Water sharing for the environment and agriculture in the Broken catchment

The Commonwealth of Australia Water Act 2007 changed the priority for water use in the Murray-Darling Basin (MDB) to first ensure environmentally sustainable levels of extraction and then to maximise net economic returns to the community from water use. The Murray- Darling Basin Authority (MDBA) is expected to deliver a draft Basin Plan in 2011 providing a framework for future water planning. The Plan will include Sustainable Diversion Limits (SDLs) which define water diversions for consumption while maintaining environmental assets and ecosystem functions. The 2009 MDBA Concept Statement acknowledged that in some areas less information is available to determine the SDLs. The 2010 MDBA Guide to the Basin Plan proposed SDLs reducing the current long-term average surface water diversions to between 25 and 34% for the Goulburn-Broken region. Representative farm-level models of irrigated dairy, horticulture and viticulture, and dryland broadacre, industries were developed to determine the likely impacts on farm income and farm enterprise mix if the price and quantity of irrigation water changes. Water for ecological benefits and ecosystem functioning was determined for a range of river health levels using a bottom-up approach identifying flow requirements for fish, riparian vegetation, invertebrates, and geomorphic and nutrient processes. A novel part of the analysis is the conjunctive use of water for both purposes, e.g. wetland filling and then pumping for irrigation. The linkages between changed land use and surface/ground water outcomes are assessed using a Catchment Analysis Tool. An experimental design of different proportions of water going to the environment and consumptive uses showed potential trade-offs between agricultural, environmental and surface/ground water outcomes. These trade-offs were examined to assess the impact of alternative water management on catchment welfare, and provide information about setting SDLs.