Since groundwater supplies are less correlated with rainfall than surface supplies, one of groundwater's key functions can be its ability to mitigate the effects of erratic rainfall or drought on agricultural production. While this function is of global importance, it may be especially so in SSA where temporal rainfall variability, as outlined earlier, is amongst the highest in the world. In fact, African pastoral societies have taken advantage of groundwater to mitigate the impact of temporal variation in rainfall supply for centuries. The focus is now on the role of groundwater in moderating the impacts of drought on domestic water supply to rural communities (Gillham, 1997) and on crops. A case in point is the considerable expansion of irrigation in general, including wells, following the 1968-1973 droughts in Sahel (Morris et a/., 1984, p. 14). There are also numerous papers that highlight that role (Amad, 1988; Calow et a/., 1997).
In contrast to valuations of groundwater supply in crop and livestock production where relatively straightforward estimates can be made based on total area, number of animals or value of output (if data are available), estimating the drought mitigation value of groundwater is complicated by two primary reasons. First, the knowledge that groundwater is available as an alternative to surface or rainwater reduces risk and makes farming and livestock production possible in areas where it would otherwise not occur. Thus the value of some production based on non-groundwater sources, especially in marginal lands, can in fact be attributed to groundwater. Second, the role of groundwater in drought mitigation highlights the issue of marginal, as opposed to average, valuation of water resources.
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