2.1.5 Surface water – groundwater interactions


Interaction between groundwater and surface water in the Galilee subregion manifests in various ways including: baseflow to rivers, discharge to springs and lakes, and groundwater-dependent ecosystems. Available streamflow gauge data for Belyando and Thomson rivers has allowed development of an approach to estimate the baseflow contribution to stream flow. A major assumption behind the approach is that all baseflow is derived via groundwater discharge from aquifers, which is currently untested in the Galilee subregion.

The Belyando River flows partly inside and partly outside of the Galilee subregion. Its flow is characterised by high discharges in response to large rainfall events followed by long periods of virtually no flow. The Belyando River hydrograph, however, exhibits significant baseflow recession which may persist for up to two months after a high-flow event. These high-flow events usually occur during the summer months. The calculated mean annual baseflow index for the Belyando River at Belyando Crossing (75 km downstream of the Galilee subregion) is 0.127, giving a mean annual baseflow of about 83,450 ML/year. The groundwater flux through the Cenozoic sediments into the Belyando River, computed using Darcy’s Law, is 7950 ML/year, which means that only 9.5% of the mean annual baseflow is generated within the subregion.

The Thomson River flows through Longreach and is the major stream draining the central part of the Galilee subregion. The Thomson River is not a single channel, but like the other major rivers of western Queensland, is strongly anastomosed with up to several parallel braided channels entrenched deeply in alluvial belts 3 to 5 km wide. The riverbed contains many elongated waterholes, some of which contain permanent water. The mean annual discharge based on 40 years (1970–2010) of records from the gauge at Longreach is 1,246,600 ML/year. The Thomson River is believed to be underlain by a paleochannel of Cenozoic age, which is incised into the Winton-Mackunda formations for a depth of up to 200 m. Like the Belyando River, flow in the Thomson River is characterised by high discharges in response to large rainfall events interspersed with long periods of virtually no flow. However, bed underflow in the paleochannel sediments continues indefinitely.

The annual baseflow index of the Thomson River at Longreach is 0.180, giving a mean annual baseflow of 224,390 ML/year. Lateral seepage to the Thomson River paleochannel and upwards leakage from the Winton-Mackunda formations is estimated to be 163,670 ML/year which comprises 73% of total baseflow.

Data that may improve the understanding of surface water—groundwater interactions includes: shallow groundwater and surface water chemistry and isotopic data; time-series of groundwater level measurements; a better understanding of degree of connectivity; geological variation in alluvial and Paleogene–Neogene sediments and variation in hydraulic properties of shallow aquifers; and an improved understanding of the water sources for the baseflow component of streamflow.

Last updated:
6 December 2018
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