In flood hydrology, the initial loss is the amount of rainfall required to wet up a catchment before runoff starts. Initial loss has major influence on the size of the flood peak produced by a storm and has long been the subject of research. Some key papers and reports are at the end of this post.

With the publication of the new version of Australian Rainfall and Runoff, there is a large compilation of loss values in the Appendix to Book 5 (Tables 5.5.12 to 5.5.14). It has long been known that initial loss is lower in urban, compared to rural areas, but this data provides an illustration of the differences. Of the 763 events in urban areas analysed by Boyd et al. (1993) (ARR Table 5.5.13) the average initial loss weighted by the number of events is 0.62 mm. For loss values from various sources (ARR Table 5.5.14) 70% of urban catchments have an initial loss of 1 mm or less. The mean initial loss value for the data provided in ARR for urban catchments (Table 5.5.14) is 1.1 mm with a standard deviation of 1.17 mm.

For the rural catchments, initial loss is highly variable. For the ARR data (Table 5.5.12) the mean initial loss is 32 mm but the variance is high (standard deviation 16.8 mm). Other studies have found similarly variable values. For example, in a study of 5 NSW catchments, El-Kafagee and Rahman (2011) reported a mean initial loss of 22 mm with standard deviation of 19 mm. For 15 catchments in Queensland, Tularam and Ilahee (2007) found a mean initial loss of 43 mm with a standard deviation of 28 mm.

A probability density plot shows the contrast between urban and rural catchments using the ARR data (Figure 1). Expect some runoff from even a few mm of rain on an urban catchment.

To choose design values of initial loss for flood modelling, check the latest advice in ARR Book 5, Section 5.5. Keep in mind, these loss values are intended for application to complete design storms not design rainfall bursts that are commonly calculated using the IFD data available from the Bureau of Meteorology.

The code to produce this plot is available as a gist.

### Bibliography

Key papers and reports on initial losses include:

- Boyd, M. J., Bufill, M. C. and Knee, R. M. (1993) Pervious and impervious runoff in urban catchments.
*Hydrological Sciences Journal***38**(6):463-478 (link). - Cordery, I. (1970) Initial loss for flood estimation and forecasting.
*Journal of the Hydraulics Division***96**(HY12):2447-2466 (link). - El-Kafagee, M. and Rahman, A. (2011) A study on initial and continuing losses for design flood estimation in New South Wales. 19th International Congress on Modelling and Simulation, Perth, Australia, 12-16 December 2011 (link)
- Hill, P.I., Mein, R. and Siriwardena, L. (1985) How much rainfall becomes runoff: Loss modelling for flood estimation. Cooperative Research Centre for Catchment Hydrology (link).
- Hill, P., Graszkiewicz, Z, Taylor, M. and Nathan, R. (2014) Australian Rainfall and Runoff Revision Project 6: Loss models for catchment simulation – Analysis of Rural Catchments. Engineers Australia. (link).
- Ilahee, M., A. Rahman and W. Boughton (2001). Derivation of new design initial losses for flood estimation in Queensland. (link).
- Ilahee, M. (2005) Modelling losses in flood estimation. PhD thesis, Queensland University of Technology (link)
- Laurenson, E. M. and D. H. Pilgrim (1963) Loss rates for Australian Catchments and their Significance
*Journal, Institution of Engineers, Australia***35**(1-2 ): 9-24. (see report here). - Phillips, B., Goyen, A., Thomson, R., Pathiraja, S. and Pomeroy, L. (2014) Australian Rainfall and Runoff Revision Project 6:Loss models for catchment simulation – Analysis of Rural Catchments. Engineers Australia. (link).
- Pilgrim, D. H. (1966) Storm loss rates for regions with limited data.
*Journal of the Hydraulics Division***92**(HY2): 193-206 (link). - Tularam, G. A. and M. Ilahee (2007). Initial loss estimates for tropical catchments of Australia. Environmental Impact Assessment Review
**27**(6): 493-504. (link) - Waugh, A. S. (1991) Design losses in flood estimation. International Hydrology and Water Resources Symposium, Perth, 2-4 October, 1991. (link)

Luke CunninghamNice post, Tony. Showed to be particularly relevant in some of our recent modeling that we worked on together. My feeling is that in hydraulic models we’re seeing lots of initial loss on the grid compared to a hydrologic approach which assumes every drop of runoff makes it to the outlet – what’s your thoughts on this? Do you think we could use different losses for hydrologic vs hydraulic modelling?

tonyladsonPost authorIn hydrology, modellers use losses as some of a number of calibration parameters to match historic rainfall to historic flood hydrographs. Can we do something similar in Rain-on-Grid hydraulic modelling? Of course, we still have the problem of selecting values for losses to use when generating hydrographs for design flood events. This is complicated because design rainfall bursts are often smaller than the complete storms used when historic floods are modelled.