Assessment of the pollution in Aghien lagoon and its tributaries (Côte d’Ivoire, West Africa)

International audienc

NOUVELLE INCERTITUDE LIÉE AUX OCCURRENCES DE SÉQUENCES AVEC OU SANS PLUIE : IMPACT SUR LES RENDEMENTS AGRICOLES DANS LA RÉGION SUD-EST

International audienceWith its large industrial plantations, the current climate change is strongly weakening the agroecological systems of Côte d'Ivoire. The study is based on daily and hourly rainfall observations at the Abidjan station. In particular, it seeks to identify how dry or rainy sequences have evolved with current climate change. The first results of this study clearly show a decrease in dry spells. However, on a monthly time resolution, this decrease of durations must be qualified according to the season. These trends, which are globally decreasing, contradict the agricultural practices of these regions, which imply being able to keep sufficiently long periods without soil humidity.Avec ses grandes plantations industrielles, le changement climatique actuel fragilise fortement les systèmes agro-écologiques de la Côte d’Ivoire. L’étude s’appuie sur des observations pluviométriques relevées à la station d’Abidjan, au pas de temps journalier et horaire. Elle cherche tout particulièrement à cerner comment les séquences sèches, ou pluvieuses, ont évolué sous la contrainte du changement climatique actuel. Les premiers résultats de ce travail montrent clairement une diminution des séquences sèches. A l’échelle mensuelle, il faut cependant nuancer cette érosion des durées selon les moments de l’année. Ces tendances, globalement à la baisse, viennent contrarier les pratiques culturales de ces régions, qui impliquent de pouvoir conserver une partie du temps des moments suffisamment longs dépourvus d’humidité

NOUVELLE INCERTITUDE LIÉE AUX OCCURRENCES DE SÉQUENCES AVEC OU SANS PLUIE : IMPACT SUR LES RENDEMENTS AGRICOLES DANS LA RÉGION SUD-EST

International audienceWith its large industrial plantations, the current climate change is strongly weakening the agroecological systems of Côte d'Ivoire. The study is based on daily and hourly rainfall observations at the Abidjan station. In particular, it seeks to identify how dry or rainy sequences have evolved with current climate change. The first results of this study clearly show a decrease in dry spells. However, on a monthly time resolution, this decrease of durations must be qualified according to the season. These trends, which are globally decreasing, contradict the agricultural practices of these regions, which imply being able to keep sufficiently long periods without soil humidity.Avec ses grandes plantations industrielles, le changement climatique actuel fragilise fortement les systèmes agro-écologiques de la Côte d’Ivoire. L’étude s’appuie sur des observations pluviométriques relevées à la station d’Abidjan, au pas de temps journalier et horaire. Elle cherche tout particulièrement à cerner comment les séquences sèches, ou pluvieuses, ont évolué sous la contrainte du changement climatique actuel. Les premiers résultats de ce travail montrent clairement une diminution des séquences sèches. A l’échelle mensuelle, il faut cependant nuancer cette érosion des durées selon les moments de l’année. Ces tendances, globalement à la baisse, viennent contrarier les pratiques culturales de ces régions, qui impliquent de pouvoir conserver une partie du temps des moments suffisamment longs dépourvus d’humidité

Statistique appliquée aux images NOAA pour le suivi du couvert végétal des bassins versants d'Aghien et de la Mé

Zone pionnière de l'agriculture de plantation cacaoyère et caféière, les bassins versants d’Aghien et de la Mé ont très vite fait l'objet de déforestation intense. Le paysage phytogéographique n'a cessé de subir des transformations depuis l’indépendance de la Côte d’Ivoire en 1960. Ces mutations paysagères sont liées aux intérêts pécuniaires des différentes cultures pérennes au fil des années (le café, le cacao, l'hévéa, le palmier à huile et la banane douce). L'étude de l'évolution du couvert végétal des bassins versants d'Aghien et de la Mé de 1982 à 2013 vise à faire le bilan des transformations spatio-temporelles des unités paysagères de ces deux bassins. Pour réaliser cette étude, ont été utilisés conjointement la télédétection, des tests statistiques de tendances et de rupture et des enquêtes de terrain. Des images journalières d'indice de végétation NDVI (Normalized Difference Vegetation Index) de NOAA (National Oceanic and atmospheric Administration) ont été compilées. Ainsi, des analyses statistiques descriptives et des tests de stationnarités ont été réalisés à partir des valeurs des pixels de ces images. Les résultats de ces tests produits sous la forme cartographique ont été investigués sur le terrain. Les résultats de l'étude indiquent, au niveau temporel, une période principale de rupture comprise entre 1993 et 1997. En outre, trois niveaux de discrimination spatiale se distinguent nettement : l'espace du bassin d'Aghien de plus en plus urbanisé ; le nord-est du bassin de la Mé, espace de grandes productions de cultures pérennes (Cacao, hévéa, café, palmier à huile, etc.) et le reste du bassin caractérisé par une mosaïque culture vivrière-culture d’exportation.A pioneering area of Ivorian cocoa and coffee plantation agriculture, the watersheds of Aghien and Mé were quickly subjected to intense deforestation. The phytogeographic landscape has been constantly changing since the country's independence in 1960. These landscape changes are linked to the pecuniary interests of different perennial crops over the years (coffee, cocoa, rubber, palm oil and sweet banana). The study of the evolution of the vegetation cover of the watersheds of Aghien and Mé from 1982 to 2013 aims to take stock of the spatio-temporal transformations of the landscape units of these two basins. To carry out this study, were used jointly remote sensing, statistical tests of trend and rupture and field surveys. Daily images of index of vegetation NDVI (Normalized Difference Vegetation Index) of NOAA (National Oceanic and atmospheric administration) were compiled. Thus, descriptive statistical analyzes and stationarity tests were carried out on the basis of the pixel values of these images. The results of these tests produced in the cartographic form have been investigated in the field. The results of the study indicate, at the temporal level, a major break between 1993 and 1997. In addition, three levels of spatial discrimination are clearly distinguishable : the increasingly urbanized area of the Aghien basin ; the northeast of the Mé basin, area of high production of perennial crops (cocoa, hevea, coffee, oil palm, etc.) and the rest of the basin characterized by a mosaic food crop-export crop

Statistique appliquée aux images NOAA pour le suivi du couvert végétal des bassins versants d'Aghien et de la Mé

A pioneering area of Ivorian cocoa and coffee plantation agriculture, the watersheds of Aghien and Mé were quickly subjected to intense deforestation. The phytogeographic landscape has been constantly changing since the country's independence in 1960. These landscape changes are linked to the pecuniary interests of different perennial crops over the years (coffee, cocoa, rubber, palm oil and sweet banana). The study of the evolution of the vegetation cover of the watersheds of Aghien and Mé from 1982 to 2013 aims to take stock of the spatio-temporal transformations of the landscape units of these two basins. To carry out this study, were used jointly remote sensing, statistical tests of trend and rupture and field surveys. Daily images of index of vegetation NDVI (Normalized Difference Vegetation Index) of NOAA (National Oceanic and atmospheric administration) were compiled. Thus, descriptive statistical analyzes and stationarity tests were carried out on the basis of the pixel values of these images. The results of these tests produced in the cartographic form have been investigated in the field. The results of the study indicate, at the temporal level, a major break between 1993 and 1997. In addition, three levels of spatial discrimination are clearly distinguishable : the increasingly urbanized area of the Aghien basin ; the northeast of the Mé basin, area of high production of perennial crops (cocoa, hevea, coffee, oil palm, etc.) and the rest of the basin characterized by a mosaic food crop-export crop

NOUVELLE INCERTITUDE LIÉE AUX OCCURRENCES DE SÉQUENCES AVEC OU SANS PLUIE : IMPACT SUR LES RENDEMENTS AGRICOLES DANS LA RÉGION SUD-EST

International audienceWith its large industrial plantations, the current climate change is strongly weakening the agroecological systems of Côte d'Ivoire. The study is based on daily and hourly rainfall observations at the Abidjan station. In particular, it seeks to identify how dry or rainy sequences have evolved with current climate change. The first results of this study clearly show a decrease in dry spells. However, on a monthly time resolution, this decrease of durations must be qualified according to the season. These trends, which are globally decreasing, contradict the agricultural practices of these regions, which imply being able to keep sufficiently long periods without soil humidity.Avec ses grandes plantations industrielles, le changement climatique actuel fragilise fortement les systèmes agro-écologiques de la Côte d’Ivoire. L’étude s’appuie sur des observations pluviométriques relevées à la station d’Abidjan, au pas de temps journalier et horaire. Elle cherche tout particulièrement à cerner comment les séquences sèches, ou pluvieuses, ont évolué sous la contrainte du changement climatique actuel. Les premiers résultats de ce travail montrent clairement une diminution des séquences sèches. A l’échelle mensuelle, il faut cependant nuancer cette érosion des durées selon les moments de l’année. Ces tendances, globalement à la baisse, viennent contrarier les pratiques culturales de ces régions, qui impliquent de pouvoir conserver une partie du temps des moments suffisamment longs dépourvus d’humidité

Activities and Uses of Aghien Lagoon (South-East of Côte d’Ivoire)

International audienc

Aghien lagoon: a sustainable resource of fresh water for the city of Abidjan (Ivory Coast)? Description of the project and preliminary results

International audienceWith more than 6 million inhabitants, Abidjan district faces tremendous difficulties in water supply. The aquifer ofthe Continental Terminal which is actually the only drinking water source of the city shows a decline of resourcesand water demand is increasing due to the population growth. Moreover significant evidences of chemical andbiological pollution of the groundwater are observed The Aghien lagoon, the largest freshwater pool located nearAbidjan, has been identified by the State of Côte d’Ivoire as a potential resource for the production of drinkingwater.. The main objective of this project is to assess the quantitative and qualitative capacity of the Aghien lagoonto complement the water supply of Abidjan city in the near future.The main components of the project are:• to assess the water budget of the lagoon and its tributaries,• to characterize the spatial and temporal variability of contaminant levels and fluxes from the tributaries towardthe lagoon,• to assess the sustainability of the water resources (quantity and quality) of the lagoon according to land usechanges in the catchments .The project started in January 2015. The first year was devoted to the set-up of hydro-meteorological gaugeswithin the lagoon watershed. Three major tributaries of the lagoon are considered, the Mé (4000 km2), the Djibi(78 km2) and Bete (206 km2) rivers. Since the start of the project, bi-monthly hydrochemical sampling surveyshave been carried out along the tributaries and in the lagoon. The data available from the surveys concern thephysico-chemical parameters, trace elements, all the forms of nitrogen and phosphorus, organic carbon, suspendedsolids.The Djibi and Bete watersheds are partly urbanized while the Mé basin is mainly rural. Baseflow has beenidentified as the major contribution to streamflow at the annual scale. The Mé flows into a channel downstream tothe Aghien lagoon but during the floods, water from the Mé River can flow up the channel and supply the Aghienlagoon. Discharge measurements have shown that over a period of 8 months Bete and Djibi Rivers renewed halfof the total volume of the lagoon. The quantification of the Mé contribution is in progress.In agreement with its high rate of urbanization, Djibi River is highly contaminated. High levels of ammonium andtotal phosphorus have been observed during both low and high flow conditions. Physico-chemical surveys haveshown also higher values of conductivity and lower values of dissolved oxygen in the Djibi River. Despite dilutioneffect due to the Mé river inflow, the Aghien lagoon is already hypertrophic. The lagoon presents a high biologicalactivity characterized by high values of pH, phosphorus concentration but above all chlorophyll a.The research in the lagoon is going on to characterize the temporal variability and to quantify the impact of themajor flood events on the water quality of the lagoon and its tributaries, urbanized or rural