STUDY OF THE MECHANISMS OF EVAPORATION UNDER ARID CONDITIONS USING A DETAILED MODEL OF THE SOIL-ATMOSPHERE CONTINUUM - APPLICATION TO THE EFEDA-I EXPERIMENT

As part of the framework of the EFEDA (Echieval field experiment in a desertification threatened areal experiment, an 11-day data set includ ing atmospheric Forcing, turbulent fluxes and soil water monitoring wa s gathered on a bare soil site, characterized by a large diurnal cycle of temperature, and with very dry conditions near the surface, A vers ion of the SiSPAT (Simple Soil Plant Atmospheric Transfer) model, rest ricted to bare soil, was applied to this data set. The first objective was to validate model results against observations. This was restrict ed somewhat by the narrow range of soil moisture variations during the 11 days of the simulation. In addition some discrepancies were observ ed between total evaporation as calculated from neutron probe measurem ents and micrometeorological ones. However, because the SiSPAT results were more in agreement with evaporation calculated from soil water ba lance, it was assumed that the model could be used as a tool to invest igate the mechanisms of the water movement and evaporation within the soil under such dry conditions. In the first 25 cm of soil, vapour flu x is the most important transfer term, and the evaporation front (chan ge of phase from liquid to vapour) can be localized at this depth. The contribution of temperature gradients to mass Bur is also found to be important, and tends to lower the total evaporation at the surface by creating a downward vapour flux component which partly cancels the up ward one induced by large matric potential gradients at the soil surfa ce. (C) 1997 Elsevier Science B.V.