2.1.5 Surface water – groundwater interactions


This section presents a baseline assessment of surface water – groundwater connectivity in the Namoi subregion representing June 2012 conditions, or as close as possible. The resultant connectivity map was compared to those previously developed using similar approaches. In previous studies, the surface water resources of the Namoi river basin and the underlying aquifers were mapped as being generally connected upstream of Wee Waa. In comparison, the June 2012 connectivity assessment has highlighted more widespread regions of disconnection between the aquifer and river systems. Areas of increased disconnection are evident in the Mooki river basin, in the region extending from Quirindi downstream to Caroona. Areas transitioning to disconnection are also evident in the mid-section of the Mooki river basin. Other areas in the Upper Namoi showing disconnected and transition classes include reaches around Carroll and a small section upstream of Maules Creek and reaches both upstream and downstream of the previously mapped disconnected reach between Mullaley and Boggabri in the Coxs creek basin.

This section also presents a hydrological assessment of the Mooki river basin for 2003 to 2012 and a comparison with pre-2003 conditions. The data indicate that baseflow is generated in the upper part of the river basin, with a notable decrease in baseflow contributions and flow duration downstream consistent with earlier interpretations. In comparison to pre-2003 conditions, the mean flows (total discharge and baseflow) over the 2003 to 2012 period are considerably reduced. The apparent reduction in flows is likely to be in part due to the drier than average rainfall conditions since 2003, as well as the influence of groundwater extractions on river connectivity and flows, although these associations require further investigation. A comparison of streamflow data for 2006 (dry year) and 2012 (wet year) demonstrates how the river reach varies in its dominant flux, with river losses dominating dry periods and gains over wet periods along the length of the Mooki River. The hydrological data indicate that connectivity is maintained along the length of the Mooki River. The exception is Quirindi Creek, which the hydrological analysis confirms is a disconnected system.

To better understand the changes in depth to watertable over time, and to provide a context to the 2012 baseline connectivity investigation, a comparison has been made to assess changes in depth to watertable between 2006 (dry year) and 2012 (wet year). The comparison shows that there were widespread increases in watertable level throughout the Upper Namoi river basin between 2006 and 2012, coinciding with higher rainfall years. Increases of up to 11 m were measured in some areas of the Upper Namoi river basin. The area between Caroona and Breeza in the Mooki river basin (Upper Namoi groundwater management zone 8), however, shows only modest increases in the watertable level (0.8 m or less) as evidenced by typical bore hydrographs. These smaller increases in watertable level appear to have been insufficient to return sections of the Mooki River mapped as transitional and disconnected reaches in this connectivity investigation, to a previous state of connection (based on earlier interpretations). Bore hydrographs in the area between Caroona and Breeza in the Mooki river basin have shown water level declines by about 11 to 12 m between the 1970s and 2012. These water level declines appear to be impacting on surface water – groundwater connectivity in the Mooki river basin. Downstream of Narrabri there were widespread areas where water levels decreased by up to 6 m over the 2006 and 2012 period. These declines may be as a result of time lag effects from groundwater extractions further up the catchment, over the previous decades, which have had the effect of reducing groundwater throughflow, in combination with the current levels of extraction in this area. The causes of widespread water level decline in the Lower Namoi require further investigation. There are limited data available to characterise the hydraulic properties and groundwater processes of the consolidated sedimentary (and minor volcanic) rocks surrounding and underlying the alluvium in the Namoi river basin. Analysis of a single bore hydrograph in the Mooki river basin suggests that there is an upward vertical hydraulic pressure gradient, most likely from the underlying basalt bedrock aquifer into the overlying alluvial aquifers. This finding is consistent with previous investigations indicating that upward vertical recharge in the Namoi river basin is not uncommon, although the extent of vertical connectivity and upward flow flux is unknown.

The findings presented here will be of value when analysing the results of the surface water and groundwater modelling, and provide a local-scale, detailed analysis to complement the broader regional modelling. The results here provide a means of ‘ground truthing’ the modelling results particularly the direction of surface water – groundwater flux, an important element of the water balance.

A summary of existing knowledge on surface watergroundwater interactions in the Namoi subregion was presented in companion product 1.1 (context statement) for the Namoi subregion (Welsh et al., 2014). This section builds upon that initial contextual information by presenting:

  • a more detailed review of several important surface water – groundwater investigations in the Namoi river basin, including summarising their key findings
  • the results of new hydrological analysis undertaken for this bioregional assessment (BA).

This section aims to expand on the more detailed findings of several important studies, summarising what the main surface water – groundwater controls are, and then presents new hydrological analysis to better understand surface water – groundwater interaction in the Mooki river basin, an area of Namoi subregion that is of critical importance for this BA due to the high level of expected coal resource development.

In the context of this BA, the review and analysis of surface water – groundwater interactions presented here is critical to inform the conceptual understanding of both the groundwater system, as investigated through the numerical groundwater modelling (see companion product 2.6.2 for the Namoi subregion (Janardhanan et al., 2018)), and the rivers and creeks, investigated through the surface water numerical modelling (see companion product 2.6.1 for the Namoi subregion (Aryal et al., 2018)). Surface water – groundwater interactions in the Namoi subregion are complex, varying spatially and over time, and respond to a range of drivers, both natural and anthropogenic. These factors are further discussed in the sections that follow.

Last updated:
6 December 2018
Thumbnail of the Namoi subregion

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