I am responding to Sandy Grant's recent challenging comments (May14) about the Notwane Dam catchment area. First of all, I suggest that the combined volume of all the small dams in the area covered by the UB survey must be insignificant when compared with the volume of the Gaborone Dam itself. I wonder if, together, they could even supply Gaborone for 15 days at current consumption rates?
The UB study only "suggests" a 30 percent interception rate of run-off by these dams. I suspect this figure is not based in any way on hydrological records and is a guess. From nearly 20 years of living in rural Botswana, my guess would be even less than three percent, and for this reason: most rainfall in Botswana is local and sometimes generates prodigious (given the usual denuded state of the land) amounts of run-off in a small area. This flows for a while, but, given the slight gradients, its flood soon dissipates and such flows rarely reach the Gaborone Dam.
