Delineating Flood-Flow Regions for the North Island of New Zealand

Abstract

With recent acknowledgment that New Zealand requires updated guidelines in predicting rainfall and runoff, this case study compares three approaches in forming flood regions for the North Island of New Zealand. With comparison to regions in the literature, new regions are delineated on the basis of administrative boundaries, similar catchment climate and dominant topographies, and comparable linear moment ratios of the coefficient of variation (L-CV) of the annual maximum flow series. Across 204 individual flow gauging sites, the regions on the basis of L-CV were observed to provide the best predictors of flood discharges when compared with those derived using at-site analyses, with relative errors of 6.9 and 21% for 10- and 100-year peak discharges, respectively. Although these regions are delineated on the basis of spatial proximity and similar hydrologic flood flow regimes, concerns are raised regarding the homogeneity of these regions as a whole. Although further breakdowns of regions could be beneficial, these regions are still recommended for use in ungauged catchments over those previously published.