Evapotranspiration model for semi-arid shrub-lands tested against data from SE Spain

A model was developed that predicted evaporation based on combined information on the physiology of overstorey and substrate, micrometeorological conditions and spatial distribution of plants. Predicted evapotranspiration was verified at stand level for Retama sphaerocarpa shrubs, and model parameters were tested to determine their importance in controlling evaporation. Model predictions were compared with evapotranspiration, measured by a Bowen Ratio Energy Balance system (BREB), and transpiration, measured by sap flow of the stems of the shrubs in south eastern Spain. Modifications made to the original two source clumped model in the tested model significantly improved agreement between predicted surface evapotranspiration rates and rates measured by the Bowen Ratio method. The modifications made to the model were improved parameterisation of soil surface conductance, a more detailed description of the radiation balance, and improved parameterisation of the soil aerodynamic conductance terms. Improvements in the soil surface conductance estimates made the most significant change to model predictions, the second two modifications showed no significant improvement in prediction of evapotranspiration, A sensitivity analysis indicated that relatively large variations of leaf area index or albedo caused little variation in evapotranspiration during the period measured, whereas variations in soil water content caused large changes in predicted evapotranspiration. Transpiration rates of shrubs (measured and modelled) indicated an independence from surface soil moisture (0-25 cm) supporting the view that R. sphaerocarpa had access to reserves of water deep in the soil which enabled it to survive and grow vigorously in this type of semiarid environment. Thus, it was concluded that land use changes which affect redistribution of water resources (overland and subsurface flow) may threaten the stability survival of R. sphaerocarpa stands. (C) 1999 Elsevier Science B.V. All rights reserved.