Regional frequency analysis of low flows in New Zealand Rivers

Annual minimum low flow series (1-day, 7-day and 30-day mean flows) from nearly 500 catchments are used to investigate regional patterns and frequency distributions of low flows within New Zealand. Maps of logarithms of mean annual minimum 7-day specific mean flows demonstrate broad regional patterns and suggest estimation of this statistic from contour maps may be worthwhile for many regions. Catchment characteristics (rainfall, soil porosity, vegetation, slope, elevation, hydrogeology) are used to help explain regional variations. Probabilities of zero flows in annual minimum series are estimated using logistic regression on river catchment area and mean annual precipitation. L-moment ratios of non-zero low flow series are used to test homogeneity of regional and non-geographic groupings of river catchments, and to identify candidate statistical distributions for each group. Regional groupings based on Hutchinson's (1990) low flow regions are of varying homogeneity. A homogeneous Bay of Plenty / Rotorua sub-region is identified from the heterogeneous North Island central volcanic region. Most of the groupings analysed are heterogeneous so a wide range of frequency distributions is required to describe adequately New Zealand annual low flow series.