Global estimation of evapotranspiration using a leaf area index-based surface energy and water balance model

Studies of global hydrologic cycles, carbon cycles and climate change are greatly facilitated when global estimates of evapotranspiration (E) are available. We have developed an air-relative-humidity-based two-source (ARTS) E model that simulates the surface energy balance, soil water balance, and environmental constraints on E. It uses remotely sensed leaf area index (L _ai) and surface meteorological data to estimate E by: 1) introducing a simple biophysical model for canopy conductance (G _c), defined as a constant maximum stomatal conductance g _smax of 12.2mms ^-1 multiplied by air relative humidity (R _h) and L _ai (G _c=g _{s max}×R _h×L _ai); 2) calculating canopy transpiration with the G _c-based Penman-Monteith (PM) E model; 3) calculating soil evaporation from an air-relative-humidity-based model of evapotranspiration (Yan & Shugart, 2010); 4) calculating total E (E ₀) as the sum of the canopy transpiration and soil evaporation, assuming the absence of soil water stress; and 5) correcting E ₀ for soil water stress using a soil water balance model.This physiological ARTS E model requires no calibration. Evaluation against eddy covariance measurements at 19 flux sites, representing a wide variety of climate and vegetation types, indicates that daily estimated E had a root mean square error=0.77mmd ^-1, bias=-0.14mmd ^-1, and coefficient of determination, R ²=0.69. Global, monthly, 0.5°-gridded ARTS E simulations from 1984 to 1998, which were forced using Advanced Very High Resolution Radiometer L _ai data, Climate Research Unit climate data, and surface radiation budget data, predicted a mean annual land E of 58.4×10 ³km ³. This falls within the range (58×10 ³-85×10 ³km ³) estimated by the Second Global Soil Wetness Project (GSWP-2; Dirmeyer et al., 2006). The ARTS E spatial pattern agrees well with that of the global E estimated by GSWP-2. The global annual ARTS E increased by 15.5mm per decade from 1984 to 1998, comparable to an increase of 9.9mm per decade from the model tree ensemble approach (Jung et al., 2010). These comparisons confirm the effectivity of the ARTS E model to simulate the spatial pattern and climate response of global E. This model is the first of its kind among remote-sensing-based PM E models to provide global land E estimation with consideration of the soil water balance.