Adjustment factors for restricted rainfall

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).

Adjustment factor
1 1.15
2 1.11
3 1.07
4 1.05
5 1.04
6 1.03
7 1.02



Boughton, W. and Jakob, D. (2008) Adjustment factors for restricted rainfall.  Australian Journal of Water Resources 12(1): 37-48.  A copy is available for purchase for $4 here or $US50 here.

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.



4 thoughts on “Adjustment factors for restricted rainfall

  1. JD

    The values presented in these studies for the 24h fixed periods are fairly consistent to the 1.13 that Hershfield presented way back in 1961 for US data. Am I interpreting this right, that across the other side of the world (between US, Hong Kong and Aus), with vastly different rainfall behaviours there is less than 2% variance in the three adjustment factors?

  2. tonyladson Post author

    Your are right JD, there have been previous studies that came up with similar numbers. The studies reviewed by Boughton and Jakob (2008) include:
    Hershfield (1961) recommended an adjustment factor of 1.13 for 24 hour and 60 min intervals.
    Kerr et al. (1970) recommended 1.12 for 24 hour and 1.16 for 60 min (based on rainfall data from Pennsylvania.
    Miller et al. (1973) used 1.13 for 24 hours (based on data for Western US)
    Harishara and Tripathi (1973) found an average value of 1.15 for 24-hours (data from India)
    UK flood studies report used 1.11 for 24-hour and 1.15 for 60-min intervals
    Huff and Angel (1992) used 1.13 for 24-hour (data from Indiana and Illinois)
    Dwyer and Reed (1995) suggest 1.15 to 1.17 (based on sites in the UK)
    Young and McEnroe (2003) recommended 1.13 for 24-hours based on data from Kansas City.

    It would be interesting to do a meta-analysis to compare, and potentially combine, all the estimates.

    References and links
    Hershfield (1961)
    Kerr et al. (1970)
    Miller et al. (1973)
    Harishara and Tripathi (1973) Indian J. Met. Geophys. 24:279:282.
    UK flood studies report (1975)
    Huff and Angel (1992)
    Dwyer and Reed (1995)
    Young and McEnroe (2003)


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