A new report has identified six risks that may reduce the volume of water in the rivers and streams of the Murray-Darling Basin.

They are: climate change, large scale tree planting, groundwater extraction, irrigation management, farm dams and bushfires.

Initial evidence suggests climate change poses the greatest risk because it could potentially reduce stream flow by 1100 gigalitres in 20 years ( five per cent of annual flow) and by 3300GL in 50 years (15pc of annual flow).

Trees use more water than pasture, so new forests will reduce annual water yields.

An additional 141 000 hectares of commercial plantations are expected to be established across the Basin by 2020, reducing water yield by up to 700GL per year.

Groundwater extraction erodes surface water flows, threatening downstream water security.

One estimate is that extraction is likely to reduce annual stream flow in the Basin by about 580GL in 2012.

The Basin has more than 10 000 kilometres of channels that lose up to 30pc of water through leakage.

While irrigators are becoming more efficient, some decline in stream flow due to irrigation is anticipated.

Farm dams account for around 40pc of the total volume of water stored.

Modelling suggests Basin dams currently reduce stream flows by about 1900GL a year.

Future reductions in stream flow will depend on farm dam legislation.

After bushfires, stream flows initially increase, and groundwater systems recharge because vegetation is not there to use the rainfall.

As vegetation regrows, it uses more water than a mature forest.

Modellers estimate that in catchments burnt in the 2003 bushfires, stream flows will rise until about 2010, then there will be some decline.

The report suggests that if present trends continue, the total reduction in stream flow from all six risks in the next 20 years is likely to range between 2500-55 000 GL/year, a reduction of 10-23pc of average annual flow.

You can read the report Risks to the shared water resources of the Murray-Darling Basin at http://www.mdbc.gov.au/nrm/risks_to_shared_water_resources/report_part2_Risks.

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