Geoscience Reference
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experiments was to evaluate the capabilities of satellite microwave sensors for soil
moisture mapping on a planetary scale. The experiments to date have all utilized a
combination of aircraft microwave measurements and in situ measurements to
achieve the goal.
Soil moisture was recognized by the NASA Post 2002 Program as a critical
measurement. Operational determination of soil moisture spatial-temporal vari-
ability plays a leading role in regional and global studies of the water, biogeo-
chemical and energy cycles. The efforts are targeted on the improvement of
spacecraft microwave radiometric assessments of soil moisture to the level of data
acceptability when reliable predictions of regional to global hydrologic regimes can
be provided. Soil moisture is a key state variable in hydrology: it is the switch that
controls the proportion of rainfall that percolates, runs off, or evaporated from the
land. It is the life-giving substance for vegetation. Soil moisture integrates pre-
cipitation and evaporation over periods of days to weeks and introduces a signifi-
-
cant element of memory in the atmosphere-land system. There is strong
climatological and modeling evidence that the fast recycling of water through
evapotranspiration and precipitation is the primary factor in the persistence of dry or
wet anomalies over large continental regions during summer. As a result, soil
moisture is possibly one of the most signi
cant land surface boundary conditions
controlling the exchange of water and energy with the atmosphere.
Remote sensing of the environment by means of passive microwaves or
microwave radiometry is based on retrieval algorithms that relate the environmental
parameters (such as soil moisture, vegetation type and biomass, forest stem volume,
water equivalent, physical temperature, and others) to the remotely sensed radiation
characteristics (spectral brightness temperature, polarization). Microwave emission
from a natural object is governed by the interaction of electromagnetic waves with a
natural medium that is determined by its geometrical structure and dielectric and
electric properties of the medium constituents. In turn, these properties are con-
nected with geophysical and biophysical parameters of the medium that enables the
retrieval of these parameters from remotely measured characteristics of electro-
magnetic waves emitted or scattered by the object. Therefore, dielectric and electric
properties of natural media constituents and their relationships to geophysical and
biophysical parameters play a crucial role in microwave remote sensing of the
environment. Besides, the dielectric permittivity of a medium or its real part and its
imaginary part determines the penetration depth of electromagnetic waves in the
medium that is very important for subsurface remote sensing of soil moisture and
for soil moisture sensing under vegetative canopies.
The impact of microwave radiometer pixel size on the reliability of soil moisture
assessments for different landscapes and minimization of the number of in situ
locations for sampling soil moisture, temperature and vegetation parameters are key
elements of the study. As a result, a combined approach for soil moisture mapping
from a regional to global scale using microwave radiometric observations, labo-
ratory tests, in situ measurements and mathematical modeling of the soil-vegetation
system inherent in different landscapes is to be developed as a highly calibration
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