What does I30 tell us? An assessment using high-resolution rainfall event data from two Australian locations

The maximum depth of rainfall in any 30-minute period - I30 - has been used as an index of erosive rainfall events for more than half a century, and is a key component of the R factor in the Universal Soil Loss Equation (USLE) and its derivatives. Other indices are in common use, including I5, I10, and I15. However, there has been little study of the ability of I30 to capture the erosive nature of a rainfall event in regions beyond the foundation experimental work in the USA. This paper examines several apparently neglected issues: how reliance on fixed clock-periods might diminish the efficacy of the various indices of short-interval intensity; how well a single 30-minute period is able to reflect the intensity-duration properties of long enclosing rainfall events, and hence, whether the influence of event duration should be evaluated in applications of I30 and related indices. Results from two Australian sites with high-resolution rainfall records show that within the enclosing rainfall events, actual intensities corresponding to the I30 value (which is an equivalent intensity or ‘rainfall rate’, calculated from rainfall depth in 30 min) commonly occur for both less and >30 min. Often the I30 interval within a rainfall event excludes the most intense periods of rain, and in long events the index reflects ≪1% of the event duration. The I30 intensity can occur for total durations of 1–3 h in the events studied, or for just a few minutes. An alternative class of indices is proposed, in which fixed clock-periods are not used, and instead, the duration of rainfall within an event that exceeds a nominated intensity is recorded as an index of intensity. This has a number of advantages, including the ability to work with events shorter than 30 min, which are frequently intense but which cannot yield an I30 index that is strictly comparable to those of longer events. Illustrative results for this new index are presented.