Defining Sustainable Diversion Limits in Unregulated South-west Western Australian Catchments

In Western Australia, the Department of Water determines the water sharing arrangement in surface water systems. The Department does this by defining the sustainable yield of individual catchments. The sustainable yield is the maximum quantity of water available for extraction from a surface water resource after environmental water provisions have been set. Currently the Department of Water uses a regional model to estimate annual sustainable yields in south-west Western Australia. This paper describes a more defensible process for defining sustainable limits on diversions from unregulated catchments in this region during the winter-fill period. A study team was assembled to help develop the recommendations, the objective being to define a sustainable diversion limit (SDL) beyond which there is an unacceptable risk that additional extractions would degrade the environment. The team was comprised of researchers and practitioners with knowledge of stream ecology, water quality, hydrology, wetlands, and catchment management. SDLs were developed by trialling various rules on streamflow data obtained for 142 gauged sites from across south-west Western Australia. The suitability of different rules was tested by investigating their impacts on diversion volumes and a range of hydrologic criteria across all parts of the study area. Particular attention was given to investigating the impacts of diversions during drought periods. A similar process was applied to develop the SDLs for Victoria in 2001-2002. Four rules were used to define SDLs: the period over which extractions are allowed, the minimum flow threshold below which extractions should cease, a maximum extraction rate, and an annual (licensed) volume associated with a specified reliability of supply. This paper describes the SDL rules adopted for south-west Western Australia, and compares the resulting distribution of SDLs to those derived for Victoria's gauged catchments.