Exorheism growth as an explanation of increasing

Catena 131 (2015) 130–139

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Exorheism growth as an explanation of increasing flooding in the Sahel Ibrahim Mamadou a, Emmanuèle Gautier b, Luc Descroix c,⁎, Ibrahim Noma d, Ibrahim Bouzou Moussa d, Oumarou Faran Maiga d, Pierre Genthon e, Okechukwu Amogu f, Moussa Malam Abdou a, Jean-Pierre Vandervaere g a

Dept of Geography, FLSH, University of Zinder, Zinder, Niger CNRS UMR 8591 LGP, Paris, France IRD, UMR PaLoc (MNHN/IRD), Dakar, Senegal d Dept of Geography, FLSH, Université Abdou Moumouni, Niamey, Niger e IRD, UMR HSM (IRD/UM1/UM2), Montpellier, France f Coyne et Bellier, Tractebel Engineering France, F-92622 Gennevilliers, France g UJF-Grenoble 1/CNRS/G-INP/IRD, LTHE UMR 5564, F-38041 Grenoble Cedex 9, France b c

a r t i c l e

i n f o

Article history: Received 14 July 2014 Received in revised form 18 March 2015 Accepted 23 March 2015 Available online xxxx Keywords: Sahel Exorheism Endorheism Runoff coefficient Horton-type runoff

a b s t r a c t For two decades, the Niamey area, in Niger, has undergone the creation of several new wadis (“koris” in hausa, the most spoken language in West Africa). The significant runoff increase in the Sahelian reach of Niger river makes us interested in the behavior of the basins of the tributary koris of Niger River in the Niamey area, in Niger. These koris generally formed during a single storm event, within depressions previously occupied by ponds; these ponds are overflown creating a new “kori”. This study examines in detail the causes of this new exorheism mechanism. The main explanation of this evolution has been determined as being the strong runoff increase, related to an extension of crusted soils due to agricultural practices, mostly the reduction of fallow duration. The degradation of their structural stability leads to crusting and a strong reduction of their hydraulic conductivity. This is linked to water and sediment balance at the catchment scale. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Despite the persistent climatic and agronomical drought in West Africa, the discharge increase of the main Sahelian rivers is going on (Descroix et al., 2013a). Concerning the Niger River, the discharge of its main Sahelian tributaries has been shown as increasing since the beginning of the drought (1970s), an unexpected observation that constitutes the “Sahelian paradox”. Albergel made this observation in 1987 by analyzing data from several experimental (small) catchments in Burkina Faso (Albergel, 1987). Then Mahé et al. (2003, 2005, 2011) observed that this increase in runoff applied in larger basins (the Nakambé River at Wayen, 20,000 km2); Mahé (2009), Mahé and Paturel (2009), Amogu et al. (2010), Descroix et al. (2012b) noticed the generalization and regionalization of this paradoxical fact in the whole Sahelian stripe (Figs. 1 and 2). Annual runoff coefficients of the Gorouol, Dargol and Sirba rivers (main tributaries of the Middle Niger River), are three times higher, and discharge 150% higher, at present than those observed fifty years ago (Fig. 3), as a consequence of land use change (Descroix et al., 2012b).

⁎ Corresponding author. E-mail address: [email protected] (L. Descroix).

http://dx.doi.org/10.1016/j.catena.2015.03.017 0341-8162/© 2015 Elsevier B.V. All rights reserved.

Otherwise, within an endorheic area near Niamey, this runoff increase was shown to cause a groundwater level rise (Leduc et al., 1997), due to the increase in the number, area and duration of ponds, ponds being the main groundwater recharge points. The Niger River has a double flooding pattern downstream from the Niger Inner Delta (NID) in Mali (Fig. 1) and the confluence with its Sahelian tributaries. Most of the stream flow comes from the mountains of Guinea (Fig. 1), and causes the main annual flood. It needs a few months to cross the NID. Therefore, the rainy season on the Sahelian area of the Middle Niger Basin triggers the appearance of a previous and secondary flood, before the main one. The most part of the Sahelian catchment of Niger River is located downstream from the NID and upstream from the city of Niamey (capital city of Niger Republic) (Fig. 1). Except the Niger River, all the tributaries have temporary flows, named “koris” (= wadi) in hausa (the most spoken language in West Africa): during the wet season (summer), water discharge rapidly increases after the monsoon onset. Most of the tributary koris of Niger River in its Middle reach are recent, less than 50 years (see Section 4.2) most of them were dug during only one single storm event in depressions previously occupied by ponds. The main aim of this paper is to determine whether there exist other land factors explaining the changes in the discharge and regime of

I. Mamadou et al. / Catena 131 (2015) 130–139

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Fig. 1. The Niger River Basin, and its two main providing water areas: the mountains of Guinea (West) in its Upper basin, and the Liptako–Gourma shield in its middle basin.

Sahelian Niger River and its tributaries. For this reason the contribution of the new koris is evaluated. Recent studies (Descroix et al., 2013b) show that there is an increase in extreme rainfall events unless reaching the number observed before the beginning of the dry period in 1968 (see Section 4.1).

2. Study area The right bank of the Middle Niger River area, named Liptako Gourma Massif (Fig. 1), is an exorheic area, contrary to the Iullemeden sedimentary basin lying on the left bank; the latter region, located

Fig. 2. The Niger River middle basin.

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The Sahelian environment consists of a mosaic of three distinct units: remaining shrub bush in the laterite plateaux (referred to as tiger bush because of its banded pattern), a patchwork of fallow savannah (Guiera senegalensis dominated) and rain-fed millet fields (D'Herbès and Valentin, 1997; Loireau, 1998). The two latter replace the original bushy and woody savannah vegetation (Piliostigma reticulatum, Bauhinia rufescens, Acacia sp., Balanites aegyptiaca, Prosopis sp.) of the large valleys due to increasing land clearance of most of the sandy slopes (Galle et al., 1999; Leblanc et al., 2008). G. senegalensis is accompanied by P. reticulatum and Combretum glutinosum in old fallow, and by Aristida mutabilis, Cenchrus biflorus and Digitaria gayana in the grass fallow. Pearl millet (Pennisetum glaucum) is the major crop; fallow and crops include some remaining big trees, mostly the “gao”, Faidherbia albida, known to improve the field fertility by providing nitrogen. The banded vegetation patch of the tiger bush is very dense and includes mainly Combretaceae (Combretum micranthum, C. glutinosum) and Boscia sp. (Descroix et al., 2012b). Fig. 3. Increase in runoff coefficient of the three main tributaries of the Middle Niger River. From Descroix et al. (2012b)

eastward from Niamey, is mostly endorheic and traditionally did not provide water to the main streams. The Liptako Gourma shield is the most water contributing area of the Middle Niger River basin (Fig. 3). The Liptako–Gourma shield is composed by a granitic basement where the main right bank tributaries of the Niger have their source (Fig. 3), as well as the main components of the Volta basin. The landscape on both banks of the Niger valley is dominated by dissected plateaux. Most of the topsoil has less than 15% of silt and clay (as measured with laser granulometer; Descroix et al., 2012a); however, it is very vulnerable to crusting (Van de Watt and Valentin, 1992; D'Herbès and Valentin, 1997), as observed in other semi arid areas (Sela et al., 2012).

3. Material and methods In order to determine the river discharge and regime factors of changes, two kinds of analyses are proposed. - An analysis of the hydrological data of the Niger river and its tributaries at the following stations: Kandadji and Niamey (Niger river), Alcongui (Gorouol river), Kakassi (Dargol river) and Garbey Korou (Sirba river) (Fig. 2); daily data of each station are available since 1957; they are provided by the Niger Hycos program of the NBA (Niger Basin Authority); rainfall data are supplied by the National Met Offices of Burkina Faso, Mali and Niger through the daily rainfall data of twelve long term data stations (Fig. 2). - A field monitoring of the boundary between endorheic and exorheic areas, including a census of cases of endorheism bursting, the so-called

Fig. 4. Newly exorheic basins in the vicinity of Niamey.


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Fig. 5. Evolution of rainfall index through the Middle Niger River basin; average index of the 12 stations indicated in Fig. 2.

endorheic areas becoming exorheic mostly due to pond overflowing. In this paper, this is applied only within the Niamey area (60 km upstream and downstream), as the more overexploited area of

Table 1 Temporal segmentation of the rainfall and discharge series 1957–2010 according to the Hubert procedure.

Rainfall Gorouol Annual rainfall/SD Rainfall Dargol Annual rainfall (mm)/SD Rainfall Sirba Annual rainfall (mm)/SD Rainfall 3 basins Annual rainfall (mm)/SD Runoff coefficient Gorouol Runoff coefficient %/SD Runoff coefficient Dargol Runoff coefficient %/SD Runoff coefficient Sirba Runoff coefficient %/SD Runoff coefficient 3 basins Runoff coefficient %/SD

1st period

2nd period

3rd period

4th period

1957–1965 479/46 1957–1967 573/85 1957–1967 763/86 1957–1967 602/57 1957–1979 1.8/0.6 1957–1987 5.6/2.7 1957–1983 2.5/1.3 1957–1981 2.45/1

1966–1982 356/50 1968–2010 436/81 1968–2010 570/82 1968–1982 448/46 1980–2010 3.6/1.6 1988–2010 13.4/5.4 1984–2006 4.3/1.8 1982–2001 4.11/1.6

1983–1990 257/44

1991–2010 413/81

1983–1987 332/22

1988–2010 486/76

2007–2010 8.9/2.5 2002–2010 6.4/1.8

the basin (Fig. 4). Since this demonstration needs heavy field works, extension or extrapolation of this research out of this area is not proposed for the moment.

4. Results 4.1. Hydrological changes The significant observed runoff increase (see Fig. 3) cannot be attributed to the rainfall evolution, as rainfall has strongly decreased since 1968 (Fig. 5). Fig. 5 shows that the reduction of rainfall began in 1968 while the partial return to the previous rainfall mean is almost observed from 1995. A set of statistical tests was performed on rainfall and discharge annual data on the three main right bank tributaries of the Niger and the three gathered, in order to detect ruptures and to determine a segmentation (Hubert and Carbonnel, 1987; Hubert et al., 1989; Paturel et al., 1996). During the first decade, runoff coefficients remain unchanged despite the severe decrease in rainfall; furthermore, discharge significantly increased at the beginning of the 1980s, when the drought became more accentuated (Table 1).

Fig. 6. Decadal hydrographs of Niger River at Niamey station.

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I. Mamadou et al. / Catena 131 (2015) 130–139 Table 2 Water balance of “small koris” after 1997.

Fig. 7. Comparison of the average hydrographs for the Niger at Kandadji and Niamey. After Amogu et al. (2010)

Otherwise, an increase in extreme rainfall event occurrence is observed since the second half of the 2001–2010 decade (Descroix et al., 2012b; Panthou et al., 2013; Descroix et al., 2013b). The proportion of annual rain amount fallen in high amount rainfall events increased. However, neither the number of rainy events above certain high thresholds, nor the amount of rain fallen during these events has exceeded those observed during the 1950s and 1960s, before the dry period. Thus it cannot explain the increase in runoff observed since the beginning of the drought, unless determining that the relative decrease of number and rainfall amount in small events should affect the vegetation cover? Analyzing the river discharge and regime highlights three main changes observed in the evolution of the middle Niger River basin. i) The first change is the general decrease of the stream flow of the Niger River. Fig. 6 shows that two “humid” decades (1951–1970) were followed by two “dry” ones (1971–1990). Then, the 1991– 2010 decades were characterized by a relative increase in discharge, which remains lower than during the wet previous period.

Year

Rainfall (mm)

Runoff depth (mm)

Runoff coefficient

1998–99 2001–02 2006–07 2007–08 2008–09 2009–10 2010–11

573 364 323 406 410 410 400

31.6 26.4 67.4 95.9 90.0 125.9 73.7

0.06 0.07 0.21 0.24 0.22 0.31 0.18

ii) The second important change lies in the relative contribution of the two floods registered in the Sahel. The Niger regime downstream of the NID is bimodal; two successive floods are observed, although they were uneasy to distinguish in the yearly hydrograph. The first flood begins with the heaviest monsoon rainfall occurring in August, being mostly provided by three Sahelian tributaries (Gorouol, Dargol, Sirba) on the right bank of the Niger River; this first local flood is called “red flood” because of its sediment load arising from the erosion of local iron oxide rich soils. It lasts generally until October and it is clearly separated, due to their respective color, from the main, second, flood that starts approximately at the beginning of November. The second flood (November–March) has a low sediment load and originates from rainfall in the upstream part of the Niger River (Fig. 1). This second flood, called “Guinean flood” or “black flood”, lasts until March and then the Niger River discharge decreases until the next monsoon (rainy season) (Descroix et al., 2012b). Usually, the Guinean flood was longer and larger than the local red flood. For the last two decades, an earlier occurrence and a rise in the first “red” flood are observed. Fig. 6 clearly shows that the summer flood occurs one month earlier than during the 1950s, and it is stronger since the 1990s. This trend is reinforced during the 2001–2010 decade. At present (2001–2010 period), the first flood can easily be distinguished from the Guinean flood, with a peak in early September reaching 1400 m3 s−1, this discharge value being unknown before the 2000's. As it was noticed by Olivry (2002), traditionally, the Niger river discharge was diminishing downstream of Koulikoro (Mali)

Fig. 8. Water balance between Kandadji and Niamey 1975–76/2010–11 (updated from Amogu et al., 2010), without the discharges of Dargol and Sirba rivers.


(Fig. 1) due to water evaporation within the NID, and up to Niamey due to infiltration and evaporation higher than the new water incomes to the river balance. Fig. 7 compares hydrographs at Kandadji and Niamey for the periods before and after 1997 (Amogu et al., 2010). Hydrographs for Kandadji and Niamey gauging stations were relatively similar before 1997, except a lower discharge in Niamey due to evaporation and infiltration between the two stations. The last period (1998–2007) clearly undergoes a differentiation during the summer flood that is much stronger at Niamey station. During the local flood, there is clearly a new stream flow input. The difference expresses the contribution of the tributaries of the middle Niger River. The water surplus must be due to the water supplied by the two right bank tributaries (Dargol and Sirba) as well as by the right and left bank small “koris” tributaries. iii) The third change, close from the latter, is an increasing contribution of the small tributaries located in the middle valley (“small koris tributaries” in Fig. 2). Fig. 8 shows the water balance between Kandadji and Niamey without the discharge of Dargol and Sirba rivers, in order to evaluate the contribution of the small koris (years without data in Fig. 8 are due to a lack of data at the Kandadji hydrometrical station). Then it only includes the incomes of the “small koris” tributaries of the two banks. Fig. 8 highlights that, before 1997, the balance was negative except for a few cases with very weak surplus. After 1997, only positive data are observed. This means that the “small koris” are mostly responsible to the recently observed increase in runoff of Niger River between Kandadji and Niamey. Table 2 gives, for the few years where all data are available and accounting the catchment area (27,050 km2), the runoff coefficient of this area of “small koris”. Values are large, around 20%, and significantly higher than that observed in the great right bank tributaries. Are land use changes observed in the Niamey area able to explain such a runoff coefficient? This is also likely due to the small size of the basins; as it is well known that runoff coefficients decrease with the basin area increase. The Niamey region shows important discharge changes, characterized by an increasing discharge during summer due to an important

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growth of the contribution of the small seasonal wadis in the middle valley. 4.2. The formation of new tributaries, “koris” A meticulous study of the topography in the vicinity of Niamey allows us to determine the occurrence of modifications in the boundary between endorheic and exorheic areas. Endorheism is intrinsically linked to the water cycle in arid and semiarid regions. The Niamey area shows new landforms (koris valleys) connecting ancient endorheic depressions directly with the Niger River. Fig. 9 shows the location of some of these events that occurred in the Niamey area. The appearance of some of them (named endorheism bursting in Fig. 9) could be dated according to local witnesses. For some of the “endorheism bursting” cases mapped in Fig. 9, the approximate formation date is indicated; this was determined owing to an inventory based on the analysis of aerial photographs of 1950, 1975 and 1992 (provided by IGN France and IGN Niger). The more recent was observed by the authors; the Kourtéré kori formation is described in Chinen (1999). The koris channels occupy ancient depressions that were previously occupied by temporary ponds, and separated by sandy dunes; the dunes were formed during dry stages of the early Holocene on both sides of the Niger valley. In some places close to Niamey, kori beds were rapidly dug as ponds overflew, eroding the sandy plugs that isolated the ponds. Kori beds were generally initiated during a single storm event, and were deepened and widened during the following wet seasons. By that way, the mechanism has connected the previous endorheic depressions to the Niger valley. This endorheism bursting led to transform ancient endorheic areas into exorheic valleys. About 12 cases of new exorheism were observed or supposed in the sole area of Niamey (60 km around the city, Figs. 4 and 9); they are characterized by the presence of: – an ancient pond area, occupied by black soils (high organic matter content) (Fig. 10); – a deep rapidly widening channel cut in sandy dunes with steep banks (Fig. 11) by the sudden emptying of the pond; and

Fig. 9. Location of endorheic bursting observed and supposed occurrence in the last decades in the area of Niamey.

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Fig. 10. Aerial view of the new Leli Maman Niaulé kori, dug in 1998; blue circles surround the location of two ancient ponds, emptied by the erosion of the dunes that separated them. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

– at the confluence with the Niger River, a large alluvial fan due to the deposit of large volumes of sand, transported during flash floods (Figs. 12 and 13).

In fact, the occurrence of endorheism bursting is linked to the runoff increase observed over the whole Sahel region. Since the annual number of extreme rainy events did not reach the one observed during the 1950s and 1960s decades, it is supposed that the overflowing of the

ponds in the vicinity of Niamey is due to an increase of the runoff coefficient rather than a change in rainfall. Most of them occurred after the onset of the 1970s' drought. Runoff increase is caused by soil degradation and by the decrease of the soil water holding capacity resulting from the extension of cropping and the fallow shortening. Most of the area of the newly connected kori basins is constituted by bare and degraded soils. Sometimes, crusted soil areas increased significantly in recent decades and also explain the runoff increase. The Kori Boubon basin, located 25 km NW from Niamey (Fig. 4), illustrates the evolution of soils, runoff and kori dynamics (Fig. 14). On this catchment, bare soils and crusted areas represent 26% and 3% respectively of the total area in 2005 (Mamadou, 2012). The diachronic analysis based on aerial pictures reveals that during the 1950s the Boubon basin was endorheic. The 1975 aerial picture shows a continuous kori bed connected to the Niger Valley (Fig. 14).

Fig. 11. Riverbed of the new Leli Maman Niaulé kori, appeared in 1998.

Fig. 12. Alluvial fan of the Kourtéré at its confluence with Niger River, just upstream from Niamey.

No one of these new "koris" appeared in the aerial photographs of 1950; many of them appeared in the one of 1975, some others only in the one of 1992, two appeared in September 1998, two more in recent years. 4.3. Causes and consequences of exorheic expansion


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Fig. 13. Evolution of the Kourtéré kori alluvial fan at its confluence with Niger River.

Even if it is difficult to estimate precisely the limit of the kori catchments, their total area around Niamey exceeds 1700 km2 (Figs. 4 and 9), without accounting for the Ouallam valley that is semi-exorheic (Northward from Niamey, see Fig. 9). Therefore, the recently connected kori catchments represent an important extension of the Middle Niger River basin, explaining the increasing water contribution of these tributaries. Thus it is proposed here that the water surplus registered between Kandadji and Niamey is partly related to the extension of exorheic surfaces. 5. Discussion: other hydrological factors of inundation and endorheism bursting West Africa is undergoing important hydrological and geomorphic changes (Amogu, 2009; Mamadou, 2012), triggered by the drought and human pressure. The specificity of the middle Niger valley is the creation of new tributaries mainly controlled by the increasing runoff on slopes and plateaus. These processes strongly impact local populations, mainly in terms of risks and loss of soil fertility. In recent decades, two main factors have aggravated the flooding intensity and then the inundation hazard in the Niger valley: – Soil degradation is provoking a silting up of Niger riverbed which reduces the stream section and enhances the water level for a given discharge (Amogu, 2009; Mamadou, 2012); this has been

noticed in the stream gauge station of Niamey of which rating curve was modified in 2011. Fig. 13 shows an example of the rapid rise of an alluvial fan on the Kourtéré kori (see also Fig. 12) since 1984. On this fan, the flood of 1st September 1998 broke the bridge of the Niamey–Namaro road. Deposition on several alluvial fans caused a narrowing of the Niger riverbed and a significant decrease of the Niger River stream section (Fig. 13). The alluvial fans are constructed during flash floods occurring on the koris during the monsoon and the main river is not able to remobilize all sediment loads, because of the shortening of the Guinean flood. Then, Niamey undergoes more frequent inundations especially during summer (Sighomnou et al., 2013), for two main reasons: the discharge increase during this period and the sedimentation on the riverbed. Fig. 15 shows the inundated right bank of the Niger River in Niamey in August 2012. Recent increase in flooding hazard is mostly due to land use changes (resulting in a reduced soil water holding capacity in the catchments) and to recent increasing of extreme rainfall occurrence; however, the inundation is aggravated by the silting up which reduced the maximum possible stream flow within the Niger channel. – A second factor could also explain the hydrological behavior change of the Niamey region. The CT3 (as Continental terminal 3rd) water table has been clearly rising for the last five decades (Desconnets, 1994; Desconnets et al., 1997; Leduc et al., 1997; Leblanc et al., 2008). The water table rising-up could provoke a saturation in the

Fig. 14. Extension of crusted soil area since 1975 in the Boubon basin.

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Fig. 15. 18th August 2012 flooding, right bank of the Niger River, Niamey.

“bas-fonds” (depressions) and enhance the possibility of Hewlettian processes, contributing to the runoff increase. The groundwater rise in the sedimentary, mostly endorheic Iullemeden basin is linked to the increase in stream flow, which led to an increase in the number, the size and the duration of the ponds. As ponds are the most important groundwater recharge points, its results in a water table level rise (4 m in 3 decades; Leblanc et al., 2008). This favors the saturation, particularly in the “bas fonds”, the natural bottom of the sandy valleys. Fig. 16 shows that on the plateaus surrounding the Niger valley, the water table is very shallow (Monfort, 1996, cited by Favreau, 2000). This is evidenced by: ○ the presence of ponds due to water table outcropping. Two types of ponds can be distinguished: “bangou bi” that are black ponds, mainly associated to water table, and “bangou kirey”, red ponds, due to runoff; ○ the presence of springs in at least four valleys on the right bank of the Niger River near Niamey. – A local factor of endorheism bursting lies in the existence of a dune ridge on the right bank of the Niger River (Fig. 4), some hundred meters or few kilometers westward from the river; it should have retained water coming from the Liptako shield, commonly exorheic. This formed numerous ponds, thus small (some km2 or tens of km2) endorheic basins; many of them were overflown in last decades, increasing the exorheic area.

Fig. 17. Runoff coefficient vs catchment in granitic regions of middle Niger River basin. Catchments areas: Gorouol 45,000 km2, Sirba 38,000 km2, Dargol 7000 km2, Boubon 160 km2, catchment MH (as Mélé Haoussa experimental site, near Boubon) 6 ha each one, plot MH 10 m2 each one.

The runoff coefficient measured (Table 2) for the gathered tributary koris is significantly higher than the ones observed for the right bank tributary basins. Fig. 2 shows that for the last decade, the Gorouol basin has a runoff coefficient around 5%, the Dargol 15% and the Sirba 7.5%; furthermore these basins are entirely located in crystalline area, known to have higher stream flows than the mostly endorheic sedimentary area (Descroix et al., 2012b). There is an inverse relationship between the area of the watershed (in log in order to consider very different spatial scales) and its runoff coefficient in granitic basement region (Fig. 17). The “small” tributary basin areas range from 30 to 1000 km2; catchments have a runoff coefficient of 18–25%, close to the coefficient measured on the Boubon basin and then consistent with the mean values evaluated in Table 2 for the last five years. Mamadou (2012) estimated the runoff coefficient of the kori Boubon around 20%. Moreover, these direct tributaries have small basins (see above) flowing directly to Niger River with mean slope of 2 to 3 m·km− 1, at least one order of magnitude higher than the slope of the three right bank tributaries. In the same way, it is worth to notice that the runoff coefficient of the Dargol basin is significantly higher than this of its two greater neighbor basins. Finally, the area near Niamey has been strongly overexploited in recent decades; this explains the endorheism bursting occurrences. All this area is likely to have higher runoff coefficients than the surrounding Sahelian areas, due to land use change causing extension of bare and crusted soils.

Fig. 16. Outcropping of the water table. In the valley bottoms in the Niamey area. Adapted from Monfort (1996), cited by Favreau (2000)


6. Conclusion Land use changes have been identified as the main factor of the “Sahelian hydrological paradox”, the increase in runoff observed since the beginning of the drought. In recent years, an increase in extreme rainfall events probably contributes to increase stream flows and to generate inundations. However, some other factors contribute, in the region of Niamey, to enhance the stream flow and the inundation hazard: i — the increase in the catchment area by endorheism bursting, resulting in an extension of exorheic functioning, has been determined in this area where soil overexploitation led to the appearance of large areas of crusted soils; ii — the silting up is aggravating the water level of the Niger channel during floods, provoking more frequent inundations; and iii — finally, we hypothesize that the shallow water table, with a rising up to several tens cm per year for three decades at least, could trigger soil saturation in the “bas-fonds”, yielding a “Hewlettian” behavior on the bottom of the tributary valleys, prone to avoid infiltration and strongly increase runoff processes. Acknowledgments This study was partially funded by the ANR ECLIS (Contribution of livestock to the reduction of rural population vulnerability and to the promotion of their adaptability to climate and society changes in Sub-Saharan Africa) (ANR-08-VULN-003-01/ECLIS) French program, and by the ANR ESCAPE ANR-10-CEPL-005 (Environmental and Social Changes in Africa: Past, present and future) French program. The French Spatial Research Center CNES funded the acquisition of satellite data through the ISIS procedure numbers 396 and 719. The JEAI SAPALOTE also funded some field works. Hydrological data of great basins were provided by Niger HYCOS program of the NBA (Niger Basin Authority) which is fully acknowledged here. References Albergel, J., 1987. Sécheresse, désertification et ressources en eau de surface — Application aux petits bassins du Burkina Faso. The Influence of Climate Change and Climatic Variability on the Hydrologic Regime and Water Resources (Proceedings of the Vancouver Symposium, August 1987). IAHS Publ. no. 168 (http://iahs.info/redbooks/ a168/iahs_168_0355.pdf). Amogu, O., 2009. La dégradation des espaces sahéliens et ses conséquences sur l'alluvionnement du fleuve Niger Moyen. (PhD thesis), Joseph Fourier University, Grenoble (420 pp., title in French but the document is written in English). Amogu, O., Descroix, L., Yéro, K.S., Le Breton, E., Mamadou, I., Ali, A., Vischel, T., Bader, J.-C., Moussa, I.B., Gautier, E., Boubkraoui, S., Belleudy, P., 2010. Increasing river flows in the Sahel? Water 2 (2), 170–199. Chinen, T., 1999. Recent accelerated gully erosion and its effects in dry savanna, southwest of Niger. Human Response to Drastic Changes of Environments in Africa. Faculty of Economics, Ryutsu Keizai University Publication, Hirahata, Ryugasaki, Japan, No 120, pp. 67–102. Desconnets, J.C., 1994. Typologie et caractérisation hydrologique des systèmes endoréiques en milieu sahélien (degré carré de Niamey - Niger). (PhD thesis), de l'Université des Sciences et Techniques du Languedoc (326 pp., 110 fig., 54 Tabl.). Desconnets, J.-C., Taupin, J.-D., Lebel, T., Leduc, C., 1997. Hydrology of the HAPEX-Sahel Central Site: surface drainage and aquifer recharge through the pool systems. J. Hydrol. 188–189, 155–178. Descroix, L., Laurent, J.-P., Boubkraoui, S., Ibrahim, B., Cappelaere, B., Bousquet, S., Mamadou, I., Le Breton, E., Quantin, G., Boulain, N., 2012a. Experimental evidence of deep infiltration under sandy flats and gullies in the Sahel. J. Hydrol. 424–425, 1–15. Descroix, L., Genthon, P., Amogu, O., Rajot, J.-L., Sighomnou, D., Vauclin, M., 2012b. Change in Sahelian Rivers hydrograph: the case of recent red floods of the Niger River in the Niamey region. Glob. Planet. Chang. 98–99, 18–30.

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