Use of meteorological satellites for water balance monitoring in Sahelian regions

Abstract

This study represents part of an EEC research programme to monitor crop water conditions during the south Saharan rainy season using satellite data. Primarily focused on the Senegalese area, it was mainly performed using Meteosat daytime thermal infrared data. The purpose was to establish a relationship between the satellite-derived land surface temperatures (T_s ) and the actual evapotranspiration (ET). After a preliminary study based on data from 1979 and 1983, the emphasis was put on the summers of 1984 and 1985. About thirty dates were selected for cloud-free conditions for each rainy season. Resulting surface temperatures were compared to measured rainfall and computed ET from the IRAT water-balance model for ten agrometeorological stations on a north-south transect depicting a steep water-balance gradient. The linear relationship obtained between Σ T_s and the cumulative rainfall is shown and the possible applications to rainfall mapping are discussed. This methodology is shown to be feasible, in spite of problems with the computation of absolute T_s , values. ET maps and rough estimates of millet yield so derived are presented tentatively. An extension to the whole Sahelian zone is proposed, using both Σ T_s , products elaborated from Meteosat routine processing and Global Vegetation Index data from NOAA-AVHRR (Advanced Very High Resolution Radiometer). Their time integrations over the whole rainy season are linearly related to cumulative rainfall which displays their complementary potential for monitoring crop water status.