(1986) notes the resiliency of the land with an 
increasing net primary productivity for much of 
the Sahelian region following a period in 1985 of 
increased rainfall. For this effort, a dry 
northerly site near Podor (16c>30’ N 14024’ W) and 
a semi-dry/wet southerly site near Tambacounda 
(13047’N 13°40’ W) were selected for analyses. 
Image pixel test sites were selected 10-20 km 
away from the city center for reducing urban 
related effects. An illustration of the study 
site region is in Figure 1 with mean annual 
isohyets included. 3 
Figure 1. Study site location with mean 
annual isohyets (Compiled :AGPE/FA0). 
3. APPROACH AND ANALYSES FOR PARAMETER DERIVATION 
3.1 Shortwave Radiometric Calibration 
There is no on-board calibrator for the AVHRR 
shortwave visible and near-IR bands. Instead a 
vicarious calibration technique is typically used 
to determine post-launch calibration changes. 
Frouin and Gautier (1987) estimates a 15% post 
launch decrease in radiometric gain for the 
shortwave NOAA—7 AVHRR bands. Holben et al. 
(1989a) through analysis of Saharan derived 
reflectivities indicates a time varying gain 
change to 20% for both NOAA—'7 and NOAA—9 
shortwave bands. The Holben et al. (1989a) 
radiometric adjustments were used to correct the 
AVHRR spectral data. 
The NOAA AVHRR calibrations to exoatmospheric 
reflectance were converted to radiance for use in 
atmospheric corrections for deriving a surface 
reflectance - The exoatmospheric derived 
calibrations were converted from the NOAA 
provided (Thekaekara 1973) solar irradiance data 
to that of the more widely recommended data of 
Neckel and Labs (1984) following recommendations 
by Price (1987) . For the AVHRR visible band 
there is a change in reflectance of 5.9% and for 
the near-IR band there is a change of 2.3%. Pre 
flight versus post-flight calibrations indicates 
a time varying change, worsening with launch for 
both NOAA-7 and NOAA-9 AVHRR. The coefficient of 
variation of the calibration difference is 31.4% 
for the visible reflectance, 29.6 for the near-IR 
reflectance, and 29.6% for the NDVI. 
3.2 Shortwave Atmospheric Correction 
The relationship between a Lambertian surface 
reflectance (Psur) and "the upward spectral 
radiance (L sa t) measured by the satellite 
(Chandrasekhar 1960) is given next with a 
spectral dependence (A) implied. 
Lsat = Lq + (FsurA") * (T*/?sur/ (l - s*Psur)) (1) 
Lsat ~ satellite calibrated radiance 
(W-m~2 si—1) 
Lq - atmosphere to satellite path radiance 
(W-m~2 sr~l) 
Fsur - surface, solar irradiance (W-m _ 2) 
T - surface to satellite total transmittance 
_ (direct + diffuse) 
s - atmosphere to surface counter-reflectance 
Psur - isotropic surface reflectance 
The measured satellite radiance for bands 1 and 2 
were converted to reflectance using the 
relationship given next. 
Psur = (Lsat ~ Lo)/(T’*F 0 (l/7r)+s(Lsat~Lc>)) (2) 
T’ - Total downwelling times upwelling 
atmosphere transmittance (Ahmad 
and Fraser 1982) 
The Lsat was derived from the NOAA AVHRR 
calibrated radiance. The atmospheric correction 
was developed using the radiative transfer model 
described by Ahmad and Fraser (1982) . The model 
incorporates a multiple scatter radiative 
transfer procedure after Dave (1972) with the 
addition of atmospheric polarization related 
effects. The model assumes a spherical aerosol 
for solution using Mie theory. In addition, the 
model assumes a horizontally homogeneous 
atmosphere bounded by a Lambertian reflecting 
surface. The atmosphere is assumed cloud free. 
The size distribution of the aerosols was 
represented by a power law distribution. The 
index of refraction is assumed to be m = 1.54- 
0.003i. The real number of 1.54 is considered 
typical of the sub—Saharan (e. g., Carlson and 
Caverly 1977) and the imaginary part at 0.003 is 
less known but was selected by inspecting results 
by Patterson et al. (1977) and a personal 
communication from Holben (1988) . 
Gaseous absorption for input to the radiative 
transfer model of Ahmad and Fraser (1982) were 
estimated using the opt i cad. properties 
(McClatchey et al. 1971) specified in the 
tropical model (15oN) from Kneizys et al. (1983) . 
The model includes estimates for pressure, 
temperature, aerosols, water vapor and ozone. 
The pressure, temperature and water vapor 
vertical distributions were normalized for 
conditions at the time of the satellite overpass 
for Tambacounda and Podor by using the near 
hourly surface meteorological data frem NCDC 
(i.e., ambient air temperature, surface 
atmospheric pressure, and dew point temperature). 
Holben and Eck (1989) indicates that atmospheric