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More rain tipped

Wednesday, 7th April, 2010

* Water continues to flow into Lake Menindee as more rain is forecast this week for most of the Murray Darling Basin. * Water continues to flow into Lake Menindee as more rain is forecast this week for most of the Murray Darling Basin.

Floodwaters from Queensland have crossed the border and are expected to reach Menindee Lakes this month or next, according to the NSW Office of Water.

Meanwhile, the Weather Channel said yesterday that widespread rain and thunderstorms would hit the Murray Darling Basin this week. The NSW Office of Water said the flood peak had crossed the border and was expected to arrive at Bourke today. Once it has arrived at Bourke, firmer predictions about its progress can be made.

Water continues to be transferred from Lakes Wetherell and Pamamaroo into Lake Menindee at about 14 gigalitres a day, to make room for the impending flows and water is being let into the lower Darling at a rate of two gigalitres a day. A State Emergency Service (SES) flood bulletin advises that the river is expected to peak at Bourke on about April 14 at a height of 10.8 metres, causing moderate flooding.

At Tilpa the river is expected to peak at 11.2 metres late in the fourth week of April, causing major flooding. Minor flooding is expected at Wilcannia in the third week of April, when the river exceeds nine metres. Further flooding is anticipated in early May when overflow from the Paroo River arrives.

Tom Saunders, a meteorologist at The Weather Channel, said yesterday that two separate weather systems would be over the region this week. The first rain event was to spread over NSW and Victoria during the next few days, as moist tropical air interacts with an eastwardly moving trough of low pressure that has already brought thunderstorms and flash flooding to South Australia.

The second system is already affecting Western Australia and will bring rain to southeastern States from Thursday to Saturday.

"Together these two weather systems will bring widespread falls of over 25mm to most of the Murray Darling Basin this week," said Mr Saunders. "This week's drenching follows heavy rain earlier in the year, and so totals across most of the Murray Darling Basin so far in 2010 are already 100mm higher than levels this time last year."

On the Paroo River the bulk of floodwater is now in the lower reaches, to the north of Mount Macpherson, where extensive systems of channels, flood-runners and billabongs have been replenished. The complex network of streams in this area is providing significant storage for the floodwaters. This is slowing down the water and spreading it out across the floodplain. Whilst this reduces flows to downstream areas it also provides tremendous benefits for the environment and graziers in the region.

Flows from the Paroo catchment are likely to sustain flows in the Barwon-Darling River above Wilcannia at relatively low rates for an extended period of time. On the Warrego River a significant volume of water has pooled near the Cuttaburra Channels, 80km northwest of Fords Bridge. Although some appears to be draining to the Paroo River, a lot of water will be lost to evaporation and seepage.

Water will enter the Darling River at Toorale Station, between Bourke and Louth, throughout April. It is estimated that over 100 GL passed Fords Bridge during March and that a further 50 GL will pass over the next few weeks. Floodwaters in the Culgoa System are also slowing down and spreading out and as a result flood peaks are lower than were originally expected. It is thought that the Christmas/New Year floods, and the vegetation growth that resulted may have had a significant effect on flood behaviour.

Satellite imagery shows extensive areas of inundation, much of which will not contribute to flows in the Darling River. The total storage volume of the Menindee Lakes is now 538 GL and falling due to the high losses from the initial wetting of the dry lake bed and evaporation. It is expected that the volume in storage will increase in the weeks ahead as the anticipated Darling River flows arrive.

The 640 GL volume whereby the management of releases from the Menindee Lakes reverts to the Murray-Darling Basin Authority is likely to be reached by mid April. Control of the water will again revert to NSW when the volume in storage next falls below 480 GL.

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