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Influence of well preparation on field-saturated hydraulic conductivity measured with the Guelph Permeameter
uaV'l~ ,7)q11 ELSEVIER
GEODEP~MA Geoderma 80 (1997) 169-180
Influence of well preparation on field-saturated hydraulic conductivity measured with the Guelph Permeameter V i n c e n z o Bagarello Dipartimento E.I.T.A., Settore ldraulica, UniL,ersith degli Studi, Viale delle Scienze, 90128 Palermo, Italy Received I May 1996; accepted 14 May 1997
Abstract
An investigation on the influence of the well preparation technique on field-saturated hydraulic conductivity, Kr~, measured with the Guelph Perrneameter (GP) method was performed in a sandy loam soil. In particular, the influence of adopting the following procedures to prepare the well was investigated: use of a wire screen insert to prevent sinking of the water outlet tip of the GP into the base of the well during a measurement; use of a metallic, sharpened rod to remove the smear layer created during the digging process from the well walls. The influence of the well radius and the antecedent soil water content on the Kr, estimates was also evaluated. Five different types of wells were prepared. Higher and less variable KI:~ values were obtained from the wells treated with the wire screen insert and the metallic rod than from the non-treated ones. These results were consistent with the hypothesized occurrence of sinking and smearing phenomena in the non-treated wells and with the effectiveness of the used procedures to prevent or reduce the influence of these phenomena on the Kr~ measurement. The estimates of field-saturated hydraulic conductivity increased with the radius of the well. Differences between the sampled soil volumes and a more pronounced influence of sinking phenomena in the small-size wells likely contributed to produce this result. In relatively dry soil conditions, the antecedent soil water content did not affect the Kr~ measurements, independently of the well preparation technique. @ 1997 Elsevier Science B.V.
Keywords." saturated hydraulic conductivity; Guelph Permeameter; site preparation; soil moisture
1. Introduction
Establishing reliability and usefulness of field and laboratory methods for measuring saturated hydraulic conductivity often involves comparisons among estimates obtained by different methods (Dorsey et al., 1990; Gallichand et al., 0016-7061/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S001 6-706 1(97)0005 I-7
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V. Bagarello / Geoderma 80 (1997) 169-180
1990; Gupta et al., 1993; Kanwar et al., 1989; Mohanty et al., 1991; Paige and Hillel, 1993). Since several factors affect the results of a particular method, these comparisons often show different trends under different soil types and field conditions (Mohanty et al., 1994; Reynolds and Zebchuk, 1996). Taking into account the influence of factors affecting conductivity estimates is therefore necessary to individuate sources of data variability and, as a consequence, to increase confidence in the results of comparisons among methods. The Guelph Permeameter (GP) method (Reynolds and Elrick, 1986) is one of the most widely applied in-situ constant head well permeameter methods [or determining field-saturated hydraulic conductivity, Kf~ (L T-~). This method involves digging a small, vertical, cylindrical well and determining the steady water discharge, Q~ (L 3 T - l ) , when a constant depth of water, H (L), is maintained in the well. Smearing a n d / o r compaction of the well surface during the digging process, sinking of the water outlet tip of the instrument into the base of the well during a measurement, and radius of the well can affect field-saturated hydraulic conductivity measured with the Guelph Permeameter. Auger-induced smearing and compaction of the well surfaces result in artificially low Kt.~ values, especially when the soil is fine-textured and the soil water content is high at the time of augering (Koppi and Geering, 1986; Asare et al., 1993; Reynolds, 1993; Bagarello and Provenzano, 1996). Reynolds (1993) suggested the following steps for minimizing the influence of smearing and compaction on Kt.~: augering when the soil is relatively dry; using a very sharp auger; applying very little downward pressure on the auger; taking only small bites with the auger before emptying it. If inspection of the well reveals smearing within the measurement zone, a small, spiked roller mounted on a handle should be used to pluck off the smeared/compacted layer. Recently, Campbell and Fritton (1994) showed in a structured clayey soil that a smear layer can be removed from well walls using an ice pick to expose individual peds and, therefore, to enhance representativeness of GP results. A quick-setting epoxy resin was used by Koppi and Geering (1986) to prepare unsmeared, hemi-spherical or flat surfaces for infiltration measurement. Sinking of the water outlet tip into the base of the well during a measurement can produce an underestimate of Kfs because the actual H-level is lower than the assumed level, and because outflow from the GP can be reduced by a smaller effective infiltration area. Reynolds (1993) suggested that subsidence of the GP can be prevented by clamping the GP reservoir to a rigid tripod so that the weight of the instrument is carried by the legs of the tripod rather than by the outflow tip. GP subsidence can be also prevented by placing a layer of pea gravel under the tip, and then recalibrating the H-level (Reynolds, 1993). To our knowledge, however, sinking effects on Kf~ have not been quantified. Finally, there is some evidence that Kt ~ values determined by the GP method tend to increase with the radius of the well (Reynolds and Elrick, 1985; W.D.
V. Bagarello / Geoderma 80 (1997) 169-180
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Reynolds, pers. commun., 1994). As reported by Reynolds and Elrick (1985), these results can reflect an auger a n d / o r a sample size effect. Although well preparation has been discussed by different authors, few investigations have been carried out in coarse-textured soils, probably since well preparation is less critical in these soils than in the fine-textured ones. Moreover, there are still relatively few data on the effect of the antecedent soil water content on measured field-saturated hydraulic conductivity. For example, it has been proved only recently that soil structural variability can produce a negative trend of Kf~ versus antecedent soil water content (Reynolds and Zebchuk, 1996). In this study, an experimental investigation was carried out on the influence of the well preparation technique on field-saturated hydraulic conductivity measured with the GP in a relatively coarse-textured soil. In particular, the influence of adopting the following procedures to prepare the well was investigated: use of a wire screen insert to prevent sinking of the tip of the instrument during a measurement; use of a metallic, sharpened rod to remove the smear layer from the well walls. The influence of the well radius on the Kfs estimates was also evaluated. Finally, the relationship between field-saturated hydraulic conductivity and soil water content at the time of augering the well was examined.
2. Materials and methods The study was conducted from May to July 1995 in a 200-m 2 experimental area, supporting a citrus orchard, neighbouring to the site already considered by Bagarello and Provenzano (1996). Table 1 shows minimum, maximum, and mean (/x) values of clay, silt, sand and gravel content of fifteen soil cores taken at a depth of 0.15 m. The soil texture of most samples was classified as sandy loam; some samples were classified as sandy or sandy clay (ISSS classification) (Gee and Bauder, 1986). Two hand-held augers of different radius (0.025 m and 0.030 m), a wire screen insert (Fig. !), and a metallic, sharpened rod were used to prepare the
Table 1 Soil textural characteristics at the experimental site Size class Clay ( < 2 Ixm) Silt (2-20 Ixm) Sand (20-2000 Ixm) Gravel ( > 2000 txm)
Mean (,a)
Minimum
Maximum
(%)
(%)
(%)
14,2 22,2 63.6 16.4
10.7 19.1 58.0 9.6
19.4 25.7 67.3 23.8
V. Bagarello / Geoderma 80 (I 997) 169-180
172
@
N+o.o6+i Fig. 1. Schematic representation of the wire screen insert.
wells. The wire screen insert, connected to a wooden surface support, was used to prevent a possible sinking of the water outlet tip of the permeameter into the base of the well during a measurement. The metallic rod was used to remove any smeared/compacted area from the well wall, in an attempt to expose undisturbed soil at the wall of the well. Five different well treatments (15 wells for each treatment) were randomly prepared in the study area. Table 2 summarizes the preparation method corresponding to each well treatment. The wells were dug to a depth of 0.15 m. In each well, steady discharge values corresponding to the H~ = 0.03 m and H 2 = 0.05 m depth of ponding values were measured. The rate of fall of the water level in the GP reservoir was monitored until it did not change in four consecutive 2-rain time intervals. Each measurement was carried out immediately after having excavated the well. As an index of the antecedent soil wetness, the gravimetric moisture content, MC (kg k g - 1), of the last volume of disturbed soil removed by the soil auger during well excavation was determined. After each field measurement, a mixture of plaster and quick-setting cement was poured into the well to obtain a mould of the well. After hardening, the Table 2 Summary of the well preparation methods used for each type of well Well treatment
Radius of the auger
Metallic rod
Wire screen insert
no no yes no yes
no no no yes yes
(m) F5 F6 F5P F6R F5PR
0.025 0.030 0.025 0.030 0.025
Y e s / n o : the treatment was u s e d / n o t used.
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mould was taken out and sawn into two parts; the lower end, 0.06-0.07-m-long, was saturated and then plunged in a known volume of water to determine an actual, average radius of the well in the measurement zone, a m (m). The a m values and the single height (SH) analysis (Elrick and Reynolds, 1992) were used to estimate Kfs (m s-l). A constant ratio a * = 36 m ( a * = Kfs/ck m, ~bm being the matric flux potential) was assumed for this analysis (Elrick and Reynolds, 1992; Bagarello and Provenzano, 1996). For each well, results of the two single height analyses (one with H~ = 0.03 m, and the other with H 2 = 0.05 m) were averaged to obtain an estimate of Kf~ (Elrick and Reynolds, 1992). According to the results of the Kolmogorov-Smirnov test, the normal frequency distribution of the In Kfs values was assumed for the statistical data analysis. The following approaches were compared to inspect the differences among lnKf~, a m and MC mean values calculated for each well treatment: (i) two groups of data were considered at a time; a t-test or a modified t-test were applied after verifying the homogeneity of variance by an F-test (Helsel and Hirsch, 1992; Piccolo and Vitale, 1984); (ii) all groups of data were considered at the same time and the Tukey Honestly Significant Difference test (Daniel, 1996) was conducted. For subsequent analysis, results of the Tukey Honestly Significant Difference test were considered. This test was the least prone to reject the hypothesis that two mean values were equal. Common statistical techniques (F- and t-tests) were used to analyse the correlation between field-saturated hydraulic conductivity and antecedent soil moisture content (Daniel, 1996). A 0.05 probability level was chosen for all the tests. 3. Results and discussion Table 3 summarizes arithmetic mean (/~), standard deviation (o-), coefficient of variation (CV), and geometric mean ( M ) of Kf~ values obtained in each type Table 3 Arithmetic mean (/z), standard deviation (o-), coefficient of variation (CV), and geometric mean ( M ) values of field-saturated hydraulic conductivity (Kf~), actual well radius ( a m), and gravimetric water content (MC) Well treatment F5 F6 F5P F6R F5PR
106 Kf~ (m s - I )
a m (m)
MC
/z
cr
CV
M
#
o-
CV
/x
22.9 39.5 52.4 59.3 73.2
14.9 20.8 27.3 19.3 23.7
0.651 0.527 0.521 0.325 0.324
16.6 a 35.1 b 46.0 bc 55.8 bc 68.0c
0.0232 a 0.0301 b 0.0283 c 0.0313 b 0.0307b
0.0013 0.0011 0.0020 0.0013 0.0015
0.056 0.037 0.071 0.042 0.049
0.127 0.128 0.114 0.110 0.118
a a a a a
o-
CV
0.034 0.034 0.027 0.016 0.025
0.268 0.266 0.237 0.145 0.212
Means in a column followed by the same lower case letter are not significantly different at P < 0.05.
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V. Bagarello / Geoderma 80 (1997) 169-180
of well. The statistics of the corresponding a m and MC values and the results of the Tukey Honestly Significant Difference test are also given. Antecedent gravimetric soil water content values ranging from 0.08 to 0.21 were measured during the runs. The mean MC values calculated for each well treatment were not statistically different. Some differences between the mean values of the actual well radii were found to be significant. Notwithstanding this, the range in well radii was very narrow: except for the F5 wells, the mean a m values fell between 0.028 and 0.031 m. For each well treatment, a low variability of the measured radii was recognized (CV _< 7%). For the desmeared wells (F5P, F5PR), this result confirmed that a uniform thickness of soil was removed from the well walls. The moulds of the unprotected wells (i.e., wells in which the wire screen insert was not used) occasionally showed a short basal protuberance, representing the track of the GP's tip, sunk a few millimetres into the base of the well during the run. This shape of the base of some moulds produced in the F5 wells a mean a m value of 0.023 m, which was less than the theoretical one of 0.025 m. Geometric m e a n Kfs values ( M ) ranging from 1.66 X 10 -5 m s -I to 6.80 X 10 -5 m s - ~ and coefficients of variation ranging from 0.32 to 0.65 were measured in the different well treatments. The lowest mean and the highest variability of Kf~ were obtained in the untreated wells (i.e., wells prepared by only using the hand-held auger). The highest mean and the lowest variability of Kf.~ were measured in the wells prepared by using the wire screen insert and the metallic rod. With the exception of the F5 wells (in which Kf~ values ranging from 3 . 9 4 × 10 - 6 m s ~ to 4.51X 10 -5 m s -~ were measured), similar minimum (1.5-2.3 X 10 -5 m s-~) and maximum (0.9-1.0 × 10 -4 m s- f) K~ values were obtained from the different well treatments. However, high Kfs values were more frequently obtained from the wells prepared by using both the metallic rod and the wire screen insert compared to the wells prepared by a single treatment and, to a greater extent, to the untreated wells (Fig. 2). The geometric mean Kfs values obtained from the three treated wells (F5P, F6R, F5PR) were not significantly different. The two untreated wells produced significantly different geometric mean estimates of Kf~. Most of the comparisons between the untreated wells and the treated ones showed significantly different geometric mean values of Kf~. The geometric mean of Kf~ values measured in the F6 wells for MC < 0 . 1 2 was equal to 3 . 9 9 × 10 -5 m s -~ (sample size N = 9); this value was not significantly different from the mean Kf~ value obtained in 1993 by Bagarello and Provenzano (1996) in a neighbouring area for low values of antecedent soil water content (see their tables 2 and 3: mean Kf~ = 3.90 X 10 -5 m s - I ; N = 36). Although the small size of the first Kf~ sample could affect the result of this comparison, the negligible difference between the two series of measurements performed in similar wells and in different years gave some support to the hypothesis (Bagarello and Provenzano, 1996) that the structure of the investigated soil was quite stable.
V. Bagarello / Geoderma 80 (1997) 169-180
175
/O
-.-F6 ]i i ~ - F5P o-F6R i • F5PR'
0.75
g .~_ 0.5
t~ c ~
D-f
-'/~
!
0/
/ /'
0.25
0 -13
I
I
I
-12
-11
-10
-9
In Kf~ Fig. 2. Empirical cumulative frequency distribution of the natural logarithms of field-saturated hydraulic conductivity, In Kf~, for each type of well.
The results summarized above showed that, for given well preparation technique (F5 and F6 wells), the well radius affected the measured Kes values. Reynolds and Elrick (1985) attributed the well radius effect to heterogeneity and macroporous structure of the soil. As a matter of fact, different soil volumes were sampled by the two types of well; soil volumes sampled by the F5 wells were probably too small and, therefore, unrepresentative of the soil structure at the field site. The occasional occurrence of sinking of the water outlet tip of the GP into the base of the well contributed to produce lower and more variable Kt:~ results in the small size wells. In fact, visual inspection of the moulds of the F5 and F6 wells showed a more pronounced influence of this phenomenon in the F5 than in the F6 wells. The comparison, for each well treatment, of the mean actual radii to the theoretical ones supported this conclusion. For a practically constant well size (mean of a m ---- 0 . 0 2 8 - - 0.031 m; F6, F5P, F6R and F5PR wells), the measured Kfs values showed a dependence on the well preparation technique. In particular, alteration of the infiltration surface was not enough to significantly change the geometric mean value of Kt:~ measured in the untreated F6 wells when only the metallic rod or the wire screen insert were used. However, using both treatments to prepare the well did modify the well characteristics in such a way that, on the average, water flow was greater than that measured in the untreated wells. Variability of Kfs measurements was minimized by applying the wire screen insert. These results suggested that smearing and sinking phenomena did significantly affect the mean GP results when they occurred simultaneously and that the changes in the predictions of Kf~ variability among the treatments were mainly attributable to the occurrence
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V. Bagarello / Geoderma 80 (1997) 1 6 9 - 1 8 0
Table 4 Intercept (b o), slope ( b i ) and coefficient of determination ( r 2) obtained by least-squares fitting a straight line through natural logarithm of field-saturated hydraulic conductivity versus antecedent soil water content (MC) or amount of fine-textured soil particle data for each well treatment Well Independent 0,08 < MC < 0.21 treatment variable b0 bi
0.08 < MC < 0.14 r2
sample b o
bL
r2
size F5 F6 F5P F6R F5PR F5PR F5PR
MC MC MC MC MC clay clay + silt
-8.2348 -21.8352s -9.3989 - 6 . 7 2 5 2 ns -8.9028 - 9 . 4 7 4 5 ns - 8 . 6 9 1 6 - 10.0308 ns -8.0411 -13.1843s -9.1889 - 0 . 0 2 8 8 ns - 11.6244 0.0557 ns
0,618 0.210 0.232 0.187 0,532 0.022 0.099
11 II 13 14 13
-8.5412 -18.7557ns -9.0997 - 9 . 5 6 6 2 ns -9.2128 - 6 . 2 5 6 0 ns - 10.5892 8.0784 ns -8.7772 -6.0545ns
0.156 0.045 0.034 0.066 0.118
s: significantly different from zero. ns: not significantly different from zero. P _< 0.05.
of tip sinking. The differences, 6 (m s-~), between the geometric mean Kf~ measured in the treated wells and the geometric mean Kf~ obtained from the untreated F6 wells were calculated. The 6 value corresponding to the F5PR wells was quasi-equal to the sum of the ~ values calculated for the F5P and F6R wells, suggesting an additive Kf~ underestimating effect of the mutually independent sinking and smearing phenomena. For each well treatment, an apparent linear trend of lnKf~ versus MC was observed. To quantify this trend, a straight line was fitted to the In Kt-~, MC data pairs. For each well treatment, Table 4 gives the least-squares estimates of the b 0 (intercept), b 1 (slope) and r 2 (coefficient of determination) values. For the F5PR wells, Table 4 also gives the fitting parameters and the coefficient of determination obtained by considering the amount of fine material (percentages of clay and clay plus silt) taken from the well bottom as the independent variable. For the F6, F5P and F6R wells, the In Kf~ and MC data were found to be not significantly correlated. For the F5 and F5PR wells, a statistically significant dependence of In K~, on MC was recognized (Fig. 3). In these wells, an increase of MC from 0.10 to 0.20 produced a decrease of Kf~ by a factor of 4 (F5PR wells) to 9 (F5 wells). The slopes of the In Kt:~ versus MC relationships were found to be not significantly different, suggesting that the dependence of In Kt~ on MC was similar in the two types of wells although the well preparation procedures were very different. The amount of fine material was not significantly correlated to the values of ln Kf~, showing that the low Kt~ values measured in the F5PR wells for high MC values were not the result of lower soil
V. Bagarello / Geoderma 80 (1997) 169-180 -9
ooeo
-10
•
OO ~• •
8b'-.
0
"°'''''''--..
O
...q
O
" " " " " -..0
-11
! '! • L......
-12
-13 0.08
177
I 0. I
[ 0.12
I 0.14
I 0.16
I 0.18
best fit (F5 wells) F5PRwells best ~t eFSVRwells) j
0.2
MC Fig. 3. Relationships between the natural logarithms of field-saturated hydraulic conductivity, In K~, and antecedent gravimetric soil water content, MC, for the F5 and F5PR wells.
permeability stemming from a finer soil texture. A trend between the well preparation technique and the statistical significance of the correlation between In Kfs and MC was not observed, Data inspection showed that most Kfs data were collected in dry soil conditions. In particular, 11 to 14 experiments (73% to 93% of all the experiments) were conducted for MC values less than 0.14 (the approximate mid point of the MC range), depending on the well type. The low number of ln Kfs and MC data pairs obtained in wet soil conditions likely influenced the contradictory results summarized above. The correlation between In Kfs and MC in the more evenly investigated range 0.08 < MC < 0.14 was also evaluated for each well treatment (Table 4). In all cases, the two variables were found to be not significantly correlated (0.034 < r 2 < 0.156), showing that the antecedent soil water content did not affect the Kf~ measurements in the range of relatively low MC values. For practical purposes, GP measurements are analysed by using a theoretical radius, a t ( L ) , equal to the radius of the auger. In this study, a t values could be used for the F5, F6 and F6R wells only since in the F5P and F5PR wells a thickness of soil was removed from the well walls. For the first three series of wells, some differences between the a m and the corresponding a t values were recognized (Table 3). The effect of these differences on the calculations of Kf~ was examined. For each F5, F6 and F6R well, Kes was estimated by using both the a t value and the measured well radius; the difference between the two estimates of Kfs, expressed as a percentage of the second one, was then calculated. For the F6 and F6R wells, the absolute values of the differences between the two series of Kf~ were always less than 8%; for the F5 wells, slightly higher differences (up to 14%) were observed. The magnitude of these differences is probably negligible, especially if compared to the errors produced
178
V. Bagarello/ Geoderma 80 (1997) 169-180
by many other factors (Elrick et al., 1990). As a consequence, measuring the actual radius of the well was not crucial, at least when a thickness of soil was not removed after well excavation; however, using a mould of the well was tound to be very useful to indicate irregular wells (for example, wells in which sinking phenomena occurred), potentially producing unrepresentative Kf~ estimates.
4. Summary and conclusions The objective of this study was to investigate the influence of the well preparation technique on field-saturated hydraulic conductivity, Kt~,, measured with the Guelph Permeameter (GP) method in a sandy loam soil. In particular, the influence of adopting procedures to prevent sinking of the tip of the instrument during a measurement and to remove the smear layer from the well walls on the Kf~ estimates was examined. The influence of the well radius on Kf~ was also evaluated. Finally, the relationship between Kt~ and antecedent soil water content was examined. A wire screen insert was used to support the weight of the instrument; a metallic, sharpened rod was used to expose natural soil aggregates at the well walls (i.e., to remove the smear layer caused by the auger). For each well, the actual, mean radius of the well was determined after the GP test by using moulds of the well. Using the wire screen insert and the metallic rod produced increasing estimates of mean Kf~ and decreasing estimates of Kf~ variability, suggesting that sinking of the water outlet tip of the instrument and smearing a n d / o r compaction of well walls affected the GP results in the non-treated wells. On the average, an additive Kf~ underestimating effect of the two phenomena was recognized. Inspection of the moulds of the unprotected wells showed that sinking of the GP tip occurred occasionally. Estimates of field-saturated hydraulic conductivity increased with the radius of the well. Differences between the sampled volumes and a more frequent occurrence of sinking phenomena in the small-size wells likely contributed to this result. Notwithstanding that statistically significant differences were found, the mean Kf~ values obtained from different well types sampling similar soil volumes were relatively similar. As a consequence, it can be concluded that the influence of the factors considered in this study on the Kf~ estimates could be considered negligible for some practical applications concerned only with the order of magnitude of Kfs. However, using both the wire screen insert and the metallic rod to prepare the well for a GP measurement seems useful to enhance representativeness of the estimates (mean values, variability).
V. Bagarello / Geoderma 80 (1997) 169-180
179
Removing a thickness of soil from the well walls implies that a measurement of the actual well radius has to be performed. This could be accomplished by preparing a mould of the well after the GP test; this procedure also allows to individuate irregular wells which can produce unrepresentative Kfs results. Although an apparent, decreasing trend of field-saturated hydraulic conductivity versus antecedent soil water content was observed for each type of well, a significant correlation was observed only occasionally. This result was attributed to the unbalanced sample composition. In relatively dry soil conditions, the antecedent soil water content did not affect the Kr.~ measurements, independently of the well preparation technique.
Acknowledgements This research was supported by a grant of the Italian Ministero dell'Universith e della Ricerca Scientifica e Tecnologica. The author sincerely thanks Dr. W.D. Reynolds and the anonymous reviewers for their helpful comments and for the help in improving the quality of the manuscript.
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Koppi, A.J., Geering, H.R., 1986. The preparation of unsmeared soil surfaces and an improved apparatus for infiltration measurements. J. Soil Sci. 37, 177-181. 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. Mohanty, B.P., Kanwar, R.S., Everts, C.J., 1994. Comparison of saturated hydraulic conductivity measurement methods for a glacial till soil. Soil Sci. Soc. Am. J. 58, 672-677. Paige, G.B., Hillel, D., 1993. Comparison of three methods for assessing soil hydraulic properties. Soil Sci. 155 (3), 175-189. Piccolo, D., Vitale, C., 1984. Metodi statistici per l'analisi economica. I1 Mulino, Bologna, 747 pP. Reynolds, W.D., 1993. Saturated hydraulic conductivity: field measurement. In: Carter, M.R. (Ed.), Soil Sampling and Methods of Analysis. Canadian Society of Soil Science, Lewis Publishers, Boca Raton, pp. 599-613. Reynolds, W.D., Elrick, D.E., 1985. In situ measurement of field-saturated hydraulic conductivity, sorptivity and the a-parameter using the Guelph permeameter. Soil Sci. 140 (4), 292-302. Reynolds, W.D., Elrick, D.E., 1986. A method for simultaneous in situ measurement in the vadose zone of field-saturated hydraulic conductivity, sorptivity and the conductivity-pressure head relationship. Ground Water Monit. Rev. 6 (1), 84-95. Reynolds, W,D., Zebchuk, W.D., 1996. Hydraulic conductivity in a clay soil: two measurement techniques and spatial characterization. Soil Sci. Soc. Am. J. 60, 1679-1685,
GEODEP~MA Geoderma 80 (1997) 169-180
Influence of well preparation on field-saturated hydraulic conductivity measured with the Guelph Permeameter V i n c e n z o Bagarello Dipartimento E.I.T.A., Settore ldraulica, UniL,ersith degli Studi, Viale delle Scienze, 90128 Palermo, Italy Received I May 1996; accepted 14 May 1997
Abstract
An investigation on the influence of the well preparation technique on field-saturated hydraulic conductivity, Kr~, measured with the Guelph Perrneameter (GP) method was performed in a sandy loam soil. In particular, the influence of adopting the following procedures to prepare the well was investigated: use of a wire screen insert to prevent sinking of the water outlet tip of the GP into the base of the well during a measurement; use of a metallic, sharpened rod to remove the smear layer created during the digging process from the well walls. The influence of the well radius and the antecedent soil water content on the Kr, estimates was also evaluated. Five different types of wells were prepared. Higher and less variable KI:~ values were obtained from the wells treated with the wire screen insert and the metallic rod than from the non-treated ones. These results were consistent with the hypothesized occurrence of sinking and smearing phenomena in the non-treated wells and with the effectiveness of the used procedures to prevent or reduce the influence of these phenomena on the Kr~ measurement. The estimates of field-saturated hydraulic conductivity increased with the radius of the well. Differences between the sampled soil volumes and a more pronounced influence of sinking phenomena in the small-size wells likely contributed to produce this result. In relatively dry soil conditions, the antecedent soil water content did not affect the Kr~ measurements, independently of the well preparation technique. @ 1997 Elsevier Science B.V.
Keywords." saturated hydraulic conductivity; Guelph Permeameter; site preparation; soil moisture
1. Introduction
Establishing reliability and usefulness of field and laboratory methods for measuring saturated hydraulic conductivity often involves comparisons among estimates obtained by different methods (Dorsey et al., 1990; Gallichand et al., 0016-7061/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S001 6-706 1(97)0005 I-7
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1990; Gupta et al., 1993; Kanwar et al., 1989; Mohanty et al., 1991; Paige and Hillel, 1993). Since several factors affect the results of a particular method, these comparisons often show different trends under different soil types and field conditions (Mohanty et al., 1994; Reynolds and Zebchuk, 1996). Taking into account the influence of factors affecting conductivity estimates is therefore necessary to individuate sources of data variability and, as a consequence, to increase confidence in the results of comparisons among methods. The Guelph Permeameter (GP) method (Reynolds and Elrick, 1986) is one of the most widely applied in-situ constant head well permeameter methods [or determining field-saturated hydraulic conductivity, Kf~ (L T-~). This method involves digging a small, vertical, cylindrical well and determining the steady water discharge, Q~ (L 3 T - l ) , when a constant depth of water, H (L), is maintained in the well. Smearing a n d / o r compaction of the well surface during the digging process, sinking of the water outlet tip of the instrument into the base of the well during a measurement, and radius of the well can affect field-saturated hydraulic conductivity measured with the Guelph Permeameter. Auger-induced smearing and compaction of the well surfaces result in artificially low Kt.~ values, especially when the soil is fine-textured and the soil water content is high at the time of augering (Koppi and Geering, 1986; Asare et al., 1993; Reynolds, 1993; Bagarello and Provenzano, 1996). Reynolds (1993) suggested the following steps for minimizing the influence of smearing and compaction on Kt.~: augering when the soil is relatively dry; using a very sharp auger; applying very little downward pressure on the auger; taking only small bites with the auger before emptying it. If inspection of the well reveals smearing within the measurement zone, a small, spiked roller mounted on a handle should be used to pluck off the smeared/compacted layer. Recently, Campbell and Fritton (1994) showed in a structured clayey soil that a smear layer can be removed from well walls using an ice pick to expose individual peds and, therefore, to enhance representativeness of GP results. A quick-setting epoxy resin was used by Koppi and Geering (1986) to prepare unsmeared, hemi-spherical or flat surfaces for infiltration measurement. Sinking of the water outlet tip into the base of the well during a measurement can produce an underestimate of Kfs because the actual H-level is lower than the assumed level, and because outflow from the GP can be reduced by a smaller effective infiltration area. Reynolds (1993) suggested that subsidence of the GP can be prevented by clamping the GP reservoir to a rigid tripod so that the weight of the instrument is carried by the legs of the tripod rather than by the outflow tip. GP subsidence can be also prevented by placing a layer of pea gravel under the tip, and then recalibrating the H-level (Reynolds, 1993). To our knowledge, however, sinking effects on Kf~ have not been quantified. Finally, there is some evidence that Kt ~ values determined by the GP method tend to increase with the radius of the well (Reynolds and Elrick, 1985; W.D.
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Reynolds, pers. commun., 1994). As reported by Reynolds and Elrick (1985), these results can reflect an auger a n d / o r a sample size effect. Although well preparation has been discussed by different authors, few investigations have been carried out in coarse-textured soils, probably since well preparation is less critical in these soils than in the fine-textured ones. Moreover, there are still relatively few data on the effect of the antecedent soil water content on measured field-saturated hydraulic conductivity. For example, it has been proved only recently that soil structural variability can produce a negative trend of Kf~ versus antecedent soil water content (Reynolds and Zebchuk, 1996). In this study, an experimental investigation was carried out on the influence of the well preparation technique on field-saturated hydraulic conductivity measured with the GP in a relatively coarse-textured soil. In particular, the influence of adopting the following procedures to prepare the well was investigated: use of a wire screen insert to prevent sinking of the tip of the instrument during a measurement; use of a metallic, sharpened rod to remove the smear layer from the well walls. The influence of the well radius on the Kfs estimates was also evaluated. Finally, the relationship between field-saturated hydraulic conductivity and soil water content at the time of augering the well was examined.
2. Materials and methods The study was conducted from May to July 1995 in a 200-m 2 experimental area, supporting a citrus orchard, neighbouring to the site already considered by Bagarello and Provenzano (1996). Table 1 shows minimum, maximum, and mean (/x) values of clay, silt, sand and gravel content of fifteen soil cores taken at a depth of 0.15 m. The soil texture of most samples was classified as sandy loam; some samples were classified as sandy or sandy clay (ISSS classification) (Gee and Bauder, 1986). Two hand-held augers of different radius (0.025 m and 0.030 m), a wire screen insert (Fig. !), and a metallic, sharpened rod were used to prepare the
Table 1 Soil textural characteristics at the experimental site Size class Clay ( < 2 Ixm) Silt (2-20 Ixm) Sand (20-2000 Ixm) Gravel ( > 2000 txm)
Mean (,a)
Minimum
Maximum
(%)
(%)
(%)
14,2 22,2 63.6 16.4
10.7 19.1 58.0 9.6
19.4 25.7 67.3 23.8
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172
@
N+o.o6+i Fig. 1. Schematic representation of the wire screen insert.
wells. The wire screen insert, connected to a wooden surface support, was used to prevent a possible sinking of the water outlet tip of the permeameter into the base of the well during a measurement. The metallic rod was used to remove any smeared/compacted area from the well wall, in an attempt to expose undisturbed soil at the wall of the well. Five different well treatments (15 wells for each treatment) were randomly prepared in the study area. Table 2 summarizes the preparation method corresponding to each well treatment. The wells were dug to a depth of 0.15 m. In each well, steady discharge values corresponding to the H~ = 0.03 m and H 2 = 0.05 m depth of ponding values were measured. The rate of fall of the water level in the GP reservoir was monitored until it did not change in four consecutive 2-rain time intervals. Each measurement was carried out immediately after having excavated the well. As an index of the antecedent soil wetness, the gravimetric moisture content, MC (kg k g - 1), of the last volume of disturbed soil removed by the soil auger during well excavation was determined. After each field measurement, a mixture of plaster and quick-setting cement was poured into the well to obtain a mould of the well. After hardening, the Table 2 Summary of the well preparation methods used for each type of well Well treatment
Radius of the auger
Metallic rod
Wire screen insert
no no yes no yes
no no no yes yes
(m) F5 F6 F5P F6R F5PR
0.025 0.030 0.025 0.030 0.025
Y e s / n o : the treatment was u s e d / n o t used.
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mould was taken out and sawn into two parts; the lower end, 0.06-0.07-m-long, was saturated and then plunged in a known volume of water to determine an actual, average radius of the well in the measurement zone, a m (m). The a m values and the single height (SH) analysis (Elrick and Reynolds, 1992) were used to estimate Kfs (m s-l). A constant ratio a * = 36 m ( a * = Kfs/ck m, ~bm being the matric flux potential) was assumed for this analysis (Elrick and Reynolds, 1992; Bagarello and Provenzano, 1996). For each well, results of the two single height analyses (one with H~ = 0.03 m, and the other with H 2 = 0.05 m) were averaged to obtain an estimate of Kf~ (Elrick and Reynolds, 1992). According to the results of the Kolmogorov-Smirnov test, the normal frequency distribution of the In Kfs values was assumed for the statistical data analysis. The following approaches were compared to inspect the differences among lnKf~, a m and MC mean values calculated for each well treatment: (i) two groups of data were considered at a time; a t-test or a modified t-test were applied after verifying the homogeneity of variance by an F-test (Helsel and Hirsch, 1992; Piccolo and Vitale, 1984); (ii) all groups of data were considered at the same time and the Tukey Honestly Significant Difference test (Daniel, 1996) was conducted. For subsequent analysis, results of the Tukey Honestly Significant Difference test were considered. This test was the least prone to reject the hypothesis that two mean values were equal. Common statistical techniques (F- and t-tests) were used to analyse the correlation between field-saturated hydraulic conductivity and antecedent soil moisture content (Daniel, 1996). A 0.05 probability level was chosen for all the tests. 3. Results and discussion Table 3 summarizes arithmetic mean (/~), standard deviation (o-), coefficient of variation (CV), and geometric mean ( M ) of Kf~ values obtained in each type Table 3 Arithmetic mean (/z), standard deviation (o-), coefficient of variation (CV), and geometric mean ( M ) values of field-saturated hydraulic conductivity (Kf~), actual well radius ( a m), and gravimetric water content (MC) Well treatment F5 F6 F5P F6R F5PR
106 Kf~ (m s - I )
a m (m)
MC
/z
cr
CV
M
#
o-
CV
/x
22.9 39.5 52.4 59.3 73.2
14.9 20.8 27.3 19.3 23.7
0.651 0.527 0.521 0.325 0.324
16.6 a 35.1 b 46.0 bc 55.8 bc 68.0c
0.0232 a 0.0301 b 0.0283 c 0.0313 b 0.0307b
0.0013 0.0011 0.0020 0.0013 0.0015
0.056 0.037 0.071 0.042 0.049
0.127 0.128 0.114 0.110 0.118
a a a a a
o-
CV
0.034 0.034 0.027 0.016 0.025
0.268 0.266 0.237 0.145 0.212
Means in a column followed by the same lower case letter are not significantly different at P < 0.05.
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of well. The statistics of the corresponding a m and MC values and the results of the Tukey Honestly Significant Difference test are also given. Antecedent gravimetric soil water content values ranging from 0.08 to 0.21 were measured during the runs. The mean MC values calculated for each well treatment were not statistically different. Some differences between the mean values of the actual well radii were found to be significant. Notwithstanding this, the range in well radii was very narrow: except for the F5 wells, the mean a m values fell between 0.028 and 0.031 m. For each well treatment, a low variability of the measured radii was recognized (CV _< 7%). For the desmeared wells (F5P, F5PR), this result confirmed that a uniform thickness of soil was removed from the well walls. The moulds of the unprotected wells (i.e., wells in which the wire screen insert was not used) occasionally showed a short basal protuberance, representing the track of the GP's tip, sunk a few millimetres into the base of the well during the run. This shape of the base of some moulds produced in the F5 wells a mean a m value of 0.023 m, which was less than the theoretical one of 0.025 m. Geometric m e a n Kfs values ( M ) ranging from 1.66 X 10 -5 m s -I to 6.80 X 10 -5 m s - ~ and coefficients of variation ranging from 0.32 to 0.65 were measured in the different well treatments. The lowest mean and the highest variability of Kf~ were obtained in the untreated wells (i.e., wells prepared by only using the hand-held auger). The highest mean and the lowest variability of Kf.~ were measured in the wells prepared by using the wire screen insert and the metallic rod. With the exception of the F5 wells (in which Kf~ values ranging from 3 . 9 4 × 10 - 6 m s ~ to 4.51X 10 -5 m s -~ were measured), similar minimum (1.5-2.3 X 10 -5 m s-~) and maximum (0.9-1.0 × 10 -4 m s- f) K~ values were obtained from the different well treatments. However, high Kfs values were more frequently obtained from the wells prepared by using both the metallic rod and the wire screen insert compared to the wells prepared by a single treatment and, to a greater extent, to the untreated wells (Fig. 2). The geometric mean Kfs values obtained from the three treated wells (F5P, F6R, F5PR) were not significantly different. The two untreated wells produced significantly different geometric mean estimates of Kf~. Most of the comparisons between the untreated wells and the treated ones showed significantly different geometric mean values of Kf~. The geometric mean of Kf~ values measured in the F6 wells for MC < 0 . 1 2 was equal to 3 . 9 9 × 10 -5 m s -~ (sample size N = 9); this value was not significantly different from the mean Kf~ value obtained in 1993 by Bagarello and Provenzano (1996) in a neighbouring area for low values of antecedent soil water content (see their tables 2 and 3: mean Kf~ = 3.90 X 10 -5 m s - I ; N = 36). Although the small size of the first Kf~ sample could affect the result of this comparison, the negligible difference between the two series of measurements performed in similar wells and in different years gave some support to the hypothesis (Bagarello and Provenzano, 1996) that the structure of the investigated soil was quite stable.
V. Bagarello / Geoderma 80 (1997) 169-180
175
/O
-.-F6 ]i i ~ - F5P o-F6R i • F5PR'
0.75
g .~_ 0.5
t~ c ~
D-f
-'/~
!
0/
/ /'
0.25
0 -13
I
I
I
-12
-11
-10
-9
In Kf~ Fig. 2. Empirical cumulative frequency distribution of the natural logarithms of field-saturated hydraulic conductivity, In Kf~, for each type of well.
The results summarized above showed that, for given well preparation technique (F5 and F6 wells), the well radius affected the measured Kes values. Reynolds and Elrick (1985) attributed the well radius effect to heterogeneity and macroporous structure of the soil. As a matter of fact, different soil volumes were sampled by the two types of well; soil volumes sampled by the F5 wells were probably too small and, therefore, unrepresentative of the soil structure at the field site. The occasional occurrence of sinking of the water outlet tip of the GP into the base of the well contributed to produce lower and more variable Kt:~ results in the small size wells. In fact, visual inspection of the moulds of the F5 and F6 wells showed a more pronounced influence of this phenomenon in the F5 than in the F6 wells. The comparison, for each well treatment, of the mean actual radii to the theoretical ones supported this conclusion. For a practically constant well size (mean of a m ---- 0 . 0 2 8 - - 0.031 m; F6, F5P, F6R and F5PR wells), the measured Kfs values showed a dependence on the well preparation technique. In particular, alteration of the infiltration surface was not enough to significantly change the geometric mean value of Kt:~ measured in the untreated F6 wells when only the metallic rod or the wire screen insert were used. However, using both treatments to prepare the well did modify the well characteristics in such a way that, on the average, water flow was greater than that measured in the untreated wells. Variability of Kfs measurements was minimized by applying the wire screen insert. These results suggested that smearing and sinking phenomena did significantly affect the mean GP results when they occurred simultaneously and that the changes in the predictions of Kf~ variability among the treatments were mainly attributable to the occurrence
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V. Bagarello / Geoderma 80 (1997) 1 6 9 - 1 8 0
Table 4 Intercept (b o), slope ( b i ) and coefficient of determination ( r 2) obtained by least-squares fitting a straight line through natural logarithm of field-saturated hydraulic conductivity versus antecedent soil water content (MC) or amount of fine-textured soil particle data for each well treatment Well Independent 0,08 < MC < 0.21 treatment variable b0 bi
0.08 < MC < 0.14 r2
sample b o
bL
r2
size F5 F6 F5P F6R F5PR F5PR F5PR
MC MC MC MC MC clay clay + silt
-8.2348 -21.8352s -9.3989 - 6 . 7 2 5 2 ns -8.9028 - 9 . 4 7 4 5 ns - 8 . 6 9 1 6 - 10.0308 ns -8.0411 -13.1843s -9.1889 - 0 . 0 2 8 8 ns - 11.6244 0.0557 ns
0,618 0.210 0.232 0.187 0,532 0.022 0.099
11 II 13 14 13
-8.5412 -18.7557ns -9.0997 - 9 . 5 6 6 2 ns -9.2128 - 6 . 2 5 6 0 ns - 10.5892 8.0784 ns -8.7772 -6.0545ns
0.156 0.045 0.034 0.066 0.118
s: significantly different from zero. ns: not significantly different from zero. P _< 0.05.
of tip sinking. The differences, 6 (m s-~), between the geometric mean Kf~ measured in the treated wells and the geometric mean Kf~ obtained from the untreated F6 wells were calculated. The 6 value corresponding to the F5PR wells was quasi-equal to the sum of the ~ values calculated for the F5P and F6R wells, suggesting an additive Kf~ underestimating effect of the mutually independent sinking and smearing phenomena. For each well treatment, an apparent linear trend of lnKf~ versus MC was observed. To quantify this trend, a straight line was fitted to the In Kt-~, MC data pairs. For each well treatment, Table 4 gives the least-squares estimates of the b 0 (intercept), b 1 (slope) and r 2 (coefficient of determination) values. For the F5PR wells, Table 4 also gives the fitting parameters and the coefficient of determination obtained by considering the amount of fine material (percentages of clay and clay plus silt) taken from the well bottom as the independent variable. For the F6, F5P and F6R wells, the In Kf~ and MC data were found to be not significantly correlated. For the F5 and F5PR wells, a statistically significant dependence of In K~, on MC was recognized (Fig. 3). In these wells, an increase of MC from 0.10 to 0.20 produced a decrease of Kf~ by a factor of 4 (F5PR wells) to 9 (F5 wells). The slopes of the In Kt:~ versus MC relationships were found to be not significantly different, suggesting that the dependence of In Kt~ on MC was similar in the two types of wells although the well preparation procedures were very different. The amount of fine material was not significantly correlated to the values of ln Kf~, showing that the low Kt~ values measured in the F5PR wells for high MC values were not the result of lower soil
V. Bagarello / Geoderma 80 (1997) 169-180 -9
ooeo
-10
•
OO ~• •
8b'-.
0
"°'''''''--..
O
...q
O
" " " " " -..0
-11
! '! • L......
-12
-13 0.08
177
I 0. I
[ 0.12
I 0.14
I 0.16
I 0.18
best fit (F5 wells) F5PRwells best ~t eFSVRwells) j
0.2
MC Fig. 3. Relationships between the natural logarithms of field-saturated hydraulic conductivity, In K~, and antecedent gravimetric soil water content, MC, for the F5 and F5PR wells.
permeability stemming from a finer soil texture. A trend between the well preparation technique and the statistical significance of the correlation between In Kfs and MC was not observed, Data inspection showed that most Kfs data were collected in dry soil conditions. In particular, 11 to 14 experiments (73% to 93% of all the experiments) were conducted for MC values less than 0.14 (the approximate mid point of the MC range), depending on the well type. The low number of ln Kfs and MC data pairs obtained in wet soil conditions likely influenced the contradictory results summarized above. The correlation between In Kfs and MC in the more evenly investigated range 0.08 < MC < 0.14 was also evaluated for each well treatment (Table 4). In all cases, the two variables were found to be not significantly correlated (0.034 < r 2 < 0.156), showing that the antecedent soil water content did not affect the Kf~ measurements in the range of relatively low MC values. For practical purposes, GP measurements are analysed by using a theoretical radius, a t ( L ) , equal to the radius of the auger. In this study, a t values could be used for the F5, F6 and F6R wells only since in the F5P and F5PR wells a thickness of soil was removed from the well walls. For the first three series of wells, some differences between the a m and the corresponding a t values were recognized (Table 3). The effect of these differences on the calculations of Kf~ was examined. For each F5, F6 and F6R well, Kes was estimated by using both the a t value and the measured well radius; the difference between the two estimates of Kfs, expressed as a percentage of the second one, was then calculated. For the F6 and F6R wells, the absolute values of the differences between the two series of Kf~ were always less than 8%; for the F5 wells, slightly higher differences (up to 14%) were observed. The magnitude of these differences is probably negligible, especially if compared to the errors produced
178
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by many other factors (Elrick et al., 1990). As a consequence, measuring the actual radius of the well was not crucial, at least when a thickness of soil was not removed after well excavation; however, using a mould of the well was tound to be very useful to indicate irregular wells (for example, wells in which sinking phenomena occurred), potentially producing unrepresentative Kf~ estimates.
4. Summary and conclusions The objective of this study was to investigate the influence of the well preparation technique on field-saturated hydraulic conductivity, Kt~,, measured with the Guelph Permeameter (GP) method in a sandy loam soil. In particular, the influence of adopting procedures to prevent sinking of the tip of the instrument during a measurement and to remove the smear layer from the well walls on the Kf~ estimates was examined. The influence of the well radius on Kf~ was also evaluated. Finally, the relationship between Kt~ and antecedent soil water content was examined. A wire screen insert was used to support the weight of the instrument; a metallic, sharpened rod was used to expose natural soil aggregates at the well walls (i.e., to remove the smear layer caused by the auger). For each well, the actual, mean radius of the well was determined after the GP test by using moulds of the well. Using the wire screen insert and the metallic rod produced increasing estimates of mean Kf~ and decreasing estimates of Kf~ variability, suggesting that sinking of the water outlet tip of the instrument and smearing a n d / o r compaction of well walls affected the GP results in the non-treated wells. On the average, an additive Kf~ underestimating effect of the two phenomena was recognized. Inspection of the moulds of the unprotected wells showed that sinking of the GP tip occurred occasionally. Estimates of field-saturated hydraulic conductivity increased with the radius of the well. Differences between the sampled volumes and a more frequent occurrence of sinking phenomena in the small-size wells likely contributed to this result. Notwithstanding that statistically significant differences were found, the mean Kf~ values obtained from different well types sampling similar soil volumes were relatively similar. As a consequence, it can be concluded that the influence of the factors considered in this study on the Kf~ estimates could be considered negligible for some practical applications concerned only with the order of magnitude of Kfs. However, using both the wire screen insert and the metallic rod to prepare the well for a GP measurement seems useful to enhance representativeness of the estimates (mean values, variability).
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Removing a thickness of soil from the well walls implies that a measurement of the actual well radius has to be performed. This could be accomplished by preparing a mould of the well after the GP test; this procedure also allows to individuate irregular wells which can produce unrepresentative Kfs results. Although an apparent, decreasing trend of field-saturated hydraulic conductivity versus antecedent soil water content was observed for each type of well, a significant correlation was observed only occasionally. This result was attributed to the unbalanced sample composition. In relatively dry soil conditions, the antecedent soil water content did not affect the Kr.~ measurements, independently of the well preparation technique.
Acknowledgements This research was supported by a grant of the Italian Ministero dell'Universith e della Ricerca Scientifica e Tecnologica. The author sincerely thanks Dr. W.D. Reynolds and the anonymous reviewers for their helpful comments and for the help in improving the quality of the manuscript.
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Koppi, A.J., Geering, H.R., 1986. The preparation of unsmeared soil surfaces and an improved apparatus for infiltration measurements. J. Soil Sci. 37, 177-181. 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. Mohanty, B.P., Kanwar, R.S., Everts, C.J., 1994. Comparison of saturated hydraulic conductivity measurement methods for a glacial till soil. Soil Sci. Soc. Am. J. 58, 672-677. Paige, G.B., Hillel, D., 1993. Comparison of three methods for assessing soil hydraulic properties. Soil Sci. 155 (3), 175-189. Piccolo, D., Vitale, C., 1984. Metodi statistici per l'analisi economica. I1 Mulino, Bologna, 747 pP. Reynolds, W.D., 1993. Saturated hydraulic conductivity: field measurement. In: Carter, M.R. (Ed.), Soil Sampling and Methods of Analysis. Canadian Society of Soil Science, Lewis Publishers, Boca Raton, pp. 599-613. Reynolds, W.D., Elrick, D.E., 1985. In situ measurement of field-saturated hydraulic conductivity, sorptivity and the a-parameter using the Guelph permeameter. Soil Sci. 140 (4), 292-302. Reynolds, W.D., Elrick, D.E., 1986. A method for simultaneous in situ measurement in the vadose zone of field-saturated hydraulic conductivity, sorptivity and the conductivity-pressure head relationship. Ground Water Monit. Rev. 6 (1), 84-95. Reynolds, W,D., Zebchuk, W.D., 1996. Hydraulic conductivity in a clay soil: two measurement techniques and spatial characterization. Soil Sci. Soc. Am. J. 60, 1679-1685,