Space-time-frequency analysis of rainfall, runoff and temperature in the Casamance River basin, southern Senegal, West Africa

Using long-term data on rainfall and annual runoff, an investigation was made of the spatial and temporal variability of rainfall and runoff in the Casamance River basin located in southern Senegal, West Africa. A 5-year moving average was employed to identify trends in the data. Monthly and annual rainfall tends to have been decreasing, and the annual maximum temperature rising from around the mid-1960s. In addition, there has been a decreasing rainfall gradient from the coast to the interior. The analysis also showed a simultaneous runoff decline as a result of the rainfall decline. Available data on rainfall, temperature, and surface runoff from two upstream stations were used to develop a procedure for estimating runoff from the annual surplus values of precipitation minus reference evaporation. The resulting equations accounted for 74% to 77% of the variation in runoff. Spatial and temporal distribution of rainfall and runoff affects salinity migration inland and data are, therefore, useful for salinity management in the Casamance River basin.