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

[摘要] A numerical groundwater flow model was used to assist with the assessment of a preliminary extraction limit for groundwater pumping from the Upper Namoi alluvial aquifer. Predictive models were developed and run for seven fifty-year scenarios and the results assessed against four criteria aimed at determining the sustainability of each particular scenario. These criteria are:• Criterion 1. Stabilisation of groundwater levels (productive base, key environmental assets). To meet this sustainability criterion groundwater levels must have stabilised or be rising at the completion of the scenario model run – i.e. after 43 years of extraction at the defined preliminary extraction limit – at the resource condition indicator (RCI) sites.• Criterion 2. Stabilisation of extraction (productive base). The groundwater extraction rates are automatically reduced in groundwater models if a model cell from which groundwater is being extracted dries out due to excessive drawdown. To meet this criteria the model must maintain pumping at the required rate for the duration of the scenario model run.• Criterion 3. Prevention of dewatering confined aquifers (productive base, key environmental outcomes). To meet this criterion the predicted groundwater levels at all RCI sites must remain above the top of a confined aquifer.• Criterion 4. Maintenance of current environmental river flows (key ecosystem functions). This criterion mandates that the sustainable extraction limit must be equal to or less than the current level of groundwater extraction. In this regard the current level of groundwater extraction is defined as the average of the last five years of groundwater extraction.The results of this investigation suggest that groundwater extraction allowed under the current New South Wales Water Sharing Plan (71.4 GL/year including an allowance of 1.0 GL/year for stock and domestic use) will lead to minor or localised breaches of sustainability criterion 1 (stabilisation of groundwater levels) and criterion 2 (maintenance of the required extraction rate).Breaches of criteria 1 and 2 occur at and around a single RCI site (GW036415.3) located in an area relatively isolated from the remainder of the model domain.It is likely that the failure to meet the criteria in this area is a product of its limited connectivity to the remainder of the aquifer and it is assumed that the breaches can be managed through the effective implementation of local area management rules.A preliminary extraction limit of 71.4 GL/year is recommended for the modelled part of the Upper Namoi Alluvial Aquifer. Modelling results indicate that the groundwater levels stabilise rapidly in all scenarios. This outcome generally arises from the fact that the aquifer is a relatively narrow alluvial strip that follows the current river channel and that there is a strong hydraulic connection between the aquifer and the river. Increases in groundwater extraction are therefore derived mostly from reductions in river flow rather than from changes in storage, evapotranspiration or lateral fluxes into the aquifer. It may be concluded that the preliminary extraction limit for the model domain should be in the order of 71.4 GL/year, factored down to 61.0 GL/year to account for model uncertainty.The groundwater model only covers about 63 percent of the entire sustainable diversion limit (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 25.3 GL/year. The proposed preliminary extraction limit for the entire SDL unit is therefore 86.3 GL/year.

[发布机构] CSIRO

[学科分类] 地球科学(综合)