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Regarding the assimilation of time series of observations, we want to underline that the a priori knowledge of the
canopy gives a better understanding of the satellite signal, and then allows the filtering of very noisy data on
biological bases. Some important progresses are expected about the understanding and the modeling of
atmospheric and directional effects, and the future generations of sensors are very promising regarding the
interest of acquired measurements for both these domains. However, some weaknesses will always appear in the
“cleaning” or in the interpretation of the data. The use of the temporal dimension is a way to compensate for the
lack of accuracy of the physical values retrieved from the signal. The coherency of the model with a temporal
profile of measurements is more useful than the accuracy of each individual measurement. It is then able to
correctly simulate the temporal profile of the LAI, which will be always considered as an important variable for
caracterizing the canopy. In man y cases, we have tendency to be mené confident in such a LAI behavior than in a
LAI profile derived from satellite data by mean of inversion technique at the dates when data are acquired. In
such a case, saturation of reflectances and soil influence may result in important shifts in the LAI retrieved by
inversion.
Classically, the LAI was the only variable related to the visible and near infrared reflectances. This is. of course a
kind of reduction in what a functional model can give as description of the canopy useful far the understanding of
the reflectances. As it was shown, the description of the cover fraction by the model itself is a first attempt to
these considerations, and a brief description of the canopy structure in terms of orientation of leaves and their
change in time could help for the link with the radiative transfer model. Other wavelengths must also be taken
into account, especially for the study of evergreen canopies without evident annual periodicity of the LAI. The
work of Bouman (1991) over sugarbeets fields showed the interest of the longwaves measurements, sensitive to
the vegetation water content, and thus to the biomass. In that spectral demain, the control of parameters related to
the soil humidity, if possible, is a very attractive idea because of the great importance of soil hydrologic
properties for the vegetation behavior. Consequently, we absolutely need to better understand the biological
mechanisms of the various functi onal types regarding hydric budget, as well as their adaptation to various
conditions. Thermal infrared radiances can also be a control of évapotranspiration processes, but in that case we
are faced to variables with a hi gh temporal variability during the day, and the consistency between the time step
of that process within the model and the availability and temporal represen ta tivity of the satellite data used is a
problem which we need to think about.
Lastly, we must notice that the effect of the temporal distributions of the observations on the retrieval of die
various kinds of parameters must be quantified. This requires studies in order to keep a critical vision on this
approach. In fact, we must avoid to assimilate data which are absolutely not relevant far the parameterization of
the mechanisms that we hope to better understand.
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