18.4 Discharge to surface water

Supporting information

This item represents discharge to surface water from groundwater. This volume could not be quantified in a way that was complete, neutral, and free from material error.

Discharge to surface water from groundwater occurs when groundwater levels are higher than the surface water levels. This generally occurs in upland areas of the landscape, and can be constant, seasonal, or episodic depending on the conditions.

A knowledge gap exists regarding groundwater–surface water interactions in the Melbourne region. Information about groundwater discharge to surface water was not reported in the Port Phillip Catchment Management Authority, groundwater model (Department of Sustainability and Environment 2010a) or in the Groundwater Resource Appraisal for Southeast Melbourne (Southern Rural Water Authority 2010).

Together with evapotranspiraton, groundwater discharge to surface water is one of the major natural groundwater discharge processes in the Melbourne region. The Port Phillip and Western Port Catchment Management Authority reports that the main watercourses in the Melbourne region gain flow from groundwater discharge along the majority of their length (Department of Sustainability and Environment 2010a).

Calculated baseflow indices range from 0.69 for the Yarra River at Millgrove to 0.2 for the Little River at Little River with an average of 0.4. This indicates that, in general, 40% of streamflow in the Melbourne region is comprised of groundwater inflow into the river; however, the widespread development of water storage schemes, diversions and other flow regulations is likely to significantly bias observed total flows and hence the calculated baseflows as stated in the report (Department of Sustainability and Environment 2010a).

Groundwater–surface water interactions are considered to be important for the regional groundwater balance, but currently a knowledge gap exists in developing a suitable methodology to estimate surface water and groundwater interactions.