Hydrology and Earth System Sciences Discussions www.hydrol-earth-syst-sci-discuss.net 1812-2108 1812-2116 6 1 2009 10.5194/hessd-6-627-2009 http://www.hydrol-earth-syst-sci-discuss.net/6/627/2009/ http://www.hydrol-earth-syst-sci-discuss.net/6/627/2009/hessd-6-627-2009.html http://www.hydrol-earth-syst-sci-discuss.net/6/627/2009/hessd-6-627-2009.pdf 627 665 2009-01-30 On the role of the runoff coefficient in the mapping of rainfall to flood return periods A. Viglione viglione@hydro.tuwien.ac.at R. Merz G. Blöschl Institut für Wasserbau und Ingenieurhydrologie, Technische Universität Wien, Wien, Austria While the correspondence of rainfall return period <i>T_P</i> and flood return period <i>T_Q</i> is at the heart of the design storm procedure, their relationship is still poorly understood. The purpose of this paper is to shed light on the controls on this relationship examining in particular the effect of the variability of event runoff coefficients. A simplified world with block rainfall and linear catchment response is assumed and a derived flood frequency approach, both in analytical and Monte-Carlo modes, is used. The results indicate that <i>T_Q</i> can be much higher than <i>T_P</i> of the associated storm. The ratio <i>T_Q/T_P</i> depends on the average wetness of the system. In a dry system, <i>T_Q</i> can be of the order of hundreds of times of <i>T_P</i>. In contrast, in a wet system, the maximum flood return period is never more than a few times that of the corresponding storm. This is because a wet system cannot be much worse than it normally is. The presence of a threshold effect in runoff generation related to storm volume reduces the maximum ratio of <i>T_Q/T_P</i> since it decreases the randomness of the runoff coefficients and increases the probability to be in a wet situation. 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