Spatio-temporal variability of hydrological regimes around the boundaries between Sahelian and Sudanian areas of West Africa: A synthesis

Journal of Hydrology 375 (2009) 90–102

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Spatio-temporal variability of hydrological regimes around the boundaries between Sahelian and Sudanian areas of West Africa: A synthesis L. Descroix a,*, G. Mahé b, T. Lebel a, G. Favreau b, S. Galle a, E. Gautier c, J-C. Olivry d, J. Albergel e, O. Amogu a, B. Cappelaere b, R. Dessouassi f, A. Diedhiou a, E. Le Breton c, I. Mamadou c, D. Sighomnou f a

LTHE-IRD (Laboratoire d’étude des Transferts en Hydrologie et Environnement, Institut de Recherche pour le Développement), BP 53, 38041 Grenoble cedex 9, France HSM-IRD HydroSciences, MSE, 34095 Montpellier cedex 5, France c LGP, CNRS-UMR 8591 Laboratoire de Géographie Physique, 1, Place Aristide Briand – 92195 Meudon, France d IRD Montpellier, Agropolis, BP 5045 Montpellier cedex 2, France e IRD c/o ICRAF P.O. Box 30677, 00100 Nairobi, Kenya f ABN, NBA Niger Basin Authority, Niamey, Niger b

a r t i c l e

i n f o

s u m m a r y

Keywords: Endoreism Sahelian climate Scale effect West Africa Hortonian runoff Land-use change

Abundant information is available on West African drought and its hydrological and environmental impacts. Land-use and climatic changes have greatly modified the conditions of Sudanian and Sahelian hydrology, impacting the regime and discharge of the main rivers. Human pressure on the environment (significant increase in crops and disappearance of natural bushes and landscapes, for example) has led to severe soil crusting and desertification throughout Sahelian regions. Despite recent increases in rainfall, the drought has not ended, resulting in two different hydrological evolutions. In the Sudanian areas, stream flows have been reduced, sometimes as much as twice the rainfall reduction rate. In the Sahelian regions, runoff coefficients have increased to such a degree that discharges are increasing, in spite of the reduced rainfall. The main goal of this paper is to synthesize the recent advances in the Sahelian and Sudano-Sahelian West African hydrology. The other objectives are two fold: First, to discuss the ‘‘Sahelian Paradox” (the increase in runoff in most of the Sahel during the drought, at least during the 1968–1995 period, as described in the 1980s) and paradox of groundwater highlighted in the square degree of Niamey (the rise in water table levels in some endorheic areas during the same drought, evidenced in the 1990s), and second, to attempt to define the application of their respective geographical areas. The land-use changes act as a general factor of hydrological evolution of soils and basins, while some spatial factors explain the great variability in the response to environmental evolution, such as endorheism, geological context, latitudinal climate gradient, and local hydrodynamic behaviour of environment. This paper is literature-based, and incorporates current research advances in the field, as well as a prospective focused on resources and socio-economic impacts. Ó 2009 Elsevier B.V. All rights reserved.

Introduction

Similar trends were observed on smaller river systems (Le Barbé et al, 1993; Mahé et al., 2000) while, at the opposite, other studies pointed to a runoff increase in some Sahelian catchments (Albergel, 1987; Cappelaere et al., 2009; Mahé et al., 2005; Séguis et al., 2004). This paper is a factual update on these different reactions of the West African river systems to the drought with the aim of discussing the behind the scene factors involved in this variability, land surface changes being the main factor. Land-cover changes, most often resulting from land-use changes, trigger alterations in hydrodynamic soil surface behaviour – for instance increase in bare soils areas causes erosion and soil crusting – and can impact the local and regional water budget. Studying the relationship between the evolution of the vegetation cover and the evolution of discharges or runoff, a number of papers show that deforestation and soil compaction cause an

The severe drought, and its hydrological impact, endured by West Africa over the past 40 years or so was the key motivation for setting up the AMMA-CATCH observing system (Lebel et al., 2009). The intent of this paper is to provide a general perspective on how the long lasting rainfall deficit impacted runoff and large river discharges at the regional scale. The decrease of the mean annual discharge of the largest rivers of the region, namely the Senegal and Niger rivers, in proportions almost twice as large as the decrease in rainfall is a well established fact for the period 1970–2000 (see e.g. Andersen et al., 2005; Lebel et al., 2003). * Corresponding author. Tel.: +33 476 82 70 89; fax: +33 476 82 52 86. E-mail address: [email protected] (L. Descroix). 0022-1694/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2008.12.012

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Fig. 1. Location of basins and rivers cited in the text; boundary of the contributing watershed of Niger river at Niamey station.

increase in runoff. This seems to hold for very different climates, from humid and tropical (Fritsch, 1990; Calder et al., 1995; Scott Munro and Huang, 1997), to Mediterranean (Sorriso-Valvo et al., 1994; Croke et al., 1999) and temperate (Cosandey et al., 1990; Hudson and Gilman, 1993; Andreassian, 2004). Arid or semi-arid environments are especially sensitive in this respect as shown by Snelder and Bryan (1995), Bergkamp (1998), Casenave and Valentin (1992).

Trees and litter constitute a stream flow regulator and a protective screen for the soil against the splash of rain drops (Croke et al., 1999; Descroix et al., 2002; D’Herbès and Valentin, 1997; Fritsch, 1990; Scott Munro and Huang, 1997; Sorriso-Valvo et al., 1994) and against evaporation (Braud et al., 1997). Inversely, soil crusting provokes an increase in runoff, conforming to observations made by Valentin and Casenave (1992) and Vandervaere et al. (1997) in the Sahel, and Janeau et al. (1999) in Mexico.

Fig. 2. The Niger River Basin (after Olivry, 2002) and boundaries of ‘‘Sahelian paradox” and Hewlettian environments. The 750 mm separates roughly Sahel (northward) and Sudan, between the Upper Niger river basin and the Lake Chad.

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In conclusion, natural vegetation – and to a lesser extent fallow land – enhances the water holding capacity of watersheds, thus reducing runoff. Enhanced soil water holding capacity increases the buffering effect of the soil on rainfall and delays the runoff processes, hours or days after a rainfall event. At the basin scale, dense vegetation causes soil attenuating low flows and floods, generating higher base flows than bare soils or cultivated areas with overexploited (or crusted) soil characteristics do. The response of West African catchments has evolved over the last decades The West African environment has had to cope with two major changes over the past decades: (i) the drought, which struck the whole region in the 1970s and 1980s, evolving later in different ways according to the sub-regions concerned (Lebel and Ali, 2009); (ii) dramatic land cover changes (see e.g. Loireau, 1998; Anyamba and Tucker, 2005; Herrmann et al., 2005). The studies

of Ada and Rockstrom (1993), Loireau (1998) and those carried out more recently in the framework of the AMMA-CATCH program show that 10% of the Nigerien Sahel was covered by cultivated areas in the 1950s and close to 80% nowadays (Cappelaere et al., 2009). Similar observations were made by Diello et al (2006) on the Nakambé catchment of Burkina Faso, Hauchart (2008) in Western Burkina, Liénou et al. (2005) in Northern Cameroon. Fig. 1 shows the main rivers basins studied here as well as the location of their stream gauging stations. Regarding the hydrological environment of the region, one important point to underline is the importance of endorheism, which is a phenomenon where surface runoff and stream flow do not reach the large rivers draining to the ocean. These endorheic areas cover almost half of the area of the Niger river basin (Fig. 2); these areas do not contribute to the discharge of the river itself, thus reducing its hydrologically effective area to 1,100,000 km2, from a topographical catchment of more than 2,000,000 km2, since the end of Atlantic period (about 4500 or 4000 BP, Gasse et al., 1990).

Fig. 3a. Evolution of discharges of some Sahelian rivers (right bank tributaries of the Niger river).

Fig. 3b. Evolution of discharges of a Sahelian river: the Nakambé (one of the tributaries of the Upper Volta).

L. Descroix et al. / Journal of Hydrology 375 (2009) 90–102

Figs. 3 and 4 illustrate how the river systems of these different hydrological environments have evolved over the past 50 years or so (see Fig. 1 for the location of these rivers). Two main points are highlighted by these figures. – The first point concerns the increase in runoff coefficient and, generally, the increase in discharge of the Sahelian rivers, despite the reduction in rainfall. This was first noticed by Albergel (1987) in Burkina Faso, thus the term ‘‘Sahelian Paradox” used in the following to designate this anti-intuitive behaviour. Similar observations were made for the Nakambé River, one of the main tributaries of the Upper Volta river in Burkina Faso (Mahé et al., 2005), and for the right bank tributaries of the Niger River during its Sahel crossing (Amani and Nguetora,

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2002). In these two areas, the increase was mainly observed during the drought years from 1968 to 1995 (Fig. 3a and Fig. 3b). – The second observation is a decrease in annual discharge (a more ‘‘common sense” behaviour since it is consistent with the strong decrease in rainfall) in the Sudanian areas, as shown at the south of Niamey (Fig. 4a). In the intermediary area of north Sudanian climate, there is no clear trend in the temporal evolution of stream flows (see Mekrou, Tapoa and Goroubi rivers in Fig. 4b). Even though this paper is focusing on surface water, it is worth noting here that these different behaviours in surface water response to the drought are mirrored in the response of the ground

Fig. 4a. Evolution of discharges of some right bank Sudanian tributaries of the Niger River and the Ueme river.

Fig. 4b. Evolution of discharges of some right bank North-Sudanian tributaries of the Niger River.

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water: the water table level in the endorheic areas of the Niamey region has been rising since at least the 1950s (Leduc et al., 2001; Favreau et al., 2002), while at the same latitude under the Lake Chad, groundwater level is decreasing (Gaultier, 2004) (Fig. 5). Figs. 3 and 4 show the opposition between the pure ‘‘Sahelian” climate rivers, where runoff is clearly increasing (Fig. 3a and b), and the ‘‘Sudanian” (Fig. 4a) or ‘‘Sudano-Sahelian” (Fig. 4b) rivers, where base flow was reduced. This base flow reduction caused a clear and lasting reduction in total annual discharge. A few other points are worth noting from Figs. 3 and 4: – The increase in discharge is higher in the Dargol and the Gorouol basin than in the Sirba basin. But the runoff coefficient of the Sirba basin increased more after 1972 than those of the Gorouol and Dargol basins (Table 1) though they are located north of the former, likely due to the impact of agriculture. The two latter basins are located north of the isohyet of 400 mm, and are mostly covered by pastures and savannah. The Sirba Basin receives annual rainfall amounts greater than 500 mm, and it is therefore included in the area of which land-use has mostly changed from natural vegetation to cultivated area. – In the past few years, there has been an increase in discharge from southern basins (the Mekrou and the Tapoa Basins, Fig. 4b). It is too early to be certain that this is a new trend, but it is worth monitoring in the years to come in order to assess whether land-use change (and a possible increase in rainfall) begins to provoke increase in runoff in Sudanian (and supposedly ‘‘Hewlettian”) basins. Hauchart (2008) hypothesized that increasing runoff in the Mouhoun basin (Volta basin) was caused by intensive cotton cultivation. – When comparing the evolution of the 1991–2000 period with that of the 1970–1990 period, a rising evolution of discharges is measured in the Benué and Ouémé Basins. We have no data which allows us to determine whether this is due to rainfall increase or rather to the degradation of the vegetation in the Benue basin as suggested for the Bénué by a recent study; Sigha Nkamdjou, 2007, Table 2); but in the Ouémé basin, there is a slight increase in rain fall noticed since 1988 (Le Lay and Galle, 2005). – Land clearing is the most obvious candidate for explaining the ‘‘Sahelian Paradox” (Séguis et al., 2004). At the same time not all catchments are reacting in the same way. We thus need to understand why some processes hold true up until a certain spatial, temporal, or numerical threshold, but not after. Why does discharge increase, in spite of the decreasing rainfall, in some

Table 1 Evolution of runoff coefficient in the western tributaries of Niger river before and after 1972 (in Mahé et al. 2003). River

Area of watershed (km2)

Mean rainfall 1955–1998 (mm)

Runoff coefficient before 1972

Runoff coefficient after1972

Ratio RCa before and after 1972 (%)

Gorouol Dargol Sirba Goroubi Diamangou Tapoa Mékrou

44850 6940 38750 15350 4030 5330 10500

422 503 634 691 746 785 1000

1.6 3.9 2 2.3 3.5 0.9 11

2.2 6.2 3.2 2.3 2.3 1 7

+40 +57 +61 0 35 +10 33

a

RC = runoff coefficient.

Sahelian areas, and not in the Sudanian region, and why has there been a rise in water table recharge in Western Niger and not in the other areas of the Sahel? Since rainfall intensity and rainfall distribution in space and time display little change – if any – in the last few decades (Le Barbé and Lebel, 1997; Le Barbé et al., 2002; Lebel and Ali, 2009; Tapsoba et al., 2004), the focus has to be on land cover changes. However, as described above, the climatic, hydrologic and geological environments are not homogeneous over the whole region, and this must be accounted for in the analysis.

Explaining factors: some antagonistic processes Vegetation degradation The degradation of vegetation cover in West Africa is caused by several factors: – The continuous drought which began in 1970, with paroxysm events in 1972/1973 and 1982/1984, resulting in the death of a significant number of trees. – The demographic pressure of annual growth rates of 3% in Sahelian countries (3.5% in Niger, 5.5% in rural areas of the Niamey region), pushing people to remove the bush in order to increase the cultivable areas (Loireau, 1998; Guengant and Banoin, 2003), after having shortened the fallow periods; Fig. 6 shows the evolution of land cover in Western Niger from 1950. – The reduction in soil productivity and land yields (Guengant and Banoin, 2003; Valentin et al., 1999; Massuel et al, 2006) due to the persistent shortage of rainfall, erosion, soil nutriment loss, and the decrease in soil water holding capacity. – The increase in forest harvesting in both Sudanian and Sahelian areas, wood fuel being the only energy available in West Africa. Spatial factors Fig. 2 shows a series of oppositions which capture the principal situations observed in the Sahel and the Sudano-Sahelian region of West Africa.

Fig. 5. Rise in water table in the Dantiandou basin (Niamey’s square degree), falling water table in the Lake Chad Basin (Gaultier, 2004).

An opposition of climates It seems necessary to differentiate between the northern and southern right bank tributaries of the Niger River in its Niger Rep crossing. The northern tributaries are characterized by a typically ‘‘Sahelian” functioning, with increasing discharges (40–60%, see Table 1), despite a high reduction in their mean annual rainfall amount (25–30%). The southern tributaries come from catchments

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L. Descroix et al. / Journal of Hydrology 375 (2009) 90–102 Table 2 Evolution of runoff at the basin scale of main West African rivers and the main Niger tributaries. River station

Niger Niamey

Senegal Bakel

Gambia Gouloumbou

Chari N’Djamena

Bani Douna

Benue Garoua

Milo Kankan

Niandan Baro

Niger Kouroussa

Niger Siguiri

Niger Koulikoro

Ouémé Beterou

Basin area (km2) Period 1 mean discharge 1951/1970 Period 2 mean discharge 1971/1990 Ratio period 2/period 1 Period 3 mean discharge 1991–2000 Ratio period 3/period 1 Period 4 mean discharge 1991–2005 Ratio period 4/period 1

700,000 1233.69

218,000 848.45

42,000 344.93

600,000 1627.50

102,000 676.55

64,000 384.49

9620 210.70

12,530 269.45

16,560 299.60

67,600 1225.20

120,000 1695.80

10,050 68.86

945.25

387.73

149.68

1107.00

200.00

228.26

159.25

180.40

207.00

739.55

1030.35

30.98

0.77 933.95

0.46 480.60

0.43 176.62

0.68 1000.83

0.30 221.29

0.59 283.27

0.76 142.80

0.67 174.70

0.69 151.60

0.60 807.30

0.61 1065.90

0.45 50.57

0.76 951.43

0.57 489.83

0.51

0.61

0.33

0.74

0.68

0.65

0.51

0.66

0.63

0.73 46.29

0.77

0.58

within the Sudanian climate and suffer a more ‘‘expected” decrease in discharges (except the Tapoa with a slight increase). However, it is worth noticing that the latter experienced a lower decrease in annual rainfall amount (15–20%) during the years 1971–1990 as compared to the years 1951–1970. The main hydrological difference between the ‘‘Sahelian” and ‘‘Sudanian” eco-climates is the amount of rainfall: less than 700 mm in Sahelian areas; between 700 and 1300 mm in Sudanian regions (these are rough numbers, especially given the fact that the isohyets migrated 150–200 km southward during recent periods, as mentioned by Lebel and Ali (2009)). This results in a strong eco-geographical gradient, between: – Bush and savanna in a Sahelian environment (with large trees in southern areas) characterized by crusted soils, low clay and organic matter contents, and low structural stability. Weak infiltration and soil water holding capacity lead to the dominance of Hortonian hydrological behaviour, defined by runoff provoked by rainfall intensity which is higher than soil infiltration capacity (Horton, 1933). – Tree-savannas and clear forest in the Sudanian regions. In these areas, soils are richer in organic matter and clay content, inducing better structural stability and hydraulic conductivity, with fewer patches of crusted topsoil. Sudanian vegetation covers a high percentage of the soil, dissipating kinetic energy of drops and preventing crusting of the soil. These environments are characterized by ‘‘Cappus-type” (Cappus, 1960) or ‘‘Hewlett-

0.67

type” (Hewlett, 1961 and Hewlett, 1972) behaviour, i.e. higher soil water holding capacity, presence of temporary water storage areas, perched water tables, under flows, and preferential infiltration paths, etc. In this hydrological pattern, the soil is saturated from the bottom, acting like a reservoir filled from its deepest layer. Runoff only appears when the soil is full and can no longer continue to infiltrate more water. However, it is worth noticing that areas of Hortonian runoff exist in the Sudanian region and, inversely, some ‘‘Hewlett-type” processes areas are observable under certain circumstances in the Sahelian region. Fig. 2 indicates only the main boundaries, without these details. Furthermore, at the same latitude, the climate is dryer through the East (Lebel and Ali, 2009, Fig. 3). The Gambia and the Sirba rivers basins both have an area close to 40,000 km2. They are located at the same latitude but display very different behaviours, partially due to the decrease in rainfall from West to east at a given latitude (Fig. 7, Table 3). The first one (Fig. 7a) presents an evolution of discharge very close to that of the rainfall index. At the opposite, the Sirba basin (Fig. 7b) is a good example of the ‘‘Sahelian paradox” showing a strong increase in runoff despite the reduction of rain fall. Comparing Fig. 7b with Fig. 3a one can deduce that, due to the rainfall reduction trend, the runoff coefficient has more increased than the discharge. Rainfall indexes and rainfall depths in Fig. 7a and b and are recomputed from Ali and Lebel (2009) and Lebel and Ali (2009) for the Sahelian belt (11–16°N), and from Mahé et al (2005) for the Niger basin. Endorheism and exorheism Increasing runoff in the Sahelian region, generally attributed to land-use changes, does not always produce the same effects: – In exorheic areas, it causes an increase in fluvial streams and discharges and a falling water table level linked to the reduction of water infiltration. – In some endorheic regions, the rise in runoff provides more water to fill ponds. Each year, as vegetation disappears, these ponds become more numerous, larger and deeper, and thus more durable, causing a raise in water table level, observed, for example, in the region of Niamey (Fig. 5).

Fig. 6. Land-use evolution in South West Niger (Loireau, 1998).

Finally, the boundary between endorheism and exorheism is not so easily defined because of the multiplicity of scales. Currently, the increase in runoff is causing some classically endorheic basins to become exorheic, for example, the Karma catchment, located north of Niamey. In the same region, other exorheic koris (temporary ‘‘wadis”) seem to have become endorheic in the last few years due to the silting up and the closure of certain valleys by sand excess.

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Fig. 7a. Rain fall vs runoff relationship in the Gambia river basin (1950–2007).

Fig. 7b. Rain fall vs runoff relationship in the Sirba river basin (1950–2007).

Despite this local variability, extended areas are endorheic in the Sahel, particularly in the Niger River basin (see #2). An opposition of scales? There exists another opposition between the medium size Sahelian basins whose discharges have been increasing for several decades, and the great West African rivers whose discharges decreased by 35% (the Niger River at Niamey, including minus 70% for its tributary, the Bani River) or 55% (Niger at Koulikoro, near Bamako; Olivry, 2002), or 60% (Senegal at Bakel; Hubert et al, 2007) from the period of 1951–1970 to the period of 1971–1990 (Table 2). The low runoff yield of Sahelian areas explains the severe general decrease of flows in the Senegal and Niger Rivers. Equivalent

total areas of the basins are located in the Sahel and in the Sudanian and Guinean region; however, natural yield in the latter is so Table 3 Comparison of main data of Gambia and Sirba basins.

Mean discharge (m3 s 1) (1950–2007) Area (km2) Mean specific discharge (l s 1 km 2) (1950–2007) Mean rain fall depth (mm) (1950–2007) Mean runoff depth (mm) (1950–2007) Runoff coefficient Latitude of basin Longitude of basin

Gambia

Sirba

208.7 42,000 4.97 758 156.7 0.252 11.5–15°N 11–16.5°W

24.5 38,750 0.63 549 18.3 0.033 12.2–14.5°N 1.4°W–1.7°E

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much higher than in the Sahel. Therefore, a strong increase of Sahelian runoff does not compensate for the difference. Does the scale effect (see Vischel and Lebel, 2007, for a sensitivity analysis on these effects) explain the great difference in behaviour between two sizes of endorheisms? The small Sahelian basins, a few km2 large, and Lake Chad, the area of which is more than 3  106 km2 are both are located at the same latitude and have approximately the same climate, but the water table is rising in the region of Niamey on the left bank of the Niger river, while it is diminishing in the Lake Chad basin (Fig. 5). An opposition between plutonic and sedimentary rock-constituted areas There is also a geological difference observed between the two banks of the Niger River when crossing the Niger Rep. The right bank (the western side) is mainly constituted of plutonic rocks, and the left bank (the eastern side) is made of sedimentary rocks. This leads to a strong difference in density of hydrographic network (Cappelaere et al, 2009). Increasing runoff coefficients versus decreasing discharge

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Olivry (2002) hypothesized that the lasting change in rainfall– runoff relation of the Niger River at the Koulikoro station (comparison of the period before and after 1968) is due to a hysteretic reduction of base flow, and that returning to previous stream flows necessitates the reconstitution of groundwater, which is possible when there are successive wet years (Olivry, 2002; Andersen et al., 2005). Previously, Olivry (1987) studied the duration of the hydrological deficit caused by the river depletion coefficient in Senegal and Casamance rivers: After 1971, this coefficient has been increasing significantly, providing evidence for the emptying of aquifers in the Sudanian and Guinean regions where they commonly play a considerable role in the hydrological cycle. This has been verified for many rivers in West and Central Africa as well (Bricquet et al., 1997). Mahé et al. (2000) found a very close relationship between the ground water level variability (from about 30 wells in the national network) and the rainfall inter-annual cumulative deficit, taking into consideration the low flows variability as well, over the Bani river in Mali (101,600 km2 at the Douna station). This example reinforces Olivry’s hypothesis about the impact of the strongly decreasing groundwater flows as a means of explaining part of the amplified reduction of discharges with regard to that of rainfall, since the 1970s in the Sudanian basins.

Non linearities and scale considerations The «Sahelian paradox»: is there an upper scale limit? On the grand scale of West Africa, the hydrological impact of the lasting drought was primarily a decrease in discharges of rivers, which has been estimated as twice the decrease in rainfall for the Niger River at Koulikoro by Olivry (2002), for the period 1905–2000. However, looking at the Niger Basin at Niamey for the 1929–2006 period (Fig. 8), one remarks that rainfall and runoff have decreased at roughly the same rate; the difference is attributable to the increase of runoff in the Sahelian part of the catchment between the inner delta and Niamey (see also Fig. 7 in Lebel and Ali, 2009, showing that the Sahelian component of the annual discharge of the Niger river at Niamey is gaining weight as compared to its Sudanian component). Otherwise, the discharges of rivers seem to experience a long lag time before they begin again to increase. Whatever the cause, it seems that runoff is far from recovering its previous rates from the pre-drought period (before 1968) in the Sudanian part of West Africa (Fig. 9), even if there is a slight increase of the discharges during the end of the 1990s and the beginning of the 2000s, which reach values similar or slightly higher than during the 1970s.

The main explanations for differences in the hydrological response to the conjunction of drought and severe land-use changes (mostly increase of runoff in Sahelian regions, mostly reduction of runoff in Sudanian areas) are a combination of land cover change and scale effects: (i) at the local scale, the hydrological impact of changing land cover and consequent soil hydrodynamic properties is dominant; (ii) at the mesoscale and beyond, the connection with integrating groundwater systems plays a major role: when the surface flow becomes significantly influenced by the discharge of ground water, then the longer time and large space scales of the aquifer play an essential role and may act in an opposite way to the local scale factors. The compaction of the soil and the reduction of hydraulic conductivity explain the rise in runoff in the Sahel, compensating for the decrease in rainfall. Soil functioning is also the cause of runoff reduction in Sudanian regions, where temporary water storage sectors disappeared because of the drought. Crops commonly cause a reduction of soil bulk density and an increase in porosity; however,

Fig. 8. Comparison of the rainfall index in the Niger river basin at Niamey station, vs the discharge index of Niger at Niamey.

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Fig. 9. Evolution of streamflow of the main West African rivers (1900–2007).

Sahelian peasants generally do not practice tillage, and the shortening of alternations between crops and fallows leads to soil compaction and severe reduction of infiltration. Observing the changes in the runoff coefficient of Sahelian catchments in Burkina Faso, Albergel (1987) first suggested the hypothesis that, ‘‘The reduction of rainfall amount during the 1969–1984 period (he wrote in the middle of the great drought period) seems to be widely compensated for by the changes in soil surface features in the hydrological behaviour of small catchments; these modifications led to conditions more favourable to runoff, overall for catchments located northward from the isohyet of 800 mm”. Albergel described a phenomenon now observed in other areas of the Sahel, writing that, ‘‘Perennial graminaceaes diminish the benefit of annual plants; in tree species, the scarce young trees are all thorny ones, taking the place of combretaceaes”. In Niger, it was observed that Guiera senegalensis replaces the combretaceae due to its resistance to clearance (Seghieri et al., 2005). While Albergel (1987) thought that the effect of changing land cover was limited to the local scale, the Gorouol (46,000 km2) and Sirba (38,000 km2) examples show that in fact large basins, can experience an increase in both runoff coefficient and total discharge (Table 1). This increasing runoff coefficient also explains why some small Sahelian endorheic catchments are characterized by a rise in the water table. This type of rise in water table level is also noticed near Dakar, Senegal, where the Niayes area is now frequently flooded (observation by the authors). An increase in number of ponds, and their extension and duration is also established in the Malian Gourma (Mougin et al., 2009) although there is no noticeable land-use change there. Therefore, other factors, which are yet to be determined, are likely involved in this process and in this particular instance it must not be excluded that more intense rainfall at small time steps over the past ten years is one of these factors. Upscale effects are clearly a dominant factor when considering the case of the water table in the Lake Chad cuvette, which is at the same latitude as Niamey (See Fig. 1). Its level has been deepening for more than three decades (Fig. 5). Whereas Sahelian catchments in the Niamey region are filled by local water only, Lake Chad is filled by a tropical, humid Guinean-type river (the Chari), a Sudanian-type river (the Logone), and a Sahelo-Sudanian-type river (the Komadougou). The discharges of the two first rivers – which are

also the main contributors – have severely decreased since the beginning of the ‘‘great drought” in 1968. This explains the spectacular regression of the lake area and the deepening of the aquifer, even though, as Gaultier remarked (2004), the proportion of water table recharge infiltrating from Lake Chad is currently not clearly defined. Geological factors Le Barbé et al. (1993) noticed a hydrological opposition between plutonic and sedimentary areas in the Sudanian region of Northern Benin, in a Sudanian climate environment. Recent studies carried out in the framework of the AMMA-CATCH observing system by Kamagaté et al. (2007) have shown that, for catchments on plutonic rock substratum, the main discharge contribution comes from surface and subsurface flow, with minimal groundwater contribution. One instance is the Donga River Basin, a 586 km2 tributary basin of the Ouémé River studied. Kamagaté et al. (2007) estimate that, on this catchment, the contribution of subsurface flow to discharge ranges from 68% during a wet year, to 83% during a dry year. Upstream catchments in the Sudanian region may thus be much more sensitive to changing land cover influencing directly surface and subsurface flows, than in regions of dominant sedimentary geology. These considerations extend to the main difference observed between the two banks of the Niger River in its middle course. The plutonic bedrock on the western bank of the Niger River renders impossible the constitution of large aquifers, while the Iullemmenden sedimentary basin on the eastern side provides an important source of groundwater. In the Liptako Gourma area, in western Niger, runoff is favoured by exorheism and by the impermeable bedrock (both are linked). As a conclusion: (i) the reduction of the water holding capacity, closely linked to the degradation of the vegetation cover is the most important factor explaining the observed changes in the hydrology of the region; (ii) the geological and hydrographic context (endorheism versus exorheism) are spatial factors that combined with the land cover change effects to produce increasing or decreasing runoff coefficients and/or discharge in a period of strong rainfall deficit; (iii) different patterns of rainfall evolution have emerged in the last decade an are also acting as mitigating factors.

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Related effects Water resources: river regimes and aquifers Despite the increasing discharge of the Sahelian tributaries, the annual flood peak of the Niger river at the Niamey’s station has steadily decreased since the end of the 1960s (Lebel and Ali, 2009, Fig. 7; Amani and Nguetora, 2002; Amogu, 2009). The duration of high waters has also been reduced significantly, making water management more difficult. The Niger river’s yearly flood at Niamey (downstream of the Inner delta) is known to be composed firstly by the local flood (due to the contribution of the mid Niger basin tributaries, flowing in Sahelian areas) that occurs during the rainy season (traditionally June to early October); and secondly by the stronger, ‘‘Guinean” flood, coming from the Upper Niger basin and delayed 2–3 months as it traverses the inner delta. The Sahelian areas of mid-basin are constituted by right bank basins ranging from 5000 to 50,000 km2. These basins experienced an increase in their runoff coefficient and even their annual discharge (Fig. 3a and b). Thus, the first flood at Niamey is becoming shorter and stronger. This led to a decrease in discharge before the arrival of the main flood. Fig. 10 shows that the trough between the two floods is more frequent in recent years than before. Consequently, since 1984, the peak of the ‘‘local” flood at Niamey has been observed seven times to be higher than the peak of the main January flood, a situation which was observed for the first time in 1984, although flow records begin at Niamey in 1923. The increase in runoff coefficient and the linked reduction in soil water holding capacity of the Sahelian areas are due to land-use changes, mainly to clearance: this has been demonstrated recently as being the main cause of increase in runoff and the rise in groundwater level in the endorheic Sahelian area of the left bank of Niger river (Leblanc et al., 2008). The aquifer modification has also consequences in terms of water resources management. For one, the decrease of groundwater level in Sahelian plutonic areas lead to plan further surface storage as ‘‘tanks” due to the uncertainty in finding water with boreholes. On the other hand, the rise in water table observed in the Western part of the Iullemmenden basin allows the providing of more groundwater thanks to the digging of wells at maximum 80 m deep (the traditional wells can reach 120–150 m). However this is not to be considered as a benefice for the future, because it is due to the degradation of soil and vegetation. Furthermore, in some areas, it provokes the permanent appearance of water table, making traditional agriculture impossible.

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and rills are more numerous and widespread each year, as are sediment deposit cones and spreading areas. These factors have led to a set of environmental changes in endorheic areas which mainly affect hydrographical patterns: – The process of ‘‘pond migration”, due to the silting up of the valley bottoms: the temporary streams (named kori in the Hausa language) transport important quantities of sand and material in the bas fonds, cutting some ponds into two or more sections, increasing the ‘‘enclosure” of some valleys (natural sand dams). – Instability of kori bed: the important deposits of sand on midslope provoke the lateral migration of the kori course. – Changes in catchments: because of their instability, the koris outlet changes rapidly: the same kori can provide water to one or several ponds, generating a rapid evolution of the limits of ponds and their associated catchments. In northern Cameroon, Liénou et al. (2005) shows an increase in the concentration of sediment transported by the Sahelian Mayo Tsanaga River (1535 km2 at Bogo station) during the recent period of 2002–2004, in comparison with previous measurements taken before 1970. Amogu (2009) observed the same evolution in the mid Niger basin. At the hydrological basin scale, the rise in erosion results in a series of morphological changes in the hydrographical pattern: – A rise in river bed level due to the substantial accumulation of sandy material. – Strong widening of the same river bed, observed by the location of trees, which were previously located at the edge, and are presently found in the middle of the river (Fig. 11).

Related sedimentary balance An increase in erosion and sediment transport due to land degradation is one of the major evolutions currently observed in Sahelian hydrology (Chinen, 1999; Karambiri et al., 2003; Amogu, 2009). The cutting of vegetation, with the increase in runoff, is causing a severe increase in erosion and sedimentation. Gullies

Fig. 11. Kori of Hamdallaye (30 km Eastward from Niamey); some trees reached by the river bed evidence its widening.

Fig. 10. The increasing role of the Niger river local flow at Niamey’s station.

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– An accumulation of sand at the bottom of each valley, from the smallest kori to the Niger River, which can no longer get rid of all of the sediments carried out by its tributaries. The sediment budget evolution of a watershed is strongly linked to land-use changes. In the Niger Basin, silting up has been noticed for several decades and is probably due to the increase in cultivated areas and the parallel decrease in natural vegetation-covered areas. However, recent observations (E. Gautier and O. Amogu, march 2007, oral communication) show that, for the moment, there is still no morphological change noticeable in the Niger River bed stream’s central area (in the Republic of Niger), where the erosion and sedimentation of small creeks are the strongest, if compared to the situation in 1975. This could be due to the lag time in the response of the river pattern to the basin changes (sedimentary stock takes several decades to move several kilometres downstream, making changes in equilibrium in large rivers a long term process). The increase in sediment load is clearly closely related to land-use change, to the runoff coefficient increase, and to the hydrological changes in regime and discharge at the basin scale. Increasing pressure on the soils and environment:socio-economic impact – The mean millet crops productivity in Niger was 400 kg ha 1 in 1950 and only 330 at the beginning of the 2000’s (Guengant and Banoin, 2003), due to the scarcity of soil nutrients and the minimal use of fertilizers (Ada and Rockstrom, 1993). – Forest harvesting is strengthened when famine forces people to sell wood to buy millet when their own crops become insufficient. However, replacing bush with agricultural areas became no longer possible since nearly all of the available space is already cultivated; shortening the fallow period is not possible since in most areas, it has been suppressed. The last two causes mentioned above will become more widespread in the future, even if annual rainfall rises significantly in future years. It is worth noting that, in both cases, most of the water is lost via evapotranspiration in the semi-arid tropics. This PE is significantly reduced by the decrease in biomass and soil water holding capacity; the latter explains the low soil water availability after rainfall events and thus, the low level of evapotranspiration (which can potentially affect the climate as a feed back effect).

Conclusion Demographic pressure and the related overexploitation of the environment and natural resources cause changes in catchment water balance and sediment budget. In West Africa, the hydrological behaviour of the Sahelian and north Sudanian catchments and their evolution, facing considerable climatic change currently – characterized by a severe drought – depend on their geographical context: – Runoff has been increasing in Sahelian basins for three decades, despite a 20–25% decrease in observed rainfall during the period of 1968–1997. On the contrary, in the Sudanian climate, a 15% reduction in rainfall has led to a more intuitive reduction in runoff and annual discharges; for almost all rivers in the area, decrease of discharges has been greater than that of rainfall. As a result, the large rivers, mainly the Niger and the Senegal, have suffered a significant decrease in flows despite their considerable extension into Sahelian areas because their discharges are mainly provided by regions in the Sudanian climatic region.

– A raise in water table level has been observed for at least five decades in some endorheic areas in the Sahelian climate, due to the strong increase in the number of ponds, their size, and their duration, which explains an acceleration of infiltration. Conversely, the water table is deepening in other contexts, for example, in exorheic areas, or in the plutonic basement rocks of the western side of the Niger River (right bank), or in the large endorheic basin of Lake Chad. The most important contrast is between the Sahelian regions, whose hydrological behaviour is of the Horton-type, and the Sudanian areas which are characterized by ‘‘Hewlettian” hydrological processes, a system where water infiltration and water holding in soils and in temporary or perennial water tables plays an important role in water runoff and balance. It is possible to localize an area with few changes in the runoff coefficient near the 750 mm annual rainfall isohyet. This corresponds to the boundary between the Sudanian climate to the south, where rainfall in excess of evapotranspiration is available for runoff (and where stream flows have been diminishing during the last several decades), and the Sahelian climate to the north, where the runoff coefficient increases naturally due to the aridification of the climate and consequent modifications of the soil surface hydrodynamic properties. Due to its continuous evolution, linked to the continuous changes in regional land-use, the various factors influencing the variability of Sudano-Sahelian hydrology remain difficult to grasp. This is a strong motivation for continuing specific observations on the AMMA-CATCH sites for years to come: the climate and the environment being in a transient state, the hydrological regimes will likely continue to evolve with great consequences on water resources. Where runoff has increased, one important point requiring further investigation is the percentage of this increasing runoff that can be explained by the reduction of evapotranspiration (due to clearance), versus the percentage that can be explained by the reduction in the soil water holding capacity. This is key for extrapolating in the future under various scenarios of further vegetation clearance and rainfall regime modifications. Another important factor not addressed in this paper is the possible feedback of a changing environment and related water budget components on mesoscale and regional atmosphere dynamics. While the early work of Taylor and Lebel (1998) showed the existence of persisting rainfall patterns at the seasonal scale, possibly linked to feedback effects on the boundary layer, further modelling studies had difficulty to produce convincing evidence of the mechanism involved. Obviously if feedback effects do exist they will further affect the river regimes in West Africa, since land-use changes remain strong and widespread. Acknowledgements We warmly acknowledge our colleagues of the Niger Basin Authority (NBA), cellule Hydroniger, for providing us with the data on Niger River and rainfall in the basin, as well as the OMVS (Organisation pour la Mise en Valeur du Bassin du fleuve Sénégal), the OMVG (Organisation pour la Mise en Valeur du fleuve Gambie) and Dr. Lamagat of IRD for data of Senegal and Gambia river basins, our colleagues of the AMMA program in Benin for data of the Ouémé basin and the DGRE (Water Resources Service) of Burkina Faso for information on the Nakambé. This study was carried out in the framework of the AMMA project (African Monsoon Multidisciplinary Analyses). Based on a French initiative, AMMA was built by an international scientific group and is currently funded by a large number of national agencies, especially from France, UK, US and Africa. It has been the beneficiary of a major financial contribution from the European

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