A Novel Method to Estimate Actual Evapotranspiration Using Soil Moisture and Meteorological Measurements

Evapotranspiration is a critical component in the hydrological cycle and natural water-energy nexus. ET rate depends on energy input, water availability, land cover, and near-surface turbulent transport. The widely used Penman-Monteith method for estimating ET assumes homogeneous surface condition and the stomatal conductance in the equation is difficult to measure. In this study, we proposed a novel approach to estimate ET based on the atmospheric demand of potential evaporation and a physically-based reduction coefficient over a variety of vegetated land surfaces. This reduction coefficient couples the determinative factors to ET explicitly by soil moisture and implicitly by the wet patch radius and the blending height. The model is tested against the dataset from eddy covariance sites in the AmeriFlux network. The results show that both water availability and atmospheric demand play determinative roles in ET over a vegetated surface. In water-limited condition, ET rate is sensitive to soil moisture since the plant controls the transpiration rate to conserve water due to water stress. While in energy-limited condition, increasing soil moisture will not promote ET rate as it is bounded by the lower atmospheric demand. The proposed new approach can be potentially extended to predict the spatial and physical patterns of ecosystem services under different hydroclimatic conditions.