Review of methods for deriving areal reduction factors

The design of hydraulic structures requires knowledge of how much rain is likely to fall within a certain amount of time and over a specific area. Point rainfalls are only representative for a very limited area, and for larger areas, the areal average rainfall depth is likely to be much smaller than at the point of the maximum observed depth. The estimation of areal reduction factors (ARFs) is concerned with the relationship between the point and areal rainfalls. This relationship has been found to vary with, for example, predominant weather type, season and return period. Traditionally, ARF estimates are based on empirical methods, although, more recently, a range of analytical methods have been applied. The review has found that no method is unambiguously correct. However, the traditional data-intensive, empirical, fixed-area methods still have advantages, including probabilistically correct ARF estimates and applicability over a comprehensive range of spatial and temporal scales. Although the analytical techniques attempt to place ARF estimation on a sounder scientific basis, they tend to rely on simplified assumptions and/or are only applicable within limited scales. The use of radar is problematic because of inhomogeneities and short data records, as well as possible biases in the ARF estimates.