A hydrogeological-economic modelling approach to dryland salinity in the North Stirling Land Conservation District, Western Australia

Abstract

Associated with the problems of rising groundwater levels and dryland salinity are the conflicting economic and environmental objectives which aim to maximise net revenue from agriculture whilst still conserving the ecological standards of the natural environment. From an economic perspective, the central objective of salinity management should be to obtain the socially optimum value of resource use (including conservation) over time. Economics can contribute to the resolution of natural resource conflicts, by concentrating specifically on the allocation of scarce resources among competing demands, and determining which actions are economic from a catchment perspective. From an environmental standpoint, it is important to ensure that the natural environment is conserved not only today, but also for future generations. Despite the importance of managing the natural resource base from both an environmental and economic perspective, numerical models, which simulate the spatial and temporal movement of saline groundwater, have not been used extensively in economic analyses. Consequently, the focus in this thesis, is on the economic-physical system modelling of dryland salinity, in Western Australia. A transient hydrogeological-economic model has been developed and applied to the North Stirling Land Conservation District, a severely salt-affected area located in south-west Western Australia, to assess the current and future impacts of varying a catchment's land use pattern on groundwater levels and agricultural production. Several alternative salinity management options which may be applied to the District have been simulated, including an agronomic practice and an agroforestry strategy. Results of the simulations show that the choice of alternative land uses that should be adopted in order to reduced losses in gross margins in an area affected by dryland salinity, will depend primarily upon the effects of the various crop, pasture and tree species planted on the dynamic and spatial variations in groundwater levels, and the relative economic returns of those species. The 1995 value of potential gross margin receipts over the nine-year period (1992 to 2000, inclusive) is expected to be $10 million. By maintaining current land use practices across the study area, groundwater levels are estimated to rise by 1 m and the opportunity cost of salinity, in terms of foregone agricultural production, is estimated to be almost $6.7 million over this period. By adopting a discharge zone planting (or agronomic) strategy, whereby salt-tolerant shrubs are established on saline areas th a t can no longer su s ta in traditional agricultural production, piezometric heads are estimated to decrease by 0.3 m over the same nine-year period and the opportunity cost of salinity is estimated to decline by $720,000 compared to the 'do-nothing' approach. Alternatively, if agroforestry techniques (in this case alley farming) are introduced to those areas within the NSLCD study area that currently support traditional crops and pastures, average piezometric heads at the end of the forecasting period (year 2000) are estimated to be 0.4 m lower than average heads predicted under the agronomic option (or 0 .7 m com pared to the do-nothing' approach). Gross margins are expected to be $790,000 higher under the agroforestry approach than if existing farming practices are maintained. However, unlike the alternative scenarios modelled in this study, the estimated gross margins are expected to increase over the forecasting period as the higher water-using trees planted throughout the study area cause groundwater levels to decline further.

On a more general note, the modelling approach developed here is a generic one that can be successfully applied to many areas affected by salinity. Such models are beneficial as they provide additional information which helps the community, farmers, Landcare groups and the government make important decisions on how best to manage the agricultural and natural resources upon which we rely for productivity and environmental amenity. Show more