Evaluation of Sahel ground climatology from 1982 to 1990 based on satellite derived LAI, 200 raingauge stations, and a vegetation model (SSIB)

The Sahel has experienced a severe and persistent drought since the beginning of the 1960s. Moreover in the same period, the land surface degradation associated with human pressure dramatically increased. In this study, we use observational precipitation data, satellite derived LAI, and a land surface model (SSiB) to evaluate the ground climatology over this region. 217 raingauge stations have been selected from the IRD (Institut de Recherche pour le Developpement-France) daily rainfall database over West Africa, tested and added with other sources. The choice of the stations is based on three different criteria: the maximization of the spatial coverage (2°N-20°N and 18°W- 25°E), the maximum period length without any gap (1982-90), and the maximum variation of different associated vegetation types (7 types are included). The stations have been interpolated into 1°x1° grid boxes. The land cover map used to determine standard surface parameter values was from the global land cover database of Hansen et al. (2000). Leaf Area Index (LAI) and vegetation cover parameters were derived from available AVHRR NDVI satellite data (Los et al., 2001). The remaining forcing variables were derived from NCEP/NCAR Reanalysis data (Kalnay, 1996) and interpolated from 6-hour values to hourly values during this time period. These forcing data will be used to drive an offline version of the model SSiB (Xue et al., 1991) to produce surface meteorological and hydrological variables. This paper discusses the seasonal and interannual variability of surface water and energy balances in West Africa. The spatial and temporal patterns of surface fluxes are analyzed. In addition, a comparison is made with reanalysis variables to evaluate the coherence of these two products. Moreover, correlation analyses are done with the main SST signals in the Pacific and Atlantic oceans and LAI data. To investigate the impact of land surface degradation in West Africa, sensitivity experiments are conducted. They are performed separately on the Sahel zone and Guinea zone and focuses on the impact of land cover change on water and energy balances in the west and east parts of each zone. These experiments show the importance of the modification of the land surface characteristics on the land surface-atmosphere relationship