How do the rain rates of sub-event intervals such as the maximum 5-and 15-min rates (I5 or I30) relate to the properties of the enclosing rainfall event?

The intensity-duration properties of rain have been explored using various approaches, including daily or hourly data, IDF and event-based analyses, and the study of rain properties across intra-event durations such as the widely used I-30 indicator of peak event rain rate. This article explores the way in which intra-event rain rates (IERRs) such as I-30 vary with the duration of the enclosing event and with the approach used to delineate events. Pluviograph records from two Australian sites are subdivided into events using the minimum inter-event time (MIT) approach, and the duration, depth and rain rate properties of the events determined. The IERRs for intervals from 1 to 60 min are extracted for each event, and generally these rates (including I-30) exhibit statistically significant increases in longer enclosing rain events. The mean value of parameters such as I-5 and I-30 across all rain events also varied in more complex ways with the MIT criterion used to define the enclosing events. Thus, measures of IERR are significantly affected by the data processing protocols used to derive them. Wider exploration of these phenomena from other rainfall climates will assist in the interpretation of hydrologic, soil erosion, agrochemical wash-off and other data that in published literature are interpreted differently in relation to event mean rainfall properties or to selected intra-event properties such as I-30. Understanding of these issues is important in the selection of an appropriate rainfall descriptor in event-based hydrologic and erosional research. Copyright (C) 2010 John Wiley & Sons, Ltd.