The seasonal change of the water budget in the Congo river basin is investigated by using hydrometeorological data averaged over long-term periods. Vapor flux convergence is calculated using the global objective analysis data of the ECMWF from 1985 to 1988. Precipitation and river discharge data mainly cover the periods 1920-1960 and 1932-1959, respectively. Evapotranspiration is estimated as precipitation minus vapor flux convergence on the monthly basis. The atmospheric water balance terms are related to the Normalized Difference Vegetation Index (NDVI) derived from the NOAA/AVHRR averaged from 1985 to 1987.
On the monthly basis, the NDVI and evapotranspiration are in phase with the seasonal change of precipitation in the evergreen forest region, which mainly covers the northern part of the basin. In contrast, the NDVI and evapotranspiration lag precipitation by one month in the southern deciduous forest region covering the southern part of the basin. As for the entire basin, the lag-relationship between the NDVI/evapotranspiration and precipitation is similar to that for the southern deciduous forest region.
In the dry season of the southern deciduous forest region, evapotranspiration exceeds precipitation in the entire basin, causing a decrease of the basin storage to its minimum value. In addition, from the viewpoint of the seasonal change of precipitation and evapotranspiration, it is concluded that the feature of the seasonal change of the water budget in the entire basin mainly reflects the characteristics of the southern deciduous forest region.