Characterising the location and rarity of annual maxima rainfall in the GTSM-R coastal zone

© CURRAN-CONFERENCE. All rights reserved. Estimates of the frequency and severity of rainfall over an individual catchment can be informed by analysing long time-series of historic rainfall over a larger region with similar hydrometeorological characteristics. This is important, because the likelihood of observing rare storms increases as the size of the region increases. Therefore, if the observed storms can be defensibly transposed to the catchment of interest, less extrapolation is required to estimate rainfall depths of extremely rare annual exceedance probabilities (AEPs). This substitution of space for time to characterise extreme rainfall probabilities is currently being applied to seven catchments within the GTSM-R coastal zone of Australia, for storm burst durations of 1-7 days. Given the size of the GTSM-R coastal zone, automated GIS-based searches of gridded, daily rainfall data are being used to find the rarest storms in the historic record (1900 to date). Using gridded, daily rainfall data from the Australian Water Availability Project (AWAP) to analyse the location and rarity of historic storms in the GTSM-R coastal zone poses some technical challenges. These include selection of an appropriate index value to assess storm severity, accounting for differences in prevailing storm orientations across the country, and correcting for the potential bias inherent in the gridded rainfall estimates. This brief paper discusses these technical challenges, and presents selected results from storm searches that may be useful to other hydrologists for their catchments of interest. These results include the location, type and estimated rarity of annual maxima storms of varying durations and size, and differences in the ratio of 1 to 3-day and 7 to 3-day 2% AEP rainfall across the GTSMR-zone. Comment is also made on the “effective independent record length” of rainfall records within high, moderate and low gauge density regions of the GTSM-R coastal zone.