The effect of intersite dependence on regional flood frequency analysis

Abstract

Regional flood frequency analysis usually assumes that flood records from different sites are statistically independent. This assumption is unlikely to be valid in practice, so it is important to know how intersite dependence affects flood quantile estimates obtained by regional analysis. We use Monte Carlo simulation to assess the effect of realistically specified intersite dependence on the regional probability weighted moment algorithm, a robust and efficient procedure for regional flood frequency analysis. Our principal conclusions are as follows. Any bias in flood quantile estimates is unchanged by the presence of intersite dependence. The accuracy of flood quantile estimates decreases when intersite dependence is present, but this effect is less important for practical applications than the bias in flood quantile estimates due to heterogeneity (inequality of the flood frequency distributions in the region). Even when both heterogeneity and intersite dependence are present and the form of the flood frequency distribution is misspecified, regional flood frequency analysis is more accurate than at‐site analysis.