Abstract
The Sahel region is particularly exposed to the variability of the West African Monsoon, which may lead to dramatic socio-economical consequences. This region also has one of the highest demographic growth rates, resulting in an ever-increasing pressure on the scarce natural resources and fragile environment. In this context, a major challenge lies in our ability to provide appropriate tools for the monitoring of hydrological and vegetation resources. These tools should also be suitable for the prediction of climatic and anthropogenic impacts in the medium term. This requires a better understanding of energy and matter transfer processes at the earth-atmosphere interface. Indeed, the latter both play a role in the regulation of the monsoon and also drive the local hydrological cycle and vegetation development. The present research follows such a framework and consists specifically in analyzing the water and energy cycles in the Sahel region under the combined effects of climate and human activity. This is undertaken by developing a methodology combining the use of in situ observations and physically-based modelling. The study was conducted in the central Sahel, where traditional agricultural systems are formed by the association of pastoralism and rain-fed crops in rotation with fallow cycles. This study was based on the network of eco-hydrological and energy data acquired continuously during 7 years (2005-2012) by the South-West Niger AMMA-CATCH Observatory. Quality and consistency of these observations allowed analyzing the main features of the eco-hydrological functioning of the two main land-covers in the region: millet and fallow savannah. However, observations alone were not sufficient to compute comprehensive water and energy budgets at all the different time scales of interest (sub-daily to inter-annual). A detailed modelling of coupled water and energy cycles was therefore undertaken for these two land-covers, using the soil-vegetation-atmosphere transfer model SiSPAT. The model was first calibrated on a 2-year period, and further validated on the remaining 5-year observations, by constraining model parameters to physically realistic values. This multi-year modelling was in good agreement with the observations, and provided a precious analysis tool that integrated the relevance, richness and consistency of the dataset. Thanks to the representativeness of the studied period, results served at the different temporal scales to (1) analyze the impact of climatic variability on water and energy budgets and (2) produce a preliminary climatology for the water and energy fluxes and storages at the soil-vegetation-atmosphere interface. Similarities and differences in eco-hydrological and energy functioning between ecosystems were evidenced. For instance, evapotranspiration represented more than 80% of annual precipitations and close to half of the global radiation at the heart of the monsoon for both sites. Seasonal distribution and partitioning of evapotranspiration between soil evaporation and plants transpiration differed between the two ecosystems, as well as the runoff, and the drainage below the root zone which appeared significant for the millet field but not for the fallow site. A sensitivity analysis of the energy and water budgets to soil and vegetation characteristics was conducted. Robustness of the produced results should enable them to serve as reference for studies of water and energy processes in this region. The resulting calibrated model showed an obvious potential for prospective studies, such as those on climate change or on the evolution of agricultural practices.