regional flood frequency analysis for small new zealand basins 1.

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four groups according to rainfall Iz¿ and slope S. Two distinct mimima areevident, indicating that a six-way split of the I 17 basins at partitions Izq = 105 mmand 155 mm and S = 19o is optimum. Table I defines the six groups and givesHosking and Wallis (1991) homogeneity test statistics. Each group is homogeneousat least with respect to V¡, and so Wiltshire's (1985) method using SSL hasachieved its objective.GROUP 2Iow I2a, High STABLE l-Six-way partition for flood frequency of ll7 small New Zealandcatchments based on rai¡fall .Iz¿, slope S and L-moments of flooddata.2Group DefinitionNumber Weighted averageof L-moment ratios HomogeneousBæins \llz l4ll, Vr Vz V¡ EY2kÉi'FGROUP 3Medium I21, Low SGROUP 4Mcdium l2{, High SI2J456Iz¡(l05mmIz¿(l05mm105
6800F5.tc)U)c.)!U)6a.)á400800-r*-r-f'Rainfall I1 Gnm)\3FC}2-ofi6005008001214Soil Porosity DWP (Vo)18 20 22r [= 1/AEP]5 10 2A 50 100 200 500 1000oç600500800Slope S (deg)25 30Gumbel Reduced Va¡iate [= lnCln(1_AEp))]FIG. 5-Comparison of dimensionless Wakeby flood frequency curves for six rainfallslope(Izq-S) groups defined in Table 1.and for fixed rainfall, basins with higher average slopes have steeper floodfrequency curves.TABLE 2-Dimensionless flood frequency quantiles for the six flood frequency$oups defined in Table l.Group Average Average 1/ AEP (=l years):lzoS51020(mm) (deg)I2345679.486. I127.5r27.6205.6198.5to SSL above),t4.226.512.024.812.0'2.6.81.38 I .78t.25 1.841.45 1.901.34 1.761.32 1.591.35 1.68502.20 2.802.63 4.ll2.33 2.872.2t 2.841.82 2.092.OO 2.40MEAN ANNUAL FLOOD GROUPSt0010003.28 5.135.66 15.73.25 4.423.36 5.322.26 2.682.68 3.52used contour maps of Q*1Ao't to estimateedure is used with Q./Aut to investigateproposed Q-/Ao't variability measure forartitioned by basin characteristics is (similar84600 l-FIG. 6 Variation in Q./AoS variability measure SSQ as the partition point for twowaygrouping changes for basin characteristics Izr, DWP, S and H.SSQ =soo L---1mnjttj=l i=l(Q-/Ao 8l¡¡ - [Q./e"1*,¡where there are m groups, each comprising nj basins with group recordlengthweightedaverage [Q./AoI*t. Figure ó shows the variation of SSQ with differentpartition points for two-way groups based separately on rainfall Iz+, soil DWP,hydrogeology H and slope S basin characteristics. lzq aîd DWP SSQ tracesexhibit minima. Four-way partitions using these two characteristics lead to welldefinedand reduced SSQ minima. Again, the best grouping found was a sixwaygrouping, defìned in Table 3.Table 3 shows desirable patterns between the group definitions and thebehaviour of recordlength-weighted average Q-/Ao't values for each group. Asrainfall Iz¿ increases, Q./Ao'' increases, and as soil porosity DWP increases,Q./ Ao t decreases.The Q. predictive powers of the six-way grouping are compared with theMcKerchar and Pearson (1989, 1990) contour map approach. Computing relativeerrors between either group or map approach estimators and observed Q. valuesfrom the ll7 basins, gives bias and root-mean-square-error values of 52Va and132/s respectrvely for the six-way grouping approach, and, 16/o and,75Vo forthe contour map approach. Therefore, the map approach is superior for estimatingQ.. Improvement in Q. estimation for small New Zealand drainage basins is85
Page 1 and 2: REGIONAL FLOOD FREQUENCY ANALYSIS F
Page 3 and 4: METHODData compílationTo assemble
Page 5 and 6: 1 llTa. Standard error for 44 "low
Page 7 and 8: APPENDIX Continued. Basin character
Page 9: (n,J)u!tt(ic)-o,ttIa)o55:4a250r |5{
Page 13 and 14: in this flood-set, nearest neighbou
Page 15: Cunnane, C., 1989: Statistical dist

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