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

A model was developed that predicted evaporation based on combined informat ion on the physiology of overstorey and substrate, micrometeorological cond itions and spatial distribution of plants. Predicted evapotranspiration was verified at stand level for Retama sphaerocarpa shrubs, and model paramete rs were tested to determine their importance in controlling evaporation. Mo del 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 t he original two source clumped model in the tested model significantly impr oved agreement between predicted surface evapotranspiration rates and rates measured by the Bowen Ratio method. The modifications made to the model we re improved parameterisation of soil surface conductance, a more detailed d escription of the radiation balance, and improved parameterisation of the s oil aerodynamic conductance terms. Improvements in the soil surface conduct ance estimates made the most significant change to model predictions, the s econd 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 evapotra nspiration during the period measured, whereas variations in soil water con tent caused large changes in predicted evapotranspiration. Transpiration ra tes of shrubs (measured and modelled) indicated an independence from surfac e soil moisture (0-25 cm) supporting the view that R. sphaerocarpa had acce ss to reserves of water deep in the soil which enabled it to survive and gr ow vigorously in this type of semiarid environment. Thus, it was concluded that land use changes which affect redistribution of water resources (overl and and subsurface flow) may threaten the stability survival of R. sphaeroc arpa stands. (C) 1999 Elsevier Science B.V. All rights reserved.