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SHORTWAVE RADIATIVE BUDGET OF SAHELIAN VEGETATION DURING 
HAPEX-SAHEL EXPERIMENT. RESULTS OF MEASUREMENT AND MODELING 
A. BEGUE 0), J.L. ROUJEAN( 2 ), N.P. HANANC 1 ), S.D. PRINCE0), WJ.D. VAN LEEUWEN( 3 ), 
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. 
Abstract 
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. 
1. INTRODUCTION 
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.