Remote Sensing of Soil Moisture
373
emittance or emissivity! in the microwave range, defined as the ratio between the flux emitted
by a surface element and the flux emitted by a black body at the same temperature. Its value
ranges between 0 (perfect reflector) to 1 (black body).
14.2.1 Visible and infrared bands
Experimental research has demonstrated that, in theory, observations in the visible, thermal
infrared and microwave regions of the electromagnetic spectrum, are able to give some information on the surface SMC of bare soils. In the optical range, soil spectral reflectance decreases as
water is added to soil; however these variations of the reflectance are often in the same order or
even lower than those caused by the physical or morphological differences existing among natural bare soils. Moreover, the observed variations in reflectance are in the order of about 10%
for soil moisture values ranging between 10% and 35% and the optical response is influenced by
a very thin soil layer (Idso et aI., 1975). Considering that on canopy covered soil the situation
is more complicated a widespread application of this technique is somewhat invalidated.
Thermal infrared sensors can be more useful than optical ones, because soil surface temperature
is influenced more markedly than reflectance by the moisture conditions of a relatively deeper
layer of soil. Observations in the thermal infrared band (8-14 Jlm) allow the measurement of
surface temperature, which depends on repartition of net radiation flux between latent (evapotranspiration) and sensible heat. Infrared thermal emission depends on SMC in that wet
regions are generally cooler than dry ones during the day and warmer at night. But even in
this case the uncertainty in the relation between surface temperature and moisture is very high.
Surface temperature, mainly if compared with air temperature, can give qualitative information on the evapotranspirative conditions of the surface. If quantitative information concerning
the real amount of water contained in the soil is required, a method consists in computing the
thermal inertia of the soil obtained by measuring the maximum and minimum daily surface
temperature. As a matter of fact the amplitude of the diurnal range of soil surface temperature
is a function of thermal conductivity and heat capacity (both dependent on SMC) as well as
of meteorological factors which drive the diurnal variations of surface temperature. In most
cases evaporation reduces the amplitude of diurnal surface temperature, so that the day/night
temperature difference depends on some combination of SMC and evapotranspiration.
During the past decade many authors have reported on methods to derive the thermal inertia of
bare and vegetated surfaces from infrared satellite data (e.g. Rosema et al., 1978; Becker et al.,
1988), and most of the efforts in using these data have been addressed to the direct estimate of
evapotranspiration. On this subject much research has been performed at the Institute National
de la Recherche Agronomique in Versailles and Avignon (France) (e.g. Seguin et al., 1983), and
by several groups in the Netherlands. The HCMM satellite, launched in April 1978, was the first
satellite devoted to acquiring high-resolution thermal data at optimum period of the diurnal
cycle. The HCMM carried out a two-channel radiometer (0.55-1.1 and 10.5-12.5 Jlm) in a sunsynchronous orbit at an altitude of 620 km and collected data at approximately 02.30 and 13.30
local standard time with a repeat cycle of 5 or 16 days. (Heilman and Moore, 1982). HCMM
data collected at different dates over US test-sites were analyzed in order to evaluate the thermal
data used in estimating near-surface soil moisture in a complex agricultural landscape. From
this experiment a highly significant correlation (Fig. 14.3) was obtained between estimated soil
temperature and soil moisture content.
During the TELL-US project which was set up as a collaboration agreement among several European institutes under the leadership of the JRC, in several European countries the volumetric
linstead of" emissivity" often the term" emittance" is used.
373
emittance or emissivity! in the microwave range, defined as the ratio between the flux emitted
by a surface element and the flux emitted by a black body at the same temperature. Its value
ranges between 0 (perfect reflector) to 1 (black body).
14.2.1 Visible and infrared bands
Experimental research has demonstrated that, in theory, observations in the visible, thermal
infrared and microwave regions of the electromagnetic spectrum, are able to give some information on the surface SMC of bare soils. In the optical range, soil spectral reflectance decreases as
water is added to soil; however these variations of the reflectance are often in the same order or
even lower than those caused by the physical or morphological differences existing among natural bare soils. Moreover, the observed variations in reflectance are in the order of about 10%
for soil moisture values ranging between 10% and 35% and the optical response is influenced by
a very thin soil layer (Idso et aI., 1975). Considering that on canopy covered soil the situation
is more complicated a widespread application of this technique is somewhat invalidated.
Thermal infrared sensors can be more useful than optical ones, because soil surface temperature
is influenced more markedly than reflectance by the moisture conditions of a relatively deeper
layer of soil. Observations in the thermal infrared band (8-14 Jlm) allow the measurement of
surface temperature, which depends on repartition of net radiation flux between latent (evapotranspiration) and sensible heat. Infrared thermal emission depends on SMC in that wet
regions are generally cooler than dry ones during the day and warmer at night. But even in
this case the uncertainty in the relation between surface temperature and moisture is very high.
Surface temperature, mainly if compared with air temperature, can give qualitative information on the evapotranspirative conditions of the surface. If quantitative information concerning
the real amount of water contained in the soil is required, a method consists in computing the
thermal inertia of the soil obtained by measuring the maximum and minimum daily surface
temperature. As a matter of fact the amplitude of the diurnal range of soil surface temperature
is a function of thermal conductivity and heat capacity (both dependent on SMC) as well as
of meteorological factors which drive the diurnal variations of surface temperature. In most
cases evaporation reduces the amplitude of diurnal surface temperature, so that the day/night
temperature difference depends on some combination of SMC and evapotranspiration.
During the past decade many authors have reported on methods to derive the thermal inertia of
bare and vegetated surfaces from infrared satellite data (e.g. Rosema et al., 1978; Becker et al.,
1988), and most of the efforts in using these data have been addressed to the direct estimate of
evapotranspiration. On this subject much research has been performed at the Institute National
de la Recherche Agronomique in Versailles and Avignon (France) (e.g. Seguin et al., 1983), and
by several groups in the Netherlands. The HCMM satellite, launched in April 1978, was the first
satellite devoted to acquiring high-resolution thermal data at optimum period of the diurnal
cycle. The HCMM carried out a two-channel radiometer (0.55-1.1 and 10.5-12.5 Jlm) in a sunsynchronous orbit at an altitude of 620 km and collected data at approximately 02.30 and 13.30
local standard time with a repeat cycle of 5 or 16 days. (Heilman and Moore, 1982). HCMM
data collected at different dates over US test-sites were analyzed in order to evaluate the thermal
data used in estimating near-surface soil moisture in a complex agricultural landscape. From
this experiment a highly significant correlation (Fig. 14.3) was obtained between estimated soil
temperature and soil moisture content.
During the TELL-US project which was set up as a collaboration agreement among several European institutes under the leadership of the JRC, in several European countries the volumetric
linstead of" emissivity" often the term" emittance" is used.