What does I ₃₀ tell us? An assessment using high-resolution rainfall event data from two Australian locations

The maximum depth of rainfall in any 30-minute period - I ₃₀ - 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 I ₅ , I ₁₀ , and I ₁₅ . However, there has been little study of the ability of I ₃₀ 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 I ₃₀ 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 I ₃₀ 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 I ₃₀ 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 I ₃₀ 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 I ₃₀ index that is strictly comparable to those of longer events. Illustrative results for this new index are presented.