UNIDIMENSIONAL MODELING OF A FALLOW SAVANNA DURING THE HAPEX-SAHEL EXPERIMENT USING THE SISPAT MODEL

In the framework of the HAPEX-Sahel experiment, a data set was gathere d on a fallow savannah site of the Central East Supersite. This includ es 54 days of atmospheric forcing (air temperature and humidity, wind speed, solar and long-wave radiation and rainfall), net radiation, sen sible, latent and soil heat fluxes and soil temperature series at a ti me step of 20 min. Furthermore, 17 soil moisture profiles, the evoluti on of the leaf area indices and some soil characteristics were availab le. The data set was used, at the field scale, to calibrate and valida te the SiSPAT (simple soil plant atmosphere transfer) model, a 1D mode l of coupled heat and mass transfer in the soil-plant-atmosphere conti nuum. The objectives of the study were (i) to assess the performances of the model in the prediction of the diurnal cycle of net radiation, turbulent fluxes, soil temperatures and the evolution of soil water co ntent over a period of 54 days (day of the year 239-292, 1992), charac terized by early stage intense rainfall events and fast drying afterwa rds, (ii) to analyse the influence of soil surface crust on the water balance and (iii) to identify the 1D modelling limits when the surface area consists of two strates: a ground sparse herb laver, characteriz ed by a large spatial variability of surface properties and water cont ent with scattered bushes. The model was calibrated over a 2-week peri od and then nln over the whole 53-day period. We were able to reproduc e the main characteristics of the observed net radiation, turbulent fl uxes, soil temperature and soil moisture for the intense rainfall even ts and for an elongated dry period. Nevertheless, when the crust was n ot taken into account, the rainfall-runoff-infiltration process and th e evapotranspiration after rain were poorly predicted (overestimation of evapotranspiration and infiltration). When a crust was considered t o model the water balance at the field scale, its influence was found to be substantial on the runoff generation and the infiltration, and c onsequently on the bare soil evaporation. However, runoff predictions were much larger than the observations, indeed, at the field scale, no runoff was generally observed. Lateral redistribution of water betwee n crusted and non-crusted zones was observed in the plot. However, thi s cannot be taken into account with the presented 1D deterministic mod elling. Hence further model development is needed to yield a better re presentation of soil water fluxes at the field scale.