The Thomson Dam, completed in 1983, is tipped to fall below 20 per cent of capacity this week. Photo: Craig Abraham By Peter Ker MELBOURNE’S dwindling water storages are on the verge of a historic low, a quarter of a century after the Thomson Dam was promised to drought-proof the city. The nine major dams are expected to fall to 28.4 per cent of capacity today, matching the record low set in June 2007. Despite rain being forecast for today, a new record is expected to be set within days, depending on consumption rates. The grim statistics mean Melbourne’s dams are holding enough water to supply the city for about 500 days — without further rainfall — given typical water consumption rates. The Wonthaggi desalination plant is still about 900 days from supplying water, and while the north-south pipe will supply water from early next year, pessimistic forecasts mean it is unlikely to boost storages by more than 100 days’ worth of water before the desalination plant starts in late 2011. That means rainfall will have to boost dams by about 300 billion litres between now and late 2011 for the city to avoid running out of water. … Dam water levels dwindling to historic low