18.4 Discharge to surface water
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 from 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 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 comprises 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 calculated baseflows (Department of Sustainability and Environment 2010a).
Groundwater–surface water interactions were not considered to be important for the regional groundwater balance, and their omission does not have a material impact on this report.