When you
dive into that salad full of lettuce grown in the American West, there's a good
chance you are enjoying the product of irrigation from an underground water
source. These hidden ground water systems are precious resources that need
careful management, but regulatory groups have a hard time monitoring them,
owing to a lack of accurate data.
Now,
scientists at Stanford have found a way to monitor aquifer levels in
agricultural regions using data from satellites that are already in orbit
mapping the shape of Earth's surface with millimeter precision.
The amount
of water in a ground water system typically grows and shrinks seasonally.
Rainfall and melted snow seep down into the system in the cooler months, and
farmers pull water out to irrigate their crops in the warmer, drier months.
In
agricultural regions, ground water regulators have to monitor aquifer levels
carefully to avoid drought. They make do with direct measurements from wells
drilled into the aquifers, but wells generally are few and far between compared
to the vast size of most ground water systems.
"Ground
water 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. 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.
Reeves
presented her results at the American Geophysical Union annual meeting in San Francisco.
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.
Now, a team
of scientists led by Reeves has found a way around this "growing"
problem.
The study
began as a collaboration between Reeves' faculty advisers, Rosemary Knight, a
geophysicist who studies ground water systems, and Howard Zebker, a
geophysicist and electrical engineer who uses satellite-based remote sensing
techniques to study the Earth's surface. Knight and Zebker hoped that the
combination of their expertise, and the efforts of their graduate student,
would lead to new ways of using satellite data for ground water management.
Reeves
analyzed a decade's worth of surface elevation data collected by satellites
over the San Luis
Valley in Colorado.
Although the valley is rich with growing crops, Reeves and her advisers hoped
that recent advances in data-processing techniques would allow her to gain an
understanding of the aquifer that lay below.
As part of
her analysis, Reeves 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.
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.
The researchers
argue that compared to drilling wells for monitoring ground water aquifers, using
InSAR data would be much cheaper, and provide many more data points within a
given area. They further contend that traditional methods rely on wells that
were not built with scientific data sampling in mind and their results can be
inconsistent. Moreover, they assert that the number of wells drilled into any
particular aquifer is much too small to be able to cover the entire ground water
system. Hydrologists and regulatory bodies looking for more data to better
understand their ground water 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. "I think it really
has potential to change the way we collect data to manage our ground water,"
says Reeves.
