Web Extra  Friday, May 3 

Satellite will watch soil moisture

NASA plans to launch AQUA, the latest addition to its series of Earth Observing System (EOS) satellites, tomorrow from the Vandenberg Air Force Base in California. Part of NASA’s Earth Science Enterprise, the satellites will make long-term observations of the land surface, biosphere, solid Earth, atmosphere and oceans. AQUA will collect information about Earth’s water cycle.

Following is a preview of a story that will appear in July in the Geotimes annual Highlights issue, which will feature about 20 short stories summarizing the latest research and trends in several earth science disciplines. One trend in hydrology is remote sensing of Earth’s water cycle. Here, authors Venkat Lakshmi of the University of South Carolina, Eni G. Njoku of the California Institute of Technology, and Thomas J. Jackson of the USDA Hydrology and Remote Sensing Lab in Beltsville, Md., describe how NASA’s AQUA satellite will collect data on the amount of water in the top layer of soil.

Soil moisture is a key variable in land-surface hydrological processes as it controls infiltration (movement of water into the subsurface), runoff (movement of water on the ground) and evapotranspiration (loss of water to the atmosphere from bare soil as evaporation and from vegetation as transpiration through the stomata). The amount of soil moisture in the unsaturated zone (a mixture of air and water) determines the fate and transport of chemical contaminants (such as those from leaky gasoline tanks), and the transport of nutrients (such as nitrogen and phosphorous).

Direct observation of the soil moisture will be key in studies of processes in agriculture, meteorology, to environmental sciences, hydrology, water supply and water resources. However, direct observations of soil moisture are currently restricted to discrete measurements at specific locations, such as those made with the U.S. Department of Agriculture (USDA) Soil Climate Analysis Network (SCAN). But such point-based measurements do not reveal large-scale soil moisture and are therefore inadequate to carry out regional and global studies. Use of satellite data for inferring soil moisture is the most practical means to acquire global coverage continuously over time.

For a few decades, various satellites have been remotely measuring soil moisture. Notable missions are:

These host sensors that span a range of frequencies from 1.4 gigahertz to 89 gigahertz. In general, the lower frequencies are more suitable for remote sensing observations of the land surface because the higher microwave frequencies are significantly weakened by the presence of moisture in the vegetation canopy and the atmosphere.

Among other sensors, AQUA will carry the Advanced Microwave Scanning Radiometer for EOS (AMSR-E), which will collect data about soil moisture, sea surface temperature, column water vapor, cloud water content, ocean wind speed, rainfall over oceans and land, sea-ice concentration, temperature and snow depth, and snow-water equivalent. Scanning Earth’s surface for water content in six microwave frequencies ranging from 6.9 gigahertz to 89 gigahertz, AMSR-E will also view the atmospheric column, land vegetation canopy and the soil surface; it is expected to be sensitive to the top few centimeters of the soil. Therefore, the retrieved values of soil moisture reflect an average over the topsoil layer only.

The soil moisture derived from AMSR-E will be validated with data collected on the ground from sites in regions of differing climates and vegetation cover. Researchers will compare the soil moisture measurements retrieved from the AMSR-E to the soil moisture measured at field sites by probes buried 5 centimeters deep. AMSR-E soil moisture data can be used as inputs to land process models that can “assimilate” these values and continually evolve the soil moisture states.

AMSR-E data will help us improve the performance of these process models to simulate other variables in the water cycle. These include runoff and fluxes such as evapotranspiration, and near-atmosphere variables such as the height of the planetary boundary layer — the lowest part of the atmosphere that is directly influenced by Earth’s surface.

Soil moisture data from AMSR-E will benefit studies covering a wide range of hydrologic research problems. Many of these studies address how global change, both from human activities and natural variability, affect the global water cycle. The launching of the AMSR-E instrument on the Aqua spacecraft later this month will provide exciting new data and information.

Venkat Lakshmi, Eni G. Njoku and Thomas J. Jackson

Specs on AMSR-E
The AQUA Web site
NASA’s Earth Observing System
The USDA Soil Climate Analysis Network
The Geotimes January 2002 issue on soils
Last year's Geotimes Highlights issue

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