By convention, daily rainfall is read at 9 am representing a restricted 24 hour period – 9 am to 9 am. When considering large falls, totals for unrestricted 24 hour periods would be higher. Therefore, we need to adjust – factor up – daily rainfall to get correct maximum totals for 24 hour periods.
Work on adjustment factors was published by Boughton and Jakob in 2008, recommending an adjustment factor of 1.14 for 24-hour annual maximum rainfalls and 1.16 for sub-daily rainfall. Specific values are provided for Australian capital cities. There is also some work for Western Australia by Durrant and Bowman (2004) and similar values were obtained for Hong Kong.
This work has now been updated in the draft Australian Rainfall and Runoff (Book 2, Section, 3.4.3) to produce adjustment factors for rainfall totals for 1 to 7 days. They are summarised here, but check ARR for any updates. The values from 1 to 5 days are very similar to those recommended by Durrant and Bowman (2004).
Durrant, J., Bowman, S. (2004) Estimation of rare design rainfalls for Western Australia. Application of the CRC-FORGE method. Department of Environment, Surface Water Hydrology Report Series Report No. HY17. Dec 2004. link.
Jiang and Tung (2013) Establishing Rainfall Depth-Duration-Frequency Relationships at Daily Raingauge Stations in Hong Kong Journal of Hydrology 504:80-93. http://dx.doi.org/10.1016/j.jhydrol.2013.09.037