9 Soil Moisture
203
35
30
25
20
15
10
5
0 .1
0 .2
0.3
0.4
0 .5
0.6
VOL METRIC WATER CONTENT (cm 3 /cm 3 )
Fig. 9.2. A comparison of laboratory measurements of the real and imaginary parts of the dielectric constant and model predictions (smooth curves) for three soils as functions of volumetric
soil moisture at 1.4 GHz (after Wang and Schmugge, 1980). Wt is the transition moisture content where the dielectric constant approaches that for a liquid
stant. According to Wilheit (1975), the layer of soil would be on the order of a
tenth of a wavelength or less. Mo et al. (1980) detennined that the radiometric
sampling depth is between 0.06 and 0.1 times the wavelength. In an experiment
comparing dry-down measurements of soil layers at three frequencies, Newton et
al. (1982) found that for L-band (21 cm wavelength) the sampling depth was about
two-tenths of the wavelength. The fact of the matter is that measurement depth is
not a constant but related to the total amount of water in the soil layer, and thus to
the moisture content, and to the operational frequency of the sensor. A reasonably
good idea about the measurement depth can be obtained from penetration depth,
dp, inside the soil. This is given by
dp = k ·IIm.J£]
(9.8)
where k = 2p/l is free space propagation constant, e is dielectric constant of soil,
1m denote the imaginary part and the vertical bars refer to the absolute value. The
203
35
30
25
20
15
10
5
0 .1
0 .2
0.3
0.4
0 .5
0.6
VOL METRIC WATER CONTENT (cm 3 /cm 3 )
Fig. 9.2. A comparison of laboratory measurements of the real and imaginary parts of the dielectric constant and model predictions (smooth curves) for three soils as functions of volumetric
soil moisture at 1.4 GHz (after Wang and Schmugge, 1980). Wt is the transition moisture content where the dielectric constant approaches that for a liquid
stant. According to Wilheit (1975), the layer of soil would be on the order of a
tenth of a wavelength or less. Mo et al. (1980) detennined that the radiometric
sampling depth is between 0.06 and 0.1 times the wavelength. In an experiment
comparing dry-down measurements of soil layers at three frequencies, Newton et
al. (1982) found that for L-band (21 cm wavelength) the sampling depth was about
two-tenths of the wavelength. The fact of the matter is that measurement depth is
not a constant but related to the total amount of water in the soil layer, and thus to
the moisture content, and to the operational frequency of the sensor. A reasonably
good idea about the measurement depth can be obtained from penetration depth,
dp, inside the soil. This is given by
dp = k ·IIm.J£]
(9.8)
where k = 2p/l is free space propagation constant, e is dielectric constant of soil,
1m denote the imaginary part and the vertical bars refer to the absolute value. The