SENEGALESE LAND SURFACE CHANGE ANALYSIS AND BIOPHYSICAL PARAMETER ESTIMATION 
USING NOAA AVHRR SPECTRAL DATA 
David L. Toll 
NASA/Goddard Space Flight- Center 
Greenbelt, MD 20771 
Fred M. Yukovich 
Research Triangle Institute 
Research Triangle Park, NC 
ABSTRACT 
Surface biophysical estimates were derived 
from analysis of NOAA Advanced Very High Spectral 
Resolution (AVHRR) spectral data of the 
Senegalese area of west Africa. The derived 
surface parameters were of solar albedo, spectral 
visible and near-infrared band reflectance, 
spectral vegetative index, and ground 
temperature. Wet and dry linked AVHRR scenes 
from 1981 through 1985 in Senegal were analyzed 
for a semi-wet southerly site near Tambacounda 
and a predominantly dry northerly site near 
Podor. Related problems were studied to convert 
satellite derived radiance to biophysical 
estimates of the land surface. Problems studied 
were associated with sensor miscalibration, 
atmospheric correction, surface anisotropy of 
reflected radiation, narrow satellite band 
reflectance to broad solar band conversion, and 
ground emissivity correction. 
Solar albedo derived estimates for both sites 
were indicated not to change markedly with 
significant antecedent precipitation events or 
correspondingly from increases in green leaf 
vegetation density. The bright soil/substrate 
contributed to a high albedo for the dry related 
scenes, whereas the high internal leaf 
reflectance in green vegetation canopies in the 
near-IR contributed to high solar albedo for the 
wet related scenes. Furthermore, the 
relationship between solar albedo and ground 
temperature was poor, indicating the solar albedo 
had little control of the ground temperature. 
NDVI and the derived visible reflectance were 
more sensitive to antecedent rainfall amounts and 
green vegetation changes than were near-IR 
changes. The information in the NDVI related to 
green leaf density changes primarily was from the 
visible reflectance. The contribution of the 
near-IR reflectance to explaining green 
vegetation is largely reduced When there is a 
bright substrate or the canopy has significant 
woody vegetation amounts. 
KEYWORDS: Albedo, NDVI, AVHRR, Senegal, 
Land Change 1 
1- TMRCJXX7TEDN 
The western sub-Saharan region has over the 
last decade undergone periods of severe drought 
followed by periods of near—average rainfall 
(Motha et al. 1980 and Nicholson 1985) . Analysis 
of the polar orbiting NOAA Advanced Very High 
Resolution Radiometer (AVHRR) spectral data may 
provide a means to study regional land surface 
changes associated with drought and anthropogenic 
activities and provide further insight to land 
surface and atmosphere interactions. 
However, the extraction of biophysical 
parameters of the land surface from the AVHRR 
using directional and spectral radiance is 
associated with numerous difficulties that were 
examined for this study: 1) radiometric 
calibration of the optical bands (i.e., the AVHRR. 
bands without an on-board calibrator) (Hoiben et 
al. 1989a); 2) land surface anisotropy sensed by 
directional derived AVHRR sensors (Kimes et al. 
1985); 3) solar albedo estimation from AVHRR 
narrow band passes (Toll 1989); 4) heterogeneous 
ground emissivity complicating the land surface 
temperature estimate (Becker 1987); and 5) 
atmospheric aerosol and water vapor temporal and 
spatial variability affecting ground surface 
derivations (D’Almeida 1986). 
The primary objective of the study is to use 
AVHRR derived spectral data to evaluate land 
surface changes and to provide biophysical 
information derived from the satellite: radiance 
to examine land surface-climatology interactions. 
To characterize the landscape, surface derived 
estimates of solar albedo, spectral vegetation 
index and ground temperature are extracted from 
the AVHRR spectral data. Surf ace meteorological 
data of the western sub-Saharan for Tambacounda 
and Podor (available from the NOAA National 
Climate Data Center, .NCDC) were used to report 
climatic conditions and to describe the state of 
the atmosphere (e.g., temperature, rain, aerosol 
loading, wind, and clouds) for the time of the 
satellite overpass. 
2. STUDY SUB 
The area selected for study is the Senegalese 
area in the sub-Saharan encompassing two 
ecological areas from north to south, the Sahel 
(averaging 100-400 mm of precipitation) and the 
Soudan (400-800 mm) (Nicholson 1983) . The study 
region area is diverse because of a steep south 
to north precipitation gradient, with mean annual 
isohyets occurring parallel to equal lines of 
latitude. The vegetation is largely steppe, with 
changes to desert in the north and changes to 
savannah in the the south (Monad 1986). The area 
has extensive pastoral lands, largely from 
insufficient rainfall to support agriculture. 
Man induced effects from overgrazing of livestock 
and gathering of firewood has compounded the 
environmental effects from drought. Justice and 
Hiemauz (1986) report these changes in addition 
to drought have led to an overall lower net 
primary productivity as derived through analysis 
of NOAA AVHRR spectral data. Tucker et al.