Quantifying risk for water harvesting under semi-arid conditions

Rainfall intensity is the most important factor for runoff estimations, however rainfall intensity data is often unavailable. Rainfall disaggregation techniques are appropriate for use in problems where design storms are required. Rainfall intensity patterns within storms and then the validity of a stochastic rainfall intensity model were, therefore, studied on the highveld of South Africa. Rainfall intensity data collected for 30 years at two semi-arid sites, Bloemfontein (29°S, 26°E) and Pretoria (26°S, 28°E) were analysed using dimensionless hyetographs. No difference was found in either seasonal or geographic variability for the two sites, indicating that intensity patterns within rainfall events across the highveld was identical. The stochastic rainfall intensity model was shown to produce acceptable results. From these results, it has been concluded that one model can be used to generate rainfall intensities for the highveld.