18 th World IMACS / MODSIM Congress, Cairns, Australia 13-17 July 2009 http://mssanz.org.au/modsim09 Quantifying environmental water demand to inform environmental flow studies Marsh N. 1,2,3 and T. Pickett 1,2 1 eWater Cooperative Research Centre 2 CSIRO, Land and Water, 3 Australian Rivers Institute, Griffith University Email: nick.marsh@csiro.au Abstract: The process of undertaking an environmental flow study often results in the identification of ecologically important components of the flow regime, some of these flow components are ultimately implemented as formalised dam release or water harvesting rules. The quantification of environmental water is often not realised until specific flow rules or dam release rules are implemented within a detailed hydrologic model. However, this quantification of environmental water is valuable to the experts conducting the initial environmental flow study to 1) be able to communicate the priority of environmentally significant components of the flow regime; 2) consider undesired ecological effects such as unnatural looking saw-tooth flow regimes and 3) inform the dam release rules by advising on the ecological trade offs between release strategies such as augmenting existing high flows (often called piggy-backing) or relating the release rules to historical seasonal cycles. We have developed a software utility ‘eFlow tool’, which can construct flow time series via augmentation of a current-case of flow by reconstructing environmentally important components of the flow. The user can adopt a range of flow augmentation strategies such as mimicking the natural frequency of events, augmenting existing flows, or waiting until the last possible day in the season of interest before allowing augmentation to commence. Augmentation strategies can also vary through time to reflect the observation that as the time since successfully achieving a specific flow component environmental flow increases, the importance in having the criteria met also increases. The output from the eFlow tool allows users to quantify the additional water requirements of meeting specific environmental flow components and to then investigate the sensitivity of the water cost to different trigger flow thresholds or event durations. Keywords: environmental flow, decision support, water demand 3844