Determining trade-off between sustainable yield and baseflow in the Kulnura - Mangrove Mountain aquifer system using simulation optimisation modelling

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The public water supply in the Gosford-Wyong area of New South Wales is reliant on streams that originate in elevated sandstone country. About half of the stream flow is believed to be baseflow from the sandstone aquifer system in the Kulnura - Mangrove Mountain area. At the same time as the population is growing steadily on the coast, there is increased demand for groundwater for horticultural, agricultural and industrial purposes along the sandstone ridges. Hence, good groundwater management is critical, to ensure that stream baseflow is not jeopardised. A management model that couples a simulation model with an optimisation model has been developed for the Kulnura-Mangrove Mountain aquifer system to evaluate the trade-offs between increased aquifer yields and baseflow reduction. The project has been successful in developing trade-off curves for sustainable yield versus reduction in baseflow. It is believed that this is the first time that rigorous trade-off curves for sustainable yield have been developed for a stream-aquifer system in Australia. The objectives of this research were to determine the sustainable yield(s) of the aquifer system in relation to extraction limits from both groundwater and surface water; to determine the magnitude, distribution and dynamics of baseflow to the streams which drain the Kulnura - Mangrove Mountain aquifer; to determine groundwater entitlement limits that would preserve baseflow to streams in order to facilitate groundwater allocation policy; and to explore how groundwater extraction limits would change for tolerable reductions in baseflow. The simulation model is necessarily coarse, with 500 m spatial resolution, as replication of a very large regional aquifer was required. Given the wide variation in vertical relief in the area, approximately 400 metres, it was necessary to divide the vertical profile into 30 layers. Otherwise, it would not have been possible to track the many baseflow-receiving creeks that descend from high elevations to the sea. The calibration results of the simulation model show that the model performs very well in representing the values and the patterns of groundwater level for both steady state and transient conditions, is able to reproduce large vertical hydraulic gradients between aquifer layers, and also replicates baseflow reasonably well. The optimisation model was developed with the objective of preserving stream baseflow within tolerable limits while maximising the pumping rates from the aquifer system. Constraints were designed in terms of hydraulic gradient, with reduction tolerance ranges from 0.1 % to 10 %. Conversion from hydraulic gradient reduction to baseflow reduction was achieved by running reported optimal production patterns through the model in simulation mode. This work differs from that of previous researchers in not making a pre-emptive assumption of linearity between groundwater pumping and stream baseflow. A very large optimisation problem has been solved in this study, consisting of up to 5700 decision variables and 8000 constraints. The study has been successful in generating trade-off curves that will provide a scientific basis for government / community decisions on responsible water allocation between computing users.
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