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HAPEX-Sahel: the Hydrology-Atmosphere Pilot Experiment in the Sahel, 1990-1992.

(The following text is an extract of : Geographical, biolo gical and remote sensing aspects of the Hydrologic Atmospheric Pilot Experiment in the Sahel (HAPEX SAHEL) by S.D. Prince, Y.H. Kerr, J.-P. Goutorbe, T. Lebel, A. Tinga, J. Brouwer, A.J. Dolman, E.T. Engman, J.H.C. Gash, M. Hoepffner, P. Kabat, B. Monteny, F. Said, P. Sellers, J. Wallace, to be published in RSE)

HAPEX-Sahel was funded by a wide range of agencies in seven participating countries. Over 200 scientists visited and worked in the field site. An interdisciplinary approach has been adopted with contributed studies in hydrology and soil moisture, surface fluxes and vegetation, remote sensing science, and meteorology and mesoscale modelling. Detailed field measurements were concentrated in 3 "super sites" and two ancillary sites. Conditions in 1992 turned out to be average for the last decade with good gradients of precipitation and a variety of vegetation productivities in the study sites. An information system (HSIS) has been established to provide a data base to diseminate the measurements. An active program of meetings, workshops and interdisciplinary studies is now in progress.

General Overview

HAPEX-Sah el (Hydrological and Atmospheric Pilot Experiment in the Sahel) is an international land-surface-atmosphere observation program that was undertaken in western Niger, in the west African Sahel region. The overall aims were to improve our understanding of the role of the Sahel on the general circulation, in particular the effects of the large interannual fluctuations of land surface conditions in this region and, in turn, to develop ideas about how the general circulation is related to the persistent droughts that have affected the Sahel during the last 25 years. The field program obtained measurements of atmospheric, surface and certain sub-surface processes in a 1deg x1deg area that incorporates examples of many of the major land surface types found throughout the Sahel. An important consideration was that the data must to be applicable to the scales of current general circulation models (GCM).

In order to obtain data for this large area, an extensive measurement program was undertaken including field, aircraft, and satellite remote sensing measurements, mainly between mid1990 and late1992. An intensive operations period was undertaken for 8 weeks from mid to late growing season of 1992. HAPEX- Sahel was initiated in response to the World Climate Research Programme (WCRP) Global Energy and Water Cycles Experiment (GEWEX) which aims to determine the fluxes of water and energy globally in order to quantify the energetic processes of the Earth's climate system and the forcing functions on the ocean, land, ice and vegetation. GEWEX recognizes the need for more realistic measurements of major global environments, especially critical areas that are thought to have strong influences on the general circulation.

There is particular interest in areas that are thought to be undergoing large, rapid changes at the present time or that have been identified as vulnerable to future changes under various global change scenarios. The Sahel forms the border of the Sahara desert which is a major source of sensible heat for the atmosphere. Any extension or reduction of desert-like conditions due to fluctuations in the rainfall and vegetation of the Sahel are therefore likely to have strong feedbacks on the general circulation through the energy balance and other processes. HAPEX- Sahel is also relevant to the International Geosphere-Biosphere Programme (IGBP) which is an evolving international program that aims to understand the interactive physical, chemical and biological processes that regulate the global environment and the changes that are taking place in the biosphere. Several IGBP core projects are expected to benefit from HAPEX- Sahel, including the International Global Atmospheric Chemsitry Project (IGAC), Biospheric Aspects of the Hydrological Cycle (BAHC), Global Change and Terrestrial Ecosystems (GCTE), and Global Analysis, Interpretation and Modelling (GAIM). In addition the French IGBP program, Savannas on the Long Term (SALT), has designated the HAPEX- Sahel site as one of its study areas.

The approach adopted in HAPEX-Sahel was pioneered by HAPEX Mobihly in SW France (André et al. 1988), by the First ISLSCP Field Experiment (FIFE) in tallgrass prairie in Kansas, USA (Sellers et al. 1988), and by the European Field Experiment in Desertification threatened Areas (EFEDA) in Spain (Bolle et al. 1993). As in these earlier campaigns, the aim of HAPEX-Sahel was to make simultaneous measurements of relevant variables at the micro, and meso scales, but with the noteable addition in HAPEX of a much greater effort than before at the macro scale. In the case of fluxes of water vapor, for instance, measurements were made at the leaf, canopy, stand, landscape and 1degx1deg scales, using porometers, eddy correlation fluxes from towers and aircraft, radiosondes and teathered baloons. Similar designs were adopted for all aspects of the energy, water, and carbon balances. Aircraft and satellite remote sensing was undertaken to extend the measurements to the entire region. HAPEX-Sahel faced several particular challenges. The heterogeneity of surface types and seasonal variation in the region are much greater than in the areas studied in any previous measurement campaign of this type.


The Sahel and the Study area

The Sahelian environment

The Sahel occupies a narrow zone between the Sahara to the north and the Sudanian vegetation zone to the south (White 1983), forming a strip about 400-600 km from north to south, 3x106 km2 in area, that stretches nearly 6,000 km across the entire African continent (Le Hourou 1989).

The region is characterized by a single, short, annual rainy season associated with the northward movement of the Inter Tropical Convergence Zone (ITCZ). The ITCZ is the junction of the dry, mid-continental air mass and humid, maritime air from the Atlantic that forms the ascending branch of the Hadley cell. Dry, dust-laden, cooler air from the northeast is undercut by a wedge of warm, humid air from the Gulf of Guinea. Disturbances develop between the two air masses and give rise to storms. Some storms are caused by organized, easterly waves that propagate from east to west between the Tropical Easterly and the African Easterly Jets (Farmer 1989). In addition local convective storms are common. The locations of storms are unpredictable and they typically cover only 1-10% of the region at any one time. Thus rainfall is highly variable in time and space, as in other semi-arid regions of the world. The ITCZ is the "thermal equator" and it migrates north and south with the apparent movement of the sun so that rainfall decreases from south to north in the Sahel (Lebel et al. 1992), in fact the boundaries of the Sahel are generally defined with reference to the rainfall gradient; the 100 mm isohyet forms the northern boundary and the 600 mm isohyet the southern boundary in many definitions (Le Hourou 1989).

The landscape of the Sahel is generally flat or gently undulating at altitudes below 600m. Large areas, particularly in the north but also in parts of the south Sahel, are covered with Pleistocene, aeolean sand sheets. Fossil dunes formed in the sand sheets create an undulating relief with 2-3 km between dune crests. The sand is vegetated and active dunes only occur locally in response to disturbance. Clay plains and alluvial flood plains, fossil valleys (wadis and dalols), pediplains, and plateaux are occasional geomorphological features. The few mountainous regions are found at the northern border of the Sahel, the Adrar of Mauritania, Adrar of Iforas, Air, Tibesti, and Ennedi; exceptions are Jebel Gourgeil and Jebel Marra in western Sudan which occur in the Sahel proper. Much of the non-sand sheet area is coated with a sand veil of varying thickness from a few centimeters to meters (Le Hourou 1989).

The current drainage systems in the Sahel were mostly developed during the wetter Pleistocene periods and are now not active. Virtually all drainage is endoreic and surface flow rarely occurs over more than a few hundred meters. Ephemeral pools form during the rainy season and gradually evaporate between rain events. Stream-flow methods are thus not applicable at a catchement scale in much of the region. The major rivers of the Sahel, the Niger, Senegal and Nile, mainly carry water that comes from outside the region and not from Sahelian rainfall.

The vegetation of the Sahel consists of annual grass and scattered bush steppe in the north, gradually merging into Sudanian savannas with perennial grasses, scattered trees, and extensive rain-fed cultivation in the south (White 1983). The vegetation is strongly seasonal and virtually all woody species are deciduous, all herbs are either annual or die back to the ground each dry season. In the north herbs are green for approximately one month, increasing to three months in the south. Potential evapotranspiration (Penman) is generally about 2,000 mm and so the actual evapotranspiration and plant production is a function of the rainfall.

The Sahel is extensively utilized by humans, however intensive inputs such as artificial fertilizers, fencing, and irrigation are rare. Traditionally the Sahel was a pastoral zone with nomadic and transhumant herding in the north and more settled mixed herding and agriculture in the south, but the population of the Sahel has more than doubled in the last 40 years (van den Oever 1989) and sedentary agriculture has moved into much of the central Sahel that was previously regarded as unsuitable. Thus much of the landscape of the southern and more favorable mid-Sahel consists of fields and extensive areas of fallow bushland in various stages of regrowth.

The HAPEX-Sahel study site

Several considerations enterred into the selection of western Niger as the HAPEX- Sahel study site. Important amongst these was the capital city, Niamey, present actually in the Sahel zone, unlike the capitals of all other Sahelian countries, thus minimizing the logistical difficulties. Niamey and its region is also the location of research centers belonging to international orgnizations, such as the Comit Permanent Inter-Etats de Lutte contre la Scheresse dans le Sahel (CILLS) agriculture, hydrology and meteorology centre (AGRHYMET), the International Crops Research Institute for the Semi Arid Tropics(ICRISAT) Sahelian Centre, and the African Centre of Meteorology Applied to Development (ACMAD), also several bilateral research programs such as those of the French Institute for Scientific Reseach for Development and Cooperation (ORSTOM), UK Institute of Hydrology (IH), and the United States Agency for International Development (USAID). National organizations such as the University of Niamey, Institut Radio-Isotope (IRI), Institut Recherche Agricole Nigerien (INRAN), and the Direction Meteo Nigerien (DMN) also have relevant activities.

A 1 deg x 1 deg region (approximately 100x100 km) was selected in which regional measurements would be made and within which three supersites would be located for detailed measurements. The 2deg-3degE, 13deg-14degN square that contains Niamey was chosen. The Niger river flows across the SW quadrant of the square and the fossil Dalol Bosso valley forms the eastern margin. Within these bounds the landscape is a dissected plateau of Tertiary, Continental Terminal, fluvio-lacustrine deposits capped by a thick, ferruginous duricrust formed in the Quartenary. The plateaux form isolated mesas with steep edges rising 30 to 60 m above the sand-filled valleys.

The landscape in the 1degx1deg region consists of repeated toposequences from plateaux to valley bottoms. The ferruginous duricrust outcrops extensively on the plateaux forming a dark, gravelly surface. In less disturbed areas vegetation arcs are found consisting of 3-5m tall, 10m wide stripes of varying length with their long axis aligned across the very slight slopes that dip towards the plateau edge. Finer grained, lighter colored soil material accumulates in the stripes but between adjacent stripes there is generally no soil or vegetation. The resulting landscape has the appearance from the air of a tiger's coat, hence the term "tiger bush" (Aboucar & other refs). In some areas sand lenses occur on the plateaux which have vegetation indistinguishable from the sand-filled valleys. The edges of the plateaux can be abrupt with a steep escarpment of outcrops of ironstone falling 10-20m to a sloping sand skirt heaped up against the plateau by runoff and wind action. Alternatively the plateaux may end in a more gentle transition to the aeolian sands that fill the valleys. Typically these upper sands are red in color. The sand skirt, where present, has a steep slope and is drained by parallel gullies filled with bushes. Little vegetation occurs between the gullies on the upper slopes, but millet cultivation is common on the lower slopes. The sand skirt gives way to an undulating, sandy or loamy valley floor covered with millet fields and fallow, and finally a valley bottom with bleached sands that often has temporary pools during the rainy season. In some areas laterites in the Continental Terminal deposits form narrow or extensive shelfs in the toposequence. Plateaux form about 20% of the region of which about one third is sand covered. The Niger river valley is essentially similar but with deeper sands and some irrigated rice cultivation along the banks. The Dalol Bosso to the east is a fossil, sand-filled valley with an undulating surface and some areas of more persistent pools during the rainy season. The Dalol Bosso is intensively cultivated and, where extensive pools occur, sometimes there are small rice paddies.

The valley floor vegetation consists of a fine-grained patchwork of cover types and this presented a particular challenge and constraint to the selection of study sites. For certain types of measurements, such as surface fluxes, there is a minimum site size that is dictated by factors such as the roughness of the vegetation. Certain types of airborne measurements are difficult in this type of terrain, particulaly those made with profiling instruments, since it is difficult to avoid contamination by neighboring cover types.

The climate of the study ar ea is typical of the southern Sahel. Mean rainfall at Niamey is about 560mm (1905-1989) with a north to south gradient of about 1mm km-1 (Lebel et al. 1992). The last 25 years have been marked with persistent drought and the mean for this period is 495mm. The average date of beginning of rains at Niamey is 12 June and the average length of the growing season is 94 days (Sivakumar 1989). Potential evapotranspiration is about 2000mm and the annual deficit between evapotranspiration and rainfall increases northwards by about 200 mm per degree of latitude (Sivakumar 1989). Winds are strongly linked to the seasons. During the dry season the harmattan blows from the desert area to the north east and during the rainy season humid winds blow from the south west. Wind speeds can be very high for short periods associated with violent thunderstorms. Dust storms often precede thunderstorms and enormous amounts of dust from the bare, loose sandy soils are carried in the air and partially re-deposited in the subsequent rain. Mean of minimum and maximum temperatures in the Sudano-Sahelian region for the rainy season are 22deg and 34degC respectively, increasing northwards (Sivakumar 1989).

Conditions during the measurement campaign

The main feature of the 1992 rainfall was the early start of the rains followed by a dry spell from early June until mid July. Following the break, rainfall was above average in August and early September. The rainfall total at the southern supersite was 787mm, whereas at the central east site it was only 410mm. Over the whole study area rainfall was 537mm, only marginally below the 1950-1990 average of 550mm. No rain fell in the measurement square after 16 September. Thus the HAPEX-Sahel achieved one of its major aims which was to make measurements over a wide range of rainfall conditions.

The migration of the ITCZ is clearly shown in the rise of the dew point temperature in May. Solar radiation varied somewhat in the dry season, possibly due to atmospheric dust. Air temperature rose with the increase in solar radiation towards the solstice but then declined during the rainy season with the shift of energy transfer to latent from sensible heat. Nightime temperatures were low in the dry season but increased during the growing season to approximately 25degC. Daytime temperatures reached a maximum of approxinately 42degC before the rains started and fell to about 32degC in the mid rainy season. Soil surface temperature reached 50deg-55degC at the time of high solar radiation before the rains.

The vegetation development in 1992 was close to the average for the past 12 years. The Normalized Difference Vegetation Index (NDVI) (Justice 1986) calculated using the Global Area Coverage (GAC) data from the Advanced Very High Resolution Radiometer (AVHRR) carried on the NOAA-11 meteorological, polar-orbiting satellite for the four 50x50km quadrants of the 1degx1deg region shows that, although 1992 did not reach the same peak values as in 1981, 1983, and 1988, it nevertheless was similar to the other years with the exception of the severe drought year of 1984. One contrast of 1992 with the average year was the marked delay in the start of the growing season. The expected north-south gradient of increasing vegetation is also evident with higher values in the growing season in the southern quadrants. The individual supersites differed quite markedly in their vegetation developement as indicated by their NDVI. The NDVI in the southern site increased from June and remained above 0.2 for three months. The central east site NDVI increased one month later and the central west a month later again, in August. In both central sites the NDVI was above 0.2 for only two months. At Dangey Gorou the NDVI declined sooner than at the other sites and was above 0.2 for only about 6 weeks.

The first significant leaf canopy development was onGuiera senegalensis, the bush that formed most of the woody component of the fallow in all of the 1degx1deg region. In the west central site, the herb layer in the fallow developed as much as two months later, at about the same time as millet emerged. The herb layer was initiated by dicotyledonous plants and followed by the grasses. The dry period in June caused the millet crop in most fields sown in May to fail except in the south west. Fields elsewhere were resown, sometimes more than once. In the southern site millet was harvested between 10 and 15 September but not until about 10 October in the central sites.

Early in the growing season latent heat fluxes were determined largely by the surface moisture. At this time rainfall events were widely separated in time and the soil surface dried quickly, but later the frequency of rain increased and the soil remained wetter for longer periods. In addition plant transpiration added significantly to the evapotranspiration. Bowen ratios in a bush fallow at the west central site reached values approaching 2 occurred in early August and declined steadily to a minimum of 0.2 in mid September. By 10 October the Bowen ratio had risen again to 1.0, indicating that the observations captured the main features of the annual cycle.

The trend in the late season from latent to sensible heat flux was also indicated in the depth of the atmospheric boundary layer, which grew from an average altitude of 1 km in the wetter, mid part of the growing season, to 1.8km towards the end of the rainy season when sensible heat dominated the energy flux.


A three-layer hierachy of sites was established in order to sample the study area and to provide the appropriate size and uniformity of sites needed for each of the wide variety of measurement techniques that were used in HAPEX- Sahel. First, one 1degx1deg square was selected; second, three supersites were identified inside the 1degx1deg square, each approximately 20x20km; third, within each supersite, three or more subsites were selected, one in each of the principle landscape components of the supersite. Below the level of the subsite ,two further, less formalized, subdivisions were used. First, the subsites were themselves divided into a number of stands, one of which was generally a tower flux site, but others were used for other types of measurements where these were thought to interfere with the flux measurements. Second, below the stand level, some studies employed individual plants or specific soil sampling locations. This formalized sampling structure was intended to assist in the subsequent scaling of field measurements to the landscape and regional scales.

The selection of supersites was determined by the particular needs of the types of measurements that were planned. The southern and central west sites were intended primarily for surface flux and energy balance studies and this demanded uniform stands large enough to measure the fluxes associated with the surface type within the stand, without significant influences from surrounding surface types. The central east supersite was selected primarily as a catchment for hydrological studies. Historical factors also played a part, in that the southern site had been used in previous studies by the UK Institute of Hydrology and the central east site was established at a very early stage by ORSTOM for its continuing hydrology program. The supersites were not arranged to capture the north-south gradient of rainfall and associated variables since rainfall is so variable spatially at the scale of a 1degx1deg square. The north-south gradient of rainfall is a phenomenon that is expressed at the spatial scale of the entire Sahel and locally only in the long term average rainfall, not in every single year. It had originally been intended to place one supersite outside the 1degx1deg square well to the north near Ouallam, and measurements were undertaken at this site in 1991, however poor security associated with the Tuareg revolt caused the Government of Niger to advise against using the site in 1992.

Three principal landscape components were selected at the subsite scale, viz. tiger bush on the plateaux, fallow bush grassland on the valley sands, and millet fields. Each supersite had subsites placed in extensive stands of each component.

Two ancillary sites were instrumented to improve the surface flux coverage of the 1degx1deg square. These were not linked to the three super sites. First a flux tower was established at Danguey Gorou in the northwest of the 1degx1deg square, in extensive millet cultivation. Second a tethered baloon was flown occassionally at a site in the south east corner of the 1degx1deg square in the Dalol Bosso.


Observations were made from aircraft to bridge the gap between ground observations and satellite data and they therefore formed an essential part of the HAPEX- Sahel measurement design. There were two roles for the aircraft; first, acquisition of data over the subsites and supersites to enable the relationships between the ground and airborne scales of measurement to be determined; second, measurements at the scale of the 1degx1deg square in order to scale the supersite results to the entire study area. Four aircraft took part in the study, viz. the ARAT Fokker 27 operated by the French Institut National des Sciences de l'Univers, the METEO-FRANCE Merlin IV, the NASA C-130 operated by Ames Research Center in the United States of America, and a Piper Saratoga operated by Sudan Interior Mission Air Services in Niger. The instruments carried by each aircraft are described in the experimental plan, together with the principal variables that were measured. Details of the instruments can be found in the Experiment Plan (Goutorbe et al. 1992)

The aircraft program consisted of over 20 flight plans, several of which used more than one aircraft, sometimes with coordinated baloon ascents.

The flux measurements made by the METEO-FRANCE Merlin IV involved measurement of; variations in surface fluxes within the 1degx1deg square; integrated fluxes for the entire 1degx1deg square; the effects of recent rainfall on fluxes, rainfall being detected by the microwave sensors on the C130 and ARAT; area-averaged fluxes over an area larger than the 1degx1deg square in order to assess the contribution of convective rainfall cells to the fluxes; and the north-south flux gradient.

The Fokker 27 (ARAT) operated as a remote sensing platform during the first month of the experiment(23/8--18/9) with a video camera, a passive microwave radiometer (PORTOS) and a visible/near-infrared radiometer (POLDER). It was for PORTOS its first operation on an aircraft while POLDER had already been flown in previous specific campaigns. Flights were made with POLDER at an altitude of 4500 m while specific PORTOS missions were flown at 450 m. The flight plans for POLDER consisted in 5 or 7 parallel lines in the solar principle angle and a complementary line perpendicular to the sun 's principal angle. The patterns were centered on subsites and were performed over the three supersites. Such a pattern allows to make acquisitions over the considered subsite with a variety of viewing conditions. During all POLDER flights, PORTOS was operated in a nadir viewing mode. During the second phase of the campaign (29/9 - 8/10), POLDER remained on the ARAT and flew several missions to sample high aerosol loading in the atmosphere. Polder missions were organized so as to sample the vegetation cycle and the atmospheric conditions (notably aerosol loading. Flights were supported by atmospheric and vegetation measurements, principally in the central west supersite.

Specific PORTOS missions were flown at an altitude of 400 m so that the size of a footprint corresponded to a "WAB". As the instrument is only a profiler, mapping was performed by making series of parallel lines separated by the length of a footprint (3dB). Most of the flights were performed with a fixed incidence angle (45deg). The three supersites as well as Danguey Gourou were covered several times during the deployment so as to sample the soil's drying out sequence. On one occasion a subset of the East and West central sites (1 km wide) was covered with several viewing angles.

After a week's break to reconfigure the aircraft, the ARAT operated during the later part of the intensive observation period as a flux measurement platform in conjunction with the Merlin. Finally, the video was systematically operated during data acquisition sequences. And for each flight a radiosounding was performed unles the flight occurred within two hours of a routine sounding.

Whenever possible, PORTOS and PBMR were operated almost synchronously. The flights were decided upon the reports of soil moisture evolution so as to sample the drying out. One one occasion, we manage to fly minutes after the end of a storm on the south supersite. Flights were supported by extensive soil moisture measurements, and ground measurements of brightness temperature (infrared and microwaves at 4.3 GHz), coupled with routine measurements of the vegetation and surface fluxes. On several occasions a POLDER and a PORTOS flight occurred within Hours, thus providing microwave measurements at two different scales.

The C-130 was used to make measurements with all instruments along four north-south transects of the 1degx1deg square, spaced between 2deg and 3deg E. Over the southern and combined east and west central supersite, flights at 1,000 ft were made in order to achieve a spatial resolution of the PBMR similar to the size of the subsites for validation of the airborne remote sensing measurements of surface conditions. Optical measurements of the supersites were made at 15,000 ft where there is less turbulence in order to achieve better platform stability. The laser profiler was flown at 800 ft. During clear sky conditions optical and thermal missions were given first priority since microwave measurements do not require clear skies. The Zeiss metric cameras were used extensively to provide color and false color infrared stereo aerial photography of the supersites.

The Piper Saratoga was used to carry a lightweight package of instruments that provided nadir and 35deg views in a range of bands from visible to thermal, together with photographs and video. By using a local aircraft it was cost effective to make measurements throughout the entire growing season and to take advantage of clear sky conditions whenever they occurred. Measurements were made of all subsites and supersites at approximately two week intervals in order to study the growing season from its start until the end of the Intensive Observation Period (IOP).

Some important measurements opportunities were offered by the range of sensors available on the several aircraft deployed for the experiment. The instruments made measurements covering the useful electromagnetic spectrum (i.e., visible to microwave) for resolutions ranging from a few meters up to 2 km. The planning of the flights was thus established so as to make full use of the synergisms as well as collect adequate data to study the scaling problem. The synergisms possible are either between several microwave frequencies or between microwaves and visible/near infrared/thermal infra red. In the microwave spectrum the two intruments available: PBMR (1.4 GHz) and PORTOS (5 to 90 GHz) are highly complementary. PBMR is a one frequency imaging radiometer whose measurements are sensitive to soil moisture with a low sensitivity to biomass. Portos has higher frequencies but is only a profiler. It is thus more sensitive to vegetation characteristics (wet biomass, structure), surface roughness. The visible/near infrared data will be used to quantify vegetation cover and the percentage of bare soil. Thermal infra red will be used to assess the surface temperature and to investigate how well passive microwaves could be used to assess surface temperature when clouds are present. With this information and data collected by PBMR and PORTOS, the following geophysical parameters will be retrieved: soil moisture, vegetation biomass, surface roughness. This will be done whenever possible at the two flight altitudes so as to study the influence of scale. As several flights were coordinated with ERS-1 overpasses and were systematically supported by ground measurements of soil moisture, HAPEX SAHEL will be the first large scale experiment where passive microwaves will be fully investigated and retieved parameters confronted with mesoscale and global circulation models (assimilation and validation).


The satellite data acquired during HAPEX- Sahel As anticipated, clear sky conditions were rare during the mid rainy season and optical and thermal data are therefore sparse for this period. Nevertheless several clear data sets were acquired. This experiment is the first large, integrated field program to benefit from ERS-1 data, including both ATSR and WINDSCATT. Meteosat and AVHRR data were acquired from receivers in Niamey at AGRHYMET and DMN. Landsat TM4 data were acquired by EOSAT using the TDRS data-relay satellites and a special arrangement was made with the EURIMAGE to acquire some TM5 data from ESA's Mas Palomas receiving station.

The rationale for satellite data acquisition is the following.


HAPEX- Sahel was a collaborative, interdisciplinary study with contributed investigations on a wide variety of topics, generally directed at addressing the broad aims of the HAPEX program. There was no "staff science" in HAPEX as there was in FIFE, in which contractors made certain routine measurements requested by the scientific management group. Thus essential measurements needed to achieve the aims of HAPEX, that were not proposed at the outset, were identified by the Coordination Committee and its special topic groups and appropriate teams were requested to add them to their existing program. Much of the HAPEX program therefore depends on formal and informal collaborations between science groups. This has already proved particularly fruitful in initiating interdisciplinary discussions and research. The most complete list of investigations is given in the experiment plan. The lists do give a reasonable indication of the range of studies undertaken. The studies were organized into four disciplinary groups, each under the leadership of a member of the Coordination Committee. The groups were; Remote Sensing Science; Flux and Vegetation Studies; Meteorology and Mesoscale Studies; and Hydrology and Soil Moisture Studies. Further details can be found in Goutorbe et al. (1992).

Short description of POLDER and PORTOS

The POLDER instrument is a visible/near infrared radiometer which has several specificities. It is equipped with a rotating wheel allowing to sequentially make measurements at several wavelengths as well as at several polarizations, enabling to make polarization measurements. Its detector is a Charged Coupled Device (CCD) matrix, acquiring a whole frame in one measurement period. Consequently, as the plane travels forward, any point of the surface is seen sequentially at different wavelengths and polarizations, as well as different angles. By flying parallel lines, several sets of viewing angles are collected, allowing to sample the bidirectional functions of the pixels in azimuth and elevation, and this for all the frequencies and polarizations. The measurements are used to quantify atmospheric characteristics (and particularly the aerosols) as well as the vegetation and soil BRDF. It is a good complement to ASAS.

PORTOS is a passive microwave instruments with 5 frequencies (5, 10.7, 23.8, 36.5, 90 GHz) dual polarized. The instrument resolution is linked to its mid power beam (11deg). Only one beam is acquired but its incidence can be chosen between 0 and 50deg. The range of frequencies available corresponds to that of the Multichannel Imaging Microwave Radiometer (MIMR) which is to be flown on the EOS-pm platform as well as on the European METOP platform. These frequencies are sensitive to different contributions to the signal (soil moisture, vegetation biomass, surface roughness, atmospheric integrated water content, surface equivalent temperature), and the combined use of the measurements made by PORTOS with those of PBMR should allow to retrieve the above mentioned quantities.

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