Résumé
During the last three decades of the 20th century, the Sahelian region suffered an unprecedented drought period, both in intensity and duration. Despite a regain in annual precipitation, measurements show an intensification of climatic events, with a significant rise in the frequency of extreme events, both on temperatures and precipitation. This evolution deeply impacts the continental surfaces’ biogeochemical cycles, like the water and energy cycles. Moreover, the skyrocketing demography of Sahel makes it particularly vulnerable on the quantitative and qualitative management of the water resources, the food safety and the protection against hydroclimatic events. Proposing methods to insure the monitoring and projection of the water and vegetation resources is thus highly needed. Such method must rely on a deep understanding of the thermo-hydric behaviour of the main ecosystems and their response to global changes, climatic and/or anthropogenic. In this context, our study aims at analysing the impact of agricultural practices in a Soil-Vegetation-Atmosphere-Transfer Model (SVAT). Such model is a reference tool to quantify the surface waterand energy budget and assess their future evolution. The objectives are: i) to identify the key processes in the surface fluxes partition in Sahelian agrosystems, ii) to assess the durability of the actual practices and iii) to evaluate the levers for a durable adaptation of these practices. Our study focuses on the impact of the crop rotation cycles and associated soil practices traditionally used in agropastoral Sahel. The SiSPAT model, used in this study, has been calibrated on the flux measurement site of Wankama, in South-Western Niger. This site is located on a plot with an alternance of millet and shrubby savannah, which is representative of Sahelian agropastoral practices. The calibration is performed on a variety of hydrometeorological measurements, as the turbulent fluxes, the radiation measurements, the soil heat conduction and the temperature and humidity down to 2.5m in the soil. It helps constraining the different processes within the model. Performed year by year including the crop rotation, the calibration has highlighted the effect of the soil practices on the hydrodynamic properties of the superficial horizons, particularly the porosity and hydraulic conductivity. Results pointed a long-lasting (few years) improvement of the soil infiltration capacity. The partition between evaporation and transpiration is also affected, in favour of the transpiration due to the deeper storage of water in the root zone. The analyse of the inverse rotation (savannah to millet) is also ongoing. It will help understand the long-term impact of the entire rotation cycle on soil properties as well as the processes of infiltration and diffuse recharge of the aquifers. It will allow to assess the combined effect of the climate scenarios and agricultural practices scenarios on the future evolution of water and vegetation resources in Sahel.