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HAPEX - SAHEL

Objective

To improve the predictions of climate change in the semi-arid
tropics, through a coordinated programme of measurements and
model development. In particular the project aims to improve the
parametrization of land surface process in climate models for
semi-arid areas and the relationship of remotely sensed
quantities indicating changes in the above processes and those
measured on the ground.


The project is organised around a major field program scheduled
for 1992 in the African Sahel (Republic of Nigeria). A large
area, comparable to a general circulation model grid element
(100km x 100km) is considered for the investigation. A
description of the vegetation and land use over this domain will
be done using SPOT data and ground survey. Rainfall,
meteorological data and meteorological satellite data will also
be available at this scale. For a special observations period
more intensive measurements will be conducted in 3 representative
areas thereafter called supersites, selected over the dominant
vegetation type of the area (millet, fallow and tiger bush).
This intensive period will take place at the end of the rainy
season and extend into the dry season. A micrometeorological
station will be installed at each of these locations to measure
turbulent fluxes of sensible heat, of water vapour, of CO2 and
momentum. Fluxes will be measured using the direct, eddy
correlation method, but also the profile method, the bowen ratio
method and simplified methods such as the so-called sigma-T and
sigma-q methods. Soil moisture surveys using neutron probes, TDR
and capacity sensors will be performed. The measurement of the
ecophysiological functioning of the vegetation (photosynthesis,
stomatal resistance, etc..) will be made in conjunction with
biometric measurements (leaf area index, wet and dry biomass,
proportion of green leaves). Spectrometers and radiometers will
be used to measure the spectral characteristics of the vegetation
and soil. At some locations bidirectional reflectivity will be
measured. Surface temperature and emissivity will be measured.
The boundary layer will be monitored using radiosondes and
aircraft. Imagery in the visible and near infrared, thermal
infrared, and microwaves domains will be acquired from airborne
remote sensing.

Local transfer models will be calibrated at the homogeneous plot
scale, for which all the necessary measurements will be available
and also for larger surfaces with non-homogeneous land use.Remote
sensing inverse models using as input airborne remote sensing
imagery will be validated at homogeneous plot scale and then used
to extrapolate local measurements at supersite scale. The result
will then be checked using direct flux measurement by aircraft.
This will give 3 replications of surface energy balance and water
balance. Using stratification methods, evaporation can then be
inferred for the whole area.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE
Address
Avenue Gustave Coriolis 42, C.n.r. Mtttorologique
31057 Toulouse
France

Participants (10)

FREIE UNIVERSITAET BERLIN
Germany
Address
Carl-heinrich-becker-weg 6-10
12165 Berlin
Institut Français de Recherche Scientifique pour le Développement en Coopération - ORSTOM
France
Address
Rue La Fayette 213
75010 Paris
Landbouwuniversiteit Wageningen
Netherlands
Address
10,Duivendaal
6701 AR Wageningen
Landbouwuniversiteit Wageningen
Netherlands
Address
2,Duivendaal
6701 AP Wageningen
NATURAL ENVIRONMENT RESEARCH COUNCIL
United Kingdom
Address
Maclean Building, Crowmarsh Gifford
OX10 8BB Wallingford
UNIVERSITAT DE VALENCIA
Spain
Address
50,Doctor Moliner 50
46100 Burjasot
UNIVERSITY OF COPENHAGEN
Denmark
Address
10,Oester Voldgade 10
1350 Koebenhavn K/copenhaegen
UNIVERSITY OF READING
United Kingdom
Address
Earley Gate 2, Whiteknights, Palmer Building
RG6 2AU Reading / Silchester
WAGENINGEN UNIVERSITY
Netherlands
Address
11,Nieuwe Kanaal 11 De Nieuwelanden
6709 PA Wageningen
Winand Staring Centre of Integrated Land, Soil and Water Research
Netherlands
Address
125,Marijeweg
6700 AC Wageningen