200
E.T. Engman
where DTs is the diurnal temperature difference between the afternoon surface
temperature Ts(PM) and the early morning surface temperature, Ts(AM), and Dis
the diurnal thermal inertia given by
D=w·Qc·k
(9.3)
where w corresponds to the day length, Qc is the volumetric heat capacity and k is
the thermal conductivity. The diurnal thermal inertia D describes the ability for the
soil to resist temperature change. For example, a dry sand has a relatively low
value of D compared to a wet clay because the thermal conductivity of the sand is
lower than that for the wet clay and the volumetric heat capacity for dry soil is
lower than that for wet soils. Thus, by measuring the amplitude of the diurnal temperature change, one can develop a relationship between the temperature change
and soil moisture. However, the relationship between the diurnal temperature and
soil moisture depends upon soil type and is largely limited to bare soil conditions
(van de Griend et aI., 1985).
Microwave Techniques. Microwave techniques for measuring soil moisture include both the passive and active microwave approaches, with each having distinct
advantages. The theoretical basis for measuring soil moisture by microwave techniques is based on the large contrast between the dielectric properties of liquid
water and of dry soil. The large dielectric constant for water is the result of the
water molecule's alignment of the electric dipole in response to an applied electromagnetic field. For example, at L-band frequency the dielectric constant of
water is approximately 80 compared to that of dry soils which is on the order of 35. Thus, as the soil moisture increases, the dielectric constant can increase to a
value of 20, or greater (Schmugge, 1983). Figure 9.1 illustrates the change in dielectric constant for soil at several microwave frequencies.
For passive microwave remote sensing of soil moisture from a bare surface, a
radiometer measures the intensity of emission from the soil surface. This emission
is proportional to the product of the surface temperature and the surface emissivity
which is commonly referred to as the microwave brightness temperature (TB) and
can be expressed as follows (Schmugge, 1990):
TB = t(H)·lrTsky +(I-r)TsoiIJ+ Ta,m ,
(9.4)
where t(H) is the atmospheric transmissivity for a radiometer at height H above the
soil, r is the smooth surface reflectivity, T soil is the thermometric temperature of
the soil, T atm is the average thermometric temperature of the atmosphere, and
Tsky is the contribution from the reflected sky brightness. For typical remote
sensing applications using longer microwave wavelengths (greater than 5 cm,
which are better for soil moisture), the atmospheric transmission will approach
99%. The atmospheric, T atm, and sky, T sky, contributions are both on the order of
5°K, each of which are small compared to the soil contribution. Thus neglecting
these two terms, Eq. 9.4 can be simplified to
TB = (1- r )Tsoil = eTsoil
(9.5)
E.T. Engman
where DTs is the diurnal temperature difference between the afternoon surface
temperature Ts(PM) and the early morning surface temperature, Ts(AM), and Dis
the diurnal thermal inertia given by
D=w·Qc·k
(9.3)
where w corresponds to the day length, Qc is the volumetric heat capacity and k is
the thermal conductivity. The diurnal thermal inertia D describes the ability for the
soil to resist temperature change. For example, a dry sand has a relatively low
value of D compared to a wet clay because the thermal conductivity of the sand is
lower than that for the wet clay and the volumetric heat capacity for dry soil is
lower than that for wet soils. Thus, by measuring the amplitude of the diurnal temperature change, one can develop a relationship between the temperature change
and soil moisture. However, the relationship between the diurnal temperature and
soil moisture depends upon soil type and is largely limited to bare soil conditions
(van de Griend et aI., 1985).
Microwave Techniques. Microwave techniques for measuring soil moisture include both the passive and active microwave approaches, with each having distinct
advantages. The theoretical basis for measuring soil moisture by microwave techniques is based on the large contrast between the dielectric properties of liquid
water and of dry soil. The large dielectric constant for water is the result of the
water molecule's alignment of the electric dipole in response to an applied electromagnetic field. For example, at L-band frequency the dielectric constant of
water is approximately 80 compared to that of dry soils which is on the order of 35. Thus, as the soil moisture increases, the dielectric constant can increase to a
value of 20, or greater (Schmugge, 1983). Figure 9.1 illustrates the change in dielectric constant for soil at several microwave frequencies.
For passive microwave remote sensing of soil moisture from a bare surface, a
radiometer measures the intensity of emission from the soil surface. This emission
is proportional to the product of the surface temperature and the surface emissivity
which is commonly referred to as the microwave brightness temperature (TB) and
can be expressed as follows (Schmugge, 1990):
TB = t(H)·lrTsky +(I-r)TsoiIJ+ Ta,m ,
(9.4)
where t(H) is the atmospheric transmissivity for a radiometer at height H above the
soil, r is the smooth surface reflectivity, T soil is the thermometric temperature of
the soil, T atm is the average thermometric temperature of the atmosphere, and
Tsky is the contribution from the reflected sky brightness. For typical remote
sensing applications using longer microwave wavelengths (greater than 5 cm,
which are better for soil moisture), the atmospheric transmission will approach
99%. The atmospheric, T atm, and sky, T sky, contributions are both on the order of
5°K, each of which are small compared to the soil contribution. Thus neglecting
these two terms, Eq. 9.4 can be simplified to
TB = (1- r )Tsoil = eTsoil
(9.5)