regional flood frequency analysis for small new zealand basins 1.

TABLE 3-Six-way partition for Q. of ll7 small New Zealand catchmentsbased on rainfall (Iz¿) and soil (DWP) basin characteristics.Group DefrnitionI23456lzq1l25mmIzq(125mm12520%oNumber of WeightedAveraseBasins Q-i Aõt37732ll255t.570.813.521.5 I5.492.31TABLE 4-Regional flood-frequency results from randomly resampling (100times) l0 annual maximum floods (from basins with 20 or morefloods) for different groups.GroupIzJ456Number ofbæinst2920292423Q:oo/Q.Number of bæins(Table 2) with n > 203.285.663.253.362.262.684l*9125l0Mean Qroo/Q.Estimate 95% Confidence2.685.943.023.632.552.58Interval+0.11+0.42+0.09+0.1210.0910.09further discussed by McKerchar (this issue); he recommends map Q. estimatesfor small ungauged basins.Alrfi1274*GEV distribution used in place of Wakeby as only one basin in group.+0 04DISCUSSIONThe robustness and accuracy of the Wakeby regional procedure for the silflood frequency groups may be illustrated using the longer annual maximumflood series (similar to Potter and Lettenmaier, 1990). Subsamples of l0 floodsare randomly selected from each drainage basin with annual maximum seriesof length 20 or more yeaß. Groups of the subsamples are used with the Wakebyregional procedure to obtain group Qroo/Q* estimates. This is repeated 100 timesfor each group. Mean Qroo/Q. estimates, w\th957o confidence intervals for themean, are given in Table 4. Although only l0 floods are used from each siteper run, the results for the six groups are not significantly different from groupQroo/Q- estimates in Table 2.Larger differences for groups I and 5 are causedby smaller group sizes in the resampling exercise. The result for all basins with20 or more years of record, considered as one group, emphasises the need formore than one group: at least three group Qroo/Q. resampling means (groups2,3,4) are significantly different to the one-group mean, and to each other.Both rainfall and slope influence upper-tail steepness of flood frequency curvesfor small New Zealand basins. Lower rainfall I2a groups are associated withsteeper frequency curves (Fig. 5) than higher Izr firoups. As explained by Wiltshire(1985), annual flood peaks from basins in drier regions are more variable thanthose from wetter regions. G¡eater variability in annual floods is directly relatedto steepness of flood frequency curves. A physical explanation for steeper averagebasin slopes relating to steeper flood frequency curves might be that steeperslopes are faster draining, and offer less storage opportunities, and henceantecedent wetness conditions are more variable, translating into more variableannual flood peaks and so steeper flood frequency curves.The flood-set of 117 small New Zealand basins displays much variability inthe L-skewness-L-kurtosis plane (Fig. l) and in the dimensionless plots of theobserved floods (Fig. 4). This is expected from small basins where the ratioof maximum current-meter-gauged stage to maximum automatic waterlevelrecorded stage is usually low (since the flashiness of small basin flooding means86there is little time for held teams to be on site for current-meter flood gaugings),implying that variabi-lity of annual floods derived from stage-discharge ratingcuryes is high between basins (Potter and \ilalker, 1985). The lengths of recordfor these basins are relatively short, ranging from l0 to 29 years, with an averageof 16 years, also implying greater flood frequency variability. Therefore averagingmethods will be superior to looking at individual sites, and so at-site floodfrequency methods will be less reliable than the robust, accurate and efhcientregional scheme (Wallis 1980, 1988) used for the six groups in this paper. Thecloseness of group flood frequency curves (Fig.5) for groups I and 4, and,to a lesser extent group 3, and the high variability of the annual flood series,indicates that these groups could be amalgamated into one group for practicalpurposes.87