How is the intensity of rainfall events best characterised? A brief critical review and proposed new rainfall intensity index for application in the study of landsurface processes

In many studies of landsurface processes, the intensity of rainfall events is expressed with clock-period indexes such as I₃₀, the wettest 30-minute interval within a rainfall event. Problematically, the value of I₃₀ cannot be estimated for rainfall events shorter than 30 min, excluding many intense convective storms. Further, it represents a diminishing proportion of increasingly long rainfall events, declining to <2% of the duration of a 30-hour event but representing 25% of the duration of a two-hour event. Here, a new index termed ED_f5 is proposed: It is the rainfall depth in the wettest 5% of the event duration. This can be derived for events of any duration. Exploratory determinations of ED_f5 are presented for two Australian locations with contrasting rainfall climatologies-one arid and one wet tropical. The I30 index was similar at both sites (7.7 and 7.9 mm h^-1) and was unable to differentiate between them. In contrast, ED_f5 at the arid site was 7.4 mm h^-1, whilst at the wet tropical site, it was 3.8 mm h^-1. Thus, the ED_f5 index indicated a greater concentration of rain at the arid site where convective storms occurred (i.e., the intensity sustained for 5% of event duration at that site is higher). The ED_f5 index can be applied to short, intense events that can readily be included in the analysis of event-based rainfall intensity. I₃₀ therefore appears to offer less discriminatory power and consequently may be of less value in the investigation of rainfall characteristics that drive many important landsurface processes.