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

Cheikh Bécaye Gaye , Moctar Diaw, Raymond Malou

Department of Geology, Faculty of Sciences and Techniques, University Cheikh Anta Diop, BOX: 5005 Dakar, Senegal

1 Introduction

The Senegal River Basin (SRB) is located in West Africa and covers a total area of 340,000 km2in North Senegal, West Mali, South Mauritania and the high plateaus of the Fouta Djallon massif in Guinea. The basin is positioned on a large Southwest–Northeast band within the area of 10°20'N–17°00'N and 7°00'W–12°20'W. This basin, drained by the Senegal River, is 1,800 km long and is the second largest shared watershed in West Africa after the Niger. Its three main tributaries––Bafing, Bakoye and Falémé all take their sources in the Massif of Fouta Djallon(Republic of Guinea). The Senegal River Basin includes three main regions (the upper basin, valley, and delta), with each region clearly characterized by distinct environmental conditions (Figure 1).

The river flow rate is determined mainly by rainfall in the upper basin, with a high-water stage from July to October and a low-water stage from November to June. The high-water season peaks at the end of August or beginning of September and quickly ends during October. An important feature of the Senegal River flow volume historically was its inter-annual irregularity, which posed a major water resource constraint. In particular, it posed a major problem for the valley, as it decreased the potential for guaranteed agricultural production in this narrow geographic area. The arable land area that could effectively be farmed after the flood varies between 15,000 and 150,000 ha, depending on the magnitude and duration of the flood.

Exceptionally high water levels caused widespread devastation in 1890, 1906, and 1950. Conversely, years with extremely reduced water flow were also disastrous, since they did not provide for sufficient agricultural production in the valley. In particular, the 1968–1974 drought in the Sahel zone was particularly devastating for the populations and economy of the riparian region (Nicholson, 1989).

Figure 1 Location of the Senegal River Basin (Source: OMVS, 2002)

To address the problems associated with this water scarcity, three of the four bordering countries (Mali, Mauritania,and Senegal) agreed to form in 1972, the Senegal River Authority, the Organisation pour la Mise en Valeur du Fleuve Sénégal (OMVS). In fact, OMVS is the result of a long process of initiatives dating back to the colonial era, with the establishment of the Mission d'Aménagement du Fleuve Sénégal (MAS) aimed at developing the basin of the Senegal River.

To meet these goals, OMVS has built a regional infrastructure consisting of two major dams and related facilities and structures. The Diama Dam, in Senegal, was completed in 1986, at the river’s mouth for anti-saline intrusion from the ocean. The Manantali Dam was completed a year later as a reservoir dam to control river discharge for agriculture,energy and navigational purposes.

The Senegal River Basin represents a good illustration of sensitivity to climatic variations and opportunities for adaptation. It is undergoing fundamental environmental, hydrologic, and socioeconomic transitions (Venemaet al., 1997).The ongoing drought highlights the vulnerability of food-producing systems to climate change and variability.Adaptation to climate change should therefore increase the sustainability of agriculture under a long-term drought. Progress towards sustainability and adaptation in the Senegal River Basin is hampered by an existing set of social and ecological relationships that define the control over the means of production and how people interact with their environment. These relationships are sensitive to technological inputs and the administration of food production, or factor bias in different policy alternatives for rural development.

Recognizing this, the three governments through OMVS have embarked on the implementation of a program called PASIE (Plan d'Atténuation et de Suivi des Impacts sur l'Environnement). Although, specifically designed to address,monitor and mitigate environmental issues raised by, and related to, the development and distribution of power from the Manantali generating station, PASIE will also support the development of revised operating procedures (through the development of a "Water Charter") which will provide the opportunity for an integrated approach to river management. A number of trans-boundary issues including groundwater evaluation, land degradation and its related impacts (deforestation, erosion, overgrazing and desertification), food security through irrigation, biodiversity conservation, wetland management and conservation, environmental health concerns were identified by OMVS and its partners including civil societies, as priority concerns.

The objectives of the issue paper are to:

(1) compile available data, information and knowledge on the water resources of the Senegal River Basin;

(2) provide understanding regarding the role of groundwater in the Senegal River Basin’s water resources systems under stress from climate variability and change;

(3) assess the implications for sustainable utilization and management of this shared groundwater resources.

2 Water resources

The major part of the Senegal River Basin has a sub-Saharan desert climate, which has been aggravated by more or less long periods of drought during the 1970s. Annual rainfall varies largely over the basin, with high variability between wet and dry seasons and also from year to year.The river’s flow regime depends mainly on rainfall in the upper basin of Guinea (about 2,000 mm/a). In the valley and delta, rainfall is generally low and there is rarely more than 500 mm/a. The climatic regime in the basin can be divided into three seasons: rainy season from June to September,cold-dry off-season from October to February, and a hot-dry off-season from March to June. This creates a high-water period or flood stage between July and October and a low-water period between November and May/June. Table 1 summarizes the major climatic parameters while the main hydrological characteristics are given in Table 2.

Table 1 Major climatic parameters in Senegal River Basin

Table 2 Hydrological characteristics of the Senegal River Basin

Despite the presence of a potentially large groundwater supply, riverine communities throughout the Senegal River Basin remain dependent economically and socially upon the river’s floods. Although studies on the Maastrichtian and other Cretaceous sub-surface water bodies are underway, the process is unsustainable due to the interruption in monitoring and deterioration of the network just after the OMVS project, despite possible benefits from its use, and this should be rectified. However, since early 2000 new efforts are being carried out under OMVS which put in place environmental observations that include surveying of groundwater. Groundwater resources are stored in the following aquifer systems (Figure 2):

(1) Maastrichtian Formation: is present in the Senegalese and Mauritanian portion of the basin in the regions of Aleg,Bogue and Kaedi, throughout the valley on the right bank and between Ourossogui and Thilogne on the left bank;

(2) Eocene aquifers: they are mainly sandy limestone or marly formations and are present in Mauritania (the Brakna aquifer) and in Senegal;

(3) Continental Terminal formations: formed in sand,sandstone with a discontinuous network of clayey lenticular aquifers, and are present in the right bank (Trarza Formation)and left bank (Ferlo Formation);

(4) Alluvial aquifers: made partly of clay and fine sand,and represent Post Nouakchottian deposits, also they form coarse and gravelly alluvium, clayey sand dated from the Ogolian Period and the lower and middle Quaternary. The alluvial aquifer is the principal bed of the river and its water flow varies with the high and low flow stages of the river;

(5) Base rock aquifers identified in Mali and Guinea. In Mali, they form discontinuous fractured or inter-granular types of deep aquifers and in Guinea they represent semi continuous fractured types of in place aquifers with an inter-granular shallow aquifer.

The effects of the Senegal River Basin development project introduced by OMVS, on groundwater resources are particularly appreciable to the alluvial aquifer. Understanding the effects of these water projects together with climatic changes on groundwater resources requires long-term monitoring of the aquifers. Previous investigations (Thiandoum,1994) indicate an increase of groundwater recharge further away from the river and a decrease in the area of natural recharge zones near the river after completion of the dams(Figure 3). The development of irrigated areas such as rice paddy fields that are water filled for several months, promotes deep percolation of surface water to groundwater. The estimated infiltration rate (beyond the root layer) in irrigated areas ranges between 30 and 100 mm/a depending on the type or scope of the irrigated zone. The delta area is experiencing a high rate of mineralization (marine salt intrusion in the groundwater) during high water flows. In the valley and upper basin, the waters are less mineralized. However, the exploitation of traditional wells has caused local pollution along with activities in the upper basin near the catchment areas. These activities include: gold-mining, anthropogenic pollution, pollution of the river banks when they are impermeable and during flood periods. This degradation has profound effects on animal and human waterborne diseases as well as on hydro-agricultural activities.

Figure 2 Hydrogeological map of the Senegal River Basin (Source: OMVS- SOE/C13, 2007)

3 Socio-economic conditions

3.1 Population

The population of the Senegal River Basin is estimated to be about 3,500,000 people, up to which 85% lives very close to the river banks. A large ethnic diversity characterizes the basin’s population, with, among others, Peuls,Toucouleurs, Soninkes, Malinkes, Bambaras, Wolofs and Moors. In connection with extended drought and the development of irrigated areas for rice that followed the building of hydro-agricultural facilities, an important migratory flux has been noticed towards the valley which represents about a tenth of the whole surface of the basin.This influx was especially favored by the implementation of a new agrarian and institutional policy (agro-industry, availability of agrarian credit, incentive measures) and the existence of a propitious infrastructural environment. Thus, the rate of population increase in the valley (around 1,350,000 inhabitants in 1999) was comparable to that of new developments and control functioning of the river and explains the vital nature of the region and its water resources, around which all traditional activities (agriculture, fishing and farming) are organized.

3.2 Agriculture

Rural communities in the Senegal Valley have traditionally engaged in a mix of farming, fishing, and herding activities, relying heavily on a customary form of recessional floodplain farming.

The predominant economic activity in the basin was rain fed agriculture (production of millet, sorghum, corn and sweet potatoes) in the uplands (Jeeri) and flooding agriculture (production of sorghum) in the lowlands (Waalo). These traditional agricultural practices based on rain and flood agriculture were deemed not to be productive enough to support the population. After the dry spells of the seventies,traditional agricultural practices in the river basin were progressively replaced with an intensive irrigated agriculture,especially in the valley. Irrigated agriculture saw a period of significant expansion thanks to the creation of the Société Nationale d'Aménagement et d'Exploitation des Terres du Delta (SAED) in Senegal in 1965 and the Société Nationale pour le développement Rural (SONADER) in Mauritania in 1975. These governmental organizations were created with the intention of developing irrigated rice production, assuming the functions of supervising the farmers, promoting new techniques and providing training and support to the rural organization. The aim was therefore to increase production but also to maintain the rural population in a context of strong rural exodus.

Figure 3 Comparison of water levels in the alluvial aquifer from 1971–1973 to 1989–1993 (Thiandoum, 1994). This figure shows a decline in water levels of 2 to 5 m as a consequence of the Sahelian drought of the 1970s.

At present, after completion of the dams (1986–1988),irrigation constitutes the basin’s main mode of development,notably in the valley and delta, thanks to technological advancements, but also due to the emergence and diversification of production patterns. This included the growing of tomatoes, green beans, onions and sweet potatoes in the cold season, and rice and peanuts in the dry season. In the context of agricultural policy reforms in Senegal in 1984 and in Mauritania in 1989 and under the pressure of international backers, the para-statal development agencies progressively withdrew their efforts to the advantage of rural organizations and Economic Interest Groups (GIE). The latter benefited from a more flexible institutional and juridical environment which favored the emergence and expansion of different types of production patterns. As a result, around 100,000 hectares (ha) of land has been developed, with 60,000 ha cultivated in the wet season (June to September), and 20,000–40,000 ha in the dry season (March to June).

3.3 Farming

Animal husbandry was always a major economic activity in the valley. Through the rather high capacity of pastures located in the grassy plateaus and lowlands, riverside residents, as well as more distant populations, use transhumance practice and make extensive livestock farming a major focus of their activities.

3.4 Fishing

Fishing is the most important economic activity in the zone after agriculture, particularly for populations living near the river, in the valley and delta. Nevertheless, this sector is experiencing difficulties which threaten its future,since for many years the volume of catch in the entire OMVS area has been consistently decreasing. Observers tend to link this phenomenon with development projects in the river basin, particularly the dams and their environmental impacts which include lowering water salinity, as well as the proliferation of floating aquatic vegetative matter. Despite an effort to develop these activities, continental fishing and fish farming have not experienced expected growth.

3.5 Other economic activities

The hydroelectric plant of Manantali, which started production in September 2001, was supposed to generate 200 megawatts of electricity in order to provide the three OMVS countries with an average of 800 Gigawatt-hour(GWh) every year. However, these estimations were made based on hydrological data from 1950 to 1954. Estimates based on data from 1974 to 1994 suggest a much more conservative production figure of only 547 GWh. Considering these revised estimations, this hydroelectric plant will only provide 17% savings on energy expenditures instead of 22%as was originally expected by the OMVS States.

At present, the waterway network has not been extensively developed. Nevertheless, the OMVS States are very aware of the strategic importance of developing this network,with projects already under consideration aiming to expand this network. The opportunities that exist for using this network such as low-cost transportation of heavy goods as well as providing an outlet to the sea for Mali could provide a valuable new stimulus to the regional economy.

4 Climate change in West Africa

A consistent picture is now emerging for the climatic history of the region, notably periods of wetter or drier years,based upon careful multidisciplinary studies of the faunal record (diatoms, shells and other palaeoecological indicators), archaeological records, sedimentological records (notably offshore sediment accumulations on land are sparse),geochemical records (radiocarbon and stable isotope data)and historical records (from early chronicles and records of explorers and settlers).

At the scale of millennia, the palaeoclimatic history may be elegantly summarized by the diatom and sedimentation records of Lake Chad (Servant and Servant-Vildary, 1980)for which lake levels have been reconstructed over a period of 40,000 years. This is probably the most complete record for northern Africa and may be used to derive a palaeohydrological record expressed as P/E in Figure 4, wet climatic phases in order of decreasing importance have been identified as occurring between 9,000–8,000 a B.P., at 6,000 a B.P., between 3,000–35,000 a B.P., at 11,000 a B.P.,and from 40,000–20,000 a B.P. The intervening periods are considered periods of arid or hyper-arid climatic conditions,including the period around 3,000 a B.P. which has been identified as an extended period of relative aridity. These late Quaternary and Holocene climatic fluctuations are considered the results of either an incursions of polar air due to lower latitudes, or conversely the movement of the Inter-tropical Convergence Zone (ITCZ) further north over the Sahara. For the Central Sahara, tropical depressions are considered the likely source of rain up to the early Holocene (6,500 a B.P.) and with monsoon rains up to 4,400 a B.P. (Maley, 1977; Fabre and Petit-Mare, 1988). Evidence of the shift in climatic belts is also clearly demonstrated by the existence of stable late-Pleistocene sand dunes in areas now receiving 600–800 mm rainfall,e.g., in the Dakar area(Faure and Williams, 1977).

An important factor in the paleoenvironment of Senegal during the late Pleistocene/Holocene has been the lower sea level (minimum of 1.00 m around 18,000 a B.P.) rising to near present day levels (0 m) at 7,000 a B.P. (Lezine, 1986). Sedimentation in the "niayes" has been used to record the environmental changes in the region of central coastal Senegal.

When considering changes that occurred at the scale of centuries and in particular changes that occurred within the past 1,000 years, Lake Chad (Figure 5) offers the most complete climatic record (Maley, 1973; Servant and Servant-Vildary, 1980). Evidence from Senegal and Mauritania has also been compiled by Nicholson (1980) from various authors and is compared against data from Lake Chad,which acts as a "rain gauge". A major dry period occurred around 1,750 a B.P. in West Africa, and a dry period is also found to have occurred for Senegal around 200 A.D. These periods have been confirmed in other record (Edmundset al.,1991). Variations during the past millennium have been built up from sedimentological data and records from superficial deposits, showing that the period from 1600 A.D. to 1800 A.D. was much more humid than present day (Edmundset al., 1991).

The record for the past 200–300 years has been constructed from archives, chronicles and colonial reports (Nicholson, 1980). In Senegal and Gambia the chronology prior to 1850 has been established by Curtin (1968). In these records, no famines or droughts are mentioned before 1640.However, Curtin does confirm that the 16th Century was characterized by increased rainfall relative to the present in Senegal and southern Mauritania. In the 17th Century, this climatic situation persisted with the significant exception of a drought of ca. 1640. During the 16th and 18th centuries,Michel (1969) describes the period as the last "pluvial" in Senegal. Mangrove stands were recorded along the banks of the Senegal River and a more humid climate extended at least as far as southern Mauritania.

Figure 4 Paleoclimatic evolution of Lake Chad Basin over the last 40,000 years (Servant and Servant-Vildary, 1980)

Figure 5 Fluctuations diagram for the level of Lake Chad during the last millennium. A shift of one meter between the modern shoreline and that of 900 A.D. reflects the accumulation of sediment in the southern basin of the lake during the last 1,000 years (Maley, 1972).The numbers 1–8 correspond to the position of palynological samples, the Roman numbers I, II and III correspond to levels dated by radiocarbon and the letters a through i, represent various data historically dated.

Adanson’s description of Senegal in 1749–1755 suggests that humid conditions lasted until early 18th Century(Adanson, 1759). He records that the island of Sor, near the mouth of the Senegal River, was bordered by a very thick wood and thorny bushes. He describes forests on the river banks near Podor including tamarisks, redgum and thorny acacias, plus thriving imported citrus fruits. His maps show certain lakes in northern Senegal and southern Mauritania which have since dried up, plus a forest in southern Mauritania to where precipitation today is below 200 mm. He mentions the great drought of ca. 1749, a time which corresponds to the extremely severe Sahel drought of 1736–1758. He qualifies his statement by stating that the area near Saint-Louis and Podor had been rainless from December to June or July; a situation that is normal today.These descriptions imply both that the ITCZ advanced more rapidly northward to produce an earlier summer rainy season and that there was a more frequent occurrence of the "heug"rains.

The scientific records of climate history are also very good for Senegal and Gambia and are based upon rainfall measurements of Saint-Louis and Banjul plus Senegal River flow at Bakel, data which have been compiled by Olivry(1982). Rainfall records for Saint-Louis are the longest for West Africa, dating from 1854, although interrupted around 1880 due to an epidemic of yellow fever. For the years 1854–1892, it seems the period was generally wetter with an average rainfall of 400 mm/a, with the lowest rainfall recorded as 141 mm, in 1863, and 188 mm, in 1872. Reliable data is provided in Figure 6 for the period of 1893–2005.

Figure 6 Inter-annual variability in rainfall in the Senegal Basin at St. Louis (1893–2005) (Sources: Kane, 1985; H. Dacosta, Pers., comm.)

The periods of low rainfall in the 20th Century, were identified as those of 1913–1914 and 1941–1942, but both of these periods were accentuated on a regional scale by the record flow level of the Senegal River. A clear correspondence does, however, exist in the major drought period from 1968–1986. The long-term mean rainfall of 356 mm contrasts markedly with the 1968–1986 mean of 223 mm,which is a decrease of 37% on the long-term average from 1893–1986.

A comparison of annual rainfall amounts at seven regional locations in the Senegal Basin, including the most eastern one at Kayes, shows a dry phase decline of about 30%to 40% with the exception of the station at Bakel, which appears to remain stationary over this period (Figure 7).

Along with the global decrease in precipitation over the last 30 years, general climatic models predict an increase in the mean annual temperature of 1.0 to 1.9 °C by 2050 relative to baseline (1961–1990).

Average annual discharge of the Senegal River at Bakel is summarized for the period of 1903–2005 in Figure 8,where the weighted mobile mean value is also given, taking into account the variability between adjacent years. Two distinct periods of low flow are noticed around 1913 and 1943, in addition to the prolonged period of drought from the early 1970s. Magistro and Lo (2001) indicated an approximate halving of the annual flow from 30,000 to 15,000 million m3over the century.

Stream flow recorded at Bakel for the period of 1971–1990 is about half the flow recorded for 1951–1970(Lazenby and Sutcliffe, 1994). Three periods of low stream flow for the basin correspond to those of low rainfall:1911–1915, 1940–1944, and 1972–1992. This most recent phase of prolonged dryness (1972–1997) represents a deficit of 44% in annual average flow (12,373 million m3) when compared to the average (22,106 million m3) for the period of 1904–1997. Eighteen of the 20 driest years for the century were recorded during the period of 1972–1992, with average flow for 1984 (6,695 million m3) being the lowest for the century (Albergelet al., 1993).

The Sahel drought of 1968 falls well within the range of short and medium-term variability, that has been proven to have occurred over the past few centuries, and indirectly shown to have occurred for the past 12 millennia or so.Kates (1981) argued that the human impact of a similar drought in 1911–1914 was of similar or greater proportions than the 1968 drought. Other authors, however, believed that colonial and postcolonial changes in land use and sustained population growth since the turn of the century have dramatically increased pressures on the ecosystem, rendering it unusually vulnerable to periodic stress (Glantz,1976; Dalbyet al., 1977). Without addressing the issue of whether rapid technological and social transformations defined as "development" serve to increase systemic fragility, it is pertinent here to explore the degree to which recent desertification has had tangible geomorphic impact.Globally, a consensus has now emerged (IPCC, 2001) on the following points:

· a rise in global temperatures, that may be between 1.4 and 5.8 °C in 2100 compared to 1990;

· a decrease in rainfall of between 0.5% and 40%,with an average of 10% to 20% by 2025;

· an increase in extreme events (floods and droughts),but with considerable uncertainty as to where and when;

· a rise in sea level (0.5 to 1 m).

These changes as predicted by the General Atmosphere Circulation Models (MCGAs) will impact water resources in different ways including an intensification of the hydrological cycle; an increase in the frequency and scale of flooding; increasingly severe droughts; a depletion of groundwater (particularly of the alluvial aquifers); and a deterioration in water quality.

Figure 7 Inter-annual variability in rainfall in the Senegal Basin at seven regional stations (Sources: Kane, 1985; H. Dacosta, Pers., comm.)

Figure 8 Annual discharges (m3/s) of the Senegal River at Bakel, 1903–2005 (Sources: Kane, 1985; H. Dacosta, Pers., comm.)

5 Impacts and vulnerability to climate change in the SRB

Recent climatic research on variability, particularly of Glantz (1992), underscores the importance of situating climate change within the context of studies on seasonal and inter-annual climate variability. In the long term, linking climate change issues to climate variability issues will provide the best approach to the development of national and continent-wide strategies to deal with the yet-unknown regional consequences of a yet-unknown global climate change that may occur decades in the future.

The degradation of the environment in the West African Sahel, including the Senegal River Basin, is usually attributed to recent droughts, population pressures, or overgrazing, with little appreciation of the duration and spatial dimensions of man’s impact on Sahelian ecosystems. There is evidence of human occupation in the Sahel as early as 600,000 a B.P. Since that time, selective hunting and gathering, bush fires, agriculture, herding, charcoal production, the destructive exploitation of forest products, and other activities have contributed greatly to the modification of Sahelian ecosystems. These activities have led to progressive reductions in biological diversity and productivity and, in recent years, serious breakdowns in essential ecological processes.The restoration, enrichment, and sound management of Sahelian ecosystems will be basic to sustainable economic development in the region.

Because climatic change and variability are regular features of the Sahel, the native plant and animal communities of the region are generally well adapted to the range of climatic variation existing in the region. While the Sahelians are the victims of drought, they have also contributed greatly to their growing vulnerability. Many efforts in "development" or modernization have also contributed to their plight.An appreciation of man’s impact on Sahelian ecosystems is of basic importance, both to our understanding of the declining ability of the region to support human populations,and to the development of strategies to correct this trend.

In the Senegal River Basin, the Great Depression of 1934 led to the creation of the Mission d'Etude du Fleuve Senegal to study and comprehensively plan the economic development of the Senegal River Valley. This agency was later replaced by the Mission d'Aménagement du Sénégal.Initial plans concerned the production of cotton and in 1945 it was decided to shift exclusively to the mechanical cultivation of rice in the delta.

The earlier earthen barrages, built annually by traditional farmers of the region, were replaced by a permanent barrage completed near Richard-Toll in 1948. Water was then pumped, at considerable cost, from behind the barrage into approximately 100 km of main canals to 5,402 ha of mechanically cultivated and artificially fertilized rice fields. In 1968, some 5,100 ha produced 10,200 tons of paddies––a very low yield (Church, 1980). As with all large irrigation schemes in the Sahel, the Richard-Toll Senegal Delta Irrigation Scheme encountered great physical and economic difficulties. Quelea, ducks, and other granivorous birds, as well as insects and rodents consumed large quantities of the rice grown; the control of wild rice became a serious problem;the soils became saline; sheet erosion was caused by dry-season winds; and the costs of fertilizer and pumping became burdensome.

The Sahelian drought of the 1970s lead the three riparian countries to create OMVS and to proceed with the construction of two major dams in an attempt to develop irrigated agriculture, hydroelectric power and encourage river navigation.

5.1 Impacts of the dams

Niasse (2005) emphasized the ambiguous response of dams as a mean to cope with climate change. It is true that,by storing freshwater during seasons and years of abundance and making it available when needed, dams are a means to address scarcity and unreliability of water and achieve a dependable water supply. However, the operation of a dam can significantly affect the patterns and modalities of access to water as well as to other resources depending on it, and ultimately impact the conditions of access and use of water resources at the entire basin level. In the context of a trans-boundary basin, building of dams could result in changes in water allocation and use creating tensions and conflicts between countries.

So far, the dams have primarily benefited irrigation,thereby playing a key role in preventing a massive exodus of the valley’s population in the face of severe drought and desertification. Flow regulation from the Manantali Dam has ensured a year-round supply of irrigation water, while the Diama Dam prevents this supply from mixing with intruding sea water and reduces pumping costs by raising the upstream water level.

The creation of a significant fisheries resource in the Manantali Reservoir has led to the seasonal settlement of fishing communities. Also, controlled flows from the Diama Reservoir have contributed to the maintenance of three important wetlands at Diawling, Djoudj and Trois Marigots/Ndiael.

On the other hand, the dams and associated dykes on the flood plain have brought about major ecological changes in the floodplain on both the Mauritanian and Senegalese sides of the river. Filling of the Manantali Reservoir reduced the volume and duration of annual floods, which led to a diminution of the inundation of the floodplain resulting in weakening the ecosystems dependence on prolonged seasonal submersion. It also resulted in a reduced area suitable for flood-recession cropping, and curtailed groundwater recharge. The Diama Dam has created a permanent and fairly stable freshwater body whose shores have been invaded by a dense growth of unwelcomed aquatic plants (Typha australis,Pistia stratiotesandSalvinia molesta). These plants proliferate in the river’s tributaries and in the irrigation canals reducing flow velocities, encouraging insects and disease,displacing other species, reducing fish production and impeding fishing.

One of the invasive plants’ most damaging effects is the habitat they create for vectors of waterborne diseases. An explosion of mosquito and snail populations has brought malaria and bilharzia to epidemic proportions. Despite efforts by the international aid community and national public health services, the re-infection process causes the prevalence of these diseases to remain at an unacceptably high level. The pest plants and their consequences for public health clearly call for corrective measures to be implemented in a concerted fashion on both sides of the river.

The practice of drawing water from the river is also becoming increasingly hazardous, particularly in the dry season, as the irrigated area expands and the use of agro-chemicals intensifies. The river’s water quality is to be addressed through the Dakar Long Term Water Sector Project, but it has not been addressed in respect of the riverside population.

The river discharges into the Atlantic Ocean through a somewhat deltaic estuary, the mouth protected by a sandy spit on which sits the town of St. Louis. The Diama Dam has drawn a clear line between fresh and salt water where none previously existed. There was once a highly productive zone of intermingled flora and fauna and an important marine fish spawning ground. There are still some linkages between salt and fresh water zones and there is potential for improved management of these zones to increase production of fish and bird life and to generally enhance the ecology of the estuary. It is recognized that effective water management requires consideration of the continuum from fresh water springs, all the way downstream to the ocean, where evaporated water is recycled as precipitation to replenish the water sources.

5.2 Groundwater role in alleviating impacts of climate change in the SRB

In the 1970s, at the peak of the Sahelian drought, investigations were carried out to develop irrigation schemes that would permit cultivation of two crops per year in the upper part of the Senegal River. These studies revealed the existence of an important groundwater reservoir (Bajsarowicz and Welton, 1979). An analytical model was developed to estimate long-term yields and average drawdown of the aquifers.

Based on these assessments, an entirely new irrigation scheme using a combination of wells and surface water sources was developed. This scheme offered several significant advantages over the original surface water scheme including:

· staged development with little pre-investment costs required for future stages;

· flexibility and uniformity in system layouts, irrigation unit designs, and development schedules;

· opportunity for double-cropping (prior to the construction of the Manantali Dam upstream);

· reduced flood levels and velocities in the Senegal River, leading to lower dike levels;

· lower maintenance costs;

· reduced immediate impacts on traditional flood recession agriculture;

· reduced long-term water table levels and associated subsurface drainage problems;

· aquifer storage use, rather than total reliance on surface storage which is subject to meteorological conditions and evaporation losses.

Other environmental and social implications include:

· reduced health hazards related to bilharzia, whose rapid spread has been linked to the increase in the number of surface water storage facilities in tropical and semi-tropical areas;

· groundwater can be safely used for drinking, with only minimal treatment (chlorination);

· a cautious and flexible-phased development, which is possible with the use of groundwater offers a viable alternative to the costly irrigation schemes based on pumping water from the Senegal River;

· no need for vast initial capital layouts;

· use of local groundwater resources would mitigate the numerous adverse environmental impacts of vast surface irrigation schemes.

In spite of this successful experience, actions to better characterize and monitor the groundwater resources of the Senegal River Basin as a whole, are very limited. The history of groundwater development/evaluation in the basin could be summarized in two periods, before and after commissioning of the dams.

During early stages of the valley’s development, different localized investigations have been carried out to characterize the hydrodynamics and recharge mode of the alluvial aquifer in the valley. The pre-dam situations has been established for the delta (Audibert, 1970) and valley (Illy, 1973)indicating some complex relationship between the Senegal River and the aquifer system.

Subsequent investigations carried out during the post-dam period (OMVS/USAID, 1990; EQUESEN,1990–1991, Diagana, 1994; Touzi, 1998; Diaw, 2008) have confirmed this inter-relationship (Figure 9) and shed some light on the possible impacts of the uses and management of surface water resources on the groundwater and its sensitivity to changing climate conditions. In most cases the river feeds the aquifer, but there are periods where the situation is reversed (Touzi, 1998).

However, due to the high heterogeneity of the alluvial formations and the differences in morphologies, these relationships need to be further investigated in order to better benefit from groundwater resources in alleviating the impacts of climate changes. The rather complex behavior of the aquifer is illustrated in Figures 10 and 11 that show the parallel evolution of river levels and the alluvial aquifer,depending on the location of the observation borehole.

Figure 10 Alluvial aquifer water level fluctuations compared to river levels (Touzi, 1998). The impact of the Manantali Dam in the general increase of the aquifer levels downstream; The aquifer water level fluctuations in the inundation zone do not indicate a clear impact,no notable difference is seen between the two piezometers located in a potential area of flooding and an area without any flooding

Figure 11 Alluvial aquifer water level fluctuations compared to river levels (Touzi, 1998). The top diagram depicts a piezometer located at a distance of 3,300 m from the bank and does not show any fluctuation; in contrast, the bottom diagram shows an observation borehole situated at a distance of 3,500 m at Saldé, with a variation in the water levels that reach 0.9 m for the season during 1987–1988.

5.3 Adaptation strategies in the SRB

Developing countries have limitations in capacity, making adaptation difficult. Limitations include both human capacity and financial resources (Brauneet al., 2009). The most effective adaptation approaches for developing countries are those addressing a range of environmental stresses and factors. Strategies and programs that are more likely to succeed need to link with coordinated efforts aimed at poverty alleviation, enhancing food security and water availability, combating land degradation and reducing loss of biological diversity and ecosystem services, as well as improving adaptive capacity. Sustainable development and the Millennium Development Goals are necessary backdrop to integrating adaptation into development policy, strategy and programs.

The Senegal River crosses four countries and has been the focus of national and international attention for more than six decades. There have been, and there continues to be,numerous interventions that relate to water resource management. National priorities sometimes supersede regional priorities and may conflict with each other. Countries naturally tend to pursue their own interests first. These interests are seldom evaluated for the long term, but receive responses for the immediate need. The availability of information is poor, data-sharing is limited, communication remains poor within the basin, there are diminished flows in the river since the 1960s and there are increasingly complex and conflicting demands from the increasing population. It is therefore necessary to improve the response to these trans-boundary issues through basin-wide cooperation; improved management and response capacity;improved data and information flow; in-depth analysis and design of appropriate actions; some immediate action on the ground; improved public participation and establishment of a sustainable long term program for improved water and environment management in the basin. It is based on these observations that the governments of the riparian countries have decided to change the status of OMVS which is now acting both an agency that constructs the infrastructure necessary for the sustainable development of the basin and one that functions as an integrated water resources management agency (GWP-INBO, 2009).

The trans-boundary Diagnostic Analysis carried out(Balde, 2001; Barry, 2001; Camara, 2001; Diarra, 2001;Dieng, 2001) has identified the following key issues as a priority for intervention and assistance in order to ensure the long-term sustainable management of the Senegal River Basin:

(1) Management capacity and institutional strengthening.It is necessary to improve the capacity of OMVS and the riparian states to address trans-boundary environmental issues. So far OMVS has focused its efforts towards the management of dams and related activities, without placing required attention on specific and broader basin management and environmental and social issues that were not previously recognized as an essential part of river management. National actions in the areas of water resources and environment management are also typically oriented to solve problems of immediate national concerns. There is a need to enhance their capacity to deal with trans-boundary and international issues that are inherent in the management of the shared basin.

(2) An inclusive framework. Optimum management of the river basin requires the development of an inclusive framework of cooperation which clearly identifies benefits that each riparian country, including Guinea, would receive from its participation to the cooperation.

(3) Data and information flow––the knowledge base.Data has been collected for many projects, but the resulting databases are incompatible or have simply been lost or abandoned on completion of the project. This is a particular need in the upper basin, including Guinea, where lack of data is a concern not only for the government of Guinea but for the whole basin. In order to optimize management of river flows, not only for generation but also for the maintenance of livelihoods in the headwaters as well as downstream, it is essential to have much improved baseline data.The need for improved hydrometric, climatologic, land resources and land use data is clear. There is a need to support capacity building, agreement on data parameters and platforms for data management and exchange.

(4) Land degradation and desertification. Land degradation and desertification issues are of high concern in the basin and cause much negative impact. Inappropriate land use in sloping, hilly and mountainous areas of the Fouta Djallon and the Manding Plateau have caused soil erosion,land degradation and loss of soil fertility leading to the creation of vast denuded areas. The spread of deforestation throughout the basin in combination with overexploitation of natural resources is affecting the basin dynamic in terms of human settlement patterns and resource conflicts. In addition, phenomena such as a decrease in rainfall, increase in the frequency of severe droughts, the occurrence of the harmattan dust-bowl, sand dune movement and associated loss of arable land as well as livestock death are indicators of an increase in land degradation and desertification progression toward the south of the basin. All basin countries have signed the Convention to Combat Desertification (CCD) and have CCD National Action Programs in place; however,jointly planned trans-boundary coordination will need to be increased if the degradation processes are to be halted.

(5) Siltation and erosion. Siltation and erosion are cross border issues and must be addressed through cooperative action. These processes are tied to overgrazing and poor agricultural practices. The Fouta Djallon region provides 13 tons/km2of eroded and siltated material per year of which 30% is derived in the Bafing Basin. The increase in siltation and erosion in many areas of the basin represents a serious threat for water resource availability. Aside from the loss in land productivity, increased siltation also increases the dangers of floods and interferes with infrastructure. A number of national programs are underway to enhance soil protection and conservation; and these important initiatives are making an impact at the local levels. However, there is an urgent need to support trans-boundary cooperation targeting specific areas such as the Fouta Djallon and fertile soil of the"walo" or flood plains, where this continuing trend will cause severe damage to the basin water resource and food production.

(6) Point and non-point source pollution. Both point and non-point sources of pollution are basin-wide and of a trans-boundary character. Growing urbanization combined with lack of sanitation facilities has increased water pollution. Increased use of pesticides and fertilizers in irrigation schemes in the valley have led to high levels of pollution.These impacts have the potential to affect the regional basin economy and could jeopardize development progress achieved thus far. Data and information as well as awareness related to national and trans-boundary impacts of this pollution are scarce. To date, only limited trans-boundary action is underway to address these issues and the need for a holistic and basin-wide approach is of high priority.

(7) Water weeds infestation. The infestation of aquatic weeds has increased tremendously throughout the basin,leading to a decline in fisheries and a threat to the available water supply sources for large cities such as Dakar. Moreover, the infestation contributes to the spread of waterborne diseases.TyphaandSalvinia molestainfestations have recently been recorded among the main environmental impacts within the Senegal Valley. Although some pilot initiatives are underway to mitigate this infestation, there is an urgent need for a trans-boundary approach so that the basin-wide dimensions can be tackled. It would be of value to learn from the integrated approach for plant management in Florida waters which allowed the control of weeds such asHydrilla,Salvinia,Water Hyacinth,andWater Lettuce(http://plants.ifas.ufl.edu/manage/).

(8) Public participation, awareness and stakeholder capacity. A fundamental issue identified in the basin has been the needs for greater public participation in the decisions that are made and which significantly affect their livelihoods.During the preparation of the present project, a significant effort has been made to involve as wide a segment of the public as possible. There is a clear need to ensure that the voice of civil society is heard as developments are discussed and options assessed. Moreover, a greater awareness of inter-linkage, of trans-boundary impacts and of the priorities of neighboring states and communities, will greatly enhance and strengthen the spirit of cooperation between the four countries. Communities need access to information and assistance to build their capacities so that they can more effectively participate in decision-making processes. Support is needed for the poorest and least educated among the communities and the special needs of the largest segment of the population, women and children, are particularly important in this respect. After a series of consultations a technical program has been elaborated that focuses on establishing a viable integrated resource management strategy that focuses on water, biodiversity and the environment. The program focuses on establishing a series of activities at national levels that together form a cohesive strategy for the river basin.However, to date insufficient attention has been given to the need for a wider consultation process and improved participation of all stakeholders for the implementation of a sustainable management of the Senegal River Basin.

(9) Sustainable management of wetlands. There is an urgent need to increase the knowledge and awareness of important economic and ecological functions which the wetlands in the basin play. These fragile environments are being degraded due to drainage, expansion of agriculture,and unplanned developments. Although specific regulations has been prepared for the protection of these areas and although protected areas have been created and/or extended,more effective enforcement is needed combined with awareness and capacity building. Examples of important wetlands in the Senegal Delta include the Jodi and Diawling which are important reserves for Palaearctic migratory species, and the Goalie Forest along the valley which plays a crucial role as a filter and nursery for fish species. Other important wetlands include the Kemeba-Ko and Baoule Game Reserve in Mali, both of which form important habitats as well as nesting areas for migratory birds.

(10) Waterborne diseases. Waterborne diseases such as bilharzia, Guinea worm, diarrhea and malaria are prevalent in the basin and remain a major concern. The construction of dams and the increase in the water level have contributed to increasing the spread of waterborne diseases, including urinary bilharzia which has especially stricken the population of the Senegal Valley. National health projects have been launched in Mauritania and Senegal to address waterborne diseases. Both of these projects seek to put in place coordinated actions to reverse the present situation. In addition, a WHO/UNICEF/UNDP regional program combating the Guinea worm is also in place supporting the two countries.

6 Conclusion and recommendations

Climate change scenarios for West Africa, including the Senegal River Basin, indicate that the current climatic variability will increase and intensify. Droughts and floods will increase not only in frequency but also in scope. The trend of declining rainfall and annual average flows experienced over the last three decades is expected to continue.

Over the last few decades, accrued efforts have been made to mitigate the impacts of these unfavorable climatic changes on the lives of the inhabitants of the region. A major step has been the creation of OMVS and the building of two dams upstream and downstream on the Senegal River. The initial objective of these water works was mainly to improve the socio-economic condition of the region through enhanced food production and a better sharing of the trans-boundary water resources of the Senegal River.

The critical evaluation of OMVS achievements and its limitations has established the necessity to address the environmental and social impacts of the dams at the same time under the current climate change framework. Therefore,OMVS has established the Environmental Observatory, to build capacities and develop a knowledge base for environmental management of the river basin as a whole, including its Guinean portion.

Groundwater now seems to be given the place it deserves with the implementation of the project on"Trans-boundary groundwater management" being implemented with the support of Global Environment Fund(GEF). A major objective of this project is the installation of an optimal monitoring network in order to propose solutions for improved management of this strategic resource.

In the framework of this project, it is recommended to put an emphasis on finding ways and means to increase the knowledge base in order to jointly identify, reduce and mitigate the trans-boundary risks from changing land and water use as well as from climate change in the trans-boundary aquifer systems of the Senegal River Basin. This will assist the joint management of risk and uncertainty in the Basin.

A specific programme needs to be developed to:

· Develop comprehensive water-management,land-use and energy-development strategies that fully recognize groundwater’s important role in the hydrologic cycle and prepare a Trans-boundary Diagnostic Analysis, to define threats and root causes. This requires better characterization of the groundwater reservoirs in the basin, their interrelations with surface water and ecosystems, and a better understanding of the response of the regional hydrologic system to natural and human-induced stresses.

· Identify the most critical ecosystem, surface and groundwater threats based on current and expected environmental conditions, local resource management policies, laws, practices and mechanisms of conflict resolution, capacity needs as well as possible bottlenecks in addressing these problems. This phase of the project will deal particularly with human and environmental threats in the trans-boundary context.

· Identify and evaluate selected areas that would be used as pilots for case studies. This phase of the project will deal with observation of environmental,social and political trends (such as groundwater utilization and degradation, impacts on the popula-tion, efficiency of existing water management agencies and local regulations, potential for inter-community and international conflicts of interest and mechanisms for their resolution). In addition, future predictions will be made by projecting trends in climate change, population growth, development, and environmental degradation. The case study may feed back the estimation of groundwater balance.

· Improve the understanding of physiographic, geological, pedological and hydrogeological parameters that determine the vulnerability of the aquifer systems to pollution in the Senegal Basin and provide information support for development of policies and measures to enable protection of groundwater from pollution in the basin.

· Build institutional capacity that would promote collaborative research in sustainable water resources management in the Senegal River Basin,both at the national and regional levels.

· Establish a joint development and conservation strategy for the trans-boundary aquifer system of the Senegal River Basin and implement a common legal and institutional management framework for joint formulation and implementation and enforcement of risk mitigation policies.

· Develop public awareness and stakeholder participation programs, along with capacity building for intergovernmental communication.

This paper has been prepared within the framework of the UNESCO-IHP and its focal area 1–5 "global change climate variability in arid and semi-arid regions", and within the MoU between UNESCO Nairobi Office and UNEP on case studies of climate and human activity impact on groundwater system in several regions in Africa. It was presented at the workshop on vulnerability assessment of Trans-boundary waters held in Pretoria (November 4–6,2009). Revisions of a previous manuscript by S. Diop and E.Naah, and kind support from I. Mall in the final editing of the paper, are gratefully acknowledged.

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