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correlations between (logs oÐ FOREST and 1224, and
FOREST and MARAIN were 0.90 and 0.65 respectively;
this was one region where estimates of l2?A were suspected
to be unreliable.
The preferred equations of Table 4.10 are shown in Figure
4.10 with their appropriate regions, and also the residual
errors for logarithms. In general the equations seem
more reliable in the north and east of the island, a situation
also noted in the South Island. The distribution ofresiduals
generally appears reasonably random in space. The region
for the West Coast includes a number of catchments tributary
to the Wanganui River and a number originating on
the slopes of the central North Island volcanoes. Many of
the larger residuals for the island are clustered here, presumably
because of the variety of soil types, including pumice,
and the sparse coverage of rainfall intensity measurements.
The equations for the East Coast and West Coast regions
suggest that they should be combined into one, but
doing so resulted in regional biases. Hence the separate regions
should be maintained.
4.7 Discussion of results
The equations derived in sections 4.5 and 4.6 are intended
for application to rural catchments where flood
storage is not excessive or where other dampening effects
are not dominant. All the nine regional equations use catchment
area, and all but one use area and one or two of the
rainfall statistics; other physical catchment characteristics
used in this study appear to be of little consequence. This is
an important finding, since results obtained in overseas
countries (see section 4.8) have suggested that these other
physical characteristics are relatively important. The
dominance of rainfall statistics is possibly due to the
generally steep nature of New Zealand catchments (see section
4.8). The two sets of rainfall statistics considered in the
study have a large range of values; both cover more than
one order of magnitude in the South Island, though the
range is less in the North Island.
Since this study was completed, an updated analysis of
rainfall intensity data has become available (Tomlinson
1980; Coulter and Hessell 1980). Tomlinson used 16000
years of data from 940 manual daily raingauges and 3500
years from 180 recording raingauges. Comparison of revised
1224 estimates with those from Robertson (1963) for a
sample of 87 stations did not reveal statistically significant
differences. This suggested that the revised estimates may
also be used for estimating 1224. Sirce the revision used
more than twice the number of stations, more accurate estimates
of 1224 lor individual catchments should be possible.
For convenience, revised 1224 estimates for the 94O daily
gauges used by Tomlinson are included as Appendix D.
- The revised intensity data were also mapped by Tomlinson.
However, in high altitude areas, especially along the
Southern Alps, use of the mapped values is not recommended.
In mapping, rainfall is assumed to increase with
altitude, This increase was not allowed for when catchment
estimates for l2A were made, so the maps will give catchment
estimates of 1224 greater than the estimates used in
deriving the equations for Q. Thus inflated Q estimates
may result from
The 2-year rec
uration intensitY
estimated from
auges was used
principally because of the better national coverage of daily
iead manual gauges for observing this duration rainfall
compared with shorter duration rainfall statistics estimated
from automatic rainfall recorder data.
As noted in section 4.4, estimates of catchment mean annual
rainfall have a low accuracy for many South Island
and this is possibly why inclusion of mean annual rainfall
improves the estimate in three of the five North Island regional
equations.
Little can be saicl of the physical significance of the exponents
for the regional equations. Whilst values of the intensity
exponent around unity for two of the South Island
regions might have some physical interpretation, the meaning
of exponents of the rainfall statistics which exceed 2.0 is
unclear, even though their statistical significance is undoubted.
Obviousl),, in using the equations, particular care
must be given to the estimation of rainfall statistics, because
estimates outside of the range of the values of the
sample used in developing the equations could result in
severe errors in estimates of Q. Tables 4.1 and 4.2 are a
guide to typical valtues of the rainfall statistics.
Where a catchment lies near a regional boundary and
each regional equation gives different estimates, the precise
location ofthe boundary is bound to create difficulties. For
some regions, dominant flood-producing weather patterns
are thought to apply, and a decision about which region a
catchment fits into could be based on the weather conditions
which are believed to produce the most flooding.
An example is a number of Marlborough and Canterbury
rivers in the Inland region, where the flood-producing
weather conditions are possibly southerly. Their headwaters
near the divide flood in nor'westerly weather condi
tions and fit in the West Coast region. The rivers flow
through the East Coast region where flood-producing weather
patterns are thought to be easterly.
In the case of the North Island Pumice region, catchments
have been included where pumice was thought to
have a dominant influence on the flood hydrology, but two
coastal Bay of Plenty catchments were tentatively included
here because this was where they fitted best, even although
pumice is not dominant on these small catchments. rÙVithin
the Pumice region, there appears to be a gradation in the effect
of the pumice. For instance, pumice lies to great depths
on the Kaingaroa Plateau within which much of the catchment
of the Rangitaiki River lies. The 28 years of record for
this river at Murupara (AREA : ll84km', Station
15408), but which was not used because it exceeds
ll00 km', gives_a Qous : 41.6 m'/s but the estimate from
the equation is Q"rt = 202 m3/s, representing a residual error
of - 0.69. Clearly, the dampening effects of pumice are
severe in this case.
The annual flood regions delineated in this chapter are'
in general, very similar to the flood frequency regions defined
in Chapter 3. An exception to this is in the eastern
area of the South Island (see Figure 4.7). From a design
characteristics, on the other hand, is also concerned with
the coefficients of variation and skewness of the flood record,
and these can be regarded as the slopes and curvatures
respectively of the individual dimensionless-magni-
Therefore it was not unexof
the flood frequencY chargions
that differed in Places
from the set deveioped for estimating Q.
4.8 Compar¡son with other results
Similar equations
given in Table 4.ll l.
favourably in terms
ent of determination
this needs to be balanced against the use of a relatively
small range of catchment areas (0'2 to ll00 km'); poorer
fits may be obtained if larger catchments are used' The
other notable feature of Table 4'll is the range of expon-
estimated with reasonable accuracy for the North Island, ents for AREA. The values tend to be less than the values
Water & soil technical publication no. 20 (1982)
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