The IDF curves give the rainfall intensity at a point. For larger catchment, the uneven spatial distribution of a storm is important.
Areal reduction factors are applied to design point rainfall intensities to account for the fact that it is not likely that rainfall will occur at the same intensity over the entire catchment area of a storm.
The point estimates of design storms are adjusted for the catchment area by following the procedure recommended in HP1 (DID, 1982), which is similar to the United States Weather Bureau’s method.
The design rainfall is calculated from the point rainfall intensity as follows (Equation 13.1 in MSMA, 2000):
F is the areal reduction factor which is expressed as a factor less than 1.0.
The values of F for catchment areas of up to 200 km2 and durations of up to 24 hours are given in Table 1.2 and Figure 1.2 below (Table 13.1 and Figure 13.1, respectively):
Table 1.2 Areal Reduction Factors (Table 13.1)
Figure 1.2 Plot of Areal Reduction Factors (Figure 13.1)
Note that Table 1.2 is limited to catchment areas up to 200 km2 only. However, for larger catchment, it may be necessary to refer to Figure 1.2 which gives the areal reduction factors for areas up to 1000 km2.