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Throwing away Brisbane water?

SUNDAY NOVEMBER 20, 2011

NEWS: About 8,000 megalitres of water will be released from Wivenhoe dam each day over the next week

 http://www.weatherzone.com.au/news/authorities-begin-wivenhoe-dam-release/19405

Transcript of interview with Channel Seven, Today Tonight 16 November 2010
What do you think of the Queensland government’s decision to release water from Wivenhoe?
Whilst I understand that a repeat of last year’s calamity should be prevented if possible, it looks like an ill-informed and knee-jerk reaction to a horse that has bolted, and with later shortages likely there is not any reason to let water out of Lake Wivenhoe.  In 2010, La Niña and low pressure systems in the spring resulted in unusually high levels of increased rainfall throughout Oct - Dec.  These rains had saturated the catchments and the ground was unable to absorb or hold any more water. January’s flood situation came about because the heavy spring rains had filled the lake to over-capacity.  By the time January arrived, the dam was nearly twice its capacity (193%) and remained so at the height of the rains, and could not hold any more water had the rains kept falling.  Somerset dam, the other main dam in the catchment, was at 160%. The engineers waited until it was too late, even though the BoM had forecast heavy rain.  But this spring has been different.  This spring has not filled the lakes in the same way as last year, with current levels down by 20-25%.   Even the BoM are saying that levels will not be as high this season.  The dam is already below its full capacity whereas this time last year there was double the capacity in Wivenhoe and half capacity again in Somerset. That’s a huge difference. Plus, I am expecting some very warm temperatures this summer season, particularly over January, including sustained heatwaves at times, so the demand for water consumption will be high during the coming months.  Added to this, I don't expect sustained heavy rain events over the rest of this year, and in 2012 from April to the end of the year, and rainfall levels are likely to be down considerably compared to last year.

Should we actually be conserving water?
Yes, I believe so.   I feel the only above average rainfall intervals for Brisbane are likely to be between mid-January and mid-February, and then mid-March to mid-April, but these will only be due to isolated heavy fall days, quite typical of the summer season anyway because of passing cyclones, and are unlikely to feature sustained heavy rain.  Lake levels are unlikely to be anywhere near full, over a summer that I expect to be 15% drier than average.  During the rest of the period from about March - Dec 2012, Brisbane should have below average rainfall figures, with June-Sept well below average.  So yes, Brisbane needs to conserve water for the dry months that will follow. The whole east of the country may be facing drought conditions later in the new year, and there may be very little rain in Lake Wivenhoe from mid-March onwards.

What can you tell us about rainfall in south east Queensland over summer?
Around Wivenhoe the biggest dump of the whole season may be around mid-Dec, the result of a coastal trough moving thru SE dists, producing showers and thunderstorms that should mostly affect Lockyer Valley, and will undoubtedly deposit water into nearby Lake Wivenhoe, but amounts are unlikely to be enough to cause flooding in the Brisbane River.  There are also isolated heavy falls expected around the end of January, mid-February as the result of a cyclone, end of Feb, and mid-March as the result of a cyclone, but again unlikely to be any more than one or two day affairs and should not be enough to cause flooding. For Brisbane itself a storm around 18th-20th December may bring about three days of heavy rain, but with only isolated to light showers on around 7 other days over the whole month.  Thunderstorms around 5 Dec may also bring a heavy downpour. 
January,may bring a prolonged dry spell over the first 3 weeks of the month before some overnight rain around 23rd, and a passing thunderstorm band may bring some storms and heavy rain over the last week of the month that may be enough to cause some surface flooding over the Moreton / Brisbane region around 31st and into the first couple of days of February. February brings the promise of some regular rain or showers throughout the month with only about 8-9 dry likely, but no flooding rains are expected.  March for Brisbane may also bring some good rainfall, with light showers over the first few days, mainly fine most of the second week, but from 11th-21st, rain or showers almost daily, including thunderstorms around 11th-13th that may bring heavy downpours and possible surface flooding, but all of that should dissipate quickly.

What about high temperatures and heatwave conditions in south east Queensland?
 I expect January to bring some rather high temps and sustained heat-wave conditions covering much of the month, peaking around 18th-20th, with temps possibly hitting the 40s, with heat generated thunderstorms likely over this period, but not much rainfall over the whole month until the last week. February is also expected to start on high temperatures (in the lower 30’s) for the first couple of days, then settling in at around 25-29°C on average each day, and that’s a temp trend expected to continue well into March, when even a couple more days at over 30°C are likely.  Daily minimas should be fairly high throughout the summer season, with Dec averaging around 16°C, and Jan and Feb both averaging around 21°C.  The March minima average is expected to only drop by around 2°C by the end of the month.

How dry do you expect this summer to be?
I expect a fairly average summer for rainfall this year but overall drier by around 10-12% - with December drier by about 9-10%, January wetter by about 25% but only because of one particular rain day, and February drier by about 30%.   Only the heavy rainfall at the end of Jan is likely to be the reason for an aberage result for the season, but if that thunderstorm band and rainfall falls short of the coast, then the Brisbane region may end up being drier overall by considerably more.

Final
Why not just allow water to spill over by itself once the level passes a certain % capacity? That would ensure full lakes and prevent flooding. After average-to-drier weather, lots of heat, water restrictions, weak and ineffectual cyclones, followed by dry months, there may be a too-late restrospective wish to have conserved water, not thrown it away.

 

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