Catchment Response to Farm Scale Land Use Change

Land use change is one of the primary management options available for the prevention or mitigation of dryland salinity but it also impacts stream water yield. This project sought to assist grain grower and catchment management groups target land use change so that catchment salt and water targets could be met at least-cost. The research was undertaken in the Simmons Creek catchment which exports salt into Billabong Creek near Walbundrie in southern New South Wales. The project developed and applied catchment bio-economic analysis that combined a conceptual model of catchment groundwater and salinity, farming systems modelling, and spatially explicit economic optimisation. This framework gave insight into the interactions and tradeoffs between catchment environmental services (water yield) and costs (salt load), and farm level production and economics. It showed where land use change is needed, the degree of land use change, and the cost of land use change to meet given salinity and water yield targets at least-cost. The model selects an arrangement of land use that preserves as much highly productive and profitable land use as possible. The maximum gross margin for the catchment, while maintaining the baseline salt load, is predicted to be about $3,000,000 per year. Shifts in land use to reduce salt load into Billabong Creek reduce farm income from this optimal level'although the reductions in gross margin are modest (<5%) at least until salt load has been reduced by ~50%. The marginal cost (cost per each extra tonne of salt load reduction) of reducing salt load gets progressively higher as greater reductions in salt load are sought. Land use change mainly in three (southern) subcatchments of Simmons Creek was identified as the least-cost means of reducing catchment salt load whilst minimising impact on water yield. These are the subcatchments with saline groundwater and where deep drainage reduction will have the most direct impact in reducing salt l