The River Ecological Sustainable Yield Model (RESYM)

Balancing the competing demands for water from consumptive users and the needs of the environment is the goal of sustainable water resource planning in Western Australia (WA). In the south west of WA, irrigated agriculture accounts for approximately 70% of total water use. To support water planning, estimates of consumptive demands for agriculture have been based on licensed entitlements, or where this information is not available, combinations of GIS-based land use mapping and published estimates of crop demand. Numerical models of the yield and reliability of large reservoirs supplying consumptive users have been available for some time. A range of approaches have been used in Australia to estimate the environmental flow (EF) requirements of rivers. The methods used in Australia (and elsewhere) over the past two decades differ in detail but are essentially habitat approaches that identify flows that achieve a required water depth in important aquatic and riparian habitats. These threshold flows are then used to 'construct' an EF regime. Ecologically important flow thresholds include those that provide access to spawning habitats, or maintain river pools and riffles. High winter flows inundate the river channel and maintain river channel morphology. Other flows drown out barriers and allow upstream breeding migration of native fish. High winter flows inundate riparian vegetation and fill wetlands on the river floodplain. There has been no standard way however, to create an EF regime that approximates the natural seasonal pattern, frequency and duration of spells above ecologically important thresholds. Nor has there been a standard way to use the results of ecological studies to quantify the ecologically sustainable yields (ESYs) from surface waters and inter-annual variation in these yields. Previously in WA, researchers have used flow gauging records to calculate the historic frequency and duration of spells above ecologically important thresholds, and used these calculations to construct average annual and seasonal EF regimes. Recent water resource planning in south west WA has clearly showed that statistical analysis of historic time- series is by itself inadequate when identifying the EF regime needed to maintain the ecological health of rivers. These analyses have generated static estimates of EF requirements, which have been used by water managers to set environmental release rules for large water storages. The same volumes of environmental releases are made every year or month according to an agreed invariant schedule, with little regard to inter- annual variation in rainfall and flows. This approach essentially ignores the importance flow regime variation has in maintaining the evolutionary and future adaptive capacity of river ecology, and the impact of constant environmental releases on reservoir levels during drought periods. A better approach was needed for modelling the desired EF regimes of rivers in south west WA, and for optimising the operating rules of large water storages. To address this problem, the authors developed the River Ecological Sustainable Yield Model (or RESYM). RESYM is a FORTRAN program with a Windows based graphical user interface, which uses a water balance of inflows, storage attributes and operating rules to estimate the volume of water that can be harvested given a specified set of environmental releases. Time- series of natural and impacted flows can then be used to assess changes in the frequency and duration of flows above ecologically important thresholds. Time-series of abstractions from the storage can be used to assess operational aspects of dam management and water supply, such as the frequency and duration of restrictions on consumptive use. RESYM is designed to be used in an iterative fashion by expert panels in a workshop setting to construct non-static EF regimes. This paper discusses the development of RESYM, and its use in determining the ESY and environmental release rules of large reservoirs in south west WA.