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Testing laboratory methods to determine the anisotropy of saturated hydraulic conductivity in a sandy–loam soil
Geoderma 154 (2009) 52–58
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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a
Testing laboratory methods to determine the anisotropy of saturated hydraulic conductivity in a sandy–loam soil V. Bagarello ⁎, S. Sferlazza, A. Sgroi Dipartimento di Ingegneria e Tecnologie Agro-Forestali, Università degli Studi, Viale delle Scienze, 90128, Palermo, Italy
a r t i c l e
i n f o
Article history: Received 30 December 2008 Received in revised form 18 September 2009 Accepted 28 September 2009 Available online 25 October 2009 Keywords: Saturated soil hydraulic conductivity Anisotropy Constant-head laboratory permeameter
a b s t r a c t Anisotropy, a (the log of the ratio of horizontal to vertical conductivity, log10(Kh/Kv)), of saturated soil hydraulic conductivity, Ks, affects transport processes in soil but is not routinely measured, probably because practical and validated methods are lacking. The objective of this investigation was to determine the effects of different constant-head laboratory and sampling procedures on anisotropy of saturated hydraulic conductivity measurements. The sequence of Ks measurements was varied (vertical conductivity, Kv, first, then horizontal, Kh, second and vice versa) for an experimental set-up considering five variables: 1) water ponding type (Mariotte or siphon); 2) saturation state prior to experiment start (unsaturated or saturated); 3) experiment duration (long or short); 4) sample geometry or extraction (cube or core); and 5) sample volume. The Mariotte, unsaturated/saturated, long experiment for a single soil cube resulted in unreliable mean anisotropy results, where a differed in sign. Generally, the sequence of measurements had a negligible impact on a for a siphon, saturated, short experiment for a soil cube. Furthermore, different a were obtained by varying the undisturbed soil sample collection procedure (cube vs. core). The conclusion of this investigation was that using a siphon and a short-duration run on an initially saturated cube of soil encased in foam is generally expected to yield reliable bi-directional Ks results. However, a check of the independence of the estimated anisotropy on the order of measurements for the sampled soil is recommended. An alternative procedure to determine a mean anisotropy for an area of interest would be to measure Kv and Kh on different soil cubes. Finally, the ratio between the mean Kh and Kv results varied from a not statistically significant factor of 1.02 to a statistically significant factor of 1.95 during the one-year investigation period (five sampling dates). Therefore, anisotropy of this sandy–loam soil varied with time but it was always low or negligible. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The hydraulic conductivity of saturated soil, Ks, is one of the most important soil properties controlling many hydrological processes. This property depends on soil texture, particle arrangement, and structure and can vary in space, time, and flow direction. In anisotropic soils, the vertical saturated hydraulic conductivity, Kv, of a given volume of soil differs from the horizontal saturated conductivity, Kh, of the same volume of soil (Beckwith et al., 2003). The anisotropy of Ks has been measured for peat (Chason and Siegel, 1986; Schlotzhauer and Price, 1999; Beckwith et al., 2003; Surridge et al., 2005) and mineral soils (Bouma and Dekker, 1981; Dabney and Selim, 1987; Bathke and Cassel, 1991; Caris and Van Asch, 1991). However, anisotropy is not routinely determined because practical and validated methods are still lacking.
⁎ Corresponding author. E-mail address: [email protected] (V. Bagarello). 0016-7061/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2009.09.012
Furthermore, anisotropy of Ks may change with time (Petersen et al., 2008), but data on temporal variability of anisotropy is scarce. The constant-head laboratory permeameter (CHP) method (Klute and Dirksen, 1986) is widely used for measuring Ks. There are two primary methods for obtaining undisturbed soil samples: 1) Two-Core Method (TCM, Dabney and Selim, 1987; Bathke and Cassel, 1991; Dörner and Horn, 2006; Petersen et al., 2008) and 2) Cube Method (CM, Bouma and Dekker, 1981) or Modified Cube Method (MCM, Beckwith et al., 2003). Anisotropy measured on soils extracted using TCM involves measuring Kv and Kh on two different samples where cylinders in both the vertical and the horizontal direction are pushed into exposed soil surfaces. On the other hand, CM and MCM measure Kv and Kh on a single soil sample where the soil sample is obtained by carving a soil cube in situ by gently removing soil along its sides. With CM, all except two opposing cube faces are encased in a slurry of gypsum. When the gypsum is cured, the soil cube encased in gypsum is removed from the pit and water flow rates through open sides are measured in the laboratory, yielding a measurement of Kv. The faces are then sealed with gypsum and the cube is rotated. Two faces at right angle to the original
V. Bagarello et al. / Geoderma 154 (2009) 52–58
faces are exposed and Kh is measured. With MCM, all faces of the cube are encased with gypsum, molten wax (Surridge et al., 2005) or expandable polyurethane foam (Bagarello and Sgroi, 2008) to ensure that all faces receive the same treatment and to minimize smearing of the exposed faces (Beckwith et al., 2003). Using a cube with removable faces is reasonably practical for sampling near-surface soil but it may become impractical at larger depths. More reliable anisotropy measurements may be expected with CM and MCM than TCM for several reasons. One reason is that Kv and Kh measured on a single soil sample is only affected by scale-dependent anisotropy whereas both anisotropy and small scale heterogeneity may influence Ks measurements made on different cores (Beckwith et al., 2003). The potential advantage of CM and MCM is particularly important when anisotropy has to be determined at a fixed sampling point but it is less important when an area of interest has to be characterized by a mean anisotropy. Another reason is that, in several cases, more representative soil samples for a selected site may probably be obtained with CM and MCM than TCM. When a cylinder is used to collect a soil core, shattering, puddling, or compaction of the soil may occur during sampling (e.g., Bouma et al., 1976; Topp et al., 1993). As a result, short-circuiting flow along edges of the soil core may occur during Ks measurements due to the presence of gaps between the soil column and the rigid cylinder (e.g., Cameron et al., 1990; Hoag and Price, 1997). Therefore, soil core disturbance due to the TCM can yield unreliable Ks results. On the other hand, the soil volume obtained by CM and MCM does not receive any particular disturbance prior to Ks measurement. Moreover, the cube is enclosed in a tightly fitting cast that conforms to any irregularities in the exposed soil surface before hardening and prevents edge flow (Bouma and Dekker, 1981; Beckwith et al., 2003; Surridge et al., 2005; Bagarello and Sgroi, 2008). In spite of the advantages of CM and MCM, experimentally induced alterations to the soil sample may occur that causes the second measurement to be affected by the first measurement when Kv and Kh are measured, in a pre-established sequence, on a single soil cube. Beckwith et al. (2003) suggested that a peat sample may not wet fully before the first measurement but may be closer to complete saturation before the second measurement. In addition, migration, rearrangement, and removal of soil particles within the porous medium (i.e., selffiltration) can significantly affect Ks measurements (Dikinya et al., 2008). If self-filtration occurs, the Ks measurement varies with the duration of the experiment and as a result the second measurement is conducted on a soil sample altered to some degree by the first measurement. Therefore, the estimated anisotropy may depend on the sequence of Ks measurements but the degree of this dependency is uncertain. To our knowledge, the reproducibility of Kv and Kh on a single cube was investigated only in a few studies (e.g., Beckwith et al., 2003). In general, specific assessments regarding the effects of the experimental set-up and method on the estimated anisotropy are rare in research literature and it is evident that the development of standardized procedures for determining anisotropy is necessary. The general objective of this investigation was to test different procedures for measuring anisotropy of Ks for a sandy–loam soil using the CHP method. The objectives are to: i) establish the dependence of the measured anisotropy by the MCM on the sequence of Ks measurements for different CHP procedures; ii) determine the effect of different soil sampling procedures and sample volume on anisotropy measurements; and iii) determine the temporal variability of anisotropy for the sampled area. 2. Materials and methods 2.1. Location A flat area supporting a citrus orchard was used for this study at the Faculty of Agriculture of the Palermo University. The study was conducted on a soil (Typic Rhodoxeralf) having a relatively high sand
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(>50%) and gravel (13%) content. According to the USDA classification (Gee and Bauder, 1986), most textures of 12 replicated samples of soil material (i.e., particles smaller than 2 mm in diameter) were sandy– loam (Bagarello and Sgroi, 2008). The experiment was carried out in the period ranging July 2007 (JUL07) to October 2008 (OCT08). The experimental site was not disturbed (i.e., no tillage was performed) from 2002 until sampling was completed. A few additional experiments were carried out with the clay soil (clay = 62.4%, silt = 28.0%) of the Sparacia experimental installation for soil erosion measurement (Bagarello and Ferro, 2004; Bagarello et al., 2008). In this case, sampling was carried out on a bare surface having a 15% slope. 2.2. Soil sampling procedure Soil samples were collected on seven sampling dates in the sandy– loam soil (Table 1). In particular, 20 soil cubes were randomly collected on each sampling date within an area of approximately 10 m2. A few cm of soil were removed from the surface and a soil prism (0.11 × 0.11 × 0.14 m3) was carved out. A four-sided wooden frame (side length = 0.17 m), with an open bottom and top with inner walls covered by plastic wrap to avoid foam adhesion to the frame (so that the same frame may be re-used in other experiments), was placed around an excavated and exposed soil prism. Water was sprayed on the inner walls of the box and expandable polyurethane foam was injected between the box and soil and on the exposed soil surface (Bagarello and Sgroi, 2008). Then, wood and a weight, not more than 40–50 N, were placed on top of the frame to confine foam expansion. The cube was marked to preserve direction. Next, the cured cast with the enclosed soil was excavated and turned upside down. A layer of soil 0.03 m thick was removed from the bottom of the excavated prism to achieve a cube of soil and also encased in expandable foam. On both June 2008 (JUN08) and OCT08, 10 vertically-oriented and 10 horizontally-oriented soil cores were collected in randomly selected locations within the sampled area by using stainless steel cylinders of 0.085 m in diameter by 0.115 m in height (long soil cores). On OCT08, additional 10 vertically-oriented and 10 horizontally-oriented soil cores were collected using stainless steel cylinders of 0.05 m by 0.05 m (short soil cores). On each sampling date, a total of 16 undisturbed soil cores (0.05 m dia. by 0.05 m long) were also collected at a depth of 0.02 to 0.07 m and 0.07 to 0.12 m near the location of the soil cubes that were collected to determine soil bulk density, ρb, and initial volumetric soil water content, θi, using the ovendrying method, and were averaged over the two depths. Ten soil cubes for Ks measurement and eight undisturbed soil cores for ρb and θi measurement were collected in the clay soil on July 2008 (JUL08). 2.3. Laboratory methods In the laboratory, the cube was taken from the wooden box and the foam was removed from both the top and bottom faces using a box cutter. A photographic sequence of the application procedure of the MCM with expandable foam was given by Bagarello and Sgroi (2008). The vertical conductivity was measured before the horizontal conductivity for ten randomly chosen cubes, and the symbols Kv1 and Kh2 were used to denote the Kv and Kh results for these cubes. The opposite sequence (i.e., horizontal measurement before the vertical) was applied for the remaining soil cubes, and the symbols Kv2 and Kh1 were used in this case. Each soil cube was placed on a nylon guard cloth and a wire net to support the weight of the soil before starting the Ks test. In all cases, the CHP method with an established ponding depth of 0.015 m was applied to measure Ks. An important point that has to be taken into account is the possible effect of the wetting fluid on the measurement. Using a fluid maintaining soil structure during the test is desirable. On the other hand, water is generally the permeating fluid, and soil structure alteration may be promoted by this fluid in the field. Previous investigations showed that various SAR levels did not cause appreciable
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Table 1 Mean, mA, and coefficient of variation, CV, of the soil bulk density, ρb (Mg m− 3), and the initial soil water content, θi (m3 m− 3), for each sampling date. Texture
Sandy–loam
Clay
#
1 2 3 4 5 6 7 8
Sampling date
JUL07 SEP07 NOV07 JAN08 MAR08 JUN08 OCT08 JUL08
ρb
θi
mA
CV
mA
CV
1.169 1.200 1.306 1.238 1.200 1.236 1.223 1.240
0.075 0.065 0.074 0.075 0.061 0.079 0.064 0.075
0.116 0.091 0.253 0.228 0.223 0.116 0.179 0.460
0.369 0.194 0.101 0.077 0.135 0.287 0.134 0.195
Experimental set-up
Sample type
# of samples
MUL MSL SSS SSS SSS SSS SSS SSS
Cube Cube Cube Cube Cube Cube + Long Core Cube + Long Core + Short Core Cube
20 20 20 20 20 20 cubes + 20 cores 20 cubes + 20 long cores + 20 short cores 10
clay dispersion or change in soil structure of the sampled sandy–loam and clay soils (Bagarello et al., 2006). Local municipal tap water is often used for measuring Ks since it is an adequate approximation to native water, i.e. water extracted from the porous medium (Reynolds et al., 2002). For these reasons, tap water (Ca2+ + Mg2+ and Na+ concentrations equal to 2.20 and 5.49 mmol L− 1, respectively; SAR = 3.7; pH= 7.0) was used in this investigation. Three different CHP procedures were used in this investigation with three factors including water ponding type (Mariotte, M, or siphon, S), 2) saturation state prior to experiment start (unsaturated, U, or saturated, S), and 3) experiment duration (long, L, or short, S). The acronyms MUL, MSL and SSS were used, depending on the combination of these factors. MUL was applied for the sampling of JUL07 where a Mariotte (M) bottle was used to establish and maintain the constant depth of ponding on the soil surface (Fig. 1e in Bagarello and Sgroi, 2008) since this apparatus is cheap, easily manufactured, and simple to use. The soil was unsaturated (U) before performing the Ks measurement because most natural and man-made infiltration processes result in significant air entrapment within the porous medium (Reynolds, 1993). In other words, the so-called field-saturated hydraulic conductivity, or satiated conductivity, was measured (e.g., Odell et al., 1998). A long-duration (L) experiment (six or more hours) was carried out for measuring Kv and Kh to be reasonably sure of considering steady-state flux density in the calculations. After the first measurement of Ks, the cube was allowed to drain under gravity for three days. The second procedure, MSL, was applied for soil cubes collected on September 2007 (SEP07). The Mariotte bottle was used and a longduration experiment was carried out. However, the soil samples were slowly saturated (S) from the bottom before measuring both Kv and Kh. With an initially unsaturated soil sample, the second measurement of Ks is likely carried out in wetter soil conditions than the first Ks run. In particular, the soil may be initially dry or almost dry for the first run but it has an initial water content close to “field capacity”, FC, for the second experiment. If soil is saturated before measuring Ks, the initial water content does not vary between the two experiments. The third procedure, SSS, was applied for soil cubes and cores collected on five sampling dates between November 2007 (NOV07) and OCT08 (Table 1). Soil samples saturated (S) from the bottom were used but a siphon (S) was applied instead of the Mariotte bottle to establish the constant ponded head on the soil surface. Also a short-duration (S) experiment (i.e., approximately 30–40 min) was conducted instead of a long-duration. Using the Mariotte bottle with small ponding depths may promote turbulences close to the soil surface, potentially resulting in clogging of exposed pores. With a siphon, undesired surface soil alteration should be reduced. A shortduration experiment was considered to prevent or, at least, reduce soil alteration due to possible occurrence of time-dependent selffiltration during the experiment (Dikinya et al., 2008). The CHP method was run with water flowing from the soil surface downward. The last 15–20 min (SSS procedure) or 30–45 min (MUL and MSL procedures) of measured flow rates were used to calculate
Ks. According to Arya et al. (1998), initial soil saturation and alteration of soil surface during the experiment may have an appreciable impact on Ks values measured by the CHP method. The SSS procedure was also applied with soil cubes collected in the clay soil. In this case, Kv was measured before Kh in five randomly chosen cubes whereas the opposite sequence was applied for the other soil cubes. A reduced number of cubes were used for this soil because the objective was to check the ability of the tested procedure to detect soil effects on measured anisotropy. 2.4. Data analysis For a soil cube, the anisotropy estimated with a given measurement sequence was expressed as a = log10(Kh/Kv) (Beckwith et al., 2003). An a value of 0 indicates Kh =Kv. Positive a values indicate Kh >Kv whereas negative a values indicate Kv >Kh. Thus, a equal to 0.3 indicates that Kh is twice Kv, and −0.3 indicates that Kv is twice Kh. The a value has the same magnitude but changes sign when the factor of difference is the same, which would not be the case with the untransformed ratio (the scores would be 2 and 0.5, respectively) (Beckwith et al., 2003). Therefore, two sets of a results were obtained for a given sampling date, i.e. a1 = log10(Kh2/Kv1) and a2 = log10(Kh1/Kv2). Ks data were assumed to be log-normally distributed since the statistical distribution of these data is generally log-normal (Lee et al., 1985; Warrick, 1998). The geometric mean, mG, and the associated coefficient of variation, CV, were calculated to summarize Ks values (Lee et al., 1985). The arithmetic mean, mA, was calculated for ρb, θi and a, a1 and a2. Statistical comparison between two sets of data was conducted using two-tailed t-tests, whereas the Tukey Honestly Significant Difference test was applied to compare three or more sets of data. The ln-transformed Ks data were used in the statistical comparison. A onetailed t-test was applied to evaluate statistical significance of the correlation between two variables. A probability level, P = 0.05, was used for all statistical analyses. 3. Results and discussion 3.1. Bulk density and initial soil water content The mA(ρb) values of the sampled sandy–loam soil ranged between 1.17 and 1.31 Mg m− 3 during the investigation period whereas mA (θi) ranged from 0.091 to 0.253 m3 m− 3 (Table 1). According to Iovino (1998), in the sampled area, the soil water content corresponding to FC (i.e., pressure head value, h = − 0.033 MPa), and the permanent wilting point, PWP (i.e., h = −1.5 MPa), is equal to 0.256 m3 m− 3 and 0.140 m3 m− 3, respectively. Therefore, the soil water content during the experiment varied from substantially less than the PWP value (on JUL07, SEP07 and JUN08) to a value close to FC (on NOV07), and thus provided a good range of soil water content values for examining temporal variability of Ks anisotropy in the experimental area. For the clay soil, mA(ρb) = 1.24 Mg m− 3 and mA(θi) = 0.460 m3 m− 3 was obtained (Table 1).
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3.2. Sequential effect on anisotropy For both MUL and MSL, Table 2 shows that mG(Kv1) ≠mG(Kv2), mG (Kh1) ≠mG(Kh2), and mA(a1) ≠mA(a2). In particular, the sequence of measurements was found to have a noticeable effect on the estimated anisotropy, and on both sampling dates a > 0 (Kh >Kv) was obtained when Kh was measured before Kv whereas a < 0 (Kv >Kh) was obtained when Kh was measured after Kv. For a given variable (e.g., Kv or Kh), the second measurement always yielded lower results than the first measurement. Similar or slightly larger CV results were obtained for Kv2 and Kh2 as compared with the corresponding CVs for Kv1 and Kh1, respectively. The SSS procedure (NOV7, JAN08, MAR08, JUN08, and OCT08) yielded mG(Kv1) = mG(Kv2) or mG(Kh1) = mG(Kh2) for eight of ten considered data comparisons and mA(a1) =mA(a2) for four of the five sampling dates (Tables 3–5). However, a positive a (Kh >Kv) was obtained on all sampling dates for both measurement sequences. The CV for Kv2 and Kh2 were similar to the corresponding CVs of Kv1 and Kh1, respectively, for four of the 10 considered data sets (Kh, NOV07 and OCT08; Kv, JUN08 and OCT08). In the other cases, larger CV values were obtained for a given variable when this variable was measured in second. For the clay soil, mG(Kv1) =mG(Kv2), mG(Kh1) =mG(Kh2) and mA (a1) =mA(a2) (Table 6). Therefore, the statistical equivalence of two sets of data included in the comparison (Kv1 vs. Kv2, Kh1 vs. Kh2, a1 vs. a2) largely prevailed in the sandy–loam soil and it was also detected for a different soil, suggesting that, in general, the mean anisotropy results did not vary with the sequence of measurements. In a few cases, however, the second measurement yielded lower results than the first measurement. Frequently, the second measurement was more variable than the first measurement. At the scale of the small plot, as the one considered in this investigation (10 m2), a random sampling of N = 10 soil cubes should be sufficient to determine a representative mean value of anisotropy of Ks given, for example, that N = 10 may be a practical sample size for characterizing soil hydraulic conductivity at a much larger scale, such as the agricultural field scale (Reynolds et al., 2002). Therefore, two different random samplings with N = 10 for each sampling should yield mean results that do not vary significantly between the two sets of data. In other words, if mean anisotropy obtained by measuring Kv before Kh does not coincide with mean anisotropy obtained by measuring Kv after Kh, the possibility that the detected differences depend on soil heterogeneity (i.e., different results were obtained because two inherently different sets of data were collected) cannot be excluded but it should be reduced. Another possible conclusion is that the applied experimental procedure did not allow an accurate measurement of Ks. It is likely that MUL and MSL procedures yielded unreliable Ks results because the long experimental duration was responsible for particle detachment and transport processes within the soil sample, altering appreciably the initial configuration of soil porosity (Dikinya et al., 2008). Prolonged turbulence on the soil surface induced an additional delivery of soil particles clogging exposed pores (Bagarello et al., 2000) or migrating through the sample. As a result, the second experiment was
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carried out on an altered soil sample and it determined Ks of a modified porous medium. The SSS procedure was generally accurate (i.e., mean anisotropy results not affected by the sequence of measurements) and it yielded more reliable results than the MUL and MSL procedures when the CHP method was used with cubes of soil encased in foam for determining anisotropy of Ks. Therefore, using a siphon instead of a Mariotte bottle and conducting a short-duration run instead of a longduration run on an initially saturated soil sample was generally effective to control soil degradation processes at the surface of, or within, the sample. In practice, applying the SSS procedure and a bi-directional measurement of Ks on a single soil cube to determine mean Ks anisotropy for an area of interest needs collecting a reasonably large number of cubes, to obtain a representative result, and also checking the independence of mean anisotropy results of the sequence of measurements. As suggested by this investigation, this independence should be the most common result. However, if an effect of measurement sequence is recognized, or feared, an estimate of mean Ks anisotropy can probably be obtained by measuring Kv and Kh on different soil cubes given that, in this case, each Ks measurement is obtained with a reduced disturbance of the sampled soil. Obviously, a larger number of soil samples have to be collected in the field to apply this alternative procedure. 3.3. Effect of soil sampling procedure and sample volume Neither the procedure used to collect an undisturbed soil sample (soil cubes vs. soil cores) nor the soil core volume (long soil cores vs. short soil cores) had a statistically significant effect on the measured Kv values (Tables 4 and 5). Although the soil core volume did not affect Kh, the soil cubes did yield higher Kh values than the soil cores on both sampling dates. Anisotropy was detected with the soil cubes (i.e., Kv1 ≠ Kh1) but not with the long and short soil cores (Kv = Kh). Therefore, the procedure applied to collect an undisturbed soil sample was another factor affecting the estimated anisotropy of Ks. An effect of the sampled soil volume on the detected differences for Kh cannot be excluded because the volume of the soil cube, long soil core and short soil core was of 1331, 653 and 98 cm3, respectively. However, this effect was not considered to be substantial in this investigation because cores of different sizes yielded statistically equivalent results for both Kv and Kh, and Kv did not vary significantly with the sampled volume. Another possible explanation is that appreciable soil compaction occurred when undisturbed soil cores were collected in the horizontal direction but not for a vertically-oriented sampling. When a vertical soil core is collected, the soil surrounding the external surface of the cylinder is removed as deepening proceeds and this reduces friction during insertion. Therefore, a relatively low pushing force is enough to drive the cylinder into the soil. With a horizontal soil core, the soil surrounding the cylinder is removed after that insertion has been completed. More significant frictional effects are expected with horizontal sampling because greater forces have to be applied on the top of the cylinder to drive it into the soil. Maybe other, more convincing,
Table 2 Geometric mean, mG, arithmetic mean, mA, and coefficient of variation, CV, of the vertical, Kv1 and Kv2, and horizontal, Kh1 and Kh2, saturated hydraulic conductivity, Ks (mm h− 1), for measurements made with the MUL (JUL07 sampling date) and MSL (SEP07) procedures. Experimental set-up
Sampling date
Statistic
Kv1
Kv2
Kh1
Kh2
MUL
JUL07
mG mA CV mG mA CV
372.2(a)†
144.6(a)
244.5(b)
190.5(b)
0.168 380.3(a)
0.266 184.8(a)
0.223 465.6(b)
0.287 182.9(b)
0.255
0.260
MSL
SEP07
0.248
0.378
a1 = log10(Kh2/Kv1)
a2 = log10(Kh1/Kv2)
− 0.29(c) 0.572‡
0.23(c) 0.644
− 0.32(c) 0.362
0.40(c) 0.311
† For a given sampling date, a statistical comparison was made between those values followed by the same letter. Means followed by the same letter enclosed in parentheses are significantly different (P = 0.05); means followed by the same letter, but not enclosed in parentheses, are not significantly different. ‡Calculated by considering the absolute value of the mean.
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Table 3 Geometric mean, mG, arithmetic mean, mA, and coefficient of variation, CV, of the vertical, Kv1 and Kv2, and horizontal, Kh1 and Kh2, saturated hydraulic conductivity, Ks (mm h− 1), for measurements made with SSS procedure on NOV07, JAN08 and MAR08 sampling dates. Experimental set-up
Sampling date
mG(Kh1)/mG(Kv1)
Statistic
Kv1
Kv2
Kh1
Kh2
SSS
NOV07
1.86
mG mA CV mG mA CV mG mA CV
526.2a(d)†
457.5a
980.2b(d)
1030.5b
0.434 591.8ad
3.119 627.6a
0.704 914.9bd
0.723 735.8b
0.688 933.5(a)d
1.381 368.5(a)
0.484 949.8bd
1.186 1210.4b
1.443
0.593
0.904
SSS
JAN08
SSS
1.55
MAR08
1.02
0.426
a1 = log10(Kh2/Kv1)
a2 = log10(Kh1/Kv2)
0.29c 0.899
0.33c 2.066
0.09c 4.597
0.16c 2.150
0.11c 2.073
0.41c 1.327
† For a given sampling date, a statistical comparison was made between those values followed by the same letter. Means followed by the same letter enclosed in parentheses are significantly different (P = 0.05); means followed by the same letter, but not enclosed in parentheses, are not significantly different.
explanations could be suggested. However, it should be noted that interpreting Ks results is a difficult enterprise because there is no independent Ks datum or benchmark upon which evaluations and judgments can be made (Reynolds et al., 2000). For a given experimental set-up (i.e., CHP method, undisturbed soil, fixed water quality and ponding type, etc.), the estimated Ks anisotropy showed a dependence on sampling procedures. This result may have practical interest since varying sampling procedures and sample volumes were applied by different authors to determine anisotropy. For example, Bouma and Dekker (1981) used large soil cubes (25 × 25× 25 cm3) covered with gypsum whereas Dörner and Horn (2006) and Petersen et al. (2008) used small soil cores (100 cm3 volume). According to this investigation, a comparison of results from different sources and/or use of these results for developing a data-base of anisotropy data should be carried out with some caution. 3.4. Temporal variability of Ks anisotropy The Kv1 and Kh1 results obtained with the SSS procedure were not significantly different on both JAN08 and MAR08 (1.02 ≤ mG(Kh1)/mG (Kv1) ≤ 1.55), whereas significant differences between the two sets of data were detected on the other sampling dates (NOV07, JUN08, OCT08; 1.66 ≤ mG(Kh1)/mG(Kv1) ≤ 1.95) (Tables 3–5). The correlation between the mG(Kh1)/mG(Kv1) ratio and either mA(ρb) or mA(θi) was
not statistically significant (determination coefficient, r2 ≤ 0.43, N = 5). Much higher discrepancies between Kv and Kh than the ones obtained in this investigation (i.e., by also a factor of 46000) were reported in the literature (Bouma and Dekker, 1981; Bathke and Cassel, 1991; Caris and Van Asch, 1991). In particular, mean conductivities differing with the considered direction by a factor of also 38 were obtained in another sandy–loam soil (Petersen et al., 2008). Therefore, the mean Ks anisotropy of this sandy–loam varied during the investigation period, but it was always low or negligible. Moreover, neither the bulk density nor the soil water content at the time of the measurements had a statistically detectable effect on the anisotropy results. The mean values of both Kv1 and Kh1 varied by a similar factor (i.e., 2.3–2.4) during the investigation period. In particular, the mG values of Kv1 differed according to the sequence OCT08 = MAR08≥ JUN08 = JAN08= NOV07, whereas the mG values of Kh1 differed according to the sequence OCT08 = JUN08 ≥ NOV07 = MAR08 = JAN08. Neither mG (Kv1) nor mG(Kh1) were significantly correlated with either mA(ρb) or mA(θi) (r2 ≤ 0.40, N = 5). Therefore, a statistically significant temporal variability was detected for both Kv1 and Kh1. The temporal patterns of these two variables did not coincide but they showed similarities, given that higher results were obtained on the last sampling dates than the first ones in both cases. The relationship between mG(Kv1) and mG(Kh1)
Table 4 Geometric mean, mG, arithmetic mean, mA, and coefficient of variation, CV, of the vertical, Kv1, Kv2, and Kv, and horizontal, Kh1, Kh2, and Kh, saturated hydraulic conductivity, Ks (mm h− 1), for measurements made with SSS procedure on JUN08. mG(Kh1)/mG(Kv1) or mG(Kh)/mG(Kv) Soil cubes
Long soil cores
1.95
0.95
Statistic
Soil cubes
Long soil cores
Kv1
Kv2
Kh1
Kh2
mG mA CV
701.7(a)(d)e†
402.9(a)
1371.3b(d)(f)
999.4b
0.387
0.366
0.313
0.559
a1 = log10(Kh2/Kv1) 0.15(c) 1.216
a2 = log10(Kh1/Kv2) 0.53(c) 0.144
Kv
Kh
583.7eg
552.5(f)g
0.685
0.421
† For a given sampling date, a statistical comparison was made between those values followed by the same letter. Means followed by the same letter enclosed in parentheses are significantly different (P = 0.05); means followed by the same letter, but not enclosed in parentheses, are not significantly different.
Table 5 Geometric mean, mG, arithmetic mean, mA, and coefficient of variation, CV, of the vertical, Kv1, Kv2, and Kv, and horizontal, Kh1, Kh2, and Kh, saturated hydraulic conductivity, Ks (mm h− 1), for measurements made on OCT08 by applying the SSS procedure to soil samples collected with different procedures. mG(Kh1)/mG(Kv1) or mG(Kh)/mG(Kv)
Statistic
Soil cubes
Long soil cores
Short soil cores
1.66
0.73
1.10
mG mA CV
Soil cubes
Long soil cores
Kv1
Kv2
Kh1
Kh2
1288.0a(d)e†
1440.1a
2137.8b(d)(f)
1951.9b
0.543
0.528
0.429
0.396
a1 = log10 (Kh2/Kv1)
a2 = log10 (Kh1/Kv2)
0.18c 1.387
0.17c 0.941
Short soil cores
Kv
Kh
Kv
Kh
806.2eg
586.7 fg
953.0eh
1045.9fh
0.385
0.871
0.474
0.475
† For a given sampling date, a statistical comparison was made between those values followed by the same letter. Means followed by the same letter enclosed in parentheses are significantly different (P = 0.05); means followed by the same letter, but not enclosed in parentheses, are not significantly different.
V. Bagarello et al. / Geoderma 154 (2009) 52–58
57
Table 6 Geometric mean, mG, arithmetic mean, mA, and coefficient of variation, CV, of the vertical, Kv1 and Kv2, and horizontal, Kh1 and Kh2, saturated hydraulic conductivity, Ks (mm h− 1), values obtained with the SSS procedure at the Sparacia sampling area. mG(Kh1)/mG(Kv1)
Statistic
Kv1
Kv2
Kh1
Kh2
7.35
mG mA CV
106.0a(d)
187.3a
778.7b(d)
1080.8b
2.021
2.278
0.455
1.147
a1 = log10(Kh2/Kv1)
a2 = log10(Kh1/Kv2)
1.01c 0.505
0.62c 0.866
† For a given sampling date, a statistical comparison was made between those values followed by the same letter. Means followed by the same letter enclosed in parentheses are significantly different (P = 0.05); means followed by the same letter, but not enclosed in parentheses, are not significantly different.
was scattered but also statistically significant (r2 = 0.67). A much higher r2 result, equal to 0.96 (N = 4, r > 0), was obtained by neglecting the data point corresponding to the lowest mA(ρb) result for the investigation period. Therefore, a hypothesis deserving testing with a larger data set is that a one-directional measurement of Ks is enough to predict Ks anisotropy in relatively compacted soil conditions. An appreciably larger horizontal anisotropy was detected for the clay soil (mG(Kh1)/mG(Kv1) = 7.35, Table 6) than the sandy–loam one (mG(Kh1)/mG(Kv1) ≤ 1.95), showing that the applied procedure was able to detect differences in anisotropy between different soils. Larger Ks results than the ones obtained in this investigation (Table 6) are not uncommon for clay soils when macroporosity effects on soil water transport phenomena are significant (Bouma and Dekker, 1981).
investigation is important to develop an improved bi-directional measurement procedure of Ks, avoiding in particular the need for checking the independence of anisotropy results on the measurement sequence. For example, using alternative fluids for the experiment could be effective to establish the role of the wetting fluid on experimentally determined Ks data. Finally, the Ks anisotropy of the sampled sandy–loam soil varied significantly during an approximately one-year investigation period. However, it was found to be always low or practically negligible since the maximum difference between corresponding mean values of Kv and Kh was of a factor of 1.95.
4. Conclusions
This study was supported by grants from the Università degli Studi di Palermo (fondi ex 60%, Dottorato di Ricerca in Idronomia Ambientale) and the Sicilian Region (Progetto “DIFA – Digitalizzazione della Filiera Agroalimentare”, A.P.Q. Società dell'Informazione, Regione Siciliana). V. Bagarello and A. Sgroi set-up the research and wrote the paper. S. Sferlazza conducted most of the experimental work. All authors analyzed the results. S. Sferlazza developed this research for his PhD activity, which is co-funded by the European Social Fund of the European Community. The authors wish to thank the anonymous reviewers for the constructive comments and the time they were willing to expend in the revision of the manuscript.
In a sandy–loam soil, both the application procedure of the constanthead laboratory permeameter method and the undisturbed soil sampling procedure affected the estimated mean anisotropy of saturated soil hydraulic conductivity, Ks. Using a Mariotte bottle to establish a constant ponding depth on the soil surface and conducting a long experimental duration yielded questionable results that differed depending on the sequence of measurements carried out on a single soil cube encased in foam (vertical conductivity, Kv, measured before the horizontal one, Kh, or vice versa). This result was probably due to the occurrence of structure alteration at the surface of, and within, the sampled soil. The dependence of anisotropy on the sequence of measurements was greatly reduced when a siphon and a short-duration run were used to measure Kv and Kh of an initially saturated soil cube (SSS procedure). However, an occasional effect of the sequence of measurements on the mean anisotropy was also detected in this last case. The estimated anisotropy of Ks varied with sampling procedure (soil cube, soil core). Therefore, comparing results obtained with different sampling procedures was not recommended. The SSS procedure and a bi-directional measurement of Ks on a single cube of soil was suggested as the best procedure among the tested ones for determining mean Ks anisotropy of an area of interest. A reasonably large number of soil cubes have to be used to deduce a representative result, and the independence of estimated mean anisotropy of the order of measurements must be verified. An alternative procedure could involve measuring Kv and Kh on different soil cubes given that disturbance of the sampled soil is expected to be substantially reduced if a single, short-duration experiment is conducted on a single soil cube. In general, a procedure minimizing any possible alteration of the tested soil sample should be applied to obtain reliable Ks values, but this recommendation is particularly important for a bi-directional measurement of Ks on a single soil sample. In this case, the risk of altering the sampled soil during the measurement process is particularly high since stresses are doubled. Several factors that may affect the laboratory measurement of Ks (for example, chemical composition of the water solution used for the experiment, upward or downward flow, established head, soil sample size) were not considered in this investigation since common and practical procedures and sample sizes were used. Testing the effects of these additional factors and including other soils in the
Acknowledgements
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Dörner, J., Horn, R., 2006. Anisotropy of pore functions in structured Stagnic Luvisols in the Weichselian moraine region in N Germany. J. Plant Nutr. Soil Sci. 169, 213–220. Gee, G.W., Bauder, J.W., 1986. Particle-size analysis, In: Klute, A. (Ed.), Methods of Soil Analysis, Part 1. Physical and Mineralogical Methods, 2nd ed. Agronomy, vol. 9. American Society of Agronomy, Madison, WI, pp. 383–411. Hoag, R.S., Price, J.S., 1997. The effects of matrix diffusion on solute transport and retardation in undisturbed peat in laboratory columns. J. Contam. Hydrol. 28, 193–205. Iovino, M., 1998. Applicazione della tecnica “Multistep Outflow” per la determinazione delle proprietà idrauliche del terreno col metodo inverso. Irrig. Dren. 4, 25–34 (in Italian). 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, 2nd ed. Agronomy, vol. 9. American Society of Agronomy, Madison, WI, pp. 687–734. Lee, D.M., Reynolds, W.D., Elrick, D.E., Clothier, B.E., 1985. A comparison of three field methods for measuring saturated hydraulic conductivity. Can. J. Soil Sci. 65, 563–573. Odell, B.P., Groenevelt, P.H., Elrick, D.E., 1998. Rapid determination of hydraulic conductivity in clay liners by early-time analysis. Soil Sci. Soc. Am. J. 62, 56–62. Petersen, C.T., Trautner, A., Hansen, S., 2008. Spatio-temporal variation of anisotropy of saturated hydraulic conductivity in a tilled sandy loam soil. Soil Tillage Res. 100, 108–113. Reynolds, W.D., 1993. Chapter 56. Saturated hydraulic conductivity: Field measurement. In: Carter, M.R. (Ed.), Soil Sampling and Methods of Analysis. Lewis Publishers, Boca Raton, Canadian Society of Soil Science, pp. 599–613.
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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a
Testing laboratory methods to determine the anisotropy of saturated hydraulic conductivity in a sandy–loam soil V. Bagarello ⁎, S. Sferlazza, A. Sgroi Dipartimento di Ingegneria e Tecnologie Agro-Forestali, Università degli Studi, Viale delle Scienze, 90128, Palermo, Italy
a r t i c l e
i n f o
Article history: Received 30 December 2008 Received in revised form 18 September 2009 Accepted 28 September 2009 Available online 25 October 2009 Keywords: Saturated soil hydraulic conductivity Anisotropy Constant-head laboratory permeameter
a b s t r a c t Anisotropy, a (the log of the ratio of horizontal to vertical conductivity, log10(Kh/Kv)), of saturated soil hydraulic conductivity, Ks, affects transport processes in soil but is not routinely measured, probably because practical and validated methods are lacking. The objective of this investigation was to determine the effects of different constant-head laboratory and sampling procedures on anisotropy of saturated hydraulic conductivity measurements. The sequence of Ks measurements was varied (vertical conductivity, Kv, first, then horizontal, Kh, second and vice versa) for an experimental set-up considering five variables: 1) water ponding type (Mariotte or siphon); 2) saturation state prior to experiment start (unsaturated or saturated); 3) experiment duration (long or short); 4) sample geometry or extraction (cube or core); and 5) sample volume. The Mariotte, unsaturated/saturated, long experiment for a single soil cube resulted in unreliable mean anisotropy results, where a differed in sign. Generally, the sequence of measurements had a negligible impact on a for a siphon, saturated, short experiment for a soil cube. Furthermore, different a were obtained by varying the undisturbed soil sample collection procedure (cube vs. core). The conclusion of this investigation was that using a siphon and a short-duration run on an initially saturated cube of soil encased in foam is generally expected to yield reliable bi-directional Ks results. However, a check of the independence of the estimated anisotropy on the order of measurements for the sampled soil is recommended. An alternative procedure to determine a mean anisotropy for an area of interest would be to measure Kv and Kh on different soil cubes. Finally, the ratio between the mean Kh and Kv results varied from a not statistically significant factor of 1.02 to a statistically significant factor of 1.95 during the one-year investigation period (five sampling dates). Therefore, anisotropy of this sandy–loam soil varied with time but it was always low or negligible. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The hydraulic conductivity of saturated soil, Ks, is one of the most important soil properties controlling many hydrological processes. This property depends on soil texture, particle arrangement, and structure and can vary in space, time, and flow direction. In anisotropic soils, the vertical saturated hydraulic conductivity, Kv, of a given volume of soil differs from the horizontal saturated conductivity, Kh, of the same volume of soil (Beckwith et al., 2003). The anisotropy of Ks has been measured for peat (Chason and Siegel, 1986; Schlotzhauer and Price, 1999; Beckwith et al., 2003; Surridge et al., 2005) and mineral soils (Bouma and Dekker, 1981; Dabney and Selim, 1987; Bathke and Cassel, 1991; Caris and Van Asch, 1991). However, anisotropy is not routinely determined because practical and validated methods are still lacking.
⁎ Corresponding author. E-mail address: [email protected] (V. Bagarello). 0016-7061/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2009.09.012
Furthermore, anisotropy of Ks may change with time (Petersen et al., 2008), but data on temporal variability of anisotropy is scarce. The constant-head laboratory permeameter (CHP) method (Klute and Dirksen, 1986) is widely used for measuring Ks. There are two primary methods for obtaining undisturbed soil samples: 1) Two-Core Method (TCM, Dabney and Selim, 1987; Bathke and Cassel, 1991; Dörner and Horn, 2006; Petersen et al., 2008) and 2) Cube Method (CM, Bouma and Dekker, 1981) or Modified Cube Method (MCM, Beckwith et al., 2003). Anisotropy measured on soils extracted using TCM involves measuring Kv and Kh on two different samples where cylinders in both the vertical and the horizontal direction are pushed into exposed soil surfaces. On the other hand, CM and MCM measure Kv and Kh on a single soil sample where the soil sample is obtained by carving a soil cube in situ by gently removing soil along its sides. With CM, all except two opposing cube faces are encased in a slurry of gypsum. When the gypsum is cured, the soil cube encased in gypsum is removed from the pit and water flow rates through open sides are measured in the laboratory, yielding a measurement of Kv. The faces are then sealed with gypsum and the cube is rotated. Two faces at right angle to the original
V. Bagarello et al. / Geoderma 154 (2009) 52–58
faces are exposed and Kh is measured. With MCM, all faces of the cube are encased with gypsum, molten wax (Surridge et al., 2005) or expandable polyurethane foam (Bagarello and Sgroi, 2008) to ensure that all faces receive the same treatment and to minimize smearing of the exposed faces (Beckwith et al., 2003). Using a cube with removable faces is reasonably practical for sampling near-surface soil but it may become impractical at larger depths. More reliable anisotropy measurements may be expected with CM and MCM than TCM for several reasons. One reason is that Kv and Kh measured on a single soil sample is only affected by scale-dependent anisotropy whereas both anisotropy and small scale heterogeneity may influence Ks measurements made on different cores (Beckwith et al., 2003). The potential advantage of CM and MCM is particularly important when anisotropy has to be determined at a fixed sampling point but it is less important when an area of interest has to be characterized by a mean anisotropy. Another reason is that, in several cases, more representative soil samples for a selected site may probably be obtained with CM and MCM than TCM. When a cylinder is used to collect a soil core, shattering, puddling, or compaction of the soil may occur during sampling (e.g., Bouma et al., 1976; Topp et al., 1993). As a result, short-circuiting flow along edges of the soil core may occur during Ks measurements due to the presence of gaps between the soil column and the rigid cylinder (e.g., Cameron et al., 1990; Hoag and Price, 1997). Therefore, soil core disturbance due to the TCM can yield unreliable Ks results. On the other hand, the soil volume obtained by CM and MCM does not receive any particular disturbance prior to Ks measurement. Moreover, the cube is enclosed in a tightly fitting cast that conforms to any irregularities in the exposed soil surface before hardening and prevents edge flow (Bouma and Dekker, 1981; Beckwith et al., 2003; Surridge et al., 2005; Bagarello and Sgroi, 2008). In spite of the advantages of CM and MCM, experimentally induced alterations to the soil sample may occur that causes the second measurement to be affected by the first measurement when Kv and Kh are measured, in a pre-established sequence, on a single soil cube. Beckwith et al. (2003) suggested that a peat sample may not wet fully before the first measurement but may be closer to complete saturation before the second measurement. In addition, migration, rearrangement, and removal of soil particles within the porous medium (i.e., selffiltration) can significantly affect Ks measurements (Dikinya et al., 2008). If self-filtration occurs, the Ks measurement varies with the duration of the experiment and as a result the second measurement is conducted on a soil sample altered to some degree by the first measurement. Therefore, the estimated anisotropy may depend on the sequence of Ks measurements but the degree of this dependency is uncertain. To our knowledge, the reproducibility of Kv and Kh on a single cube was investigated only in a few studies (e.g., Beckwith et al., 2003). In general, specific assessments regarding the effects of the experimental set-up and method on the estimated anisotropy are rare in research literature and it is evident that the development of standardized procedures for determining anisotropy is necessary. The general objective of this investigation was to test different procedures for measuring anisotropy of Ks for a sandy–loam soil using the CHP method. The objectives are to: i) establish the dependence of the measured anisotropy by the MCM on the sequence of Ks measurements for different CHP procedures; ii) determine the effect of different soil sampling procedures and sample volume on anisotropy measurements; and iii) determine the temporal variability of anisotropy for the sampled area. 2. Materials and methods 2.1. Location A flat area supporting a citrus orchard was used for this study at the Faculty of Agriculture of the Palermo University. The study was conducted on a soil (Typic Rhodoxeralf) having a relatively high sand
53
(>50%) and gravel (13%) content. According to the USDA classification (Gee and Bauder, 1986), most textures of 12 replicated samples of soil material (i.e., particles smaller than 2 mm in diameter) were sandy– loam (Bagarello and Sgroi, 2008). The experiment was carried out in the period ranging July 2007 (JUL07) to October 2008 (OCT08). The experimental site was not disturbed (i.e., no tillage was performed) from 2002 until sampling was completed. A few additional experiments were carried out with the clay soil (clay = 62.4%, silt = 28.0%) of the Sparacia experimental installation for soil erosion measurement (Bagarello and Ferro, 2004; Bagarello et al., 2008). In this case, sampling was carried out on a bare surface having a 15% slope. 2.2. Soil sampling procedure Soil samples were collected on seven sampling dates in the sandy– loam soil (Table 1). In particular, 20 soil cubes were randomly collected on each sampling date within an area of approximately 10 m2. A few cm of soil were removed from the surface and a soil prism (0.11 × 0.11 × 0.14 m3) was carved out. A four-sided wooden frame (side length = 0.17 m), with an open bottom and top with inner walls covered by plastic wrap to avoid foam adhesion to the frame (so that the same frame may be re-used in other experiments), was placed around an excavated and exposed soil prism. Water was sprayed on the inner walls of the box and expandable polyurethane foam was injected between the box and soil and on the exposed soil surface (Bagarello and Sgroi, 2008). Then, wood and a weight, not more than 40–50 N, were placed on top of the frame to confine foam expansion. The cube was marked to preserve direction. Next, the cured cast with the enclosed soil was excavated and turned upside down. A layer of soil 0.03 m thick was removed from the bottom of the excavated prism to achieve a cube of soil and also encased in expandable foam. On both June 2008 (JUN08) and OCT08, 10 vertically-oriented and 10 horizontally-oriented soil cores were collected in randomly selected locations within the sampled area by using stainless steel cylinders of 0.085 m in diameter by 0.115 m in height (long soil cores). On OCT08, additional 10 vertically-oriented and 10 horizontally-oriented soil cores were collected using stainless steel cylinders of 0.05 m by 0.05 m (short soil cores). On each sampling date, a total of 16 undisturbed soil cores (0.05 m dia. by 0.05 m long) were also collected at a depth of 0.02 to 0.07 m and 0.07 to 0.12 m near the location of the soil cubes that were collected to determine soil bulk density, ρb, and initial volumetric soil water content, θi, using the ovendrying method, and were averaged over the two depths. Ten soil cubes for Ks measurement and eight undisturbed soil cores for ρb and θi measurement were collected in the clay soil on July 2008 (JUL08). 2.3. Laboratory methods In the laboratory, the cube was taken from the wooden box and the foam was removed from both the top and bottom faces using a box cutter. A photographic sequence of the application procedure of the MCM with expandable foam was given by Bagarello and Sgroi (2008). The vertical conductivity was measured before the horizontal conductivity for ten randomly chosen cubes, and the symbols Kv1 and Kh2 were used to denote the Kv and Kh results for these cubes. The opposite sequence (i.e., horizontal measurement before the vertical) was applied for the remaining soil cubes, and the symbols Kv2 and Kh1 were used in this case. Each soil cube was placed on a nylon guard cloth and a wire net to support the weight of the soil before starting the Ks test. In all cases, the CHP method with an established ponding depth of 0.015 m was applied to measure Ks. An important point that has to be taken into account is the possible effect of the wetting fluid on the measurement. Using a fluid maintaining soil structure during the test is desirable. On the other hand, water is generally the permeating fluid, and soil structure alteration may be promoted by this fluid in the field. Previous investigations showed that various SAR levels did not cause appreciable
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V. Bagarello et al. / Geoderma 154 (2009) 52–58
Table 1 Mean, mA, and coefficient of variation, CV, of the soil bulk density, ρb (Mg m− 3), and the initial soil water content, θi (m3 m− 3), for each sampling date. Texture
Sandy–loam
Clay
#
1 2 3 4 5 6 7 8
Sampling date
JUL07 SEP07 NOV07 JAN08 MAR08 JUN08 OCT08 JUL08
ρb
θi
mA
CV
mA
CV
1.169 1.200 1.306 1.238 1.200 1.236 1.223 1.240
0.075 0.065 0.074 0.075 0.061 0.079 0.064 0.075
0.116 0.091 0.253 0.228 0.223 0.116 0.179 0.460
0.369 0.194 0.101 0.077 0.135 0.287 0.134 0.195
Experimental set-up
Sample type
# of samples
MUL MSL SSS SSS SSS SSS SSS SSS
Cube Cube Cube Cube Cube Cube + Long Core Cube + Long Core + Short Core Cube
20 20 20 20 20 20 cubes + 20 cores 20 cubes + 20 long cores + 20 short cores 10
clay dispersion or change in soil structure of the sampled sandy–loam and clay soils (Bagarello et al., 2006). Local municipal tap water is often used for measuring Ks since it is an adequate approximation to native water, i.e. water extracted from the porous medium (Reynolds et al., 2002). For these reasons, tap water (Ca2+ + Mg2+ and Na+ concentrations equal to 2.20 and 5.49 mmol L− 1, respectively; SAR = 3.7; pH= 7.0) was used in this investigation. Three different CHP procedures were used in this investigation with three factors including water ponding type (Mariotte, M, or siphon, S), 2) saturation state prior to experiment start (unsaturated, U, or saturated, S), and 3) experiment duration (long, L, or short, S). The acronyms MUL, MSL and SSS were used, depending on the combination of these factors. MUL was applied for the sampling of JUL07 where a Mariotte (M) bottle was used to establish and maintain the constant depth of ponding on the soil surface (Fig. 1e in Bagarello and Sgroi, 2008) since this apparatus is cheap, easily manufactured, and simple to use. The soil was unsaturated (U) before performing the Ks measurement because most natural and man-made infiltration processes result in significant air entrapment within the porous medium (Reynolds, 1993). In other words, the so-called field-saturated hydraulic conductivity, or satiated conductivity, was measured (e.g., Odell et al., 1998). A long-duration (L) experiment (six or more hours) was carried out for measuring Kv and Kh to be reasonably sure of considering steady-state flux density in the calculations. After the first measurement of Ks, the cube was allowed to drain under gravity for three days. The second procedure, MSL, was applied for soil cubes collected on September 2007 (SEP07). The Mariotte bottle was used and a longduration experiment was carried out. However, the soil samples were slowly saturated (S) from the bottom before measuring both Kv and Kh. With an initially unsaturated soil sample, the second measurement of Ks is likely carried out in wetter soil conditions than the first Ks run. In particular, the soil may be initially dry or almost dry for the first run but it has an initial water content close to “field capacity”, FC, for the second experiment. If soil is saturated before measuring Ks, the initial water content does not vary between the two experiments. The third procedure, SSS, was applied for soil cubes and cores collected on five sampling dates between November 2007 (NOV07) and OCT08 (Table 1). Soil samples saturated (S) from the bottom were used but a siphon (S) was applied instead of the Mariotte bottle to establish the constant ponded head on the soil surface. Also a short-duration (S) experiment (i.e., approximately 30–40 min) was conducted instead of a long-duration. Using the Mariotte bottle with small ponding depths may promote turbulences close to the soil surface, potentially resulting in clogging of exposed pores. With a siphon, undesired surface soil alteration should be reduced. A shortduration experiment was considered to prevent or, at least, reduce soil alteration due to possible occurrence of time-dependent selffiltration during the experiment (Dikinya et al., 2008). The CHP method was run with water flowing from the soil surface downward. The last 15–20 min (SSS procedure) or 30–45 min (MUL and MSL procedures) of measured flow rates were used to calculate
Ks. According to Arya et al. (1998), initial soil saturation and alteration of soil surface during the experiment may have an appreciable impact on Ks values measured by the CHP method. The SSS procedure was also applied with soil cubes collected in the clay soil. In this case, Kv was measured before Kh in five randomly chosen cubes whereas the opposite sequence was applied for the other soil cubes. A reduced number of cubes were used for this soil because the objective was to check the ability of the tested procedure to detect soil effects on measured anisotropy. 2.4. Data analysis For a soil cube, the anisotropy estimated with a given measurement sequence was expressed as a = log10(Kh/Kv) (Beckwith et al., 2003). An a value of 0 indicates Kh =Kv. Positive a values indicate Kh >Kv whereas negative a values indicate Kv >Kh. Thus, a equal to 0.3 indicates that Kh is twice Kv, and −0.3 indicates that Kv is twice Kh. The a value has the same magnitude but changes sign when the factor of difference is the same, which would not be the case with the untransformed ratio (the scores would be 2 and 0.5, respectively) (Beckwith et al., 2003). Therefore, two sets of a results were obtained for a given sampling date, i.e. a1 = log10(Kh2/Kv1) and a2 = log10(Kh1/Kv2). Ks data were assumed to be log-normally distributed since the statistical distribution of these data is generally log-normal (Lee et al., 1985; Warrick, 1998). The geometric mean, mG, and the associated coefficient of variation, CV, were calculated to summarize Ks values (Lee et al., 1985). The arithmetic mean, mA, was calculated for ρb, θi and a, a1 and a2. Statistical comparison between two sets of data was conducted using two-tailed t-tests, whereas the Tukey Honestly Significant Difference test was applied to compare three or more sets of data. The ln-transformed Ks data were used in the statistical comparison. A onetailed t-test was applied to evaluate statistical significance of the correlation between two variables. A probability level, P = 0.05, was used for all statistical analyses. 3. Results and discussion 3.1. Bulk density and initial soil water content The mA(ρb) values of the sampled sandy–loam soil ranged between 1.17 and 1.31 Mg m− 3 during the investigation period whereas mA (θi) ranged from 0.091 to 0.253 m3 m− 3 (Table 1). According to Iovino (1998), in the sampled area, the soil water content corresponding to FC (i.e., pressure head value, h = − 0.033 MPa), and the permanent wilting point, PWP (i.e., h = −1.5 MPa), is equal to 0.256 m3 m− 3 and 0.140 m3 m− 3, respectively. Therefore, the soil water content during the experiment varied from substantially less than the PWP value (on JUL07, SEP07 and JUN08) to a value close to FC (on NOV07), and thus provided a good range of soil water content values for examining temporal variability of Ks anisotropy in the experimental area. For the clay soil, mA(ρb) = 1.24 Mg m− 3 and mA(θi) = 0.460 m3 m− 3 was obtained (Table 1).
V. Bagarello et al. / Geoderma 154 (2009) 52–58
3.2. Sequential effect on anisotropy For both MUL and MSL, Table 2 shows that mG(Kv1) ≠mG(Kv2), mG (Kh1) ≠mG(Kh2), and mA(a1) ≠mA(a2). In particular, the sequence of measurements was found to have a noticeable effect on the estimated anisotropy, and on both sampling dates a > 0 (Kh >Kv) was obtained when Kh was measured before Kv whereas a < 0 (Kv >Kh) was obtained when Kh was measured after Kv. For a given variable (e.g., Kv or Kh), the second measurement always yielded lower results than the first measurement. Similar or slightly larger CV results were obtained for Kv2 and Kh2 as compared with the corresponding CVs for Kv1 and Kh1, respectively. The SSS procedure (NOV7, JAN08, MAR08, JUN08, and OCT08) yielded mG(Kv1) = mG(Kv2) or mG(Kh1) = mG(Kh2) for eight of ten considered data comparisons and mA(a1) =mA(a2) for four of the five sampling dates (Tables 3–5). However, a positive a (Kh >Kv) was obtained on all sampling dates for both measurement sequences. The CV for Kv2 and Kh2 were similar to the corresponding CVs of Kv1 and Kh1, respectively, for four of the 10 considered data sets (Kh, NOV07 and OCT08; Kv, JUN08 and OCT08). In the other cases, larger CV values were obtained for a given variable when this variable was measured in second. For the clay soil, mG(Kv1) =mG(Kv2), mG(Kh1) =mG(Kh2) and mA (a1) =mA(a2) (Table 6). Therefore, the statistical equivalence of two sets of data included in the comparison (Kv1 vs. Kv2, Kh1 vs. Kh2, a1 vs. a2) largely prevailed in the sandy–loam soil and it was also detected for a different soil, suggesting that, in general, the mean anisotropy results did not vary with the sequence of measurements. In a few cases, however, the second measurement yielded lower results than the first measurement. Frequently, the second measurement was more variable than the first measurement. At the scale of the small plot, as the one considered in this investigation (10 m2), a random sampling of N = 10 soil cubes should be sufficient to determine a representative mean value of anisotropy of Ks given, for example, that N = 10 may be a practical sample size for characterizing soil hydraulic conductivity at a much larger scale, such as the agricultural field scale (Reynolds et al., 2002). Therefore, two different random samplings with N = 10 for each sampling should yield mean results that do not vary significantly between the two sets of data. In other words, if mean anisotropy obtained by measuring Kv before Kh does not coincide with mean anisotropy obtained by measuring Kv after Kh, the possibility that the detected differences depend on soil heterogeneity (i.e., different results were obtained because two inherently different sets of data were collected) cannot be excluded but it should be reduced. Another possible conclusion is that the applied experimental procedure did not allow an accurate measurement of Ks. It is likely that MUL and MSL procedures yielded unreliable Ks results because the long experimental duration was responsible for particle detachment and transport processes within the soil sample, altering appreciably the initial configuration of soil porosity (Dikinya et al., 2008). Prolonged turbulence on the soil surface induced an additional delivery of soil particles clogging exposed pores (Bagarello et al., 2000) or migrating through the sample. As a result, the second experiment was
55
carried out on an altered soil sample and it determined Ks of a modified porous medium. The SSS procedure was generally accurate (i.e., mean anisotropy results not affected by the sequence of measurements) and it yielded more reliable results than the MUL and MSL procedures when the CHP method was used with cubes of soil encased in foam for determining anisotropy of Ks. Therefore, using a siphon instead of a Mariotte bottle and conducting a short-duration run instead of a longduration run on an initially saturated soil sample was generally effective to control soil degradation processes at the surface of, or within, the sample. In practice, applying the SSS procedure and a bi-directional measurement of Ks on a single soil cube to determine mean Ks anisotropy for an area of interest needs collecting a reasonably large number of cubes, to obtain a representative result, and also checking the independence of mean anisotropy results of the sequence of measurements. As suggested by this investigation, this independence should be the most common result. However, if an effect of measurement sequence is recognized, or feared, an estimate of mean Ks anisotropy can probably be obtained by measuring Kv and Kh on different soil cubes given that, in this case, each Ks measurement is obtained with a reduced disturbance of the sampled soil. Obviously, a larger number of soil samples have to be collected in the field to apply this alternative procedure. 3.3. Effect of soil sampling procedure and sample volume Neither the procedure used to collect an undisturbed soil sample (soil cubes vs. soil cores) nor the soil core volume (long soil cores vs. short soil cores) had a statistically significant effect on the measured Kv values (Tables 4 and 5). Although the soil core volume did not affect Kh, the soil cubes did yield higher Kh values than the soil cores on both sampling dates. Anisotropy was detected with the soil cubes (i.e., Kv1 ≠ Kh1) but not with the long and short soil cores (Kv = Kh). Therefore, the procedure applied to collect an undisturbed soil sample was another factor affecting the estimated anisotropy of Ks. An effect of the sampled soil volume on the detected differences for Kh cannot be excluded because the volume of the soil cube, long soil core and short soil core was of 1331, 653 and 98 cm3, respectively. However, this effect was not considered to be substantial in this investigation because cores of different sizes yielded statistically equivalent results for both Kv and Kh, and Kv did not vary significantly with the sampled volume. Another possible explanation is that appreciable soil compaction occurred when undisturbed soil cores were collected in the horizontal direction but not for a vertically-oriented sampling. When a vertical soil core is collected, the soil surrounding the external surface of the cylinder is removed as deepening proceeds and this reduces friction during insertion. Therefore, a relatively low pushing force is enough to drive the cylinder into the soil. With a horizontal soil core, the soil surrounding the cylinder is removed after that insertion has been completed. More significant frictional effects are expected with horizontal sampling because greater forces have to be applied on the top of the cylinder to drive it into the soil. Maybe other, more convincing,
Table 2 Geometric mean, mG, arithmetic mean, mA, and coefficient of variation, CV, of the vertical, Kv1 and Kv2, and horizontal, Kh1 and Kh2, saturated hydraulic conductivity, Ks (mm h− 1), for measurements made with the MUL (JUL07 sampling date) and MSL (SEP07) procedures. Experimental set-up
Sampling date
Statistic
Kv1
Kv2
Kh1
Kh2
MUL
JUL07
mG mA CV mG mA CV
372.2(a)†
144.6(a)
244.5(b)
190.5(b)
0.168 380.3(a)
0.266 184.8(a)
0.223 465.6(b)
0.287 182.9(b)
0.255
0.260
MSL
SEP07
0.248
0.378
a1 = log10(Kh2/Kv1)
a2 = log10(Kh1/Kv2)
− 0.29(c) 0.572‡
0.23(c) 0.644
− 0.32(c) 0.362
0.40(c) 0.311
† For a given sampling date, a statistical comparison was made between those values followed by the same letter. Means followed by the same letter enclosed in parentheses are significantly different (P = 0.05); means followed by the same letter, but not enclosed in parentheses, are not significantly different. ‡Calculated by considering the absolute value of the mean.
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V. Bagarello et al. / Geoderma 154 (2009) 52–58
Table 3 Geometric mean, mG, arithmetic mean, mA, and coefficient of variation, CV, of the vertical, Kv1 and Kv2, and horizontal, Kh1 and Kh2, saturated hydraulic conductivity, Ks (mm h− 1), for measurements made with SSS procedure on NOV07, JAN08 and MAR08 sampling dates. Experimental set-up
Sampling date
mG(Kh1)/mG(Kv1)
Statistic
Kv1
Kv2
Kh1
Kh2
SSS
NOV07
1.86
mG mA CV mG mA CV mG mA CV
526.2a(d)†
457.5a
980.2b(d)
1030.5b
0.434 591.8ad
3.119 627.6a
0.704 914.9bd
0.723 735.8b
0.688 933.5(a)d
1.381 368.5(a)
0.484 949.8bd
1.186 1210.4b
1.443
0.593
0.904
SSS
JAN08
SSS
1.55
MAR08
1.02
0.426
a1 = log10(Kh2/Kv1)
a2 = log10(Kh1/Kv2)
0.29c 0.899
0.33c 2.066
0.09c 4.597
0.16c 2.150
0.11c 2.073
0.41c 1.327
† For a given sampling date, a statistical comparison was made between those values followed by the same letter. Means followed by the same letter enclosed in parentheses are significantly different (P = 0.05); means followed by the same letter, but not enclosed in parentheses, are not significantly different.
explanations could be suggested. However, it should be noted that interpreting Ks results is a difficult enterprise because there is no independent Ks datum or benchmark upon which evaluations and judgments can be made (Reynolds et al., 2000). For a given experimental set-up (i.e., CHP method, undisturbed soil, fixed water quality and ponding type, etc.), the estimated Ks anisotropy showed a dependence on sampling procedures. This result may have practical interest since varying sampling procedures and sample volumes were applied by different authors to determine anisotropy. For example, Bouma and Dekker (1981) used large soil cubes (25 × 25× 25 cm3) covered with gypsum whereas Dörner and Horn (2006) and Petersen et al. (2008) used small soil cores (100 cm3 volume). According to this investigation, a comparison of results from different sources and/or use of these results for developing a data-base of anisotropy data should be carried out with some caution. 3.4. Temporal variability of Ks anisotropy The Kv1 and Kh1 results obtained with the SSS procedure were not significantly different on both JAN08 and MAR08 (1.02 ≤ mG(Kh1)/mG (Kv1) ≤ 1.55), whereas significant differences between the two sets of data were detected on the other sampling dates (NOV07, JUN08, OCT08; 1.66 ≤ mG(Kh1)/mG(Kv1) ≤ 1.95) (Tables 3–5). The correlation between the mG(Kh1)/mG(Kv1) ratio and either mA(ρb) or mA(θi) was
not statistically significant (determination coefficient, r2 ≤ 0.43, N = 5). Much higher discrepancies between Kv and Kh than the ones obtained in this investigation (i.e., by also a factor of 46000) were reported in the literature (Bouma and Dekker, 1981; Bathke and Cassel, 1991; Caris and Van Asch, 1991). In particular, mean conductivities differing with the considered direction by a factor of also 38 were obtained in another sandy–loam soil (Petersen et al., 2008). Therefore, the mean Ks anisotropy of this sandy–loam varied during the investigation period, but it was always low or negligible. Moreover, neither the bulk density nor the soil water content at the time of the measurements had a statistically detectable effect on the anisotropy results. The mean values of both Kv1 and Kh1 varied by a similar factor (i.e., 2.3–2.4) during the investigation period. In particular, the mG values of Kv1 differed according to the sequence OCT08 = MAR08≥ JUN08 = JAN08= NOV07, whereas the mG values of Kh1 differed according to the sequence OCT08 = JUN08 ≥ NOV07 = MAR08 = JAN08. Neither mG (Kv1) nor mG(Kh1) were significantly correlated with either mA(ρb) or mA(θi) (r2 ≤ 0.40, N = 5). Therefore, a statistically significant temporal variability was detected for both Kv1 and Kh1. The temporal patterns of these two variables did not coincide but they showed similarities, given that higher results were obtained on the last sampling dates than the first ones in both cases. The relationship between mG(Kv1) and mG(Kh1)
Table 4 Geometric mean, mG, arithmetic mean, mA, and coefficient of variation, CV, of the vertical, Kv1, Kv2, and Kv, and horizontal, Kh1, Kh2, and Kh, saturated hydraulic conductivity, Ks (mm h− 1), for measurements made with SSS procedure on JUN08. mG(Kh1)/mG(Kv1) or mG(Kh)/mG(Kv) Soil cubes
Long soil cores
1.95
0.95
Statistic
Soil cubes
Long soil cores
Kv1
Kv2
Kh1
Kh2
mG mA CV
701.7(a)(d)e†
402.9(a)
1371.3b(d)(f)
999.4b
0.387
0.366
0.313
0.559
a1 = log10(Kh2/Kv1) 0.15(c) 1.216
a2 = log10(Kh1/Kv2) 0.53(c) 0.144
Kv
Kh
583.7eg
552.5(f)g
0.685
0.421
† For a given sampling date, a statistical comparison was made between those values followed by the same letter. Means followed by the same letter enclosed in parentheses are significantly different (P = 0.05); means followed by the same letter, but not enclosed in parentheses, are not significantly different.
Table 5 Geometric mean, mG, arithmetic mean, mA, and coefficient of variation, CV, of the vertical, Kv1, Kv2, and Kv, and horizontal, Kh1, Kh2, and Kh, saturated hydraulic conductivity, Ks (mm h− 1), for measurements made on OCT08 by applying the SSS procedure to soil samples collected with different procedures. mG(Kh1)/mG(Kv1) or mG(Kh)/mG(Kv)
Statistic
Soil cubes
Long soil cores
Short soil cores
1.66
0.73
1.10
mG mA CV
Soil cubes
Long soil cores
Kv1
Kv2
Kh1
Kh2
1288.0a(d)e†
1440.1a
2137.8b(d)(f)
1951.9b
0.543
0.528
0.429
0.396
a1 = log10 (Kh2/Kv1)
a2 = log10 (Kh1/Kv2)
0.18c 1.387
0.17c 0.941
Short soil cores
Kv
Kh
Kv
Kh
806.2eg
586.7 fg
953.0eh
1045.9fh
0.385
0.871
0.474
0.475
† For a given sampling date, a statistical comparison was made between those values followed by the same letter. Means followed by the same letter enclosed in parentheses are significantly different (P = 0.05); means followed by the same letter, but not enclosed in parentheses, are not significantly different.
V. Bagarello et al. / Geoderma 154 (2009) 52–58
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Table 6 Geometric mean, mG, arithmetic mean, mA, and coefficient of variation, CV, of the vertical, Kv1 and Kv2, and horizontal, Kh1 and Kh2, saturated hydraulic conductivity, Ks (mm h− 1), values obtained with the SSS procedure at the Sparacia sampling area. mG(Kh1)/mG(Kv1)
Statistic
Kv1
Kv2
Kh1
Kh2
7.35
mG mA CV
106.0a(d)
187.3a
778.7b(d)
1080.8b
2.021
2.278
0.455
1.147
a1 = log10(Kh2/Kv1)
a2 = log10(Kh1/Kv2)
1.01c 0.505
0.62c 0.866
† For a given sampling date, a statistical comparison was made between those values followed by the same letter. Means followed by the same letter enclosed in parentheses are significantly different (P = 0.05); means followed by the same letter, but not enclosed in parentheses, are not significantly different.
was scattered but also statistically significant (r2 = 0.67). A much higher r2 result, equal to 0.96 (N = 4, r > 0), was obtained by neglecting the data point corresponding to the lowest mA(ρb) result for the investigation period. Therefore, a hypothesis deserving testing with a larger data set is that a one-directional measurement of Ks is enough to predict Ks anisotropy in relatively compacted soil conditions. An appreciably larger horizontal anisotropy was detected for the clay soil (mG(Kh1)/mG(Kv1) = 7.35, Table 6) than the sandy–loam one (mG(Kh1)/mG(Kv1) ≤ 1.95), showing that the applied procedure was able to detect differences in anisotropy between different soils. Larger Ks results than the ones obtained in this investigation (Table 6) are not uncommon for clay soils when macroporosity effects on soil water transport phenomena are significant (Bouma and Dekker, 1981).
investigation is important to develop an improved bi-directional measurement procedure of Ks, avoiding in particular the need for checking the independence of anisotropy results on the measurement sequence. For example, using alternative fluids for the experiment could be effective to establish the role of the wetting fluid on experimentally determined Ks data. Finally, the Ks anisotropy of the sampled sandy–loam soil varied significantly during an approximately one-year investigation period. However, it was found to be always low or practically negligible since the maximum difference between corresponding mean values of Kv and Kh was of a factor of 1.95.
4. Conclusions
This study was supported by grants from the Università degli Studi di Palermo (fondi ex 60%, Dottorato di Ricerca in Idronomia Ambientale) and the Sicilian Region (Progetto “DIFA – Digitalizzazione della Filiera Agroalimentare”, A.P.Q. Società dell'Informazione, Regione Siciliana). V. Bagarello and A. Sgroi set-up the research and wrote the paper. S. Sferlazza conducted most of the experimental work. All authors analyzed the results. S. Sferlazza developed this research for his PhD activity, which is co-funded by the European Social Fund of the European Community. The authors wish to thank the anonymous reviewers for the constructive comments and the time they were willing to expend in the revision of the manuscript.
In a sandy–loam soil, both the application procedure of the constanthead laboratory permeameter method and the undisturbed soil sampling procedure affected the estimated mean anisotropy of saturated soil hydraulic conductivity, Ks. Using a Mariotte bottle to establish a constant ponding depth on the soil surface and conducting a long experimental duration yielded questionable results that differed depending on the sequence of measurements carried out on a single soil cube encased in foam (vertical conductivity, Kv, measured before the horizontal one, Kh, or vice versa). This result was probably due to the occurrence of structure alteration at the surface of, and within, the sampled soil. The dependence of anisotropy on the sequence of measurements was greatly reduced when a siphon and a short-duration run were used to measure Kv and Kh of an initially saturated soil cube (SSS procedure). However, an occasional effect of the sequence of measurements on the mean anisotropy was also detected in this last case. The estimated anisotropy of Ks varied with sampling procedure (soil cube, soil core). Therefore, comparing results obtained with different sampling procedures was not recommended. The SSS procedure and a bi-directional measurement of Ks on a single cube of soil was suggested as the best procedure among the tested ones for determining mean Ks anisotropy of an area of interest. A reasonably large number of soil cubes have to be used to deduce a representative result, and the independence of estimated mean anisotropy of the order of measurements must be verified. An alternative procedure could involve measuring Kv and Kh on different soil cubes given that disturbance of the sampled soil is expected to be substantially reduced if a single, short-duration experiment is conducted on a single soil cube. In general, a procedure minimizing any possible alteration of the tested soil sample should be applied to obtain reliable Ks values, but this recommendation is particularly important for a bi-directional measurement of Ks on a single soil sample. In this case, the risk of altering the sampled soil during the measurement process is particularly high since stresses are doubled. Several factors that may affect the laboratory measurement of Ks (for example, chemical composition of the water solution used for the experiment, upward or downward flow, established head, soil sample size) were not considered in this investigation since common and practical procedures and sample sizes were used. Testing the effects of these additional factors and including other soils in the
Acknowledgements
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