Runoff and salt transport modelling to maximise environmental outcomes in the upper south east of South Australia

Water resource management often requires an integrated assessment of resource use options, including local and regional impacts on the environment and stakeholders. The Upper South East of South Australia is one such location, where a large drainage network has been constructed to manage flooding, dryland salinity and environmental outcomes. Within the drainage network, there are a number of regulators that enable water to be directed along different natural watercourses or drainage paths to meet management objectives. Given the recent dry period, there is a desire to manage the limited water available to maximise the ecosystem response at the many wetlands in the region. This paper describes the methodology undertaken to assist the planning of regulator settings within the drainage network for an upcoming water year. Coupled component models have been used to assess the outcomes expected for different regulator operating regimes. A hydrologic model representing the 1 million ha contributing to the drainage network was used to calculate the runoff expected from each catchment, as well as the movement of this water through the drainage network and into the wetland systems. The outputs from the hydrologic model, including catchment surface flows, baseflows, and wetland storage volumes were used as inputs to a water quality model, simulating the transport of salt at the catchment scale through the system. The hydrologic model and water quality model are ultimately used to produce relationships between the winter rainfall and the volume that can be delivered to target wetlands in the network, for a range of maximum salinity thresholds. There are uncertainties in each step of the coupled component modelling procedure. One large source of uncertainty is the climate for the upcoming year is unknown. This will obviously impact the water available for environmental purposes. Secondly, the groundwater salinity, required as an input for salt transport modelling, is sparsely gauged in space and time in the region. A 20 year period has been simulated to represent a range of climate inputs to the model, and various values of groundwater salinity have been considered to provide an indication of the impact these uncertainties have on the simulated volumes. A total of 128 regulator settings were considered to determine the configuration that can maximise the volume of water delivered to each target wetland for a range of maximum salinity thresholds. It was found that the optimal regulator settings were largely insensitive to the variability in the climate or groundwater salinities, with the same groups of settings identified as performing the best for each target wetland. The most upstream regulators were also found to have little impact on the volume of water available at the most downstream target wetlands, and hence the upstream regulators could be set to satisfy the requirements of the closest target wetlands. However, the downstream target wetlands were in competition with each other for the water resource, and therefore these regulators must be set based on the highest water requirement in a given year. Relationships were derived between the driving rainfall variable and the volume available at each target wetland, for a range of maximum salinity thresholds. This information can be used to determine the likelihood of meeting environmental objectives in the region, based on a forecast of the winter rainfall for the upcoming year. This work highlights the importance of keeping the modelling objectives in mind when considering the outputs and uncertainties involved in integrated modelling assessment.