From space, keep tabs on groundwater

STANFORD (US) — Researchers have found a way to use satellites to see past crops and check water systems underground for signs of overuse.

In agricultural regions, groundwater regulators use direct measurements from wells—which are often few and far between—to monitor aquifer levels carefully in an effort to avoid drought.

“Groundwater regulators are working with very little data and they are trying to manage these huge water systems based on that,” says Jessica Reeves, a geophysics doctoral student at Stanford University. 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, specialized 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.

A team led by Reeves has found a way around the problem.

The study began as a collaboration between Reeves’ faculty advisers, Rosemary Knight, a geophysicist who studies groundwater systems, and Howard Zebker, a geophysicist and electrical engineer who uses satellite-based remote sensing techniques to study the Earth’s surface.

Minds the gaps
Reeves analyzed a decade’s worth of surface elevation data collected by satellites over the San Luis Valley in Colorado, a region rich with growing crops. She produced maps of satellite measurements in the valley and saw a regular pattern of brightly colored 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 realized 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 center-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.

Shape shifting
The satellites use interferometric synthetic aperture radar, known as InSAR. It is a radar technique that measures the shape of the surface of Earth and can be used to track shape changes over time. Earth scientists often use InSAR to measure how much the ground has shifted after an earthquake.

While continuously orbiting, a satellite sends an electromagnetic wave down to the surface. The wave then bounces back up and is detected by the satellite. The properties of the wave tell scientists how far the wave traveled before it was reflected back. This distance is directly related to the position of the ground.

After the satellite completes a circle around the globe, it returns to the same location to send down another radar wave and take another measurement. Measurements are taken every 35 days and data collection can go on for years.
Compared to drilling wells for monitoring groundwater aquifers, using InSAR data would be much cheaper and provide many more data points within a given area.

Traditional methods rely on wells that were not built with scientific data sampling in mind and their results can be inconsistent. Moreover, the number of wells drilled into any particular aquifer is much too small to be able to cover the entire groundwater system.

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. The technique could be used in agricultural regions anywhere in the world, even those that lack modern infrastructure such as wells.

“I think it really has potential to change the way we collect data to manage our groundwater,” says Reeves, who presented her results Dec. 13 at the American Geophysical Union meeting in San Francisco.

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