Ann. Geophysicae 12. 53-63 (1994)
@
EGS
-
Springer-Verlag 1994
HAPEX-Sahel: a large-scale study of land-atmosphere interactions
in the semi-arid tropics
J-P.
outo or be',
T.
~ebel',
A.
~inga", P. Bessemoulin', J. Brouwer",
A.J.
~olman"~~,
E.T.
KnRman7, J.H.C. ~a~h",
M. ~oepffner," P. ~abat~,
Y.H.
KerrY, B. Monteny2,
S.
Princelo,
F.
Said", P. Sellers',
J.S.
Wallace"
I
CNRM. 42 Av.
G.
Coriolis. 31057. Toulouse, France
'
ORSTOM.
BP
11416, Niamey. Niger
Clniversiti. de Niamey. Niamey. Niger
ICRISAT.
BP
12404. Niamey. Niger and Dept. Soil Science
Clc
Geology.
P.O.
Box 37. 67oU
AA.
Wagen~~~gen. Thc Netherlands
"nstitute of Hydrology. Wallinpford, OX10
XBU.
UK
Winand Staring Centre (SC-D1.O). P.O.
Box
125.
6700
AC'.
Wugenitlgen. Thc Ncthcrlnnds
'
NASA GSPC Greenbelt. MD 20771. USA
'
ORSTOM
BP
5045.
F-34031 Montpcllier. France
'
LEKTS. 18 Av.
E.
Belin.
3
1055 Toulouse. Fraiicc
"'
University of Maryland. College
Park.
MU 20742-8225,
LISA
'I
UPS 1.aboratoire d'Aerologie, 3 1069 Toulouse. France
Recc~ved
4
June 1993
/
Rev~wd:
4
October
1995
!
Accepted: 11 October 1993
1
Introduction
Abstract. The Hydrologic Atmospheric Pilot
EXperi-
ment in the Sahel (HAPEX-Sahel) was carried out in
Niger, West Africa, during
1991-1992, with an inten-
sive observation period (IOP) in August-October 1992.
It aims at improving the parameterization of land surface
atmosphere interactions at the Global Circulation Model
(GCM)
gridbox scale. The experiment combines remote
sensing and ground based measurements with hydrologi-
cal and meteorological modelling to develop aggregation
techniques for use in large scale estimates of the hydro-
logical and meteorological behaviour of large areas in
the Sahel.
The experimental strategy consisted of a period of
intensive measurements during the transition period of
the rainy to the
dry
season, backed up by a series of long
term measurements in a
lo
by
lo
square in Niger. Three
"supersites" were instrumented with a variety of hydro-
logical and
(micro) meteorological equipment
to
provide
detailed information on the
surfacq energy exchange at
the
local
scale. Boundary layer measurements and
air-
craft measurements were
used
to provide information at
scales
of .100-500
km2.
All
relevant remote sensing im-
ages were obtained for
this
period.
This
ptogramme of
measurements
is
now being analyzed
and
an
extensive
modelling
programme
is
under way to
aggregate
the
in-
formation at
all
scales
up
to
the
GCM
grid
box
scale.
The experimental
strategy
and
some
pre1imiaar)r
results
of
the
IOP
are
described.
Correspondence
to:
A.J.
Dolman
GCMs used for climate change prediction are based on grids
several hundred kilometres square. At each point on the
grid the land surface energy. water and momentum balance
needs to be calculated at each time step
in
the model. This
must be achieved with what amounts to a single sub-model
which must represent the complete grid square, Increased
understanding
of
land-surface atmosphere interaction has
led to these sub-models continually becoming more realis-
tic. This creates a need for data and for measurement and
modelling techniques which can be used to quantify the
varied landscape likely to be contained
withir~ areas at the
required scale.
A
series
of
large-scale international exper-
iments have now been executed, (see Shuttleworth, 1991
for a review and discussion of these experiments
in
the
context of various international programmes): for example
HAPEX-MOBILHY in France (Andre
et
a/..
1988). FIFE in
the USA (Sellers
et
ul.,
1992), and
EFEDA
in Spain (Bolle
rt
al.,
1993). These have proved successful 'in developing
the
experimental strategies necessary to provide such data
and the subsequent modelling efforts aimed at integrating
all these data up to the
GCM
grid square scale (Bougeault
et
al.,
1991, Noilhan and LacarrItre, 1993). HAPEX-Sahel is an
experiment designed to fulfil the requirement for modelling
the climate and hydrology of the semi-arid,
sahelian, region
of sub-Saharan Africa.
The Sahel comprises
an
area of some
3
x
lo6
km2
lying
between the wet, humid, equatorial zone of Africa to the
south, and the Sahara
~esek to the north. This results in
a strong north-south rainfall gradient and a climate with a
notoriously unreliable rainfall. The rainfall is now generally
accepted to have been declining for the past two decades
(Nicholson,
1989). and attempts to understand this phe-
nomenon have been one of most active areas of application
Goutorbe
et
a/.:
HAPEX-Sahel
for GCMs (Rowntree, 1988). Previous studies (e.g. Char-
ney, 1975, Laval and Picon, 1986) have shown a sensitivity
to albedo, with a reduction in rainfall resulting from the
increased albedo associated with the removal of
vegetation.
Other studies (e.g., Folland
el
al.;.
1991) have indicated that
external factors such as changes in Atlantic sea-surface tem-
perature may also be responsible. Understanding the reasons
for the decline in rainfall and the need to predict how the
climate may change in the future
-
either in response to
local vegetation change, or in response to external factors
such as global
C02 increase
-
have provided the incentive
for
HAPEX-Sahel.
HAPEX-Sahel consists of 66 separate studies. Clearly
in a single paper it is not possible to describe each of
these studies in detail and no attempt will be made to do
so. This paper describes the design of the experiment, in
terms of-the sampling strategy, the measurements taken
and the modelling philosophy aimed at integrating local
scale measurements up to the GCM grid square scale. A
broad description of the experimental sites is given, together
with some data to indicate the range of climatic conditions
experienced during the intensive observation period
(IOP).
2
Methodology
2.
I
Experimentul strutegy
The HAPEX-Sahel experiment attempts to bridge the gap
between the local and
thc GCM grid box scale. The strategy
developed to achieve this ambitious goal is
bascd on a two-
stream approach, which draws heavily on experiences from
previous large scale field experiments such as
HAPEX- MO-
BILHY and
FIFE.
The two-stream approach is based on thc
integration of remote sensing and ground measurements, and
hydrological and meteorological modelling. It
aims to pro-
vide a full assessment of the potential of these techniques to
produce large scale estimates of relevant land surface char-
acteristics and
sul-fdce energy, water, C02 and momentum
fluxes. The experimental design of
HAPEX-Sahel reflects
this philosophy
(e.g. Goutorbe
el
nl.,
1992).
2.2
The
Suheliun envirorzntent
The climate of co;ltinental west Africa is influenced by thc
interaction of the air masses over large homogeneous areas
of sea and land. The two air
mb,ses meet in a zone which
is often referred to as the intertropical convergence zone
(ITCZ), the ascending branch of the
Hadley cell. The tropical
continental air mass containing warm, dry air and which is
generally hazy, originates from the Sahara desert. It slopes
upwards to the south over the
warm, humid tropical maritime
air originating over the Atlantic which forms a wedge under
the continental air mass. The seasonal migration of the ITCZ
is of fundamental importance in understanding the climate
of West Africa.
The north-south migration of the ITCZ is related to the
seasonal shifts in the relative positioning of the sun. During
June and July, a large
part
of West Africa is under the
influence of moist southwesterly air masses, giving rise to
the rainy season. The maximum northward extent of the
ITCZ is reached in August, when the maximum rainfall
occurs in the Sahel. Depending on the depth of the humid
air mass, thunderstorms with heavy rainfall can occur.
The Sahelian region comprises the semi-arid vegetation
belt south of the Sahara desert and north of the dry forest
zone. The duration of the wet season decreases from about
5
months in the south of the Sahel (12" N) to 3 months in
the north
(18" N). The annual rainfall is closely related to
the duration of the rainy season, varying from
800
mm in
the south to only 200 mm in the north, following a regular
gradient of
1
mm
km-'
(averages from 1950-1989; Lebel
el
nl.,
1992).
The highly variable and unreliable rainfall governs the
growth and distribution of the vegetation. Total rainfall is
everywhere less than potential evaporation, which is of the
order of 2000
mm per year. This results in a highly sparse
vegetation
cover with large areas of bare soil. The interlac-
ing of bare soil and vegetation creates a large variability
of surface conditions at the 0.01-1
km' scale. However,
the somewhat uniform geological and pedological structures
generate a relatively homogeneous regional landscape.
Another major aspect of the Sahelian environment is
the marked degradation of drainage networks, as a result
of highly
intermittent streamflow characteristics. Sahelian
hydrology is therefore
priniarily the hydrology of endoreic
(inward draining) areas, the surface areas of which rarely
exceed a few square kilometres. This results in the formation
of ponds and pools in the beginning of the rainy season,
which dry out 2-3 months after the last rainfall. The areas
which drain externally do not behave as watersheds in the
classical sense either: runoff rapidly dissipates as the
s,lopes
beconie too weak. It is therefore generally impossible to rely
on measured streamflow at the watershed outlet to provide
spatially integrated values of the residual (evaporation)
term
in the water balance. The emphasis must thus be placed on
meteorological techniques to obtain the (spatially) integrated
evaporation.
The experiment is based on a
1"
x
1"
square (2-3' E,
13-14
N).
This area (approximately 100 kn~
x
100 km)
is
corriparable in si~e to a GCM grid square and large
enough to contain a representative sample of the variability
of Sahelian vegetation.
Three super-sites
were selected for intensive monitoring.
Their position, chosen to capture the expected long term
climatological rainfall gradient, is shown in Fig.
1. The scale
of
tlie super-sites reflects the scale at which the atmospheric
boundary layer responds to changes in the land surface
(typically 10-20
krn).
This makes the super-sites sufficiently
large for aircraft and boundary layer measurements, while at
the same time they can be used for testing and developing
remote sensing algorithms.
TWO less heavily instrumented
sites were also established, one northwest of Niamey, at
Danguey Gourou, and one east of Niamey, in the river
valley the Dallol
Bosso. These were to extend the spatial
coverage over the square. In addition to these sites a network
of automated weather stations was established over the full
Goutorbe
el
ul.:
HAPEX-Shel
Agadez
_
--
AFRICA
lamey
NIGER
BURKINA
FA50
.-
-..-
.,-'
NIGERIA
.
t
BE
Nl
N,',
8
-,
,
Fig.
1.
Diagram showing the position
of'
the experimental square ar~d the three super-sites
1"
by
1"
square giving information on the regional-scale
variability of the main climatic variables.
At the super-site scale energy fluxes will bc estimated
using the data provided by eddy correlation instruments
mounted on low-flying aircraft and through the study of
the boundary layer development. On the ground, extensive
micrometeorological measurements of energy and carbon
fluxes, and vegetation, soil and hydrological measurements
were performed at sub-sites. There was one sub-site for each
of the main vegetation types. Within the sub-sites further
studies looked at the variation in the energy balance resulting
from the different components of the vegetation and bare
soil.
The large differences in vegetation, rainfall, soil moisture
conditions and hydraulic properties over the square require
the use of upscaling techniques to obtain an area-integrated
view of the hydrological processes operating in the
lo
by
1"
square. The vegetation within the square will be classified
and mapped using a combination of ground-based, aircraft
and satellite data; primary production has been monitored.
The EPSAT-Niger experiment, which comprises a dense
network of rain gauges in combination with weather radar
data, will be used to obtain the spatial variability of rainfall
input to the square. Stochastic and remote
seniing techniques
will then be used to integrate the local scale information on
rainfall, soil moisture condition and hydraulic properties to
the large scale, where they can be used as inputs to mesoscale
models and
GCMs. The long term input of water to the
aquifer is to be estimated under a monitoring programme
using existing wells and boreholes.
2.4
Temporal sampling strategy
The data collection took place over a period of
2
years,
1991-1993, with an IOP from 15 August to 9 October
1992. The IOP was timed to capture the transition from
the wet to the dry season, in which both soil moisture and
Goutorbe
rl
ul.:
HAPEX-Sahcl
vegetation changes combine with changing meteorological
conditions, to produce a radical
transformation in the surface
energy balance of the-region (Gash
et
al.,
199
1
).
The 2-year
data span allows investigation of hydrological processes
operating on seasonal or longer time scales, such as soil
moisture and runoff, while the'relatively short IOP provides
data for processes operating on smaller time scales such as
the surface fluxes of heat and water vapour. During the IOP
several aircraft were involved in both remote sensing and
flux measurement missions.
2.5
Aggregation
Remote sensing techniques offer the opportunity to extrapo-
late local measurements to the larger scale. Within
HAPEX-
Sahel inverse modelling techniques will be used to obtain
relevant land surface characteristics at pixel scales of, say
40 m2 to 2500
km2. Similar techniques using aircraft and
satellite measurements of, for instance, reflectance patterns,
surface temperature and soil moisture, will
be used to reach
the level of the GCM grid box scale. Essential in this attempt
is
the unique combination of ground truth data, covering a
variety of vegetation, soil types and soil moisture patterns,
with aircraft and satellite imagery, and atmospheric
profilc
data for correcting satellite measurements. Ten years of Me-
teosat B2 data and 4 ycars
of
AVHRR data are available for
this experiment.
However, interactions
betwccn the land surface charac-
teristics and the surface fluxes of
heat, water vapour and
momentum are generally non-linear. Therefore, whilst re-
mote sensing
tcchniqi~es can be used to provide information
on the spatial variability of land surface characteristics such
as albedo, aerodynamic roughness and soil
moisture,
mete-
orological modelling
must also be used to investigate the
interaction between adjacent areas having different surface
characteristics. Mesoscale models have a typical domain size
of a few hundred
kilometres and a resolution of 5-25 km
and have been successfully used in the past to investigate the
effects of different land surface cover on
the area-integrated
surface fluxes
(e.g., Bougeault
er
trl.,
1991). At the smaller
scale, relevant to the "footprint" of the atmospheric bound-
ary
layer, two dimensional boundary layer (Blyth
er
(I/.,
1993) and non-hydrostatic mesoscale models (Adrian and
Fiedler, 1991) can be used to obtain area-integrated surface
fluxes at the level of the super-site. Both these techniques
will be used in
HAPEX-Sahel so that the information which
exists at the local and super-site scale can be integrated up
to the GCM
gridbox scale.
The unique combination of large, high quality
datasets
coupled with remote sensing techniques and modelling at
a variety of spatial scales, both for hydrology and meteo-
rology, should provide fascinating new insights in the large
scale
interaction of the land surface and the atmosphere in
desertification threatened, semi-arid areas such as the
Sa-
hel, and provide new and rigorous tests for aggregation
algorithms.
3
Site description and measurements
In the following a detailed description of the area, vegetation.
the sites and the measurements taken during the campaign
will be given.
3.1
Description
of
the
trretr
A
1"
by
1"
square (2-3"
E,
13-14' N) was chosen as a
representative area of
the Sahel. It is situatcd to the east
of Niamey, the capital city of Niger, see Fig. 1. Figure 2
presents a composite image of the square, obtained from sev-
eral
Landsat overpasses, showing the variation in vegetation
cover, together with the course of the river Niger. The
lc
by
I"
square was chosen for a number of reasons, amongst
which the presence of long-term hydrological observations
by
ORSTOM (Lebel, 1990), the existence of
a
dense net-
work of rain gages (the EPSAT-Niger project, Lebel
et
nl.,
1992) and previous micrometeorological studies (e.g., Gash
et
trl.,
1991, Durand
et
al.,
1988, Frangi
et
al.,
1992) were
the
most important. The physical geography of the area
consists of the remains of the continental terminal, dissected
into ferruginous plateaux separated by sand-filled valleys.
The plateaux become less distinct towards the north. To the
east, the square is bounded by an ancient river valley, the
Dallol Bosso. The water table in the Dallol Bosso is nearly
at the surface during
the wet scason, and
4-5
rn
below the
surfacc in
the dry season, giving the area its distinct, wet
and lush character.
The vegetation within the
HAPEX-Sahel square is typ-
ical of that occurring in the southern Sahelian zone. There
arc three
main vegetation types, arable crops (almost entirely
millet), fallow savannah
and sparse dryland forest known lo-
cally as tiger bush.
The areas of millet
(Pmnisetuin
glaucum
(L.)
R.Br.) are planted at the beginning of the rainy scason.
The fallow savannah contains diverse mixtures of naturally
occurring perennial woody shrubs and herbaceous annual
plants. Throughout the square the dominant woody shrub
in the fallow savannah is
Guiertr sarregalensis
(L.).
The sa-
vannah
areas will have been previously used for growing
crops, but
form part of a rotation system with
a
cycle which
varies from
a
few years to up to
15
or 20 years. In re-
cent times the shorter rotations have
become more common.
Tiger bush only occurs on the
laterite plateaux. The tiger
bush vegetation is dominated by comparatively large woody
perennials
(e.g.,
Comhreturn rnicrrrnthurn
G.
Don) and trees
(e.g.,
Cornbretuiir ni~ricuns
Lepr, ex Guill. and Perrott.).
The vegetation grows in dense strips which are separated by
areas of completely bare soil. The proportion of vegetation
cover within the tiger bush areas varies according to the
rainfall such that the densest cover is at
the southern most
part of the square and this decreases towards the north. The
areas of tiger bush can be clearly seen in Fig. 2. Although
the occurrence of
laterite plateaux does not vary much with
the north-south gradient, the
amount of vegetation on them
does,
Goutorbe
er
dl.:
HAPEX-Sahcl
Fig.2.
Satellite image
(LANDSAT)
of
the
experimental
area (image pn)viilcd
hy
tlic lnstitulc
of
Hytlrology. Wallingli~rtl,
UK)
The monitoring of the entire experimental zone is designed
to characterize
thc e~lvironment and its evolution and to
study the large scale processes.
The first results of EPSAT-Niger
(Taupin
et
(11..
1993)
have shown that the spatial variability of rainfall is very large
at all time scales. They concluded that a density of roughly
one gauge per 100
km2 was needed to estimate rainfall
inputs over areas in excess of 500
km2. Areas smaller than
500
km2 required an increasing number of gauges with
decreasing area. To capture the spatial variation in rainfall,
a network of 107 recording gauges was installed on a
12 km
square basic mesh, with additional gauges at the super-sites
to capture the local variability. This network was operated
in conjunction with a C-band weather radar system located
at Niamey airport. The weather radar provided
50
m
radial
resolution at 1.5" angular resolution.
Assessment of upper aquifer recharge requires a direct
measurement of aquifer levels
-
because the aquifer in the
area is not continuous and the recharge is a point process
both in time (individual rainfall events) and space. Two
hundred access points to the upper aquifer to the east of
the river were monitored every
3
months and recording
piezometers provided a continudus survey in time at six
locations. On the western side of the river, 50 wells have
been monitored monthly since 1991.
Synoptic conditions
were monitored during the
10P
with
a
network of 12 autonlatic weather stations, evenly spread
over the
I
by
I'
square.
The last component of the large scale monitoring refers
to the role of the vegetation in controlling the water bal-
ance of the area. It aimed at mapping the surface conditions
and land units, classifying
the fallow land according to
various types of vegetation cover and monitoring the
phe-
nology of the main types in the study area. The sites of
the EPSAT-Niger network were used as sampling points
for both the surface conditions and vegetation
mapping and
phenology monitoring. The fieldwork, which began in 1990
and consisted of a local characterisation
oj the vegetation
and radiometric measurements on the ground, was used for
calibration of SPOT imagery.
It
should
be
stressed that, with
the exception of the synoptic network, all the large scale
monitoring started in 199011991 and continued until 1993.
This provides essential long term background data for the
experiment.
3.3
The
super-sites
The location of the southern super-site is shown in Fig.
1.
The site was on the right bank of the river Niger about
45
km
south of Niamey, close to the
ICRISAT Sahelian Centre at
Sadore. There were three contrasting sub-sites: a millet site,
a fallow site and a tiger bush site. The millet site consisted
