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ln the trials undertaken for the provisional regions
shown dashed in Figure 4.9 both 13901 (Mangawhai) and
15534 (Wairere) showed as outliers in the regions to which
they were initially assigned. As they fitted best into the
adjacent pumice region, they were placed there for the final
analysis. This required extension of the pumice region into
the coastal Bay of Plenty area; in terms of the hydrological
regions of Toebes and Palmer (1969) it includes Tauranga
and Opotiki regions. This assignment is tentative because
the soils in these catchments are not pumice to the extent of
others in the Rotorua/Taupo area. Their better fit with the
pumice region could be the result of inaccurate rainfall intensity
statistics.
4.6.3 Final equations for North lsland
Table 4.1O Stepwise regressions for final North lsland regions.
After a number of trials, final equations were developed
for the regions shown in Figure 4.10. The stepwise regression
results are givern in Table 4. 10. In all but one case the
best fit equation includes AREA and one or two of the
rainfall statistics. The exception is the Manawatu/WairarapalWellington
region were the equation including AREA,
1224 and FOREST provides a very good fit. However,
when MARAIN is substituted for FOREST in the equation
it is almost as good; this is preferred as it should provide a
more robust estimator. The table shows that for every region
the accuracy of estimate can be improved by including
a rainfall statistic in addition to AREA, and also, that other
catchment parameters with the possible exception of FOR-
EST in one region are not of importance. It is possible that
FOREST does not directly influence flood size; rather it
does so indirectly to the extent that correlations occur between
FOREST and the rainfall statistics (Table 4.7). For
the Manawatu/Wairarapa/Vr'ellington region where FOR-
EST was most dominant in these regional equations, the
Region
Northland/
Coromandel/
East Cape
{21 catchments}
Pumice Land
( 1 4 catchments)
East Coast
(1 1 catchments)
Manawatu/
Wairarapa/
Wellington
( 1 9 catchments)
West Coast
(25 catchments)
Number Variable
Varìables Name
1
2
1
2
1
2
1
2
AREA
AREA
MARAIN
AREA
MARAIN
ELEV
AREA
MARAIN
ELEV
LENGTH
AREA
AREA
t224
AREA
t224
MARAIN
AREA
t224
MARAIN
FOREST
AREA
AREA
t224
AREA
AREA
FOREST
AREA
FOREST
t224
AREA
t224
MARAIN
AREA
AREA
t224
AREA
t224
MARAIN
AREA
t224
MARAIN
FOREST
Coef
bl
se
of coef
R2
SE
est
o.70 0.06 10.8+ 0.927 0.86 0.237
0.64 0.o3 19.5* 0.983 0.97 0.1 19
2.33 0.31 7.6*
0.62 0.o3 18.7* 0.986 0.97 0.1 10
2.O1 0.33 6.2*
o.22 0.11 2.O
o.84 0.16 5.2* 0.988 0.98 0.107
2.O3 0.32 6.4*
o.21 0.11 2.O
-o.39 0.28 -1.4
Const Multipl¡er
loga a
0.807 6.42
-6.66 2.18 x 10'+
- 6.OB 8.24 x 'l O-7
-6.06 8.71 x 1O'
0.67 0.12 5.5* o.a44 0.71 0.348 0.0933 1.24
0.83 0.06 13.5* O.971 O.94 O.161 -7.94 1.15x1O-8
4.O2 0.60 6.7*
o.74 o.06 12.2* 0.984 O.97 Oi2A - 10.51 3.Og x 'l O{1 +
2.54 o.72 3.5*
1.75 0.64 2.7 *
O.88 O.O9 9.9* 0.989 O.98 O.111 -11.94 1.15x1O{'?
3.16 0.70 4.5*
1.80 0.56 3.2*
-o.17 0.o9 -2.O
o.80 0.o8 10.6*
0.76 0.o3 27.4*
2.24 0.29 7.8*
0.90 0.1 3 6.9*
o.72
0.46
o.82
1.53
o.94
0.06 12.2*
o.o5 8.9-
o.74 0.o4 16.7*
o.34 0.o5 6.7*
1.22 0.33 3.7*
o.o5 15.8*
o.19 4.O*
o.39 4.9*
o.85 0.o8 1 1.3*
o.az 0.o5 16.7*
2.18 0.38 5.8*
o.82 0.o5 17.7*
1.67 0.44 3.8*
0.78 0.39 1.9
o.79 0.o5 16.1 *
1.6'l O.42 3.8*
0.80 0.37 2.1
0.1 1 0.06 1.8
0.963 0.93 0.227
0.996 0.99 0.082
o.858 0.74 0.341
0.978 0.96 0.144
0.988 0.98 0.107
0.982 0.96 0.1 18
o.974 0.95 0.157
0.977 0.96 0.1 50
0.296 1.98
-4.O5 8.84 x 10{ +
0.232 1.70
o.1 58 1.44
-2.O5 8.99 x 1O-"
-5.51 3.12x10-6 +
0.920 0.85 0.258 0j82 1.52
0.969 O.94 O.1 67 - 3.83 1 .48 x 1O-1 +
- 5.41 3.87 x 10-6
-5.51 3.13x10-6
* significant at 5olo level
+ preferred equation
Water & soil technical publication no. 20 (1982)
69