Spatial variation of catchment-oriented extreme rainfall in England and Wales

This paper presents the spatial variation of the annual maximum daily rainfall (AMDR) in more than 900 catchments of England and Wales over the last century with respect to different spatial features including geographic location, elevation, size, orientation and shape of catchments. A SPER toolbox is employed to extract the spatial features of catchments where the AMDR is modelled by a well-tested Generalized Extreme Value (GEV) distribution. The results show that the GEV parameters μ and σ exhibit similar patterns and are usually larger with higher elevations. Increasing catchment size can decrease parameters due to areal averaging, however, in the middle-sized transition regions of the rainfall variation, e.g., east Wales, the trend reverses. For areas at high elevation, parameters are greater in the west-northwest-oriented catchments while parameters in west-northwest or east-northeast-oriented catchments at lower elevation are similar or smaller than those with a north-south orientation. An elongated shape catchment usually has smaller parameters than a rounded shape one. These findings reveal the heterogeneity of extreme rainfall distribution in space with respect to different spatial characteristics of catchments even under the same climate, which lays a basis for further catchment-based analysis concerning the relationship between hydrological response and geomorphic properties