20 January, 2010

Flood Water Flows in the Darling

The recent publicity about the flood flows in the Darling, the return to the river of Toorale irrigation water, the Menindee Lakes and the needs of the Lower Lakes in South Australia, highlights some of the misunderstandings that prevail.

I contend that the unique feature of the Murray Darling Basin and of Australia generally, is massive rainfall and run-off variability. This appears to be little understood as does the magnitude of our big rain/run-off events when they occur.The current event is a very good example of what happens quite frequently, but totally irregularly!

As a consequence of the heavy rains mostly in Northern NSW over the Christmas/New Year period there is now a moderate flood in the Darling River. So far some 800,000 megalitres of water have flowed past Bourke with considerably more to come.

The Federal Government have made much of their guess that Toorale would have extracted 11,000 megalitres had the water licenses not been purchased by the Government. This would be approximately correct, but would include stock and domestic retention in the Warrego Dams. In other words $23.75M was spent to save 1.4% of the flow so far!

The man engineered Menindee Lakes rank as the most inefficient water storage in Australia, surpassed only by the Lower Lakes themselves. Of water that enters Menindee Lakes, which have a total capacity of 1.8m megalitres, it is a fair to estimate to that approximately 50% will be lost to evaporation and seepage. Much is made of the fact that Broken Hill's water supply has now been secured for the next two years. I would hope so, as Broken Hill only uses approximately 10,000 megalitres per annum!

The South Australian's now claim that the dreadful condition of the Lower Lakes is due to extractions and lack of fresh water from upstream. No mention of the fact that under natural conditions the Lower Lakes were sometimes salty and sometimes fresh depending on fresh water flows, or lack of them from upstream in Victoria, NSW, and Queensland. No mention of the impact of the building of "The Barrages" at the mouth of the lakes which converted the Lakes to all times fresh with the expectation that there would always be sufficient fresh water flows from upstream.

Under natural conditions with no dams in the Upper Murray catchment and no Snowy diversions, with the minimal run-off of the recent drought years, the Murray would have stopped flowing some three years ago. Salt water would have entered the Lakes and the lower reaches of the river itself as it always did under such dry conditions in the past, prior to the building of the Barrages. So it can be argued that the dire condition of the Lakes is largely a man-made problem.

Perhaps NSW and SA should do a deal:-
1) Somehow secure Broken Hill's water supply without relying on Menindee Lakes. How did they get on in the mining hey-day before the Menindee Storage Scheme was built?
2) Engineer a much more efficient (deeper) storage at Menindee to service the needs of the Lower Darling irrigation industry.
3)Build the proposed weir at Wellington (SA) to service the Lower Lakes irrigation industry. 4)Remove The Barrages and return the Lower Lakes to more like their natural condition.

That should stir a few responses!!!