| The sensors require an antenna
that looks like an umbrella or a satellite dish. It functions as a mirror,
constantly reflecting radiation emitted from Earth onto sensors that measure
the strength of the radiation. Currently, the device is as big as can
be shipped into space in one piece6 feet in diameter. Jackson says
diameter is the limiting factor for the sensors, which could measure longer
wavelengths and be more accurate with larger dishes.
"Better soil moisture products might require a dish
18 to 36 feet or more in diameter," he says. "The only way
that could be done would be to construct it as a mesh umbrella that
could be shipped into space closed and then unfurled," Jackson
says. The Hydros satellite will use this technology. Success should
bring larger and larger dishes on successive generations of weather
satellites. Jackson hopes the Hydros soil moisture-monitoring mission
would demonstrate the value of such single-use satellites to military
and civilian weather forecasters. Moisture sensors aboard airplanes
have higher resolution than those on satellites. The airplanes are also
used to test new ideas for sensors.
A ground crew of about 40 sampled plants and soil in Alabama
and 20 flew aboard the P-3B planefairly typical-size crews for
the SMEX campaigns. The ground crew checked moisture in crop and pasture-plant
leaves and soil. They analyzed the samples at Alabama A&M University
labs and at the nearby National Space Science and Technology Center.
The center is a venture of NASA and the State of Alabama through the
Space Science and Technology Alliancea group of six research universities:
Alabama A&M, Auburn, Tuskegee, Alabama-Tuscaloosa, Alabama-Birmingham,
and South Alabama.
"The moisture measurements of plants and soil were
part of the ground "truth" readings we used to gaugeand
calibratethe accuracy of the air- and space-based sensors,"
In Alabama, the ground crew faced a dense, oak-pine forest
around the NASA Marshall Space Flight Center in Huntsville. They also
did tests around the campus of Alabama A&M University and on cotton,
corn, and soybean fields. Their farm/forest study area formed a 31-
by 62-mile area mostly in Alabama, but with a portion in Tennessee.
Alabama A&M students and faculty worked with the federal scientists.
USDA's Natural Resources Conservation Service set up seven
monitoring towers at the University of Alabama as part of the Soil Climate
Analysis Network, which currently has over 70 permanent towers around
the country to monitor soil moisture and temperature and weather data.
"Soil moisture readings from the towers closely matched
satellite and airplane sensor readings for the farm areas. But tower
readings really began deviating from air and space readings in the densest
parts of the forested land," Jackson says.
"We've found that we can see through forestthough
much of the signal is diminished after passing through all that vegetation.
Now we need to figure out what in the vegetation is causing the signal
weakening. Then we plan to see whether we can remove those obstacles
so we can read soil moisture accurately."
Jackson and colleagues worked with the ARS Southeast Watershed
Laboratory in Tifton, Georgia, and the ARS Grazinglands Research Laboratory
in El Reno, Oklahoma. This year, Jackson and crew will set up again
in America's Southwestsurveying a land mass that includes much
of Arizona and Mexicoin a National Oceanic and Atmospheric Administration
project focusing on understanding North American "monsoons."
The atmospheric disturbance caused by rapid evaporation from these sudden,
heavy rainstorms over a large land mass has a major effect on water
supply forecasts in the West.
Once soil moisture predictions and measurements are part
of daily weather forecasts, Jackson explains, "it will help predict
certain storms, because sudden changes in soil moisture over vast areas
in America's Westand similar climates worldwidecan be important
determinants of whether it rains or not. They drive the atmospheric
circulation that spawns storms."By Don
Comis, Agricultural Research Service Information Staff.
This research is part of Global Change, an ARS National
Program (#204) described on the World Wide Web at www.nps.ars.usda.gov.
J. Jackson is with the USDA-ARS Hydrology
and Remote Sensing Laboratory, Bldg. 007, 10300 Baltimore Ave.,
Beltsville, MD 20705-2350; phone (301) 504-8511, fax (301) 504-8931.
"How Wet's Our Planet?" was published in the March
2004 issue of Agricultural Research magazine.