9706403 Xue There are two distinct hydrological phenomena in the Sahel: a persistent drought, which is very likely to have been partially caused by the land surface change, and a significant seasonal variation in the surface water balance. Simulations that couple a NCEP GCM with a biosphere model (SSiB), which has been calibrated and validated against HAPEX and SEBEX data, will be conducted to investigate influences of land-surface conditions on hydrological processes and regional climate variability in the Sahel, especially the impact of land cover on long-term Sahel drought patters. The simulations will use land property data and land cover data from field measurements and satellite observations to evaluate the integration between regional and local moisture sources in determining rainfall. The impacts of heterogeneity of vegetation distribution on land-atmosphere interactions will be explored. It will be followed by the development of a methodology to aggregate land-surface parameters to GCM grid boxes or sub-grid box. The study will focus on how land condition changes seasonal patterns in rainfall, ET, Soil water, and runoff. The results will be confirmed through comparisons with observed changing rainfall, surface temperature, and in particular, the river discharge. A river routing model will be used to convert the runoff values produced by the land surface model in each grid cell of a GCM to the basin outlet. The diagnostic analyses will be made to determine sources of Sahel precipitation, and relative contributions of each evaporation source to the rainfall and its anomalies in the Sahel. Attention will also be paid to study the impacts of land surface processes on AFJ dynamics and wave propagation. The significance of the land surface effects will be tested with respect to various patterns of land forcing anomalies, which include different degrees of land surface degradation and different percentage of vegetation cover changes in one GCM grid box.
