer rainy and winter periods at both watersheds indicated that MLR models can give reasonable estimates of AET, at least under the climatic conditions in which the formulae were developed.