Groundwater modelling report - Lower Lachlan. CSIRO: Water for a HealthyCountry National Research Flagship

[摘要] A numerical model analysis was undertaken for the Lower Lachlan alluvium to assess the long-term impact of a proposed preliminary extraction limit of 90 GL/year extracted from groundwater resources. Predictive models were developed and run for eight different scenarios.Scenario 1 was run as a base case and includes no groundwater extraction. It is used to demonstrate a no-development condition under a historical median climate. Scenarios 2 and 3 were run to illustrate the impacts of extraction at the current extraction limit under the New South Wales Water Sharing Plan for the Lower Lachlan groundwater management unit (119 GL/year) and extraction at the proposed preliminary extraction limit of 90 GL/year respectively. Both of these scenarios were run under a historic median climate. Scenario 4 was equivalent to Scenario 3, but with extraction redistributed throughout the model domain to limit drawdown at resource condition indicator (RCI) monitoring sites. This scenario was not run for the Lower Lachlan model. Scenarios 5 and 6 were run to assess the impact of climate change whereby dry (Scenario 5) and median (Scenario 6) climate change projections were used to scale diffuse recharge. Scenario 7 was run to assess the impact of reduced irrigation recharge. Additional scenarios 8 and 9 were run with extraction further reduced to 61 and 35 GL/year respectively.A number of general conclusions can be drawn from the modelling results.The model results indicate that the major source of water supplying groundwater extraction is a depletion of aquifer storage. This outcome results in substantial and prolonged declines in groundwater levels predicted for all extraction scenarios. The phenomenon is largely due to the fact that the aquifer is effectively disconnected from the river throughout most of the model domain, groundwater levels being substantially lower than river bed elevation. Simulation of groundwater extraction at current plan (108 GL/year plus 11 GL/year for stock and domestic water supply), preliminary extraction limit (90 GL/year) and reduced preliminary extraction limit (61 GL/year) did not result in establishment of a new dynamic equilibrium within the fifty-year prediction period.A zone of substantial drawdown was evident in the east of the model under scenarios 2 to 8. This is the region of freshest water and the centre of groundwater extraction.The results from scenarios 5, 6 and 7 suggest that the groundwater levels and response to extractive stresses in the Lower Lachlan are not particularly sensitive to diffuse recharge variability – rainfall and irrigation – as driven by various future climate change assumptions.

[发布机构] CSIRO

[学科分类] 地球科学(综合)