Incorporation of snowmelt into joint probability event based rainfall-runoff modelling

In the Australian context, the influence of snowmelt contribution to design flood estimates is typically considered negligible due to climatic and topographic conditions. However, there are some catchments in Victoria, New South Wales and Tasmania which receive significant annual snowpack depths. Design flood estimates for these catchments must incorporate the effects of this snowpack melting during a storm and contributing to the peak flow and volume of runoff entering the reservoir. This paper describes the incorporation of an event-based snowpack melting module sourced from the United States Bureau of Reclamation into the hydrologic model RORB, and its subsequent use in a joint probability framework for the estimation of design floods. The paper describes the conceptual hydrologic processes at play in melting snowpack, and discusses how these processes are represented numerically in the USBR module. The various model parameter values are identified and discussed, and guidance is provided on how to use recorded snowpack and meteorological data to estimate the historic or design values of these parameters. The incorporation of the USBR module in RORB’s native ‘rain-only’ joint probability framework is also discussed. An example application of the model is provided for two catchments in south-east New South Wales. The process of model calibration to historic events is discussed, as well as verification of the model results to complete gauge flood frequency curves. Finally, the annual impact of the snowmelt contribution to the design flood estimates for these catchments is presented and discussed.