Mapping the N-Year design rainfall - A case study for the Western Cape

Abstract

Flooding is often associated with heavy rainfall. Hence quantifying the probability associated with heavy rainfall events is useful in hydrology for design flood estimation and mapping Rainfall varies over time and space, therefore it is anticipated that at high levels the process will also vary over time and space. The objective of this study is to regionally quantify the average size of a 24-hour winter rainfall event associated with a 2% chance of being exceeded for an area in the Western Cape, South Africa. The point process approach to extreme value theory is employed to quantify the 50-year winter rainfall return level estimate from daily rainfall data from fifteen stations in the Western Cape. Ordinary kriging is used to estimate the 50-year winter rainfall return level surface over the study area. We compare the result to those obtained when the universal kriging approach is undertaken. We present a technique used to obtain spatial correlation models used in kriging, where the return level estimates are extended spatiotemporally, to circumvent inaccurate specification of the model and its parameters as a result of the spatial sparseness of the sample.