Rainfall and river flow variability in Sub-Saharan Africa during the twentieth century

Conway, Declan, Persechino, Aurelie, Ardoin-Bardin, Sandra, Hamandawana, Hamisai, Dieulin, Claudine and Mahé, Gil (2009) Rainfall and river flow variability in Sub-Saharan Africa during the twentieth century. Journal of Hydrometeorology, 10 (1), 41-59. (doi:10.1175/2008JHM1004.1).

Abstract

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.

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