Groundwater modelling report - Lower Namoi. CSIRO: Water for a Healthy Country National Research Flagship

[摘要] A numerical model analysis was undertaken for the Lower Namoi alluvium to assess the long-term impact of a proposed preliminary extraction limit of 70 GL/year extracted from groundwater resources. Predictive models were developed and run for six 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 Namoi groundwater management unit (88 GL/year) and extraction at the proposed preliminary extraction limit of 70 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. 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.A number of general conclusions can be drawn from the modelling results.The model results indicate that the major sources of water supplying groundwater extraction are diffuse recharge (~20 GL/year), river leakage (~40 GL/year) and inflows from the Great Artesian Basinand Upper Namoi (~10 GL/year). Together these sources account for ~70 GL/year of groundwater inflow. When the extraction rate exceeds 70 GL/year (Scenario 2), a storage imbalance results and groundwater levels decline. The storage terms are more closely balanced when the extraction rate is 70 GL/year (Scenario 3), yet there can still be a slight deficit in storage terms which indicates some continued dewatering of the Gunnedah Formation (~5 GL/year) until a new dynamic equilibrium is established. The declines can be limited if extraction is redistributed throughout the model domain and removed from regions of the model where groundwater inflows are lower (Scenario 4).A zone of substantial drawdown was evident in the region to the north of the Pian Creek in the vicinity of GW025144 and GW025249 under scenarios 2, 3, 5 and 6. The extent of the drawdown is such that Gunnedah Formation becomes unconfined in places. It appears that the drawdown occurs in this zone because the area is subject to a substantial amount of extraction yet does not receive significant recharge from the river or from flooding events. The extraction in this zone was redistributed under Scenario 4 with groundwater levels recovering such that the Gunnedah Formation remains confined. This result suggests that extraction within the Lower Namoi can be optimised to deliver sustainable outcomes.The results from scenarios 5 and 6 suggest that the groundwater levels and response to extractive stresses in the Lower Namoi are not particularly sensitive to diffuse recharge variability as driven by various future climate change assumptions.It may be concluded that the model preliminary extraction limit should be in the order of 70 GL/year factored down to 58 GL/year to account for model uncertainty.The groundwater model only covers about 70 percent of the entire SDL unit. A proposed preliminary extraction limit for the region outside the model domain has been obtained from the application of a recharge risk assessment method (RRAM). The method has been applied using recharge rates included in the calibrated groundwater model. The results indicate that the preliminary extraction limit for the non-modelled areas is 4.6 GL/year. The proposed preliminary extraction limit for the entire sustainable diversion limit unit is therefore 62 GL/year.

[发布机构] CSIRO

[学科分类] 地球科学(综合)