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Mesures physiques et signatures en télédétection

A.R. HUETE (3), J. DUNCAN ( 4 ), J. FRANKLIN ( 4 )
(!) University of Maryland, Geography Department, 1113 Lefrak Hall, College Park, MD 20742, U.S.A.
( 2 ) Météo France/CNRM, 42 Avenue Gustave Coriolis, 31057 Toulouse Cedex, FRANCE
(3) Department of Soil and Water Science, University of Arizona, Tuckson, AZ 85721, U.S.A.
( 4 ) Department of Geography, San Diego State University, San Diego, CA 92182, U.S.A.
This paper reports the results of a shortwave radiative budget experiment conducted over Sahelian vegetation during the 1992
HAPEX-Sahel experiment. The investigated region is stratified into five land units: bush/grassland, grassland, degraded bushland,
millet Geld and tiger bush. For each land unit (except the tiger bush), the understore canopy of a sample plot has been equipped
with sets of optical sensors for the whole rainy season to measure intercepted radiation. In parallel, canopies characteristics such as
optical and structural properties have been acquired. A particular attention is given here to the description of the experimental
design which is a new technique adapted to sparsely vegetated surfaces. It appears that the dynamic of Sahelian vegetation is much
larger when derived from radiative fluxes measured below (interception) than above (reflectance) the canopy, as a consequence of
the strong effect of the soil background reflectance. Finally, the use of radiative transfer models allow to assess the intercepted
radiation from the local to site scale. Site estimates of intercepted PAR are then related to ground and airborne radiometric data.
Pbotosynthetically Active Radiation (PAR: 400-700 tun) intercepted and absorbed by plants are strongly correlated to
the phytomass production, and used to estimate the net primary production (Monteith, 1972). PAR intercepted and
absorbed seem related to spectral vegetation indices by near-linear relationships (Asrar et al., 1984; Sellers, 1987)
which in fact depend strongly on the optical properties of the background (Choudhury, 1987; Goward and
Huemmrich, 1992) and the foliage geometry (Baret and Guyot, 1991). Most of the experimental and theoretical
studies on radiative transfer inside vegetation were obtained for homogeneous canopies, because of the difficulties of
measuring and modeling the radiative fluxes in sparse canopies. Therefore, most previous works remain quite
questionable to study semi-arid areas as Sahelian ecosystems where the vegetation is highly discontinuous. In the
framework of Sahelian vegetation studies, Begue (1993) has shown the strong dependence of the relationship between
intercepted PAR and spectral vegetation indices with the percent vegetation cover and the geometry of irradiation.
Furthermore, as instantaneous IPAR seems to be affected by directional effects, daily estimates are a more meaningful
variable in terms of primary production for discontinuous vegetation canopies.
New techniques have made possible to investigate a large range of canopies. For instance, the amorphous silicon cells
manufactured by Solems Industry show a good sensitivity in the PAR range (Chartier et al., 1989). Moreover, their
relative small size and reasonable cost made them convenient to sample semi-arid vegetation which requires the use of
a large number of sensors. Several successful! uses of these PAR sensors in Sahelian environment (Begue, 1993) were
in favour of implementing on the same panel a NIR detector to get the shortwave radiative budget of the canopy (this
product also manufactured by SOLEMS is referred hereafter as PARIR sensor).
The goal of this paper is mainly to present the results of measured and modeled shortwave radiative fluxes measured
under and over Sahelian vegetations during the 1992 HAPEX-Sahel experiment (Goutorbe et al., 1993; Prince et al.,
1993). The field measurements were conducted in the West Central super-site (13°31’N; 2°33'E) where the area is
stratified into five land units, that is bush/grassland, grassland, degraded bushland, millet field and tiger bush. On each
land unit (except the tiger bush), a plot was sampled by equipping the understore canopy with sets of PARIR sensors
for the whole 1992 growing season. Optical and structural properties of canopies were also measured. A brief
description of the sites, sensors properties, and radiative and biological measurements appear in the first part of this
paper. While the second part displays with the main radiative measurements acquired on the plots, the third part
concerns the use of radiative transfer models to calculate, at sites scale, the PAR intercepted/absorbed. The link
between those PAR data and the NDVI, derived from airborne radiometric data, offers the possibility to map radiative
fluxes at regional scale.