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Soil Science Society of America Journal 64:1203-1210 (2000)
© 2000 Soil Science Society of America

DIVISION S-1-SOIL PHYSICS

Factors Affecting Measurement of the Near-Saturated Soil Hydraulic Conductivity

Vincenzo Bagarello, Massimo Iovino and Giuseppa Tusa

Dipartimento di Ingegneria e Tecnologie Agro-Forestali, Università degli Studi, Viale delle Scienze, 90128, Palermo, Italy

bagav{at}unipa.it


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Theory
 Materials and methods
 NOTES
 Results and discussion
 Conclusions
 REFERENCES
 
Different techniques and different technique application procedures are available to determine soil hydraulic conductivity (K). The objective of this study was to determine the dependence of the K values on the selected technique and application procedure. Soil hydraulic conductivity was determined on a sandy loam soil in the near-saturated zone by the pressure infiltrometer, the tension infiltrometer field techniques, and the inverse method applied to laboratory outflow multistep experiments. Other experiments included a study of ascending and descending pressure heads (h) on the K values measured by the tension infiltrometer technique, and the effect of contact material on ponded infiltration rates. Differences between ascending and descending K means and coefficients of variation were <20% for low pressure heads (h <= -120 mm). Three successive ponding infiltration runs produced mean field-saturated hydraulic conductivities decreasing from 42.7 to 81.8 mm h-1 to 12.9 to 20.6 mm h-1, whether or not contact material was present on the infiltration surface. The mean values of K obtained by the selected three methods ranged between 47.1 to 71.9 mm h-1 at saturation and . The K(h) relationships obtained by the pressure with tension infiltrometer and multistep techniques were essentially overlapping. The pressure infiltrometer produced K(h) relationships that were different from those determined by the two other techniques. In conclusion, ponded infiltration measurements were not usable for estimating unsaturated soil hydraulic conductivity, but contact material and ascending vs. descending direction had no substantial effect on infiltration rate.

Abbreviations: GM, geometric mean • NC, no contact material sites • SA, natural sand contact material sites • SP, Spheriglass no. 2227 spheres contact material sites


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Theory
 Materials and methods
 NOTES
 Results and discussion
 Conclusions
 REFERENCES
 
MEASUREMENT OF THE HYDRAULIC CONDUCTIVITY of soil, K, can be made either in the laboratory or in the field by using different methods, which often have different operating ranges, flow geometries, boundary conditions, sample sizes, and underlying assumptions. Selecting the proper method for particular soil and site conditions, and the proper application procedure for the selected method, is important to obtain representative estimates of K. The pressure (Reynolds and Elrick, 1990) and the tension (Perroux and White, 1988) infiltrometer methods and the inverse method applied to outflow multistep experiments (Eching et al., 1994; van Dam et al., 1994) are widely applied for determining the hydraulic conductivity of soil.

Both the pressure infiltrometer and tension infiltrometer techniques are simple, can be applied in the field, are quite rapid, and do not greatly disturb the soil at the measurement site. They are routinely used, but there still remain issues related to the pressure infiltrometer and tension infiltrometer methods that need consideration.

With the tension infiltrometer method, a sequence of (quasi) steady flow rates, Qs, is generally measured by setting a sequence of pressure heads, h0, on the soil surface. In many applications, a single-ring infiltrometer is first used for saturated infiltration measurements. Then, the ring is filled with the contact material and the tension infiltrometer is used for unsaturated infiltration measurements from low to high suction (Ankeny et al., 1991; Logsdon and Jaynes, 1993; Mohanty et al., 1994). According to Mohanty et al. (1994), a wet-to-dry measurement sequence reduces the antecedent negative head effects at low infiltration rates. By conducting saturated measurement first, the wetting front advances as rapidly as possible, and the assumption of a unit gradient below the device is more likely to be valid. On the other hand, an ascending h0 sequence (i.e., h1 < h2 < h3 ...) is sometimes applied because a decreasing sequence may cause hysteresis, with progressive drainage occurring close to the disc while wetting continues at the infiltration front (Reynolds and Elrick, 1991; White et al., 1992; Jarvis and Messing, 1995). According to Reynolds and Zebchuk (1996b), a slight positive head can be established by the tension infiltrometer at the end of the ascending h0 sequence by maintaining the layer of contact material within a circumscribed ring on the soil surface. Some tension infiltrometer experiments carried out by Logsdon et al. (1993) in a silt loam soil showed greater infiltration rates for measurements taken at descending (i.e., from low to high negative heads) than at ascending pressure head values. However, to our knowledge, the impact of the selected h0 sequence on the calculations of soil hydraulic conductivity has not been critically examined.

The presence of a thin contact layer on the soil surface should not practically influence steady-state infiltration rates provided that the water permeability of the contact material is greater than that of the soil. Everts and Kanwar (1993) experienced a decrease in ponded infiltration rates measured by a tension infiltrometer set at a positive head in presence of a layer of contact sand. This result merits consideration given that a positive head can be established either by using or not using a layer of contact material to determine the field-saturated hydraulic conductivity, Kfs.

Ponded infiltration measurements, which are used primarily to determine Kfs, have been assumed to be usable also for estimating the soil hydraulic conductivity-pressure head, K(h), relationship (Reynolds and Elrick, 1985, 1986; Ragab and Cooper, 1993; Russo et al., 1997). Ponded flow tends to maximize the gravity component of flow rather than the capillary component (Reynolds and Elrick, 1990). Consequently, accurate estimates of parameters related to soil capillarity are difficult to obtain from ponded infiltration techniques (Paige and Hillel, 1993; Reynolds, 1994; Elrick et al., 1995). The tension infiltrometer method can help to evaluate the uncertainties associated with a K(h) relationship derived only from pressure infiltrometer measurements. In fact, both methods can be applied in situ, at the same location, with minimum disturbance of the infiltration surface and establish a three-dimensional infiltration process controlled by the pressure head imposed on the soil surface. Consequently, they are similar enough to avoid misconceptions arising from method comparison.

In the last few years, the parameter estimation method, as applied to outflow multistep experiments, has become a standard laboratory technique for the simultaneous determination of the water retention, {theta}(h), and unsaturated hydraulic conductivity, K(h), functions (Eching et al., 1994; van Dam et al., 1994; Crescimanno and Iovino, 1995). With this approach, the parameters describing the soil hydraulic properties are estimated by an optimization procedure that minimizes deviations between the measured values of one or more flow variables, and the corresponding values calculated by solving numerically the Richards equation. In order to overcome the uniqueness problems experienced with the traditional multistep experiments, additional {theta}(h) equilibrium values and/or h values vs. time t collected inside the sample by a microtensiometer could be introduced into the optimization procedure (Eching et al., 1994; Iovino, 1998).

The tension infiltrometer method investigates the K(h) relationship in the near-saturated zone, while the multistep method applies to lower h values. Therefore, the overlap between the two methods is minimal. Mohanty et al. (1997) used the two methods jointly to obtain experimental K values for a wide range of pressure heads. Comparing the two methods can help to establish if a single method can be applied across a wide range of pressure heads. Moreover, the two methods generally use different K(h) relationships given that Gardner's exponential function is used in the tension infiltrometer method and the Mualem–van Genuchten model is usually selected in the parameter estimation method. Using the Gardner relationship in a piecewise form (Ankeny et al., 1991; Reynolds and Elrick, 1991) reduces the importance of this last aspect.

The objectives of this investigation were (i) to evaluate the effect of a sequence of applied pressure head values on the tension infiltrometer estimates of the soil hydraulic conductivity; (ii) to examine the dependence of ponded infiltration measurements on the presence, above the soil surface, of a layer of contact material; and (iii) to compare the hydraulic conductivity–pressure head relationships obtained by the pressure and the tension infiltrometers with multistep outflow laboratory experiments.


   Theory
TOP
 ABSTRACT
 INTRODUCTION
 Theory
Materials and methods
 NOTES
 Results and discussion
 Conclusions
 REFERENCES
 
The pressure and tension infiltrometer methods, the multistep outflow approach, and the underlying theory have already been widely described in the literature. Only brief details of these methods, as relevant for this study, are repeated here.

Both the pressure and tension infiltrometer methods allow establishment of a three-dimensional infiltration process that attains steady-state conditions after a transient phase (Philip, 1969; Elrick et al., 1990). Frequently applied procedures for analyzing field measured infiltration rates are based on steady-state conditions. Apparent, steady infiltration rates frequently overestimate true steady rates (Elrick et al., 1990; Quadri et al., 1994; Bagarello and Giordano, 1999). Consequently, field estimates of steady-state flow are often referred to as quasi, or approximate, steady-state infiltration rates. In practice, the steady-state rate overestimation is often assumed to be of little significance and compensated for by other factors that affect the accuracy of the K estimates (Reynolds, 1993a, 1993b).

Soil hydraulic conductivity, K (L T-1), as a function of soil water pressure head, h (L), can be calculated from tension infiltrometer measurements by using Wooding's (1968) analysis for unconfined steady-state infiltration from a disc into homogeneous, isotropic, uniformly unsaturated soil:

(1)
where Qs (L3 T-1) is the steady flow rate; a (L) is the radius of the infiltrometer disc or the retaining ring for the contact material (Reynolds, 1993b); ; h0 and hn (L) are the wetting and the initial pressure head, respectively; and {lambda}c (L) is the macroscopic capillary length. Wooding's solution uses a hydraulic conductivity function of the exponential form (Gardner, 1958):

(2)
where Kfs (L T-1) is the field-saturated hydraulic conductivity and the {alpha} (L-1) parameter is equal to {lambda}-1c. If the steady-flow rate is measured at two different heads, , and {lambda}c is assumed to be constant for the potential range , the following equations can be applied to calculate K(h1) and K(h2) (Ankeny et al., 1991; Cook and Broeren, 1994):

(3a)

(3b)

(3c)

From more than one rate pair, two estimates of K can be obtained for a given head. In this case the best estimate of K(h0) can be defined as the arithmetic average of the available estimates (Ankeny et al., 1991).

Steady, ponded infiltration from within a single ring into rigid, homogeneous, isotropic, uniformly unsaturated soil can be approximated by the following analytical expression (Reynolds and Elrick, 1990):

(4)
where a (L) is, in this case, the ring radius; G is a dimensionless shape factor; H (L) is the steady depth of ponding in the ring; and {phi}m (L2 T-1) is the matric flux potential. For practical purposes, the following estimate of the G factor (Ge) can be used (Reynolds and Elrick, 1990):

(5)
where d (L) is the depth of ring insertion. To solve Eq. [4] for the two unknowns, Kfs and {phi}m, the One-Ponding-Depth and Two-Ponding-Depth approaches can be applied (Reynolds and Elrick, 1990; Reynolds, 1993a). The first approach uses one H level. The second one uses two H levels ponded in ascending order (i.e., H1 ponded first, H2 > H1) and without intervening drainage to obtain the corresponding Qs values, . The One-Ponding-Depth approach requires an a priori estimate of the parameter a* (L-1), which is equal to the ratio between Kfs and {phi}m (Reynolds, 1993a).

The multistep outflow approach for determining the soil hydraulic properties involves a transient drainage experiment performed on an initially near-saturated soil core subjected to several steps of air pressure at the top and a saturated porous plate at the bottom. The experimental procedure results in a set of cumulative outflow volume vs. time, V(t), and pressure head vs. time, h(t), measurements. The unsaturated one-dimensional vertical flow equation is then numerically solved for the following initial and boundary conditions:

(6a, 6b, 6c)
where is taken at the top of the core, is the bottom of the porous plate, hL is the pressure head at the bottom of the porous plate, and ha(t) is the applied pneumatic potential head. The Mualem–van Genuchten model (van Genuchten, 1980) can be used to describe the water retention and the unsaturated hydraulic conductivity functions:

(7)

(8)
where {theta}s (L3 L-3) is the saturated water content, {theta}r (L3 L-3) is the "residual" water content; is the effective saturation; and {alpha}vg (L-1), n, {gamma}, and m are empirical parameters with . The unknown parameters are determined by minimizing an objective function in which the differences between measured and numerically estimated V(t) values together with additional h(t) and/or {theta}(h) values appear (Eching et al., 1994; Iovino, 1998).


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Theory
 Materials and methods
NOTES
 Results and discussion
 Conclusions
 REFERENCES
 
A 150-m2 experimental, flat area was used for this study at the Agricultural Faculty of the Palermo University. The study was performed in a soil (Typic Rhodoxeralf) that has a relatively high sand and gravel content and comes from a citrus orchard. According to the International Soil Science Society classification (Gee and Bauder, 1986), the soil texture of the upper layer (300 mm thick) was sandy loam (Bagarello and Provenzano, 1996; Iovino, 1998). Previous investigations have shown only small differences among the saturated hydraulic conductivity measured in different years and different times of the year by both field and laboratory methods (Bagarello, 1993, 1997; Bagarello and Provenzano, 1996; Bagarello et al., 1997; Iovino, 1998). Consequently, it can be argued that the soil has a quite stable structure.

Multistep outflow experiments were performed at the beginning of 1998 on six undisturbed soil cores (50 mm in height by 80 mm in diameter) randomly collected from the soil surface at the experimental field. Standard Tempe pressure cells (Soilmoisture, Santa Barbara, CA), with a 5.7-mm-thick, 10-m air-entry ceramic plate, were modified in order to insert at the center of the soil core a microtensiometer (5 mm in diameter by 30 mm in length) connected to a pressure transducer.1 After saturation, the samples were equilibrated with a small pneumatic pressure corresponding with a pressure head of -102 mm in order to realize initial unsaturated conditions (Hopmans et al., 1992). Four successive pressure steps were applied, corresponding with pressure heads of -510, -1020, -3060, and -6120 mm, at 24-h intervals. Cumulative outflow volumes V(t) and pressure heads inside the sample h(t) were collected by an automatic data acquisition system. The parameter estimation procedure was performed using a modified version of the FLOFIT code (Kool and Parker, 1987). The parameters {alpha}vg, n, Ks, and {gamma} were optimized, whereas {theta}s was set to its measured value and {theta}r to the value measured at h = -150 m. Further details on the experimental procedure are presented in Iovino (1998).

During the summer of 1998, a tension infiltrometer manufactured by Soil Measurement Systems (Tucson, AZ) was used in 15 randomly selected locations to evaluate the effect of the sequence of the applied pressure head values on the estimates of the soil hydraulic conductivity. A retaining ring with a radius a = 120 mm and a previously saturated nylon guard cloth having an air entry-value of -160 mm were placed on the soil surface. A contact layer having a thickness of 10 mm was prepared by using dry Spheriglass no. 2227 glass spheres (Potters Ballotini GmbH, Kirchheimbolanden, Germany; Reynolds and Zebchuk, 1996b). Water was then sprayed on the surface to wet the contact material to a volumetric water content {theta} of {approx}0.35. Finally, the tension infiltrometer plate having a radius of 115 mm with a perforated area of radius equal to 100 mm was placed on the contact material surface. A radius a = 120 mm was considered in the K calculations, assuming that the wetting front entered the soil everywhere under the retaining ring at the time to steady state.

The following sequence of h0 values was imposed at the same location and without a time gap: -120, -30, and -120 mm. Although the ascending–descending pressure head sequence used is not extensive enough to recognize hysteresis effects across the whole range of pressure head values, in this study it was preferred to consider only two values of h0 to minimize the duration of the experiment. This reduced other possible confounding factors such as a variation in water temperature or small modifications in soil structure during the experiment. Water level readings were collected at 1- to 4-min intervals. Apparent steady-state infiltration rates were calculated from the slope of the linear portion of the cumulative infiltration vs. time plot (Bagarello et al., 1999). Equations [3a–3b] were used to calculate K. Before each tension infiltrometer run, the antecedent volumetric water content, {theta}i, was determined by collecting an undisturbed soil core (50 mm high by 50 mm in diameter) near the tension infiltrometer application site. These {theta}i were related to the soil hydraulic conductivity at h0 = -120 mm determined during the wetting sequence. For comparative purposes, two series of infiltration rates measured by Logsdon et al. (1993) in a silt loam soil by a h0 sequence of -150, -75, -30, +5, -30, -75, and -150 mm were also analyzed by Eq. [3].

During the same period (summer 1998), ponded infiltration measurements were taken at the soil surface of 15 newly selected and randomly spaced locations. A pressure infiltrometer consisting of a mariotte reservoir (110 mm i.d. by 1200 mm high) sealed to a stainless steel ring (radius a = 75 mm) that was driven into the soil to a depth d = 50 mm was used. After removing a thin layer of soil, a synthetic filter cloth was placed on the infiltration surface inside the ring to minimize soil disturbance at the beginning of the experiment. At each location, steady-discharge values corresponding with the H1 = 55-mm and H2 = 123-mm depth of ponding values were measured. The rate of fall of the water level in the pressure infiltrometer reservoir, R, was monitored at 2- or 5-min intervals. Quasi steady state was assumed when R was practically constant for at least 15 min and calculated from the slope of the linear portion of the cumulative infiltration vs. time plot (Bagarello et al., 1999).

Attempting to reproduce and extend the experiment by Everts and Kanwar (1993), three successive pressure infiltrometer runs were conducted at each site. The time interval between the end of a run and the starting of the next one was {approx}1 h, so ponding was not maintained in the pressure infiltrometer ring during this intervening time interval. A 10-mm-thick contact layer was placed inside the ring before the second run and removed before the third run. A natural sand contact material was used in five locations (SA sites), while the Spheriglass no. 2227 spheres were used in other five sites (SP sites). In the last five sites, no contact material was used for the three repeated applications (NC sites). The satiated hydraulic conductivity of the contact material, determined on five soil cores 50 mm high by 80 mm in diameter by a constant head laboratory device, was 245.7 mm h-1 (CV = 0.033) for the natural sand and 265.0 mm h-1 (CV = 0.015) for the Spheriglass spheres. This latter value is 1.5 times less than that reported by Reynolds and Zebchuk (1996b). Tap water was used for the experiments. The experimental area was then thoroughly irrigated. The Two-Ponding-Depth and One-Ponding-Depth procedures were used to calculate Kfs (Reynolds and Elrick, 1990; Reynolds 1993a). According to Bagarello (1993) and Bagarello and Provenzano (1996), a value of {alpha}* = 0.036 mm-1 was used for the calculations. The presence of the contact material on the soil surface was considered to not alter the H levels.

The same experimental procedure was also applied on three undisturbed soil cores (C1–C3, 50 mm high by 80 mm in diameter) collected at the soil surface to determine the satiated hydraulic conductivity, Kst, by a constant head laboratory permeameter. After the third Kst measurement, each core was saturated from the bottom and the saturated hydraulic conductivity, Ks, was also measured. Finally, the cores were oven dried for {approx}3 h at 105°C and then the upper layer of {approx}5 mm was carefully removed. The cores were saturated again and Ks was redetermined.

One to two months after the pressure infiltrometer experiments (early fall 1998), tension infiltrometer measurements were conducted in the same locations used for the ponded experiments, after removing a thin layer of soil ({approx}10 mm). The locations used for the pressure infiltrometer experiments were deemed to be usable and representative for the unsaturated infiltration experiments because the time interval between the pressure infiltrometer and tension infiltrometer experiments was long, the tension infiltrometer experiments were negative head K tests, and the soil structure at the experimental area appeared quite stable. The soil was removed to reduce the incision after the pressure infiltrometer application and to prepare the site for the tension infiltrometer experiments. The Spheriglass spheres were used as contact material. An ascending sequence of pressure heads on the infiltrometer membrane was selected in order to obtain pressure heads at the soil surface of -120, -50, and -10 mm. Water level readings were collected at 1- to 4-min intervals. Apparent steady-state infiltration rates were calculated from the slope of the linear portion of the cumulative infiltration vs. time plot. Equations [3a–3b] were applied to calculate the K values.

For comparing the K(h) relationships, the following three procedures were used: (i) pressure infiltrometer method with the Two-Ponding-Depth approach; (ii) pressure infiltrometer method (first run, H1 = 55 mm) with the One-Ponding-Depth approach to estimate Kfs and tension infiltrometer method to determine the unsaturated hydraulic conductivity; and (iii) multistep outflow method. In this last case, K at saturation, -10 and -50 mm were extrapolated values from the K(h) function estimated from flow data at pressure heads less than -100 mm.

The statistical frequency distributions for K data were log-normal, which is common for this soil property (e.g., Warrick and Nielsen, 1980). All statistical tests of the K results were therefore carried out using the natural logs of the data. Also, geometric means, standard deviation factors and coefficients of variation were calculated using the appropriate "log-normal equations" (Lee et al., 1985). In the data statistical analysis, a probability level P = 0.05 was assumed.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Theory
 Materials and methods
 NOTES
 Results and discussion
Conclusions
 REFERENCES
 
Within an h0 sequence of -120, -30, and -120 mm conducted within a single experiment, two K-120 values were calculated. In particular, the first two infiltration runs were used to estimate K-120 during the wetting sequence (i.e., increasing h0 values); the last two experiments allowed K-120 estimation during the drying sequence. For the 15 experiments, the wetting sequence produced a geometric mean (GM) value of K-120 equal to 1.6 mm h-1 (CV = 64.2%), while the drying sequence resulted in GM = 1.9 mm h-1 (CV = 74.0%). The mean and the coefficient of variation of the K-120 values obtained during the wetting and the drying sequences showed differences of <20%. According to a paired t test, the differences between the mean K-120 values were statistically significant. For the experiments of Logsdon et al. (1993), the differences between the K values corresponding with h0 = -150, -75, and -30 mm obtained by a wetting and a drying sequence increased with h0, ranging from a minimum of 20% at h0 = -150 mm to a maximum of 120 to 160% at h0 = -30 mm. According to these results, the order of the h0 values is probably a minor factor affecting the K values measured at low pressure heads by the tension infiltrometer method.

A significant relationship was recognized between K-120 (mm h-1) (wetting sequence) and the antecedent volumetric soil water content, {theta}i (m3 m-3) (r2 = 0.848). According to the fitted relationship [K-120 = 8.803 exp (-9.712{theta}i)], K-120 increases by a factor of nearly six as {theta}i decreases from the maximum measured value of 0.29 to the minimum one of 0.11. Similar relationships have already been observed for both saturated (Reynolds and Zebchuk, 1996a) and unsaturated (Lin et al., 1998) experiments. According to these authors, a negative Kh0({theta}i) relationship can be considered indicative of the effect of the soil structure on the measured K values. A higher {theta}i value occurs in a more dense and therefore less permeable soil volume. For two pressure head values (h0 = 0 and h0 = -30 mm), Fig. 1 shows the relationships between the steady infiltration rate (is) and the initial gravimetric water content ({theta}g,i) obtained by Lin et al. (1998). Our results for h0 = -120 mm (wetting sequence) are also plotted. In the representation of Fig. 1, the slopes of the three straight lines appear relatively similar. The ratios between the maximum and minimum is value calculated for each plotted relationship were within a factor of less than two. In all but one case for ponded conditions, the coefficients of determination for the K({theta}i) relationships were higher than 0.85, which showed that {theta}i explained most of the variance of K. The one exception was an r2 of 0.52 for the relationship obtained by Reynolds and Zebchuk (1996a). Confirming that the Kh0({theta}i) relationship is essentially a relationship between K and the soil structure could have practical consequences in the development of accurate pedo-transfer functions for estimating K. Using a dynamic variable such as {theta}i could have the obvious and practical advantage of allowing a simple detailed determination of variations in K, both in space and time.



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  		Fig. 1 Relationship between the steady-state infiltration rate (is) corresponding with a pressure head h0 = -120 mm and the initial gravimetric water content ({theta}g,i). The relationships obtained by Lin et al. (1998) for pressure heads h0 = 0 and h0 = -30 mm are also plotted

 
For each of the three pressure infiltrometer runs, the field-saturated hydraulic conductivity, Kfs (mm h-1), values were determined by the Two-Ponding-Depth procedure. In any case, the satiated hydraulic conductivity of the contact material was higher than Kfs (Table 1) . The differences among the CV values calculated within each group of sites were noticeable and probably influenced by the small number of sites established within a group. For each of the three runs, these differences followed the same trend, with the CV being highest for the SA sites and lowest for the SP sites. The mean Kfs values were not significantly different within each run. For the three groups of sites, the ratios between the mean Kfs values measured in subsequent runs fell in quite narrow ranges: 2.57 to 2.96 (first and second run) and 1.23 to 1.38 (second and third run). These results were considered to be indicative of the fact that Kfs remained practically unaffected by the placement on the soil surface of a layer of contact material (second run) or by its removal (third run). The contact material did not alter the ponded infiltration measurements.


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  		Table 1 Statistics of the field-saturated hydraulic conductivity values, Kfs, measured in the NC, SA, and SP sites in each run (sample size for each site type, n = 5)

 
The decreasing trend in Kfs observed in successive ponding experiments was expected, based on observations in the literature that air entrapment and siltation phenomena may affect measurements taken at small time intervals (Reynolds and Elrick, 1986; Cislerova et al., 1988). Soil core experiments (Fig. 2) producing decreasing Kst values in the first three runs confirmed the field results. In addition, the complete saturation of the cores did not result in a substantial increase of the measured Ks. As expected, a saturated hydraulic conductivity higher than the satiated one measured in the first run was obtained only after removing the upper layer of soil from the core. The ratio of the two Ks values measured both before and after removing the upper layer of soil ranged between 0.28 and 0.60. This ratio can be considered representative of the relative influence of the short-term surface soil structure degradation on the hydraulic conductivity. The apparent inconsistency with the long-term soil structure stability discussed earlier may be explained by taking into account that structure-regenerating phenomena probably occur in the field and that the field and laboratory measurement of saturated conductivity usually involves removing a thin layer of soil.



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  		Fig. 2 Satiated hydraulic conductivity (Kst) values measured on undisturbed soil cores in three successive runs and saturated hydraulic conductivity (Ks) values measured on the same cores before (fourth run) and after (fifth run) removing a thin layer of soil from the surface of the core

 
The comparison among the K(h) relationships obtained by the three selected procedures showed that, with one exception (at h = -10 mm), the mean values of K were not statistically different (Table 2) , suggesting that the three methods produced essentially equivalent mean estimates of K at the considered pressure head values.


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  		Table 2 Summary of hydraulic conductivity values at saturation and at pressure heads of h0 = -10 mm (K-10), - 50 mm (K-50), and -120 mm (K-120) obtained by the different procedures

 
The individual estimates of the K(h) relationship determined by the pressure infiltrometer with tension infiltrometer and multistep techniques compared quite favorably in the h >= -120 mm range (Fig. 3a) . The pressure infiltrometer produced individual K(h) relationships (Fig. 3b) that were substantially different from the ones determined by the other two methods. Inspection of Fig. 3b also suggests that at least two {alpha} values (one for h >= -10 mm and the other for h <= -10 mm) are necessary to reproduce the shape of the K(h) relationship as determined by the tension infiltrometer technique. These results further support the conclusion by Reynolds and Elrick (1990) that ponded infiltration measurements alone are not reliable estimates of unsaturated K.



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  		Fig. 3 Comparison between the hydraulic conductivity (K) vs. soil water pressure head (h) relationships obtained with the pressure and tension infiltrometer technique and (a) the multistep method and (b) the pressure infiltrometer method

 
At h < 0 the differences between the CV values of K for the pressure infiltrometer with tension infiltrometer and multistep techniques were noticeable, but the estimates of K obtained by the two methods were essentially overlapping (Fig. 3a). For both methods, the variability of the K values decreased as h decreased, as also observed by Clothier and White (1981); however, for the pressure infiltrometer technique an opposite trend was observed. The CV value was very high at h = -50 mm and it further dramatically increased at h = -120 mm. At this pressure head, the pressure infiltrometer-based estimates of K varied over seven orders of magnitude, while the K values calculated by the two other methods varied over only one order of magnitude (Fig. 3).

The comparison between the pressure infiltrometer with tension infiltrometer and multistep techniques was encouraging and suggested that the two methods led to essentially equivalent results in the near-saturated zone (h >= -120 mm). However, they are very different theoretically and practically. Further comparison between the K estimates obtained, at a given site, by the tension infiltrometer and the multistep outflow approach is expected to produce more definitive results.

According to a two-tailed t test, the mean K-120 value obtained by the tension infiltrometer in the summer experiments (GM = 1.6 mm h-1) was significantly different from that obtained in early fall (GM = 2.6 mm h-1). Given the sign of the difference, this result was probably a consequence of the different site preparation procedure used for the two tension infiltrometer experiments. In fact, higher K-120 values were obtained in sites where the surface layer of soil was removed before the tension infiltrometer application.


   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Theory
 Materials and methods
 NOTES
 Results and discussion
 Conclusions
REFERENCES
 
The order of the pressure head values applied at the soil surface by the tension infiltrometer affected the K estimates. Comparison with data taken from the literature confirmed that this influence was not substantial at low pressure head values. Further specific investigations are necessary in order to extend the experimental knowledge of the sensitivity of K to the applied sequence (ascending or descending) and the dependence of this sensitivity on both the pressure head values used during the instrument application and the soil type.

The presence on the soil surface of a layer of contact material having a satiated hydraulic conductivity greater than the field-saturated hydraulic conductivity of the soil did not appreciably alter ponded infiltration measurements. A consequence of this result is that the presence of the contact material on the soil surface is not a factor affecting the infiltration rates taken at a positive head in practical use of the tension infiltrometer technique.

In the near saturated zone, the hydraulic conductivity–pressure head, K(h), relationships determined by the combined use of a pressure and a tension infiltrometer compared favorably with the ones obtained in the laboratory using multistep outflow experiments. This result is encouraging, given that the two techniques considered are becoming standard techniques for determining K. The individual K(h) relationships derived from only pressure infiltrometer measurements were very different from the ones obtained by the two other methods and a single {alpha} value of the Gardner exponential K(h) relationship was found to be not usable for reproducing the shape of the K(h) relationship as determined by the tension infiltrometer technique. These results confirmed that ponded infiltration measurements should not be used to estimate unsaturated hydraulic conductivity.


   ACKNOWLEDGMENTS
 
This study was supported by grants of the Italian Consiglio Nazionale delle Ricerche and of the Italian Ministero dell'Università e della Ricerca Scientifica e Tecnologica. V. Bagarello and M. Iovino set up the research. G. Tusa conducted most of the field work. All authors analyzed the results and participated in writing the paper. G. Tusa developed this research for her Ph.D. activity, which is co-funded by the European Social Fund of the European Community.


   NOTES
TOP
 ABSTRACT
 INTRODUCTION
 Theory
 Materials and methods
 NOTES
Results and discussion
 Conclusions
 REFERENCES
 
1 Mention of a product does not constitute endorsement by the University of Palermo. Back

Received for publication June 22, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Theory
 Materials and methods
 NOTES
 Results and discussion
 Conclusions
 REFERENCES
 




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