On the ability of surface model to reproduce Soil moisture dependencies over West Africa: The ALMIP experiments

Abstract:

From the AMMA-CATCH Observatory located in West Africa, a complete data set of surface exchanges over various vegetation landcovers is analysed. It is shown these exchanges, especially the Evapo-transpiration (ETR), primarily rely on surface soil moisture at annual time scales because of the seasonal cycle of the West African monsoon and the corresponding precipitation. However, at finer time scales, soil moisture dependencies are much more complex and no simple relation exists between ETR and soil moisture, especially when vegetation has developed. If the presence of adequate soil moisture is a prerequisite for any ETR rate, the interactions between all surface processes render the use of simple models inadequate.

Several illustrations will be presented based on data analysis and ALMIP experiments. One of them concerns the surface model response at daily time step after a rain event. It is shown that, for all the experimental sites, the immediate surface response is mainly dependent on the soil moisture content especially for bare soil conditions. The ALMIP model ensemble broadly reproduce the ETR annual cycle but depicts a broad range of relationships between Evaporative Fraction and soil moisture, with the worst results for the driest sites.

The second illustration concerns a comparison of local modelling results with in situ data. It is shown that the CLM model parameterisations are able to reproduce surface conductances. For bare soil conditions, the soil moisture dependency follows the Sakagushi law. During the vegetated period (over fallow), the evaporation rate is no longer controlled by soil moisture but by the radiative budget of the canopy and the amount of energy that reaches the ground. Finally, the Ball & Berry conductance model is able to reproduce transpiration rate reasonably well. This model is a function of incoming radiation and soil moisture, but also leaf temperature which impacts all of the surface energy exchanges.