Report No. : GEO Report No. 115

Report Title : Regional Variation in Extreme Rainfall Values (2001), 81 p.

Author : N.C. Evans & Y.F. Yu

Abstract

The standard references on extreme point rainfall values in Hong Kong use long-period data collected at the Hong Kong Observatory (HKO) in Tsimshatsui. It has long been recognised that these data may not be valid for other locations in Hong Kong.

This Report presents the results of statistical analyses of fourteen years of rainfall data recorded at 46 GEO automatic raingauges throughout Hong Kong. There are no firm rules regarding minimum periods of data for climatological extreme value analyses, but there are some guidelines. Discussions with the HKO, and a review of some of the literature, suggest that a period of only fourteen years is less than ideal, particularly when the data are not 100% complete.

Two indirect methods of estimating extreme rainfall values at these locations were also reviewed and developed for use in Hong Kong. One relies on a general relationship between extreme values for different durations and maximum 60-minute rainfall values recorded in a ten-year period, while the other postulates a correlation between extreme values and mean annual rainfall at a point.

Results from the indirect methods were compared with the values obtained from statistical analysis of the raingauge data. It is concluded that sufficient information exist to make a reasonable assessment of extreme rainfall values at any location in Hong Kong. The precise combination of data and analytical techniques which should be used will depend on the location of the site and the purpose of the assessment.

Further analysis of existing data from HKO autographic raingauges which have been set up for a long time might provide additional insights into the regional variation of extreme values. Similarly, when approximately 20 years of data become available from the GEO and HKO automatic raingauges, a review of the calculated extreme values would be beneficial.

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