8 Evaporation
181
tation canopies should improve significantly the accuracy of current estimates
of turbulent heat and momentum fluxes at land surfaces.
The advanced satellite sensors and, in the long run, DIfferential Absorption
Lidars (DIAL) should provide measurements of PBL height, temperature and
humidity to specify better boundary conditions when modeling land evaporation.
8.9 Summary and Conclusions
In this chapter the use of remote sensing to address all aspects of land evaporation
has been reviewed. Although most literature on land evaporation relates to the use
of thermal infrared observations and the land surface heat balance, a broad spectrum of multi-spectral measurements by space- and airborne instruments has been
used to estimate potential evaporation and crop water requirements. The chart
(Fig. 8.2) illustrates the evolution of methods over time: the vertical links are
meant to indicate how each type of method evolved, while as regards the heat balance approach an overall shift has occurred from left (linear relationships) to right
(improved observations and parameterizations). The reader is also referred to the
synoptic table on methods and outstanding issues ofKustas and Norman (1996).
One may say that the early expectations for a simple and general algorithm have
been replaced by the awareness of the need for a stepwise approach to address
outstanding observational and modelling issues.
Among the likely developments in the coming years the author would like to
mention:
The documented impact of the difference (T rad - Taer) on estimates of latent heat
flux will stimulate work on new observational methods. This difference depends
on the difference (Tfoliage - Tsoil) and directional thermal infrared measurements
have the potential of capturing foliage and soil temperatures, so we should see
significant developments in this direction.
Canopy architecture and local topography playa significant role in land -atmosphere interactions. Very high resolution measurements by means of laser systems
provide nearly ideal observations to capture the geometry of land surface and to
estimate related land surface variables (e.g. Menenti and Ritchie, 1994).
Estimation of land surface evaporation requires some knowledge on the conditions of the atmospheric boundary layer. It appear that two complementary approaches will have to be explored:
a) use of back-scatter and differential absorption lidars;
b) assimilation of land surface variables in high resolution models of land - atmospheric processes.
The determination of actual evaporation through the water balance of an atmospheric column should become significantly more accurate with the launch of advanced sounders and the development of new 4DDA systems.
181
tation canopies should improve significantly the accuracy of current estimates
of turbulent heat and momentum fluxes at land surfaces.
The advanced satellite sensors and, in the long run, DIfferential Absorption
Lidars (DIAL) should provide measurements of PBL height, temperature and
humidity to specify better boundary conditions when modeling land evaporation.
8.9 Summary and Conclusions
In this chapter the use of remote sensing to address all aspects of land evaporation
has been reviewed. Although most literature on land evaporation relates to the use
of thermal infrared observations and the land surface heat balance, a broad spectrum of multi-spectral measurements by space- and airborne instruments has been
used to estimate potential evaporation and crop water requirements. The chart
(Fig. 8.2) illustrates the evolution of methods over time: the vertical links are
meant to indicate how each type of method evolved, while as regards the heat balance approach an overall shift has occurred from left (linear relationships) to right
(improved observations and parameterizations). The reader is also referred to the
synoptic table on methods and outstanding issues ofKustas and Norman (1996).
One may say that the early expectations for a simple and general algorithm have
been replaced by the awareness of the need for a stepwise approach to address
outstanding observational and modelling issues.
Among the likely developments in the coming years the author would like to
mention:
The documented impact of the difference (T rad - Taer) on estimates of latent heat
flux will stimulate work on new observational methods. This difference depends
on the difference (Tfoliage - Tsoil) and directional thermal infrared measurements
have the potential of capturing foliage and soil temperatures, so we should see
significant developments in this direction.
Canopy architecture and local topography playa significant role in land -atmosphere interactions. Very high resolution measurements by means of laser systems
provide nearly ideal observations to capture the geometry of land surface and to
estimate related land surface variables (e.g. Menenti and Ritchie, 1994).
Estimation of land surface evaporation requires some knowledge on the conditions of the atmospheric boundary layer. It appear that two complementary approaches will have to be explored:
a) use of back-scatter and differential absorption lidars;
b) assimilation of land surface variables in high resolution models of land - atmospheric processes.
The determination of actual evaporation through the water balance of an atmospheric column should become significantly more accurate with the launch of advanced sounders and the development of new 4DDA systems.