rature in the range of 110 to 120 K for calm fresh water
and sea water. The emissivity of water measured by remote
sensing tests is 0.37 to 0.40. The brightness temperature of
marshland is situated between that of water bodies and of
dry land. The difference of brightness temperature of marsh
lands mainly depend on the moisture content and density of
covering vegetations. It is worthy to be noted that, firstly,
the radiometric temperature of the marginal water bodies is
higher somewhat than that of the center water bodies, as a
result of the contribution of aquatic plants growing at sh
allow waters along the bank; secondly, there is a lower tem
perature belt lies in the center parts outside the dyke,
contrasting sharply with surrounding targets. The anomaly
of the brightness temperature is probably concerned with the
seepage of the reservoir judging from the image. In situ
observations showed that this belt is a former riverbed and
became a paddy field after the dam was erected. A fully wet
of soil is kept as a result of the seepage of the reservoir
to a certain extent. Fig.3 is a grey level image of another
marshland. It can be seen that the waterlogged depressions
presenting the dark tones are scattered over the grassland
the light tones.
Fig. 3,A grey level image
of Xinlicheng marshland,
Changchun, at an altitude
of 2000 m acquired on
August 20, 1979.
A, Water bodies,
B, Grassland.
Soil and Vegetation
Microwave radiation of soils are dependent on a number of
geophysical properties. These include moisture content, sur
face roughness, vegetable layer, thermometric temperature,
etc. Compositional variations are of less importance in de
termining the microwave characteristics of soils, except in
circumstances where significant amounts of metallic or mag
netic minerals are present. With the exception of the ther
mometric temperature dependence, these parameters are all
associated with the dielectric properties of the soils. The
dielectric constant of water at microwave frequencies is
auite large. For example, the real part of the dielectric
constant of water at L band or below is about 80, and that
of most dry soils is about 3 (7). Thus the addition of water
to dry soils causes significant changes in the dielectric
properties. A set of curves in Fig. 4 are the results of
experimental measurement for various soils type of China(8).
Fig. 4, The dielectric
constant (real part) g/of
soils as function of mois
ture content. ( 1,meadow
soil, 2, drab soil, 3,rice
paddy soil, 4, red earth,
5, yellow soil, 6, brown
earth. )
10 20
moisture content (%)
1
30