A recently concluded NASA aerial field experiment, high above our
nation’s heartland, may lead to a clearer understanding of soil moisture– a
key variable in Earth’s global water cycle that profoundly influences
seasonal weather patterns and agriculture.

Flying thousands of feet above Iowa farmlands in a NASA DC-8 and
a National Center for Atmospheric Research C-130 aircraft, scientists and
engineers from multiple NASA centers, including the Jet Propulsion
Laboratory, Pasadena, Calif., participated in a three-week field experiment
using remote sensing techniques to measure soil moisture content. The
NASA scientists were joined by researchers from the Department of
Agriculture’s Agricultural Research Service, several universities and other
agencies, including the National Oceanic and Atmospheric Administration.

The experiment will help pave the way for the eventual development
of a remote global soil moisture observing system that will provide
observations every three days, or less, over most of the unfrozen, non-
forested regions of the globe (dense vegetation such as forests limits the
ability to sense the underlying soil moisture). A proposal for such an
observing system, called Hydros, was selected recently by NASA as an
alternate mission under the Earth System Science Pathfinder small satellite

“Soil moisture is a key variable in Earth’s hydrology, or water
cycle,” said Dr. Eni Njoku, a JPL scientist and co-investigator on the
experiment and a project scientist for Hydros.

“Soil moisture conditions play a vital role in controlling summer
precipitation over the central United States and provide initial information
for seasonal predictions. Persistently wet or dry soil moisture conditions
can also feed back into seasonal weather patterns that cause persistent
flooding or droughts,” Njoku said. “Today, scientists are limited to
scattered ground measurements of soil moisture. A future remote sensing
platform orbiting above Earth will enable us to better understand, on a
global basis, the factors that influence soil moisture fluctuations. This will
enable people everywhere to plant crops more intelligently and mitigate the
effects of natural hazards.”

Njoku said an additional objective of the experiment will be to
evaluate how well the Advanced Microwave Sounding Radiometer, a
Japanese National Space Development Agency instrument aboard NASA’s
Aqua satellite, can measure soil moisture of agricultural areas from space.
Data from the Aqua instrument will be compared with the more detailed soil
moisture information derived from the airborne instruments and ground
measurements. The Aqua instrument operates at wavelengths of less than 5
centimeters (about 2 inches), so it may have only limited ability to measure
soil moisture under moderate or dense vegetation cover like crops and

Two JPL airborne remote sensing instruments were an integral part
of the experiment, conducted from June 25 to July 8. The Passive and
Active L- and S-band microwave instrument flew at low altitudes (about 1.1
kilometers or 3,500 feet) on the C-130 aircraft. The JPL Airborne Synthetic
Aperture Radar flew at a higher altitude (about 7.9 kilometers or 26,000
feet) on NASA’s DC-8 aircraft. Both instruments operate at long
wavelengths (approximately 20 centimeters, or 8 inches) that are sensitive to
soil moisture and have the ability to penetrate clouds and moderate
vegetation cover.

The flights took place over agricultural fields in the Walnut Creek
watershed area south of Ames, Iowa. Exhaustive ground sampling of the soil
and vegetation (corn and soybean crops) in that region was conducted in
conjunction with the overflights.

Scientists will spend the next year analyzing data from the
spaceborne, airborne and ground-based samplings to better understand the
influence of soil moisture on water cycling between the land and
atmosphere, and to verify the accuracy of the instrument observations.

In addition to JPL’s instruments, other airborne microwave
instruments operated by NASA’s Goddard Space Flight Center, Greenbelt,
Md., and the National Oceanic and Atmospheric Administration’s
Environmental Technology Laboratory also participated in the experiment,
flying aboard the NASA P-3 aircraft. More information on the experiment
is available at http://hydrolab.arsusda.gov/smex02/smex02SCAN.html .

The experiment is part of the Terrestrial Hydrology program under
NASA’s Earth Science Enterprise, a long- term research effort to understand
and protect our home planet. Through the study of Earth, NASA will help to
provide sound science to policy and economic decision-makers so as to
better life here, while developing the technologies needed to explore the
universe and search for life beyond our home planet.

JPL is managed for NASA by the California Institute of Technology,