Mathematical optimisation of drainage and economic land use for target water and salt yields

Land managers in upper catchments are being asked to make expensive changes in land use, such as by planting trees, to attain environmental service targets, including reduced salt loads in rivers, to meet needs of downstream towns, farms and natural habitats. End-of-valley targets for salt loads have sometimes been set without a quantitative model of cause and effect regarding impacts on water yields, economic efficiency or distribution of costs and benefits among stakeholders. This paper presents a method for calculating a ‘menu’ of technically feasible options for changes from current to future mean water yields and salt loads from upstream catchments having local groundwater flow systems, and the land-use changes to attain each of these options at minimum cost. It sets the economic stage for upstream landholders to negotiate with downstream parties future water-yield and salt-load targets, on the basis of what it will cost to supply these ecosystem services.