Analysis of transpiration results from the RICE and PILPS workshop

Results from the 14 land surface parameterization schemes involved in the PILPS-RICE Workshop are compared for a soya crop growing season (from June to September). During this period, the transpiration flux dominates the total surface evapotranspiration and observed data from HAPEX-MOBILHY are available for comparison. Results indicate that during the month of June half of the models fall within the uncertainty range of the observations. The scatter between models behaviour is explained by three major reasons: The functional dependency between soil moisture and transpiration; the initial moisture content at the beginning of the period; the vertical discretization within the soil and the extension of the root system that defines the soil water holding capacity for plants Examination of diurnal cycles of evaporation reveals that formulations based on the supply-demand concept are very sensitive to the specification of the root zone. This analysis underlines the need for more sensitivity experiments to be done with the current forcing data set and more detailed datasets to be collected in future field experiments (e.g. latent heat flux during all the growing season, root zone distribution).