Assessment of runoff from arid terrain

It is difficult to calculate runoff from arid areas for various reasons. Rivers will generally only flow after storms and rapidly infiltrate into the ground. Runoff can occur along the sandy beds of the rivers undetected from the surface. The estimation and modelling of flood flow is even more difficult because the infiltration capacity of the ground is an important factor and a completely different set of criteria applies in determining runoff and flood. The spectrum of extreme floods is not as directly related to the rainfall spectrum as in the case of temperate climates. Flow records are invariably scarce and unreliable. This is because for most of the time there is no flow in the river and most of the flow would occur during floods. Gauges may be overtopped or wash away or unobserved when the rare flow does occur. The patching and extension of records in order to obtain a reliable time-series is therefore a difficult task and there is even less data for calibrating models. Experience in Botswana on synthesis of monthly flow records and prediction of extreme floods is described. The applicability of hydrograph methods and even the rational method is queried and instead hydraulic-type modular modelling is proposed as it has been used successfully on a number of catchments in the Kalahari Desert region of Southern Africa.