Inverse Estuaries in West Africa: Evidence of the Rainfall Recovery?

Type Article
Date 2020-03
Language English
Author(s) Descroix Luc1, 2, Sané Yancouba2, 3, Thior Mamadou2, 3, Manga Sylvie-Paméla2, 3, 4, Ba Boubacar Demba2, 3, Mingou Joseph2, 3, Mendy Victor3, 5, Coly Saloum3, 5, Dièye Arame2, 3, Badiane Alexandre1, 2, 3, Senghor Marie-Jeanne2, Diedhiou Ange-Bouramanding2, Sow Djiby2, 3, Bouaita Yasmin2, Soumaré Safietou2, 6, 7, Diop Awa2, 8, Faty Bakary9, Sow Bamol Ali3, 5, Machu Eric5, 10, Montoroi Jean-Pierre11, Andrieu Julien2, 7, Vandervaere Jean-Pierre12
Affiliation(s) 1 : IRD UMR PALOC MNHN/IRD/Sorbonne-Université, 75231 Paris, France
2 : LMI PATEO, UGB, BP 234, Saint Louis 32002, Senegal
3 : UASZ Université Assane Seck de Ziguinchor, Ziguinchor BP 523, Senegal
4 : Université de Lorraine, UFR des Sciences Humaines et Sociales, 54015 Nancy, France
5 : LMI ECLAIRS, UASZ, ENS, Dakar BP 5036, Senegal
6 : LaSTEE, Ecole Polytechnique de Thiès, Thiès DPA 10, Senegal
7 : ESPACE Lab, Université Côte d’Azur, UFR Espaces & Cultures Campus, 06204 Nice, France
8 : Université Versailles St Quentin en Yvelines, UFR Sciences Sociales, 78280 Guyancourt, France
9 : DGPRE, Direction de la Gestion et la Planification des Ressources en Eau, Section 2, Diamniadio 20000, Senegal
10 : IRD/Laboratoire d’Océanographie Physique et Spatiale (LOPS), IUEM, Univ. Brest, CNRS, IRD, Ifremer, 29280 Plouzané, France
11 : IRD/IEES, Institut d’Ecologie et des Sciences de l’Environnement, 93143 Bondy, France
12 : IGE/Université Grenoble Alpes, Institut des Géosciences et Environnement, 38058 Grenoble, France
Source Water (2073-4441) (MDPI AG), 2020-03 , Vol. 12 , N. 3 , P. 647 (25p.)
DOI 10.3390/w12030647
Note This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology
Keyword(s) water salinity, inverse estuaries, West Africa, drought, mangrove
Abstract

In West Africa, as in many other estuaries, enormous volumes of marine water are entering the continent. Fresh water discharge is very low, and it is commonly strongly linked to rainfall level. Some of these estuaries are inverse estuaries. During the Great Sahelian Drought (1968–1993), their hyperhaline feature was exacerbated. This paper aims to describe the evolution of the two main West African inverse estuaries, those of the Saloum River and the Casamance River, since the end of the drought. Water salinity measurements were carried out over three to five years according to the sites in order to document this evolution and to compare data with the historical ones collected during the long dry period at the end of 20th century. The results show that in both estuaries, the mean water salinity values have markedly decreased since the end of the drought. However, the Saloum estuary remains a totally inverse estuary, while for the Casamance River, the estuarine turbidity maximum (ETM) is the location of the salinity maximum, and it moves according to the seasons from a location 1–10 km downwards from the upstream estuary entry, during the dry season, to a location 40–70 km downwards from this point, during the rainy season. These observations fit with the functioning of the mangrove, the West African mangrove being among the few in the world that are markedly increasing since the beginning of the 1990s and the end of the dry period, as mangrove growth is favored by the relative salinity reduction. Finally, one of the inverse estuary behavior factors is the low fresh water incoming from the continent. The small area of the Casamance and Saloum basins (20,150 and 26,500 km² respectively) is to be compared with the basins of their two main neighbor basins, the Gambia River and the Senegal River, which provide significant fresh water discharge to their estuary.

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Descroix Luc, Sané Yancouba, Thior Mamadou, Manga Sylvie-Paméla, Ba Boubacar Demba, Mingou Joseph, Mendy Victor, Coly Saloum, Dièye Arame, Badiane Alexandre, Senghor Marie-Jeanne, Diedhiou Ange-Bouramanding, Sow Djiby, Bouaita Yasmin, Soumaré Safietou, Diop Awa, Faty Bakary, Sow Bamol Ali, Machu Eric, Montoroi Jean-Pierre, Andrieu Julien, Vandervaere Jean-Pierre (2020). Inverse Estuaries in West Africa: Evidence of the Rainfall Recovery? Water, 12(3), 647 (25p.). Publisher's official version : https://doi.org/10.3390/w12030647 , Open Access version : https://archimer.ifremer.fr/doc/00612/72434/