Poster Session 4
“Physical properties of tropical sandy soils”

Water infiltration in saline sandy soils

Hammecker, C.¹; R. Razzouk ²; J-L Maeght ¹ and O. Grünberger¹

Keyword: Salinity, sodicity, hydraulic conductivity, infiltration, sandy soils, Northeast Thailand

Introduction

In the Northeast of Thailand, the lowlands are mainly dedicated to sticky (glutinous) rice production for home consumption. However these paddy soils are seriously affected by salinisation and during the last few decades the phenomenon has increased with the rise in saline water tables. After deforestation in the recharge area, groundwater rise effectively mobilizes halite deposits of the Mahasarakham formation, which eventual reach the soil surface. However salinity does not affect soils uniformly, as it appears in discrete patches of 5 to 10 m of diameter. Water flow in soils chiefly controls the salinisation process and it is therefore crucial to understand the water dynamics in these soils, if rehabilitation or management is to be achieved.

In addition to the osmotic stress that salinisation imposes on the crop, sodicity may also modify soil structure. As soil structure chiefly governs the water flow, hydraulic conductivity is consequently affected. The influence of mixed salt solutions percolation on the soil hydraulic conductivity has been studied previously by several authors (Abu-Sharar et al., 1987; Curtin et al., 1994; Amézketa and Aragües, 1995; Abu-Sharar and Salameh, 1995) and empirical models have been established (Mc Neal, 1968; Suarez et al., 1984). These studies assert that elevated exchangeable sodium levels at low concentrations cause dispersion and swelling of the clay minerals and consequently a reduction in hydraulic conductivity of the soil. The development of this phenomenon in the presence of sodium invariably indicates that there is a threshold level for these changes to occur, especially for high Sodium Adsorption Ratio levels. However, most studies have in general been conducted in the laboratory on repacked disturbed soil columns. The results obtained with these experiments are representative of the behaviour of the clay and cannot be assumed to be indicative of the behaviour of transport properties under field conditions.

The objective of this study was to measure hydraulic conductivity of soils with different salt solution concentrations in order to evaluate water flow; whether it is natural rain water or saline water originating from the water table, and to asses the influence of organic matter management on soil hydrodynamic properties.

Material and methods

Experimental design

Infiltration properties were measured in situ with a disk infiltrometer (Figure 1) in order to evaluate the saturated hydraulic conductivity of the soil under undisturbed conditions and with solutions being representative of field conditions. These properties were measured with both deionized water, and a NaCl brine at the same concentration as the groundwater (250 meq/l) in two contiguous plots, inside and outside saline patches. The two plots have different managements: in the first one L25 the farmer follows carefully the water level during the cropping season and introduces organic matter amendments, especially on the saline patches, whereas in the second one L14, the farmer only puts very little effort in the manage­ment of the field. This is a commonly observed where yields of rice have declined significantly and farmers are not willing to invest the effort to try to remediate their fields.

Figure 1. Schematic of a disk infiltrometer showing a. the actual disk on the soil surface allowing for 3D water flow, b. the water (or brine) reservoir and, c. the Mariotte device controlling the suction imposed at the soil surface

 Saturated hydraulic conductivity was calculated with the multipotential method (Perroux and White, 1988; Smettem and Clothier, 1989). Three infiltration measurements were undertaken in each situation at two suction values and for each type of solution. These measurements were undertaken at different depths down the soil profile.

Model

Water Infiltration into soil is usually well described by the Philip equation (1957):

where S is the sorptivity, resulting from the flow due to capillary pressure head in dry soil, and A the constant infiltration rate parameter, depending on the gravity flow. As infiltration from a disc is unconfined and water flow is 3-dimensional, the hydraulic conductivity is calculated with the constant infiltration rate A according to Woodings’ equation (1968):

where r is the disc radius, A₀ is the constant infiltration rate, K₀ the hydraulic conductivity, and φ₀, the matrix flux potential for applied infiltration suction h₀, h_i is the initial pressure head of the soil. Assuming an exponential relation for conductivity with pressure head (Gardner, 1958): K (h) = K_s • exp (α • h), the matrix flux potential derives into a simple relation, φ₀ = K_s • exp (α • h₀)/α where Ks is the saturated hydraulic conductivity and α the exponential slope. Equation (1) becomes:

When performed for two pressure heads (h₀ and h₁), the exponential slope is calculated: α = In (A₁/A₀)/ (h₁ – h₀) and introduced into equation (4) to calculate the saturated hydraulic conductivity Ks.

In order to determine easily and graphically, the infiltration parameters S and A from the infiltration data, Philip’s equation can be rewritten as follows:

When representing these results graphically as I/√t versus √t diagram, the infiltration appears as a straight line defined by a slope A and an ordinate to origin S.

Location and soils

The study was established in Northeast Thailand, near Pra Yuhn, Khon Kaen region (16º27.8′N, 102º6.9′E). Saline patches in the rice fields were identified and delimited by EM38 measurements. The infiltration experiment was performed during the dry season in March 2005. The soil represents a quaternary aeolian deposits (Lesturgez, 2005), with a fine sandy loam texture, but with a low clay content. However, within the profile the clay content increases from 3% near the surface to 10% at 50-60 cm. Similarly the bulk soil density varied from 1.46 10³ kg m^-3 in the most surface layers to 1.92 10³ kg m^-3 at 50 cm. This dense layer seems to be continuous except in discrete points under the saline patches where it has not been found. Wider exploration in the area with penetrometer, confirmed this observation.

Results and interpretation

The infiltration measurements were performed during the dry season (March 2005) at different depths. A typical set of infiltration curves is presented in Figure 2, where distilled and saline water have been used to generate the data. It demonstrates clearly that in saline patches, infiltration with a saline solution is in accordance with Philip’s equation and is represented by a straight line according to equation (5). Contrasting this, for distilled water the Philip’s model does not entirely describe the infiltration kinetics, as curvilinear relation was observed (Figure 2). Based on the fact that as the distilled water enters the soil, a dilute salt solution is progressively formed that induces clay dispersion. The consequence is a reduction of hydraulic conductivity depending on the concentration of the infiltrating solution. Previous laboratory experiments on undisturbed soil monoliths (Rivallan, 2004) showed a linear positive relationship between solution concentration and saturated hydraulic conductivity, indicating that infiltration rate decreases with time when distilled water or rain water enters the soil. However, in order to calculate a saturated hydraulic conductivity, we used the tangent to the infiltration curve for an arbitrarily chosen amount of infiltrated water, equivalent for each experiment, as depicted in Figure 2.

Figure 2. Example of infiltration curves in a I/√t vs √t diagram in a saline patch. Squares infiltration with brine; diamonds infiltration with distilled water. Dotted grey line: tangent for calculating parameter S and A

As expected, when brine was used for infiltration measurement, the saturated hydraulic conductivity was systematically higher than distilled water (Figure 3). This clearly illustrates the clay dispersion phenomenon under saline soil conditions when diluted with fresh water is applied that leads to the partial blockage of the pores. The measurements performed with brine can be considered as representing the intrinsic Ks values for this soil, as the conditions are fulfilled for clays to be flocculated, and hence optimal for water flow. However, these saturated hydraulic conductivity values are very low for sandy soils as the maximum values are less than 5 cm d^-1 whereas for sandy soils Ks is usually found 500 to 700 cm d^-1 (Carsel and Parrish, 1988).

In each of the assessments irrespective of management and within or outside a saline patch, hydraulic conductivity varies with depth in the soil profile. Inside the saline patches, in both plots, the maximum saturated hydraulic conductivity was observed in the surface layers (6 cm), where the salt concentration is highest. This probably illustrates changes in soil structure when salt crystallizes in the pores. Outside saline patches, Ks in the topsoil is 3 times higher in L25 (4.2 cm d^-1) than in L14 (1.4 cm d^-1). This difference is most probably due to the soil management, as L14 does not receive routine applications of organic matter unlike L25. On the other hand, an important decrease of Ks at a depth of 17 cm represents typically the plough layer. Hydraulic conductivity of the deepest soil layer (56 cm) is not influenced in the case of plot L14 whether measurements are taken inside or outside the saline patch (Figure 3). In contrast, in the case of L25 there is a drastic decrease in Ks between inside and outside the patch at 56 cm depth. The presence of the dense layer described previously is clearly illustrated here in L25, where it fades inside the saline plot. However this difference is not marked as distinctly in L14.

Figure 3. Saturated hydraulic conductivity inside (saline) and outside (non saline) the saline patches in both plots L14 and L25, measured with saline solution (dark grey) and distilled water (light grey)

In both plots L14 and L25, the saturated hydraulic conductivity is higher inside the saline patch than outside, whether distilled water or brine is used for infiltration (Figure 3). As infiltration occurs with a limited amount of water (approximately 2 liters), the soils solution in the upper part of the profile does not get diluted sufficiently to reach the optimal dispersion concentration. However one can expect that during the cropping season the dispersion concentration is reached and that Ks declines significantly.

Basing on these first results, one can propose a functional model of water flow and solute transport in these sandy saline soils. As infiltration of fresh water tends to be limited in the saline patches, possibilities of leaching the salts downwards during the cropping season are therefore limited. On the other hand, capillary rise of saline water from the water table towards the soil surface is favoured at the end of the cropping season as Ks increases due to the presence of highly concentrated saline solutions. Consequently, if the plots are not flushed regularly during the cropping season, superficial salinity levels will increase and widespread naturally.

Conclusion

Saturated hydraulic conductivity was measured in situ with a disk infiltrometer inside and outside saline patches in two contiguous plots with different management. From this simple experiment several conclusions can be drawn about the i) intrinsic parameters of soils, ii) the techniques used to measure them and iii) the dynamics of water and salt in these soils. The intrinsic saturated hydraulic conductivity measured was very low compared to the values usually observed for sandy soils. The presence of silt is presumably contributing to these observed responses. However this feature ensures the possibility of ponding and consequently favours rice cropping. It has been shown that saturated hydraulic conductivity is dependant on the quality of the infiltrating solution, because distilled water tends to disperse the clays and block infiltration pathways. Hence under saline conditions it is important to measure hydraulic conductivity with solutions that have similar solute compositions to those found naturally. Moreover these results demonstrate that in highly managed plots the infiltration properties are higher than in the low management plot, highlighting the importance of organic amendments. Despite no actual numerical modelling being performed, it can be assumed that if no surface flushing of salts is undertaken during the cropping season, the degree and extent of salinity will increase. Nevertheless the complete water flow and salt transport has to be simulated numerically with a model, taking into account the development of saturated hydraulic conductivity with solution composition and concentration, which still has to be designed.

References

Abu-Sharar, T.M.; Bingham, F.T.; Rhoades, J.D. 1987. Reduction in hydraulic conductivity in relation to clay dispersion and disaggregation. Soil. Sci. Am. J. 51: 342-346.

Abu-Sharar, T.M.; Salameh, A.S. 1995. Reductions in hydraulic conductivity and infiltration rate in relation to aggregate stability and irrigation water turbidity. Agricultural Water Management 29: 53-62.

Amezketa, E.; Aragües, R. 1995. Hydraulic conductivity, dispersion and osmotic explosion in arid-zone soils leached with electrolyte solutions. Soil. Sci. 159: 287-295.

Carsel, R.F.; Parrish, R.S. 1988. Developing joint probability distributions of soil water retention characteristics. Water Resour. Res. 24: 755-759.

Curtin, D.; Stepphun, H.; Selles, F. 1994. Clay dispersion in relation to sodicity, electrolyte concentration and mechanical effects. Soil Sci. Am. J. 58: 955-962.

Lesturgez, G. 2005. Densification des sols sableux sous culture mécanisée. 164 pp. Thèse Univ. Henri Poincaré Nancy I.

Rivallan; E., 2004. Effets des changements de salinité sur les propriétés hydrodynamiques de sols de rizières dans deux modes de gestions paysans très contrastés, dans le Nord-Est de la Thaïlande. Mém. DESS ULP. 55 pp.

Suarez, D.L.; Rhoades, J.D.; Lavado, R; Grieve, J.D. 1984. Effect of pH on saturated hydraulic conductivity and soil dispersion. Soil. Sci. Soc. Am. J. 48: 50-55.

Smettem, K.R.J.; Clothier, B.E. 1989. Measuring unsaturated sorptivity and hydraulic conductivity using multiple disc permeameters. J. Soil Science. 40: 563-568.

Suarez, D.L.; Rhoades, J.D.; Lavado, R.; Grieve; C.M. 1984. Effect of pH on saturated hydraulic conductivity and soil dispersion. Soil. Sci. Soc. Am. J. 48: 50-55.

Perroux, K.M.; White, I. 1988. Design for disc permeameters. Soil. Sci. Soc. Am. J. 52: 1205-1215.

Philip, J.R. 1957. Theory if infiltration. 4. Sorptivity and algebraic infiltration equations. Soil Sci. 84: 257-264.

Wooding, R.A. 1968. Steady infiltration from a shallow circular pond. Water Resour. Res. 4:1259-1273.

¹ IRD Land Development Department, Office of Science for Land Development, Bangkok 10900, Thailand. E-mail address: [email protected]
² ENSAM Montpellier, France

Wind processes improve water infiltration in Sahelian sandy rangeland

Ribolzi, O.¹; M. Hermida²; J.P. Delhoume¹; H. Karambiri³ and L. Thiombiano⁴

Keywords: Wind erosion, water erosion, Burkina Faso, penetrometer, Sahel

Introduction

Dryland ecosystems develop strategies to adapt and resist adverse conditions such as drought. Naturally contracted vegetation patterns such as tiger bush is a well-known example of such strategies (Valentin et al., 1999). Dotted bush is another example that can frequently be found in the Sahelian zone of Burkina Faso (Leprun, 1999). Dotted bush is generally associated with sandy grassy microdunes. The genesis, development and evolution of these microdunes result from an array of factors including wind and water processes. These aeolian landforms are considered as “islets of fertility” (Thiombiano, 2000) where biomass production (Grouzis, 1991) and water infiltration (Ribolzi et al., 2003) are significantly higher than in other parts of the landscape. They support a primary production which is essential as a stable food supply for cattle, and are also potential initial points for the regeneration of degraded Sahelian environments.

One of the most limiting factors for natural vegetation growth within microdunes is access to water by plant root systems. It is well known that infiltration capability of Sahelian soils mainly depends on their surface features (Casenave and Valentin, 1992). The aim of this paper is to provide a better understanding of water infiltration and percolation through microdune soils. We focus on the influence of soil surface and subsurface characteristics on the spatial variability of infiltration. Water can be beneficial to the development of vegetation if it is able (i) to infiltrate through soil surfaces; (ii) to reach a sufficient depth not to be evaporated rapidly; and (iii) to be accessible to the root system. The wetting-front depth (WFD) proved a relevant indicator of the three above mentioned criteria. Our investigations were conducted at several spatial scales along an overgrazed hillslope in the Sahelian zone of Burkina Faso.

Material and methods

Study area

The study area is located in Northern Burkina Faso, some 13 km from Dori (UTM30, WGS84, 809,847 m East, 155,093 m North). The climate is of the Sahelian type, with a single rainy season which lasts from June to September. The average annual rainfall recorded in Dori is 512 mm. The dominant wind is the dry dust-laden Harmattan wind that blows from the northeast during the dry season and blows from the southwest during the wet season. Mean annual potential evapotranspiration, calculated by the Penman method, is about 2,396 mm. Most of the soils are solonetz soils (haplic Solonetz in the FAO terminology) developed from calco-alkaline granitic rocks.

Observations were conducted within three experimental plots of different sizes. The largest one (14,000 m²) was a small catchment (BV1) composed of five main soil surface types (Figure 1a) according to the classification by Casenave and Valentin (1992): (1) bare erosion surfaces (ERO) accounted for 33.6% of the total catchment area, (2) pavement surfaces (G), which were also bare, covered 0.4% of the catchment area, (3) sedimentation surfaces (SED) covered the bottom of ponds and depressions, accounted for 1.2% of the catchment area, (4) runoff type surfaces (RUN) which mainly consisted of laminated materials of various textures deposited within rills represented 4.2% of the catchment area, and (5) the drying type surfaces (DRY), covered the leeward area of sandy microdunes represented 59.9% of the catchment area. Microdune soils accounted for 69% of the total catchment area; they supported vegetation with an herbaceous cover density exceeding 50% for about 2/3 of the total area at the high of the rainy season.

We also selected two microdunes (MD1 and MD2) for detailed investigations. MD1 and MD2 had respectively a medium (376 m²) and a small (36 m²) surface area. DRY was by far the dominant surface type for these plots. The soil below DRY surface was composed of two main horizons. The upper horizon (5-7 cm thick) comprised a deposit of sand with numerous macropores formed by plant roots and soil fauna. This top layer corresponded to the sand thickness affected by cattle trampling. The total porosity of this horizon ranged between 39 and 47%, with a proportion of non-functional vesicles varying from place to place. The horizon below had a laminated structure alternating between continuous sandy and plasmic layers. This second horizon laid over a massive silty-sandy impervious horizon.

Figure 1. Maps of the largest plot (BV1 catchment) showing a) the relief and the main soil surface features, and b) the wetting-front depth (WFD) at the height of the 1999 rainy season, one day after the 16 August 1999 rainfall event (Rainfall depth = 39 mm). Surface of the drying type (DRY); Surface of the runoff type (RUN); Surface of the erosion type (ERO); Surface of the pavement type (G); Surface of the sedimentation type (SED)

Topography and soil characteristics

Topography measurements were accurately conducted on each experimental plot using an optical level. For BV1, the digital elevation model and slope gradient were estimated using a set of 1 m grid measurements covering the entire catchment (i.e. 5,890 points). For MD1, micro-relief was determined according to a 0.5 m grid (i.e. 1,484 points). For MD2, topography measurements were made every 0.1 m following an E-W transect across the microdune. We estimated surface conditions visually using the method of Casenave and Valentin (1992). Four soil pits within an adjacent microdune allowed us to estimate root density profiles within the sandy aeolian deposit.

Wetting-front depth

A soil cone penetrometer was used to estimate the wetting-front depth (WFD) within the three experimental plots. The penetrometer was made up of a steel 30-deg circular cone (ø_base = 20 mm) fastened to one end of a metallic graduated stick (ø = 15 mm, length = 1.1 m). These dimensions satisfy the standards of the American Society of Agricultural Engineers. The steel cone was inserted manually. WFD was accurate within ±1 cm of the measured value. WFD mea­surements within BV1 were conducted during the climax of the 1999 rainy season, one day after a rainfall event (16 August, 39 mm). Measurements covered the entire surface of the catchment and were made according to a 4-m grid (i.e. 1959 points). In MD1, WFD determinations were also conducted in 1999, but following a different rainfall event (12 July, 76 mm), which was the first and the most important event of the rainy season (rainfall depth = 39 mm). As for BV1, measurements covered the entire plot, but with finer grid of 0.5 m (i.e. 755 points). For MD2, data were performed 2 days after another rainfall event (24 June 2002, 20 mm), every 0.2 m along a representative E-W transect of 17.2 m long.

Results and discussion

Influence of soil surface type at the catchment scale

WFD within BV1 ranged between nil and 0.87 m (Figure 1b). The mean value was 0.23 m with a standard deviation of 0.17 m. This result reveals the extreme variability of infiltration within this small catchment. The zones of deeper infiltration coincide with surfaces of the drying and runoff types. In contrast, infiltration through bare crusted surfaces (ERO, G) was very limited or even nil. Figure 2 shows the median, first and third quartiles, maximum and minimum values of WFD within the main soil surface types (DRY, ERO, RUN, G and SED). The infiltration depth ranking was DRY>>RUN>ERO=G=SED. The difference between DRY and RUN was highly signi­ficant (threshold of significance: a = 0.050, Mann-Whitney unilateral test, P <0.0001) and significant between RUN and ERO (P = 0.0003). No significant difference was found between ERO and G (P = 0.108) and between G and SED (P = 0.091). Compared to DRY surface with lower vegetation cover, the surfaces with a vegetation cover of more than 50% had a significantly higher WFD values (P <0.0001).

Our results at the catchment scale clearly showed that sandy aeolian deposits had by far the highest WFD, and demonstrated the positive effect of the herbaceous cover density on water penetration depth. Microdune soils allowed a rapid and sufficiently deep infiltration so that the water did not evaporate rapidly (Karambiri, 2003) and was therefore accessible to plant roots. These results are not surprising for semi-arid areas. They confirm the influence of soil crusting (e.g. Hoogmoed et al., 1984) and vegetation on water infiltration in soils. It was also observed that pathways of runoff (RUN surfaces) were privileged zones where the WFD could be relatively high, particularly when concentrated runoff dissected pervious sandy aeolian deposits. This is consistent with the findings of Peugeot et al. (1997) who assumed significant stream bed infiltration in order to balance the hydrological budget of a small Sahelian catchment in Southwestern Niger.

Figure 2. Box-Whiskers plot of wetting-front depths (WFD) measured within the main soil surface features (drying-type: DRY; runoff-type: RUN; erosion-type: ERO; pavement-type: G; sedimentation type: SED) of the largest plot (BV1) at the high of the 1999 rainy season, one day after the 16 August rainfall event (Rainfall depth = 39 mm)

Influence of subsurface conditions at the microdune scale

MD1 and MD2 had similar morphological and surface features. Figure 3 shows some bio-physical characteristics and WFD along MD2. Three ERO sub-units were identified within the bare windward area. Starting at the eastern end of the microdune, the first unit was covered by a continuous erosion crust with slopes ranging between 5 to 20 degrees. The second unit, also bare, was characterised by a fragmented surface resulting from wind erosion, with a degree of fragmentation increasing with slope angle and elevation (i.e. crumbling down of the laminated structure of plasmic and sandy layers). The third unit, also exposed under the direct influence of the wind action, was located on the upper ridge of the microdune. It was a narrow fringe of grass stubble (dead roots exposed at the base) dating back to the previous year. Three major soil surface units were also observed on the leeward side. The first one was a narrow sand-accumulation unit (about 50 cm wide) colonized by Bracharia villosa, with a cover density exceeding 90% at the high of the rainy season (Figure 3). The second soil unit was a DRY surface with a more scattered herbaceous cover. The third leeward unit was similar to the first windward one. It was bare and covered with an erosion crust. The density of live roots along the DRY surface was moderate and almost homogeneous near the soil surface (0-10 cm). In contrast, higher live root densities were found within the deeper horizons (10-40 cm) of the sand-accumulation unit colonized by Bracharia villosa.

Figure 3. Soil surface types (erosion-type: ERO; drying-type: DRY), herbaceous cover and root densities, relief and wetting-depth front (WDF) across the small microdune (MD2) measured two days after the 24 June 2002 rainfall event (Rainfall depth = 20 mm)

In the most eastern unit (crusted ERO surface) of MD2, WFD was close to the soil surface (Figure 3). The two following windward units (steep fragmented ERO surfaces), WFD increased sharply and reached the silty-sand layer below the microdune. There, drainage was limited by this impervious layer. In the leeward side, WFD values remained high below the sand-accumulation unit with Bracharia villosa. Beyond this unit (i.e. drying type surfaces), WFD showed a wavy-like pattern and did not reached the impervious layer. In the last unit at the western end of the transect, the WFD was very close to the soil surface.

As for BV1 and MD2, the WFD of MD1 appeared extremely variable (Figrue 4). The minimum and the maximum values observed were 0.0 and 0.50 m respectively. The mean value was 0.22 m with a standard deviation of 0.11 m. This variability could be related to the sand deposit thickness (SDT). Figure 4 shows the WFD measurements as a function of SDT. Two situations can be distinguished. The first situation, when WFD≥SDT, WFD and SDT were very well correlated (r = 0.91) and data were fitted with a linear regression (R² = 0.82). In the second situation, when WFD < SDT, the correlation coefficient between the two parameters remained high but lower than previously (r = 0.70). Data could not be fitted satisfactorily with a linear regression (R² = 0.49) due to the dispersion of WFD values within the areas with high SDT. To explain the dispersion of WFT for the highest SDT, we compared windward and leeward WFD values with SDT ≥0.3 m. This assessment led to the conclusion that the steep windward areas located in the eastern part of the plot had higher WFD values compared to the leeward areas (threshold of significance: α = 0.050, Mann-Whitney unilateral test, P <0.0001).

Figure 4. Wetting-front depth (WFD) measurements as a function of sandy deposit thickness (SDT) within the medium-size microdune (MD1) one day after the 12 July 1999 rainfall event (Rainfall depth = 76 mm)

Our measurements clearly showed that the impervious horizon below the microdune controlled and limited vertical drainage. When WFD<SDT (the wetting front still did not reach the impervious horizon), our results showed an increasing dispersion of WFD with the increase of SDT (Figure 4). This wetting front dispersion might be the consequence of preferential flows resulting from heterogeneous surface or subsurface characteristics. At the soil surface level, we observed local variability: discontinuous vegetation cover characterised by micromounds of annual plants alternating with patches of bare, crusted surfaces (drying type) or of loose sand, trampled by livestock. Uneven wetting front might also be the consequence of heterogeneous subsurface conditions due to the presence of: (i) coarse sand lenses (Boll et al., 1996); (ii) non-functional vesicular porosity near the surface; (iii) macroporosity caused by ant and termites activity (Léonard and Rajot, 2001); (iv) vertical shrinkage cracks. Textural interfaces are also known to force water to flow laterally (Ribolzi et al., 2003).

Why was wetting-front deeper within steep windward zones?

In the West African Sahel, convective rainstorms are often preceded by strong windstorms (Visser et al., 2004). During these windstorms, the steep windward side of microdunes is exposed to wind deflation: sand grains transported by wind hit the soil surface, a process through which they weaken or even fragment water-related erosion crusts (e.g. Neuman et al., 2005), hence improving soil water infiltrability at the onset of the rainfall event which often follows the sandstorm. During rainfall, the crumbled materials generated by wind deflation (residues of plasmic microlayer and free grains of sands) are easily removed by raindrop impacts (i.e. splash erosion). The destroyed erosion crust forms progressively again during the following rainfall event. We hypothesise that this crust reconstitution process is slower on steeper micro-slopes. Such a tendency (i.e. more infiltration on steeper slopes) was already described in tropical environments on steep loamy soils (Janeau et al., 2003).

Most of the time in the Sahel, rainfall is accompanied with a strong northeastern wind, which diverts raindrops from their vertical trajectory, so that windward sides of microdunes receive more rainwater per unit area and are more subjected to splash. Splash preferentially translocates soil particles downhill, which prevents the development of crusts and maintain a good water infiltrability. The previous hypothesis is still valid in the case of raindrops with a vertical trajectory. In this case, the trend for increased infiltration with slope gradient is ascribed to weaker crusting on steeper slopes: raindrops would hit the soil at a more acute angle, and thus with less vertical kinetic energy per unit area (Poesen, 1986). Lateral shearing forces due to raindrop impact might increase with slope radient, and lead to the detachment of soil particles, hampering soil crusting.

Conclusion

Wetting-front depth measurements using a simple cone penetrometer allowed us to better understand infiltration and water storage capability in Sahelian rangelands. Our main results can be summarized according to five main points:

1. At the catchment scale, the wetting-front depth ranged from nil to 1 m. The deepest infiltration occurred within sandy aeolian deposits (drying type surface) with an herbaceous cover of more than 50%.
2. Pathways of concentrated runoff (rills) favoured water infiltration, especially when they dissected the surface of pervious sandy soils.
3. At the microdune scale, the most important penetration depths were observed in bare windward surfaces with steep slopes. This unexpected result is attributed to the combined effect of wind deflation and splash erosion which prevent the development of impervious crusts.
4. Wavy-shape water fronts observed under leeward slopes were related to soil surface characteristics; e.g. the wetting-front depth was deeper along the ridge of assymetric microdunes where sand-accumulation units colonized by Bracharia villosa developed.
5. Internal heterogeneities of sandy soil layers limited infiltration and/or forced the water to flow laterally: e.g. the wetting-front depth in the thinnest part of microdunes was controlled by the impervious layer below the sand deposit.

Acknowledgment

The present work was supported by the Département des Ressources Vivantes of the Institut de Recherche pour le Développement (IRD). We acknowledge with gratitude INERA (Institut National de l’Environnement et de Recherche Agricole, Burkina Faso) for providing access to the site.

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Janeau, J.L., Briquet, J.P., Planchon, O. and Valentin, C. 2003. Soil crusting and infiltration on steep slopes in Northern Thailand. European Journal of Soil Science, 54, 543-553.

Karambiri, H. 2003. Crue et érosion hydrique au Sahel: étude et modélisation des flux d’eau et de matières sur un petit bassin versant pastoral au nord du Burkina Faso. Doctorat Thesis, Université Paris VI, 341 p.

Léonard, J. and Rajot, J.L. 2001. Influence of termites on runoff and infiltration: quantification and analysis. Geoderma, 104, 17-40.

Leprun, J.C. 1999. The influences of ecological factors on tiger bush and dotted bush patterns along a gradient from Mali to Northern Burkina Faso. Catena, 37, 25-44.

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¹ Institut de Recherche pour le Développement (IRD), Centre de recherche de Ouagadougou, 01 BP 182 Ouagadougou 01, Burkina Faso, [email protected]
² Institut National de l’Environnement et de la Recherche Agricole (INERA), BP 80 Dori, Burkina Faso.
³ Ecole Inter-Etats d’Ingénieurs de l’Equipement Rural (EIER), 03 BP 7023 Ouagadougou 03, Burkina Faso.
⁴ Food and Agricultural Organisation (FAO), Regional Office for Africa, P.O. Box 1628.

Hardpan formation of some coarse-textured upland soils in Thailand

Anusontpornperm, S.¹; S. Nortcliff ² and I. Kheoruenromne¹

Keywords: Hardpan development, tillage, root penetration, cassava yield

¹ Department of Soil Science, Faculty of Agriculture, Kasetsart University, Bangkok, Thailand.
² Department of Soil Science, School of Human and Environmental Science, the University of Reading, UK.

The effect of salinity on sandy soils physical characteristics and rice root system development: a case study from Northeast Thailand

Hartmann, C.¹; H. Hao²; J.L. Maeght³; A.D. Noble⁴; A. Yuvaniyama³ and A. Polthanee⁵

Keywords: Sandy soils, saline patches, root development, soil porosity

¹ IRD – UR 176 ‘SOLUTIONS’ – UMR 7618 ‘BIOEMCO’ 46 rue d’Ulm – 75230 Paris Cedex 05 – France.
² Institute of Soil , Chinese Academy of Science, Nanjing, PR China.
³ Land Department Development, Office of Science for Land Development/IRD, Phaholyothin Road, Chatuchak, Bangkok 10900, Thailand.
⁴ International Water Management Institute (IWMI), c/o WorldFish Center Jalan Batu Maung, Batu Maung 11960 Bayan Lepas, Penang, Malaysia.
⁶ Associated Professor, Department of Agronomy, Faculty of Agriculture, Khon Kaen University, Thailand.