Groundwater recharge estimation using chloride,stable isotopes and tritium profiles in the sands of northwestern Senegal

A study of environmental chloride, deuterium, oxygen-18, and tritium in deep sand profiles (35 m) has been carried out in order to estimate their relative value for measuring average groundwater recharge. The investigation was located at a 0.1-km² site in Quaternary sands near the northwestern coast of Senegal in a zone of rainfed agriculture. By using a steady-state model for duplicate unsaturated zone chloride profiles, the long-term average recharge at the site was estimated to be 30 mm yr^–1 or around 10% of the average precipitation (290 mm). The chloride concentration of adjacent shallow groundwater was relatively uniform and comparable to the unsaturated zone average, while the spatial variability in the depth distribution of Cl^– in the unsaturated zone was considerable. Stable isotope (deuterium and oxygen-18) data show that there is some isotopic enrichment due to direct evaporation through the soil surface. The degree of heavy isotope enrichment is proportional to the extent of evaporative loss and there is good correspondance with the chloride enrichment. Nevertheless, stable isotopes cannot be used quantitatively to estimate the recharge. The excellent preservation of the peak in thermonuclear tritium in precipitation in the unsaturated zone at depths between 12 and 20 m enables an estimated annual recharge of 24 mm yr^–1 in this area to be calculated, using the piston flow model. Agreement therefore between Cl and³H as tools for recharge measurement is reasonable over the site.     

Feeding ecology and ontogenic diet shifts of juvenile fish species in an inverse estuary: The Sine-Saloum,Senegal

We examined the diet of juvenile fish of four species – Eucinostomus melanopterus (Gerreidae), Ethmalosa fimbriata (Clupeidae), Monodactylus sebae (Monodactylidae), and Sarotherodon melanotheron (Cichlidae) – between their appearance on the nursery ground and the end of the recruitment season, when they began to shift to deeper parts of the estuary. Fish were collected from the Sine-Saloum, an inverse estuary in Senegal, West Africa. Sampling was conducted at six sites located along the salinity gradient. The study was conducted in the context of an inverse estuary where increasing salinity leads to the disappearance of mangroves.     

Spatial and temporal variability of salinity in Casamance River Basin,Southern Senegal,West Africa

Salinity in the Casamance River Basin has been on the rise for quite some time. This is believed to be further accentuated by declining rainfall. The salinity patterns in time, space and frequency domains are analysed. These patterns are useful in designing measures not only for arresting the rise in salinity but also for mitigating it. © 1998 John Wiley & Sons, Ltd.     

Localities visited on Excursions during the Nordic Symposium on Frozen Ground Morphology

Johnsson, G.: Localities visited on Excursions during the Nordic Symposium on Frozen Ground Morphology. Geografisk Tidskrift 82: 99–103. Copenhagen, October 15, 1982.

The localities showing periglacial phenomena of various kinds that were demonstreated on September 7, 8 and 10 during the frozen ground symposia are described in this report with text and illustrations. The objects demonstrated were, i.a., periglacial valleys formed chiefly through gelifluction, an ice-wedge cast with infilling of lime, large ice-wedge casts of the most common type in south Sweden, lateglacial aeolian sand underlain by wind eroded stones and boulders, fossil periglacial ground surfaces with overlying sand with small ice-wedge casts and asymmetrical periglacial valleys.     

The Guinea-Bissau–Senegal maritime boundary dispute

This article discusses the role of negotiation, arbitration, and that of the International Court of Justice (ICJ) in resolving maritime boundary disputes in the Gulf of Guinea region. Primarily using the cases of Guinea-Bissau and Senegal, the paper highlights that joint maritime development agreements could be a better option for resolving existing maritime boundary disputes in the region rather than outright delimitation requests.     

F) Personalia

Abstract

Knowledge of the natural resources of a country is essential for the assessment of its development possibilities. This is particularly true for the evaluation of groundwater resources in a country like Uruguay, where less than 5 per cent of the potential is exploited and this in an inefficient way, and where no drastic future increase in demand is predicted.

In the present paper the generalized hydrogeological model of the country was reconstructed, and recharge values, obtained by an approach adapted from nearby areas, were introduced.     

Assessing the impacts of climate change on water resources of a West African trans-boundary river basin and its environmental consequences (Senegal River Basin)

The Senegal River Basin (SRB) is a shared watershed in West Africa which includes regions (the upper basin, valley, and delta), characterized by distinct environmental conditions. An important feature of the Senegal River flow volume historically was its inter- annual irregularity, which caused a major water resource constrain. This situation has been accentuated during the long-term drought (1969–1984) in the Sahel zone which highlights the vulnerability of food-producing systems to climate change and variability. SRB is undergoing fundamental environmental, hydrologic, and socioeconomic transitions and represents a good illustration of sensitivity to climatic variations and opportunities for adaptation. This paper aims to study water resources systems under stress from climate variability and change in the Senegal River Basin. The results show (1) through the compilation of available data, information and knowledge (sedimentological, climatical, geological, environmental, archeological, etc.), the chronological consequences of climate change during the past millennium in West Africa, and also (2) an analysis of the recent impacts and vulnerability to climate change in the SRB and finally (3) the adaptation strategies in the SRB in order to identify and resolve problems associated with this water scarcity and to address the potential for guaranteed agricultural production in this narrow geographic area.     

Understanding pastoral mobility: the case of Senegalese Fulani

Based on a case study from Sahelian Senegal, this paper analyses how various actors perceive the importance of pastoral mobility and presents issues of importance for understanding the use of mobility among Fulani of Ferlo. One knowledge system is a scientific one, the 'new rangeland paradigm'. According to this, pastoral mobility is a means to balance variability in dryland resources; hence, 'nature' is the point of departure. Another knowledge system is local pastoral knowledge. For the pastoralists, the well-being of their animals is the point of departure and mobility is used to ensure that the livestock are in good condition. The paper shows that it is important to distinguish between mobility of pastoralists and of their herd; even though the pastoralists of northern Senegal have become semi-sedentary, their herds are still quite mobile. The pastoralists are willing to move around within a small territory, which they consider their place, but are unwilling to employ large-scale mobility themselves. Mobility is not of importance for their ethnic identity and some use paid herders to care for their livestock. By looking at both knowledge systems, we achieve a better understanding of pastoral mobility and how this may change in the future.     

Impact of salinity on early life history traits of three estuarine fish species in Senegal

The adaptive mechanisms on the early life stages of fishes to hypersaline stress are still poorly understood and probably determine the resistance of a population to disruption, compared with other less plastic species. The Casamance River in Senegal is an ideal location to test the adaptation to salinity as a dam was built in 1998 to exclude saline water intrusion. This lowered the salinity from 70 to 5 upstream and 60 downstream. The salinity influence on the growth in the early life of three West African fish species (Ethmalosa fimbriata, Sarotherodon melanotheron, and Tilapia guineensis) was studied using the width of microstructures in the otoliths and the individual migratory behaviour analysed from strontium (Sr) to calcium (Ca) ratios in the otoliths. The Sr:Ca ratio was quantified along individual transects measured from the posterior edge of the otolith to the core. The fishes were sampled on both sides of the dam that separated water with low salinity upstream from metahaline and hyperhaline water downstream.The results showed that salinity has different influence on the growth of each species. Ethmalosa fimbriata has the highest growth during the first 180 days in the freshwaters, indicating growth inhibition in the hyperhaline areas. For the two other species no growth difference were found. The Sr/Ca ratio varied widely, in Tilapia and Sarotherodon from below the dam. Individual life histories were more heterogeneous than upstream and showed a crossing of the dam for some individuals which could reach half of the fishes analysed. On the contrary in E. fimbriata, despite the large range of salinity, identical Sr/Ca profiles were found both upstream and downstream. This indicated that Sr/Ca ratio was not appropriate to evaluate life history patterns linked to salinity for this specie.     

The influence of natural and anthropogenic factors on mangrove dynamics over 60 years: The Somone Estuary, Senegal

Although such ecosystems are fragile, this study shows that the anthropogenic damages inflicted on the mangrove forests of West Africa can be reversed over a relatively short time period if environmental conditions are favorable. The mangrove ecosystem of the microtidal Somone Estuary, Senegal, has undergone extreme changes during the last century. The area occupied by mangrove forest was estimated with a diachronic study by GIS for the period 1946-2006. Between 1946 and 1978, 85% of the area was progressively replaced by unvegetated mudflats in the intertidal zones and by barren area in the supratidal zones. Until 1990, this was mainly a result of traditional wood harvesting. The impact was exacerbated by the closing off of the estuary to the sea (1967-1969 and 1987) and by an extended drought (1970 onwards), which resulted in a lack of renewal of water, hypersalinization and acidification. The main factors controlling mangrove evolution in the Somone ecosystem, however, are anthropogenic. Until 1990, traditional wood cutting (for wood and oyster harvesting) was practiced by the local population. Between 1978 and 1989, a small area occupied by the mangroves was stabilized. Since 1992, a modification of mangrove logging and a new reforestation policy resulted in an exponential increase of mangrove area progressively replacing intertidal mudflats. Such success in the restoration of the ecosystem reforestation is supported by favorable environmental conditions: tidal flooding, groundwater influence, rainfall during the wet season, low net accretion rate of about 0.2-0.3 cm year⁻¹, and a ban on the cutting of mangrove wood. The rate of mangrove loss from 1946 to 1978 was 44,000 m² year⁻¹, but this has been offset by restoration efforts resulting in an increase in mangrove area from 1992 to 2006 of 63,000 m² year⁻¹.