Change in Sahelian Rivers hydrograph: The case of recent red floods of the Niger River in the Niamey region

Global and Planetary Change 98–99 (2012) 18–30

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Change in Sahelian Rivers hydrograph: The case of recent red floods of the Niger River in the Niamey region Luc Descroix a,⁎, Pierre Genthon b, Okechukwu Amogu a, c, Jean-Louis Rajot d, e, Daniel Sighomnou f, Michel Vauclin g a

IRD/UJF-Grenoble 1/CNRS/G-INP, LTHE UMR 5564, BP 53, 38041, Grenoble cedex 9 France IRD/UM2-UM1-Montpellier/CNRS, HSM UMR 5569, MSE place Eugène Bataillon, 34095 Montpellier cedex 5, France Tractebel Engineering France, 92622 Gennevilliers, France d IRD/UPMC-CNRS-INRA-ENS-AgroParisTech-UPEC, BIOEMCO UMR IRD 211, 61, avenue du Général de Gaulle, 94010 Créteil Cedex, France e UPEC/U-Paris Diderot/CNR, LISA, UMR 7583, 94010 Créteil Cedex, France f NBA Niger Basin Authority, projet Niger-Hycos, BP 729, Niamey, Niger g CNRS/UJF-Grenoble 1/IRD/G-INP, LTHE UMR 5564, BP 53, 38041, Grenoble cedex 9 France b c

a r t i c l e

i n f o

Article history: Received 11 May 2012 Accepted 31 July 2012 Available online 5 August 2012 Keywords: red flood monsoon West Africa land use change endorheism bursting soil crusting

a b s t r a c t Changes in the hydrological regime of Sahelian Rivers are considered based upon the example of the Middle Niger River and its exceptional flood in 2010 near the city of Niamey. It is shown that rainfall in 2010 was only average with respect to the long term record, with neither the monthly rainfall distribution in terms of the amount of rainfall nor the distribution of rainy events changing significantly in the last few decades. Particularly, no increase in the number of extreme rainfall events is observed. In spite of this, the Niger River's right bank tributaries have shown a sharp increase in runoff since the 1970s, which is still ongoing, and has resulted in a modification of the Niger River's regime from a single hydrograph to a two flood hydrograph, the local flood, occurring during the rainy season being the more pronounced one. This modification is likely due to an increase of bare soils and crusted soil areas as a consequence of human pressure, resulting mostly from the spatial extension of crop areas and the shortening of fallow periods. Changes in connectivity of the river networks on both banks of the Niger such as endorheism bursting events also caused an increase in the contributing basin area. Policy makers should be alerted to the effects of intensive cropping, land clearing and overgrazing in some areas, on the hydrological regimes of Sahelian Rivers. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Large rivers, spanning whole continents provide a sensitive marker of hydrological changes induced by climate or mostly by man. For example Raymond et al. (2008) showed that agricultural development changed the chemical balance of the entire Mississippi basin. Delgado et al. (2010), using statistical analyses demonstrated the increasing frequency of flooding events in the Mekong basin, while Lacombe et al. (2010), showed the importance of deforestation resulting from bombing during the Vietnam War in the middle Mekong basin. Conversely, Liang et al. (2010) found that the drastic change of the Yellow River in China involving zero discharge events was mainly influenced by climate, i.e. precipitation and temperature. West Africa suffered recently – during the 1970s and the 1980s – from a severe drought which is considered as one of the most significant

⁎ Corresponding author. Tel.: +33 456 52 09 97. E-mail address: [email protected] (L. Descroix). 0921-8181/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gloplacha.2012.07.009

climatic events worldwide (Hulme, 2001). In the Eastern part of West Africa, the intensity of the drought has been largely attenuated since the 2000s, but persists in the western part of the region (Nicholson, 2005; De Wit and Stankiewicz, 2006; Ali and Lebel, 2009). The drought resulted in contrasted effects in the Sudanian (1300 mm> annual rainfall amount (P) > 700 mm) and Sahelian regions (Pb 700 mm). The flow rates of Sudanian rivers were found to be decreasing (Amogu et al., 2010) which was suggested by Mahé (2009) as resulting from a drop in aquifer water levels that sustain these rivers during the low flow period. Conversely, an increase in discharge of Sahelian rivers was observed by Mahé et al. (2005) and Descroix et al. (2009) and was attributed to changes in the runoff properties of soils in the Sahel resulting both from overcropping and from non reversible effects of the drought (Gardelle et al., 2010). The Niger River spanning a large part of West Africa has its source in the humid mountainous Fouta Djallon area of Guinea, and then flows northwards towards the northern area of the Malian Sahel before turning southwards towards Niamey and then Nigeria (Fig. 1). In its northern part, the Niger River does not receive water from

L. Descroix et al. / Global and Planetary Change 98–99 (2012) 18–30

19

3° E

Gao

16° N

Inactive Niger Basin

Stream gauge

Active Niger Basin

Rain gauge (daily rainfall) Rain gauge (annual rainfall)

Kandadji Sm

al

Active basin of the small tributaries Inner Niger Delta (IND)

lt

Gorouol

rib ut

ar

Darg

Niger

ie

ol

Origin areas of the two floods

s

Niamey

13° N Sirba

KOULIKORO

0

Bani

NIAMEY

100

200 km

1° W

e nu

Be

0

250

500 km

Fig. 1. The Niger River basin. Location of the two main providing areas explaining the two floods in the Niger's hydrograph at the Niamey station. The Guinean flood originates from the highlands of Guinea, Southern Mali and Northern Ivory Coast included in the western blue circle, while the local flood originates from the surrounding of the Niger River immediately upstream Niamey (eastern blue circle). Enlargement: the main Sahelian tributaries of Niger River influencing the regime and discharge at the Niamey station; location of cited rain and stream gauge stations is indicated.

any tributary, and active tributaries are only found south of the Niger-Mali border. Therefore, in Niamey, the Niger River has two flood peaks; the first one, termed the red or local flood, arises from local rainfall, drained by a series of tributaries (Fig. 1, enlargement) and occurs between August and September. The second one, named the Guinean flood originates from water fallen in the Fouta Djallon during the rain season (June–September). It is delayed by its transport and the crossing of the Niger Inner Delta (see Fig. 1) in Malian territory and takes place around January. The existence of these two distinct floods makes it possible to separate the local Sahelian effect from the remote trend due to the Guinean flood. An extensive hydrological database is now available for the Niger River spanning back to the 1950s based upon the work of the AMMA project (Lebel et al., 2009) and on the setting up of the Niger Basin Authority (NBA) in 1981. This database also includes data from the Meteorological services in Niger, Burkina Faso and Mali. NBA data are graphically displayed on the NBA website, with full data accessibility requiring a data use agreement with the NBA (NBA, 2012). This database provides insight on hydrograph changes of the Niger River. After a description of the regional setting and a presentation of the materials and methods, we show that the Niger River's regime at the Niamey station has changed from a continuous flood to a two-flood hydrograph as a result of a drastic increase in the discharge of local tributaries of the Niger River near Niamey. It is also shown that this is neither

due to changes in the amount of rainfall nor to the distribution of rainfall. Furthermore, we present a summary of land cover changes that have probably enhanced runoff in the Sahelian area near Niamey and led to endorheism bursting events that have increased the size of the hydrological basin drained by the Niger River upstream of Niamey. The main findings of this paper are discussed and summarized in Sections 5 and 6. 2. A dromedary becomes a camel in the Niger River Hydrograph The hydrograph of the Niger River has changed significantly since the beginning of the drought of the 1970s and 1980s, from a one-peak hydrograph to a twin-peak hydrograph, with an enhanced early flood which is the flood phase that severely damaged the area of Niamey (the capital city of the Republic of Niger) in August and September 2010. This first flood begins with the heaviest monsoon rainfall occurring in August, being mostly transported by three Sahelian tributaries on the right bank of the Niger River (Fig. 1). The Gorouol River (basin area of 44,900 km2), the Dargol (6940 km2) and the Sirba (38,750 km2) are temporary rivers, termed as “koris”, and they run dry a few weeks or months (depending on basin size) at the end of the rainy season. They are supported by a system of smaller koris on both banks (total contributing area of 27,020 km2) (Fig. 1). This first “local” flood (Fig. 2) is also known as the “red flood” because of its sedimentary load arising from

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L. Descroix et al. / Global and Planetary Change 98–99 (2012) 18–30

a

b 2500

2500

single year

Discharge of the Niger River (m3/s)

10 yrs-average

2000

2000

Climate

Years

Partial recovery

2000 - 10

2010-11

1990 - 00 1980 - 90 Drought

1500

1500

1970 - 80 1960 - 70 1958-59

1000

1000

Wet 1965-66

1950 - 60

1994-95

500

500

0

0

1979-80 1988-89

1-Jun. 1-Aug. 1-Oct. 1- Dec. 31-Jan. 2-Apr.

1-Jun. 1-Aug. 1-Oct. 1-Dec. 31-Jan. 2-Apr.

2-Jun.

2-Jun.

Fig. 2. The hydrograph of the Niger River at Niamey. a. Mean decennial hydrograph b. Individual hydrographs referring to the year with the earliest first flood in a given decade.

r

Small

ive

rR

ge

tributa

Ni

Gao

the two driest decades (1970s and 1980s) show that the second flood was less marked than during the previous period and that the two floods were linked without rupture (Fig. 2a). The peak of the second flood occurred on average in mid-December and the hydrograph's falling limb was at least two months earlier than during the previous wet period (1950–1968). During the last two decades, the main flood had mostly the same pattern, confirming that the rainfall deficit remains severe in

ries

the erosion of local iron oxide rich soils; it lasts generally until October and is clearly separable, due to the colour difference, from the main flood that starts in November, having a low sediment load and originating from rainfall in the upstream part of the Niger River (Figs. 1 and 2). This second flood, also referred to as the Guinean flood or “black flood”, lasts until the end of February and then the Niger River's flows diminish until the next monsoon (rainy season). Mean decennial hydrographs for

Hombori

Menaka

Alcongui Djibo

Ayorou

Gorom Gorom

Kandadji

Gorouol

Kakassi

Darg

ol

Dori

Sebba

Niamey

Garbey Korou

Sirba

Fada N’Gourma

er r Riv

Nige

Bogandé

Ougadougou Stream gauge Rain gauge (daily rainfall) 0

100

200 km

Rain gauge (annual rainfall) Infiltration

Fig. 3. Location of cited basins, rain gauges stream gauges and infiltration experiment stations in the middle Niger River basin; the names of the stream gauge stations are in bold characters; those of daily rainfall stations are in regular characters.

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Table 1 Periods and segmentation of the time series for rainfall, runoff and onset flood date according to the Hubert procedure.

Rainfall in middle Niger River basin

Years Mean/St- Deva Years Mean/St -Dev Years Mean/St-Dev Years Mean/St-Dev Years Mean/St-Dev

Runoff coefficient Dargol Runoff coefficient Gorouol Runoff coefficient Sirba Onset flood dateb Niger a b

Period 1

Period 2

Period 3

Period 4

1950–1967 569 +/− 55 1956–1981 5 +/− 2,1 1957–1979 1.9 +/− 0.6 1956–1983 2.6 +/− 1.3 1950–1970 195 +/− 12

1968–1982 421 +/− 45 1982–1993 8.9 +/− 3.7 1980–2009 3.7 +/− 1.5 1984–2009 4.6 +/− 1.9 1971–1989 179 +/− 10

1983–1987 311 +/− 22 1994–2009 13.6 +/− 4

1988–2010 460 +/− 71

2010 14.4 1990–2010 155 +/− 10

St-Dev = standard deviation. Number of the first day of the local flood.

the western part of the Sahelian and Sudanian areas where most of the discharge comes from. Moreover, the flood recession became much faster than in the previous wet periods. The first or local flood was, on the contrary, more marked in last decades than previously. This first flood is becoming a distinct and independent flood, reinforced and occurring earlier (overall for the last decade 2001–2010, see Fig. 2a) suggesting an increase in runoff during the monsoon in the Sahelian zone. Individual hydrographs for the earliest first flood of each decade are given in Fig. 2b, providing hints on interannual variability which is smoothed in decennial hydrographs. The aim of this paper is to determine the respective roles of climate change and land use change in the explanation of such a modification in hydrological functioning.

3. Regional setting The Niger River runs in a southeasterly direction for hundreds of kilometres after it exits the Inner Delta (NID) in Mali, and again traverses most of the central Sahelian belt. We focus here on the reach located between Kandadji and Niamey stream gauge stations (see Figs. 1 and 3). The Middle Niger River basin is characterised by a relatively flat topography without steep slopes. Landscape is composed of plateaux and long hillslopes connecting with “bas-fonds” (low lying areas) frequently occupied by ponds. The river runs at an altitude close to 200 m and the maximum observed altitude is less than 400 m. This area has a Sahelian, semiarid climate, with a potential evaporation near 3500 mm yr − 1 (Descroix et al., 2012), and annual precipitation ranging from 300 mm at Gao (northward) to 570 mm (mean annual value 1905–2003 at Niamey Airport). 90% of the annual rainfall, mostly of convective origin, occurs between June and September (Leblanc et al., 2008). The landscape of the Middle Niger River basin is characterised in its western bank by the granitic basement of the Liptako-Gourma Massif, while the eastern bank lies on the sedimentary basin of Iullemeden, the contact area being located more or less along the Niger River. The Eastern bank of the river is dominated by the Miocene terrigenous deposits of the Continental Terminal. It covers the Precambrian crystalline basement complex, part of the pan-African shield (D'Herbès

Table 2 p value of Student test on the difference of runoff coefficients. Basin

Gorouol

Sirba

Dargol

Periods 1957–1979 and 1980–1994 Periods 1957–1979 and 1995–2010 Periods 1980–1994 and 1995–2010

0.013 0.022 0.55

0.019 0.086 0.32

0.079 0.0004 0.1

and Valentin, 1997). The landscape on both banks of the Niger valley, is dominated by dissected plateaux; it is extensively cultivated with pearl millet. Most of the topsoil has less than 15% of silt and clay (as measured with laser granulometer; Descroix et al., 2012); however, it is very vulnerable to crusting (Van de Watt and Valentin, 1992; D'Herbès and Valentin, 1997), as has been observed in other semi arid areas (Sela et al., 2012). 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 savanna (Guiera senegalensis dominated) and rain-fed millet fields (D'Herbès and Valentin, 1997; Loireau, 1998). The two latter vegetation types have replaced 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 soil fertility by providing nitrogen. The banded vegetation patch of tiger bush is very dense and includes mainly combretaceae (Combretum micrantum, C. glutinosum) and Boscia sp. Soil water balance is controlled more by surface than by deep soil conditions (Collinet and Valentin, 1979; Chevallier and Valentin, 1984; D'Herbès and Valentin, 1997). In particular, soil crusts, which develop even in very sandy soils, impede infiltration (Vandervaere et al., 1997; Descroix et al., 2009, 2012). Rainfall is characterised by short and high intensity events which contribute to crusted soils and result in Hortonian runoff (Leblanc et al., 2008). The rural population, largely dependent upon rain-fed cropping, is spread over numerous villages with a few hundreds of inhabitants. The population increase for the second half of the 20th century was shown to be representative of West Africa (i.e., near + 1.5% yr −1 in the 1950s, increasing to 3% yr −1 in the 1980–2000s) (Leblanc et al., 2008). 4. Material and methods The data coverage used for the present study is outlined in Fig. 3. Runoff is measured at the stream gauges of the 3 sub-basins and at Kandadji and Niamey stations for Niger River. Runoff coefficient is measured for hydrological years, lasting from June 1st to May 31st of the following year, as the ratio of the total annual discharge of a basin by the total annual volume of rain fall in the same basin. The validity of stream gauge rating curves was verified and found to be stable, having been installed in places where there is no sedimentation or modification of the channel. In the case of Niamey area, where silting up could exaggerate the flood of 2010 (Mamadou, 2012), the station is not affected by

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L. Descroix et al. / Global and Planetary Change 98–99 (2012) 18–30

downloading at http://www.hydrosciences.org/spip.php?article239 and the reader is referred to (Lubès-Niel et al., 1998; L'Hôte et al., 2002) for an extensive discussion of the methods and for additional references. This led us to regression analyses on the subset of the rainfall/runoff data defined by the rupture analysis.

Table 3 Evolution of runoff coefficients. Basin

Gorouol

Sirba

Dargol

Period 1957–1979 Periods 1980–1994 Periods 1995–2010

1.9 3.6 4.3

2.6 4.2 6.0

5.0 8.8 14.2

5. Results According to the statistical analyses described in the above section, neither rainfall series, nor discharges (runoff depth) and runoff coefficients can be considered as randomly distributed; on the contrary, annual flood duration is random except for the Gorouol basin. The flood onset date is also random except for the main basin, the Niger at Niamey station. Table 1 gives the time segmentation according to the Hubert procedure on the same data series. It indicates the different periods segregated by the test, with the average and standard deviation of the series. The periods chosen in the Fig. 5a, b and c are those most frequently detected by these procedures. The Student test was applied to the data of runoff coefficients in order to compare the runoff coefficients associated with the three different time periods (Table 2). For the three basins, this test rejects with a high confidence level the null hypothesis that two successive periods have the same runoff coefficient, except after 1994. This means that the main rupture occurred at the end of the first period, i.e. in 1979, in the middle of the Great Drought. Clearly, the increase in runoff coefficients commenced in the driest period.

sedimentation due to the presence of two obstacles upstream of the stream gauge (a weir and an ancient ford). The other stream gauges were installed in places where the channel flows over bedrock. Rainfall data are recorded by a network of 55 rain gauges from which at least 45 are running each year (see location on Fig. 3). Trends on daily and monthly rainfall are measured only on 12 stations where there is a long time series of data (Fig. 3). Rainfall depths on the study basins are measured using the 55 rain gauges, areal rainfall being estimated by linearly interpolated kriging. Hydraulic conductivity is measured using disc infiltrometers (4 and 12 cm radius) at multiple suctions. Four suctions are applied (− 100, − 50, − 25 and − 10 mm) at each test and 20 to 40 tests were performed in each kind of land use (in two different representative experimental sites; Fig. 3) (Vandervaere et al., 1997). Land use evolution is estimated by remote sensing and by the visual interpretation of aerial photographs. Aerial photographs, some of them taken by a PIXY® (Asseline et al., 1999) drone and satellites scenes are also used to evaluate the evolution of the endorheism bursting. The results of linear regression analyses indicate an ongoing increase in runoff coefficients, but also show the existence of a severe hydrological change for the drought period. Statistical analyses of randomness and detection of trends and ruptures were therefore performed using the Khronostat ® software. This software allows the study of stationarity by the use of five statistical tests. First, the “rank correlation” test determines whether the chronological series is random; then the test of Pettitt, the U-statistic of Buishand, and the Bayesian estimation of Lee and Heghinian are used to detect possible discontinuities (statistical ruptures). Finally, the procedure of segmentation of Hubert with a significance level of the Scheffé test at 1% points out changes in a series and defines the different statistically homogeneous periods. The Khronostat software is available for

5.1. Change in runoff near Niamey For the main Sahelian tributaries, the Gorouol, Dargol and Sirba rivers, in addition to the trends and ruptures demonstrated in Tables 1 to 3 the following common hydrological functioning features are observed (Figs. 4a and 5): (1) the runoff coefficient has been increasing since the mid 1950s in spite of the high rainfall deficit observed after 1968 (Table 3); (2) the flood intensity was accentuated by the association of an increase in runoff coefficient with the observed decrease in flow duration (Fig. 4b). The evolution of the start and end dates of the annual flood is shown in Fig. 4b. The beginning of the annual flood takes place earlier; the red

b

a 20

10 15

8 6

10

4 5 2 0 1955

1965

26/2 Gorouol, begin.

Gorouol Sirba Dargol

1975

1985

1995

2005

0 2015

Beginning and ending of the flood

12

25

Runoff coefficient (Dargol)

Runoff coefficient (Sirba and Gorouol)

14

Gorouol, end.

7/1

Sirba, begin. Sirba, end.

18/11

29/9

10/8

Dargol, begin. Dargol, end. Niger, begin. local flood Gorouol 204 d. Sirba 183 d. Dargol 144 d. Gorouol 180 d. Sirba 165 d. Dargol 130 d.

21/6

2/5 1925 1935 1945 1955 1965 1975 1985 1995 2005 2015

Fig. 4. Evolution of the hydrological regime of the main tributaries. a. Evolution of runoff coefficient per basin (runoff coefficient is defined as the ratio of the annual river discharge to the total rainfall on its watershed); the lines are linear regressions for this evolution. b. Evolution of the starting and ending dates of the annual flood for the Niger River and its main tributaries. The duration of the flood of the main tributaries at the beginning (1956) and the end (2010) of the measurement period is indicated in the coloured boxes.

L. Descroix et al. / Global and Planetary Change 98–99 (2012) 18–30

flood begins at Niamey 40 days earlier than fifty years ago (see also Table 1); the annual flood also ends earlier. For the small tributaries contributing directly to the Niger River between Kandadji and Niamey stations (see enlargement of Fig. 1),

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it is shown that there is a great difference in the hydrological behaviour before and after 1997 (Fig. 6). 5.1.1. At the main tributaries' scale Fig. 5 shows that in the three basins, runoff increased significantly after 1980, which is obviously not climate-related, the 1980s being the driest decade in the last century. This is obvious in the Sirba and Dargol basins; however, in the Gorouol River basin, the main change seems to occur after 1995. On the contrary, Table 3 indicates that the main change in runoff coefficient in the Gorouol basin occurred in 1980; this is explained by the high runoff coefficient observed for dry years during the 1980–1994 period. The increase in runoff coefficient is observed overall in the driest years of the drought (1980–1994), and it remains at least as high during the following period (Table 3). 5.1.2. At the small tributaries' scale An indication of the annual discharge of the small tributaries of the Niger River arises from the water balance between the Kandadji and Niamey stations (Fig. 6; see location on Fig. 3). In general, between 1975 and 1996 the input volume of the Niger River at the Kandadji station was greater than the output volume at Niamey, after subtraction of the discharge of the two main right bank tributaries (Dargol and Sirba), presumably due to infiltration and evaporation losses. Since 1998, the reverse seems to occur (it can only be detected for the years with complete discharge data). This is interpreted here as resulting from the new input of the small tributaries, where crusted soils were widespread in recent years – some of them were endorheic basins until the end of the 1990s and became exorheic in recent years (see Section 5.3 below) – increasing the catchment of the Niger River with degraded areas producing high runoff rates. These small tributaries have provided several billions of m 3 per year to the Niger River discharge. However, the same analysis as for the main tributaries is unachievable due to the lack of continuous data, but a significant difference in the hydrological behaviour was detected around 1996–1998 (Amogu et al., 2010). 5.2. Change in rainfall In West Africa, rainfall distribution in space and time before, during and after the Great Drought exhibits a high variability (Le Barbé et al., 2002) – which could be impacted by global warming (Min et al., 2011) – and an East–west gradient: it was shown that rainfall has been increasing again since the mid 1990s in West Africa, but only in the central and eastern part of the sub-region (Ali and Lebel, 2009). However, it was recently shown that land use change could directly influence the mesoscale soil-moisture patterns and contribute to increase in storm frequency over the Sahel (Taylor et al., 2011). The albedo (Charney, 1975), the soil moisture (Koster et al., 2004; Taylor et al., 2011) and the vegetation dynamics (Wang and Eltahir, 2000) likely play a role in rainfall distribution and occurrence which could generate a feedback effect to be accounted in hydrological forecast.

Fig. 5. Evolution of rainfall-runoff relationship in the three main Sahelian tributaries basins of the mid Niger River for the 1957–79, 1980–84 and the 1995–2010 periods, for the Sirba River basin, the Gorouol River basin and the Dargol River basin. For each period, a linear regression curve labelled with its equation and the corresponding amount of explained variance. The two dashed lines around the regression line indicate a 95% confidence interval.

5.2.1. The event (daily) scale It can be seen in Fig. 7 that, in Niamey station: (1) there is no significant trend in rainy event size distribution since 1950, date of the beginning of the available recorded data; (2) a decrease in the total amount of rainy events > 20 mm in the last decade (2001–2010) is observed; (3) a general decrease in trend is noticeable in the number of events > 30 mm and > 40 mm; (4) the same trend is observed for events > 60 mm but only since the 1960s. The other eleven stations showed the same evolution (not shown here).

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Fig. 6. Evolution of the remaining water balance between Kandadji and Niamey from 1975 to 2010, after subtraction of the discharge of the two main right bank tributaries (Dargol and Sirba); discharge of the Dargol and Sirba Rivers are measured at the Kakassi and Garbey Korou stations (see Fig. 3).

The increase in runoff and discharges in the last decades cannot be therefore explained by either the distribution or the number of rainfall events per class of rainfall amount during the 1951–2010 period. 5.2.2. The annual scale Monthly distribution of rainfall during the year has not changed significantly since 1950, for twelve representative stations in the “local flood” originating area (see location in Fig. 3). It is shown in Fig. 8 that no increase in the monthly rainfall recorded during the first two months of the rainy season (June and July) has been observed for the last two decades (1990–2010 par ex.) in comparison to the monthly rainfall records of the wettest decade (1950s and 1960s) which could explain the earlier occurrence of the red flood. Fig. 9 shows this evolution as an index, and it appears that there is no significant change of monthly rainfall distribution in the Middle Niger River area. There is only a slight relative shift of rain fall amount from August

Fig. 7. Evolution of the number of daily rainfall events ranged by rainfall amount, per decade at Niamey station.

to July and June (Nicholson, 2005; Ali and Lebel, 2009), observed since the 1980s. This change cannot be related to the reinforcement of the red flood, since the total rainfall amount has slightly decreased in June (Fig. 8). In every sub basin, Fig. A1 (see Appendix A) shows that the amount of rainfall decreased at the beginning of the monsoon between the 1950s and the 2000s. The only increase at the beginning of the monsoon is observed in the number of rainfall events >40 mm. During the 1990s and the 2000s, the number of rainfall events (>40 mm) increased significantly in June in the Middle Niger River basin; but it remained lower than during the 1960s, and cannot then explain the fact that the flood onset of Niger River at Niamey occurs nowadays 40 days earlier than during the 1960s. Table A1 (Appendix A) shows that the rainfall amount of 2010 which provoked the highest first flood of the Niger River was average and only the 20th highest amount of rainfall recorded in the basin since 1951. In conclusion, the increase in runoff and the earlier occurrence of the annual flood cannot be explained by the evolution of rainfall intensity or time distribution either at the event scale or at the yearly scale.

Fig. 8. Evolution of the monthly distribution of rainfall in the middle Niger River basin. Computed according to Thiessen polygons based on the 12 daily stations displayed on in Fig. 3.

L. Descroix et al. / Global and Planetary Change 98–99 (2012) 18–30

Fig. 9. Evolution of the rainfall monthly index per decade from 1950 to 2010 in the middle Niger River basin area. The mean rainfall amount of each month is divided by the total mean annual rainfall amount; rainfall is spatialised by Thiessen polygons based on the 12 daily stations of the middle Niger River basin (Fig. 3).

Table 4 Comparison of the hydrodynamic properties of non-crusted soils with those of crusted soils (see also Section 4). Soil surface feature

Runoff coefficient %

Saturated hydraulic conductivity mm.h−1

Millet on common sandy soil Fallow on common sandy soil Millet and fallow erosion (ERO) crusted soils

4 +/− 1.4 10 +/− 4 60 +/− 8

172 +/− 79 (20)a 79 +/− 41 (20) R10 +/− 5 (30)

a

Number of repetitions.

5.3. Land use change, land cover change (LUC and LCC) and increase in runoff Although rainfall was observed to be decreasing from 1968 to 1995, runoff increased from the beginning of the 1970s in Sahelian experimental catchments in Burkina Faso, as a hydrological paradox, while in the Sudanian catchments of the same country the discharges were decreasing (Albergel, 1987). An increase in discharge during the Great Drought was observed in the Sahelian Nakambé River, in Burkina Faso (Mahé, 2009), contrary to model predictions (De Wit and Stankiewicz, 2006). A regionalisation of hydrological processes compared Sudanian areas, where runoff decreased strongly during the 1955–2008 period, to Sahelian ones where runoff increased significantly (Descroix et al., 2009). In southern Sudanian areas (Tschakert et al., 2010), and in Sahelian

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regions (Tarhule, 2005), an ongoing increase in the occurrence of inundations was highlighted in recent years. Having begun during the Great Drought, the increase in runoff in the Sahel cannot be attributed to climatic factors; however, in this area, land cover has significantly changed in the last few decades due to demographic growth (Raynaut, 2001). In the last 50 years this has led to a significant increase in cropped areas (mostly pearl millet in the Sahel). This land use is characterised by a high soil hydraulic conductivity (Table 4) while the soil structure remains good. However, association of extension of the crop/fallow alternation and fallow shortening frequently causes soil degradation mostly characterised by superficial crusting, which in turn results in a reduction in infiltration capacity (Albergel, 1987) and an increase in runoff and erosion (Casenave and Valentin, 1992). Satellite data should provide powerful observations on land use/ land cover changes, but interpretation of these data is subject to an ongoing debate. (1) Some studies based on satellite data vegetation indices (Rasmussen et al., 2001; Herrmann et al., 2005; Prince et al., 2007; Govaerts and Lattanzio, 2008; Bégué et al., 2011) have noticed a re-greening of the Sahel after 1994; (2) However, other studies (Hein and De Ritter, 2006 among others) have highlighted the limitations of vegetation indices for the determination of land use and land use changes when using remote sensing at coarse resolution. Land cover studies based on aerial photograph analyses (Leblanc et al., 2008) (some examples at different spatial scales are given in Figs. 10 and 11) show on the contrary a decrease in vegetation cover in south western Niger; (3) Although the Sahel is probably re-greening, the western part of Niger and the eastern part of Burkina Faso are still suffering land degradation (see maps in Prince et al., 1998; Fensholt and Rasmussen, 2011). Furthermore, a newly “treeless” state could have a feedback climatic impact (Hirota et al., 2011). We observed, from the local to the regional scale, a significant increase in bare soil areas. An increasing part of these areas are crusted and constitute an “erosion crust” (Casenave and Valentin, 1992). An erosion crust consists of a thin, smooth, clay-rich surface layer overlying a 20–50 cm thick surface of erosion type (ERO) sand layer, degraded by splash, runoff and desiccation (Casenave and Valentin, 1992). This ERO induces a severe reduction in soil hydraulic conductivity (see Table 4) and a consequent significant acceleration of runoff. Figs. 10 and 11 show the recent extension of crusted bare soils at the small Tondi Kiboro basin (Fig. 10) and the mid size (Fig. 11) Boubon basin. At the basin scale, the increase in bare soil areas on the main right bank Sahelian tributaries of the Niger River (example for the Sirba basin, 38,750 km 2, in Fig. 12) is the main explanation of the increase in runoff coefficient (Amogu et al., 2010; Descroix et al., 2012). Figs. 10 and 11 are based on interpretations of aerial photographs, allowing the distinction of crusted soils, with field validation. These results cannot be extended to the scale of a catchment basin, due to the inability of satellite data to discriminate between

Fig. 10. Evolution of the extension of crusted soils (erosion crust) in the Tondi Kiboro experimental catchments (0.24 km2, 75 km east of Niamey) from 1993 (20% of crusted soil) to 2007 (40% of crusted soils) (based on aerial photographs).

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Fig. 11. Evolution of bare crusted soil area (erosion crust) in the Boubon basin (160 km2, 25 km northwest of Niamey) between 1975 and 2005 (based on aerial photographs).

bare non-crusted soils and crusted soils. Mapping of Fig. 12 is based on Landsat satellite scenes. However Fig. 12 shows that at the scale of the Sirba basin, bare soil areas have increased between 1975 and 1999, and probably a significant part of these bare soils actually consists of crusted soils. The same evolution is also observed in parts of the Dargol and Gorouol rivers basins (Amogu et al., 2010); these areas are representative of their whole respective basins by their latitude and by their demographic and land use evolution. At the whole Sahel scale, cultivated areas have been increasing significantly for more than 50 years due to the demographic growth and the permanence of very low crop yields (see Fig. 13 for the Niger Republic) (Guengant and Banoin, 2003). Crop yields are decreasing in a long term trend due to poor soils and low fertility. Exhaustion of soils has contributed to the significant increase in bare crusted soil areas in the whole Sahel. Sahelian hydrology on sedimentary basins is characterised by endorheism, resulting in the formation of numerous temporary ponds, which are filled during the rainy season and dry up progressively during the dry season (Desconnets et al., 1997). The acceleration of runoff at

the hillslope and the basin scale has led to an increase in the number, the volume and the duration of the typical Sahelian ponds. In the vicinity of the Niger River valley, some valleys suffered endorheism bursting, due to the overfilling of their pond. This created new tributaries of the Niger River, increasing significantly the area of its basin. 6. Discussion: Anthropic or climatic effect? Since annual rainfall amounts are yet to attain pre-drought levels, there is presently no evidence of any climate change which could explain the increase in runoff, either in temporal distribution of rainfall during the year or in the occurrence of high amounts of rainfall or increase in rainfall intensity. However, the decrease in rainfall observed after 1968 and the increasing demographic growth seem to have resulted in a general soil clearing and local soil crusting, observed at different scales (Figs. 10–13), which promoted runoff. It is shown here that the increase in the river discharge of the first Niger River flood since the beginning of the drought is attributable mainly to the increasing runoff on the large right bank tributaries of the Middle Niger River, due to an observed decrease in soil water

Fig. 12. Evolution of bare soil area in the lower Sirba basin, representative of the whole basin (38,750 km2) between 1975 and 1999 (Amogu et al., 2010); based on Landsat satellite data.

L. Descroix et al. / Global and Planetary Change 98–99 (2012) 18–30

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decrease in lag time and in base flow volume and duration, as well as to an enhancement and a widening of river beds (Viramontes and Descroix, 2003); this already led the authors to investigate the boundaries between Hortonian and Hewlettian processes (Descroix et al., 2007). This runoff increase process can be reversed at least for temperate climates: in northern Mediterranean areas in Europe, rural abandonment led to a progressive afforestation since the end of the 19th century. Discharge, flooding and flood flow were reduced significantly, while lag time and base flow increased progressively; simultaneously, a narrowing and an entrenchment of riverbeds were observed (Descroix et al., 2005).

7. Summary and conclusion

Fig. 13. Evolution of crop area and population in Niger since 1950 (Guengant and Banoin, 2003).

holding capacity (see Table 4), and since the end of the 1990s to the contribution of both the former and the new contributive areas of the small koris (and marginally to the slight increase in rainfall). Some of them have been newly connected to the Niger River between Kandadji and Niamey stations. North of our study area, in eastern Mali, an observed increase in runoff has been attributed to the removal of vegetation during the drought and its inability to recover during recent wetter years due to soil erosion and degradation (Gardelle et al., 2010). The local flood of the middle Niger River in 2010 is the highest ranked in magnitude since the beginning of records in 1929, without any exceptional rainfall (Table A1 in Appendix A). It could still be exceeded if extreme events, which are forecast by several climatic models, occur with an increased frequency. No direct link is showed here between the decline of the natural vegetation and the increase in runoff. However, we show that in the Niamey area, this decline is associated to land uses which induce the increase in runoff. There are strong indications that the same LUCs have similar hydrological effects in other parts of the Sahel (Le Barbé et al., 2002; Prince et al., 2007; Tschakert et al., 2010). The distinction between Sudanian decrease and Sahelian increase in runoff was observed by Albergel (1987) at the experimental catchment scale; it was reinforced by observations made by Amani and Nguetora (2002), Diello et al. (2005), Mahé and Paturel (2009), and Mahé et al. (2011) in Sahelian areas, by Liénou et al. (2009) in Sudanian regions, and the difference highlighted by Mahé (2009) comparing previous works on the Bani (Sudanian) and on the Nakambé (Sahelian) rivers. The regionalization attempted by Amogu et al. (2010) highlighted the fact that Sahelian regions are characterised by Hortonian infiltration excess runoff and Sudanian areas are dominated by Hewlettian saturation excess runoff which explains the severe decrease in runoff (Olivry, 2002, personal communication) in the Upper Niger River basin where discharges were decreasing twice the rate of rainfall decrease. While crusted soils are widespread in Sahelian regions, in Sudanian areas, soil water holding capacity is preserved; as a consequence, when rainfall decreased, the part of rain water previously dedicated to runoff reduced. Changes in water balances due to land use changes on other great basins were already observed by Raymond et al. (2008) in the Mississippi basin and by Delgado et al. (2010) in the Mekong basin, among others. In endorheic basins of Northern Mexico, deforestation and overgrazing did not lead to an increase in runoff but to a

The analysis of the hydrological evolution of the Middle Niger River basin highlighted a significant and ongoing increase in discharge in this Sahelian area; the discharge of the main tributary rivers is at least twice those observed fifty years ago, runoff coefficients showing an even higher increase. The lag time of the basins and the total duration of stream flow were reduced, both demonstrating a decrease in the water holding capacity of soils and basins. This increasing discharge clearly cannot be caused by a climatic evolution, since the major climatic change that has been observed is a marked and persistent rainfall deficit. Furthermore, no change has been noticed in rainfall intensity and neither changes in seasonal time distribution of rainfall nor the number of extreme rainfall events could explain any increase in runoff or the earlier occurrence of the annual flood. As has been supposed since the end of the 1980s, the change in the hydrological functioning and the new twin peak hydrograph are linked to human factors. These factors are largely related to land use changes, particularly the land clearing and the extension of crops due to demographic pressure; this leads to soil clearing, fallow shortening and soil crusting that result in a severe decrease of soil infiltrability. These modifications cause an increase in flood hazard, which is strengthened by endorheism bursting. Some endorheic valleys became exorheic in recent years and decades, enlarging the catchment area of the Niger River and its tributaries with surfaces of high runoff coefficient. The observed evolution seems to be widespread in the larger Sahelian area; a region with the highest demographic increase since the beginning of the 1990s, and this situation is forecast to continue for the decades to come. Therefore it is important for policy makers to be sensitized about the effects of consequent land use changes on the hydrological regimes of Sahelian Rivers. The flood hazard is obviously increasing and must be taken into account in the context of rapid urbanization in Sub Saharan Africa. It is all the more urgent since extreme events are forecast to occur with an increased frequency during the 21th century by several climatic models.

Acknowledgements We warmly acknowledge the colleagues of the Niger Basin Authority's “Projet Niger-Hycos” (NBA; http://nigerhycos.abn.ne/user-anon/htm/ listStationByGroup.php) and the Niger Water Resources Department (DRE) for providing the Niger River discharge data. The Directors, and the colleagues of the Burkina Faso, the Mali and the Niger Meteorological Offices (Direction de la Météorologie Nationale) are also acknowledged. The SIEREM data were provided by A. l'Aour-Crès, N. Rouché and C. Dieulin (IRD-HSM), and the FRIEND animators provided rainfall data. This work was partly funded by French ANR projects ECLIS and ESCAPE, as well as by the AMMA project. The authors warmly acknowledge the reviewers who assessed the manuscript, improving its quality.

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Appendix A

Fig. A1. Evolution of the rainfall monthly index (left) and the monthly mean rainfall depth (right) per decade from 1951 to 2010 in four sub basins of the Central Niger River: a. Gorouol, b. Dargol, c. Sirba, d. small koris. Rainfall data computed according to Thiessen polygons.

L. Descroix et al. / Global and Planetary Change 98–99 (2012) 18–30 Table A1 Rank of the 2010 monsoon within the last 60 rainy seasons according to rainfall amount. For each basin, the first column gives the year and the second the rainfall amount of the considered year 2010 and its corresponding rainfall amount is specified in bold for each basin. Rank

Sirba

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

1958 1964 1959 1952 1965 1994 1953 1962 1956 1961 2003 1957 1954 1967 1963 2008 2005 1950 1976 1955 2010 1960 1969 1966 1988 1991 1998 1951 1999 1978 1974 2009 1968 1975 2007 1996 1979 1992 2001 2002 1972 1977 1995 1982 1989 1973 1980 1993 2006

925 875 825 790 790 780 775 775 750 750 727 725 720 710 700 692 683 680 680 675 675 670 670 650 650 650 650 640 640 625 610 607 600 600 600 580 575 575 570 564 550 550 550 540 540 530 530 525 523

Gorouol

Dargol

1953 1950 1958 1952 1994 1954 1957 1963 1964 1991 1996 1956 1955 1998 2005 1961 1965 2010 2003 1959 1966 1999 2009 2006 2007 1978 1951 1960 2001 1969 2008 1962 1967 1975 1968 1981 2002 1976 1974 1979 2000 1970 1973 1989 1977 1980 1997 1982 1995

1961 1964 1952 1953 1959 1954 2003 1958 1965 2005 1967 1950 2009 1956 1994 1957 1998 1979 1991 1992 1988 1966 1976 1996 1955 1962 1963 1978 1969 2010 2002 1960 1980 2001 2007 1995 2008 1999 1951 1977 1968 1974 1982 1973 1985 1989 1970 2006 1975

605 575 550 540 530 520 520 500 500 500 500 490 480 480 478 475 475 475 471 470 450 450 450 444 440 435 425 425 425 420 420 400 400 390 385 360 354 350 340 340 340 335 330 330 325 325 325 310 310

700 690 680 680 660 610 608 600 600 591 575 565 565 550 550 540 525 520 520 520 510 500 500 500 490 490 490 490 470 465 461 460 460 460 460 450 447 445 440 430 420 420 420 410 410 410 400 397 390

Small koris

Gathered

Rank

1952 1998 1953 1964 1950 1959 2005 1957 1967 1961 1955 2003 1954 1965 1962 1956 2008 2009 1958 1978 2007 2010 1976 1980 1994 1991 1969 1989 1999 1951 1988 2001 1977 1979 2000 1966 1960 1963 1968 2004 1992 1995 2006 1982 2002 1975 1974 1993 1972

1958 1953 1952 1964 1959 1950 1994 1965 1957 1954 1961 1956 2003 1998 2005 1963 1962 1955 1991 2010 1967 2008 1966 1969 2009 1999 1978 1960 1951 2007 1996 1976 2001 1968 1975 2006 1988 1979 1974 2002 1980 1977 1989 1992 1995 2000 1982 1981 1973

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

616 573 555 529 512 492 473 470 451 445 439 439 436 430 429 417 410 410 407 406 406 400 397 396 395 395 386 376 376 374 368 364 364 359 352 350 344 343 342 340 335 330 323 313 311 308 304 296 280

662 661 649 649 610 601 597 586 586 582 580 573 564 559 556 544 543 543 537 533 528 516 507 506 505 505 501 500 494 492 490 484 468 456 451 449 447 437 434 427 417 417 417 416 406 401 398 396 395

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), 168. IAHS Publ. http://iahs.info/redbooks/a168/iahs_168_0355.pdf. Ali, A., Lebel, T., 2009. The Sahelian standardized rainfall index revisited. International Journal of Climatology 29, 1705–1714 http://dx.doi.org/10.1002/joc.1832. Amani, A., Nguetora, M., 2002. Evidence d'une modification du régime hydrologique du fleuve Niger à Niamey. In: Van Lannen, H., Demuth, S. (Eds.), FRIEND 2002 Regional Hydrology: Bridging the Gap between Research and Practice, Proceedings of the Friend Conference, Cape Town, South Africa, 18–22 March, 2002, 274. IAHS publication, Wallingford, UK, pp. 449–456. http://iahs.info/redbooks/a274/iahs_274_449.pdf. Amogu, O., Descroix, L., Souley 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 http://dx.doi.org/10.3390/w2020170. Asseline, J., Noni, G.D., Chaume, R., 1999. Design and use of a low speed, remotelycontrolled unmanned aerial vehicle (UAV) for remote sensing [French]. Photo Interprétation 37, 3–9 http://www.documentation.ird.fr/hor/fdi:010026172. Bégué, A., Vintrou, E., Ruelland, D., Cladern, M., Dessay, N., 2011. Can a 25-year trend in Soudano-Sahelian vegetation dynamics be interpreted in terms of land use change? A remote sensing approach. Global Environmental Change 21, 413–420 http://dx.doi.org/10.1016/j.gloenvcha.2011.02.002.

29

Casenave, A., Valentin, C., 1992. A runoff capability classification system based on surface features criteria in semi-arid areas of West Africa. Journal of Hydrology 130, 231–249 http://dx.doi.org/10.1016/0022-1694(92)90112-9. Charney, J.G., 1975. Dynamics of deserts and drought in the Sahel. Quarterly Journal of the Royal Meteorological Society 101 (248), 193–202. Chevallier, P., Valentin, C., 1984. Influence des microorganisations pelliculaires superficielles sur l'infiltrabilité d'un type de sol sahélien. Bulletin du Groupe Français d'Humidimétrie Neutronique 17, 9–22. Collinet, J., Valentin, C., 1979. Analyse des différents facteurs intervenant sur l'hydrodynamique superficielle. Nouvelles perspectives. Applications agronomiques. Cahiers ORSTOM Serie Pedologique 17, 283–328. De Wit, M., Stankiewicz, J., 2006. Changes in surface water supply across Africa with predicted climate change. Science 311, 1917–1921 http://dx.doi.org/10.1126/ science.1119929. Delgado, J.M., Apel, H., Merz, B., 2010. Flood trends and variability in the Mekong river. Hydrology and Earth System Sciences 14, 407–418 http://dx.doi.org/10.5194/hess14-407-2010. Desconnets, J.-C., Taupin, J.-D., Lebel, T., Leduc, C., 1997. Hydrology of the HAPEX-Sahel Central Super-Site: surface water drainage and aquifer recharge through the pool systems. Journal of Hydrology 188–189, 155–178 http://dx.doi.org/10.1016/S00221694(96)03158-7. Descroix, L., Gautier, E., Besnier, A.-L., Amogu, O., Viramontes, D., Gonzalez Barrios, J.-L., 2005. Sediment Budget as an Evidence of Land-use Changes in Mountainous Areas: Two Stages of Evolution. : Sediment Budgets 2, 292. IAHS Publ., pp. 262–270. Descroix, L., Viramontes, D., Estrada, J., Gonzalez Barrios, J.-L., Asseline, J.-P., 2007. Investigating the spatial and temporal boundaries of Hortonian and Hewlettian runoff in Northern Mexico. Journal of Hydrology 346, 144–158 http://dx.doi.org/ 10.1016/j.jhydrol.2007.09.009. Descroix, L., Mahé, G., Lebel, T., Favreau, G., Galle, S., Gautier, E., Olivry, J.-C., Albergel, J., Amogu, O., Cappelaere, B., Dessouassi, R., Diedhiou, A., Le Breton, E., Mamadou, I., Sighomnou, D., 2009. Spatio-temporal variability of hydrological regimes around the boundaries between Sahelian and Sudanian areas of West Africa: a synthesis. Journal of Hydrology, AMMA Special Issue 375, 90–102 http://dx.doi.org/10.1016/ j.jhydrol.2008.12.012. Descroix, L., Laurent, J.-P., Vauclin, M., Amogu, O., Boubkraoui, S., Ibrahim, B., Galle, S., Cappelaere, B., Bousquet, S., Mamadou, I., Le Breton, E., Lebel, T., Quantin, G., Ramier, D., Boulain, N., 2012. Experimental evidence of deep infiltration under sandy flats and gullies in the Sahel. Journal of Hydrology 424–425, 1–15 http:// dx.doi.org/10.1016/j.jhydrol.2011.11.019. D'Herbès, J.M., Valentin, C., 1997. Land surface conditions of the Niamey region: ecological and hydrological implications. Journal of Hydrology 188–189, 18–42. Diello, P., Mahé, G., Paturel, J.-E., Dezetter, A., Delclaux, F., Servat, E., Ouattara, F., 2005. Relations indices de végétation-pluie au Burkina Faso: cas du bassin versant du Nakambé. Hydrological Sciences Journal 50, 207–221 http://dx.doi.org/10.1623/ hysj.50.2.207.61797. Fensholt, R., Rasmussen, K., 2011. Analysis of trends in the Sahelian ‘rain-use efficiency’ using GIMMS NDVI, RFE and GPCP rainfall data. Remote Sensing of Environment 115, 438–451 http://dx.doi.org/10.1016/j.rse.2010.09.014. Galle, S., Ehrmann, M., Peugeot, C., 1999. Water balance in a banded vegetation pattern. A case study of tiger bush in western Niger. Catena 37, 197–216. Gardelle, J., Hiernaux, P., Kergoat, L., Grippa, M., 2010. Less rain, more water in ponds: a remote sensing study of the dynamics of surface waters from 1950 to present in pastoral Sahel. (Gourma region, Mali). Hydrology and Earth System Sciences 14, 309–324 http://dx.doi.org/10.5194/hess-14-309-2010. Govaerts, Y.M., Lattanzio, A., 2008. Estimation of surface albedo increase during the eighties Sahel drought from Meteosat observations. Global and Planetary Change 64, 139–145 http://dx.doi.org/10.1016/j.gloplacha.2008.04.004. Guengant, J.-P., Banoin, M., 2003. Dynamique des populations, disponibilités en terres et adaptation des régimes fonciers: le Niger. 142 pp. FAO-CICRED, Publ., Roma, Paris. http://www.cicred.org/Eng/Publications/pdf/MonoNiger.pdf. Hein, L., De Ritter, N., 2006. Desertification in the Sahel: a reinterpretation. Global Change Biology 12, 751–758 http://dx.doi.org/10.1111/j.1365-2486.2006.01135.x. Herrmann, S.M., Anyamba, A., Tucker, C.J., 2005. Recent trends in vegetation dynamics in the African Sahel and their relationship to climate. Global Environmental Change 15, 394–404 http://dx.doi.org/10.1016/j.gloenvcha.2005.08.004. Hirota, M., Holmgren, Van Nes, E.M., Scheffer, M., 2011. Global resilience of tropical forest and savanna to critical transitions. Science 334, 232–235 http://dx.doi.org/10.1126/ science.1210657. Hulme, M., 2001. Climatic perspectives on Sahelian desiccation : 1973–1998. Global Environmental Change 11, 19–29 http://dx.doi.org/10.1016/S0959-3780(00)00042-X. Koster, R., Dirmeyer, D., Guo, P.A., Bonan, Z., Chan, G., Cox, E., Gordon, P., Kanae, C.T., Kowalczyk, S., Lawrence, E., Liu, D., Lu, P., Malyshev, C.-H., McAvaney, S., Mitchell, B., Mocko, K., Oki, D., Oleson, T., Pitman, K., Sud, A., Taylor, Y.C., Verseghy, C.M., Vasic, D., Xue, R., Yamada, Y., “The GLACE Team”, 2004. Regions of strong coupling between soil moisture and precipitation. Science 305, 1138–1140 http:// dx.doi.org/10.1126/science.1100217. Lacombe, G., Alain, Pierret, Hoanh, C.T., Sengtaheuanghoung, O., Noble, A.D., 2010. Conflict, migration and land-cover changes in Indochina: a hydrological assessment. Ecohydrology 3 (4), 382–391 http://dx.doi.org/10.1002/eco.166. Le Barbé, L., Lebel, T., Tapsoba, D., 2002. Rainfall variability in West Africa during the years 1950–1990. Journal of Climate 15, 187–202 http://dx.doi.org/10.1175/ 1520- 0442(2002)015b0187:RVIWAD>2.0.CO;2. Lebel, T., Cappelaere, B., Galle, S., Hanan, N., Kergoat, L., Levis, S., Peugeot, C., Vieux, B., Descroix, L., Gosset, M., Mougin, E., Peugeot, C., Seguis, L., 2009. AMMA-CATCH studies in the Sahelian region of West-Africa: an overview. Journal of Hydrology, AMMA Special Issue 375, 3–13 http://dx.doi.org/10.1016/j.jhydrol.2009.03.020.

30

L. Descroix et al. / Global and Planetary Change 98–99 (2012) 18–30

Leblanc, M., Favreau, G., Massuel, S., Tweed, S., Loireau, M., Cappelaere, B., 2008. Land clearance and hydrological change in the Sahel : SW Niger. Global and Planetary Change 61 (3), 135–150 http://dx.doi.org/10.1016/j.gloplacha.2007.08.011. L'Hôte, Y., Mahé, G., Somé, B., Triboulet, J.-P., 2002. Analysis of a Sahelian annual rainfall index from 1896 to 2000; the drought continues. Hydrological Sciences Journal 47 (4), 563–572 http://dx.doi.org/10.1080/02626660209492960. Liang, S., Ge, S., Wan, L., Zhang, J., 2010. Can climate change cause the Yellow River to dry up? Water Resources Research 46, W02505 http://dx.doi.org/10.1029/2009WR007971. Liénou, G., Mahé, G., Paturel, J.E., Servat, E., Ekodeck, G.E., Tchoua, F., 2009. Variabilité climatique et transport de matières en suspension sur le bassin de Mayo Tsanaga : Extrême-Nord Cameroun. Sécheresse 20 (1), 139–144 http://dx.doi.org/10.1684/ sec.2009.0167. Loireau, M., 1998. Espaces, ressources, usages : spatialisation des interactions dynamiques entre les systèmes écologiques au Sahel nigérien. PhD Thesis of Geography, Université Montpellier 3, 410 p. http://eclis.get.obs-mip.fr/index.php/productionsscientifiques/ Documentation Lubès-Niel, H., Masson, J.M., Paturel, J.E., Servat, E.S., 1998. Climatic variability and statistics. A simulation approach for estimating power and robustness of tests of stationarity. Revue des Sciences de l'Eau 3, 383–408 http://www.rse.inrs.ca/art/ volume11/v11n3_383.pdf. Mahé, G., 2009. Surface/groundwater interactions in the Bani and Nakambe rivers, tributaries of the Niger and Volta basins, West Africa. Hydrological Sciences Journal 54, 704–712 http://dx.doi.org/10.1623/hysj.54.4.704. Mahé, G., Paturel, J.-E., 2009. 1896–2006 Sahelian annual rainfall variability and runoff increase of Sahelian rivers. Comptes Rendus Geoscience 341, 538–546 http:// dx.doi.org/10.1016/j.crte.2009.05.002. Mahé, G., Paturel, J.E., Servat, E., Conway, D., Dezetter, A., 2005. Impact of land use change on soil water holding capacity and river modelling of the Nakambe River in Burkina-Faso. Journal of Hydrology 300, 33–43 http://dx.doi.org/10.1016/j.jhydrol.2004.04.028. Mahé, G., Lienou, G., Bamba, F., Paturel, J.E., Adeaga, O., Descroix, L., Mariko, A., Olivry, J.C., Sangare, S., 2011. Niger River and climate change over 100 years. In: Franks, S.W., Boegh, E., Blyth, E., Hannah, D.M., Yilmaz, K.K. (Eds.), Hydro-climatology: Variability and Change. : Proceedings of Symposium J-H02 Held During IUGG2011, 344. IAHS Pub., Melbourne, Australia, pp. 131–137. Mamadou, I., 2012. la dynamique des koris et l'ensablement de leur lit et de celui du fleuve Niger dans la région de Niamey. PhD thesis, Paris 1 Panthéon Sorbonne University and Abdou Moumouni University of Niamey, 260 p. Min, S.K., Zhang, X., Zwiers, F.W., Hegerl, G.C., 2011. Human contribution to moreintense precipitation extremes. Nature 470, 378–381 http://dx.doi.org/10.1038/ nature09763. NBA (Niger Basin Authority), 2012. Discharge of the Niger River on the 1951–2010 period, Kandadji, Alcongui, Garbey Kourou, Kakassi and Niamey Stationsavailable for downloading at http://nigerhycos.abn.ne/user-anon/htm/listStationByGroup.php.

Nicholson, S., 2005. On the question of the “recovery” of the rains in the West African Sahel. Journal of Arid Environments 63, 615–641 http://dx.doi.org/10.1016/ j.jaridenv.2005.03.004. Prince, S.D., Brown de Colstoun, E., Kravitz, L., 1998. Evidence from rain-use efficiencies does not indicate extensive Sahelian desertification. Global Change Biology 4, 359–374 http://dx.doi.org/10.1046/j.1365-2486.1998.00158.x. Prince, S.D., Wessels, K.J., Tucker, C.J., Nicholson, S.E., 2007. Desertification in the Sahel: a reinterpretation of a reinterpretation. Global Change Biology 13, 1308–1313 http://dx.doi.org/10.1111/j.1365-2486.2007.01356.x. Rasmussen, K., Bjarne, F., Madsen, J.-E., 2001. Desertification in reverse? Observations from northern Burkina Faso. Global Environmental Change 11, 271–282 http:// dx.doi.org/10.1016/S0959-3780(01)00005-X. Raymond, P.A., Oh, N.-H., Turner, R.E., Broussard, W., 2008. Anthropogenically enhanced fluxes of water and carbon from the Mississippi River. Nature 451, 449–452 http://dx.doi.org/10.1038/nature06505 (24 January 2008). Raynaut, C., 2001. Societies and nature in the Sahel: ecological diversity and social dynamics. Global Environmental Change 11, 9–18 http://dx.doi.org/10.1016/S09593780(00)00041-8. Sela, S., Svoray, T., Assouline, S., 2012. Soil water content variability at the hillslope scale: impact of surface sealing. Water Resources Research 48, W03522 http:// dx.doi.org/10.1029/2011WR011297. Tarhule, A., 2005. Damaging rainfall and floodings: the other Sahel hazards. Climatic Change 72, 355–377 http://dx.doi.org/10.1007/s10584-005-6792-4. Taylor, C.M., Gounou, A., Guichard, F., Harris, P.P., Ellis, R.J., Couvreux, F., De Cauwe, M., 2011. Frequency of Sahelian storm initiation enhanced over meso-scale soil-moisture patterns. Nature Geoscience 4, 430–433 http://dx.doi.org/10.1038/ngeo1173. Tschakert, P., Sagoe, R., Ofori-Darko, G., Codjoe, S.M., 2010. Floods in the Sahel: an analysis of anomalies, memory, and participatory learning. Climatic Change 103, 471–502 http://dx.doi.org/10.1007/s10584-009-9776-y. Van de Watt, H.V.H., Valentin, C., 1992. Soil crusting: the African view. In: Summer, M.E., Stewart, B.A. (Eds.), Soil Crusting, Chemical and Physical Processes. : Advances in Soil Science. Lewin Publ., Boca Raton, USA, pp. 301–338. Vandervaere, J.-P., Peugeot, C., Vauclin, M., Angulo Jaramillo, R., Lebel, T., 1997. Estimating hydraulic conductivity of crusted soils using disc infiltrometers and minitensiometers. Journal of Hydrology 188–189, 203–223. Viramontes, D., Descroix, L., 2003. Variability of overland flow in an endoreic basin of northern Mexico : the hydrological consequences of environment degradation. Hydrological Processes 17, 1291–1306 http://dx.doi.org/10.1002/hyp. 1285. Wang, G., Eltahir, E.A.B., 2000. Role of vegetation dynamics in enhancing the lowfrequency variability of the Sahel rainfall. Water Resources Research 36 (4), 1013–1021 Paper number 1999WR900361. 0043-1397/00/1999WR900361$09.00.