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1 School of Geography, Oxford University, Mansfield Road, Oxford OX1 3TB, UK
2 British Geological Survey, Maclean Building, Wallingford, Oxon OX10 8BB, UK
Data from about 1500 boreholes in an area of African erosion surface in central Malawi were statistically analysed to attempt to correlate terrain characteristics, profiles and yields. The objectives were to seek terrain criteria which (a) allow regional evaluation of hydrogeological potential and (b) facilitate borehole site selection. From the 26 1:50 000 topographic maps analysed, a close map-by-map correlation was shown between extensive areas of dambo (seasonally waterlogged bottomland), low stream frequency and low relief as expressed by mean, median and modal values of the relative relief of individual km squares. Using this map-by-map approach no strong correlations were established between yield, profile and terrain characteristics.
There were only slight variations in mean yield of the two borehole types: those which finished in the regolith and those penetrating the underlying hard rock (55 1/min and 68 1/min respectively). Division of the total borehole data into relative relief subsets showed clear associations of low relative relief, thicker total and thicker saturated regolith and higher yields, where relative relief is <250 ft/km2. Subsetting the data with respect to the eight minor erosion surfaces which here comprise the African surface also showed significant variations in mean yield associated with these surfaces. Low relative relief emerged as the prime target for borehole siting, with sites at altitudes between the surfaces more likely to be higher yielding than comparably low relative relief sites within the altitudinal range of the minor surfaces. Sites close to the base of inselbergs yielded less well than those in the lowest relief category. The emergence of strong relationships between terrain and borehole performance, regardless of lithological variations, places the effects of rock type on borehole performance of secondary importance to those of geomorphology and leaching history in the study area.
This study allows regional assessment of hydrogeological potential in terms of terrain type. It also provides borehole site selection options within 1 km of a given site, using tabulated mean yields in the relative relief classes in various positions in relation to the minor erosion surfaces. Only the information available on the 1:50 000 topographic maps, i.e. altitude and relative relief as represented by the 50 ft contours, is necessary to apply the siting options.
