In discussions about water allocation in the Murray Darling Basin it is generally assumed that runoff now is equivalent to what it was at the time of European settlement. Consider, for example, the following table from a Murray Darling Basin Commission Facts Sheet.
At the same time there is a perception that there are a lot fewer trees now than there were at the time of European settlement. As Ian Mott points out in the following comment, first posted at ‘Banking in the Macquarie Marshes’, if there are fewer trees now, then there is more water now:
“If you really want to correct the misconceptions that threaten you water allocations then you should correct the fallacy that the pre-irrigation runoff into the marshes was 460,000 megalitres which has since been reduced to 395,000ml by irrigation.
The pre-settlement runoff into the marshes would have been much less than 460,000ml and most likely less than 395,000ml because much of the upper catchment has been cleared for pasture. And this has substantially increased the catchment yield.
But don’t expect the MDB Mafia or the Land and Water audit people to concede this willingly. The work of Robert Vertessy and the CRC for Catchment Hydrology makes it very clear that the switch from forest to pasture increases water yield.
So your group has to determine the exact amount of clearing that has taken place in your catchment and overlay the rainfall data so you can find out the real historical water footprint for the marshes.
There is not the slightest room for doubt that the volume of water taken out of the system by irrigators, given that extractions are only 14% of current runoff, is less than the improvement in yield produced by clearing.
My understanding is that the upper Macquarie is not subject to widespread thickenning like the mulga and brigalow country so cleared land has been more likely to stay cleared.“

Jennifer Marohasy BSc PhD has worked in industry and government. She is currently researching a novel technique for long-range weather forecasting funded by the B. Macfie Family Foundation.