A hydrologic Modelling tool for investigating enviornmental flow options

The major issue in water resources management in Australia is the need to stop the degradation of the rivers that is occurring chiefly as a consequence of the diversion of water for consumptive use. This issue is most important in the Murray-Darling Basin (Figure 1) where diversions for consumptive use are greatest. Median outflows from the basin are now approximately 20% of natural outflows (MDBC, 1995). Seasonal flow patterns, low flows and the frequency, volume and duration of small to medium floods have been most affected. Major water resources developments have occurred in the Murray-Darling Basin since the late 19th Century, principally since 1948. These have been mainly intended to provide reliable water supplies for irrigated agriculture, and also to provide water for hydro-electric power generation in the case of the Snowy Mountains Scheme. Developments include major storage dams, re-regulating weirs and irrigation infrastructure. Main irrigated crops are cotton in the north of the basin and rice, horticulture and pastures in the south. Publicly funded structural development is now largely complete and emphasis is now on improvements in the management of existing systems and resolution of conflicts between users competing for the scarce water resources. In the state of New South Wales (NSW), which includes approximately two thirds of the basin area and accounts for almost 60% of the water use, environmental flow policies are being developed which aim to improve the health of the rivers and wetlands through a partial restoration of the flow regime to that which existed prior to development. Policy options under consideration include rules for sharing of small to medium flood flows between consumptive users and the environment, augmentation of floods to prolong wetland inundation, and reproducing low flow patterns, including complete cessation of flow where appropriate. Simulation modelling is extensively used for development of these environmental policies and to assess the trade offs involved. This paper discusses the major environmental flow related issues and the model being used for carrying out the studies needed.