9 Soil Moisture
207
the 21 cm data the radiometer was sensitive to soil moisture even under the tallest
sorghum.
Basharinovand Shutko (1975) and Kirdiashev et al. (1979) studied a variety of
crops in the USSR with wavelengths varying from 3 cm to 30 cm. For wavelengths
greater than 10 cm, their results indicate that one can expect a decrease in sensitivity of about 10-20% for small grains over what would be expected for bare soil.
With broad leaf crops such as com, the sensitivity could decrease by as much as
80% for wavelengths shorter than 10 cm, and 40% for a 30 cm wavelength. Thus,
from these studies the wavelength effect can be seen, that is, a vegetation canopy is
more transparent for longer wavelengths than for shorter wavelengths.
Jackson et al. (1982) developed a parametric approach based on a theoretical
model proposed by Basharinov and Shutko (1975). This model treats the vegetation as an absorbing layer that can be quantified in terms of the water content of
the vegetation by the following relationship:
Mv = 78.9-78.4[1+(e-1)exp(0.22W)]
(9.11 )
where Mv is the volumetric soil moisture (0-2.5 cm), e is the measured emissivity,
and W is the water content of the vegetation (kg/m2). Figure 9.5 illustrates the
effect of vegetation on soil moisture. Jackson and Schmugge (1991) have analyzed
a large amount of published data to verify previous findings and they have defined
a vegetation parameter that is based on the optical depth of the canopy. This parameter appears to be inversely related to the wavelength and can represent four
types of vegetation classes (leaf dominated, stem dominated, grasses, and trees and
shrubs). However, at longer wavelengths, a single value of the parameter might be
~
e.....
e
'"
or.
M
40
d>
(oJ
a:
;:;l
....
;a 30
0
~
~
'-"
"" u
c:2
....
(oJ
2: ;:;l
~
;;
NORMALIZED BRIGHTNESS TEMPERATURE
Fig. 9.5. The relationships between normalized brightness temperature and volumetric soil
moisture for different types of vegetation, L-band, H polarization (after Jackson et al. 1982)
207
the 21 cm data the radiometer was sensitive to soil moisture even under the tallest
sorghum.
Basharinovand Shutko (1975) and Kirdiashev et al. (1979) studied a variety of
crops in the USSR with wavelengths varying from 3 cm to 30 cm. For wavelengths
greater than 10 cm, their results indicate that one can expect a decrease in sensitivity of about 10-20% for small grains over what would be expected for bare soil.
With broad leaf crops such as com, the sensitivity could decrease by as much as
80% for wavelengths shorter than 10 cm, and 40% for a 30 cm wavelength. Thus,
from these studies the wavelength effect can be seen, that is, a vegetation canopy is
more transparent for longer wavelengths than for shorter wavelengths.
Jackson et al. (1982) developed a parametric approach based on a theoretical
model proposed by Basharinov and Shutko (1975). This model treats the vegetation as an absorbing layer that can be quantified in terms of the water content of
the vegetation by the following relationship:
Mv = 78.9-78.4[1+(e-1)exp(0.22W)]
(9.11 )
where Mv is the volumetric soil moisture (0-2.5 cm), e is the measured emissivity,
and W is the water content of the vegetation (kg/m2). Figure 9.5 illustrates the
effect of vegetation on soil moisture. Jackson and Schmugge (1991) have analyzed
a large amount of published data to verify previous findings and they have defined
a vegetation parameter that is based on the optical depth of the canopy. This parameter appears to be inversely related to the wavelength and can represent four
types of vegetation classes (leaf dominated, stem dominated, grasses, and trees and
shrubs). However, at longer wavelengths, a single value of the parameter might be
~
e.....
e
'"
or.
M
40
d>
(oJ
a:
;:;l
....
;a 30
0
~
~
'-"
"" u
c:2
....
(oJ
2: ;:;l
~
;;
NORMALIZED BRIGHTNESS TEMPERATURE
Fig. 9.5. The relationships between normalized brightness temperature and volumetric soil
moisture for different types of vegetation, L-band, H polarization (after Jackson et al. 1982)