Evaluation Of Machine Learning Classification Methods For Rice Detection Using Earth Observation Data: Case Of Senegal

Agriculture is considered one of the most vulnerable sectors to climate change. In addition to rainfed agriculture, irrigated crops such as rice have been developed in recent decades along the Senegal River. This new crop requires reliable information and monitoring systems. Remote sensing data have proven to be very useful for mapping and monitoring rice fields. In this study, a rice classification system based on machine learning to recognize and categorize rice is proposed. Physical interpretations of rice with other land cover classes in relation to the spectral signature should identify the optimal periods for mapping rice plots using three machine learning methods including Support Vector Machine (SVM), Random Forest (RF), and Classification and Regression Trees (CART). The database is composed of field data collected by GPS and high spatial resolution (10 to 30 m) satellite data acquired between January and May 2018. The analysis of the spectral signature of different land cover show that the ability to differentiate rice from other classes depends on the level of rice development. The results show the efficiency of the three classification algorithms with overall accuracies and Kappa coefficients for SVM (96.2%, 94.5%), for CART (97.6%, 96.5%) and for RF (98% 97.1%) respectively. Unmixing analysis was made to verify the classification and compare the accuracy of these three algorithms according to their performance

Identification of suitable zones for manual drilling using borehole data, thematic maps and remote sensing

Manual drilling is a possible option to increase access to safe water with low cost techniques, but it can be applied only where hydrogeological conditions are suitable. To improve the method to produce maps of suitable zones for manual drilling, a research project has been carried out in Senegal and Guinea. The main objective is to elaborate a new method of interpretation of hydrogeological data and integrate indirect environmental information obtained from public data, available all over the world. The final results are more reliable and detailed maps to support manual drilling implementation, as well specific tools and method to process water point data. This paper presents the results obtained in Senegal and suggests some recommendations for future application

Multi-Parameter Statistical Analysis of K, Th, and U Concentrations in Eastern Senegal: Implications for the Interpretation of Airborne Radiometrics

In geological mapping, maps of K, Th, and U concentrations provided by airborne radiometric surveys are widely used to delineate geological units in tropical regions from the few rare outcrops. Indeed, thanks to their specific geochemical properties and behaviors, K, Th, and U allow us to trace geological processes. However, the combination of the concentrations of these radioelements does not allow us to determine the lithology in a unique way. We examined the potential of delineating the statistical parameters of K, Th, and U concentrations for geological mapping using the purpose airborne radiometric data in eastern Senegal. The mean, standard deviation, skewness, and kurtosis were calculated and mapped at a baseline of 3000 m. We noted the narrow dispersion of skewness and kurtosis values away from the expected curve for the log-normal distribution, implying that log-normal distributions dominate at the scale of analysis. The higher moments (kurtosis and skewness) varied more over shorter distances than lower order moments (mean and standard deviation). Mixtures of log-normal distributions across some lithological contacts with large differences in statistical parameters may account for this behavior. The area covered by the airborne radiometric data was classified into eight units according to the statistical parameters. The eight clusters do not show obvious correlations with geological units, but they may be interpreted in terms of the superposition of lithology and recent superficial processes (erosion and weathering)

Do Agrometeorological Data Improve Optical Satellite-based Estimations of Herbaceous Yield in Sahelian Semi-Arid Ecosystems?

Quantitative estimates of forage availability at the end of the growing season in rangelands are helpful for pastoral livestock managers and for local, national and regional stakeholders in natural resource management. For this reason, remote sensing data such as the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) have been widely used to assess Sahelian plant productivity for about 40 years. This study combines traditional FAPAR-based assessments with agrometeorological variables computed by the geospatial water balance program, GeoWRSI, using rainfall and potential evapotranspiration satellite gridded data to estimate the annual herbaceous yield in the semi-arid areas of Senegal. It showed that a machine-learning model combining FAPAR seasonal metrics with various agrometeorological data provided better estimations of the in situ annual herbaceous yield (R² = 0.69; RMSE = 483kg•DM/ha) than models based exclusively on FAPAR metrics (R² = 0.63; RMSE = 550kg•DM/ha) or agrometeorological variables (R² = 0.55; RMSE = 585kg•DM/ha). All the models provided reasonable outputs and showed a decrease in the mean annual yield with increasing latitude, together with an increase in relative inter-annual variation. In particular, the additional use of agrometeorological information mitigated the saturation effects that characterize the plant indices of areas with high plant productivity. In addition, the date of the onset of the growing season derived from smoothed FAPAR seasonal dynamics showed no significant relationship (0.05 p-level) with the annual herbaceous yield across the whole studied area. The date of the onset of rainfall however, was significantly related to the herbaceous yield and its inclusion in fodder biomass models could constitute a significant improvement in forecasting risks of a mass herbaceous deficit at an early stage of the year

Utilisation des données des capteurs MODIS et SPOT-VGT pour l’analyse de la dynamique des feux dans deux territoires (réserve protégée et unités pastorales) au Ferlo (Sénégal)

article publié en français (p. 16-28), avec un résumé étendu en anglais (p. 29-30), et 11 planches couleur hors-texte (p. 55-65)International audienceVegetation dynamics in the Sahelian area of Ferlo, Senegal is mainly driven by fire occurrence. Extent and intensity of savannah fires are dependent on land-use patterns; in this context, two studied areas were selected : South Ferlo (natural protected area) and Kanel – UP (managed grazing units). Two data sources have been used to monitor vegetation dynamics from 2001 to 2013 in Ferlo area : burned area maps derived from MODIS data and NDVI from SPOT-VEGETATION. MODIS data from Terra and Aqua satellites are available for active fire detection and burned areas assessment. SPOT VEGETATION dekadal NDVI data (S10) allow seasonal vegetation phenology monitoring and year to year comparison. Vegetation cover trends must be related to rainfall trend in the area, which can be described as a decrease of rainfall amount since the middle of 20th century. However, there is a relative stabilization of annual rainfall from 1990 to present. Main vegetation units are of savannah type : herbaceous shrubby savannah, wooded shrubby savannah, wooded or forested savannah. Other land-cover types are wooded shrubby steppe, degraded gallery forest and bare soil, rainfed crops and fallow land. Methodological approaches are the following :- detection of fires is based upon MODIS binary burned areas maps of maximum montly extent of fires : october (South Ferlo) or november(Kanel-UP;- cumulated fire occurrences from 2001 to 2013 is obtained to derive a multi-annual fire pattern in both areas;- ISODATA classification of monthly NDVI in 2 classes (low and high NDVI);- comparison of NDVI and fire seasonal dynamics (dekadal data) along 2 transects (North-South and West – East) in the 2 studied areas.Results of burned areas study indicate a higher extent of burned areas in the protected area of South Ferlo (excepted for years 2003-2005 and2012-2913. This may be due to the fact that vegetation patches are smaller and more diversified in the grazing units. Results of monthly extents of fires in the 2 areas show maximum values in october or november. Cumulated interannual fire occurrences have also been mapped. It may be concluded that fire occurrences are higher in the Kanel-UP grazing units, especially in the south of the studied area. On an annual basis,results indicate large variations from one year to an other. It is noticed that the extent of fires is inversely related to rainfall amount : in rainy years, vegetation extent and density are is higher, inducing larger burned areas. From results of monthly NDVI classification in 2 classes and associated statistics, it can be concluded that NDVI is generally higher in the grazing units of Kanel-UP. This difference is noticed in the dry season, and less significant in the rainy season. Results of a comparative analysis of NDVI and fire occurrences along the 2 transects in the 2 studied indicate that the impact of fires is significant along the West-East transect, showing a delayed effect on vegetation regrowth and vegetation amount after the rainy season following the fires. In conclusion remote sensing data appear helpful to monitor vegetation dynamics in the Ferlo area and allow better understanding of complex interactions between fire disturbances, climate conditions and grazing pressure.La dynamique des feux au Sénégal s’opère dans des écosystèmes déjà fragilisés par des facteurs environnementaux passés et actuels. Dans cette étude, deux zones d’étude aux caractéristiques territoriales différentes sont choisies : le Ferlo-Sud (réserve protégée) et le Kanel-UP (zone d’unités pastorales) et deux types de données géospatiales couvrant la période 2001-2013 sont exploités : les données matricielles de feux (MODIS) et d’indice de végétation NDVI (SPOT VEGETATION). Les données MODIS sont adaptées pour la détection des feux actifs ; les données NDVI (synthèses décadaires) sont adaptées pour évaluer la dynamique de la couverture végétale. La méthodologie se base sur deux approches principales : une première approche cherche à évaluer les occurrences des patrons les plus brûlés au cours des 13 années de la série des données ; une seconde approche superpose pixel par pixel des images NDVI et des feux à partir de transects est-ouest et nord-sudpour chaque zone d’étude. Les résultats révèlent une occurrence plus significative de la répétition des feux annuels dans la zone de Kanel-UP, bien que, pour 8 années sur les 13 considérées, les surfaces brûlées s’avèrent plus importantes dans la zone protégée de Ferlo-Sud. L’analyse du NDVI moyen mensuel (2001-2013) montre une dichotomie entre une zone de Kanel-UP caractérisée par la classe de fortesvaleurs de NDVI et la zone du Ferlo-Sud qui recense un couvert végétal moins dense. Enfin, l’analyse comparative des feux et du NDVI traduit une forte pression des feux sur l’activité chlorophyllienne au Ferlo-Sud selon le transect est-ouest. En revanche, sur le transect nord-sud, la relation est quasi inexistante en raison de la faiblesse du nombre de feux détectés

ÉTUDE DE LA SAISONNALITÉ DES MESURES DES DIFFUSIOMÈTRES SCAT : APPORT AU SUIVI DE LA VÉGÉTATION AU SAHEL, CAS DU FERLO AU SÉNÉGAL

International audiencePrevious studies involving ERS scatterometer data have shown the potential of these instruments for monitoring surface parameters. The coarse spatial resolution of the data is widely compensated with the high temporal frequency of the measurements. These data could have a major importance for vegetation monitoring in western Sahel area. This region of Sahel is indeed characterized by a high variability of the parameters of the ground surface, changing from bare and dry soil surface in dry seasons to a dense herbaceous biomass during the rainy season. These changes should greatly influence the answers of scatterometer sensors. The continuity of ERS measures is obtained from the ASCAT scatterom-eter on board the METOP satellite. This sensor presents the same characteristics (polarization, wavelength resolution and angle of measure) as the ERS-1 and ERS-2 scatterometer. Our study consists in looking at the potential of the SCAT (ERS and ASCAT) data for the surveillance of a semi-arid Sahel zone, the Ferlo catchment, Senegal.Les études antérieures impliquant les données des diffusiomètres ERS ont montré le potentiel de ces instruments pour le suivi des paramètres de surface. Ces études tirent parti de la fréquence temporelle élevée de ces capteurs. Ces données pourraient être d'une importance capitale pour le suivi de la végétation dans la partie ouest du Sahel. En effet, cette région est caractérisée par une forte variabilité des paramètres au sol, passant d'un sol nu et sec en saison sèche à une biomasse herbacée dense pendant les saisons des pluies, ce qui influencerait énormément les réponses de ces capteurs. La continuité des mesures du diffusiomètre ERS est assurée depuis 2007 avec les acquisitions du diffusiomètre ASCAT à bord du satellite METOP. Ce capteur présente les mêmes caractéristiques (polarisation, longueur d'onde résolution et angle de mesure) que les diffusiomètres de ERS-1 et ERS-2. Notre étude consiste à regarder le potentiel des données SCAT (ERS et ASCAT) pour la surveillance d'une zone sahélienne semi-aride, la vallée du Ferlo, Sénégal

ÉTUDE DE LA SAISONNALITÉ DES MESURES DES DIFFUSIOMÈTRES SCAT : APPORT AU SUIVI DE LA VÉGÉTATION AU SAHEL, CAS DU FERLO AU SÉNÉGAL

International audiencePrevious studies involving ERS scatterometer data have shown the potential of these instruments for monitoring surface parameters. The coarse spatial resolution of the data is widely compensated with the high temporal frequency of the measurements. These data could have a major importance for vegetation monitoring in western Sahel area. This region of Sahel is indeed characterized by a high variability of the parameters of the ground surface, changing from bare and dry soil surface in dry seasons to a dense herbaceous biomass during the rainy season. These changes should greatly influence the answers of scatterometer sensors. The continuity of ERS measures is obtained from the ASCAT scatterom-eter on board the METOP satellite. This sensor presents the same characteristics (polarization, wavelength resolution and angle of measure) as the ERS-1 and ERS-2 scatterometer. Our study consists in looking at the potential of the SCAT (ERS and ASCAT) data for the surveillance of a semi-arid Sahel zone, the Ferlo catchment, Senegal.Les études antérieures impliquant les données des diffusiomètres ERS ont montré le potentiel de ces instruments pour le suivi des paramètres de surface. Ces études tirent parti de la fréquence temporelle élevée de ces capteurs. Ces données pourraient être d'une importance capitale pour le suivi de la végétation dans la partie ouest du Sahel. En effet, cette région est caractérisée par une forte variabilité des paramètres au sol, passant d'un sol nu et sec en saison sèche à une biomasse herbacée dense pendant les saisons des pluies, ce qui influencerait énormément les réponses de ces capteurs. La continuité des mesures du diffusiomètre ERS est assurée depuis 2007 avec les acquisitions du diffusiomètre ASCAT à bord du satellite METOP. Ce capteur présente les mêmes caractéristiques (polarisation, longueur d'onde résolution et angle de mesure) que les diffusiomètres de ERS-1 et ERS-2. Notre étude consiste à regarder le potentiel des données SCAT (ERS et ASCAT) pour la surveillance d'une zone sahélienne semi-aride, la vallée du Ferlo, Sénégal

Cartographie De La Dynamique Historique Du Trait De Côte Des Plages De Guédiawaye Et Malika (Dakar, Sénégal)

L’érosion côtière est une des causes de la disparition des plages, du recul des falaises et de la perte de terrains à valeur socio-économique et écologique. La compréhension de la dynamique côtière est donc primordiale avant d’entreprendre toute politique d’aménagement du littoral. Cette étude se propose de retracer la dynamique historique du trait de côte de la zone de Malibu à Malika au Nord de Dakar qui subit actuellement une forte pression humaine avec l’extension de la Voie de Dégagement Nord. Elle est basée sur l’analyse de données de télédétection au moyen du logiciel ArcGIS 10.4 et de l’extension DSAS 4.3 (Digital Shoreline Analysis System). Les résultats obtenus laissent apparaitre, entre 1942 et 2011, une évolution du trait de côte en faveur d’une érosion avec une vitesse moyenne de -0,15 m/an. Cette évolution, qui n’est pas uniforme d’un secteur à un autre mais aussi d’une période à une autre, serait essentiellement due à l’action des agents hydrodynamiques qui gouvernent les mouvements sédimentaires. Néanmoins, la pression humaine commence à se faire sentir avec l’extension de la Voie de Dégagement Nord, les extractions de sables et la coupe des filaos au profit des constructions humaines dont les conséquences sur la dynamique côtière peuvent être alarmantes. Coastal erosion is one of the causes of the disappearance of beaches, retreat of cliffs and loss of land with socio-economic and ecological value. Understanding coastal dynamics is therefore essential before undertaking any coastal development policy. This study aims to retrace the historical dynamics of the coastline from the Malibu to Malika area located in the north of Dakar which is currently under strong human pressure with the extension of the Northern Clearance Way. It is based on the analysis of remote sensing data using ArcGis 10.4 and the DSAS 4.3 extension (Digital Shoreline Analysis System). The results obtained show, between 1942 and 2011, an evolution of the coastline in favor of erosion with an average speed of -0.15 m/year. This evolution, which is not uniform from a sector to another, but also from one period to another, is essentially due to the hydrodynamic agents which govern sediment movements. However, human pressure is starting to be felt with the extension of the Northern Clearance Way, sand extraction and the cutting of casuarinas for human constructions benefit. The consequences of such practices on coastal dynamics can be alarming

Bosumtwi impact structure, Ghana: Evidence for fluidized emplacement of the ejecta

International audienceThe about 10.5 km diameter Bosumtwi impact crater is one of the youngest large impact structures on Earth. The crater rim is readily noticed on topographic maps or in satellite imagery. It defines a circular basin filled by water (Lake Bosumtwi) and lacustrine sediments. The morphology of this impact structure is also characterized by a circular plateau extending beyond the rim and up to 9-10 km from the center of the crater (about 2 crater radii). This feature comprises a shallow ring depression, also described as an annular moat, and a subdued circular ridge at its outer edge. The origin of this outermost feature could so far not be elucidated based on remote sensing data only. Our approach combines detailed topographic analysis, including roughness mapping, with airborne radiometric surveys (mapping near-surface K, Th, U concentrations) and field observations. This provides evidence that the moat and outer ring are features inherited from the impact event and represent the partially eroded ejecta layer of the Bosumtwi impact structure. The characteristics of the outer ridge indicate that ejecta emplacement was not purely ballistic but requires ejecta fluidization and surface flow. The setting of Bosumtwi ejecta can therefore be considered as a terrestrial analog for rampart craters, which are common on Mars and Venus, and also found on icy bodies of the outer solar system (e.g., Ganymede, Europa, Dione, Tethys, and Charon). Future studies at Bosumtwi may therefore help to elucidate the mechanism of formation of rampart craters