A complementary relationship between actual and potential evapotranspiration and soil effects

A complementary relationship (CR) between potential (ETp) and actual (ETa) 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 ETp and ETa 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 ETa and ETp values, MODIS_ET tends to underestimate ETa in wet years and systematically overestimate ETp across Nebraska, which leads the wet surface evapotranspiration (ETw) to decrease with increasing annual humidity index Phi (i.e., the ratio of annual precipitation over annual ETp). In comparison, ETw estimated from the vadose zone model is similar to ETw calculated from the Priestley-Taylor equation, and remains relatively constant over a wide range of annual Phi with a mean of 105.1 cm/year. Both approaches show that ETw 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. (C) 2012 Elsevier B.V. All rights reserved.