In tropical West Africa, distribution patterns of forest islands in savannas are influenced by fires which occur regularly in the grass stratum. Along continuous forest–savanna transects in the Comoé National Park, the change in the amount and composition of non-woody phytomass was investigated from savanna to forest interior. This was correlated with the cover of vegetation strata above, soil depth, and the occurrence of seasonal surface fires. Phytomass mainly consisted of leaf litter in the forests (about 400 g m−2 at the end of the rainy season, and about 600 g m−2 at the end of the dry season) and of grasses in the savanna (about 900 g m−2). Low grass biomass appeared to be primarily the result of suppression by competing woody species and not of shallow soil. The occurrence of early dry-season fires seemed to be determined mainly by the amount of grass biomass as fuel because fires occurred in almost all savanna plots while forest sites remained unaffected. However, late dry-season fires will encounter higher amounts of leaf litter raising fire probability in forests. Due to the importance of the amount of combustible phytomass, fire probability and intensity might increase with annual precipitation in both savanna and forest.

Résumé Ce travail a pour objectif de proposer une méthodologie de cartographie des accidents régionaux, basée sur une analyse combinée d'images radar et optique, afin de déterminer leur rôle dans les écoulements souterrains. Cette méthodologie est mise en oeuvre en zone de socle, dans la région de Dimbokro-Bongouanou (Côte d'Ivoire). Dans un premier temps, cette démarche méthodologique consiste à spatialiser les linéaments régionaux susceptibles d'être en relation avec la fracturation du socle à partir de l'analyse combinée d'images radar (Radarsat) et optique (Landsat), puis à valider ces linéaments aux moyens de données géologiques. Dans un second temps, une analyse permettant d'identifier le rôle de ces linéaments dans les circulations souterraines est proposée. Celle-ci consiste à étudier les corrélations entre la position des forages et le tracé des linéaments régionaux. Plus précisément, le débit des forages est analysé en fonction de leur distance au linéament le plus proche, ce qui permet de constater une augmentation du débit inversement proportionnelle à la distance entre forages ayant un débit supérieur à 5 m 3 /h à l'air lift (forages à fort débit) et linéaments majeurs. Aucune corrélation avec le débit des forages ayant un débit inférieur à 5 m 3 /h (forages à faible débit) n'est constatée. Cette corrélation négative, entre la productivité des forages à fort débit et la distance au linéament le plus proche, permet de confirmer que ces linéaments sont les expressions surfaciques d'accidents majeurs de la région et jouent un rôle primordial dans les écoulements souterrains.Mots clefs fracturation; images radar; Landsat; linéaments régionaux; productivité de forage; Dimbokro-Bongouanou, Côte d'IvoireMapping regional discontinuities and identification of their role in underground hydrodynamics in a base zone: case study of the Dimbokro-Bongouanou region, Ivory Coast Abstract This study aims to investigate a methodology to map regional discontinuities, based on a combined analysis of radar and optical images, to assess if they play a noticeable role as groundwater flow paths. This method is implemented in the region of Dimbokro-Bongouanou (Ivory Coast). In a first step, the methodological approach consists of spatial analysis of the regional lineaments likely to be related to the fracturing of the basement by means of combined analysis of radar (Radarsat) and optical (Landsat) images, and then validating these lineaments with the mean geological data. In the second step, analysis is proposed that enables the role of these lineaments in groundwater circulation to be identified. This consists of determining the correlation between the major lineament traces and the location of boreholes. The yield of boreholes is analysed as a function of their distance from the nearest lineament, which shows that borehole yield decreases proportionally with the distance between the lineament and high-yield boreholes (discharge greater than 5 m 3 /h). No correlation between distance to the lineament and lowyield boreholes ...

In granitic rocks, various models of weathering profile have been proposed, but never for the hard rocks of West Africa. Besides, in the literature there is no description of the weathering profile in volcanosedimentrary rocks. Therefore, we propose three models describing the weathering profiles in granites, metasediments, and volcanic rocks for hard rock formations located in West Africa. For each of these models proposed for granitic and volcanosedimentary rocks of the Dimbokro catchment, vertical layered weathering profiles are described, according to the various weathering and erosion cycles (specific to West Africa) that the geological formations of the Dimbokro catchment experienced from the Eocene to the recent Quaternary period. The characterization of weathering profiles is based on: i) bedrocks and weathering profile observations at outcrop, and ii) interpretation and synthesis of geophysical data and lithologs from different boreholes. For each of the geological formations (granites, metasediments, and volcanic rocks), their related weathering profile model depicted from top to bottom comprises four separate layers: alloterite, isalterite, fissured layer, and fractured fresh basement. These weathering profiles are systematically covered by a soil layer. Though granites, metasediments and volcanic rocks of the Dimbokro catchment experience the same weathering and erosion cycles during the palaeoclimatic fluctuations from Eocene to recent Quaternary period, they exhibit differences in thickness. In granites, the weathering profile is relatively thin due to the absence of iron crust which protects weathering products against dismantling. In metasediments and volcanic rocks iron crusts develop better than in granites; in these rocks the alterite are more resistant to dismantling.

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RESUMELes eaux de 61 puits au sud du District d'Abidjan ont été caractérisées afin de connaître la variabilité spatiale de la minéralisation des eaux du Quaternaire. Le variogramme et le krigeage ordinaire ont été utilisés pour analyser la distribution spatiale de la minéralisation. La détermination des facteurs qui gouvernent cette minéralisation s'est faite à partir de l'algorithme de Kohonen et du calcul de l'indice d'échange de base. Les eaux des zones urbanisées (Treichville, Marcory, Koumassi et Port-Bouët) et celles situées en bordure de mer et des lagunes sont fortement minéralisées tandis que les zones rurales sont caractérisées par des eaux de faible minéralisation. Quatre facteurs expliquent cette répartition spatiale de la minéralisation : les échanges de bases et la dissolution des sels marins sont observés sur l'ensemble du site ; en revanche, la pollution anthropique et l'intrusion d'eaux marine et lagunaire traduites respectivement par les fortes concentrations de nitrate et de chlorure, affectent les eaux des zones urbanisées et celles en bordure des lagunes et de l'océan atlantique.

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The pollution generated by human activities in the area "Vridi canal-Grand Bassam" puts more at risk the quaternary groundwater that supplies water to a large part of the population of the southern part of Abidjan city. This paper deals with the combined approach of geographic information systems and multicriteria analysis to delineate areas at risk of pollution of groundwater in the study area. The multicriteria analysis also incorporated the statistical and geostatistical techniques for defining the factor ratings and weightings of all the parameters under a GIS environment. The criterion for these factor ratings and weightings was the correlation coefficient of each parameter with the nitrates concentration in groundwater. Two indicators were combined for creating situations of pollution risks according to their importance: the intrinsic vulnerability indicators which combined the depth to groundwater, the net recharge, the slope, the soil media, the unsaturated hydraulic conductivity and extrinsic vulnerability characterized by the land use. The results showed that all to the site presents nitrate pollution risks with 50.7% of moderate risk, and 49.3% of high and very high risks. The areas which present the highest risks of pollution are the densely urbanized communes of Treichville, Marcory, Koumasi, Port-Bouët and Grand-Bassam. The sensitivity analysis indicates that parameters causing high sensitivity on the nitrate pollution risks map are the depth to groundwater and the recharge. The errors committed on the maps establishment are ±0.15%, ±0.40% and ±6% respectively for IV map, EV map and pollution risk map.

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