The influence of climate change on short duration rainfall in the Western Cape

[摘要] ENGLISH ABSTRACT: The Western Cape has been subject to extreme rainfall storms in the last decade, which hasbeen responsible for millions of Rands worth of damage to public infrastructure and humansettlements. Current climate model projections under climate change suggest that the WesternCape can expect greater rainfall intensities, with magnitude of rainfall events increasing, but thefrequency of occurrence of events decreasing. The aim of this research is to assess if historicalshort duration rainfall data provides any evidence to support increasing intensities in rainfall inthe Western Cape.Short duration rainfall data (< 24 hours), consisting of digitised autographic- and automaticweather station data (AWS), was selected from the South African Weather Services' databasefor the Western Cape and wider South African region, in order to combine the data for anextension of the effective record length. Numerous difficulties were encountered with the datathat required the application of editing and quality control procedures. The digitised autographicdata contained many errors and generally compared poorly to standard gauge daily rainfalltotals of the same period. After the application of editing and quality control procedures, sevenstations were selected and their autographic and AWS data was combined for further detailedanalysis.Analysis was divided into two sections, one for the analysis of the magnitude of the rainfall,the other for the frequency of occurrence of rainfall events. For the magnitude analysis, nonstationaryextreme value theory was applied by implementing a parametric and non-parametricnon-stationary approach to both the generalised extreme value distribution (GEV) with an annualmaximum series, and a generalised Pareto distribution with a peaks over threshold series.The parametric approach entailed fitting linear models to the parameters of the extreme valuedistributions, and the non-parametric approach tested if return levels of the distributions remainedconstant over a moving window period. The frequency analysis entailed the recordingof the annual number of rainfall events exceeding predetermined threshold values.For the magnitude analysis, the parametric approach only produced two stations with significantnon-stationarity and only for the GEV, whilst the non-stationary approach produced results thatcan indicate non-parametric behaviour, but this was most likely because of the combination ofautographic and AWS data. The frequency analysis produced results that indicated no observableresults for some stations, while others produced trends that are consistent with a changefrom the autographic to AWS data.This lead to the conclusion that with the available data, no clear or significant evidence supportingincreasing intensities or any other change in short duration rainfall was found regarding themagnitude and frequency of occurrence of rainfall events.

[发布机构] Stellenbosch University