Review of methods for deriving areal reduction factors. Report to Defra, project WS194/2/39 (Reservoir Safety - Long Return Period Rainfall)

Svensson, Cecilia. 2007 Review of methods for deriving areal reduction factors. Report to Defra, project WS194/2/39 (Reservoir Safety - Long Return Period Rainfall). Wallingford, Centre for Ecology and Hydrology, 23pp. (CEH Project Number: C02760)

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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 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. Methods for estimation of areal reduction factors include empirical and analytical methods. The current design guidelines in the UK are based on an empirical method, but since they were issued in 1975 (NERC, 1975), several new analytical methods have been proposed. In addition to the more than 30 years of raingauge data collected since 1975, radar rainfall data have also become available for large parts of the UK, and at increasing spatial and temporal resolutions. Recently developed analytical methods attempt to put areal reduction factor estimation on a sounder scientific basis. However, they are generally based on assumptions that are not entirely true descriptions of the real rainfall process, which causes concern and uncertainty regarding the results. This is compounded by the limited amount of actual rainfall data that so far has been used to verify them. Results from methods based on fractal or scaling relationships seem to agree with empirical estimates within a limited scaling regime, but may not be appropriate for application to a comprehensive set of temporal and space scales. Storm-centred methods, such as some correlation-based methods and the so-called annual maxima-centred method, do not result in probabilistically correct areal rainfall estimates. That is, when multiplying a storm-centred ARF with a T-year point rainfall, the resulting areal rainfall does not necessarily have the same T-year return period. These methods can therefore not be recommended for use with rainfall frequency estimates. The use of radar data is still problematic. Differences between the results of a sequence of studies can be expected because of heterogeneities in the data, as resolution and radar data processing improves with time. Other problems are short records and biases in the ARF estimates. Since the publication of the Flood Studies Report (NERC, 1975), more than 30 years of additional raingauge data have been collected. An empirical method may be used to update the current ARF estimates for the UK, perhaps using a regionalisation scheme to take into account the differing rainfall characteristics in different parts of the country.

Item Type: Publication - Report
Programmes: CEH Programmes pre-2009 publications > Water > WA01 Water extremes > WA01.1 New methodologies to quantify floods, flows and droughts
UKCEH and CEH Sections/Science Areas: Boorman (to September 2014)
_ Hydrological Risks & Resources
Funders/Sponsors: DEFRA
Additional Keywords: rainfall, frequency analysis, areal reduction factor
NORA Subject Terms: Hydrology
Date made live: 26 Feb 2008 10:48 +0 (UTC)

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