Regional analyses of precipitation annual maxima in Washington State

Abstract

Regional analyses of precipitation data were conducted using an index flood type methodology and probability weighted moments parameter estimates for the generalized extreme value distribution. Annual maximum series data were collected at 115 stations for durations of 2 and 6 hours and at 315 stations for the 24‐hour duration. Because the climate in Washington State varies from arid to rain forest, the issues of homogeneity and region definition posed major problems. Those problems were circumvented by considering the state to be a heterogeneous superregion. Climatologically homogenous subregions within the superregion were defined in terms of mean annual precipitation (MAP) rather than geographic location. The subregional values of the coefficients of variation C v and skew γ were found to vary systematically with MAP across the superregion. This allowed the superregional values of C v and γ to be expressed as continuous variables instead of conventional fixed values and eliminated the boundary problems normally associated with subregion definition. The values of C v and γ for the superregion were found to be largest for arid areas and shorter durations. Smaller values of C v and γ were associated with humid and rain forest environments. All subregional solutions were within, or near, the extreme value type II family.