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H.-J. Bolle, S. I. Rasool, P. Try
radiation at the surface (if the sky emission can assumed to be constant). The
temperature data in addition inform us whether vegetation species may be under
stress. Microwave sensors are capable to provide information about the soil moisture
in the uppermost soil layer and if this information is combined with soil-water
models and precipitation data series also soil moisture profiles can be estimated.
Combining further these various bits of information one can approach the question
of the surface energy fluxes and evaporation. With an elaborate method Bastiaanssen
(1995) and Bastiaanssen et al. (1997) were able to determine these fluxes for a test
area. Roerink and Menenti (1999) and Menenti et al. (1989) simplified the method
for quasi-operational application using the fact that if albedo is low and if at the
same time the temperature is low, then evaporation is close to potential evaporation
and if on the other hand temperature and visible reflectance are high then there
cannot be much vegetation and the sensible heat flux will be maximum which is
close to the difference between the downwelling radiation flux minus the soil heat
flux. In the temperature versus albedo diagram the line for the maximum sensible
heat flux starts high and slopes down because with increasing albedo less energy
becomes available and the surface temperature sinks. The line of the maximum
latent heat flux starts low and gently slopes up because with increasing albedo and
decreasing net radiation the evaporative flux becomes smaller which reduces the
cooling effect and the surface temperature slightly rises. The distances of a pixel in
this diagram from these two lines allows to compute the ratio between the sensible
and latent heat fluxes (the Bowen Ratio) and by solving the energy budget equation
the heat fluxes can be computed. The soil heat flux can be estimated with the
experience gained from field experiments. A cloudy atmosphere makes budget
estimates much more difficult but if there is no precipitation one can assume that the
partitioning of the net radiation flux does not change under clouds and if one can
infer the net radiation flux at the ground also the absolute magnitude of the heat
fluxes (and evaporation) can be derived. By building up this tree of information it
finally becomes feasible to draw a fairly good picture of the land-surface processes
at a large spatial scale.
The new generation of satellites will in addition provide more information about
the composition of the atmosphere then so far is possible. Trace constituent
concentrations are becoming available for the troposphere and improved information
about the water vapour distribution is expected which will help to explain and to
remove discrepancies in the modeling of this quantity.
With a number of ground measurements at "Anchor Stations" this information
can be validated and the algorithms be improved which are used for the evaluation
of the data. If furthermore the analysis of the measurements made in space can be
continued for long time periods trends and extents of changes could be analysed for
large areas. The intensive evaluation and use of observations made from space
combined with research at "Anchor Stations" will support the evaluation of existing
ground based data and may ring in a first phase of a basin wide Mediterranean
research area.
H.-J. Bolle, S. I. Rasool, P. Try
radiation at the surface (if the sky emission can assumed to be constant). The
temperature data in addition inform us whether vegetation species may be under
stress. Microwave sensors are capable to provide information about the soil moisture
in the uppermost soil layer and if this information is combined with soil-water
models and precipitation data series also soil moisture profiles can be estimated.
Combining further these various bits of information one can approach the question
of the surface energy fluxes and evaporation. With an elaborate method Bastiaanssen
(1995) and Bastiaanssen et al. (1997) were able to determine these fluxes for a test
area. Roerink and Menenti (1999) and Menenti et al. (1989) simplified the method
for quasi-operational application using the fact that if albedo is low and if at the
same time the temperature is low, then evaporation is close to potential evaporation
and if on the other hand temperature and visible reflectance are high then there
cannot be much vegetation and the sensible heat flux will be maximum which is
close to the difference between the downwelling radiation flux minus the soil heat
flux. In the temperature versus albedo diagram the line for the maximum sensible
heat flux starts high and slopes down because with increasing albedo less energy
becomes available and the surface temperature sinks. The line of the maximum
latent heat flux starts low and gently slopes up because with increasing albedo and
decreasing net radiation the evaporative flux becomes smaller which reduces the
cooling effect and the surface temperature slightly rises. The distances of a pixel in
this diagram from these two lines allows to compute the ratio between the sensible
and latent heat fluxes (the Bowen Ratio) and by solving the energy budget equation
the heat fluxes can be computed. The soil heat flux can be estimated with the
experience gained from field experiments. A cloudy atmosphere makes budget
estimates much more difficult but if there is no precipitation one can assume that the
partitioning of the net radiation flux does not change under clouds and if one can
infer the net radiation flux at the ground also the absolute magnitude of the heat
fluxes (and evaporation) can be derived. By building up this tree of information it
finally becomes feasible to draw a fairly good picture of the land-surface processes
at a large spatial scale.
The new generation of satellites will in addition provide more information about
the composition of the atmosphere then so far is possible. Trace constituent
concentrations are becoming available for the troposphere and improved information
about the water vapour distribution is expected which will help to explain and to
remove discrepancies in the modeling of this quantity.
With a number of ground measurements at "Anchor Stations" this information
can be validated and the algorithms be improved which are used for the evaluation
of the data. If furthermore the analysis of the measurements made in space can be
continued for long time periods trends and extents of changes could be analysed for
large areas. The intensive evaluation and use of observations made from space
combined with research at "Anchor Stations" will support the evaluation of existing
ground based data and may ring in a first phase of a basin wide Mediterranean
research area.