A study area was selected in West Africa in order to establish a relationship between rainfall and grassland productivity, and also to provide ground measurements and support for the GIMMS investigation of the use of AVHRR vegetation index data for monitoring grassland biomass and productivity. Field data collection, image and collateral data acquisition, model development, and data analysis and processing are briefly discussed

Simonett, D.

English

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CANOPY REFLECTANCE MODELING IN A
TROPICAL WOODED GRASSLAND
Principal Investigator: David Simonett, Professor of Geography
Department of Geography, University of California, Santa Barbara, CA 93106
Semi-annual Report
NASA Award NAGW-788
January 31, 1986
™
PICAf WOODED'GEASSLAMD N86-20932
Oniv.) 5 p
CSCL 02F Onclas
G3/43 05383
https://ntrs.nasa.gov/search.jsp?R=19860011461 2020-03-20T15:17:44+00:00Z
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CANOPY REFLECTANCE MODELING IN A TROPICAL WOODED
GRASSLAND
Introduction
The following brief report summarizes the research accomplished on NASA Grant
NAGW-788 during the period 7/1/85 to 1/31/86. The topics to be addressed are; 1)
field data collection, 2) image and collateral data acquisition, 3) model development and
4) data analysis and processing. Based on progress in this period, we have formulated
appropriate goals for the second half of the year (2/1/86 through 7/31/86), and for the
second year of the grant (8/1/86 through 7/31/87). A complete description of this
research can be found in our original proposal.
Field Reconnaissance and Data Collection
A study area was selected in the Gourma region of Mali, West Africa. A study is
already being conducted there by the Centre International pour 1'Elevage en Afrique
(CIPEA), Pierre Hiernaux, Principal Investigator, in collaboration with the GIMMS (Glo-
bal Inventory, Monitoring and Modeling) Project at NASA/Goddard Space Flight Center
(Chris Justice, Brent Holben and Jim Tucker, Investigators). This area has been moni-
tored for two years in order to establish a relationship between rainfall and grassland
productivity, and also to provide ground measurements and support for the GIMMS
investigation of the use of AVHRR vegetation index data for monitoring grassland
biomass and productivity.
Janet Franklin (UCSB Graduate Student and research assistant on this grant) visi-
tied Mali from 10/17/85 to 11/24/85. This visit was deemed necessary in order to estab-
lish a working relationship with our colleagues there, locate suitable test sites, collect
some preliminary field data and acquire collateral data that is difficult to get outside of
Mali (aerial photographs, maps, and so fort'h).
Ms. Franklin was able to visit the Gourma field sites with Mr. Hiernaux and other
CIPEA personnel, and observe their data collection procedures. In the Gourma study,
thirty one-kilometer line transects have been located along a north-south trans-Sahelian
climatic gradient from near Douna in the south (14° 40' N, 1° 35' W, average annual
rainfall approximately 500 mm), to Gourma-Rharous in the north (17° 45' N, 1° 50' W
average annual rainfall approximately 250 mm). These transects are being monitored for
aboveground biomass through the growing season, a census is being taken of the seed
bank, and spectral reflectance measurements are being taken from a hand-held radiome-
ter. The CIPEA team has also estimated woody cover by the line intercept method,
and sampled tree height, circumference, and crown diameter for certain dominant species
within the sites. These measurements can be used in part for parameterizing our canopy
model, although more field measurements will be required.
We have requested the tree cover and dimensional data from CIPEA so that a rela-
tionship can be established between tree height, circumference and crown diameter, for
calibrating the canopy reflectance model. Additional calibration data will be required,
and will be collected in the next field season.
An additional field area was visited in Mali. It is located in the Se'gou region of the
Sudanian bioclimatic zone. The area between Tamani and Konodimini (6° 50' W and 6°
20' W) and the Niger River and Nango (13° 25' N and 13° 10' N) is being used by Mr.
Roy Cole, a graduate student in the Department of Geography, Michigan State Univer-
sity, in his study of the changes in land use practices in response to the drought since
the early 1970's. This area lies within an extensive bioclimatic region where rainfed
PRECEDING PAGE BUNK 89T FTUMED
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naturally occurring trees which are preserved because of their economic and agricultural
value. This long history of land use results in a partly artificial vegetation structure,
consisting of sparse tree cover with an understory of crops. The structure and extent of
this vegetation type make it suitable for testing the canopy model.
Ms. Franklin visited this area with Mr. Cole during her stay in Mali, and collected
some preliminary tree structural measurements, as none were available for this area. In
sixteen 50 m radius plots (eight in each on two canopy types) the diameter at breast
height (dbh) of each tree was recorded, and height, and shape parameters were measured
for a subsample of the trees. A total of 482 trees were measured.
Image and Collateral Data
Thematic Mapper data for the study areas have been ordered and recently received.
The satellite imagery was not ordered until Ms. Franklin returned from the field, which
delayed the progress of research somewhat, but was a necessary and prudent precaution,
so that appropriate dates and subscenes could be ordered, based on first-hand knowledge
of the areas.
In addition, invaluable aerial photography and maps were acquired by Ms. Frank-
lin in Mali. Topographic maps at several scales (1:200,000 and 1:1,000,000) were
acquired for both the Gourma and Se'gou sites. Black and white aerial photographs are
available for the whole country at a scale of 1:60 000, but they date from 1956. These
are the only small-scale photographs available in the Gourma region, and will be
required for image registration, location of study sites, strata labeling, and so forth.
Therefore, partial coverage for the Gourma region has been ordered. In the Se'gou
region, 1:50,000 black and white panchromatic photos from 1974 are available for part of
the region due to the presence of "Projet Riz" (an extensive irrigation project for rice
growing) in this area. These photos have been purchased. Current (1985-1986) low-level
color air photos for some of the study sites in both regions were made available to us by
CIPEA. All of these maps and photo data sources will be used for model parameteriza-
tion (calculating tree spatial pattern, measuring actual density and cover for sample
stands to be used in accuracy assessment, etc.), image registration (to help interpret
from satellite imagery to topographic maps) and strata labeling during the image
stratification step.
Model Development
Li and Strahler at Hunter College have continued to refine their family of canopy
models, and have modified the simple variance-dependent model to run for tree shapes
appropriate to these study sites (hemiellipsoid and inverted cone). Sensitivity analysis of
the model with modified geometric parameters shows that the model is less sensitive to
changes in tree shape than it is to changes in other parameters, such as background sig-
nature, for an area with small trees and sparse cover.
The next step (see below) is to run the model with its modified tree shape parame-
ter using field data collected this year, and produce a preliminary evaluation of the
model for a few sample stands. Other modifications underway by Li and Strahler, also
of relevance to our work, is incorporation of canopy transmissivities, rather than total
reflectance. The sparse canopies of many semi-arid tree species, and different species-
specific phenological patterns (dry season vs. wet-season deciduous) also need to taken
into consideration when applying the model.
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Data Analysis and Processing
Analysis of the Se'gou field data has been initiated. The objective is to come up
with reasonable shape parameters for the woodland types sampled, and product popula-
tion statistics for these trees that can be used in the canopy model (e.g., allometric rela-
tionships among tree dimensional measurements, tree size distribution and spatial pat-
tern). Preliminary results are encouraging. In the crop/woodland vegetation of the
Se'gou region, the trees have a very predictable shape (because they are pruned for
fodder), and a Gaussian size class distribution (which would be unusual for a natural
population, but for these preserved trees with very little natural regeneration, is not too
surprising). Unfortunately, due to the pruning practices, the correlation between tree
height and diameter is not good, which will make fieldwork more difficult. Diameter
cannot be measured to predict height, but rather height will have to be measured
directly. However, it was observed that these stands have low spatial variability, so
perhaps a smaller subsample of trees can be chosen for field measurement.
Because of our deliberate delay in ordering image data, image processing has only
proceeded as far as a cursory examination the data for quality. We expect to progress
rapidly with image pre-processing and stratification in the second half of this year (see
below).
Goals for Second Half of First Year
In the second half of the first year, the period from 2/1/86 through 7/31/86, the
following tasks will be accomplished:
(1) Data Analysis and Processing: The analyses of tree population
statistics and spatial point pattern that have been initiated for the
Se'gou sites will be completed. When the raw field data is received
from CIPEA, a similar analysis will be performed for the Gourma
test sites.
The Thematic Mapper data for the test sites will be registered to a
map base, corrected and enhanced in other ways if necessary. One
exploratory task will be to perform image spatial analysis by looking
at image variograms for different vegetation types in the study
region. The image will then be stratified according to the procedures
specified in our original proposal. An appropriate sampling scheme
will be worked out for assessing the accuracy of the image
stratification and inventory during the next field sampling period.
The temporal pattern of the vegetation index from AVHRR data for
the study sites will also be analyzed in conjunction with the GIMMS
study in this area. This is to examine the possibility of using
AVHRR data to identify areas of woody cover based on phenological
patterns. This analysis has been carried out by Hiernaux and Jus-
tice for 1984, which was a very bad rain year. The analysis will be
repeated with the AVHRR vegetation index data for 1986.
(2) Model Development: Once the above processing tasks are com-
pleted, spectral data for sample stands can be extracted from the
images, and the canopy model can be tested for some sample stands
of varying densities.
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Goals for the Second Year
Our goals for the second year are essentially those outlined in our original proposal,
but will be refined and elaborated on in the renewal proposal we will be sending within
the month.
Publications
No publications have been produced as yet in conjunction with this project, but we
anticipate presenting a paper on the initial testing of the canopy model at the Interna-
tional Symposium of Remote Sensing of Environment, in Nairobi, Kenya, December
1986. This paper will become the first is a series of manuscripts that will be produced
based on this work.


Canopy reflectance modeling in a tropical wooded grassland

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