Site-focused nonparametric test of regional homogeneity based on flood regime

In regional flood frequency analysis, the proper identification of homogeneous pooling groups is a key factor for reliable quantile estimation. A new test of regional homogeneity based on flood regime is proposed in this paper. In contrast to other tests, this test does not use flood magnitude data and is focused on a site of interest and a target return period. The test relies on a nonparametric comparison of the regional variability of local flood regimes within a given pooling group with the variability that could be expected in a theoretically homogeneous group with the same hydrological properties as the original group. The performance of the test in the detection of regional heterogeneity is evaluated in a set of simulation experiments, each with different pooling group size, record length, and degree of regional heterogeneity. The test is then applied to a large set of real data, and the pooling measures and quantile errors are compared with the results obtained from the Hosking and Wallis's (HW) homogeneity test included in the same pooling algorithm. The simulation results demonstrated that the power of the test rapidly improves as the record length, pooling size, and degree of regional heterogeneity increase. In our study area the new test rejected fewer similar sites and led to similar quantile errors in comparison with the approach based on the HW test. The test is a sound alternative to the existing tests of regional homogeneity and is particularly convenient for site-focused pooling algorithms that require homogeneity testing at each step of the pooling.