PROPERTIES OF EXTREME POINT RAINFALL .3. IDENTIFICATION OF SPATIAL INTER-SITE CORRELATION STRUCTURE

A general stochastic model that describes the observed variability of extreme rainfall properties estimated at different sites in a region i s developed. The observed variability is divided into sampling variabi lity and variability due to regional heterogeneity. A method for estim ating the inter-site correlation structure of the at-site sampling err ors based on partial duration series theory is developed, and geostati stical theory is introduced to estimate the spatial correlation struct ure of the true, but unknown, variables. On this basis the variance of the regional mean value can be determined and used in a regional desi gn approach. The theory is applied to extreme values of rainfall depth and maximum 10 minutes intensity per rainfall event measured at 43 ga uging stations in Denmark. Adding one design standard deviation to the regionally averaged T-year event estimates yields increased design va lues between 8 and 18%, depending on the return period and the rainfal l characteristic considered. For small return periods this compares to results obtained from traditional non-parametric statistics, but for large return periods the suggested approach yields significantly lower and more accurate design values.