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

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.

La population d’Avicennia germinans du delta du Saloum est-elle relictuelle depuis la dernière période humide ?

“…De 2015 à 2018, un ensemble de mesures de salinité ont été réalisées dans le delta du Saloum et du Casamance à l'aide d'un réfractomètre (Descroix et al, 2020) modèle PCE© 0100. Pour les valeurs les plus faibles, la conductivité a été mesurée au conductimètre (Hanna Hi92130©) pour vérifier et affiner au besoin la teneur en sel.…”

“…Les fluctuations plus récentes de la pluviosité de la région qui nous intéressent ici peuvent être ainsi résumées : une période humide de 1950 à 1967, une période sèche de 1968 à 1994 et une période normale par rapport à une moyenne sur le siècle de 1994 à aujourd'hui (Nicholson, 2005(Nicholson, , 2013Descroix et al, 2015). Ces fluctuations de la pluviosité modifient la salinité de l'eau libre (Descroix et al, 2020), mais aussi l'évaporation de l'eau dans les sols et la capacité de lessivage par les eaux de pluie. Durant la période sèche, la mangrove a reculé au profit des tannes (Marius, 1985 ;Bâ et al, 1999).…”

Monitoring land use and soil salinity changes in coastal landscape: a case study from Senegal

“…These results confirm the reduction of the extent of salt-affected areas registered in the different land use type in 2017. Such decrease of salinity could be related to the improvement of rainfall recorded in the area (Descroix et al, 2020 ) as well as the various adaptation and mitigation measures (e.g., anti-salt dams, revegetation and conservation of trees, use of manure and mulching, etc.) implemented by the local communities and some NGOs.…”

Special Issue “Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology”

“…The topics studied cover a large panel of hydrological questions. Many of them deal with the study of time series variability, rainfall [16], discharges [19,20], sediment transport [11][12][13]21], recession coefficient [22,23], climate change impacts [17,24], or anthropogenic impacts [2,11,20,21]. Drought is also a topic of notable interest in Mediterranean and tropical areas [23][24][25].…”

“…Descroix et al [24] explore the evolution of the salinity in the inverse estuaries of two Senegalese rivers in West Africa since the end of the 1968-1993 drought. The results show that in both estuaries, the mean water salinity values have markedly decreased since the end of the drought.…”