Dust front ahead of a line squall
in a monsoon period (Senegal).
Unprecedented
mobilization
Le journal de l'IRD
Chronicle of the African monsoon
© IRD/B. Mougenot
simultaneous documentation of all
the components of the monsoon,
from regional down to local scales.
Amma therefore includes an observation programme unique of its kind
aiming to gain a clear picture of
the earth-ocean-atmosphere system,
from the south of the Sahara to the
Equator, from the Sudan across to
the middle of the tropical Atlantic
and, vertically, from underground
water level up to the tropopause at
20km altitude. The successful setting-up of an observation programme
of such size and scope, operating
over a number of years (2005-2007),
is particularly remarkable for its
implementation in a climate difficult
both for men and equipment, with
infrastructures and operations networks much less developed than
elsewhere.
●
Banizoumbou weather
station (Niger).
Amma experiment
system hinges on three
mesoscale sites,
including this one
in the Niamey area
(13-14°N), which
covers 16 000 km2
and has been
monitored by IRD
teams since
the Hapex-Sahel
experiment in 1990.
© IRD/T. Lebel
had a catastrophic impact on food
security and water resources, the
River Niger ceasing to flow at
Niamey in 1985 and the surface area
of Lake Chad shrinking from
25 000 km2 in the 1950s to a
present-day 2 500 km2.
Some monsoon-inhibiting processes
have been identified, in particular a
warming of the tropical ocean, which
amounted to a 0.5° increase since
the 1950s as a consequence of global
warming and the disappearance of
non-anthropized forests and savannah. For all that, the complex interactions which regulate the variability of the monsoon are still
insufficiently understood. Prediction
of such variations, whether at seasonal or overall climatic scale, is
therefore still highly uncertain. The
main stumbling block to improvement of prediction models is a deficiency of observations pertinent for
o the non-specialist, the
monsoon is an Indian
affair. Monsoon systems
really are enormous sea
breezes on a regional scale, associated with thermal and dynamic contrasts between tropical oceans and
neighbouring continental surfaces.
West Africa, bordering the tropical
Atlantic Ocean, is no stranger to this
phenomenon: its climate is governed
by the West African monsoon whose
strong fluctuations –which are really
difficult to predict– rule over the
lives of 300 million people living in a
space of 7.8 million km2.
Drought has been raging since the
beginning of the 1970s, especially in
the Sahel. This drought has no equivalent anywhere in the world, for the
surface area it extends to, its duration and severity (up to 50% rain
deficit during the period 1970-1990)
since climatic records began. It has
Contact
Thierry Lebel, IRD, Laboratoire d’études
des transferts en hydrologie et environnement, UMR LTHE, coprésident du
comité scientifique Amma France
Thierry.Lebel@hmg.inpg.fr
WEB
An African scientific plan
The Amma-Africa network now has a complement of 200 African researchers
and students from different disciplines and different countries, and also
engineers and technicians from national or regional operational services.
Africa’s scientific plan for the period
2007-2010. Seventeen African research institutions and operational
services are being backed through
this. The Ripiecsa solidarity fund,
amounting to 3.5 millions euros
raised by the French Ministry of
Foreign and European Affairs for the
period 2007-2010, represents a second major opportunity to initiate
research on climate-ecosystemssocieties interactions. Amma-Africa’s
objective is to keep operating beyond
Contact
One year after the intensive measurement campaign of 2006, which
saw an exceptional deployment of
instrument arrays over West Africa,
combining aircraft and research ship
surveys with substantial instrumentation set up on the ground, the
time has come to take stock of the
research conducted using data collected since measurements began in
2002. After the conference held in
Dakar in December 2005, the second International Amma Conference
took place in Karlsruhe in Germany
in the last week of November. About
20 plenary or parallel sessions gave
the opportunity to about 300 researchers to discuss atmospheric,
continental and oceanic processes,
integrative science, regional dynamics of the monsoon, the regional
water cycle, feedbacks into the coupled system, impacts of the variability of the monsoon on the agricultural resources and water, and also
on health.
●
Arona Diedhiou, Laboratoire d’études des transferts en hydrologie et
environnement, UMR LTHE
arona.diedhiou@inpg.fr
Amadou Gaye, head of Laboratoire
de physique de l'atmosphère et de
l’océan Simeon Fongang, Université
Cheikh Anta Diop, Dakar - École
supérieure polytechnique.
atgaye@ucad.sn
© Météo-France
▲ The Ronsard weather radar was designated to cover the
mesoscale site of the Ouémé Basin (14 600 km2 ; 9-10°N),
with the dual aim of studying the three-dimensional
structure of rainfall-producing convective systems and
providing real-time monitoring for performing airborne
survey missions.
Preparation for launching of a radiosounding mission at
Cotonou.
http://amma.mediasfrance.
org/france/
Second
International
Amma
Conference
© AMMA
offices and the “Agence pour la sécurité de la navigation aérienne”
(Asecna) have assigned 18 forecasting specialists to the operational
centre to run the forecast unit at
installed at Acmad (African Centre
for Meteorology Applied to Development) for 4 months with the support of Météo-France and the World
Meteorological Organization. The
operational component of Amma has
thus made a strong contribution to
regional integration of research.
In spite of these accomplishments,
the problem of mobilization of
human, material and financial
resources for applying the AmmaAfrica plan is always present, like
that of establishing the activities in a
permanently running system. Recent
calls for proposals should lead to
actions based on the long term. The
European Union (through DG
Research) has thus contributed
1.2 million Euros to the “Impacts”
component of the projects in Amma-
▼
ith the aim of tackling
questions arising at local
scale, Amma-Africa devised a scientific plan
which gears the research to subject
areas that are crucial for Africa –like
desertification, management of natural resources, food security, the
socio-economic and environmental
impacts of climate change, adaptation
strategies, health or water quality.
This network, the fruit of a highly
structured international scientific
programme with which it is incorporated, is an opportunity for often-isolated young African research scientists. Individual scientists and teams
have thus found a regional framework for exchanging information,
linking together individual initiatives
and proposals and gaining effectiveness in the search for the necessary
means for implementing their projects.
From the operational point of view,
partnership is also at the core of the
Amma’s observation programme. Its
implementation necessarily implies
the close involvement of African
institutions and scientists. The most
obvious sign of this involvement was
the setting-up of Amma’s main
Operations Centre (AOC) in Niger.
And all over the subregion teams
have for the first time had the opportunity to take part in a scientific
campaign of such far-reaching
scope. The national meteorological
the initial phase of the project
(2001-2009) in order to promote
interdisciplinary research, particularly by means of greater involvement of life sciences and social sciences. This partnership should pave
the way for the creation of a true
African hub of competence on the
environment and sustainable development in Africa.
●
Contact
Jean-Luc Redelsperger, CNRS, Centre
national de recherches météorologiques
Jean-Luc.Redelsperger@meteo.fr
Elisabeth Van den Akker,
Bureau de Projet Amma.
Elisabeth.vandenAkker@ipsl.
jussieu.fr
Sciences au Sud - Le journal de l’IRD - n° 42 - november/december 2007
African monsoon
An international coalition of research scientists has been drawing up a chronicle
of the African monsoon over three consecutive years. All the climatic parameters, including
the factors at work at terrestrial, atmospheric and under-water levels have been the subject
of scrupulous recording and subsequent analysis in order to understand and anticipate the sharp
swings the West-African climate undergoes, which sometimes have deadly consequences.
Scientific discussion within the French
scientific community about the elaboration of a large-scale international
programme devoted to the causes and
impact of West Africa’s long severe
drought of 1970-1990 began in 2001.
The programme rapidly became international in character. It now involves a
community of several hundred scientists working in concert within the
Amma programme (African Monsoon
Multidisciplinary Analyses) –a force of
an estimated 600 participants made up
the non-African scientific and technical
teams that contributed in the field in
the 2006 observation campaign.
However, many European scientists
have not had this opportunity and will
have to take satisfaction from making
the data gathered yield information in
the laboratory to improve the models
which will be used for predicting the
future changes of this complex climatic
system. As described in the article
opposite, the rallying of the African
community has also been exceptional
and the “Amma community” is close
to reaching 1 000 scientists over the
world. Amma is thus a pioneer project
in many respects, at least concerning a
joint effort to establish instrumentation
of atmosphere, ocean and continent in
order to study the dynamics of a
regional climatic system over a number
of years.
1
he days during the Sop
campaigns are without
end. The detection of
situations of interest for
documentation is founded on permanent weather surveillance conducted
from the Acmad centre in Niamey,
fed by Météo-France as model outputs and satellite images by means
of a dedicated liaison set up during
the Amma programme. When a large
convective system is identified as
having the potential to reach the
zone covered by research aircraft
when the evening briefing is done, a
scientific team comes on alert and
watches with the forecasters
through the night how the situation
develops. Drawing up a flight plan
involves anticipating the trajectory
of the system and taking account of
the time necessary for pre-flight
calibration and temperature setting
of the on-board instruments. The
choice of the type of mission to perform will have been made beforehand at the evening briefing and the
scientific and technical teams (about
100 persons are concerned when the
4 aircraft are flying at the same
time) must sometimes decide at
around 3 or 4 o’clock in the morning
whether or not to continue with
preparation for the mission. If the
decision is in favour of continuing,
the technical teams handling the
instruments go to the airport at
about 4 o’clock, and are joined later
by the scientific teams and the crew,
to get the aircraft ready. Take-off will
from their missions,
the scientists carry
out a rapid check
that the measurements from the
flight have been
properly recorded
and prepare a quick
presentation for the Inside the British aircraft.
evening or the next
kind. It proved to be a centre for "on
morning. The evening briefing is,
the job training with no equivalent
moreover, the occasion for all the
for the forecasters from the various
scientists present (more than
weather centres in the countries of
600 people from a wide range of scithe subregion. The comparison and
entific and technical teams came to
synergy between the most modern
take part in the different Sop operameteorological products and the
tions) to get involved in following up
meteorological knowledge accumuthe operations under way and the
lated by the African forecasters
analysis from missions accomturned out to be highly enriching not
plished. The multi-site operations
only for the latter, but also for the
centre (Niamey, fed from the relays
Météo-France forecasters seconded
installed in Benin, Burkina-Faso,
to Niamey in the context of the operMali and Senegal) set up to coordiation and, generally, for all the
nate the Amma Sop is the first of its
researchers present in Niamey. ●
Contacts
thierry.lebel@hmg.inpg.fr
cheikh.kane@ird.fr
arona.diedhiou@hmg.inpg.fr
Laboratoire d’études des transferts
en hydrologie et environnement, UMR
LTHE
Frédérique Saïd, Laboratoire d'Aérologie, Toulouse
saif@aero.obs-mip.fr
The evening weather briefing, the
last stage before final decision on
airborne missions for the night and
day that follow.
2006: The monsoon ran late
Contact
thierry.lebel@hmg.inpg.fr
© IRD/T. Lebel
© Météo-France
UMR LTHE
The American MIT radar on the Niamey site plays
an identical role to the Ronsard radar installation
at the River Ouémé basin site.
be between dawn and late morning,
depending on how the weather
develops. At the 8 o’clock briefing
held at the Niger meteorological
office, the decisions made are
recorded (flight under way, being
prepared or cancelled), the weather
situation is updated on the basis of
the report compiled by the forecasters from Acmad and a preselection
is conducted for the next day and the
day after. Just before or just after
this briefing, contacts are established with the ground teams positioned in the subregion, including on
Atalante in the Gulf of Guinea, in
order to warn them of ongoing flights
(whether flying over their site is
planned or not) or of results from the
previous day’s flight mission. In
exchange, these teams communicate
the status of their instruments, an
important element for decisions on
forthcoming missions. Once back
© IRD/T. Lebel
2
A day in the special
observing period
The Amma observation strategy is
based on two main features: first, there
is strong interannual variation in the
West African monsoon. The event is
still largely unpredictable, therefore
records must be compiled over several
contrasting years if patterns are to be
better understood. This prompted the
creation of a regional observation system to study the fundamental variables
for the period from 2002 to 2010, several components of which have already
been reported in Sciences au (n°18,
n°32, n°33, n°34 and n°35). Secondly,
the climatic system functions at several
scales and is driven by complex interactions between local, regional and
global processes. Hence it was necessary to extend and tighten the mesh of
the system to study a complete annual
monsoon cycle in all its detail. This was
indeed done in 2006. The first measure
was an increase in the frequency of
measurements taken by instruments
already in place (for example certain
stations have progressed from 1 radiosounding to 8 soundings per day) supplemented by the deployment of new
ground-based instruments such as
radars and lidars on the land and
research vessels on the ocean. The second involved reinforcement of in situ
atmospheric observation by implementation of a programme, unique of its
type, of balloons and five European
research aircraft based at Niamey and
Ouagadougou, backed up by a Nasa
DC8 from Cape Verde. These aircraft
criss-crossed the West African sky from
January to September 2006, on the
track of aerosol clouds and the large
convective systems to study how they
generate and change relative to the
observations performed on the
ground.
This unique deployment strategy, coordinated by French survey teams, supplied a harvest of data on which hundreds of scientists from several tens of
laboratories over the world will work
over the coming years. These data are
not sufficient in themselves because
such a system, impressive as it might
be, cannot sample continuously all the
variables at work in the monsoon
regime. For this reason the strategy of
in situ observations was devised taking
account of satellites data contributions,
with several recent missions such as
Aqua Train and MSG –or others being
prepared such as Smos and MeghaTropiques – and possibilities offered by
modelling.
●
© IRD/T. Lebel
African monsoon
An exceptional
scientific
instrument
array
© IRD/T. Lebel
Each of the research aircraft is fitted out,
on the outside with sensors, on the inside
with acquisition and analysis systems.
Real-time data display enables
the mission leader to adapt the flight plan
to atmospheric conditions encountered.
beginning of June and a subsequent renewed build-up. The
convection weakened once
more between June 25 and
July 10. This reduction in
activity is typical of the transition phase associated with the
establishment of the summer
monsoon. This monsoon, centred on July 3, occurred 10
days late compared with the
date of June 24 (only 10% of
cases are later than July 3).
The convective activity then
developed at the regional scale
over the Sahel from about July
Monitoring of the monsoon’s progress by meteorological teams from the ACMAD. 10 and kept itself at a level a
little higher than the average
he build-up of the
pheric circulation above West Africa
during the summer.
African summer mon(the convection movement, a vertical
However, analysis of the circulation
soon, in response to the
movement of the heated air in conof the low layers of the atmosphere,
warming
continental
tact with the ground, induces a
characteristic of the onset of a sumsurface and the temperature condepression which triggers a current
mer monsoon, shows an earlier reintrast with the ocean, is characterof humid air coming from the Gulf of
forcement, around June 25. Thus
ized by a rapid shift in latitude of the
Guinea); the maximum activity of the
although some of the dynamic and
convection zone from 5°N to 10°N in
African convective systems over the
thermodynamic conditions favouthe space of ten or so days between
Sahel occurs in the summer.
rable for development of summer
June and July.
The situation observed in 2006 duconvective systems were present at
Over the past 40 years, the date of
ring the Amma programme operathe usual date, it was not until midthis transition phase has been centions, on the coasts of Guinea where
July that the continental monsoon
tred on June 24 with a standard
the event arises, showed a first rainy
regime set in. This delay had impordeviation of 8 days. This installation
season which began in mid-April.
tant repercussions for the hydrology
of the system is accompanied by an
Strong convective activity developed
(the River Niger flood water level
overall reinforcement of the atmosin May, followed by a fall at the
came one month late which was,
Sciences au Sud - Le journal de l’IRD - n° 42 - november/december 2007
however, made up for from midAugust) and on soil water content
and vegetation cover (similar development with an initial deficit made
up quite rapidly as the season went
on). Two preliminary trails are currently proposed to explain this delay
in the summer monsoon onset: the
local air-sea interactions with the
Gulf of Guinea upwelling occurring
about a fortnight late in 2006 compared with 2005, and a large-scale
forcing effect in the Tropics driven by
strong reinforcement of convective
activity in the Indian and Asian monsoon. These two mechanisms are not
mutually exclusive which illustrates
well the multiscale research problems facing Amma, the African
monsoon being driven by and exerting feedback on the Earth’s atmospheric circulation, whereas certain
regional factors –continental or
oceanic– also have their role. The
simultaneous measurements of the
characteristics of the atmosphere,
the ocean, and the continental surfaces performed by the Amma campaigns will bring improved graded
understanding of the causes of the
late onset, or premature stopping
(as in 2000 or 2003) of the monsoon,
with the hope of one day being able
to predict these “accidents”.
●
Balloon campaigns
These balloons are released to drift
with
the
stratospheric
winds
(20 000 m altitude) as far as offshore
zones near the South American
coasts. They regularly cast off dropsondes (airborne vertical atmospheric profiling systems) which measure the thermodynamic profile of the
atmosphere. Nearly 160 dropsondes
were released by the driftsonde balloons, supplying useful measurements about the East African waves,
the storm systems and tropical
cyclones. They gave a means for
investigation of disturbances in Africa
that gave rise to the cyclones
Florence, Gordon and Helen, and then
for tracking these as they moved
around over the Atlantic.
Finally, the open stratospheric balloons (OSB) were deployed in Niamey
(Niger). These balloon flights of the
CNRS Aeronomics Department, conducted as part of a campaign jointly
involving European projects Scout-O3
and Amma, had the objective of studying the vertical transport from the
troposphere towards the stratosphere associated with the particularly intense convective systems
(storms) that are at play in the region
in summer. The campaign saw 7 OSB
flights between July 31 and August
25, balloons carrying on board
between 100 and 150 kg worth of
instruments contributed in a multinational effort –French, British,
Italian, Norwegian and Americanlaunched as close as possible to a
Preparation for the ozone
sounding launch (Cotonou site).
3
© IRD/ J.Derrider
period when few airborne devices
were deployed, the CVBs showed the
role they can play in observing the
beginning of the rainy season and
especially the dynamics and thermodynamics of the lower troposphere
associated with the monsoon flux.
In the context of a joint FrancoAmerican engagement, scientists
from the Cnes, the Centre
national de la recherche scientifique (CNRS) and the National
Center
for
Atmospheric Research (NCAR/
launched
USA)
8 “driftsonde
balloons-” from
Zinder in Niger
between 15 August and 15 September 2006.
© AMMA / P.Taburet, Météo-France
onstant volume balloons
(CVBs) developed by the
Cnes and the Laboratoire
de météorologie dynamique (LMD) are small pressure balloons 2.5 m in diameter flying at a
constant height of between 1 000
and 1 500 m. They are equipped with
weather sensors and a GPS,
and send back their measurements by way of the
Argos tracking system.
Fifteen CVBs were launched
from
Cotonou
(Benin); between
mid-June and midJuly: the longest
flight
covered
over 7 000 km in
15 days above
the land. At a
African monsoon
For the first time in an international experiment, the three “balloon” sections
of the Centre National d’Études Spatiales (CNES) were set in operation
simultaneously. Investigation was thus made possible of different atmospheric
levels inaccessible for scientific survey aircraft, too close to the ground
in high-relief zones or located at too high an altitude.
Launching of a stratospheric balloon (Niamey site). As the balloon starts
to lift, the measuring chain can still be distinguished dragging behind on
the ground sol. Once in flight, this can hang up to 100m under the balloon.
storm system. After several hours of
flight, separation and descent by
parachute, the nacelles, or gondolas,
were retrieved in Burkina-Faso. On
this occasion, 29 ozone soundings
were also performed by the Danish
Meteorological Institute through six
of their teams for the first time, a
Russian stratospheric hygrometer
and an American apparatus for measuring ice particles. The overall result
is an unambiguous demonstration, a
first at international level, of the existence of a violent injection of lowlayer air, desert dust, ice crystals and
water up to 19 km in the stratosphere
by the storms associated with the
African monsoon.
●
Contact
Arona Diedhiou, UMR LTHE
arona.diedhiou@inpg.fr
Constant volume balloons
Claude Basdevant
basdevant@lmd.ens.fr
Driftsondes
Philippe Drobinski
philippe.drobinski@aero.jussieu.fr
and Jean-Luc Redelsperger
redels@meteo.fr
Scout-Amma
Anne Garnier
Anne.Garnier@aerov.jussieu.fr
Jean-Pierre Pommereau
Pommereau@aerov.jussieu.fr
Radiosounding station
at Agadez, Niger, 17°N.
Serge Janicot, Laboratoire d'océanographie et
du climat : expérimentation et approches numériques (Locean)
serge.janicot@locean.fr
© IRD/T. Lebel
Contact
he Amma programme undertook the updating
of the radiosounding network managed in the
region by Asecna (Agence pour la sécurité de
la navigation aérienne en Afrique et à
Madagascar), putting special focus on critical zones for
which data are rare. This action includes the deployment
of modernized telecommunications systems and automation of collection and exchange of data. Radiosoundings
remain the only in-situ atmospheric profile observations
performed according to a homogeneous protocol, once or
twice per day, over the whole of the globe. They are the
raw material essential for feeding into meteorological
models.
This operation has been successful on several levels.
• From mid-2005, a network of 17 stations has been
conducting 1 or 2 soundings per day. In a special operating period, the rhythm reached 8 soundings per day,
making West Africa the world’s most observed region over
this period.
• The African meteorological offices and Asecna now have
at their disposal a modern network which can continue to
function long after the time of the Amma programme and
thus provide data for the World Weather Watch system.
In previous years, just 45% of the data collected in the
subregion were received in the weather centres around the
world. The Amma programme, however, achieved a success rate of about 95% in West Africa, in spite of a rise in
the frequency of measurements taken: 4 times per day on
average and, in certain observing periods, up to 8 soundings per day over 6 stations, (Cotonou and Parakou
[Benin], Abuja [Nigeria], Tamale [Ghana], Niamey and
Agadez [Niger]). This work rallied the human resources of
most of the national meteorological offices of the countries
concerned and represented a real success for Asecna.
Preliminary studies conducted at Météo-France (CNRM)
have already shown the positive impact of these supplementary radiosounding data in the improvement of weather forecasting in this region when it is integrated into the
models. It is therefore important to continue this investiga-
tion in order to determine the optimal network for observing and understanding the variability of the climate in this
region. The challenge is to set in place a strategy for harnessing resources, accepted and supported by the African
institutions with the aim of making this radiosounding network permanent.
●
Contact
Serge Janicot, UMR Locean
Serge.Janicot@locean-ipsl.upmc.fr
Jean-Luc Redelsperger, CNRM
redels@meteo.fr
© IRD/T. Lebel
© IRD/T. Lebel
The radiosounding network updated
No rest at night for the radar operating teams :
powerful convective systems often lurk at night.
Sciences au Sud - Le journal de l’IRD - n° 42 - november/december 2007
The area around Mount Hombori,
Mali’s highest point,
in July (left)
and August (right).
The variability of the water cycle
and the associated mechanisms are the key
elements for understanding the African monsoon.
Sciences.au.sud@paris.ird.fr
IRD - 213, rue La Fayette F - 75480 Paris cedex 10
http://www.ird.fr
Directeur de la publication
Michel Laurent
Directrice de la rédaction
Marie-Noëlle Favier
Rédacteur en chef
Olivier Dargouge (Olivier.dargouge@ird.fr)
Dossier coordonné par
Aude Sonneville
© IRD/ E. Mougin
Contact
Christophe Peugeot, Laboratoire
HydroSciences Montpellier (HSM)
peugeot@ird.fr
n Africa, climatic fluctuations
are
expressed
essentially as variations
in rainfall and hence in
the intensity of the hydrological cycle
on that continent. The link between
the dynamics of the monsoon and the
continental water cycle comes into
play in two places. The first is in the
free atmosphere, whose thickness
can reach 15 km. The deep convection movement which develops there
(thermals generated by the energy
stored up in this region of the globe
during the boreal summer) is the
source of this rainfall. The second
place is at the Earth’s outer atmospheric layer whose thickness varies
from a few hundred meters to 2 or
3 km depending on the season and the
time of day. Within this boundary
layer, the surface layer, some tens of
meters thick, is the site of exchanges
of gas (water vapour, CO2, nitrogen)
and energy between the continent
surface and the atmosphere, largely
controlled by biological activity and
plant cover. Knowledge of these fluxes
is fundamental for understanding and
quantifying the feedback reactions
from the continental land surface that
influence the atmospheric processes,
notably convection and therefore
rainfall generation, but also the
dynamics of the vegetation and its
interactions with the hydrological
cycle.
In line with this objective, a largescale measurement system was
deployed on the different types of
cover represented within each of the
three mesoscale sites of the Amma
programme, thus constituting a
homogeneous sampling arrangement
along the latitudinal eco-climatic gradient of West Africa. In particular, it
is a means of direct measurement of
Sciences au Sud - Le journal de l’IRD - n° 42 - november/december 2007
the fluxes of water vapour (evapotranspiration), detectable heat (warming of the air by the ground) and
CO2 in the surface layer. The diagram
shows for example the swing of the
ratio of the fluxes of latent heat
(evapotranspiration) and detectable
heat on a piece of fallow land on the
arrival of the 2006 monsoon in Niger.
This illustrates how the energy
received is used primarily in re-evaporation of the rainfall as soon as the
monsoon has become installed.
Monitoring of the rainfall, of the components of the radiative balance, the
heat flux and the humidity profile in
the soil, and the parameters of the
vegetation cycle, complete the investigation strategy. There are few such
comprehensive observation systems
of this type in the world, especially in
Africa, that are spatially and temporally representative and embrace
energy, water, vegetation
and carbon related parameters. The resulting
data are crucial for the
parametrization and validation of models at different scales, from the local
to regional, whether they
be geared towards climate, hydrology or ecology. The observation
array of Amma’s combined ecological-hydrological-weather stations
can also contribute to the
continental or even planetary scale of observation
cover which are the bases
for global climatic or environmental
modelling.
Furthermore, it will bring
the host countries (Benin,
Niger, Mali) information
© N. Boulain
Luc Descroix, Laboratoire d’études des
transferts en hydrologie et environnement, UMR LTHE
descroix@ird.fr
Le journal de l'IRD
Installation of the bioclimatic station located 10 km North of Bamba
(17,03°-North 1°24-West) on the left bank of the River Niger. Burial of soil
humidity and temperature sensors arranged to trace a vertical profile
from 5 to 250 cm.
season’s monsoon more active, but
the overall mechanisms involved still
have to be elucidated.
The experimental set-up was
designed such that the key processes
participating in these interactions
could be documented at the regional
(>105km2) local (<100km2) and
meso scales (104km2), a strategy
which offers the possibility of coupling studies in the atmosphere and
on land as the working resolutions of
the models become compatible.
The international working group
which coordinates the water cycle
studies conducted a preliminary simulation exercise for rainfall that
occurred from 28-29 August 2005 at
the three experiment sites. The comparison of the results from atmospheric and hydrological models used
to simulate this event and its impact
was presented at the Second
International AMM Conference held in
Germany in November 2007.
●
Not a drop unnoticed
Contact
Wells in Mali.
edge has been acquired of the essential mechanisms of the water cycle–in
the ocean, the atmosphere and on the
surface– and the associated simulation models are powerful tools.
Nevertheless the existing information
needs to be refined and considerable
progress is necessary concerning the
interfaces of these three domains,
and in particular for the interface
continent-atmosphere, which plays a
key role but which is poorly known
with regard to monsoon processes. In
the less dry zone South of the Sahel a
high level of water storage in the soil
and vegetation at the end of one monsoon appears to make the following
© IRD/F. Timouk
he monsoon takes its
force and character from
the intensity of influxes of
humid oceanic air onto
the continent, its transformation into
precipitation and the abundance of
rainfall on the ground. With a certain
lapse of time, the continental land
surface attenuates this pluviometric
signal by returning a proportion of the
water to the atmosphere (through
evaporation from soils and transpiration by vegetation) or to the ocean
(runoff into the water courses then
into rivers that flow down to the sea),
and by storing part of it in underground reservoirs. The underground
water bodies, rivers and lakes and the
ocean are reservoirs essential for
human activities. It is important to
understand their dynamics and their
vulnerability. Water cycle and energy
balance are furthermore closely
linked. Study of the water cycle is
therefore a core line of research in
the Amma programme. Good knowl-
Measurement of turbulent and
radiative fluxes on a piece of
fallow land, Niger.
valuable for devising transnational
greenhouse gas emission control
policies.
●
Contacts
bernard.cappelaere@mpl.ird.fr
Laboratoire HydroSciences Montpellier (HSM)
sylvie.galle@hmg.inpg.fr
UMR LTHE
franck.timouk@ird.fr
Centre d’études spatiales de la
biosphère (UMR Cesbio)
© IRD/S. Galle
4
The water cycle in a state of upset
In West Africa an overall shrinkage in
the volume of available water has been
progressing since the beginning of the
drought period (1968). This is true
both at surface and underground levels. The Rivers Niger and Senegal have
suffered a fall in flow rate (of 40 and
60% respectively), Lake Chad has
nearly disappeared and most of the
groundwater tables are rapidly receding. Many ponds and wetlands are
also threatened. This comes at a time
when the population growth rate continues on a steep rise (an average of
3% per year, 3.5% in Niger).
Nevertheless, in the Sahel, runoff has
been increasing markedly. This stems
from land use changes; and from wood
and scrub clearance for crop-growing
which exposes the bare soil and causes
crust formation, in turn facilitating
greater runoff and erosion. The process
has caused an increase in flow-rate of
some right-bank tributaries of the River
Niger by 10 to 35% over the past two
decades. Moreover, in some areas of
the Sahel with endorheic drainage systems, the number of ponds is growing,
and the extent and life-span of existing
ones are increasing. The result is a new
rise of the groundwater table (more
than 20 cm per year over the past
10 years). In the sedimentary geological zones. There is great potential in
groundwater, more reliable than the
River Niger (where considerable precautions must be taken with dam construction schemes to limit the negative
impacts for health, the hydrological
system and agriculture). Small delimited areas can be supplied in all villages by digging wells without recourse
to water feed from outside.
It is in the zones lying on the basement
rock where water resources are rare,
difficult to access and falling sharply
owing to diminishing rainfall. Future
supplies will probably have to be
ensured by retention dams doted with
a few weeks’ storage capacity of the
water input from the scarce bouts of
rainfall that do occur.
●
© IRD/M.-N. Favier
African monsoon
Water,
an under-used yet
threatened resource
Change in detectable (red) and
latent (blue) heat flux on a piece of
fallow land, on arrival of the 2006
monsoon in Niger (rainfall in
black). Background: rainfall
intercepted by herbaceous
vegetation.