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Discussion papers
https://doi.org/10.5194/hess-2019-99
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2019-99
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Technical note 11 Mar 2019

Technical note | 11 Mar 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

How rainfall event characteristics affect the applicability of I30 as an index of intense or erosive rainfall: a brief review with proposed new rainfall index

David L. Dunkerley David L. Dunkerley
  • School of Earth Atmosphere and Environment, Monash University, Melbourne, Victoria 3800, Australia

Abstract. In many fields, the intensity of rainfall events is expressed using indexes such as I30, the wettest 30-minute interval within a rainfall event. Various limitations attend this usage: I30 cannot be estimated for rainfall events shorter than 30 minutes, including many intense convective storms, and it represents a diminishing proportion of increasingly long rainfall events (representing 10 % of the duration of a 5-hour event but declining to < 2 % of the duration in a 30-hour event). These and other issues connected with I30 and related indices based on fixed clock periods (I15, I60, etc.) can be eliminated if instead, a nominated fraction of the event duration is used as an index, such as the wettest 5 % of the event duration. This index (termed EDf5) can be derived for both short and long rainfall events. Illustrative results are presented for two Australian locations having high-resolution rainfall data and contrasting rainfall climatologies, one arid and one wet tropical. The value of I30 is similar at both sites (7.7 mm h-1 and 7.9 mm h-1) and fails to differentiate between them. In contrast, the average intensity of the wettest 5 % of event durations (EDf5) at the arid site is 7.4 mm h-1, whilst at the wet tropical site, the corresponding value is 3.8 mm h-1. Thus, the EDf5 index indicates a greater concentration of rain at the arid site (i.e., intensity sustained for 5 % of event duration at the wet tropical site is lower). Results exemplify the capacity of the EDf5 index to be applied to short, intense events. The use of a fixed 30-minute clock period to describe intensity at the contrasting field locations has less discriminatory power and may be of less use in the investigation of rainfall characteristics that drive landsurface processes.

David L. Dunkerley
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Status: open (until 08 Jun 2019)
Status: open (until 08 Jun 2019)
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David L. Dunkerley
David L. Dunkerley
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Short summary
This research was undertaken to critically evaluate I30, a widely-used measure of rainfall intensity. Originally proposed from field area in the USA, I30 is now used internationally, in areas having very different rainfall duration and intensity. Results show that in long rainfall events, I30 reflects only the rainfall of a brief fraction of the rainfall; likewise, it cannot be derived for short rainfall events. A versatile new index is proposed, the rainfall of the wettest 5 % of an event.
This research was undertaken to critically evaluate I30, a widely-used measure of rainfall...
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