Bernard Cappelaere

A GIS for hydrological modelling in the semi-arid, HAPEX-Sahel experiment area of Niger, Africa

During the HAPEX-Sahel experiment (1991–94), water redistribution processes were studied at the meso-scale (10 000 km2) near Niamey, Niger. A project now under way at ORSTOM aims at modelling the regional water balance through a spatial approach combining GIS data organization and distributed hydrological modelling. The main objective is to extend the surface water balance, by now available only on a few, small (around 1 km2) unconnected endoreic catchments, to a more significant part of the HAPEX-Sahel square degree, a 1500 km2 region called SSZ that includes most of the environmental and hydrology measurement sites. GIS architecture and model design consistently consider data and processes at the local, catchment scale, and at the regional scale. The GIS includes spatial and temporal hydrological data (rainfall, surface runoff, ground water), thematic maps (topography, soil, geomorphology, vegetation) and multi-temporal remote sensing data (SPOT, aerial pictures). The GIS supports the simulation of the composite effect at the regional scale of highly variable and discontinuous component hydrologic processes operating at the catchment scale, in order to simulate inter-annual aquifer recharge and response to climatic scenarios at the regional scale.

A New Land Surface Hydrology within the Noah-WRF Land-Atmosphere Mesoscale Model Applied to Semiarid Environment: Evaluation over the Dantiandou Kori (Niger)

Land-atmosphere feedbacks, which are particularly important over the Sahel during the West African Monsoon (WAM), partly depend on a large range of processes linked to the land surface hydrology and the vegetation heterogeneities. This study focuses on the evaluation of a new land surface hydrology within the Noah-WRF land-atmosphere-coupled mesoscale model over the Sahel. This new hydrology explicitly takes account for the Dunne runoff using topographic information, the Horton runoff using a Green-Ampt approximation, and land surface heterogeneities. The previous and new versions of Noah-WRF are compared against a unique observation dataset located over the Dantiandou Kori (Niger). This dataset includes dense rain gauge network, surfaces temperatures estimated from MSG/SEVIRI data, surface soil moisture mapping based on ASAR/ENVISAT C-band radar data and in situ observations of surface atmospheric and land surface energy budget variables. Generally, the WAM is reasonably reproduced by Noah-WRF even if some limitations appear throughout the comparison between simulations and observations. An appreciable improvement of the model results is also found when the new hydrology is used. This fact seems to emphasize the relative importance of the representation of the land surface hydrological processes on the WAM simulated by Noah-WRF over the Sahel.

Interactions entre surface et convection au Sahel

Strong interactions and feedbacks between nsurface processes and deep nconvection occur in the Sahel. They ntake place over a wide range of scales. nThey are found to enhance surfaceatmosphere nenergy exchanges during nthe monsoon and to generate a large nvariability of surface sensible and nlatent heat fluxes in time and space.A npositive feedback is observed between nthis variability, or more accurately soil nmoisture heterogeneities, and the frequency nof initiation of convective systems. nThis feedback operates at fine nscale, on the order of a few tens of kilometres. nThe underlying mechanisms nand their modelling are discussed.

Le cycle de l'eau dans le système de mousson d'Afrique de l'Ouest

Improving our knowledge of the water cycle in the West Africanmonsoon system and the way it is represented in numerical models is one of the major goals of the AMMA programme. The water cycle results from complex interactions between the atmosphere and the continent, with contrasted behaviour depending on the region and the space and time scales at which it is analysed. The properties of the surface, the soil and the sub-soil strongly drive water redistribution over the continent and towards the atmosphere, resulting in complex feed back loops that are still not fully understood. Process studies and water budgets computed from a mixture of observations and simulation products provided advances in the knowledge of thesemechanisms. Some of the processes still remain uncertain, such as the links between evapotranspiration, vegetation and ground water storage. Beyond a better knowledge of the monsoon system, model improvement, better numerical weather predictions and the development of tools for water resources assessment and management are among the main applications of these studies.