Experimental evidence of deep infiltration under sandy flats and gullies in the Sahel

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Experimental evidence of deep infiltration under sandy flats and gullies in the Sahel

L. Descroix a,⇑, J.-P. Laurent b, M. Vauclin b, O. Amogu a, S. Boubkraoui a, B. Ibrahim c, S. Galle a, B. Cappelaere d,S. Bousquet e, I. Mamadou f,g, E. Le Breton f,g, T. Lebel a, G. Quantin a, D. Ramier d,h, N. Boulain d,i

a IRD/UJF-Grenoble 1/CNRS/G-INP, LTHE UMR 5564, F-38041 Grenoble, Franceb CNRS/UJF-Grenoble 1/G-INP/IRD, LTHE UMR 5564, F-38041 Grenoble, Francec 2iE, rue de la Science, 01 BP 594, Ouagadougou 01, Burkina Fasod IRD/U Montpellier 1/U Montpellier 2/CNRS, HSM UMR 5569, Case MSE, Place Eugène-Bataillon, 34095 Montpellier cedex 5, Francee Department of Biology, École Normale Supérieure, 46, Rue d’Ulm , 75005 Paris, Francef U Paris 1/CNRS, LGP UMR 8591, 1, Place Aristide Briand, 92195 Meudon, Franceg UAM, Department of Geography, BP 418, Niamey, Nigerh CETE Ile-de-France 12 rue Teisserenc de Bort 78197 Trappes cedex, Francei UTS, Climate Change Cluster, Sydney, Australia

a r t i c l e i n f o

Article history:Received 24 September 2009Received in revised form 28 April 2011Accepted 9 November 2011Available online 18 November 2011This manuscript was handled by AndrasBardossy, Editor-in-Chief, with theassistance of Harald Kunstmann, AssociateEditor

Keywords:Endorheic areaSandy soilRunoffDeep infiltrationNiger

s u m m a r y

Despite the strong reduction in rainfall observed after 1968, the water table of some endorheic areas inthe Sahel has been found to be rising over the last several decades. It has been previously demonstratedthat this is due to land use changes which have led to a severe increase in runoff and erosion. In suchareas, the excess in runoff causes a strong increase in the number of ponds, their sizes and thus, theirduration. Ponds have been identified as the main zones of deep infiltration of water. The aim of this studywas to investigate whether other areas of the Sahelian region could also be defined as deep infiltrationones as well, and then, whether they were contributing to aquifer recharge.

Soil water content was surveyed for five consecutive years (2004–2008) by implementing a set of mea-surement devices at different depths. The hydrologic water balance was monitored at stream flow gaugestations located upstream and downstream of two small endorheic catchments.

The observed replacement of bush vegetation by crops and fallow areas led to the appearance ofextended bare soil areas due to both aeolian and hydric erosion, triggering a strong reduction in soil infil-trability under millet fields and fallow lands as well as in the soil water holding capacity. It also resultedin the formation of a great number of gullies and sand sediment deposits in the endorheic areas.

Measurements showed that sandy deposits correspond in fact to large areas of deep infiltration: tens ofthousands of cubic meters of water infiltrated catchments of less than 1 km2. Runoff decreased by up to50% in the sandy deposit areas, while infiltration (close to 1300 mm h�1) was observed up to depths of10 m. These factors would raise the water table and significantly modify the surface and sub-surfacecomponents of the water cycle.

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1. Introduction

For 40 years, the Sahel and most of West Africa has experiencedsubstantial drought (Le Barbé and Lebel, 1997) and significant landuse changes (Leduc and Loireau, 1997) leading to a strong increasein the runoff coefficients and stream flows in most Sahelian areas(Albergel, 1987). This phenomenon has been named ‘‘The SahelianParadox’’ (Descroix et al., 2009). The HAPEX–Sahel (Hydrologicaland Atmospheric Pilot Experiment) program, (see J. Hydrol., SpecialIssue, Vol. 188–189, 1997) provided, among many comprehensiveresults, valuable measurements dealing with the Sahelian soil

water content and its spatial and temporal variability (Cuencaet al., 1997), as well as on the infiltration of water through deepsoil layers of the vadose zone (Bromley et al., 1997; Leduc et al.,1997).

In certain endorheic areas in the Sahel, the water table level hasbeen found to be rising over the last several decades despite thestrong reduction in rainfall observed after 1968. This phenomenonhas been previously defined as the ‘‘Niamey Paradox’’ (Leduc et al.,2001; Favreau et al., 2002): the excess in runoff has significantlyincreased the number of ponds. Such ponds being the main zonesof deep infiltration (Leduc et al., 2001; Massuel et al., 2006), theirincrease explains the rise of the water table level (Leblanc et al.,2008). While this mechanism is typical of the hydrology withinthe Sahelian endorheic zones, the existence of exorheic areas inthe Sudano–Sahelian region leads to another type of hydrologic

0022-1694/$ - see front matter � 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.jhydrol.2011.11.019

⇑ Corresponding author. Address: LTHE, BP 53, 38041 Grenoble cedex 9, France.Tel.: +33 4 56 52 09 97; fax: +33 4 76 63 58 87.

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

Journal of Hydrology 424–425 (2012) 1–15

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functioning (see boundary limits in Fig. 1) (Descroix et al., 2009).As a matter of fact, the current active erosion processes are leadingto the appearance of many new gullies and new spreading areaswhere sediments extracted by aeolian and hydric erosion and thentransported in the gullies, are deposited (Chinen, 1999; Leblancet al., 2008; Le Breton, 2012). The gully beds are currently charac-terized by sand deposits ranging from 2 to 4 m wide, several tens ofcentimeters deep (up to 1–2 m) and hundreds of meters long.Spreading areas are formed by these newly created streams whenthey reach gentler slopes, because their transport capacity be-comes suddenly insufficient to carry such significant volumes ofsand. They form sandy deposits of the order of hundreds of square

meters, up to several hectares, and, in some cases, tens of centime-ters deep. As an example, Fig. 2 shows the strong spatial extensionof gullies and spreading areas since 1950 as observed by Le Breton(2012) for the Wankama catchment. This is also consistent withobservations made by Leblanc et al. (2008), who showed that thelength of the gullies increased by a factor of about 2.5 over thesame period.

The goal of this study was twofold: (i) to get further insightinto the role played by gully beds and spreading areas on theprocess of deep infiltration into endorheic zones, (ii) to comparethe spatio-temporal evolution of wetting front according to dif-ferent devices.

Fig. 1. Location of the two experimental catchments within the Western Niger; Niger River approximately separating the exorheic plutonic (westward) from the endorheicsedimentary areas (eastward).

Fig. 2. Maps of the Wankama catchment showing the evolution of gullies and spreading areas between 1950 and 2004 (from Le Breton, 2012). The Dantiandou valley is themain, fossil, tributary valley.

2 L. Descroix et al. / Journal of Hydrology 424–425 (2012) 1–15


In the following ‘‘deep infiltration’’ should be viewed as the pro-gression of the wetting front below the root system through thedeepest soil layers of the vadose zone, possibly reaching, but notnecessarily able to reach the water table.

2. Materials and methods

2.1. The study area

The research was conducted for 5 years (2004–2008) on twoendorheic catchments, named Wankama (W) and Tondi Kibori(TK) (Figs. 1 and 3). They are situated within the East CentralSupersite of the 1992 HAPEX–Sahel pilot experiment in Niger. Theywere also integrated later on into the long-term observation period(2002–2010) of the African Monsoon Multidisciplinary Analyses(AMMA) program (Lebel et al., 2009, 2010). The climate of the re-gion is semi-arid with a monsoon season lying between June andSeptember, and a dry season from October to May. The meanannual rainfall P is 560 mm, (calculated from 1905 to 1989 byLe Barbé and Lebel, 1997), and the annual potential evaporation(PE), as measured by a class A pan, is about 3400 mm. The twocatchments studied are located at 240 m ASL on average. Locally,the ground slopes eastwards for the W catchment and westwardsfor the TK catchment, at equal gradients of 2% that are sufficient toerode channels at high storm intensity. The catchments are sepa-rated by a distance of 10 km, and their surface areas are 0.32 km2

for W and 0.11 km2 for TK. Both are located in the upper half partof their respective hillslopes. At mid slope of the hillslopes (down-stream of the studied part of each catchment), as commonlyobserved in this area of the Sahel (Peugeot et al., 1997), a shelf, sev-eral hectares large, leads to sand deposition by the stream flows,constituting a sandy spreading area. In each catchment, the twostream gauges are located upstream of this shelf. The water dis-charge at both catchments is measured at two stations in eachcatchment, in order to monitor the respective contribution of theupper and the lower part of the catchments.

Soils are mainly of tropical ferralitic type, produced by thealternation of the dry and wet seasons. They are sandy (approxi-

mately 86.7% sand, 13% silt and 0.3% clay–grain size distributionmeasures with laser granulometer mean on 121 samples) andweakly structured (Cappelaere et al., 2009). On the laterite pla-teaus, soils are thin acidic lithosols directly overlaying the ferru-ginous ironpan. Below the scarp, the soils are first shallow andcontain ironstone gravel, but a few meters downslope the sanddeposits are usually very thick (up to 8 m). These sandy, aeoliandeposits at the foot of the plateau scarp are referred to as ‘sandyskirts’ in the Sahel, and are very vulnerable to crusting (D’Herbèsand Valentin, 1997). In the valley bottoms, soils are of the weaklyleached sandy ferruginous type. All these soils are rich in sesqui-oxides (Al2O3 and Fe2O3), poor in organic matter (from 0.5% to3%), and have low fertility in nitrogen and phosphate contents.Various types of surface crusts can be encountered, their develop-ment being largely favored by land cultivation and fallowingcycles (Valentin and Casenave, 1992). Climate conditions alsocontribute to exposing the soils to acidification and to severewater and wind erosion. Although they constitute 20% of the totalarea of the square degree of Niamey (Leduc and Loireau, 1997),laterite plateaus formerly covered by tiger bush were not consid-ered in the present study.

The main land cover types and their local distribution withineach catchment are presented in Fig. 3. The sandy soils are cur-rently covered by millet crops (60%, which represents 95% ofthe total croplands) and by fallow lands (40%; Loireau, 1998;Guengant and Banoin, 2003). The rainfed millet (mostly the‘‘Haini Kiré’’ variety) is sown in pockets 1–1.5 m apart withouttillage and its duration cycle (between sowing and harvesting)is about 90–100 days long. Rooting systems observed in theexperimental area reached 2-m deep on fallow and 1.5 m undermillet. It is worthwhile to note that alluvial deposits at the bot-tom of the valleys were not considered in the study, becausebeing located in the vicinity of the much-studied valley ponds;their role in the deep infiltration process is already wellunderstood.

The free water table is located, on average, at a depth of about44 m at the W-AMONT site, and at a depth of about 47 m at theTK-AVAL site (Fig. 4).

Fig. 3. Land covers of the Wankama (top) and Tondi Kiboro (bottom) catchments; the labels indicate the location of neutron access tubes used in the soil moisture profiles ofFigs. 7–9 and 11.

L. Descroix et al. / Journal of Hydrology 424–425 (2012) 1–15 3


The study was mainly focused on the hydrological conse-quences of the landforms on infiltration through the unsaturatedzone; thus, the experimental setup of both catchments was de-signed accordingly.

2.2. Instrumentation and data collection

The main features of the sampling strategy that was developedto monitor the dynamics of deep infiltration on the two catch-ments are summarized in Table 1.

2.2.1. Rainfall dataOn-site daily rainfall values per event was measured by 10 rain

gauges in the Wankama catchment and 12 gauges at the Tondi Ki-

boro catchment, mostly by the so-called ‘‘Malian peasant’’ device(SIMPLAST, Bamako, Mali). On top of these manual rain-gauges,two recording gauges in the W and two in the TK catchment re-corded automatically after every 0.5 mm of rainfall (model PM3030, Précis Mécanique, Bezons, France, connected to dataloggers,model HoBo, OnSet Computer Corp., Pocasset, MA, the USA)(Fig. 4). Every cumulative value of rainfall event was averaged bykriging, best linear unbiased estimator defined by Matheron(1963), using the two series of datasets (with SURFER-8� software).

2.2.2. Runoff measurementsRunoff was monitored since the 2004 rainy season on four

stream gauges. Each catchment was equipped with two pairs of

Fig. 4. Map of the experimental catchments and implementation of the main devices at Wankama (top) and Tondi Kiboro (bottom) catchments.



Parshall-type flumes: one pair located at the upper AMONT (with acontributing area S = 0.3 km2) and lower AMZE (S = 0.32 km2) loca-tions in the Wankama catchment, and the other pair was placed atthe upper AMONT (S = 0.049 km2) and lower AVAL (S = 0.11 km2)locations in the Tondi Kiboro catchment (Fig. 4). Each stream gaugestation was equipped with a stage recorder (model ‘‘Thalimedes’’,OTT Messtechnik, Kempten, Germany) collecting water level dataevery minute at a resolution of 0.3 cm.

The location of these stations was guided by the need to com-pare surface water flows in both parts of each catchment by col-lecting data showing the possible existence of deep infiltrationareas. For that purpose, the four stream gauges were located inthe gullies of the two catchments (Fig. 4), where there was onlyone drainage channel. For the Wankama catchment, the downhillstream gauge was installed just at the boundary between the gullyand the spreading area, the upper one being situated 150 m up-stream. For the Tondi Kiboro catchment, the downhill hydrometricstation was installed 100 m upstream from the end of the gully, theupper station being implemented 500 m upstream from the lowerone.

2.2.3. Soil water content measurementsTwo types of sensors were used to monitor the soil water con-

tent of the two catchments: water content reflectometer probes

(model CS616, Campbell Scientific Inc., Logan, UT), and soil watertension meters (model Watermark, Irrometer Co., Riverside, CA).Table 2 gives their location at different depths. Two field cam-paigns of soil gravimetric water content sampling were carriedout in order to calibrate the two series of sensors at the samedepths. The first campaign was made in the dry season, and thesecond one during the rainy season, to take advantage of a largerrange of water content and soil water tension values. Based on sev-eral tens of measurements for each set, the results were foundclose to the calibration provided by the manufacturers (RMSECS616 = 0.0207; r2 = 0.65, 89 measurements; RMSE Water-mark = 0.37, r2 = 0.57, 54 measurements).

All the sensors were automatically recorded and connected todataloggers (Campbell CR10X Campbell Scientific, Leicester, UK)at the frequency of one measurement every minute averaged ona 30 min basis. Three measurement sites (two for the Wankamacatchment and one for the Tondi Kiboro catchment see Figs. 4and 5) were equipped (see Table 2) according to the most usedland types (gully bed on the one hand and millet and fallow onthe other hand). The sites were selected so as to be located as closeas possible to either a gully or a water spreading area, dependingon the catchment.

Since the goal was to assess soil water content profiles relatedto supposed deep infiltration into semi-arid areas, the only possi-ble method (other than measurements of chloride concentrationprofiles as described by Alison and Hughes, 1978; Alison, 1988;Edmunds and Gaye, 1994; Bromley et al., 1997) was the neutronprobe (NP) technique (Hignett and Evett, 2002). The NP allowsthe measurement of soil water content in boreholes equipped withaccess tubes down to several tens of meters, depending on theprobe used. Thus, in addition to the three soil water monitoringsites in the top 2.50 m of soil, 54 NP access tubes (made of rigidpolyvinyl chloride) were bored, in order to avoid preferential infil-tration along the tube, after installing the tube, dry sand (this com-ing from the hole) was poured to fill the space between hole andtube. The sand was regularly wetted to reach a good compaction.From 0.5 m deep to the surface, a cement plug, first very sandyand then pure near the surface, was poured to 50 cm higher than

Table 1Summary of the information on the measurement program for the two catchments.

Measuredvariable

Instrument Number oflocations

Frequency Depth monitored Procedure Observation

Rainfall« Malian peasant rain gauge » 22 Event KrigingPM 3030 rain gauge 4 0.5 mm Kriging

Soil water content profileWater content sensor CS616 Model 3 1 min 0–3 m Averaged every 30 min 8 in each stationSoil water Suctionwatermarks 3 1 min 0–3 m Averaged every 30 min 10 in each stationCPN 503DR Model-Neutron Probe 39 Dry season : month 10 m 1 reading per depth layer, 28 at W; 11 at TK

Wet season : 15 days 25 m 32 s count time 35 at 10 m deep, 4at 25 m deep

Infiltration2 disk infiltrometers at multiplesuctions: 8 and 25 cm diameter

50 0 and 0–100 cm Measured with graded sandbase; applied suctions:�100, �50, �25 and�10 mm

Stream flowOTT Thalimedes on stabilizedsections

4 1 min or 1 cm 1–2 m Calibration curve based on20–50 gauging

Discharge measuredwith current meter

Water Table levelDiver Eijkelkamp (6.5 cm diameter) 1 15 min 47 m TK catchmentPiezometric probe (6.5 cm diameter) 2 15 days 44 and 47 m W and TK sites

Additional data collectionSoil samples taken along one 2.5 mdepth profile

3 Wettest and driest seasons 0–2.5 m Gravimetric Bulk density measuredby the ‘‘pool’’ method

Table 2Depth (in cm) of implementation of tensiometers and TDR probes on each monitoringsite.

Soil suction sensors (tensiometers) Soil moisture sensorsa

Watermarks fallow Watermarks gully CS616 fallow CS616 gully

25 10–4055 55 40–70 40–7085 85 70–100 70–100

115 150 100–130 100–130230 200 140–170b

250

a Installed vertically and 30 cm long.b Only for the station of the Tondi Kiboro catchment.



Fig. 5. Cross-sections of the Wankama (top) and Tondi Kiboro (bottom) catchments including locations of ponds, gullies, spreading areas, and streamgauges stations. Verticalarrows represent the measured (dark gray) and estimated (clear gray) volumes annually infiltrated in deep soil layers.

Table 3Components of the water balance values for the Wankama catchment (2004 2008).

WANKAMA catchment P N AMONT (area 300,000 m2) AMZE (area 320,000 m2) D AMONT-AMZE (estimated infiltration)

YEAR RN VR KR RN VR KR RN VR

2004 379 14 35 10,543 9 29 9269 8 6 12732005 500 14 66 19,841 13 30 9446 6 36 10,3952006 566 19 177 53,082 31 71 22,732 13 106 30,3502007 445 11 141 42,197 32 64 20,510 14 77 21,6872008 585 21 239 71,575 41 90 28,852 15 144 41,358

Total of 5 years 2475 79 658 197,238 284 90,809 379 105,064Mean per year 495 16 132 39,448 27 57 18,162 11 76 21,013CV (%) 17 62 50 47 36 72

P: rain (mm).N: number of runoff events.RN: runoff depth (mm).VR: total discharge (m3).KR: runoff coefficient (%).CV: coefficient of variation.

Table 4Components of the water balance values for the Tondi Kiboro catchment (2004 2008).

TONDI KIBORO catchment P N AMONT (area 49,300 m2) AVAL (area 110,540 m2) D AVAL-AMONT (61,240 m2)YEAR RN VR KR RN VR KR RN VR KR

2004 533 18 239 11,761 45 171 18,898 32 �117 �7138 222005 400 19 90 4436 22 65 7161 16 �44 �2724 112006 561 24 218 10,754 39 132 14,590 24 �63 �3835 112007 524 24 274 13,496 52 127 14,083 24 �10 �587 22008 611 19 317 15,624 52 193 21,261 32 �92 �5637 15Total of 5 years 2627 104 1138 56,072 688 75,993 �325 �19,925Mean per year 526 20 227 11,214 43 138 15,199 26 �65 �3984 12CV (%) 15 38 28 36 25 64 59

P: rain (mm).N: number of runoff events.RN: runoff depth (mm).VR: total discharge (m3).KR: runoff coefficient (%).CV: coefficient of variation.



the soil surface. Data of 39 from the 54 NP access tubes were col-lected for the study (Fig. 4) in order to monitor the wetting frontadvance below the root zone, that is down to at least 10 m – andin certain places down to 25 m- below the surface: 28 for the Wcatchment and 11 for the TK one, 18 of them being on fallow lands,9 in millet fields, (3 more were installed in 2004, which became fal-low in 2005) and 3 on degraded fallow (erosion crusted soils). Thelast six tubes were located in areas where deep infiltration wasthought to occur: 4 on spreading areas and 2 under gullies. Most ofthe access tubes were 10 or 12 m deep and 4 reached 25 m (Table 1).Neutron counting rates (model 503DR Campbell Pacific Nuclear,Concord, CA; 32 s count time) were taken every 0.25 m down to

1.5 m and below that at every meter down to the depth of theaccess tubes. The frequency of measurements was at least monthly(2-days long) and was increased to twice a month during the mon-soon season (until December) of each year. The measurementsstarted in August 2004 (but in January 2006 for 14 of the accesstubes used in this study) and continued until April 2009. Resultsare presented here in terms of ratios between the count rates Ctaken at the various depths into the soil in a given access tube and a‘‘standard count rate’’ C0 taken in a reference medium (water drumin this case) equipped with the same tube type (PVC). Unfortu-nately, the absence of a drilling device able to take undisturbed soilsamples at different times and depths of investigation prevented

Table 5Duration (in h) of water flow measured in each stream gauge of the catchments for the five rainy seasons.

CATCHMENT/YEAR 2004 2005 2006 2007 2008 Total of 5 years Mean 2004–2008

W-AMONT 31.2 17.4 36.5 50.6 46.4 182.1 36.42W-AMZE 24.6 13 20 18.6 31 107.2 21.44TK-AMONT 11.1 19.8 28.6 23.8 46 129.3 25.86TK-AVAL 8.5 12.2 18.7 8.9 21.8 70.1 14.02D W-AMONT/W-AMZE 6.6 4.4 16.5 32 15.4 74.9 14.98D TK AMONT/TK-AVAL 2.6 7.6 9.9 14.9 24.2 59.2 11.84

Fig. 6. Time evolution of soil water content calculated at different depths on fallow fields and their neighboring gullies for the Wankama (top) and Tondi Kiboro (bottom)sites. The total amount of rain in 2005 is 500 mm and 400 mm, respectively.



the calibration of the neutron probe for converting count ratios C/C0 into volumetric water contents. However, the evolution in C/C0

from one neutron dimensionless profile to another one allowed toidentify the water front at a given depth, and to monitor it in timeand space. Since the experimental sites were submitted to positivewater pressure heads for only a few hours in the year, measure-ment bias due to local infiltration was considered unlikely.

In addition, values of volumetric water content and correspond-ing tension close to saturation at the soil surface and to depths of afew centimeters were determined on the main soil surface condi-tions by using the tension disk infiltrometer method (Vandervaereet al., 1997) with two disks of 8 and 25 cm diameter.

3. Results

3.1. Stream flow water balance

We compared the amount of rainfall in the Wankana catch-ment, the runoff at the two stations and the infiltration estimatedfrom the runoff difference (Table 3). For the 5 years of runoff mea-

surements in the Wankama catchment, 79 rain events (out of a to-tal of 158 with P > 2 mm) triggered runoff and stream flows. Thestream flow was more than 50% smaller at the AMZE downstreamstation, than at the upper AMONT one. This difference represents atotal volume VR of 105,000 m3 (almost 21,000 m3 yr�1 or an infil-tration depth of 63 mm yr�1 over the catchment area). The dis-tance between both stations is 150 m long. However, the volumeproduced by runoff in the 20,000 m2 of the catchment contributingto stream flow between the AMONT and AMZE stations, not esti-mated here, should be added to the difference, then to the infil-trated water volume. This means that there was a stronginfiltration in this part of the gully. It is considered that the differ-ence was mostly infiltrated in the sandy deposit of the gully. Thedifference is even greater when comparing runoff coefficients ofboth stations: 27% at the AMONT station and only 11% at the AMZEstation.

For the TK catchment, where the total amount of rainfall overthe 5 years was slightly higher than (but not statisticallysignificant) for the W catchment (526 versus 495 mm with CVsof 0.15 and 0.17, respectively), 104 rainy events (over 151 total)

Fig. 7. Examples of time evolution of neutron counting ratios as function of depth of fallow land (7a) and millet crop (7b), for the Wankama catchment in 2006.



produced runoff and stream flows (Table 4). The difference incatchment areas was significant (the total area of the AVAL catch-ment was more than twice as much as the AMONT catchment). Butthere was a decrease in the yearly mean runoff coefficients (KR),from 43% at the AMONT station to 26% at the AVAL station, whichmust have been due to high infiltration in the gully between thetwo stations.

Mean values of the yearly duration of stream water at eachstation ranged from 16 to 36 h per year depending on the station(Table 5). Since air is saturated during the ‘‘stratiform’’ stage ofsquall lines, which is the period when stream flow decreases andstops, and due to the non-vegetated environment of the gulliesand spreading areas, we hypothesized that evaporation could beneglected and that most of the water lost between the upstreamgauges and the downhill ones, was infiltrating. Even if the evapo-rative demand (PE) was satisfied the entire day following a rainyevent, the evaporated volume in the gully bed would be, for exam-ple for the Wankama gully, equal to 2.25 m3 (150 m long � 3 m

wide � 5 mm with 5 mm of maximum PE measured during theday following a rainy event). Typically, the sandy deposit was driedin its first 10–20 cm at the end of the first day; thus this value of2.25 m3 multiplied by the number of runoff events (79 in 5 years)gives less than 200 m3, which is negligible compared to the105,000 m3 lost between the two stream gauges.

The total stream flow volume at the downstream Tondi KiboroAVAL station contributed about 15,000 m3 yr�1 to the spreadingarea on average over the 5 years (Table 4; Fig. 5). As a matter of fact,no water passed over the outlet of this catchment for the 5 years un-der consideration. Fig. 5 shows the infiltration measured by beingthe difference of total volume discharged between the two streamgauges (dark gray) and the estimated (clear gray) infiltration inthe new infiltration areas for the Wankama catchment; in the TondiKiboro catchment, the infiltration between the two stream gauges isestimated (clear gray) while the infiltration downstream the AVALstation is measured (dark gray), Figure 5 also indicates the locationof devices in the profiles of each catchment.

Fig. 8. Examples of time evolution of neutron counting ratios as function of soil depth of the spreading areas for the Wankama catchment in 2006 (8a) and Tondi Kiborocatchment in 2007 (8b).



For both catchments, we observed: (i) a strong decrease in therunoff coefficients between the upstream and the downstreamstations for the 5 years of measurements, discharges differing bya factor of 2 on the Wankama catchment over its 150-m long gully,(ii) a full infiltration of stream flow in the spreading area down-stream of the TK-AVAL station, and (iii) on the W catchment,stream water flows at the outlet of the spreading area, which alsoinfiltrates large volumes of water, were still observed every year.Notwithstanding the high significance of these estimated infiltra-tion volumes, it should be kept in mind that streamflow gauging,in gullies with very high sediment transport and equipped withmechanical stage sensors, is unavoidably marked by a certain levelof uncertainty, especially when considering volumes differencesthat cumulate errors from two gauging stations. It is believed how-ever, given the magnitudes of the above estimated volumes, thatthese field results do evidence the importance of infiltration inthese drainage networks, and the significant roles played by bothgully beds and alluvial flats in the water balance of thesecatchments.

3.2. Soil water content

The soil water content recorded by water content reflectome-ters at several depths (see Table 1) as a function of time in 2005on the two catchments was strongly affected by the different landsurface conditions (Fig. 6). Strong differences in the potentialdrainage between the sandy bottom of a gully and the crusted san-dy soil of the neighboring fallow are highlighted. The gully bottomshowed a high increase in soil water content down to 130 cmdepth between one or two hours after the rainfall event, whereasthe soil under the fallow shows soil water content at 120 cm, only2 or 3 weeks after the first significant rainy event in the season.

A very small temporal variation of soil water content was ob-served at depths between 2 and 10 m under the fallow (Fig. 7a);and the soil was wetted only slightly below 2 m during the mon-soon under millet (3 m on average, 5.5 m at maximum) (Fig. 7b).This is relevant to the case in almost the entire area of the Sahel,since 80% of the land is nowadays covered by fallow or millet.The year 2006 was relatively rainy, but the features of water

Fig. 9. Examples of time evolution of neutron counting ratios as function of soil depth of the gullies for the Wankama (9a) and Tondi Kiboro (9b) catchments in 2006.



infiltration shown in Fig. 7 were representative of the 5 years ofmeasurements.

3.3. Infiltration under gullies and spreading areas

It has been hypothesized that spreading areas and gullies, asnew landforms linked to the current erosion stage, are both drain-age and deep infiltration zones because of their sandy constitution.Figs. 8a and b shows the deepening of the wetting front within thetwo spreading areas of Wankama and Tondi Kiboro. At the Wank-ama site, the wetting front reaches at least 10 m, and more than8 m at Tondi Kiboro.

The evolution of wetting front under the gullies at Wankama(Fig. 9a) and Tondi Kiboro (Fig. 9b) showed that in both cases,the infiltration reached at least 10 m deep. Contributions to localinfiltration due to the measurement devices themselves were unli-kely because:

(i) Greater wetting front progression rates were observed inplaces where the first meters of soil have hydraulic conduc-tivity value larger by one or two orders of magnitude thanthose for the soil in the vicinity of the gullies;

(ii) the wetting front was recorded at nearly the same time bythe three devices (neutron probe, water content reflectome-ters and soil water tension sensors), in the 0–2.5 m deepzone where the data are comparable (Fig. 10) making doubt-ful a local infiltration due to the neutron probe access tube.Similar results were observed from 2004 to 2010; in somecases the wetting front appears earlier with one sensor com-pared to the others (e.g. fallow at Wankama in 2007; gully of

Tondi Kiboro in 2006); as these cases appeared randomly(never at the same site or the same year) we attributed theiroccurrence to the spatial variability of soil physical charac-teristics. In certain cases, this occurrence may have beendue to the fact that the neutron probe counting was madetwice a month, while the other sensors automatically mon-itored water content and water tension every 15 min.

(iii) There was little change in the annual variation between thelargest and the smallest soil water content values for the4 years of measurements (Fig. 11). This variability is proba-bly linked to rainfall variability; total infiltrated volumebetween AMONT and AVAL station in Wankama are linkedto rainfall (first and last columns in Table 3).

3.4. Synthesis on the infiltration rate

An important concern is the mean date of occurrence of thewetting front during the monsoon according to the depth mea-sured by neutron probe (Fig. 12). The first significant rainfall wasconsidered as the first one with more than 20 mm and withoutany dry period of more than 7 days before the following rain eventof more than 10 mm. The mean date (2004–2008) of this first eventfor the four rainy seasons considered was June 20 at Tondi Kiboroand July 1 at Wankama. The difference in mean dates of advance inwetting front (Fig. 12) highlights the considerable difference inpermeability, infiltration rate, and soil water content between mil-let fields and fallow ground, which are characterized by low infil-tration capacity, on one hand, and spreading areas and gullies,which exhibit high infiltration capacity, on the other hand. Satu-rated hydraulic conductivity measured with infiltrometers (Ks)

Fig. 10. Comparison of wetting front deepening as function of time between neutron probe, TDR and tensiometers devices under millet and fallow (10a) and under gullies(10b) for the Wankama and Tonki Kiboro catchments in 2006 and 2007.



was 162 mm h�1 (standard deviation SD = 47) on millet fields,108 mm h�1 (SD = 50) on fallows and 18 mm h�1 (SD = 3.6) oncrusted bare soils (10 repetitions in each class), and close to1300 mm h�1 in sandy deposits. (24 repetitions).

Clearly this difference, except for the millet at the beginning ofthe monsoon, is also due to evapotranspiration of millet fields andfallow (both being linked, because vegetation and roots enhancesstrongly the soil hydraulic conductivity).

The stronger evolution of the wetting front under the newly ex-tended landforms (sandy deposits on gullies and spreading areas)was due to larger hydraulic conductivity of the sandy native soilmaterial. But in the latter places, the wetting front cannot crosseasily the top soil because of the crust that strongly slows downthe infiltration. Conversely, infiltration is favoured within the san-dy deposits in the gullies and spreading areas because of the con-tinual erosive processes in the gullies and the continual renewal ofthe soil surfaces during flooding on the spreading areas, both ofwhich prevent crusting. Although no water table data are pre-sented here, previous results show that large volumes of waterare infiltrated below the root zone in a sandy environment, even-tually contributing to groundwater recharge.

4. Discussion

New deep infiltration areas have appeared in the last 2 or 3 dec-ades, due to erosion and land degradation. They can increase therate of rise in the water table. It is still necessary to establishwhether these ‘‘new’’ infiltration areas have a more important im-pact on the water table budget compared to the ponds, or whethertheir roles remain secondary.

Since potential evapotranspiration did not change significantlyin recent years, we cannot here state whether the minor move-

ment of the wetting front under millet is a recent phenomenonor not. It might be considered surprising, as this crop constitutedthe only extended site where there existed a surface roughness.However, millet is rarely sufficiently watered in this environment,so it is possible that nearly all rainfall was evapotranspired by thecrop. Also, Hiernaux et al. (2009) observed, on the basis of long-term historical monitoring, an increase of area cropped to milletat an annual rate of 2%. They also reported a yearly decreasingtrend in yield of 5%, due to soil degradation. Overexploitation ofsoils has been shown to lead to top soil crusting (Valentin andCasenave, 1992). This explains why, simulating land use and cli-mate changes over the 1950–1992 period, it was shown that landclearing increased runoff by nearly threefold (Séguis et al., 2004).

In recent years, bush was replaced by crops and fallow; sinceFig. 7 shows that there is very little infiltration under fallow andcrops it may be infered that more water runs to the ponds, fromwhich it infiltrates to the aquifer, thus explaining the rise in watertable. However, at the plot scale, it was determined by Le Breton(2012) that there was still little runoff on millet and almost allrainfall was infiltrated. This author monitored runoff and erosionon 20 plots (10 and 100 m2 large depending on the plots) over5 years. Mean observed runoff coefficient was 4% on millet crop,10% on fallow and 60% on crusted degraded soils. These twoassumptions (infiltration versus no infiltration under millet crop)may be consistent due to the scale effect: infiltration dominatedat the plot scale due to roughness, and runoff dominated at the re-gional scale due to the land degradation which is linked to the soilerosion because of the over-exploitation of the soil. Clearly part ofthe water is evapotranspirated on millet fields and fallow (roughlyrespectively 45% and 65% of the rain water for a wet year; Ramieret al., 2009), but this rate is punctually and regionally decreasingdue to soil crusting that reduces infiltration and contributes to

Fig. 10 (continued)



the lowering of the millet crop yield. Otherwise, although most ofthe water infiltrated under the millet on non-degraded areas, this

was not the case during the first month of the rainy season, whenthe millet did not yet cover the soil.

Fig. 11. Annual infiltration variations between maximum (August) and minimum (before Monsoon) as function of depth for the 5 years of measurements under the spreadingarea (left) and the gully (right) of the Wankama catchment; horizontal axes represent neutron count ratios.



The new landforms caused by the current erosion stage must beacknowledged as important deep infiltration areas and we hypoth-esized that they contributed to water table recharge.

Since data from only two measurement sites were available, it isclearly not sufficient to describe the regional behavior. Deepinfiltration appeared to certainly reach the water table in certaincases, since they reach much deeper that the root zone. This couldconfirm one of the assumptions made by Peugeot et al. (1997)about the probability of deep infiltration under the Tondi Kiborocatchment. It is also consistent with the results obtained on thesame catchment by Esteves and Lapetite (2003) who observed adecrease in stream flows between the AMONT and the AVALstations (the same stations as those used in this study). Theyconcluded that most of the water was infiltrating in the bottomof the gullies. It also corroborates the geophysical observationsmade by Massuel et al. (2006) in the Wankama catchment, whichshow that ‘‘deep infiltration can also occur episodically throughalluvial fans on sandy slopes, thus representing additional poten-tial sites for groundwater recharge’’.

Thus, there is a strong convergence of facts to support the infer-ence that the actual process of erosion and deposition of sandylandforms results in significant areas of deep infiltration into theunsaturated zone.

5. Conclusion

Spatial extension of both gullies and spreading areas covered bysandy deposits increased significantly in the last decades, due togeneral land use changes and land degradation in the Sahel of WestNiger. Therefore, we can state that:

1. Gullies and spreading areas are important zones of deep infil-tration. This study showed that sandy deposits allows the tem-poral storage of high volumes of stream water with probablylittle evaporation, because of the coarse grain size distributionthat causes a significant infiltration at the moment where thestream flow occurs and in the following hours. Simultaneously,it avoids water retention near the surface. Therefore, infiltratedwater reaches soil layers with neither roots nor capillarity pro-cesses sufficient to convey water upwards to the surface in afew hours. Under these sandy deposits, wetting fronts reached>10-m deep, making them likely areas of deep infiltration. They

probably contribute to the rising water table in the Sahel andimpact the continental part of the water cycle. The current ero-sion stage is creating numerous gullies in the upper part of thecatchments, and a significant number of spreading areas down-stream. Thus, the permanency of such an erosion stage willcontinually create more infiltration areas, which acceleratesthe water table recharge, as these new landforms rechargegroundwater in addition to that recharged in and around the‘‘bas-fonds’’ (the French name for valley bottom) ponds. Usingtension infiltrometers, it was possible to estimate values ofinfiltration rates in the main areas of different land uses, whichfurther reinforced the conclusion that spreading areas andgullies are currently areas of groundwater recharge by deepinfiltration. However, by following Popper (1934), we are awarethat our findings, although appearing robust enough by thelight of our assumptions, need further research studies in otherconditions to confirm or not the results presented heretofore.

2. Very little infiltration is observed under the root zone in the fal-low and millet fields; high runoff coefficient at the catchmentscale suggests that part of the non-evaporative part of rainwater provides runoff, due to soil crusting;

3. It is shown that significant water volumes are infiltrated belowthe root zone in a sandy environment, that eventually contrib-ute to groundwater recharge. The relative importance of gulliesand spreading areas sandy deposit on the one hand, and pondson the other hand in deep infiltration remains to be quantified.

The next step of our research will be devoted to the hydrologicprocesses and unsaturated area behaviors in the exorheic areas ofthe western bank of the Niger River, where the catchments are sev-eral thousands of km2 large, and the substratum mainly plutonic.Better understanding of these phenomena in most of the Sahelianconfigurations and their spatial and temporal evolution with landuse changes will hopefully help to provide all the necessaryexplaining factors to study the land surface-atmosphere feedback.

Acknowledgements

This study was carried out within the framework of the AMMAproject (African Monsoon Multidisciplinary Analyses). Based on aFrench initiative, AMMA was built by an international scientificgroup and is currently funded by a large number of agencies based

Fig. 12. Depth of the wetting front measured by neutron probe (mean 2004–2008) at different dates of occurence according to the main land features of the Wankama andTondi Kiboro catchments.



in France, the United Kingdom, the United States of America, andAfrica. It has been the beneficiary of a major financial contributionfrom the European Community’s Sixth Framework Research Pro-gram. Detailed information on scientific coordination and fundingis available on the AMMA International web site http://www.am-ma-international.org. This research was also funded by the ‘‘Pro-gramme National de Recherche en Hydrologie’’ of the French‘‘Ecosphère Continentale’’ program, and by the TOSCA programmeof the CNES space agency. We also warmly acknowledge IssoufouHassan Bil Assanou and Oumarou Halilou for their important con-tribution in the field works.

This study was also partially funded by the French ANR projetECLIS (Contribution of livestock to the reduction of rural popula-tion vulnerability and to the promotion of their adaptability to cli-mate and society changes in Sub-Saharan Africa).

The authors warmly acknowledge the reviewers who assessedthe manuscript and helped to greatly improve its English grammarand syntax.

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Experimental evidence of deep infiltration under sandy flats and gullies in the Sahel

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