Predicted salinity impacts from land use change: comparison between rapid assessment approaches and a detailed modelling framework

Abstract

This paper illustrates the hydrological limitations and underlying assumptions of 4 catchment modelling approaches representing different generic classes of predictive models. These models are commonly used to estimate the impacts of land use and management change on stream flow and salinity regimes within a target region. Three approaches are based on a simple conceptual framework that assumes a single layer groundwater aquifer and requires minimal information and calibration (Zhang-BC2C, CAT1D-BC2C and LUCICAT), whereas the fourth approach (CAT3D) adopts a fully distributed highly parameterised catchment model capable of simulating complex multi-layered groundwater aquifer systems. All models were applied to the Gardiner subcatchment within the Goulburn–Broken region of Victoria, identified as a National Action Plan for Salinity priority subcatchment. Current condition simulation results were compared with observed stream flow and groundwater hydrograph data. Results show that the simple frameworks predicted whole-of-catchment mean annual salt and water yield with minimum parameterisation. The fully distributed framework produced similar catchment-scale responses to the simple approaches, but required more intensive input data and solution times. However, the fully distributed framework provides finer temporal and spatial scale information within the catchment. The more detailed models (such as CAT3D) also have the predictive capacity to assess the within-catchment dynamics at a range of scales and account for landscape position and complex surface/groundwater interactions. This paper concludes that the simple frameworks are useful for judging the whole-of-catchment impacts of broad-scale land use change on catchment water yields and salinity and therefore provide valuable tools for community engagement. However, the within-catchment dynamics are not well represented and particular care must be taken when applying such models in those catchments where the interaction between groundwater and surface features result in saturated areas that are disconnected from streams. Adoption of a distributed groundwater modelling environment similar to that of CAT3D provides higher spatial resolution relative to the lumped broad scale groundwater glow system (GFS) based parameterisation adopted by the BC2C rapid assessment approaches. The developers of the BC2C model acknowledge that such models are currently limited to upland local and intermediate groundwater flow systems. Given that the majority of land salinisation is located in regions dominated by intermediate and regional groundwater systems, this tool is not well suited to adequately model regional processes. In contrast, the CAT3D distributed groundwater models are likely to be applicable across a range of scales and provide the capacity to assess the trade offs between salinity recharge and discharge intervention strategies. We conclude that more complex models (e.g. CAT3D) are needed to identify at the land management scale (paddock/farm) cost effective land use and land management changes within the catchment to improve catchment health.