Vegetation Dynamics. Natural versus Cultural and the Regeneration Potential. The Example of Sahara-Sahel

There is a principal and controversial debate on the so-called ‘Greening-Regreening’ of the Sahel. There still is the old philosophy of an expanding/shrinking ecosystem Sahara versus Sahel. In some concepts, it is presented as annual. Another concept is based on a general degradation of the Sahelian savannas – in some cases with a decline to a lower state of ecological equilibrium after a short period of resilience. Anyhow, there are also signs of still ongoing regeneration processes of vegetation and soil. The main problem, however, lies in the principal lack of terrestrial observation and in the confusion of terms. This mostly concern on vegetation units and their dynamics. The goal of this article is to explain the general nature of the Sahara and the Sahel based on maps and graphs. We try to analyse the dynamics of boundaries during the last 200 years. The main results are the tripartite nature of the Sahara, divided into semidesert, desert and Saharan savanna with relatively stable boundaries. A reconstruction of the vegetation for the last 200 years confirmed the position of these borderlines even under different states of the plant cover. It also revealed the nature of Sahelian savannas as cultural landscapes – in higher diversity and density. It is also possible that the North Sahelian savannas had been for long times under the dynamics of elephant landscapes. A high-resolution sediment and pollen record from the Middle Sahel of Niger evidenced the high diversity and resilience up to the severe drought of the 1970s. It was a definite stroke from which these savannas never reached again their former diversity despite a slide recovery named ‘Regreening’. The various projects for regeneration or conservation in Sahara or Sahel differ in two types of projects. The one is the installation of Nature Reserves/National Parks with special reserves for emblematic animals as keystone organisms and an auto-regeneration of vegetation and soil. The other type consists of pasture rotation projects such as in the Malian Gourma or in the Central Air Mts. The first initiative resulted in the decade-long protection against the severe degradations, which were typical for the surrounding regions. The rotation system was based on timewise open wells and of observed pasture status. It was conceived together with the local populations and has been respected until the invasion of northern cattle keepers during the peak of drought in 1984. After severe quarrels, the system collapsed and the savannas degraded heavily. A comparable project worked in the central Air Mts. for 5 years. Remarkable results have been, but the rebellion of the 1990s, put a sudden end on it. The general insecurity of the last decades caused by civil war and/or various terrrorist groups led to a re-evaluation of a great number of regeneration initiatives including the pharaonic ‘Great Green Wall’, a continent wide forest belt. However, smaller projects on the village level may better develop as they are under the responsability of local population, which can reactivate their long experience. The ‘regreening’ might be restricted to the region of the southern Sahara and the northern Sahel as well as to the traditional park systems. Anyhow, even if a long-time amelioration of production systems will happen, the former must be regarded on the background of a rapidly increasing demography

Pratiques et contraintes de l’élevage de la chèvre rousse de Maradi en milieu rural au Niger

Au Niger, l’élevage caprin occupe une place importante dans l’économie des ménages ruraux. Une étude a été conduite dans la région de Maradi afin de caractériser les pratiques de gestion des élevages des chèvres rousses et analyser leurs contraintes. Ainsi, 180 agro-éleveurs répartis dans 12 villages de la région ont été questionnés. Les thèmes abordés au cours des interviews ont porté sur : les caractéristiques du ménage, les objectifs de production, le mode de conduite des animaux, la gestion de l’alimentation, la santé animale ainsi que les contraintes auxquelles les producteurs font face. Les résultats ont révélé que la majorité (72,8%) des propriétaires des caprins sont des femmes. Les caprins sont élevés en priorité comme épargne pour subvenir aux besoins du ménage. Pendant la saison sèche, plus de la moitié (51%) des éleveurs font recours à l’achat du son de blé pour la complémentation des animaux. Diverses sources des compléments minéraux sont utilisées, notamment le sel de cuisine, les pierres à lécher, le natron et le sel de Bilma. La pénurie d’aliments a été identifiée comme la contrainte majeure de l’élevage caprin (57,8%). Elle est suivie par les maladies animales (17,2%) et la stabulation en saison de pluies (7, 8%). L’amélioration de l’élevage de la chèvre rousse passe par la prise en compte de toutes les contraintes de production. English title: Practices and constraints of Maradi red goat breeding in rural areas in Niger In Niger, goat farming plays an important role in the economy of rural households. A study was conducted in the Maradi region to characterize the management practices of red goat farms and analyze their constraints. Thus, 180 agro-pastoralists spread out in 12 villages of the region were questioned. The themes addressed during the interviews included: household characteristics, production objectives, animal husbandry, feed management, animal health and the constraints faced by producers. The results revealed that the majority (72,8%) of goat owners were women. The feeding system is based on natural pastures and crop residues. During the dry season, more than half (51%) of the farmers used wheat bran for animal complementation. Various sources of mineral supplements are used, including cooking salt, lickstones, natron and Bilma salt. Food shortage has been identified as the major constraint to goat farming (57.8%), followed by animal diseases (17.2%) and stalling during the rainy season (7.8%). The improvement of the breeding of the red goat requires the taking into account of all the constraints of production

Detection of oasis basins in east-central Niger by textural image classification based on the variance

peer reviewedCette étude évalue les capacités de détection des cuvettes oasiennes du centre-est du Niger par analyse texturale d’images-satellites, basée sur la variance. Ces cuvettes constituent les seuls endroits où l’agriculture irriguée est pratiquée durant la saison sèche. La méthode mise au point est une détection des cuvettes oasiennes à partir de la bande infrarouge d’une image SPOT5-THX (2,5 m de résolution spatiale) en analysant la courbe de la fréquence cumulée des valeurs de variances déterminées dans une fenêtre de 5*5 pixels, et en s’appuyant sur une valeur seuil. L’évaluation des résultats de la classification d’image a donné des conclusions satisfaisantes avec un coefficient Kappa égal à 0,96 et un taux de bonne classification des cuvettes de 82,7%. Cette étude permet de localiser et de mesurer la superficie des cuvettes dans la zone d’étude. L’inventaire de ces cuvettes permettra de les caractériser et de les suivre afin d’évaluer leurs potentialités agricoles

Etude de la repartition des formations sableuses et interpretation des depots eoliens dans le Liptako oriental (republique du Niger)

CNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueSIGLEFRFranc

Covid-19 et impacts du confinement sur la population de Niamey (Niger)

Appelée « virus chinois » au début de sa propagation, la maladie à coronavirus 2019 (Covid-19) a bouleversé la planète terre à cause de son ampleur sanitaire, socio-économique, culturelle, géopolitique, etc. Au moment où les pays occidentaux et autres contrées du monde, en l’occurrence l’Amérique Latine et quelques pays asiatiques comme la Chine et l’Inde, comptent leurs milliers de morts et millions d’infectés par cette maladie, en Afrique subsaharienne particulièrement au Niger la Covid-19 a eu très peu d’impact sanitaire sur la population. Mais pour des mesures préventives, les gouvernants nigériens ont pris des dispositifs coercitifs semblables à ceux des pays fortement touchés par la pandémie du coronavirus. Ainsi, il est à constater le confinement d’une importante partie de la population et l’instauration d’un couvre-feu nocturne qui restreint ipso facto les libertés individuelles fondamentales. Cette restriction n’a pas été sans conséquence sur le climat social, provoquant ainsi des remous sociaux, des vives contestations voire la désobéissance civile résultant par des violences policières hors normes surtout à Niamey dans la capitale nigérienne. Cette étude est essentiellement basée sur la méthode qualitative à travers l’usage de la grille d’observation et du guide d’entretien semi-dirigé comme outils d’enquête pour analyser les impacts du confinement contre la Covid-19 sur les personnes victimes et témoins des effets du couvre-feu et/ou des violences policières à Niamey. Leurs perceptions déterminent des comportements qui banalisent ou non les gestes barrières contre le coronavirus. Quant aux résultats de l’étude, ils montrent que les violences policières reflètent le caractère conflictuel de la gestion de cette pandémie et freinent l’adhésion pacifique et totale de la population aux mesures préventives contre la Covid-19. Ces résultats montrent aussi que l’absence des mesures d’accompagnement conséquentes des autorités politiques a considérablement contribué à l’inobservance desdites mesures par la population de Niamey. Some people name it "Chinese virus" as it spreads. The 2019 coronavirus disease (Covid-19) disrupts our planet earth because of its health, socio-economic, cultural, geopolitical scale, etc. At a time when Western countries and other parts of the world, in this case Latin America and some Asian countries such as China and India, count their thousands of deaths and millions infected by this disease, in Sub-Saharan Africa particularly in Niger, Covid-19 has very little health impact on the population. But, for preventive measures, nigérien rulers have taken coercive measures similar to those in countries strongly affected by the coronavirus pandemic. Thus, it is to be noted that locking down a large part of the population and setting of a night curfew which ipso facto restrict fundamental individual freedoms. Indeed, this restriction has consequences on social scale by causing for instance social unrest, strong protests and even civil disobedience resulting in extraordinary police violence, especially in Niamey, the capital City of Niger. This study is essentially based on the qualitative method through the use of observation grid and semi-structured interview guide as survey tools to analyze the impacts of locking down against Covid-19 of people who are victims and witnesses of the curfew effects and/or police violence in Niamey. Their perceptions determine behaviors that may or may not trivialize barrier gestures against coronavirus. As for the outcomes of our study, they show that police violence reflects conflictual nature of management of this pandemic and hinders the peaceful and total support of the population for preventive measures against Covid-19. These results also show the absence of subsequent additional measures from political authorities leading considerably to the nonobservance of these measures by the population of Niamey

Fonctions des mythes dans la gestion traditionnelle des ressources naturelles : le cas du mythe de toula ou ≪serpent d\u27eau≫en pays songhay (niger)

Studies of anthropogenic factors of the Sahelian ecosystem deterioration often put forward impacts caused by population growth, poverty or land use systems. An often ignored factor is the thorough disintegration which changes the original perception of nature these societies had. The study of the myth of Toula, allowed to assess the role and the weight of the cult of ≪water spirits≫ called ≪serpents d\u27eau≫ in the traditional water resources management of natural. Water is the main element structuring natural resources in these semiarid regions. The cult returned in Toula structured the access rules to the water resources by supervising and prioritizing the perception of the social group. The effectiveness of the regulations between intakes and available water resources relied on self-censorship principles and the pertinence of traditional knowledge. The cult is celebrated by children of the matriarchal lineages, creating equilibrium between the political power detained by the men, which is linked to the earth and the spiritual power of the matriarchal lineages linked to water. Around powers several stakes are managed in an integrated way: the management of water in social and mythical environment, the consanguinity and alliances. The fading of the cult since two generations led consequently to a deregulation of the natural resources management system which the desacralized traditional land tenure systems and the current land tenure laws could not still supplant

Détection des cuvettes oasiennes du Centre-Est du Niger par classification d’une image texturale basée sur la variance

article publié en français (p. 24-34), avec un résumé étendu en anglais (p. 35-36), et 8 planches couleur hors-texte (p. 65-71)International audienceThis study assesses the possibility of detection of oasis basins in East-Central Niger through textural analysis of satellite images based on thevariance. Texture analysis serves to find descriptive attributes that quantify images granularity characteristics, directionality or finesse. These textural characteristics are usually related to image objects properties. An image texture analysis provides information about the nature of bjects, constituting the segments of homogeneous regions, and improves image quality. The study area is located in the Sahel-Saharan region of Niger. Note that agriculture and animal husbandry are the main economic activities in this area. Irrigated and rainfed crops guarantee almost all cereal production. Irrigated crops grown in the oasis basins provide a minimum level of output less sensitive to fluctuations in rainfall thanrainfed crops. Oasis basins are closed depressions within which flow is endorheic. These basins are the only places where irrigated agriculture is practiced during the dry season. They therefore constitute the main source of income for farmers in the region.Indeed, no study has previously made an exhaustive inventory of these closed depressions. The only censuses carried were often limited to exploited oasisbasins. In this context this study has been initiated to preserve the oasis basins and assess their socio-economic roles. The developed method detects oasis basins based on the image variance of the grey levels (digital counts) in a 5x5 pixels moving window of the infrared band of a SPOT5-THX image (2.5 m spatial resolution) acquired in November 2013. This procedure is performed in three steps. The first stage analyses the cumulative frequency curve of variance values determined by a threshold value. The high window variance values, representing the contours of the basins, were determined from a threshold adapted to the classification results. This threshold value allowed with the «erase» tool of ArcGIS to make a mask of the oasis basins and their surroundings, and another mask representing the other items: dry valleys, dune edificesand hills. Both masks led to a segmentation of the composite image into two parts. To improve the accuracy of the results, a supervised classification, using the Mahanalobis distance algorithm, was performed on the first part of the image, oasis basins and some surrounding bare soil dunes. The second part of the image, representing dune edifices, dry valleys and hills, was used in step 2. In this step, a second supervised classification, using the same algorithm, was applied on the composite image without cuvettes oasis, hence allowing to distinguish between the various constituent objects. In the third step, the results from step 1 and 2 were combined. Classification evaluation is based on a confusion matrix. The evaluation indicators calculated are: Kappa coefficient, overall accuracy, rate of reliability of a class and correct classification rate. Following thresholding, binarization of the textural channel derived from the infrared band of SPOT 5 THX image first delineates the oasis basins; then this result has been improved through supervised classification of the oasis basins boundaries. The combined results of the two images show four classified geomorphic units: oasis basins (11512 ha or 7.4% of the study area), dry valleys (32386ha or 20.7% of the study area), dune edifices (112499 ha or 71.8% of the study area) and hills (218 ha or 0.1% of the study area). Classification evaluation has given satisfactory conclusions with a Kappa coefficient of 0.96 and a rate of correct oasis basins classificationof 82.7%. Our results confirm several studies showing that the performance of image classification by textural analysis is greater than using simple pixel by pixel analysis. But some confusions remain between classes. These confusions can be explained by the definition of classesidentified in the study, which are geomorphological units. Yet, these geomorphological units may have similar land cover. For example, a temporary water can be located at a top of a hill, in a dry valley or in an oasis basin. However, the performance of the oasis basin detection method can be improved by combining several textural attributes to create a textural composite image. This study demonstrates the possible identification and measurement of the surface of oasis basins in a part of the Gouré Department; this could be done even with free remote sensing software allowing computation of a textural channel. The inventory of these oasian basins resources is a necessary first step in their monitoring and the evaluation and management of their agricultural potential.Cette étude évalue les capacités de détection des cuvettes oasiennes du centre-est du Niger par analyse texturale d’images-satellites, basée sur la variance. Ces cuvettes constituent les seuls endroits où l’agriculture irriguée est pratiquée durant la saison sèche. La méthode mise au point est une détection des cuvettes oasiennes à partir de la bande infrarouge d’une image SPOT5-THX (2,5 m de résolution spatiale) en analysant la courbe de la fréquence cumulée des valeurs de variances déterminées dans une fenêtre de 5*5 pixels, et en s’appuyant sur une valeur seuil. L’évaluation des résultats de la classification d’image a donné des conclusions satisfaisantes avec un coefficient Kappaégal à 0,96 et un taux de bonne classification des cuvettes de 82,7%. Cette étude permet de localiser et de mesurer la superficie des cuvettes dans la zone d’étude. L’inventaire de ces cuvettes permettra de les caractériser et de les suivre afin d’évaluer leurs potentialités agricoles

DÉTECTION DES CUVETTES OASIENNES DU CENTRE-EST DU NIGER PAR CLASSIFICATIONS D’IMAGES-SATELLITE SPOT5-THX

Article publié en français (p. 32-46) avec un résumé étendu en anglais (p.47-48) et 9 planches couleur hors-texte (p.76-84)International audienceExtended abstract: Located in a semi-arid zone, oasis basins of east-central Niger (plate 1) are high-potential agro-ecosystems (plates 2 and 3). These oasis basins are covered by dense vegetation consisting mainly of Hyphaene thebaica, Phoenix dactylifera, Digitata andasonia reflecting an oasis-like area, thus the name of oasis basin. Oasis basins represent the last areas where farming is possible throughout the year. These areas have experienced many investments from several NGOs and projects in these recent years. However, the oasis basins are still unknown and / underused by development actors due to their extreme diversity and also due to their huge number. In this context, research has been initiated to preserve the oasis basins and assess their socio-economic roles. In order to achieve this goal a thorough knowledge of the variable characteristics of these basin would be necessary. Given their large number and diversity, remote sensing, with its synoptic vision, is a relevant tool to locate and identify and characterize (set up an inventory) these oasis basins. SPOT5-THX images (2.5m resolution) were used in this study (table 1). This study evaluates the performances of three image classification methods in detecting oasian basins in the administrative unit (« Départment ») of Gouré using a confusion matrix tool (table 2). The methods are pixel analysis, textural analysis and object analysis (plate 4). A visual analysis of image mosaics was performed to obtain a preliminary categorization of the study area and to choose a sampling design. A field campaign led to the selection of a set of regions of interest, one part of which was used to develop the classifications and another part to validate the results. As part of this study, four geomorphological classes were defined (oasis basin, dry valley, dune edifice and hill). Table 3 summarizes the number and areas of the basins obtained by the three methods. The analysis by object yielded fewer oasis basins in number while the pixel approach identified more oasis basins and with areas that are far larger than those of the two other methods. This is explained by a strong confusion between the oasis basins and other classes like dry valley, hill and dune edifice (plate 6). The distribution of oasis basins according to their size (plate 7) shows a very high proportion of oasis basins covering less than 0.5 ha. Although the oasis basins with scopes lesser than 0.5ha represent more than fifty percent, those that have an area greater than 1O ha represent more than half of the areas obtained for each method (plate 8). Evaluation of the results (tables 4-6) shows that the approach by object (global precision was 97%, Kappa coefficient was equal to 0.97) is more efficient than the textural method (overall precision 90%, Kappa coefficient 0.90) and the pixel-based approach (overall precision 82%, Kappa coefficient 0.82). Indeed, given the importance of the detected oasis basins and large differences between the results of the three approaches, the performances of the three methods have been validated through a model of prediction to determine the exact number of oasis basins which approaches the ground reality. The oasis basins probability detection (ability of a method to identify oasis basins observed in the field and photo-interpretation) of the three methods gives satisfactory results: 98.3% pixel analysis, textural analysis 98.9% and 99.2% object analysis. However, the false positive rates, which represent the probability that the oasis basin is detected by a method when it does not exist in reality in the field are important (pixel analysis: 31%, textural analysis: 22% and object analysis: 7%). Oasis basins observed in the field or by photo interpretation and which were not detected by the alternative method (false negatives) are fewer at the analysis by object.The critical success index (CSI) has been computed, taking into account the rate of false positive and false negative results, the quality of the agreement between the estimate of the oasis basin identified by a method and true oasis basins observed in the field or by photo-interpretation, apart from random errors. The object analysis gave the highest CSI value (i.e. 0.94). It is unquestionably the method whose results are similar to those observed in the field. Analysis by texture is better than that by pixel. One can even get a further improved performance of the textural analysis by combination the fourteen textural attributes of HARALICK et al. (1973). This study used a single textural attribute i.e. the variance. Our results confirm several studies showing that the performance of image classification by object analysis is greater than textural analysis and simple pixel based analysis. But some confusion remains between mapped classes. This confusion can be explained by the classes identified in the study, which are geomorphological units. Yet, these geomorphological units have similar land cover. For example, a temporary water can be located atop a hill, in a dry valley or in an oasis basin. The study provides a first original information, namely the number, the position and the surface of all the oasis basins on the study area. The total number of oasis basins in the Department of Gouré is estimated at 11,300 with an error more or less than 0.06.The inventory of these resources is a first compulsory step for the characterization of oasian basins to be used for the evaluation of their agricultural potential.Situées en zone semi-aride, les cuvettes oasiennes du centre-est du Niger sont des agroécosystèmes à haute potentialité. Cependant, elles sont souvent peu connues par ces acteurs du développement à cause de leur extrême diversité et de leur nombre. Cette étude évalue les performances de trois méthodes de classification d’images dans la détection des cuvettes oasiennes (analyse pixellaire, analyse texturale et analyse par objet) à partir des images SPOT5-THX de 2,5m de résolution spatiale. L’évaluation des résultats montre que l’approche par objet (indice critique de succès égal à 0,94) est beaucoup plus performante que celle utilisant la texture (indice critique de succès égal à 0,78) et l’analyse par pixel (indice critique de succès égal à 0,69). L’étude fournit une première information originale, à savoir le nombre (11300 cuvettes oasiennes détectées avec une erreur de plus ou moins 6%), la position et la surface de l’ensemble des cuvettes sur la zone d’étude

DÉTECTION DES CUVETTES OASIENNES DU CENTRE-EST DU NIGER PAR CLASSIFICATIONS D’IMAGES-SATELLITE SPOT5-THX

Article publié en français (p. 32-46) avec un résumé étendu en anglais (p.47-48) et 9 planches couleur hors-texte (p.76-84)International audienceExtended abstract: Located in a semi-arid zone, oasis basins of east-central Niger (plate 1) are high-potential agro-ecosystems (plates 2 and 3). These oasis basins are covered by dense vegetation consisting mainly of Hyphaene thebaica, Phoenix dactylifera, Digitata andasonia reflecting an oasis-like area, thus the name of oasis basin. Oasis basins represent the last areas where farming is possible throughout the year. These areas have experienced many investments from several NGOs and projects in these recent years. However, the oasis basins are still unknown and / underused by development actors due to their extreme diversity and also due to their huge number. In this context, research has been initiated to preserve the oasis basins and assess their socio-economic roles. In order to achieve this goal a thorough knowledge of the variable characteristics of these basin would be necessary. Given their large number and diversity, remote sensing, with its synoptic vision, is a relevant tool to locate and identify and characterize (set up an inventory) these oasis basins. SPOT5-THX images (2.5m resolution) were used in this study (table 1). This study evaluates the performances of three image classification methods in detecting oasian basins in the administrative unit (« Départment ») of Gouré using a confusion matrix tool (table 2). The methods are pixel analysis, textural analysis and object analysis (plate 4). A visual analysis of image mosaics was performed to obtain a preliminary categorization of the study area and to choose a sampling design. A field campaign led to the selection of a set of regions of interest, one part of which was used to develop the classifications and another part to validate the results. As part of this study, four geomorphological classes were defined (oasis basin, dry valley, dune edifice and hill). Table 3 summarizes the number and areas of the basins obtained by the three methods. The analysis by object yielded fewer oasis basins in number while the pixel approach identified more oasis basins and with areas that are far larger than those of the two other methods. This is explained by a strong confusion between the oasis basins and other classes like dry valley, hill and dune edifice (plate 6). The distribution of oasis basins according to their size (plate 7) shows a very high proportion of oasis basins covering less than 0.5 ha. Although the oasis basins with scopes lesser than 0.5ha represent more than fifty percent, those that have an area greater than 1O ha represent more than half of the areas obtained for each method (plate 8). Evaluation of the results (tables 4-6) shows that the approach by object (global precision was 97%, Kappa coefficient was equal to 0.97) is more efficient than the textural method (overall precision 90%, Kappa coefficient 0.90) and the pixel-based approach (overall precision 82%, Kappa coefficient 0.82). Indeed, given the importance of the detected oasis basins and large differences between the results of the three approaches, the performances of the three methods have been validated through a model of prediction to determine the exact number of oasis basins which approaches the ground reality. The oasis basins probability detection (ability of a method to identify oasis basins observed in the field and photo-interpretation) of the three methods gives satisfactory results: 98.3% pixel analysis, textural analysis 98.9% and 99.2% object analysis. However, the false positive rates, which represent the probability that the oasis basin is detected by a method when it does not exist in reality in the field are important (pixel analysis: 31%, textural analysis: 22% and object analysis: 7%). Oasis basins observed in the field or by photo interpretation and which were not detected by the alternative method (false negatives) are fewer at the analysis by object.The critical success index (CSI) has been computed, taking into account the rate of false positive and false negative results, the quality of the agreement between the estimate of the oasis basin identified by a method and true oasis basins observed in the field or by photo-interpretation, apart from random errors. The object analysis gave the highest CSI value (i.e. 0.94). It is unquestionably the method whose results are similar to those observed in the field. Analysis by texture is better than that by pixel. One can even get a further improved performance of the textural analysis by combination the fourteen textural attributes of HARALICK et al. (1973). This study used a single textural attribute i.e. the variance. Our results confirm several studies showing that the performance of image classification by object analysis is greater than textural analysis and simple pixel based analysis. But some confusion remains between mapped classes. This confusion can be explained by the classes identified in the study, which are geomorphological units. Yet, these geomorphological units have similar land cover. For example, a temporary water can be located atop a hill, in a dry valley or in an oasis basin. The study provides a first original information, namely the number, the position and the surface of all the oasis basins on the study area. The total number of oasis basins in the Department of Gouré is estimated at 11,300 with an error more or less than 0.06.The inventory of these resources is a first compulsory step for the characterization of oasian basins to be used for the evaluation of their agricultural potential.Situées en zone semi-aride, les cuvettes oasiennes du centre-est du Niger sont des agroécosystèmes à haute potentialité. Cependant, elles sont souvent peu connues par ces acteurs du développement à cause de leur extrême diversité et de leur nombre. Cette étude évalue les performances de trois méthodes de classification d’images dans la détection des cuvettes oasiennes (analyse pixellaire, analyse texturale et analyse par objet) à partir des images SPOT5-THX de 2,5m de résolution spatiale. L’évaluation des résultats montre que l’approche par objet (indice critique de succès égal à 0,94) est beaucoup plus performante que celle utilisant la texture (indice critique de succès égal à 0,78) et l’analyse par pixel (indice critique de succès égal à 0,69). L’étude fournit une première information originale, à savoir le nombre (11300 cuvettes oasiennes détectées avec une erreur de plus ou moins 6%), la position et la surface de l’ensemble des cuvettes sur la zone d’étude

Caractérisation morphologique des cuvettes oasiennes du Centre-Est du Niger

Situées en zone semi-aride à aride du Niger, les cuvettes oasiennes du département de Gouré sont actuellement menacées d'ensablement en raison de la dégradation du couvert végétal et d'un accroissement de l'efficacité érosive des vents. Afin de comprendre les processus d'ensablement autour de ces cuvettes, une caractérisation de leurs formes et orientations a été entreprise. Le coefficient de compacité de GRAVELIUS a permis de mettre en évidence des cuvettes circulaires (8,5 %), ovales (46,8 %), amiboïdes (10,9 %), étirées (17,6 %) et très étirées à tendance amiboïde (16,1 %). Les cuvettes oasiennes de forme circulaire ou ovale renvoient à des drains partiellement comblés par les apports éoliens. Les formes étirées et très étirées sont liées à un morcèlement du réseau hydrographique par des édifices dunaires. Enfin, la mesure de l'orientation a permis d'apprécier la distribution entre quatre secteurs majeurs : E-O (22,8 %), N-S (21,4 %), NE-SO (21,6%) et SE-NO (34,3 %). Les cuvettes orientées E-O et NE-SO sont associées à des dunes longitudinales et celles orientées N-S et SE-NO à des dunes transversales