Rainfall and water resources variability in sub-Saharan Africa during the 20th century
River basin rainfall series and extensive river flow records are used to characterise and improve understanding of spatial and temporal variability in sub-Saharan African (SSA) water resources during the last century. Nine major international river basins (comprising ~32% of SSA’s area), chosen primarily for their long good quality flow records are examined.
A range of statistical descriptors highlight: substantial variability in rainfall and river flows (e.g. differences in rainfall [flows] of up to -14% [-51%] between 1931-60 and 1961-90 in West Africa); marked regional differences; and modest intra-regional differences. On decadal time scales sub-Saharan Africa exhibits drying across the Sahel after the early 1970s, relative stability punctuated by extreme wet years in East Africa, and periodic behaviour underlying high interannual variability in Southern Africa. Central Africa shows very modest decadal variability with some similarities to the Sahel in adjoining basins. No consistent signals in rainfall and river flows emerge across the whole of SSA.
Detailed analysis of rainfall-runoff relationships reveals varying behaviour including: strong but non-stationary relationships (particularly in West Africa), many basins with marked variations (temporal and spatial) in strength, weak almost random behaviour (particularly in Southern Africa), and very few strong, temporally stable relationships. 20-year running correlations between rainfall and river flow tend to be higher during periods of greater rainfall station density, however, there are cases where weak (strong) relationships exist even with reasonable (poor) station coverage. Non-stationary behaviour in West African rivers is associated with Sahel desiccation and primarily reflects the non-linear runoff response to rainfall, but may include some effects from changes in land cover. We conclude for SSA that robust identification and attribution of hydrological change is severely limited by data limitations, conflicting behaviour across basins/regions, low signal-to-noise ratios, sometimes weak rainfall-runoff relationships and limited quantification of the magnitude and potential effects of land use change and other anthropogenic influences.
We identify a clear need to integrate better understanding of biophysical drivers of variability (e.g. ENSO, Sahelian desiccation) with actions to strengthen the capacity of African water managers to deal with climatic variability and extremes.
Conway, D., A. Persechino, S. Ardoin-Bardin, H. Hamandawana, M. E. Dickson, C. Dieulin, and G. Mahe
