A complementary relationship between actual and potential evapotranspiration and soil effects

Abstract

Summary A complementary relationship (CR) between potential ( ET p ) and actual ( ET a ) evapotranspiration is obtained using two independent approaches, and the role of soils in formulating the CR is also investigated. The first approach employs a process-based vadose zone model that links ET p and ET a by explicitly considering the role of soils. To drive the model, hydrometeorological, soil, and remotely sensed physiological data are utilized. The second approach utilizes the products of the MODIS_ ET (MOD16) data set that is derived from remotely sensed data by mainly considering above-ground land surface processes. Both methods are applied for 42 sites across Nebraska, USA. The results from both approaches demonstrate the CR; however, according to previously reported ET a and ET p values, MODIS_ ET tends to underestimate ET a in wet years and systematically overestimate ET p across Nebraska, which leads the wet surface evapotranspiration ( ET w ) to decrease with increasing annual humidity index Φ (i.e., the ratio of annual precipitation over annual ET p ). In comparison, ET w estimated from the vadose zone model is similar to ET w calculated from the Priestley–Taylor equation, and remains relatively constant over a wide range of annual Φ with a mean of 105.1 cm/year. Both approaches show that ET w is not sensitive to soil properties, which provides a rationale for combining historical data from different locations to study the impact of climate variability on evapotranspiration processes at annual time scales.