According to Bill Heffernan about 40% of the inflow into the Murray-Darling Basin comes from groundwater. Therefore accounting for groundwater and stream flows separately will overstate the availability of water supplies in the MDB by 40%. Indeed, in some river valleys, up to 90% of the river comes from groundwater. If the groundwater is pumped out then, ignoring recharge, there just isn’t much net contribution to the river left. But, as caps have been placed on licences to use river water, farmers have increasingly turned to harvesting groundwater.
Heffernan’s 40% figure seems to be a stab in the dark based on experience in the southern part of the basin and Malcolm Turnbull rejects the figure as being far too high. Indeed, the issue of determining the degree of connectivity between groundwater and stream flow supplies is complex. However there does seem to be an issue of concern here.
According to the AFR today (subscription required) hydrologist Richard Evans states that ‘if (groundwater) extractions continue to grow, by 2050 the loss to the River Murray will be around 711 gigalitres’. Such a loss is equivalent to half the water that needs to be put back in the River Murray to restore its health.
Recognising the link between groundwater and stream flows (something Malcolm Turnbull emphasises with his advocacy of the need to better map groundwater supplies) suggests the Federal Government’s buyback of water rights might need to be significantly increased from its current level of $3 billion. Predictably (though irrationally) the National Party oppose any further buybacks of groundwater licences. They will continue to claim that increased water supplies can be achieved by improved efficiencies in returning water to the environment - with these efficiencies funded in the main through publicd handouts.
The National Water Commission have found unsustainable extractions of water in Western NSW while the problem in Victoria and Queensland is less clearly known.
It is worth noting that water diverted from river tributaries and stored in farm dams also significantly depletes overall water supplies in the MBD. Farmers can build levies and dams to capture water flows without restriction in western NSW. Moreover, there are hundreds of thousands of such dams which divert water from rivers. A major part of the PM’s water initiative is to better calibrate such water uses via metering schemes.
A descriptive picture of the use of groundwater in the Murray-Darling is here. Of course, the use of groundwater not only depletes stream flow supplies but also increases the salinity of such supplies through rising groundwater tables.