July 31, 2002 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 program.
"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 forests.
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, Pasadena.
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