Intake towers at the Hoover Dam are shown on July 14, 2014 in the Lake Mead National Recreation Area, Arizona

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Drought drains critical US water supply


A huge volume of fresh water has disappeared from the drought-struck south west of the US in the past decade in what researchers say is a startling sign of the fragility of one of the country’s most important water supplies.

Almost 65 cubic kilometres of water has been lost since late 2004 from the Colorado River Basin, an area roughly the size of France that is a vital but heavily used source of water for more than 30m people and 4m acres of farmland.


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The amount lost was nearly double the volume of the Colorado River’s Lake Mead, the largest man-made reservoir in the US, according to a study by scientists using data from Nasa satellites that can measure changes in water levels.

“This is a lot of water to lose. We thought that the picture could be pretty bad, but this was shocking,” said the study’s lead author, Stephanie Castle, a water resources specialist at the University of California, Irvine.

More than 75 per cent of the loss was due to the rapid depletion of groundwater from underground aquifers that many farmers depend on for irrigation, especially during droughts like the one that has afflicted parts of the south west for the last 14 years.

The researchers found the rate of decline of groundwater, much of which is non-renewable and poorly managed, was roughly six times greater than the losses in Lake Mead and Lake Powell, another large reservoir further upstream on the Colorado River.

“Groundwater is already being used to supplement the gap between surface water supply and basin water demands,” said study co-author, Jay Famiglietti, adding the study revealed a surprisingly high and prolonged reliance on groundwater to bridge the gap between demand and supply.

“The water security of the western United States may be at greater risk than is fully appreciated,” he said.

Groundwater supplies are often far less well documented than more easily observable lakes and rivers. The use of Nasa satellites has helped researchers improve their understanding of the extent of these losses worldwide.

Lake Mead, Las Vegas’s main source of water, has been a highly visible victim of the drought. Its levels have fallen so much for so long that it has a prominent white ‘bathtub ring’ of exposed rock around its edges.

It has been falling by a foot a week in recent months and earlier this month fell to its lowest level since it was formed in the 1930s by the construction of the Hoover Dam.

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Authorities are spending more than $800m to tunnel under Lake Mead to maintain access to water because the stunted flow of the Colorado River means the lake has shrunk so much that two higher tunnels may no longer be able to channel water to Las Vegas and other cities.

The rapid rates of groundwater depletion are likely to lead to further falls in Colorado River stream flows.

The study’s authors say declines in the snowpack that feeds the river, and population growth, could threaten the long-term ability of the Colorado River Basin to meet water allocation commitments to the seven basin states, which includes California, and to Mexico.

Brophy Friday 25 July 2014 - 3:19 pm | | Brophy Blog, Global Warming

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