IEEE Spectrum June, 2013 - 47

SciEncE SourcE

growIng IT

Remote sensing of the world's agricultural output isn't new,
NDVI, FEWS NET uses remotely sensed estimates of rainfall
but improved software and sensors are raising this form of
as an indicator of crop performance and other data to predict
information gathering to new heights. Satellite data are now
food shortages.
used not just to monitor crop yields but also to predict them
"The major cause of food insecurity is the lack of rain," says
weeks in advance. This helps traders in international markets
Glenn Bethel, an advisor on remote sensing to the U.S. Departset reasonable pricing. More important, it has the potential to
ment of Agriculture. "The lack of good precipitation data hindrastically change how food shortages are dealt with in some of
ders the accuracy of the crop models. [But] any crop can have
the poorer parts of the world.
optimal growing conditions and still fail. That is why we always
"If you're going to have a big shortfall in wheat production, for
need to monitor."
example, the earlier you know that, the better you can plan,"
Bethel says the workhorse of crop-condition assessment is the
says Inbal Becker-Reshef, an assistant research professor in the
Terra satellite. With its polar orbit and 2330-km-wide field of view,
department of geographical sciences at the University of Maryland.
it can see every point on the planet every one to two days. But this
"If it's a food-insecure location, can you mobilize food aid early
satellite is aging, having launched in 1999. And, Becker-Reshef says,
enough to avert a [bad] situation?"
data continuity is key when it comes to agricultural monitoring.
Becker-Reshef helped start the Group on Earth ObservaLuckily, other satellites are ready to step in. Landsat 8, also
known as the Landsat Data Continuity
tions Global Agricultural Monitoring
Mission, launched this past February. It
(GEOGLAM), a G20 initiative aimed at
joins an instrument called the visible intimely dissemination of the most up-todate information about agricultural profrared imager radiometer suite, which
launched on board a satellite in 2011. And
duction. She spoke last December at the
there are plans to combine Landsat 8's
American Geophysical Union's meeting in
data with similar observations collected
San Francisco about one GEOGLAM effort,
from the soon-to-launch European Space
related to wheat-crop outputs.
Agency's Sentinel-2. The two satellites can
Using a measure called the normalized
work together, and with some correction
difference vegetation index (NDVI), sciand calibration they could be used almost
entists managed to predict annual crop
interchangeably.
yields for wheat in Kansas six weeks in
Combining the satellites' data is cruadvance and to within 7 percent. Here's
why that's impressive: The state's annual
cial, given their limitations. Landsat 8 will
Patches of Production: earth-observing
wheat output has bobbed up and down
have a repeat cycle of 16 days, meaning
satellites such as the Landsat series help
predict agricultural output. near san Luis,
between 200 million and 500 million
it will view any given point on Earth less
argentina [above] and along the okavango
bushels over the past 20 years.
frequently than once every two weeks.
river [opposite] between namibia and angola,
NDVI is measured using instrumenBethel
says that isn't enough for good
farms form a patchwork of productivity.
monitoring, and Becker-Reshef agrees.
tation on board NASA's Terra satellite:
If the Sentinel-2 data are mixed in, howspecifically, spectral bands 1 and 2
(infrared to visible light) of the Moderate-resolution Imaging
ever, that repeat period will drop drastically, to about five days.
Spectroradiometer. NDVI compares the amount of light reflected
The extent of global communications in the 21st century may
in near-infrared with that reflected in visible light; if there is more
make a second Great Grain Robbery unlikely, but we are still
infrared, the vegetation in that spot is likely to be dense and thus
a long way from perfect when it comes to understanding how
yield more of the crop. The new NDVI algorithm seems to work
the world's crops are doing. Postharvest numbers on yields
everywhere in the world-predictions came within 4 percent of
tend to take about six months to emerge in some countries, far
the yield in Ukraine last year and within 6 percent for part of Pakitoo late to be of use for managing price volatility or preventing
famine. The price spikes following droughts in 2008 and 2010
stan. Again, country outputs can vary dramatically from year to
in Australia, Russia, the United States, and elsewhere are prime
year, so predictions within 10 percent are useful.
examples. "Whatever happens in the United States impacts what
Satellites also play a big part in a program called FEWS NET-
happens in Australia and impacts what happens in Brazil," says
the Famine Early Warning Systems Network. Funded by the
Becker-Reshef. "Remote sensing is really the only option, and
U.S. Agency for International Development, FEWS NET uses
by far the most cost-effective option, for getting a global picture
both on-the-ground monitors and remote sensing to try to preof what's going on." n
dict and stave off food shortages around the world. Along with
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Table of Contents for the Digital Edition of IEEE Spectrum June, 2013