Our rivers are short of water. This may seem an odd statement but the fact is that we take out too much water for consumptive use, and this is having an effect on river health. As a rule of thumb, a river can cope with having up to one third of its water extracted and remain in reasonable health. Take more than that and trouble begins – flows drop off, wetlands dry up, water quality declines, birds and fish have trouble breeding and populations begin to decline.
The impact of our water use shows up in drier years. Most water planning is done on the basis of averages; however average years do not happen very often. Generally we either have well above average streamflows or well below.
Let’s take the year 2003/04 as an example. It is convenient because it is the first year in which comprehensive reporting on water availability became available through the publication of the State Water Report. The Report tells us that rainfall was about 80% of the long term average and streamflow across the state was 84% of long term average. It sounds as if our rivers weren’t doing too badly until we take a look at what actually happened to environmental water in individual river basins.
Water available for the environment at the basin outlet in 2003/04 compared with an average year . Red highlights indicate rivers that retained less than 66% of their flows
|Basin||2003/04 streamflow (ML)||Water available for environment in 2003/04 (ML)||Proportion of total in 2003/04 (%)||Proportion in average year (%)|
|Millicent Coast (c)||180||–||–||–|
(a) Most water flowing out of the Ovens is captured downstream for use in South Australia
(b) Excludes water extracted in NSW by Snowy Hydro
(c) Insufficient data
This table shows that in an average year four rivers (the Murray, Goulburn, Campaspe and Yarra) have more than one third of their water extracted. In 2003/04, which was not an excessively dry year (80% of the long-term average rainfall), 14 rivers gave up more than a third of their flows, and experienced flow stress. The most severely stressed river was the Wimmera, with just 2% of its water left, followed by the Campaspe, Loddon, Goulburn and Moorabool, all of which had less than 20% of their water left in the river.
This pattern is repeated in other dry years. The details might vary from year to year but the trend remains the same – rivers have too much water taken out of them and many experience flow stress as a result. At the height of the Millenium drought in 2007 the Victorian government described the situation like this “In these years, about 95% of the flow is extracted for towns and irrigation. The drought now being faced by the Campaspe River environment is 20 times harsher than a natural drought”. Our demand for water magnifies the effects of drought and means that some rivers experience extended periods of man-made drought. For example the Murray is deprived of the small floods that are essential to inundate the iconic Red Gum forests at Barmah, Gunbower and Hattah, leaving them ever more susceptible to natural or climate changed induced drought.
Most of the rivers highlighted in the table now have Victorian environmental entitlements which can be used to mitigate some of the impacts of reduced flows. However the entitlements may be small (e.g. the Moorabool has 2500ML, 2.5% of average streamflows), contingent on water availability (eg the Barwon, where environmental water is only delivered when river heights reach a particular level) or principally low reliability so rarely allocated (e.g. the Goulburn), or a combination of these characteristics (eg the Campaspe which has a small entitlement of low reliability) . Some rivers like the Maribyrnong have been promised an entitlement but are yet to receive it, while others such as the Hopkins are still waiting for even a promise.
Rivers in need of a larger share of their own water:
There are many ways to help these rivers get more water – by sharing water more fairly between the river and its users, and by changing the way we use water in irrigation, in towns and in industry.