##### Abstract

In the paper by Durrans et al. (1999), an algorithm proposed by Ormsbee (1989) is recommended for the
stochastic disaggregation of hourly rainfall in continuous flow simulation studies of urban hydrologic
systems. However, Durrans et al. found that the method produced a “severe negative bias” in the
maximum rainfall intensity of the disaggregated series, so that peak flows in urban systems are likely to
be under-estimated by the model. Here we develop a method for disaggregating hourly data to 5min
series, which addresses the problem of negative bias. A regression equation is derived for the ratio of the
maximum 5min depth to the total depth in the hour. Thus, for any given hourly depth this ratio can be
simulated and multiplied by the hourly depth to obtain a 5min maximum. The temporal location of the
maximum within the hour can be randomly placed using an appropriate distribution function, e.g. based
on a geometrical construction as developed by Ormsbee (1989). The model is developed and tested using
5min rainfall data taken from Lund (1923-39) and Torsgatan (1984-93), Sweden. The results support the
use of the model in urban drainage applications.

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Citation

Cowpertwait, P.S.P. (2001), A continuous stochastic disaggregation model of rainfall for peak
flow simulation in urban hydrologic systems, Research Letters in the Information and Mathematical Sciences, 2, 81-88

##### Date

2001

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Publisher

Massey University