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Vertical variation of near-saturated hydraulic conductivity in three soil profiles
Geoderma 126 (2005) 181 – 191 www.elsevier.com/locate/geoderma
Vertical variation of near-saturated hydraulic conductivity in three soil profiles Y. Coquet*, P. Vachier, C. Labat UMR INRA/INAPG Environment and Arable Crops, Institut National de la Recherche Agronomique/Institut National Agronomique Paris-Grignon, B.P. 01, 78850 THIVERVAL-GRIGNON, France Received 13 March 2003; received in revised form 31 August 2004; accepted 22 September 2004
Abstract Hydraulic conductivity is an important parameter of water and solute transport in soils. Hydraulic conductivity is known to be highly variable in space, but its dependence on soil horizonation has seldom been explored. We measured the nearsaturated hydraulic conductivity of three soils located in the Centre of France, a Ne´oluvisol, a Calcisol, and a Calcosol (according to the bRe´fe´rentiel Pe´dologiqueQ), using tension disc infiltrometry. The Reynolds and Elrick’s multipotential technique has been applied to the steady state infiltration kinetics measured at potentials 1.5, 0.6, 0.3, 0.1 and 0.05 kPa to derive the hydraulic conductivity values. Each horizon of the three soil profiles has been characterised at least once at three locations 50-cm apart from each other on a flat surface prepared within the same access pit. Some horizons have been characterised on a second occasion in other access pits opened at nearly 10 m from the previous ones. On average, hydraulic conductivity values increased from 1.3107 m/s at 1.5 kPa to 2.5106 m/s at 0.3 kPa. Saturated hydraulic conductivity varied between 3.6106 m/s for the ploughed layer of the Calcosol and 4.9103 m/s for the Calcosol substrate (C horizon). The various horizons of the Ne´oluvisol had hydraulic conductivity values which were not significantly different from each other, except the ploughed layer which had lower hydraulic conductivity values than the underlying horizons. The tilled layers of the Calcisol and the Calcosol also had lower hydraulic conductivity values than their respective underlying cambic horizons. Further, the hydraulic conductivity of the tilled layer of the Calcosol was lower than that of the tilled layers of the Calcisol and the Ne´oluvisol. This difference was attributed to the effect of different tillage systems, the Calcosol being superficially tilled while the Calcisol and Ne´oluvisol were ploughed each year. The structural S cambic horizons of the Calcisol and Calcosol had similar hydraulic conductivities but lower than their respective substrate (limestone). D 2004 Elsevier B.V. All rights reserved. Keywords: Soil horizon; Spatial variability; Tillage; Limestone; Loamy soil
1. Introduction * Corresponding author. Tel.: +33 1 30 81 52 04; fax: +33 1 30 81 53 96. E-mail address: [email protected] (Y. Coquet). 0016-7061/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2004.09.014
The hydraulic conductivity K of soils is known to be highly variable in space. A large number of field
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studies have been conducted to better understand the nature and the characteristics of this variability (e.g. Nielsen et al., 1973; Russo and Bresler, 1981; Rao et al., 1983; Vauclin et al., 1983; Wilson et al., 1989; Mohanty et al., 1991; Russo and Bouton, 1992; Mallants et al., 1997; Russo et al., 1997). Most of them have shown its autocorrelated nature at various scales (plot, field, catchment), thereby extending the field of application of the geostatistics to soil water physics. Analysis of the spatial variations of soil hydraulic parameters have quite often disregarded the role that soil types and soil horizonation may play. Only occasionally does the experimental setup account for soil organisation (Mallants et al., 1996; Vanderborght et al., 2001). A reason for this may be the scale at which the studies are conducted, which is mostly the field plot scale and where there is a smaller chance to encounter different soils than at a smaller scale (e.g. catchment scale). Another reason may be the tendency of experimental setups to give more importance to the horizontal directions than to the vertical one. Yet the vertical differentiation of soils may affect their hydraulic properties. Significant improvements in the prediction of soil hydraulic properties have been found when stratifying pedotransfer functions according to soil groups (Pachepsky and Rawls, 1999), horizon types (Reeve et al., 1973; McKenzie et al., 1991), and morphological descriptors (McKeague et al., 1982; Griffiths et al., 1999; Lilly, 2000) or pedogenetic origin (Bruand, 1990) of the soil horizons.
Farming practices, such as tillage (Kim and Chung, 1994; Prieksat et al., 1994), wheeling due to traffic (Logsdon and Jaynes, 1996; Heddadj and Gascuel-Odoux, 1999), have been shown to have a strong influence on K. Generally, K increases on tillage events and then decreases during the growing season due to the settling of the soil structure created by tillage (Angulo-Jaramillo et al., 1997, Azevedo et al., 1998; Messing and Jarvis, 1993). Thus, tillage is expected to have a determining influence on the eventual K differentiation between the tilled layer and the underlying untilled soil. In this study, we analyse the variations in nearsaturated K along the vertical of three soil profiles according to their pedological stratification.
2. Materials and methods 2.1. The soils Three cultivated soil profiles representative of the Beauce agricultural region of France were considered in this study (Table 1): – A Eutric Ne´oluvisol according to the French bRe´fe´rentiel Pe´dologiqueQ, RP, (Baize, 1998) or Orthic Luvisol according to the FAO Classification (FAO-UNESCO, 1989). Four horizons composed the soil profile. A 30-cm thick tilled
Table 1 Basic properties of the three soils Soil horizons
Clay 0–2 Am (g kg1)
Silt 2–50 Am (g kg1)
Ne´oluvisol LA (0–30 cm) E (30–55 cm) BT (55–100 cm)
181 184 303
783 784 676
36 32 21
Calcisol LA (0–30 cm) Sci (30–70 cm)
210 367
746 602
Calcosol LA (0–30 cm) Sca (30–60 cm)
279 313
663 468
Sand 50–200 Am (g kg1)
CaCO3 (g kg1)
pH
OM (g kg1)
CEC (cmol (+) kg1)
1 1 3
7.4 7.3 7.7
17.7 6.1 7.1
15.0 13.4 19.9
44 31
2 1
7.2 7.3
18.7 10.0
15.7 25.6
58 219
17 472
8.0 8.4
18.6 9.8
20.9 16.4
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organo-mineral horizon LA overlaid a moderately clay-impoverished non-albic eluvial E horizon between 30 and 55 cm depths, followed by a clay-enriched illuvial BT horizon between 55 and 145 cm depths. The textural differentiation index (TDI, ratio of the clay content of the BT horizon to the clay content of the E horizon) of 1.65 indicated a moderate clay translocation process within the profile. The fourth and last horizon, noted Rca according to RP, was a hard coarse-slabby limestone starting at 1.45 m depth. – A Calcisol (RP Classification) or Eutric Cambisol (FAO Classification). Below a 0–30 cm LA horizon, a weathered carbonate-free Sci horizon overlaid at 0.70 m depth a weathered C horizon corresponding to a heterogeneous highly fractured marly limestone. – A Calcosol (RP Classification) or Calcic Cambisol (FAO Classification), composed of a 0–30 cm LA horizon, followed between 30 and 45–70 cm depths by a short carbonated Sca horizon including calcareous coarse fragments, over a clayeyloamy pulverulent limestone corresponding to a C horizon. Details about the reference horizons corresponding to the letters used here may be found in the bRe´fe´rentiel Pe´dologiqueQ (Baize, 1998). These three soils were located near Ouarville within a 2500-ha area at 20 km South East of Chartres. The coordinates (WGS84) of the pits were E 1844V31.8262U; N 48821V34.6398U for the Ne´oluvisol, E 1845V24.5927U; N 48821V35.4722U for the Calcisol, and E 1846V24.9673; N 48822V4.3568U for the Calcosol. Some physical and physicochemical properties of the three soils are presented in Table 1. Each horizon, except the substrates, was characterised by its particle size distribution (g/kg dry soil) after removal of the organic matter by H2O2 and decarbonatation by HCl, according to the following fractions: clay (b2 Am), silt (2–50 Am) and sand (0.05–2 mm), its CaCO3 content (g/kg dry soil) by the volumetric method, its pH in water, its organic matter content (g/kg dry soil) by dry combustion, its cation exchange capacity (cmol (+)/kg dry soil) by ammonium exchange.
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2.2. Hydraulic conductivity measurements Tension disc infiltrometry (Perroux and White, 1988) have been used to measure the hydraulic conductivity of the soil horizons between 1.5 and 0 kPa matric potential h. A 8-cm diameter base was used and infiltrations were done at 1.5, 1.0, 0.6, 0.3, 0.1, and 0.05 kPa matric potentials successively without displacing the infiltrometer. An ascending order was used in order to discard any hysteresis effect due to drainage occurring below the disk when lowering the infiltration potential while wetting is progressing at the infiltration front (Reynolds and Elrick, 1991). Contact between the disk and the soil surface was ensured by a 1-mm thick sand layer. The fine well-sorted Fontainebleau sand was used because it corresponded well to the optimal characteristics of contact material for disk infiltrometry (Reynolds and Zebchuk, 1996). Its median grain size was 0.15 mm (Coquet et al., 2000) and its air entry potential could be estimated to less than 2 kPa and its saturated hydraulic conductivity to 0.15 m/s. Flow impedance due to the thin sand layer was therefore less than 0.007 s, which can be considered negligible considering the moderate flow rates in loamy soils (Reynolds and Zebchuk, 1996). An approximate calculation showed that any impedance effect was negligible up to flow rates of 0.5 m/s (Klute and Dirksen, 1986). Soil surfaces were prepared for infiltration by gently scraping the soil with the extremity of a large-blade knife, avoiding smearing, to create a surface as flat and levelled as possible. Soil bits were removed from the surface by blowing air with hand-held bellows. The sand was then poured slowly using a small funnel over the infiltration surface materialised by a 8-cm diameter shallow plastic ring just put on the soil surface. The sand was levelled using a little hand-held rammer until a flat surface was obtained. The thickness of the sand layer was around 1 mm but could vary between a few tenths of mm to a maximum of 2 mm. Reynolds and Zebchuk (1996) showed for loam and clay soils that the pressure head at the soil surface was simply offset from the pressure head applied at the tension infiltrometer membrane by the thickness of the contact sand layer. As the thickness of the sand layer was kept to a minimum, we considered that the matric potential
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offset due to the sand was of the same order of magnitude as the uncertainty due to the Mariotte vessel readings (F0.01 kPa) and did not modify the matric potential values applied by the tension infiltrometer. The multipotential technique of Reynolds and Elrick (1991) was applied to derive the K(h) relationship from the steady-state regimes of the successive infiltrations. For each matric potential interval, except the [0.1, 0.05 kPa] interval, we calculated K at the centre of the interval as done by Reynolds and Elrick (1991) but also at the bounds of each interval as done by Ankeny et al. (1991). For the 1.0, 0.6, 0.3 and 0.1 kPa potentials, the K values were calculated as the arithmetic average of the two estimates obtained from the left and right intervals. Values at 1.5 and 0.05 kPa were calculated directly from only the right [1.5, 1.0 kPa] or left [0.1, 0.05 kPa] interval, respectively. Saturated K was extrapolated from the [0.1, 0.05 kPa] interval. As such, it should not be considered as the actual saturated K such as measured under a ponded condition for instance because any macropore of equivalent radius N3 mm, if present, did not contribute to infiltration. Such large macropores were however not seen in the soil surfaces used for infiltration. Following Bouma (1982) and Booltink and Bouma (2002), we will note our underestimated saturated hydraulic conductivities K (sat) to distinguish them from actual saturated hydraulic conductivities K sat. Values of the parameter k m were calculated by r a km ¼ qw g where r is the surface tension of water, q w is the density of water, g is the acceleration due to gravity, and a is the slope of the lnK vs. h relationship (Gardner, 1958). The parameter k m is generally interpreted as a mean pore size characterising the soil porosity that is hydraulically active (Philip, 1969). The initial volumetric water content was measured on three small soil cores (3 cm diameter, 2.1 cm length) taken at 20 cm from the infiltrometer before infiltration. All soil horizons have been characterised at least once. A first set of measurements, set 1, included all the horizons except the LA horizons of the Calcisol and Calcosol and was taken in May 2000 from three
access pits. A second set of measurements, set 2, included the same horizons and was taken in October 2000 from three other access pits dug at approximately 10 m from the previous ones. Finally, a third set of measurements, set 3, included all the LA horizons of the three soils and was taken in March 2004 at less than 10 m from the previous measurement sites. In each data set, three replicates per horizon were done at 50 cm from each other on a flat surface prepared in the same access pit. The measurement depths, amongst all replicates and data sets, were 12–15, 42–47, 77–78, and 144–147 cm for the LA, E, BT and Rca horizons of the Ne´oluvisol, 12–15, 40–45 and 75 cm for the LA, Sci and C horizons of the Calcisol, and 10–15, 30–32 and 45–50 cm for the LA, Sca and C horizons of the Calcosol. The ranges of measurement depths have been chosen as to fall in representative zones of the soil horizons, possibly far from the transitions between horizons (although it was not possible for the Rca and C horizons due to their hardness). 2.3. Statistical analysis We used a multivariate analysis of variance (Johnson, 1998, chap. 11) to test whether the various soil horizons had different K(h) relationships. Globally, K values at each potential had empirical statistical distributions that were not significantly different from lognormal, according to the Kolmogorov-Smirnov test at the 5% level. Therefore, all K values were log10-transformed before analysis. The significance of any horizon effect was tested using the Wilks’ lambda at the 5% level. Student–Newman–Keuls tests were used to distinguish the soil horizons that had significantly (5% level) different K values.
3. Results and discussion 3.1. K(h) measurements All the K data are presented in Figs. 1–3. All K(h) characteristics show the same general pattern. log10-K increased at an increasing rate as h increased. The increase in K between h=1.5 and 0.2 kPa (equivalent diameter e.d. 1.5 mm) was
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Fig. 1. K(h) characteristics for the (a) LA, (b) E, (c) BT, (d) Rca horizons of the Ne´oluvisol. Full symbols indicate the three replicate measurements of set 1, open symbols indicate those of set 2, and the grey symbols are for set 3. The number next to each symbol in the legend indicates the initial volumetric water content of the soil before the infiltration.
about 2 orders of magnitude and was also 2 orders of magnitude between h=0.2 and 0 kPa, with the exception of the topsoil of the Calcisol (Fig. 2a) which increased by less than 1 order of magnitude between 0.2 and 0 kPa. Geometric mean K values were around 1.3107 m/s at 1.5 kPa with a minimum value of 1.8108 m/s and a maximum value of 5.6107 m/s, and reached 2.5106 m/s at 0.3 kPa with a minimum value of 3.3107 m/s and a maximum value of 1.7105 m/s. Values of K (sat) varied between 3.6106 m/s (LA horizon, Calcosol, Fig. 3a) and 4.9103 m/s (C horizon, Calcosol, Fig. 3c). The large increase between 0.2 and 0 kPa is however variable amongst replicates. This variability is consistent with the presence or absence of macropores (e.d.N1.5 mm) below the 50cm2 of the disk infiltrometer according to its location at the local scale (0.5–10 m). In many soil variability surveys, K sat was found to have coefficients of variation well over 100% (Warrick and Nielsen, 1980; Mulla and McBratney, 2002). Here, the coefficient of variation of K raised from 87% at h=0.2 kPa up to 226% for K (sat). Higher variability may be expected for actual K sat.
The LA horizons of the three soils differed from the underlying horizons by at least two aspects (Figs. 1–3): – A lower conductivity from 1.5 to 0.1 kPa. – A higher within-horizon variability of K at large water potential (hN0.1 kPa), except for the Calcisol’s LA. Fig. 4 compares the evolution of k m vs. h observed for the LA horizon to that observed for the underlying horizon for each of the three soils. The k m of the LA horizon is systematically larger than that of the underlying horizon in the 0.6 to 0.1 kPa range. In all three soils, tillage increased the hydraulically active porosity in the 0.5–3 mm e.d. range. The same effect of tillage on k m values have already been observed by Coutadeur et al. (2002). Further, the increase of k m took place within a very similar water potential range (0.5 to 0.2 kPa). Changes in k m values may be the results of modifications in the pore size distribution and/or pore connectivity. The former will alter both water retention and hydraulic conductivity while the latter
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Fig. 2. K(h) characteristics for the (a) LA, (b) Sci and (c) C horizons of the Calcisol. Full symbols indicate the three replicate measurements of set 1, open symbols indicate those of set 2, and the grey symbols are for set 3. The number next to each symbol in the legend indicates the initial volumetric water content of the soil before the infiltration.
Fig. 3. K(h) characteristics for the (a) LA, (b) Sca and (c) C horizons of the Calcosol. Full symbols indicate the three replicate measurements of set 1, open symbols indicate those of set 2, and the grey symbols are for set 3. The number next to each symbol in the legend indicates the initial volumetric water content of the soil before the infiltration.
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Fig. 4. Variations of the k m parameter with h for the LA and E horizons of the Ne´oluvisol (a), the LA and Sci horizons of the Calcisol (b), and the LA and Sca horizons of the Calcosol (c). Vertical bars represent Fone standard error of the mean values.
will essentially modify the hydraulic conductivity. The lower conductivity at low potential may be due to the disruption of pore continuity as a result of tillage (Logsdon et al., 1990; Reynolds et al., 1995; Coutadeur et al., 2002). On the other hand, the higher conductivity at high potential may be due to the porosity created by tillage and resulting in large inter-clod or inter-aggregate voids that are interconnected to the difference of the smaller-sized pores (e.d.b3 mm). One of the Ne´oluvisol LA replicates of set 2 had a low conductivity at all potentials
(Fig. 1a), especially when compared to the E horizon (Fig. 1b), and may be due to compacted soil zones within the tilled layer (Coutadeur et al., 2002). High K values have been observed in the substrates of the three soils (Fig. 1d, 2c and 3c) especially at potentials N0.1 kPa. Although corresponding to different facies of the Beauce limestone, these substrates have a good water conduction ability due to intense fracturation and weathering processes since the Miocene.
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The calcareous Sca horizon of the Calcosol had a highly variable K(h) characteristic (Fig. 3b). Such a variability can be related to a high proportion of limestone stones and blocks within the horizon, to the difference of the Sci horizon of the Calcisol for instance where these coarse elements were lacking. The influence of coarse elements on tension disk infiltrometer measurements probably depends on the relative size of the infiltration bulb compared to the size of the coarse elements. Yet, this influence has not been fully explored. Finally, no obvious differences between the data sets could be seen in Figs. 1–3 except for the Rca of the Ne´oluvisol and the C horizon of the Calcisol, where the values of set 2 were higher than those belonging to set 1 (Fig. 1d and 2c). In the BT horizon of the Ne´oluvisol (Fig. 1c) also, some values of set 2 are larger than those of set 1 at potentials higher than 0.2 kPa. 3.2. Analysis by soil horizons The Wilks’ Lambda MANOVA test on the overall effect of soil horizonation was strongly significant with a probability level of 0.0001. Single ANOVA Fisher tests done on the K values at each potential (Table 2) showed that the differences between soil horizons were significant at all potentials. Note, however, that the h=0 kPa potential corresponds to extrapolated K (sat) values and that the result of the Fisher test might have been different with actual K sat measurements. The analysis was pursued with Student–Newman–Keuls tests to group the various horizons whose K values did not differ at any potential (Fig. 5). The results show the peculiarity of the LA horizons for all three soils, especially that of the Calcosol which had the lowest K values (Fig. 6). The LA horizons of the Calcisol and the Ne´oluvisol were not significantly different from each other but differed from the rest of the soil profiles. It is
Fig. 5. Groups of soil horizons according to their K(h) characteristic. Ne´o: Ne´oluvisol, Cis: Calcisol, and Cos: Calcosol. Horizons belonging to each group do not have significantly different K values between 1.5 and 0 kPa (Student–Newman–Keuls tests at the 5% level).
worth noting that the Calcosol was cropped using a different tillage system than the other two soils. The LA horizon of the Calcosol has not been ploughed since 1999 and has only been superficially (0–3 cm) tilled, while the Calcisol and the Ne´oluvisol were generally ploughed each year. Such a difference in tillage system probably explains the difference between the two LA groups (Fig. 5), with a poorer hydraulic conductivity for the non-ploughed soil (Fig. 6). The eluvial E horizon and illuvial BT horizon of the Ne´oluvisol were not statistically different from each other or from the substrate (Rca horizon). Unlikely, the K(h) characteristics of the Sca and C horizons of the Calcosol were different. Another interesting result is the fact that the E, Sci and Sca horizons located immediately below the LA
Table 2 ANOVA results testing a bhorizon effectQ on the log10-K values (in m/s) measured at various potentials h
1.5 kPa
1.0 kPa
0.6 kPa
0.3 kPa
0.1 kPa
0 kPa
Mean Variance R2 Horizon effect ( F probability)
6.90 0.15 0.72 0.0001
6.60 0.15 0.71 0.0001
6.15 0.13 0.72 0.0001
5.60 0.13 0.68 0.0001
4.82 0.16 0.59 0.0001
3.96 0.42 0.44 0.006
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Fig. 6. K(h) characteristics for the five main groups of soil horizons identified in Fig. 5. Ne´o: Ne´oluvisol, Cis: Calcisol, and Cos: Calcosol. Vertical bars represent Fone standard error of the mean values.
horizons formed a single group from the point of view of their near-saturated K (Fig. 5). Their properties were quite similar (Table 1) with a dominating silt fraction. Indeed, all three soils have evolved from the same loess cover deposited during the Quaternary era over the intensely weathered and heterogeneous Miocene Beauce limestones (Duval and Isambert, 1992). Apparently, the differing degree of weathering and clay translocation between the three soil types was not sufficient to induce any strong differentiation in K. However, it should be noted that the hydraulic conductivity of the BT horizon of the Ne´oluvisol was significantly lower than that of the Sci horizon of the Calcisol. The hydraulic conductivity of the BT horizon of the Ne´oluvisol was also, but not significantly, lower than that of the E horizon of the Ne´oluvisol and the Sca horizon of the Calcosol, especially in the h=1.5 to 1.0 kPa range. This may be an indication of the premises of K reduction due to clay illuviation in the BT horizon of the Ne´oluvisol. The C horizons of the Calcisol and the Calcosol had the highest hydraulic conductivities (Fig. 6). This fact can be related to the structure of those horizons. The Calcisol had a heterogeneous and highly fractured substrate, and the Calcosol’s substrate was a fine pulverulent weathered limestone (Ould Mohamed et al., 1997). The less-weathered, less-fractured hard limestone of the Ne´oluvisol had a lower hydraulic conductivity.
4. Conclusion We studied the near-saturated K(h) variations of a loamy soil cover in the Centre of France according to its organisation in horizons. Our results showed a strong impact of the soil horizonation on nearsaturated K(h). Two main K contrasts could be seen. The first one was due to tillage. Tillage had a strong influence with a decrease in K up to a potential of 0.1 kPa and an increase in K close to saturation due to pore size redistribution and/or continuity modifications, probably through the creation of large inter-clods or inter-aggregates interconnected voids. Different tillage practices (ploughed vs. superficially tilled) were also responsible for a K differentiation between the ploughed layers of the Ne´oluvisol and the Calcosol and the tilled layer of the Calcisol. The second contrast was between the substrate (C or R) horizons and the overlaying (E, BT, S) horizons in the three soil profiles. The two types of horizons derived from different parent materials. The C and R horizons were derived from the Miocene Beauce limestones, while the eluvial E, illuvial BT and cambic S horizons and also the LA horizons were derived from a loess material that covered the limestones during the Quaternary times. The BT horizon had a lower, although non significantly, hydraulic conductivity than the E horizon showing that the clay-translocation pedogenetical
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process in the Ne´oluvisol was too moderate to create a strong K differentiation between the two horizons. Acknowledgements We wish to thank Pauline Panel for her help with the infiltrometer measurements while she was a student at the INAPG. We also thank MM. Chatin, Dubief and Pelard for having given us full access to their agricultural fields. The cooperation with S. Salle´ and B. Ailliot of the Chambre d’Agriculture d’Eureet-Loir was greatly appreciated. This work was financially supported by formerly Aventis CropScience France, now Bayer CropScience France. References Angulo-Jaramillo, R., Moreno, F., Clothier, B.E., Thony, J.L., Vachaud, G., Fernandez-Boy, E., Cayuela, J.A., 1997. Seasonal variation of hydraulic properties of soils measured using a tension disk infiltrometer. Soil Sci. Soc. Am. J. 61, 27 – 32. Ankeny, M.D., Ahmed, M., Kaspar, T.C., Horton, R., 1991. Simple field method for determining unsaturated hydraulic conductivity. Soil Sci. Soc. Am. J. 55, 467 – 470. Azevedo, A.S., Kanwar, R.S., Horton, R., 1998. Effect of cultivation on hydraulic properties of an Iowa soil using tension infiltrometers. Soil Sci. 163 (1), 22 – 28. Baize, D. (Ed.), A Sound Reference Base for Soils. The bRe´fe´rentiel Pe´dologiqueQ. Translation by J.M. Hodgson, N.R. Eskenazi, D. Baize. INRA, Paris. 322 p. Booltink, H.W.G., Bouma, J., 2002. Steady flow soil column method. In: Dane, J.H., Topp, G.C. (Eds.), Methods of Soil Analysis, Part 4-Physical Methods, SSSA Book Series: 5, American Society of Agronomy, Soil Science Society of America, Madison, WI, pp. 812–815. Bouma, J., 1982. Measuring the hydraulic conductivity of soil horizons with continuous macropores. Soil Sci. Soc. Am. J. 46, 438 – 441. Bruand, A., 1990. Improved prediction of water-retention properties of clayey soils by pedological stratification. J. Soil Sci. 41, 491 – 497. Coquet, Y., Boucher, A., Labat, C., Vachier, P., Roger-Estrade, J., 2000. Caracte´risation hydrodynamique des sols a` l’aide de l’infiltrome`tre a` disques. Etude Gest. Sols 7, 7 – 24. Coutadeur, C., Coquet, Y., Roger-Estrade, J., 2002. Variation of hydraulic conductivity in a tilled soil. Eur. J. Soil Sci. 53, 619 – 628. Duval, O., Isambert, M., 1992. Etude pe´dologique du secteur de Villamblain (Beauce). Rapport interne. SESCPF, INRA, Orle´ans, France. FAO-UNESCO, 1989. Soil Map of the World. Revised Legend. FAO-UNESCO, Rome. 125 p.
Gardner, W.R., 1958. Some steady-state solutions of the unsaturated moisture flow equation with application to evaporation from a water table. Soil Sci. 85, 228 – 232. Griffiths, E., Webb, T.H., Watt, J.P.C., Singleton, P.L., 1999. Development of soil morphological descriptors to improve field estimation of hydraulic conductivity. Aust. J. Soil Res. 37, 971 – 982. Heddadj, D., Gascuel-Odoux, C., 1999. Topographic and seasonal variations of unsaturated hydraulic conductivity as measured by tension disc infiltrometers at the field scale. Eur. J. Soil Sci. 50, 275 – 283. Johnson, D.E., 1998. Applied multivariate methods for data analysis. Duxbury Press, Brooks/Cole Publishing, Pacific Grove, CA, 567 p. Kim, S.J., Chung, H.W., 1994. Field evaluation of layered GreenAmpt infiltration model considering temporal variation of physical properties. Trans. ASAE 37 (6), 1845 – 1852. Klute, A., Dirksen, C., 1986. Hydraulic conductivity and diffusivity: laboratory methods. In: Klute A. (Ed.), Methods of Soil Analysis, Part 1-Physical and Mineralogical Methods, SSSA Book Series: 5, American Society of Agronomy, Soil Science Society of America, Madison, WI, pp. 687–734. Lilly, A., 2000. The relationship between field-saturated hydraulic conductivity and soil structure: development of class pedotranfer functions. Soil Use Manag. 16, 56 – 60. Logsdon, S.D., Jaynes, D.B., 1996. Spatial variability of hydraulic conductivity in a cultivated field at different times. Soil Sci. Soc. Am. J. 60, 703 – 709. Logsdon, S.D., Allmaras, R.R., Wu, L., Swan, J.B., Randall, G.W., 1990. Macroporosity and its relation to saturated hydraulic conductivity under different tillage practices. Soil Sci. Soc. Am. J. 54, 1096 – 1101. Mallants, D., Mohanty, B.P., Jacques, D., Feyen, J., 1996. Spatial variability of hydraulic properties in a multi-layered soil profile. Soil Sci. 161 (3), 167 – 181. Mallants, D., Mohanty, B.P., Vervoort, A., Feyen, J., 1997. Spatial analysis of saturated hydraulic conductivity in a soil with macropores. Soil Technol. 10, 115 – 131. McKeague, J.A., Wang, C., Topp, G.C., 1982. Estimating saturated hydraulic conductivity from soil morphology. Soil Sci. Soc. Am. J. 46, 1239 – 1244. McKenzie, N.J., Smettem, K.R.J., Ringrose-Voase, A.J., 1991. Evaluation of methods for inferring air and water properties of soils from field morphology. Aust. J. Soil Res. 289, 587 – 602. Messing, I., Jarvis, N.J., 1993. Temporal variation in the hydraulic conductivity of a tilled clay soil as measured by tension infiltrometers. J. Soil Sci. 44, 11 – 24. Mohanty, B.P., Kanwar, R.S., Horton, R., 1991. A robust-resistant approach to interpret spatial behavior of saturated hydraulic conductivity of a glacial till soil under no-tillage system. Water Resour. Res. 27 (11), 2979 – 2992. Mulla, D.J., McBratney, A.B., 2002. Soil spatial variability. In: Warrick, A.W. (Ed.), Soil Physics Companion. CRC Press LLC, Boca Raton, FL, pp. 343 – 373. Nielsen, D.R., Biggar, J.W., Erh, K.T., 1973. Spatial variability of field-measured soil–water properties. Hilgardia 42, 215 – 259.
Y. Coquet et al. / Geoderma 126 (2005) 181–191 Ould Mohamed, S., Bruand, A., Bruckler, L., Bertuzzi, P., Guillet, B., Raison, L., 1997. Estimating long-term drainage at a regional scale using a deterministic model. Soil Sci. Soc. Am. J. 61, 1473 – 1482. Pachepsky, Ya.A., Rawls, W.J., 1999. Accuracy and reliability of pedotransfer functions as affected by grouping soils. Soil Sci. Soc. Am. J. 63, 1748 – 1757. Perroux, K.M., White, I., 1988. Designs for disc permeameters. Soil Sci. Soc. Am. J. 52, 1205 – 1215. Philip, J.R., 1969. Theory of infiltration. Adv. Hydrosci. 5, 216 – 296. Prieksat, M.A., Kaspar, T.C., Ankeny, M.D., 1994. Positional and temporal changes in ponded infiltration in a corn field. Soil Sci. Soc. Am. J. 58, 181 – 184. Rao, P.S.C., Jessup, R.E., Hornsby, A.C., Cassel, D.K., Pollans, W.A., 1983. Scaling soil microhydrologic properties of Lakeland and Konawa soils using similar media concepts. Agric. Water Manag. 6, 277 – 290. Reeve, M.J., Smith, P.D., Thomasson, A.J., 1973. The effect of density on water retention properties of field soils. J. Soil Sci. 24 (3), 355 – 367. Reynolds, W.D., Elrick, D.E., 1991. Determination of hydraulic conductivity using a tension infiltrometer. Soil Sci. Soc. Am. J. 55, 633 – 639. Reynolds, W.D., Zebchuk, W.D., 1996. Use of contact material in tension infiltrometer measurements. Soil Technol. 9, 141 – 159. Reynolds, W.D., Gregorich, E.G., Curnoe, W.E., 1995. Characterisation of water transmission properties in tilled and untilled soils using tension infiltrometers. Soil Tillage Res. 33, 117 – 131.
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Russo, D., Bresler, E., 1981. Soil hydraulic properties as stochastic processes: I. An analysis of field spatial variability. Soil Sci. Soc. Am. J. 45, 682 – 687. Russo, D., Bouton, M., 1992. Statistical analysis of spatial variability in unsaturated flow parameters. Water Resour. Res. 28 (7), 1911 – 1925. Russo, D., Russo, I., Laufer, A., 1997. On the spatial variability of parameters of the unsaturated hydraulic conductivity. Water Resour. Res. 33 (5), 947 – 956. Vanderborght, J., Vanclooster, M., Timmerman, A., Seuntjens, P., Mallants, D., Kim, D.J., Jacques, D., Hubrechts, L., Gonzalez, C., Feyen, J., Diels, J., Deckers, J., 2001. Overview of inert tracer experiments in key Belgian soil types: relation between transport and soil morphological and hydraulic properties. Water Resour. Res. 37 (12), 2873 – 2888. Vauclin, M., Imbernon, J., Vachaud, G., Dancette, C., 1983. Experimental description and stochastic modelling of transfers using a scaling factor for the hydrodynamic properties of the soils (In French). Isotope and radiation techniques in soil physics and irrigation studies 1983, IAEA vol. SM-267. 1983, pp. 103 – 124. Warrick, A.W., Nielsen, D.R., 1980. Spatial variability of soil physical properties in the field. In: Hillel, D. (Ed.), Applications of Soil Physics. Academic Press, New-York, pp. 319 – 344. Wilson, G.V., Alfonsi, J.M., Jardine, P.M., 1989. Spatial variability of saturated hydraulic conductivity of the subsoil of two forested watersheds. Soil Sci. Soc. Am. J. 53, 679 – 685.
Vertical variation of near-saturated hydraulic conductivity in three soil profiles Y. Coquet*, P. Vachier, C. Labat UMR INRA/INAPG Environment and Arable Crops, Institut National de la Recherche Agronomique/Institut National Agronomique Paris-Grignon, B.P. 01, 78850 THIVERVAL-GRIGNON, France Received 13 March 2003; received in revised form 31 August 2004; accepted 22 September 2004
Abstract Hydraulic conductivity is an important parameter of water and solute transport in soils. Hydraulic conductivity is known to be highly variable in space, but its dependence on soil horizonation has seldom been explored. We measured the nearsaturated hydraulic conductivity of three soils located in the Centre of France, a Ne´oluvisol, a Calcisol, and a Calcosol (according to the bRe´fe´rentiel Pe´dologiqueQ), using tension disc infiltrometry. The Reynolds and Elrick’s multipotential technique has been applied to the steady state infiltration kinetics measured at potentials 1.5, 0.6, 0.3, 0.1 and 0.05 kPa to derive the hydraulic conductivity values. Each horizon of the three soil profiles has been characterised at least once at three locations 50-cm apart from each other on a flat surface prepared within the same access pit. Some horizons have been characterised on a second occasion in other access pits opened at nearly 10 m from the previous ones. On average, hydraulic conductivity values increased from 1.3107 m/s at 1.5 kPa to 2.5106 m/s at 0.3 kPa. Saturated hydraulic conductivity varied between 3.6106 m/s for the ploughed layer of the Calcosol and 4.9103 m/s for the Calcosol substrate (C horizon). The various horizons of the Ne´oluvisol had hydraulic conductivity values which were not significantly different from each other, except the ploughed layer which had lower hydraulic conductivity values than the underlying horizons. The tilled layers of the Calcisol and the Calcosol also had lower hydraulic conductivity values than their respective underlying cambic horizons. Further, the hydraulic conductivity of the tilled layer of the Calcosol was lower than that of the tilled layers of the Calcisol and the Ne´oluvisol. This difference was attributed to the effect of different tillage systems, the Calcosol being superficially tilled while the Calcisol and Ne´oluvisol were ploughed each year. The structural S cambic horizons of the Calcisol and Calcosol had similar hydraulic conductivities but lower than their respective substrate (limestone). D 2004 Elsevier B.V. All rights reserved. Keywords: Soil horizon; Spatial variability; Tillage; Limestone; Loamy soil
1. Introduction * Corresponding author. Tel.: +33 1 30 81 52 04; fax: +33 1 30 81 53 96. E-mail address: [email protected] (Y. Coquet). 0016-7061/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2004.09.014
The hydraulic conductivity K of soils is known to be highly variable in space. A large number of field
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studies have been conducted to better understand the nature and the characteristics of this variability (e.g. Nielsen et al., 1973; Russo and Bresler, 1981; Rao et al., 1983; Vauclin et al., 1983; Wilson et al., 1989; Mohanty et al., 1991; Russo and Bouton, 1992; Mallants et al., 1997; Russo et al., 1997). Most of them have shown its autocorrelated nature at various scales (plot, field, catchment), thereby extending the field of application of the geostatistics to soil water physics. Analysis of the spatial variations of soil hydraulic parameters have quite often disregarded the role that soil types and soil horizonation may play. Only occasionally does the experimental setup account for soil organisation (Mallants et al., 1996; Vanderborght et al., 2001). A reason for this may be the scale at which the studies are conducted, which is mostly the field plot scale and where there is a smaller chance to encounter different soils than at a smaller scale (e.g. catchment scale). Another reason may be the tendency of experimental setups to give more importance to the horizontal directions than to the vertical one. Yet the vertical differentiation of soils may affect their hydraulic properties. Significant improvements in the prediction of soil hydraulic properties have been found when stratifying pedotransfer functions according to soil groups (Pachepsky and Rawls, 1999), horizon types (Reeve et al., 1973; McKenzie et al., 1991), and morphological descriptors (McKeague et al., 1982; Griffiths et al., 1999; Lilly, 2000) or pedogenetic origin (Bruand, 1990) of the soil horizons.
Farming practices, such as tillage (Kim and Chung, 1994; Prieksat et al., 1994), wheeling due to traffic (Logsdon and Jaynes, 1996; Heddadj and Gascuel-Odoux, 1999), have been shown to have a strong influence on K. Generally, K increases on tillage events and then decreases during the growing season due to the settling of the soil structure created by tillage (Angulo-Jaramillo et al., 1997, Azevedo et al., 1998; Messing and Jarvis, 1993). Thus, tillage is expected to have a determining influence on the eventual K differentiation between the tilled layer and the underlying untilled soil. In this study, we analyse the variations in nearsaturated K along the vertical of three soil profiles according to their pedological stratification.
2. Materials and methods 2.1. The soils Three cultivated soil profiles representative of the Beauce agricultural region of France were considered in this study (Table 1): – A Eutric Ne´oluvisol according to the French bRe´fe´rentiel Pe´dologiqueQ, RP, (Baize, 1998) or Orthic Luvisol according to the FAO Classification (FAO-UNESCO, 1989). Four horizons composed the soil profile. A 30-cm thick tilled
Table 1 Basic properties of the three soils Soil horizons
Clay 0–2 Am (g kg1)
Silt 2–50 Am (g kg1)
Ne´oluvisol LA (0–30 cm) E (30–55 cm) BT (55–100 cm)
181 184 303
783 784 676
36 32 21
Calcisol LA (0–30 cm) Sci (30–70 cm)
210 367
746 602
Calcosol LA (0–30 cm) Sca (30–60 cm)
279 313
663 468
Sand 50–200 Am (g kg1)
CaCO3 (g kg1)
pH
OM (g kg1)
CEC (cmol (+) kg1)
1 1 3
7.4 7.3 7.7
17.7 6.1 7.1
15.0 13.4 19.9
44 31
2 1
7.2 7.3
18.7 10.0
15.7 25.6
58 219
17 472
8.0 8.4
18.6 9.8
20.9 16.4
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organo-mineral horizon LA overlaid a moderately clay-impoverished non-albic eluvial E horizon between 30 and 55 cm depths, followed by a clay-enriched illuvial BT horizon between 55 and 145 cm depths. The textural differentiation index (TDI, ratio of the clay content of the BT horizon to the clay content of the E horizon) of 1.65 indicated a moderate clay translocation process within the profile. The fourth and last horizon, noted Rca according to RP, was a hard coarse-slabby limestone starting at 1.45 m depth. – A Calcisol (RP Classification) or Eutric Cambisol (FAO Classification). Below a 0–30 cm LA horizon, a weathered carbonate-free Sci horizon overlaid at 0.70 m depth a weathered C horizon corresponding to a heterogeneous highly fractured marly limestone. – A Calcosol (RP Classification) or Calcic Cambisol (FAO Classification), composed of a 0–30 cm LA horizon, followed between 30 and 45–70 cm depths by a short carbonated Sca horizon including calcareous coarse fragments, over a clayeyloamy pulverulent limestone corresponding to a C horizon. Details about the reference horizons corresponding to the letters used here may be found in the bRe´fe´rentiel Pe´dologiqueQ (Baize, 1998). These three soils were located near Ouarville within a 2500-ha area at 20 km South East of Chartres. The coordinates (WGS84) of the pits were E 1844V31.8262U; N 48821V34.6398U for the Ne´oluvisol, E 1845V24.5927U; N 48821V35.4722U for the Calcisol, and E 1846V24.9673; N 48822V4.3568U for the Calcosol. Some physical and physicochemical properties of the three soils are presented in Table 1. Each horizon, except the substrates, was characterised by its particle size distribution (g/kg dry soil) after removal of the organic matter by H2O2 and decarbonatation by HCl, according to the following fractions: clay (b2 Am), silt (2–50 Am) and sand (0.05–2 mm), its CaCO3 content (g/kg dry soil) by the volumetric method, its pH in water, its organic matter content (g/kg dry soil) by dry combustion, its cation exchange capacity (cmol (+)/kg dry soil) by ammonium exchange.
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2.2. Hydraulic conductivity measurements Tension disc infiltrometry (Perroux and White, 1988) have been used to measure the hydraulic conductivity of the soil horizons between 1.5 and 0 kPa matric potential h. A 8-cm diameter base was used and infiltrations were done at 1.5, 1.0, 0.6, 0.3, 0.1, and 0.05 kPa matric potentials successively without displacing the infiltrometer. An ascending order was used in order to discard any hysteresis effect due to drainage occurring below the disk when lowering the infiltration potential while wetting is progressing at the infiltration front (Reynolds and Elrick, 1991). Contact between the disk and the soil surface was ensured by a 1-mm thick sand layer. The fine well-sorted Fontainebleau sand was used because it corresponded well to the optimal characteristics of contact material for disk infiltrometry (Reynolds and Zebchuk, 1996). Its median grain size was 0.15 mm (Coquet et al., 2000) and its air entry potential could be estimated to less than 2 kPa and its saturated hydraulic conductivity to 0.15 m/s. Flow impedance due to the thin sand layer was therefore less than 0.007 s, which can be considered negligible considering the moderate flow rates in loamy soils (Reynolds and Zebchuk, 1996). An approximate calculation showed that any impedance effect was negligible up to flow rates of 0.5 m/s (Klute and Dirksen, 1986). Soil surfaces were prepared for infiltration by gently scraping the soil with the extremity of a large-blade knife, avoiding smearing, to create a surface as flat and levelled as possible. Soil bits were removed from the surface by blowing air with hand-held bellows. The sand was then poured slowly using a small funnel over the infiltration surface materialised by a 8-cm diameter shallow plastic ring just put on the soil surface. The sand was levelled using a little hand-held rammer until a flat surface was obtained. The thickness of the sand layer was around 1 mm but could vary between a few tenths of mm to a maximum of 2 mm. Reynolds and Zebchuk (1996) showed for loam and clay soils that the pressure head at the soil surface was simply offset from the pressure head applied at the tension infiltrometer membrane by the thickness of the contact sand layer. As the thickness of the sand layer was kept to a minimum, we considered that the matric potential
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offset due to the sand was of the same order of magnitude as the uncertainty due to the Mariotte vessel readings (F0.01 kPa) and did not modify the matric potential values applied by the tension infiltrometer. The multipotential technique of Reynolds and Elrick (1991) was applied to derive the K(h) relationship from the steady-state regimes of the successive infiltrations. For each matric potential interval, except the [0.1, 0.05 kPa] interval, we calculated K at the centre of the interval as done by Reynolds and Elrick (1991) but also at the bounds of each interval as done by Ankeny et al. (1991). For the 1.0, 0.6, 0.3 and 0.1 kPa potentials, the K values were calculated as the arithmetic average of the two estimates obtained from the left and right intervals. Values at 1.5 and 0.05 kPa were calculated directly from only the right [1.5, 1.0 kPa] or left [0.1, 0.05 kPa] interval, respectively. Saturated K was extrapolated from the [0.1, 0.05 kPa] interval. As such, it should not be considered as the actual saturated K such as measured under a ponded condition for instance because any macropore of equivalent radius N3 mm, if present, did not contribute to infiltration. Such large macropores were however not seen in the soil surfaces used for infiltration. Following Bouma (1982) and Booltink and Bouma (2002), we will note our underestimated saturated hydraulic conductivities K (sat) to distinguish them from actual saturated hydraulic conductivities K sat. Values of the parameter k m were calculated by r a km ¼ qw g where r is the surface tension of water, q w is the density of water, g is the acceleration due to gravity, and a is the slope of the lnK vs. h relationship (Gardner, 1958). The parameter k m is generally interpreted as a mean pore size characterising the soil porosity that is hydraulically active (Philip, 1969). The initial volumetric water content was measured on three small soil cores (3 cm diameter, 2.1 cm length) taken at 20 cm from the infiltrometer before infiltration. All soil horizons have been characterised at least once. A first set of measurements, set 1, included all the horizons except the LA horizons of the Calcisol and Calcosol and was taken in May 2000 from three
access pits. A second set of measurements, set 2, included the same horizons and was taken in October 2000 from three other access pits dug at approximately 10 m from the previous ones. Finally, a third set of measurements, set 3, included all the LA horizons of the three soils and was taken in March 2004 at less than 10 m from the previous measurement sites. In each data set, three replicates per horizon were done at 50 cm from each other on a flat surface prepared in the same access pit. The measurement depths, amongst all replicates and data sets, were 12–15, 42–47, 77–78, and 144–147 cm for the LA, E, BT and Rca horizons of the Ne´oluvisol, 12–15, 40–45 and 75 cm for the LA, Sci and C horizons of the Calcisol, and 10–15, 30–32 and 45–50 cm for the LA, Sca and C horizons of the Calcosol. The ranges of measurement depths have been chosen as to fall in representative zones of the soil horizons, possibly far from the transitions between horizons (although it was not possible for the Rca and C horizons due to their hardness). 2.3. Statistical analysis We used a multivariate analysis of variance (Johnson, 1998, chap. 11) to test whether the various soil horizons had different K(h) relationships. Globally, K values at each potential had empirical statistical distributions that were not significantly different from lognormal, according to the Kolmogorov-Smirnov test at the 5% level. Therefore, all K values were log10-transformed before analysis. The significance of any horizon effect was tested using the Wilks’ lambda at the 5% level. Student–Newman–Keuls tests were used to distinguish the soil horizons that had significantly (5% level) different K values.
3. Results and discussion 3.1. K(h) measurements All the K data are presented in Figs. 1–3. All K(h) characteristics show the same general pattern. log10-K increased at an increasing rate as h increased. The increase in K between h=1.5 and 0.2 kPa (equivalent diameter e.d. 1.5 mm) was
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Fig. 1. K(h) characteristics for the (a) LA, (b) E, (c) BT, (d) Rca horizons of the Ne´oluvisol. Full symbols indicate the three replicate measurements of set 1, open symbols indicate those of set 2, and the grey symbols are for set 3. The number next to each symbol in the legend indicates the initial volumetric water content of the soil before the infiltration.
about 2 orders of magnitude and was also 2 orders of magnitude between h=0.2 and 0 kPa, with the exception of the topsoil of the Calcisol (Fig. 2a) which increased by less than 1 order of magnitude between 0.2 and 0 kPa. Geometric mean K values were around 1.3107 m/s at 1.5 kPa with a minimum value of 1.8108 m/s and a maximum value of 5.6107 m/s, and reached 2.5106 m/s at 0.3 kPa with a minimum value of 3.3107 m/s and a maximum value of 1.7105 m/s. Values of K (sat) varied between 3.6106 m/s (LA horizon, Calcosol, Fig. 3a) and 4.9103 m/s (C horizon, Calcosol, Fig. 3c). The large increase between 0.2 and 0 kPa is however variable amongst replicates. This variability is consistent with the presence or absence of macropores (e.d.N1.5 mm) below the 50cm2 of the disk infiltrometer according to its location at the local scale (0.5–10 m). In many soil variability surveys, K sat was found to have coefficients of variation well over 100% (Warrick and Nielsen, 1980; Mulla and McBratney, 2002). Here, the coefficient of variation of K raised from 87% at h=0.2 kPa up to 226% for K (sat). Higher variability may be expected for actual K sat.
The LA horizons of the three soils differed from the underlying horizons by at least two aspects (Figs. 1–3): – A lower conductivity from 1.5 to 0.1 kPa. – A higher within-horizon variability of K at large water potential (hN0.1 kPa), except for the Calcisol’s LA. Fig. 4 compares the evolution of k m vs. h observed for the LA horizon to that observed for the underlying horizon for each of the three soils. The k m of the LA horizon is systematically larger than that of the underlying horizon in the 0.6 to 0.1 kPa range. In all three soils, tillage increased the hydraulically active porosity in the 0.5–3 mm e.d. range. The same effect of tillage on k m values have already been observed by Coutadeur et al. (2002). Further, the increase of k m took place within a very similar water potential range (0.5 to 0.2 kPa). Changes in k m values may be the results of modifications in the pore size distribution and/or pore connectivity. The former will alter both water retention and hydraulic conductivity while the latter
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Fig. 2. K(h) characteristics for the (a) LA, (b) Sci and (c) C horizons of the Calcisol. Full symbols indicate the three replicate measurements of set 1, open symbols indicate those of set 2, and the grey symbols are for set 3. The number next to each symbol in the legend indicates the initial volumetric water content of the soil before the infiltration.
Fig. 3. K(h) characteristics for the (a) LA, (b) Sca and (c) C horizons of the Calcosol. Full symbols indicate the three replicate measurements of set 1, open symbols indicate those of set 2, and the grey symbols are for set 3. The number next to each symbol in the legend indicates the initial volumetric water content of the soil before the infiltration.
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Fig. 4. Variations of the k m parameter with h for the LA and E horizons of the Ne´oluvisol (a), the LA and Sci horizons of the Calcisol (b), and the LA and Sca horizons of the Calcosol (c). Vertical bars represent Fone standard error of the mean values.
will essentially modify the hydraulic conductivity. The lower conductivity at low potential may be due to the disruption of pore continuity as a result of tillage (Logsdon et al., 1990; Reynolds et al., 1995; Coutadeur et al., 2002). On the other hand, the higher conductivity at high potential may be due to the porosity created by tillage and resulting in large inter-clod or inter-aggregate voids that are interconnected to the difference of the smaller-sized pores (e.d.b3 mm). One of the Ne´oluvisol LA replicates of set 2 had a low conductivity at all potentials
(Fig. 1a), especially when compared to the E horizon (Fig. 1b), and may be due to compacted soil zones within the tilled layer (Coutadeur et al., 2002). High K values have been observed in the substrates of the three soils (Fig. 1d, 2c and 3c) especially at potentials N0.1 kPa. Although corresponding to different facies of the Beauce limestone, these substrates have a good water conduction ability due to intense fracturation and weathering processes since the Miocene.
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The calcareous Sca horizon of the Calcosol had a highly variable K(h) characteristic (Fig. 3b). Such a variability can be related to a high proportion of limestone stones and blocks within the horizon, to the difference of the Sci horizon of the Calcisol for instance where these coarse elements were lacking. The influence of coarse elements on tension disk infiltrometer measurements probably depends on the relative size of the infiltration bulb compared to the size of the coarse elements. Yet, this influence has not been fully explored. Finally, no obvious differences between the data sets could be seen in Figs. 1–3 except for the Rca of the Ne´oluvisol and the C horizon of the Calcisol, where the values of set 2 were higher than those belonging to set 1 (Fig. 1d and 2c). In the BT horizon of the Ne´oluvisol (Fig. 1c) also, some values of set 2 are larger than those of set 1 at potentials higher than 0.2 kPa. 3.2. Analysis by soil horizons The Wilks’ Lambda MANOVA test on the overall effect of soil horizonation was strongly significant with a probability level of 0.0001. Single ANOVA Fisher tests done on the K values at each potential (Table 2) showed that the differences between soil horizons were significant at all potentials. Note, however, that the h=0 kPa potential corresponds to extrapolated K (sat) values and that the result of the Fisher test might have been different with actual K sat measurements. The analysis was pursued with Student–Newman–Keuls tests to group the various horizons whose K values did not differ at any potential (Fig. 5). The results show the peculiarity of the LA horizons for all three soils, especially that of the Calcosol which had the lowest K values (Fig. 6). The LA horizons of the Calcisol and the Ne´oluvisol were not significantly different from each other but differed from the rest of the soil profiles. It is
Fig. 5. Groups of soil horizons according to their K(h) characteristic. Ne´o: Ne´oluvisol, Cis: Calcisol, and Cos: Calcosol. Horizons belonging to each group do not have significantly different K values between 1.5 and 0 kPa (Student–Newman–Keuls tests at the 5% level).
worth noting that the Calcosol was cropped using a different tillage system than the other two soils. The LA horizon of the Calcosol has not been ploughed since 1999 and has only been superficially (0–3 cm) tilled, while the Calcisol and the Ne´oluvisol were generally ploughed each year. Such a difference in tillage system probably explains the difference between the two LA groups (Fig. 5), with a poorer hydraulic conductivity for the non-ploughed soil (Fig. 6). The eluvial E horizon and illuvial BT horizon of the Ne´oluvisol were not statistically different from each other or from the substrate (Rca horizon). Unlikely, the K(h) characteristics of the Sca and C horizons of the Calcosol were different. Another interesting result is the fact that the E, Sci and Sca horizons located immediately below the LA
Table 2 ANOVA results testing a bhorizon effectQ on the log10-K values (in m/s) measured at various potentials h
1.5 kPa
1.0 kPa
0.6 kPa
0.3 kPa
0.1 kPa
0 kPa
Mean Variance R2 Horizon effect ( F probability)
6.90 0.15 0.72 0.0001
6.60 0.15 0.71 0.0001
6.15 0.13 0.72 0.0001
5.60 0.13 0.68 0.0001
4.82 0.16 0.59 0.0001
3.96 0.42 0.44 0.006
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Fig. 6. K(h) characteristics for the five main groups of soil horizons identified in Fig. 5. Ne´o: Ne´oluvisol, Cis: Calcisol, and Cos: Calcosol. Vertical bars represent Fone standard error of the mean values.
horizons formed a single group from the point of view of their near-saturated K (Fig. 5). Their properties were quite similar (Table 1) with a dominating silt fraction. Indeed, all three soils have evolved from the same loess cover deposited during the Quaternary era over the intensely weathered and heterogeneous Miocene Beauce limestones (Duval and Isambert, 1992). Apparently, the differing degree of weathering and clay translocation between the three soil types was not sufficient to induce any strong differentiation in K. However, it should be noted that the hydraulic conductivity of the BT horizon of the Ne´oluvisol was significantly lower than that of the Sci horizon of the Calcisol. The hydraulic conductivity of the BT horizon of the Ne´oluvisol was also, but not significantly, lower than that of the E horizon of the Ne´oluvisol and the Sca horizon of the Calcosol, especially in the h=1.5 to 1.0 kPa range. This may be an indication of the premises of K reduction due to clay illuviation in the BT horizon of the Ne´oluvisol. The C horizons of the Calcisol and the Calcosol had the highest hydraulic conductivities (Fig. 6). This fact can be related to the structure of those horizons. The Calcisol had a heterogeneous and highly fractured substrate, and the Calcosol’s substrate was a fine pulverulent weathered limestone (Ould Mohamed et al., 1997). The less-weathered, less-fractured hard limestone of the Ne´oluvisol had a lower hydraulic conductivity.
4. Conclusion We studied the near-saturated K(h) variations of a loamy soil cover in the Centre of France according to its organisation in horizons. Our results showed a strong impact of the soil horizonation on nearsaturated K(h). Two main K contrasts could be seen. The first one was due to tillage. Tillage had a strong influence with a decrease in K up to a potential of 0.1 kPa and an increase in K close to saturation due to pore size redistribution and/or continuity modifications, probably through the creation of large inter-clods or inter-aggregates interconnected voids. Different tillage practices (ploughed vs. superficially tilled) were also responsible for a K differentiation between the ploughed layers of the Ne´oluvisol and the Calcosol and the tilled layer of the Calcisol. The second contrast was between the substrate (C or R) horizons and the overlaying (E, BT, S) horizons in the three soil profiles. The two types of horizons derived from different parent materials. The C and R horizons were derived from the Miocene Beauce limestones, while the eluvial E, illuvial BT and cambic S horizons and also the LA horizons were derived from a loess material that covered the limestones during the Quaternary times. The BT horizon had a lower, although non significantly, hydraulic conductivity than the E horizon showing that the clay-translocation pedogenetical
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