Supplies conserved, only to evaporate ...

16 Jan 2006

WHILE southeast Queensland residents save 200 megalitres a day with water restrictions, 280 megalitres a day disappears from our dams in evaporation.

Worse still, evaporation has probably increased over the past year in which Australia endured its hottest 12 months on record.

SEQWater acting chief executive Rob Drury said yesterday although evaporation caused a significant loss in all dams, it was difficult to estimate exactly how much more was lost due to global warming.

The 280 megalitres is equivalent to 40 per cent of total daily water use of 700 megalitres.

Mr Drury said over the past year Wivenhoe, Somerset and North Pine dams lost about 102,000 megalitres to evaporation.

Measuring evaporation was inexact because it varied widely according to weather. Hotter conditions played a role, but other major factors were the number of windy days, wind strength, waves, clouds and surface area.

Engineers used weather bureau pan evaporation rates from sites close to dams to calculate losses.

When dams were full – and therefore had a greater surface – evaporation more than doubled to 223,300 megalitres a year or 612 megalitres a day.

"People often say we should cover the dams in some way but the areas involved are just too big," Mr Drury said.

The dams' surface area at full capacity was 170 sq km.

Mr Drury said evaporation rates also had to be offset by the natural recycling effect. Rainfall on lake surfaces put water into storages regardless of any catchment runoff.

Assuming an average annual 1m rainfall for the three dams resulted in a gain of about 78,350 megalitres a year or 215 megalitres a day at current levels.

"The net result for the three dams is thus a loss of around 65 megalitres a day for the current storage levels or 143 megalitres a day for the full supply level," he said.

The nation's annual mean temperature increase last year was 1.09C, making it the warmest year since reliable observations became available in 1910.

see - Supplies conserved, only to evaporate.