groundwater fluctuations and footslope ferricrete soils in the humid tropical zone of southern...

HYDROLOGICAL PROCESSESHydrol. Process. 19, 3097–3111 (2005)Published online 10 May 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.5834

Groundwater fluctuations and footslope ferricrete soils inthe humid tropical zone of southern Cameroon

Emile Temgoua,1,2* Henri-Bosko Djeuda Tchapnga,3 Emile Tanawa,3 Claire Guenat1

and Hans-Rudolf Pfeifer2

1 Swiss Federal Institute of Technology, Lausanne, ENAC-Pedology, Bulding GR-Ecublens, 1015 Lausanne, Switzerland2 IMG-Centre d’Analyse Minerale, Universite de Lausanne, BFSH2, 1015 Lausanne, Switzerland

3 Laboratoire Environnement et Science de l’eau, Ecole Nationale Superieure Polytechnique, BP 8930 Yaounde, Cameroun

Abstract:

This paper discusses the relationship between the differentiation of ferruginous accumulations and the variable watersaturation of footslope soil patterns. An analysis of the slope morphology of a typical hill in the forest zone of southernCameroon and a seasonal survey of the levels of groundwaters, springs and rivers were considered in relation to thepetrology of different soil patterns. The study site is a tabular hillock whose slopes present a progressive developmentfrom steep to gentle slopes. The variable residence time of water within the soil, creating an alternation of reducingand oxidizing conditions, affects soil chemistry, structure and lateral extension of the soil patterns. The ferruginoussoil patterns, being formed on the footslopes, gradually increase in extent with decreasing slope angle and the relativerise of the groundwater level. The steep footslopes, where groundwater has a shorter residence time, show a softmottled clay pattern, restricted to the bottom part of the slope. The moderate footslopes exhibit a deep permanentand a temporary perched groundwater table. The latter, with its regular capillary fringe, contributes to more reducingconditions within isolated domains in the soil patterns, and thus to the alternation with oxidizing conditions, generatinga continuous hard soil pattern (massive carapace). The more gently dipping footslopes exhibit groundwater levels nearthe surface and also a significant amplitude of groundwater fluctuation. Iron, previously accumulated in moderatefootslope patterns, is reduced, remobilized, and leached. The soil patterns formed develop into a variegated carapace,more extended along the slope, containing less iron, but nevertheless more hardened, due to the important fluctuationsof the groundwater table. These patterns are limited to the zone of groundwater fluctuation and deteriorate as the waterfluctuation zone recedes. Copyright 2005 John Wiley & Sons, Ltd.

KEY WORDS groundwater fluctuations; footslope soil genesis; tropical humid rainforest; ferricrete

INTRODUCTION

Topography is both an internal and external factor in pedogenesis, that is to say it either influences or is aconsequence of soil formation (Tricart and Michel, 1965). Geochemical conditions are different from upslopeto downslope in the same landform unit, depending upon the influence of topography on the drainage and thehydrology of the soil cover (Lucas and Chauvel, 1992), according to the conditions of circulation and renewalof the soil solution (Grimaldi et al., 1994). The most important chemical change that takes place, when a soilis saturated, is the reduction of iron and the accompanying increase in its solubility (Ponnamperuma, 1972).Iron in soils behaves according to some general principles: (a) the rise of the groundwater table within thesoil profile causes a variety of changes in the soil, due to changes in the redox state, i.e. bright, oxidized‘ferric’ forms become replaced by darker, reduced ‘ferrous’ forms (Bown and Kraus, 1987; Wright et al.,1992); (b) the zone with temporary saturation is a site of iron precipitation (Brinkman, 1970; Van Breemann,1988), in particular when the water table recedes at the end of the rainy season; (c) iron oxides occur across a

* Correspondence to: Emile Temgoua, IMG-Centre d’Analyse Minerale, Universite de Lausanne, BFSH2, 1015 Lausanne, Switzerland.E-mail: [email protected]

Received 16 September 2003Copyright 2005 John Wiley & Sons, Ltd. Accepted 19 August 2004

3098 E. TEMGOUA ET AL.

continuum of amorphous, para-crystalline and crystalline forms in soils; and (d) with regard to colour, yellowto brownish hues indicate appreciable goethite content, whereas redder hues indicate appreciable haematitecontent (Schwertmann and Taylor, 1989).

Many studies have combined hydrological (age of the water and the spatial origin of overland flow,i.e. shallow and deep groundwater) and geochemical (elemental leaching by storms, groundwater, etc.)methodologies to study the biogeochemical processes taking place in the various hydrological compartmentsof watersheds in tropical environments. Available data centre upon the geochemical interactions betweensurface or ground waters and lateritic covers, and have mostly dealt with the chemical source/sink budgets(e.g. Stallard and Edmond, 1983; Dupre et al., 1996; Eyrolle et al., 1996; Viers et al., 1997) or the plant-waterfeed (McDaniel et al., 2001). Brinkman (1970) was the first to explain the mobility and accumulation of ironduring the formation of seasonally wet soils. Studies that sought to understand the processes governing soilgenesis are scarce and for the most part focus on the effects of storms and fluctuations in the water-tablelevel on the soil chemistry (Ponnamperuma, 1972; Lucas et al., 1986; Phillips, 2000; Grimaldi et al., 1994,2003).

Landscape peneplanation is a consequence of the uphill ferricrete degradation in the tropical rainforestzone (Lucas, 1989; Tardy and Roquin, 1998). This flattening phenomenon occurs with the relative rise ofwater level towards the land surface. The mottled horizon, commonly described in lateritic soils, constitutesthe transition from weathered bedrock to the lateritic duricrust and lateritic gravel (Bilong et al., 1992;Tardy, 1993; Beauvais, 1999) or to the soft clayey level (Nahon et al., 1989; Rosolen et al., 2002). On thefootslopes of the central African rain forest zone, these mottled horizons evolve into ferruginous patterns(Temgoua et al., 1999, 2002), which are precursors of a new ferricretion. The appearance of humid climaticconditions and forest cover in the wet tropical zone are at the origin of most of the degradation of the oldsummit crusts, but also of land development and the genesis of footslope soil patterns. Thus, the rise of thewater table (Olivry, 1986) might have caused the new ferruginous accumulations in profiles of footslopesas supposed by Bilong et al. (1992). The understanding of Central African processes governing the genesisof the footslope soil would be improved by taking into account the effect of water-table fluctuations on theaffected soils. Observations of soil patterns in relation to their saturation time were carried out in southernCameroon. The specific aim of this study is to understand the role of water-table fluctuations in relation to themorphology and the development of three different types of footslope soil sequence starting from the sameparent material.

MATERIALS AND METHODS

General aspects

The site of this study is located near the village of Meyomessala, which is situated northeast of the cityof Sangmelima in southern Cameroon (Figure 1). The rock substratum is a granodiorite, consisting of quartz,calcium-sodium-feldspar, pyroxene, muscovite and hornblende minerals.

The overlying vegetation is part of the Dja forest, constituting a particularly well-protected part of thesemi-deciduous ‘Congolese Forest’. The Dja forest covers the Dja river watershed and is permanently green.The site of Meyomessala is drained by a tributary of the Dja on the right bank at the principal hairpin loopof this river (Figure 1b). The structural hierarchy of various tributaries in the hydrographic network is asfollows, from largest to smallest: Dja, Libi, Ndi, Zembe. The site is bordered by the Ndi river in the north andits Zembe tributary in the south (Figure 2), their orientation being subparallel to the lateral extension of themountain site (southwest–northeast). The site is deeply notched along its borders by the tributaries of theserivers, which subdivide it into lobes. In these notches, the valleys are narrow and the marigot headwaterscoincide with the beginning of the water channel. Downstream from these headwaters, the valleys widen andbecome marshy.

Copyright 2005 John Wiley & Sons, Ltd. Hydrol. Process. 19, 3097–3111 (2005)

GROUNDWATER FLUCTUATIONS AND FERRICRETE SOILS 3099

Nsimi

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Figure 1. (a) Location of Cameroon in Africa; (b) The Meyomessala study area in southern Cameroon. Map B also shows the main citiesand hydrographical networks of the area

The annual average temperatures in this area are between 23 and 25 °C (means over 20 years). Februaryis the hottest month, and July is the least hot. This low seasonal average temperature fluctuation, 2 °C, ischaracteristic of the Congolese Forest (Suchel, 1972). At the hydrological station in Sangmelima, there are fourdistinct seasons: two dry seasons alternate with two rainy seasons (Figure 3). The most frequent fluctuationsin precipitation are around 250 mm year�1, but a fluctuation of 700 to 1000 mm year�1 is often observed(1952–53), with the result that, from one year to another, rainfall can vary from 1300 mm for a ‘dry year’to 2000 mm or even more for a ‘wet year’. Precipitation of the year 1997 was analysed in more detail; themaximum precipitation in 24 h was 57Ð3 mm on 1 October, followed by 56 mm on 20 May, 53 mm on 20September, 34 mm on 21 June. The maximum daily precipitations for the other months are all below 30 mm.In 1997, the annual precipitation was 4% less than the 20 year mean (1600 mm).

Methods

Topographical surveys were carried out using a bubble level gauge, along transects that crossed theentire site. Disturbed and non-disturbed soil samples were described and collected as a function of depthin pits dug along the transects (Figure 2). Residual moisture was measured on non-perturbed samples bythe measurement of the weight before and after drying for 24 h at 105 °C. Other soil samples were air-dried,sieved to 2 mm and then this ‘fine earth’ was used for physical measurements and mineralogical and chemicalanalyses. Non-perturbed soil samples were impregnated with epoxy resin under vacuum prior to making thinsections for examination with a polarizing microscope. Soil mineralogy was investigated with a Philips X-raydiffractometer, using a copper anode (Cu K˛) with a characteristic wavelength of 1Ð5418 A (40 kV source)and a xenon X-ray detector. Chemical elements were analysed by X-ray fluorescence spectrometry on pressedpowder pellets, using a Philips PW 2400 instrument with a rhodium anode end window tube. To evaluatethe crystallinity of iron oxides, quantitative wet chemical methods permit one to establish the amount of thetotal free iron (Fed), extracted with strong reducing agents like dithionite–citrate–bicarbonate (DCB; Mehraand Jackson, 1960), and the amount of amorphous iron (Feo), extracted by oxalic acid–oxalate (Tamm).Supernatants from chemical extractions were decanted and analysed by inductively coupled plasma atomic



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Schematization ofMeyomessala sitelandscape morphology

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Figure 2. Typical landform units of the Meyomessala interfluve and examples of topographic profiles. Legend: (1) plateau; (2) top hill;(3) steep slope; (4) moderate slope; (5) gentle slope; (6) marshy zone; (7) outcropping rock; (8) soil profile; (9) open transects on the forest;

(10) specific localities in the springs (SA–D) and the rivers described in the text; (11) A—B, toposequence line

emission spectroscopy for iron and aluminium. A citrate–bicarbonate (CB) extraction was also carried out,supernatant iron and aluminium contents from which it was possible to calculate the rate of iron substitutionby aluminium according to the formula Al/�Al C Fe�% D �mol Ald � mol AlCB�/[�mol Ald � mol AlCB� C



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Figure 3. Annual and seasonal distribution of precipitation for the Samgmelima station, including the 20 year mean

�mol Fed � mol FeCB�] (Jeanroy et al., 1991). The CB value represents the concentration of metal adsorbedon mineral surfaces and poorly crystallized iron oxides.

Observations during the four seasons of 1997 made it possible to determine fluctuations of the groundwatertable, springs and large rivers at Meyomessala Hill. The head level of the groundwater table was detected eitherin the pedological pits, or by hand-held drill surveys, the level of rivers and springs being measured directlyby lowering a graduated rod to the bottom of the river. Sometimes, follow-up measurement of variations inthe river water-table level were made monthly, or even daily during some field survey periods, in order tospecify its fluctuations.

LANDSCAPE MORPHOLOGICAL UNITS

The Meyomessala site is an interfluve of around 250 ha. The altitude varies between 705 and 635 m abovesea level (a.s.l.) in the south, e.g. a relief of 70 m, and between 705 and 660 m a.s.l. in the south, e.g. asmaller relief of 45 m. On the basis of slope steepness, six landform units can be distinguished on the site(Figure 2):

ž The plateaus (i.e. summits with slopes less than or equal to 1%) account for approximately 20% of thesite’s hill surface. The main plateau is separated from the eastern plateau by a dry valley, which is not verydeep, and is connected to the southern plateau by a weak slope zone.

ž The top hills with weak slopes (i.e. slopes from 2 to 12%) is the most widespread landform unit on the site,accounting for 30% of the site surface.

ž The steep footslopes, varying from 30 to 50%, represent less than 5% of the hill surface studied. Theyoccupy the northern hill border on a strip of land whose width varies greatly, from 5 to 50 m.

ž The moderate footslopes, varying from 15 to 30%, account for approximately 10% of the hill surface studied.They constitute a curbing zone at the head of western, southwestern and eastern notches, on a strip of landwhose width can reach 200 m on both sides of the valley.



ž The gentle footslopes, varying from 1 to 2%, occupy the southern, eastern and western parts of the interfluve.They encircle the lobes, connecting with the moderate footslope zones towards the laterally lower part ofthe notches.

ž On the valley grounds, marshy zones occupy the valleys, getting narrow towards the headwaters andenlarging towards the main rivers.

The study will be of particular interest in the footslope domains, which exhibit the three types of morphologydescribed above.

SOIL PATTERNS CHARACTERIZATION AND DISTRIBUTION

The soils studied can be classified as ferric ‘Ferrasols’ (FAO, 1998). If we had applied classical soilterminology, most of the soil features found would be in the same horizon. Therefore, we use the term‘pattern’ to describe sub-horizons. Fifteen patterns were identified within the study site (Temgoua, 2002).Six of them are of interest to this study (Figure 4) because they are specific to the footslope. Thesepatterns are, according to the terminology of Nahon (1991), Tardy (1993) and Beauvais (1999), from thesoftest to the most hardened: yellow–grey clay, mottled clay, variegated clay, variegated massive carapace,spotted massive carapace and variegated vesicular carapace. Their distribution is related to the grade of theslope.

On the steep footslopes, the unique ferruginous accumulation pattern is represented by mottled clay(Figure 4a), being distinguished from the bottom weathering part of the soil profile by the diminution insize of yellow–orange (10YR 7/8) and whitish or light grey (10YR 8/3, 8/2) spherical domains. At the sametime, one observes a transition of the purplish red domains at the bottom of the profile to dull reddish orange(10R 6/4) domains towards the top and, within the latter, the appearance of small ferruginous hardenednodules, containing haematite and goethite minerals.

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Figure 4. Distribution of soil patterns and seasonal groundwater fluctuation along the (a) steep footslope, (b) the moderate footslope and(c) the gentle footslope. (0) granitic rock; dotted line: soil patterns (1, saprolite; 2, grey clay; 3, yellow–grey clay; 4, variegated clay;5, mottled clay; 6, variegated massive carapace; 7, spotted massive carapace; 8, variegated vesicular carapace; 9, brown–red soft clayeyhorizon, finely spotted with isolated ferruginous dark-red mottled); full line under the topographic level: mean level of groundwater table thatvaries little, according with precipitation; filled circle: perched groundwater (P) or zone of groundwater fluctuation (F). The upper horizons

(representing the old ferricrete dismantling patterns and described in Temgoua et al. (2002)) are not shown



10 µm

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Figure 5. Scanning electron microscope picture of yellow–grey clay aggregates, showing ‘booklets’ of kaolinite (K), quartz (Q), and anunidentified porous structure (V)

From the bottom to the top of the soil profiles on the moderate footslopes (Figure 4b), one distin-guishes the following: a yellow–grey clay pattern, homogeneous with kaolinite mineral within a litho-morphic plasma (Figure 5); a variegated clay pattern with large spherical domains that can be pur-plish red, goethitic, and very slightly hardened, or yellow goethitic, kaolinitic and crumbly, or lightgrey (10YR 7/1), kaolinitic and soft; a variegated massive carapace pattern directly surmounting a var-iegated clay, and distinguished by semi-hardened domains containing goethite, haematite and kaolinite,and by an increase in the hardness of the yellow and bright brown (7Ð5YR 5/8) domains; lastly, thereoccurs a spotted massive carapace pattern, being distinguished from the previous carapaced patterns bythe reduction in size of the whitish and yellow domains and a wealth of red domains; these last tendto be organized in a ferruginous purplish brown–red framework, and are haematitic, goethitic and hard-ened.

On the gentle footslopes (Figure 4c), the bottom of the profile consists of a very sandy greenish grey (7Ð5GY6/1) material. From the bottom to the top of the soil profile, one distinguishes the following: a variegatedclay pattern composed of a dull grey (7Ð5Y 6/1) matrix with kaolinite and quartz minerals, having a networkof cracks (3–5 cm in length and less than 1 cm in width) filled either with very slightly hardened purplish-red goethitic domains, or with soft, yellow, kaolinitic and goethitic domains; the variegated clay is directlysurmounted by a variegated vesicular carapace, distinguished by (1) semi-hardened domains organized in adark red ferruginous, haematitic and goethitic, very hardened network, and (2) an increase in the hardness andappearance of vesicles with soft clay; and last a mottled clay pattern connects the first two patterns upslope.

In terms of lateral distribution (Figure 4), the mottled clay pattern is restricted to the extreme parts of thesteep footslopes, and the spotted massive carapace pattern appears around the notches and at the bordersof drainage axes within roughly 200 m width. The variegated massive carapace pattern has the same landdistribution as the spotted massive carapace. The variegated vesicular carapace and the surrounding variegatedclay appear around the southern lobe, and they are replaced upslope by mottled clay for a width around theborders of drainage axes of more than 400 m.

Chemical analyses indicate up to 350 mg g�1 of total iron (FeT) in the red domains from different patterns(Temgoua, 2002), whereas the whitish and grey domains have only between 20 and 73 mg g�1 FeT (Table I).Fed contents are high in the mottled clay of steep footslopes (40 mg g�1) and above the zone of water-tablefluctuation; at the bottom of the soil profiles, the Fed contents are very low in the whitish grey domainsand increase slightly towards the top of soil profiles (Table I). The ratios Fe0/Fed �ð100�, expressing theproportion of amorphous iron compared with total free iron, are low, i.e. between 0Ð4 and 1Ð7% (Table I) andremain low compared to those which are usually obtained in the plinthic soils in Brazil (4–7%: dos Anjoset al., 1995; 1–8%: Motta and Kampf, 1992). Calculated iron substitution by aluminum is generally moresignificant in the whitish and grey domains than in the relatively iron-rich red ones, and also increases from



Table I. Concentration of the different forms of iron (FeT: total iron; Fed: total free iron; Feo: amorphous iron) in whitishand grey soil domains, situated below the water table. Moderate and gentle footslopes: indicated ranges depend on soil depth(first value: bottom; second value: top of a profile); on the steep footslope there is only one pattern. Iron substitution by

aluminium: range represents minimum and maximum value, independent of soil depth

Location of soil pattern FeT �mg g�1� Fed �mg g�1� Feo/Fed (%) Fe substitution by Al (mol%)

Steep footslope 47 39Ð6 0Ð75 7Ð94Moderate footslope 20Ð9–73Ð1 13Ð6–27Ð7 0Ð51–0Ð39 15Ð68–20Ð65Gentle footslope 30Ð1–31Ð5 8Ð2–19Ð0 1Ð70–0Ð78 19Ð15–23Ð96

steep to moderate to gentle footslopes (Table I). In contrast to FeT, which does not differentiate the whitishand grey domains of the sequences, the Fed contents decrease from steep to moderate to gentle footslopes,indicating that the more gentle slope areas likely have seen more reduction and removal of iron.

Using the petrography of the soil patterns and their degree of hardness, the evolution of Feo/Fed ratio and thelandscape morphological history, Temgoua et al. (2002) have demonstrated the existence of a chronosequencefrom steep to moderate and to gentle footslope soil patterns.

HYDROLOGY

Fluctuations of the piezometric levels and related soil patterns

The general appearance of the water-table levels along the three sequences are schematized in Figure 4.On the steep footslopes, the groundwater table deepens very quickly uphill during the two rainy seasons(Figure 4a). The mottled clay pattern came in contact with this groundwater level only at its bottom. Nogroundwater was found there during the dry seasons. In the absence of a groundwater table, the mottled claycontained only little residual water (profile 17 of Figure 6).

On the moderate footslopes (Figure 4b), there are two kinds of groundwater table. One is situated almostpermanently at the footslope and fluctuates weakly with precipitation, and the other is a temporary perchedgroundwater table (TPWT) at mid-slope. The TPWT, formed during the long rainy season, is present up to

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different depths)



Table II. Physical parameters (mean of five analyses) of horizons above and below the perched water-table level (pit 6)

Depth (cm) Soil pattern Bulk density Porosity (%) Clay (%) Silt (%) Sand (%)

550 Spotted massive carapace 1Ð7 34Ð52 28Ð7 26Ð7 44Ð6640 Variegated massive carapace 1Ð6 41Ð12 25Ð9 24Ð6 49Ð5710 1Ð5 41Ð02 18Ð7 39Ð5 41Ð8800 Variegated clay 1Ð5 44Ð69 12Ð2 39Ð5 48Ð31010 Yellow–grey clay 1Ð4 44Ð85 7Ð4 27Ð9 64Ð71050 Grey clay 1Ð5 41Ð65 11Ð2 30Ð6 58Ð2

the end of the subsequent long dry season (at least 4 months) but has receded by 1Ð40 m. The soil patternson the moderate footslopes retain a significant amount of water even during the absence of saturation (profile6 of Figure 6). The residual moisture of the pattern above the TPWT was also high and was attributed tothe capillary fringe. The yellow–grey clay pattern hosted the TPWT, and the variegated clay pattern comesinto contact with its capillary fringe zone. Downstream, the two patterns are in contact with the permanentgroundwater table. A TPWT is due to the presence of a less permeable horizon under a more permeable one.It can flow laterally because of the slope within the more permeable horizon (Gaviria, 1993; Grimaldi et al.,2003). Braun (oral communication) found other TPWT at Nsimi, 40 km from the site of Meyomessala studiedhere (Figure 1). Table II shows that particle size decreases continuously towards the top of the profile, and theincrease in clay content marks the evolution of weathering. However, the porosity decreases above and underthe variegated clay and yellow–grey clay patterns that hosted the TPWT and its capillary fringe zone. Thesehorizons might function as a kind of funnel, where water penetrates easily and leaves with difficulty. Thus,the perched groundwater zone releases water so slowly that, at the end of the subsequent long dry season,the water table has receded by the height corresponding to the yellow–grey clay pattern (1Ð40 m). During thesubsequent short rains, the water table, shallow in depth, becomes fugacious and is pulled by the jolts causedby the first drops of rain. Braun et al. (1998) show that, at Nsimi, the rainfall at this time does not seep intothe soil; instead it flows superficially (overland) and feeds the channels, which then dry immediately after therain.

On the gentle footslopes (Figure 4c), the groundwater table is permanent and its fluctuation zone is large.The minimal groundwater level is obviously observed at the end of the long dry season. At the end of theshort rainy season, in May, the upper level of the water table does not change on the top slope but increasesby 20 cm at the footslope along the marshy borders. During the short dry season, one notes a 20 cm decreasein the water-table level of the top slope (pit 4) and a simultaneous increase of 10 cm at the footslope. Duringthis period, the top slope water-table level receded and the level of the Zembe river continues to go up.This indicates that, in the absence of water supply of the regular groundwater table, the footslope is suppliedfrom the top slope water by lateral flow. During the long rainy period, the level of the water table risesthroughout the sequence, with a total amplitude of 30 cm on top slope to more than 160 cm towards thefootslope. The upper level of the water table abruptly approaches the surface, constituting the Zembe riverin the marshy zones. The variegated clay pattern remains permanently under the water table, whereas thevariegated vesicular carapace remains permanently in the zone of groundwater fluctuation. The brown–redsoft clayey horizon comes into contact with the groundwater table only during the long rainy season, whenthe groundwater table raises considerably.

Fluctuations in springs and larger rivers

Groundwater studied in the steep and moderate footslope soil profiles is situated close to the springs,whereas that of gentle footslope profiles is geographically close to the Zembe river. During the long dryseason, the springs dry out (Table III), leaving visible iron deposits as fine yellow pasty films around the



Table III. Average flow rates of the Zembe and Ndi rivers, and spring SA as a function of seasona

Season Zembe Ndi Spring SA

LRS 93 850 ndLDS 37 nd 1SRS 42 45 5SDS 47 170 20

a LRS: long rainy season; LDS: long dry season; SRS: short rainy season; SDS: short dry season; nd: not determined.

ferricrete blocks that strew the valley. During the other seasons, flow increases gradually, supplying the largerrivers (Table III). Springs generally have several outlets, where the water emerges in small rivulets.

In the rivers, the minimum flows appear during the long dry season. The Zembe river, owing to the lowand marshy surrounding slopes, has a reduced speed compared with that of the springs; this enables the localpopulation to set up muddy-fishing traps. The flow of the river increases with the arrival of the rains duringthe short rainy season. During the subsequent short dry season, when precipitation becomes less and less, theflow rate nevertheless increases, indicating a sublateral influx of soil drainage water. During the long rainyseason, the mean water level is at the maximum in all the rivers.

Seasonal cycle of groundwater supplying the Meyomessala study site

The first rains of March do not replenish the groundwater. Figure 7 shows that the runoff does not directlyfollow the rainfall, with the delay between increased rainfall leading to increased runoff being at around1 month. This phenomenon is frequently observed in southern Cameroon, and it was interpreted as arising froma significant part of the rainfall undergoing deep drainage (Olivry, 1986; Sigha-Nkamdjou, 1993). The rainiestmonth is September, and in 1997 there were high precipitations observed during that month until October (1October had the most precipitation in 24 h), whereas the maximum level of Zembe occurs between Octoberand November, and the maximum flood period of the Dja river is in November (Olivry, 1986). A portionof the rainwater is thus used in replenishing the water table. This replenishment is accumulated successivelyaccording to the amount of rain accumulated during the hydrological year. Considering a maximum runoffof 10% of the precipitation, Temgoua (2002) calculated the drainage in the Dja watershed to be between

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J F M A M J J A S O N D

Figure 7. Relationship between precipitation (black) and the runoff (white) of the Zembe river in 1997



240 and 500 mm year�1; this accounts for approximately 25% of total precipitation. This drainage is nearthat of the Oubangui basin (in the Congo basin of which the Dja forms a part), whose value is 269 mm(Beauvais, 1999). It appears that, in spite of large quantities of evapotranspiration, a significant quantity ofwater percolates through the soils. After the rains, the water retained by the ground continues to flow and tosupply resurgences.

The Dja river has a small specific low-flux, 2–3 l s�1 km�2 (Olivry, 1986), but these values represent thehighest low-fluxes compared with those of the other southern Cameroon rivers (e.g. the Sanaga or Nyongrivers; Olivry, 1986; Sigha-Nkamdjou, 1993; Ndam, 1998). The scarcity of drying out in the Dja watershedis related either to significant aquifer inputs or to the humid conditions of the Great Forest; the water stocksin the marshy zones and the occasional rains in the dry season also contribute to constant flows.

RELATIONSHIP BETWEEN HYDROLOGY AND SOIL PATTERNS

Overview

The footslope soil patterns described, to a large extent, owe their formation to the presence of the TPWTor to the fluctuation of the permanent groundwater table. Because of significant water-table fluctuations,iron reduction can occur within these soils, at least partially (Vizier, 1983). Between 5 and 50% of thefree iron oxides present in a soil may be reduced within a few weeks of submergence, depending on thetemperature, the organic matter content, and the crystallinity of the oxides (Ponnamperuma, 1972). Thepresence of colour segregation (variegation of clays) is always regarded as a hydromorphic feature in soil(Vizier, 1983; Thompson and Bell, 1996). The low chroma (GY 6/1, YR 7/1 and Y 6/1) observed at thebottom of moderate and gentle footslopes (greenish grey features, yellow–grey clay and variegated clay soilpatterns) is an indication of reducing conditions, as shown by Reuter and Bell (2003), who also noted thatprofile darkness is correlated with the duration of soil saturation. This is explained by the fact that saturationprevents atmospheric oxygen from entering the soil. Soil colours change after a soil has become saturated, bymicrobial respiration accompanying degradation of organic tissues consuming available oxygen and causingthe soil to become anoxic (Ponnamperuma, 1972; Hayes and Vepraskas, 2000). Under anaerobic conditions,iron reduction occurs (McBride, 1994) and iron reduction eliminates the red and yellow colours producedby iron(III) compounds, resulting in low chroma (�3) or grey soil colours (Vepraskas, 1994). In the bottomlevels of the Ferralsols, certain studies infer the absence of organic matter because this would be retainedby mineral surfaces in the top of the profile (Lucas et al., 1996); this is valid for the upper parts of slopes.Other studies show that the organic matter is mineralized in footslope groundwater and favours mineraldissolution (Viers et al., 1997). The low chroma is observed as mottled clay in the moderate footslopes andas background on the bottom soil patterns of the gentle footslopes. Also, Table I shows 15 to 23 mol% ironsubstituted by aluminium; Temgoua (2002) has shown that these substitutions occur mainly in goethite. Thekaolinite–goethite bands in the iron patterns (e.g. variegated clay) could be due to seasonal fluctuation ofEh (Petersen, 1971), related to seasonal climatic changes (Figure 3). Alternating dry and wet seasons haveproduced disordered and weakly structured aluminium-goethite with high contents of iron substitution byaluminium. The removal of iron observed from the moderate to gentle footslopes could then be explained byvariable saturation time, existence or not of reducing conditions, and iron migration with decreasing footslopeangle. This iron reduction has consequences for the morphology and the development of the soil profiles.

Relationship between hydrology and the current footslope ferruginous accumulation patterns

The steep footslopes are less affected by groundwater and we consider the waterlogging effect from moderateto gentle footslopes, sequences previously described (Temgoua et al., 2002) as a chronosequence.

On the moderate footslopes, the yellow–grey clay pattern occurs in the perched groundwater zone, withrelatively homogeneous colour and massive structure. Above this water table the soil features (variegated



clay pattern) are neither saturated nor dry because of the capillary fringe. The high residual moisture content,even during the dry seasons, substantiates this. Capillary fringe water creates reducing conditions and soilaggregate dryness is suitable for iron accumulation. Observations under the optical microscope show that thesoil aggregates are impregnated by iron, in a diffuse way. The mobilization and the deposition of iron inthis sequence thus occur at any time and over very short distances. Ferruginous accumulations arise withinpockets in soil aggregates; these pockets become bigger from the bottom to the top of soil profiles. Thus,variegated soil volumes are gradually formed in more oxidizing conditions, within the capillary fringe zone.The disappearance of the perched groundwater and the lowering of the capillary fringe create more oxidizingconditions and favour iron precipitation throughout the horizon; this is the source of the continuous ferruginousframework that forms the spotted massive carapace.

When slopes become progressively more gentle, the groundwater fluctuates with a higher amplitude, ironaccumulations extend on more and more of the slope area, and massive carapaces are replaced by the vesicularcarapaces. The ferruginous network in this case is very hardened, but also isolated from the movable vesicules.The former are formed in the absence of the water table (dry periods), whereas the latter are the result of clayleaching in the absence of hardened soil features in a saturated environment. During the long dry season, thesoil patterns were so hard that digging the pit near the soil profiles 4 and 16 was very difficult, despite thefact that we are within the humid forest. The strong humidity contrast is the cause of cracks in the bottomvariegated clay pattern during periods of pronounced desiccation and of ferrous clay leaching during theperiod of saturation. However, similar but larger cracks were described by Chauvel and Pedro (1978) and byAmbrosi and Nahon (1986) for soil under climates having a long, hard dry season. The absence of lithologicalcharacteristics, the relatively low content of total free iron (Fed, Table II) correlated to the high content ofamorphous iron, and the positive iron mass balance (Temgoua et al., 2002, 2003), manifesting an overall ironinflux, show that iron is always immobilized and remobilized in this sequence. Within the soil features of thissequence, the saturated conditions during the long rainy season lead to the creation of an insufficiently drainedreducing environment; this is confirmed by the discoloration of the lower part of the variegated clay patternand the vesicules of the carapace. The organic matter present in this marshy zone (Viers et al., 1997) alsofavours iron reduction. The period of saturation is extended by the internal lateral flow during the short dryseason, and this long period of saturation contributes to the transport of soft clay through the pores. Rosolenet al. (2002) described this transport phenomenon; Lucas et al. (1986) attributed the genesis of the footslopesandy horizons in Guyana to this kind of lateral supply. Wetting and drying episodes helped in the earlyhardening of the ferricrete profile (Nahon, 1991). It is in this way that the iron that originally accumulatednear the cracks subsequently developed into strong hard shells within a ferruginous network.

For instance, the ferruginous soil patterns being formed on the footslopes in Meyomessala are graduallyextended with the decreasing slope angle of the landscape and the rise of the groundwater table. The presenceof the most hardened carapace on the gentle footslopes, which is only limited to the zone of groundwaterfluctuation, and the low thickness of these new soil patterns show that they deteriorate as soon as this waterfluctuation zone recedes. This is also consistent with the upper pattern (see pattern number 9 of Figure 4)having isolated iron domains within the soft clayey soil. This represents the weathering pattern of the variegatedvesicular carapace, and is indicative of the unstable characteristic of the current ferruginous patterns.

CONCLUSIONS

The commonly accepted model of top hill ferricrete formation focuses on vertical and relative accumulations,involving concentrations of iron derived from weathering of under- or over-lying material (Nahon, 1991; Tardy,1993). Today, this ferricrete is in disequilibrium (as a consequence of climate change), and is dismantlingand simultaneously being replaced by new ferruginous carapace patterns. We have described these newferruginous patterns in the recent southern Cameroonian footslope landscape (Temgoua, 2002; Temgoua et al.,2002) and observed others at Mbalam and Mayos near Abong-Mbang (Figure 1). Other footslope ferruginous



accumulations described frequently (Bilong et al., 1992; Braun et al., 1998; Beauvais, 1999) point to anunquestionable ubiquity of current ferricretion processes in the Central African rainforest region. How arerecent footslope ferricretes formed? The case studied here shows that iron oxidation and precipitation occurwhere groundwater recedes below a fluctuating water table; hardening results from wetting and drying episodesdue to cycling between rainy and dry seasons. This explains the forming process of footslope ferruginouscarapace soils and integrates the model proposed by Pain and Ollier (1992), and Phillips (2000) involvinglateral translocation of iron-enriched water.

This study shows that the footslope soil patterns really result from processes involving both topographic andgroundwater dynamics. Owing to the shorter period of saturation of its soil materials, the steep footslopes arelittle affected and are similar to the top hill patterns. There is no iron reduction, only leaching and formationof whitish domains in an environment where iron was previously segregated in the relic rock materials. Thesequence related to saturation develops from moderate to gentle footslopes. On the moderate footslopes, ironreducing conditions are supported by perched groundwater and its capillary fringe, favouring iron segregationinto millimetric-length pockets. With a decreasing slope angle and a relative groundwater table rise, ironreduction occurs in the superficial soil layer, supported by the marshy zone always rich in organic matter;there is also leaching of the non-hardened pockets and finally production of the vesicular carapace pattern.The strong humidity contrast is the ultimate cause of ferrous clay leaching and hardening of the ferruginousnetwork. This soil pattern is substantial in lateral extent but is not very thick; it is limited to the zone ofgroundwater fluctuation. It weathers from the top when groundwater levels recede. The next step, in orderto assess this important process completely, would be to measure chemical elements in the soil percolationwaters, spring and surface waters.

ACKNOWLEDGEMENTS

We wish to thank P. Boivin from the EPF-Lausanne and C. Grimaldi, INRA-ENSA Rennes-France, for manystimulating discussions. The assistance of D. Bitom from Yaounde 1 University, Cameroon, for field datadiscussion was greatly appreciated. Thanks to Mona Wells from EPFL for the English improvement. We wishto thank the three anonymous reviewers of this paper for their constructive remarks.

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