Developing Index Maps of Water-Harvest Potential in Africa

The food security problem in Africa is tied to the small farmer, whose subsistence farming relies heavily on rain-fedagriculture. A dry spell lasting two to three weeks can cause a significant yield reduction. A small-scale irrigation schemefrom small-capacity ponds can alleviate this problem. This solution would require a water harvest mechanism at a farm level.In this study, we looked at the feasibility of implementing such a water harvest mechanism in drought prone parts of Africa.A water balance study was conducted at different watershed levels. Runoff (watershed yield) was estimated using the SCScurve number technique and satellite derived rainfall estimates (RFE). Watersheds were delineated from the Africa-wideHYDRO-1K digital elevation model (DEM) data set in a GIS environment. Annual runoff volumes that can potentially bestored in a pond during storm events were estimated as the product of the watershed area and runoff excess estimated fromthe SCS Curve Number method. Estimates were made for seepage and net evaporation losses. A series of water harvest indexmaps were developed based on a combination of factors that took into account the availability of runoff, evaporation losses,population density, and the required watershed size needed to fill a small storage reservoir that can be used to alleviate waterstress during a crop growing season. This study presents Africa-wide water-harvest index maps that could be used forconducting feasibility studies at a regional scale in assessing the relative differences in runoff potential between regions forthe possibility of using ponds as a water management tool.