Estimating groundwater-river connectivity factor for quantifying changes in irrigation return flows in the Murray–Darling Basin

ABSTRACT Concerns have been raised that the use of infrastructure and water efficiency projects to recover water entitlements for the environment in the Murray–Darling Basin (MDB) could be undermined by a reduction in return flows to rivers. Water use efficiency changes prior to 2009 had already reduced surface return flows. A dimensionless and normalised variable, connectivity factor (CF), has been used to analyse the cumulative impact on river flow from actions that alter groundwater recharge or extraction. CF is objectively estimated from existing modelling outputs for three large alluvial groundwater systems in south-eastern MDB relevant to a major water use efficiency program. Lower values of CF (0.05–0.45) imply lower reductions of return flows, less impacts on recovered water entitlements, with impacts more likely to be managed adaptively within current water management plans. The study should help allay concerns by explaining why previous estimates had been overly high.