Classification of natural flow regimes in Australia to support environmental flow management

1. The importance of hydrologic variability for shaping the biophysical attributes and functioning of riverine ecosystems is well recognised by ecologists and water resource managers. In addition to the ecological dependences of flow for aquatic organisms, human societies modify natural flow regimes to provide dependable ecological services, including water supply, hydropower generation, flood control, recreation and navigation. Management of scarce water resources needs to be based on sound science that supports the development of environmental flow standards at the regional scale. 2. Hydrological classification has long played an essential role in the ecological sciences for understanding geographic patterns of riverine flow variability and exploring its influence on biological communities, and more recently, has been identified as a critical process in environmental flow assessments. 3. We present the first continental-scale classification of hydrologic regimes for Australia based on 120 metrics describing ecologically relevant characteristics of the natural hydrologic regime derived from discharge data for 830 stream gauges. Metrics were calculated from continuous time series (15-30 years of record constrained within a 36-year period) of mean daily discharge data, and classification was undertaken using a fuzzy partitional method - Bayesian mixture modelling. 4. The analysis resulted in the most likely classification having 12 classes of distinctive flow- regime types differing in the seasonal pattern of discharge, degree of flow permanence (i.e. perennial versus varying degrees of intermittency), variations in flood magnitude and frequency and other aspects of flow predictability and variability. Geographic, climatic and some catchment topographic factors were generally strong discriminators of flow-regime classes. The geographical distribution of flow- regime classes showed varying degrees of spatial cohesion, with stream gauges from certain flow- regime classes often being non-contiguously distributed across the continent. These results support the view that spatial variation in hydrology is determined by interactions among climate, geology, topography and vegetation at multiple spatial and temporal scales. Decision trees were also developed to provide the ability to determine the natural flow-regime class membership of new stream gauges based on their key environmental and / or hydrological characteristics. 5. The need to recognise hydrologic variation at multiple spatial scales is an important first step to setting regional-scale environmental flow management strategies. We expect that the classification produced here can underpin the development of a greater understanding of flow-ecology relationships in Australia, and management efforts aimed at prescribing environmental flows for riverine restoration and conservation.