Rainfall estimates from IFD relationships are applicable strictly only to a single point and not to a wider area such as a catchment area is small, the point IFD relationships are taken to be representative of the areal IFD relationship. For this purpose, a small catchment would be defined as being less than 4km2. For larger areas, it is unrealistic to assume that the same intensity rainfall will occur over the entire area and reductions in the rainfall intensity are made. Unfortunately, at present there is limited information available regarding values for the areal reduction factor (ARF).

Due to this, Canterford et al. (1987) recommended the use of areal reduction factors developed by the US National Weather Service (NOAA, 1980) for the Chicago region as being appropriate for all regions of Australia except for the inland regions where the recommendation is to use areal reduction factors developed for Arizona (NOAA, 1984). Hence the ARFs currently recommended for use in Australia were not defined from Australian data. Since that recommendations, Srikanthan (1995), Siriwardena and Weinmann, (1996) and Catchlove and Ball (2003) have investigated ARFs using Australian data for different regions of the country. These studies have highlighted the inappropriateness of American ARFs for Australian conditions.

In addition to the need to estimate relationships between point and spatial rainfall, there is a need to estimate rainfall variation over a catchment for historical storm events. This variation occurs as the result of catchment topography, storm movement, etc. A comparison of alternative approaches for estimation of the spatial variation of rainfall has been presented by Ball and Luk (1998) who showed that these alternative approaches have varying accuracy. A subsequently study by Umakhanthan and Ball (2005) highlighted the importance of rainfall models for simulation of flows from a catchment. At present, however, no guidance is presented in Australian Rainfall and Runoff to aid practitioners in the spatial modelling of rainfall over a catchment for either historical or design events.

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