Geoscience Reference
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10 000
5000
2500
1000
500
250
100
50
25
Figure 14.9 Areal reduction
factor related to rain area and
duration. (Redrawn from
Sumner, 1988; after Rodda
et al ., 1976, published with
permission.)
10
0.25
0.5
1
2
3 4 6
Duration, D (hours)
12
24
48
96
192
analysis . In this case ARFs are calculated for entire drainage basins by relating the
area-average precipitation (calculated using the Theissen method, for example)
for a chosen storm duration and for an annual time series of extreme events in a
number of different selected catchments. In each case, the ratio between the aver-
age precipitation for the catchment and the areal maximum precipitation is calcu-
lated and an overall mean then calculated from these values for all the catchments
analyzed. Figure 14.9 shows an example of the ARFs obtained in this way in terms
of duration and rain area. Apparently in this example the ARF is considered inde-
pendent of return period. Area-average precipitation can be simply obtained by
taking the product of observed point depth precipitation with the ARF for an
appropriate duration and area.
Probable maximum precipitation
A further measure of extreme precipitation for a region that might be helpful
in infrastructure design is the concept of probable maximum precipitation
(PMP). Although the name implies PMP is a statistical measure, it is largely
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