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

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 ED_f5) 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 (ED_f5) 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 ED_f5 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 ED_f5 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.