Hong Kong's Probable Maximum Precipitation

The Hong Kong Observatory has recently completed studies on the 24-hour and the 4-hour Probable Maximum Precipitation (PMP) for Hong Kong. Work to derive the 1-hour PMP is now in progress. The 24-hour PMP was derived at the request of the Geotechnical Engineering Office of the Civil Engineering Department, and the 4-hour PMP and 1-hour PMP at the request of the Drainage Services Department.

Simply stated, PMP is the greatest amount of rain that can possibly fall in a given duration over an area of given size according to hydrometeorological principles. PMP is specialized information used by the engineering community in connection with the risk assessment of landslips and floods, the design of related engineering works as well as the formulation of the corresponding mitigation measures. The Hong Kong Observatory first calculated PMP in 1965, for the then Public Works Department.

Deriving PMP is a complicated process. It involves selecting a list of the major historical storms which had induced severe landslips or flooding, computing the rainfall amounts brought by these storms to areas of different sizes, and maximizing these rainfall amounts to those that would be obtained if the water vapour content in the storms attained the climatological maximum.

The 4-hour and 24-hour PMP for Hong Kong are shown in Table 1.