Regional rainfall frequency analysis for Botswana using L-Moments and radial basis function network

Information on rainfall magnitude at specified recurrence interval is important for agricultural and water resources planning and management besides particularly for a semi-arid country like Botswana where the annual rainfall is erratic with large coefficient of variation. This study which was based on long term annual rainfall data from 1961 to 2003 at 11 synoptic stations across Botswana, concluded that rainfall was increasing up to 1981, after which there has been a decline in the annual rainfall pattern. An overall downward trend shows that the decrease in the rainfall quantities in the recent years, i.e. after 1981, has been very high. Regional analysis showed that all the stations behaved homogeneously and followed a generalised extreme value (GEV) distribution. Rainfall quantiles computed with the total data also showed a decreasing pattern when compared with quantiles estimated using the data series up to 1981. It was found that for specific amounts of rainfall the recurrence interval increased considerably when the total period data were analysed. Overall it can be said that rainfall quantiles with a 10-year recurrence interval will decrease by 2–17%, while rainfall quantiles with a 50-year recurrence interval will decrease by 0–14%.