JULY 27, 2001 FRI  
 

 

In search of water - via satellite

Two US scientists say they have found an economical way to spot new water sources using satellite images, which should aid countries facing a shortage

By R Senthilnathan
IN TORONTO

TORONTO - As countries struggle to find new water sources to meet ever rising demand, two US scientists say they have discovered a method to spot underground water faster and more economically - using satellites.

High-resolution images taken by radar called synthetic aperture radar (SAR) fitted in satellites can pinpoint areas where underground water is found, the scientists from the US Geological Survey wrote in the Geophysical Research Letters journal of the American Geophysical Union.

Using the SAR images taken by satellites operated by the European Space Agency (ESA), they were able to see changes to an existing aquifer in southern California.

But Mr Wesley Danskin, one of the scientists involved in the project, said this technique could also be used to detect new underground water resources.

As the underground rocks and soil soak up the water from rain and run-offs, they bulge, resulting in a swelling of the land surface.

Conversely, land subsides when groundwater is pumped out excessively, exceeding the replenishment rate, as seen in Bangkok, Jakarta and Shanghai.

The changes in the land surface - which is the height of the land surface above mean sea level - can be detected and measured by screening the SAR images taken of a particular spot at different times with high-tech equipment.

This is not the first time satellite data has been used to monitor surface changes caused by groundwater, but till now it was used only to monitor land subsidence.

One great advantage of using satellite technology for water detection is monetary, Mr Danskin says.

According to him, this technology costs about a tenth, at times just 1 per cent, of the cost of traditional technology which banks on ground-based data collection from a large number of wells and aerial photography.

This would come in handy for many nations facing water scarcity.

According to the United Nations, the number of people facing water shortage will rise from 432 million in 1995 to between 1.4 billion and 2.5 billion by 2025.

Mr Danskin and his colleague, Mr Zhong Lu, studied images of the semi-arid San Bernardino area in southern California because the region's groundwater basin has been researched extensively and is monitored continuously with surface water gauges and groundwater wells.

The images used were taken between 1992 and 1995.

'We focused on a period when we knew the groundwater levels changed significantly, in the hopes of seeing a similar response in the land surface,' Mr Danskin said.

The images showed that the land surface swelled by 7 cm between December 1992 and August 1993.

More than 4 cm of the increase came during a three-month period ending June 1993, when there were high run-offs from surrounding mountains.

The scientists checked whether the swelling could have been caused by tectonic movements, but that was ruled out, Mr Danskin said.

How the method works

AS UNDERGROUND rocks and soil soak up the incoming water from rain and run-offs, they bulge, resulting in a swelling of the land surface. Conversely, land subsides when groundwater is pumped out.

The changes in the land surface can be detected and measured by analysing high resolution images taken by radar of an area at different times with high-tech equipment.

The cost of this technology is about a tenth, at times just 1 per cent, of that of traditional technology that banks on ground-based data collection from a large number of wells and aerial photography.

   
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