By Michael Perry SYDNEY, March 7 (Reuters) - Australia's longest river has lost half its natural water and it is predicted to dry up by a further 20 percent due to climate change by 2030. The 2,739 km (1,700 miles) Darling River, the lifeblood for some major farmlands, loses the equivalent of four Sydney Harbours worth of water, or a quarter of its flow each year, through evaporation, according to a new report. Large scale water infrastructure, such as dams and irrigation projects, built along the Darling River since the 1960s have also dramatically reduced water flows. The "State of the Darling" released on Wednesday paints a picture of a river under threat from global warming-induced drought, lower rainfalls and decades of poor water management. "The result is that average (Darling River) outflows to the Murray (River) are now less than half the volume they would be under natural conditions," said the report. Australia is in the grip of the worst drought in 100 years. Prime Minister John Howard announced plans in January to spend more than A$10 billion ($7 billion) to restore ailing rivers, making dwindling water supplies his priority in an election year. The 10-year plan includes A$6 billion to repair and cover irrigation channels along Australia's Murray-Darling River Basin, which accounts for 41 percent of agriculture. The report, by the Murray-Darling Basin Commission which manages Australia's two major rivers, said climate change had also affected the frequency and size of floods on the Darling. It said floods that would normally be expected every two years now only occur every five years. It also said that fish migration in the river was now possible only when water flows were high enough to drown out the many weirs on the river. The Darling River basin covers an area of 699,000 sq kms (sq miles) and makes up 70 percent of the Murray-Darling Basin. The pumping of groundwater had also resulted in the loss of surface flows in the river of 74,000 megalitres a year and this will rise to 191,000 megalitres as extraction increases. The report said that unless river management was changed and the over-allocation of water reduced then the the drying up of the Darling River presented "a serious risk to long-term socio-economic sustainability" of farms and towns on the river. Dry weather in 2006, which extended Australia's drought into its sixth year, cut crops by around 60 percent, and depressed farm exports to A$26.3 billion ($20.2 billion) in 2006/07.
