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RAINFALL AND RIVER FLOW VARIABILITY IN SUB-SAHARAN AFRICA DURING THE 20TH CENTURY

Congress: 2008
Author(s): Sandra Ardoin-Bardin, Aurélie Persechino, Claudine Dieulin, Gil Mahé
Declan Conway and Aurélie Persechino School of Development Studies, University of East Anglia, Norwich, NR4 7TJ, United Kingdom Sandra Ardoin-Bardin, Claudine Dieulin and Gil Mahé UMR HydroSciences Montpellier, IRD, BP 64501, 34394 Montpellier cedex

Keyword(s): Africa, climate, rainfall, river flows, variability, water resources
AbstractWe utilise rainfall records from a global high resolution product (CRUTS2.1) and extensive river flow records to characterise and better understand spatial and temporal variability in sub-Saharan African (SSA) water resources during the last century. Over twenty major international river basins, chosen primarily for their long good quality flow records, located in West, Central, East and Southern Africa comprising 11 million km2 (~45% of SSA’s land area) are examined. A range of statistical descriptors highlight substantial variability in rainfall and river flows (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 (greater in East and Southern Africa). No clear consistent signals in rainfall and runoff emerge across the whole of SSA. Detailed analysis of rainfall-runoff relationships reveals a wide range of results that include: strong, stable over time relationships (Blue Nile); strong but non-stationary relationships (many examples in West Africa), very weak almost random behaviour (particularly in Southern Africa, Zambezi and Olifants, but examples also occur in all other regions), and various idiosyncratic results which are difficult to explain. 20-year running correlations between rainfall and runoff tend to be higher during periods of greater rainfall station density. The early and latter decades of last century generally show weak results due to sparse coverage of rainfall stations and possibly poor data quality, particularly during recent decades for river flows. Non-stationary behaviour in West African rivers is associated with Sahel desiccation, and primarily reflects non-linear response in rainfall-runoff, but may include some effects from changes in land cover (natural and anthropogenic in origin). Non-stationary behaviour and data availability, coupled with some very poor observed rainfall-runoff relationships, present significant challenges to macro-scale hydrological modelling in Africa. Our findings confirm that rainfall variability in SSA is high but also that rainfall provides the dominant control, alongside river basin physiography and human interventions, on interannual and interdecadal variability in river flows and hence surface water availability. River flows in major basins show clear examples of significant variability that challenges the effective management of water resources and results in huge socio-economic costs. We identify a clear need to couple better understanding of the 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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