Improving evapotranspiration estimates in Mediterranean drylands: The role of soil evaporation

An adaptation of a simple model for evapotranspiration (E) estimations in drylands based on remotely sensed leaf area index and the Penman-Monteith equation (PML model) (Leuning et al., 2008) is presented. Three methods for improving the consideration of soil evaporation influence in total evapotranspiration estimates for these ecosystems are proposed. The original PML model considered evaporation as a constant fraction (f) of soil equilibrium evaporation. We propose an adaptation that considers f as a variable primarily related to soil water availability. In order to estimate daily f values, the first proposed method (f(SWC)) uses rescaled soil water content measurements, the second (f(Zhang)) uses the ratio of 16 days antecedent precipitation and soil equilibrium evaporation, and the third (f(drying)), includes a soil drying simulation factor for periods after a rainfall event. E estimates were validated using E measurements from eddy covariance systems located in two functionally different sparsely vegetated drylands sites: a littoral Mediterranean semiarid steppe and a dry-subhumid Mediterranean montane site. The method providing the best results in both areas was f(drying) (mean absolute error of 0.17 mm day(-1)) which was capable of reproducing the pulse-behavior characteristic of soil evaporation in drylands strongly linked to water availability. This proposed model adaptation, f(drying), improved the PML model performance in sparsely vegetated drylands where a more accurate consideration of soil evaporation is necessary.