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
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:
· the L-band channel on the Skylab,
· the Scanning Multichannel Microwave Radiometer (SMMR),
· the Special Sensor Microwave Imager (SSM/I) and
· the European Radar Satellite (ERS-1 and 2).
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.
As a part of the Earth Science Enterprise, Earth Observing System Satellite series,
NASA launched the AQUA satellite May 4. Among other sensors, AQUA carries 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.