Review of rainfall frequency estimation methods

This review outlines nationwide methods for point rainfall frequency estimation currently in use in nine different countries: Canada, Sweden, France, Germany, the United States, South Africa, New Zealand, Australia and the United Kingdom. For the United Kingdom, the Flood Studies Report method from 1975 is described as well as the current Flood Estimation Handbook method. The focus is on return periods relevant to reservoir design, in the region of 100 to 10,000 years. There is considerable difficulty in estimating long return period rainfalls from short data records and there is no obviously “best” way of doing it. Each country’s method is different, but most use some form of regionalisation to transfer information from surrounding sites to the target point. Several of the methods are variations of a regionalisation method that combines a local estimate of an index variable (typically the mean or median annual maximum rainfall) with a regionally-derived growth curve to obtain a design rainfall estimate. Three of the methods use regions centred on the site of interest, rather than fixed-boundary regions. Different statistical distributions and fitting methods are used, with the Generalised Extreme Value distribution being the most common.