Stanford, US: Scientists at Stanford University are using satellite data to effectively monitor aquifer levels in agricultural regions. They claim that it is a cheaper and more effective way as they get millimetre level precision of Earth’s surface through those data.
In agricultural regions, groundwater regulators have to monitor aquifer levels carefully to avoid drought.
“Groundwater regulators are working with very little data and they are trying to manage these huge water systems based on that,” said Jessica Reeves, a geophysics doctoral student. But now, Reeves has shown how to get more data into the hands of regulators, with satellite-based studies of the ground above an aquifer.
As the amount of water in an aquifer goes up and down, specialised satellites can detect the movements of the land above the water system and hydrologists can use that information to infer how much water lies below.
Previously, accurate elevation data could only be acquired on barren lands such as deserts. Plants – especially growing crops, whose heights change almost daily – create “noise” in data collected over time, reducing their quality. Reeves led a team of scientists to find a way around this “growing” problem.
The study began as a collaboration between Reeves’ faculty advisers, Rosemary Knight, a geophysicist, and Howard Zebker, a geophysicist and electrical engineer who uses satellite-based remote sensing techniques to study the Earth’s surface.
Reeves analysed a decade’s worth of surface elevation data collected by satellites over the San Luis Valley in Colorado. As part of her analysis, Reeves produced maps of satellite measurements in the valley and saw a regular pattern of brightly collared high-quality data in a sea of dark, low-quality data. After overlaying the maps with a Google Earth image of the farmland, the team realised that the points of high-quality data were in the dry, plant-free gaps between circles of lush crops on the farms.
In the San Luis Valley, the majority of irrigation is done by centre-pivot irrigation systems. Like a hand on a clock, a line of sprinklers powered by a motor moves around, producing the familiar circles seen by airline passengers. The circles don’t overlap, leaving small patches of arid ground that don’t receive any water and so don’t have any plants growing on them.
Reeves confirmed that these unvegetated data points were trustworthy by comparing the satellite data to data collected from wells in the area – exactly the kind of proof that would be important to hydrologists studying aquifers.
The satellites use interferometric synthetic aperture radar, known as InSAR.
Hydrologists and regulatory bodies looking for more data to better understand their groundwater system could one day set policies requiring farmers to leave a patch of land clear for InSAR data collection. Furthermore, the technique could be used in agricultural regions anywhere in the world, even those that lack modern infrastructure such as wells.
Source: Space Mart