384
M. Menenti
plant litter). Actual determination of stress level, therefore, may not be feasible for
partial canopies. Reflectance spectra may be used, however, to determine the
relative fractions of foliage and soil which gives a measure of irrigation
unifonnity.
E. Crop water stress using thermal infrared radiances. As described in Chap.
8, measurements of spectral radiances in the 0.4 through 12. Il region can be used
to study the land surface heat balance and to obtain estimates of actual evaporation
or of other indicators of soil water availability. Assuming weather as well as all
land surface properties, except surface temperature and crop transpiration, remain
constant, then measurements of radiometric temperature provide a measure of the
difference between actual and maximum transpiration and, therefore, of crop water
stress. This applies, for example, to a relatively homogeneous, complete canopy.
Poor irrigation unifonnity may well lead to a spatially variable water availability
and possibly to water stress measurable through surface temperature (see e.g.
Stanghellini and De Lorenzi, 1994). Spatial variability of other land surface
properties such as hemispherical reflectance, vegetation height or canopy
architecture will be significant for partial canopies or when considering larger,
heterogeneous areas. Observed spatial variability of the radiometric temperature
will be larger and related to water availability in a more complex manner. A
significant amount of research has focussed on this subject (see Current
Applications), but the scope for truly operational use of airborne or spaceborne
sensors appears constrained by the need for timely acquisition of the observations
and rapid distribution and actual use in support of irrigation management.
Continuing water stress leads to observable changes in spectral reflectance (see D),
which are easier to measure at the required spatial and spectral resolution.
F. Determination of soil moisture using active and passive techniques. Soil
water content is related to two land surface properties:
soil thermal capacity and conductivity;
soil dielectric properties;
The former determine temporal changes of soil surface temperature in response to
the heat balance at the soil surface. This subject has been discussed at length in the
Chap. 8, devoted to evaporation. It may be recalled, however, that the soil surface
can be observed at low fractional vegetation cover only. This implies that it is
unlikely to obtain useful estimates of soil water content in irrigated lands on the
basis of this physical principle. An alternative method, also based on the heat
balance approach is by relating the ratio of actual to potential evaporation
(evaporative fraction, see Chap. 8) to a water availability factor. As regards the
technical feasibility, the same comments apply as given above (item E). Soil
dielectric properties (see also Chaps. 2 and 9) are very sensitive to the soil water
content and sensitive to soil type and salinity. Moreover both active and passive
microwave instruments probe at best a limited soil depth and only the lowest
frequencies currently in use (i.e. L-band) may be relevant for irrigation and
M. Menenti
plant litter). Actual determination of stress level, therefore, may not be feasible for
partial canopies. Reflectance spectra may be used, however, to determine the
relative fractions of foliage and soil which gives a measure of irrigation
unifonnity.
E. Crop water stress using thermal infrared radiances. As described in Chap.
8, measurements of spectral radiances in the 0.4 through 12. Il region can be used
to study the land surface heat balance and to obtain estimates of actual evaporation
or of other indicators of soil water availability. Assuming weather as well as all
land surface properties, except surface temperature and crop transpiration, remain
constant, then measurements of radiometric temperature provide a measure of the
difference between actual and maximum transpiration and, therefore, of crop water
stress. This applies, for example, to a relatively homogeneous, complete canopy.
Poor irrigation unifonnity may well lead to a spatially variable water availability
and possibly to water stress measurable through surface temperature (see e.g.
Stanghellini and De Lorenzi, 1994). Spatial variability of other land surface
properties such as hemispherical reflectance, vegetation height or canopy
architecture will be significant for partial canopies or when considering larger,
heterogeneous areas. Observed spatial variability of the radiometric temperature
will be larger and related to water availability in a more complex manner. A
significant amount of research has focussed on this subject (see Current
Applications), but the scope for truly operational use of airborne or spaceborne
sensors appears constrained by the need for timely acquisition of the observations
and rapid distribution and actual use in support of irrigation management.
Continuing water stress leads to observable changes in spectral reflectance (see D),
which are easier to measure at the required spatial and spectral resolution.
F. Determination of soil moisture using active and passive techniques. Soil
water content is related to two land surface properties:
soil thermal capacity and conductivity;
soil dielectric properties;
The former determine temporal changes of soil surface temperature in response to
the heat balance at the soil surface. This subject has been discussed at length in the
Chap. 8, devoted to evaporation. It may be recalled, however, that the soil surface
can be observed at low fractional vegetation cover only. This implies that it is
unlikely to obtain useful estimates of soil water content in irrigated lands on the
basis of this physical principle. An alternative method, also based on the heat
balance approach is by relating the ratio of actual to potential evaporation
(evaporative fraction, see Chap. 8) to a water availability factor. As regards the
technical feasibility, the same comments apply as given above (item E). Soil
dielectric properties (see also Chaps. 2 and 9) are very sensitive to the soil water
content and sensitive to soil type and salinity. Moreover both active and passive
microwave instruments probe at best a limited soil depth and only the lowest
frequencies currently in use (i.e. L-band) may be relevant for irrigation and