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

[摘要] The existing groundwater flow model (SKM, 2002 and SKM, 2008) was updated and revised. The principal changes to the model include:• extending the calibration period from 1980 to 2008• updating groundwater extraction data to include pumping from wells outside current groundwater management units (unmetered extractions)• review and revision of metered groundwater extraction data included in the model• increasing the number of groundwater observation bores used for model calibration.As a result of the changes to the model it was necessary to recalibrate. The calibration process was undertaken successfully and resulted in acceptable calibration statistics. Groundwater levels predicted by the model are consistent with and provide a good match to time series water level data measured in observation bores located throughout the model domain.The groundwater model was used to assess the long-term impact of a preliminary extraction limit for groundwater pumping from the Upper Condamine 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 the particular scenario. These criteria are:• Criterion 1. Stabilisation of groundwater levels (key environmental function). 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). 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 environmental outcome). 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 current levels of groundwater extraction (52.5 GL/year) in the area of the Upper Condamine groundwater model domain will lead to continued decline in groundwater levels for the duration of the fifty-year scenario. This response breaches criterion 1 (stabilisation of groundwater levels) and criterion 2 (maintenance of the required extraction rate) and hence is considered to be unsustainable. The result is consistent with the fact that the extraction rate (52.5 GL/year) exceeds the total recharge to the model. The preliminary extraction limit of 33 GL/year was chosen as this is equivalent to the average annual recharge to the model domain in the calibration model. The model results in a minor breach of criterion 1 and this result was repeated for the lower preliminary extraction limit of 23 GL/year. The breach in criterion 1 for these scenarios (scenarios 3 to 8) occurs at a single RCI site (42231398A) that is located adjacent to the model boundary in the east of the Oakey Creek alluvium. It is assumed that the result is influenced by the model boundary and, in any event such localised declines could be controlled by local area management rules if necessary. It may be concluded that the model preliminary extraction limit should be in the order of 33 GL/year factored down to 29 GL/year to account for model uncertainty.

[发布机构] CSIRO

[学科分类] 地球科学(综合)