Vadose Zone Journal 1:172-178 (2002)
© 2002 Soil Science Society of America
The Influence of Hydraulic Nonequilibrium on Pressure Plate Data
G. W. Gee*,a,
A. L. Warda,
Z. F. Zhanga,
G. S. Campbellb and
J. Mathisonc
a Hydrology Group, Environmental Technology Division, Pacific Northwest National Laboratory, Richland, WA 99353, USA
b Decagon Devices, Pullman, WA
c Washington State University, Pullman, WA
* Corresponding author (glendon.gee{at}pnl.gov)
Received 12 March 2002.
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ABSTRACT
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Pressure plates are used routinely to measure water-retention characteristics of soils. Plates of varying porosity are used, depending on the pressure range of interest. For applied pressures up to 1.5 MPa, 15-bar porous ceramic plates with fine porosity are used because of their high bubbling pressure (>1.5 MPa), which limits airflow through the plate. The typical saturated hydraulic conductivity of the 15-bar plate is <3 x 10-11 m s-1. Low plate conductance coupled with decreasing soil hydraulic conductivities at high pressures strongly influence equilibrium times, which theoretically may extend to months or years. We measured the soil water pressures (suctions) for three soils, a sand, a silt loam, and a clay, placed on 15-bar pressure plates for 10 d or longer, with and without static loads and with and without using a kaolinite slurry to improve plate contact. Total matric suctions, inferred from peltier psychrometry data, were always <1.0 MPa. When sample height was increased from 1.5 to 3 cm, the water contents increased and total suctions decreased to 0.15 MPa for sand, 0.3 MPa for silt loam, and 0.55 MPa for clay. These data suggest that alternative methods other than pressure plates may be required to measure equilibrium water suctions of soils in reasonable times in the 1.5-MPa (15-bar) pressure range and that loading of the samples and use of kaolinite slurry appear to be ineffective in speeding equilibrium.
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INTRODUCTION
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SOIL WATER PRESSURE has been measured in soils using a variety of methods, including pressure plates, tensiometry, thermocouple psychrometers, heat-dissipation sensors, and filter-paper techniques (Klute, 1986; Gee and Ward, 2000). Soil water pressure is typically expressed in units of kilopascals or megapascals, and soil water pressure head in centimeters or meters. For unsaturated soils, these values are always negative (less than zero). For convenience, the terms soil matric suction and soil suction head are used to describe absolute (positive) values of soil water pressure and pressure head for unsaturated systems (Richards, 1965). As external pressure is applied to a wetted sample on a pressure plate, the matric suction or suction head correspondingly increases. Figure 1 lists various methods that have been used to measure the matric suction from 0 MPa (saturation) to >100 MPa (air dry). For pressure plates, a soil sample is placed on the plate, wetted and allowed to soak overnight, and then subjected to an external pressure. This causes water to flow from the soil through the pressure plate via a flow tube to the exterior of the pressure chamber where it is collected. When soil water is in equilibrium with the applied pressure, flow ceases, and the applied pressure is equal to the soil matric suction. The pressure method, developed by L.A. Richards (Richards and Fireman, 1943; Richards, 1948, 1965; Richards and Ogata, 1961), has been the method of choice for determining water-retention characteristics for literally thousands of soil samples during the past 50 yr (Clapp and Hornberger, 1978; Rawls et al., 1982). Specific details for use of pressure plates for matric suction measurements are provided by Richards (1965), Klute (1986), Topp et al. (1993), and Townend et al. (2001). Typically, soils are tested over the pressure ranges from 0 to 0.1 MPa, from 0.1 to 0.5 MPa, and from 0.5 to 1.5 MPa. The soils are tested on plates that have bubbling pressures (air-entry values) >0.1 MPa, >0.5 Mpa, and >1.5 MPa, respectively. Table 1 lists characteristics for plates that cover the pressure range from 0 to 1.5 MPa.
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Fig. 1. Methods for measuring water-retention characteristics. Note that no single measurement method spans the full range of matric suctions (after Gee and Ward, 1999).
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Table 1. Plate characteristics for selected porous plates used in pressure-plate apparatus (data courtesy of Soilmoisture Equipment Corp., Goleta, CA).
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Outflow Assessment
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In spite of extensive use of the pressure-plate method, there is little guidance for determining when equilibrium is actually achieved for a given soil. Only infrequently is the matric suction checked by an independent method (Richards and Ogata, 1961; Williams, 1968; Madsen et al., 1986; Jones et al., 1990). Typically, when soils are put on a pressure plate, pressure is applied from several days up to several weeks. When the outflow has materially decreased or stopped (i.e., too low to measure in reasonable time), equilibrium is assumed to occur. The cessation of drainage, however, often takes longer than several weeks, particularly for finer-textured soils. Stol (1965) suggested a way to estimate equilibrium water contents by using an analytical (nomograph) method that requires estimating both soil and plate conductances to predict the final drainage curve. Topp et al. (1993) suggested fitting the time-dependent weight loss or changes in volume outflow (after 48 h) to an exponential function. Using this method, Topp et al. (1993) estimated equilibrium times of up to 25 d for 76-mm-high cores. Both methods are somewhat empirical, relying heavily on initial drainage rates to predict final equilibrium. The standard Soil Survey (Soil Conservation Service, 1982), American Society of Testing and Materials (1999), or the Soil Science Society of America (SSSA) (Klute, 1986) procedures do not endorse either the Stol or Topp procedures, so there is no routine method to estimate equilibrium water contents. There are at least four conditions that make it difficult to predict equilibrium from drainage alone.
Reduced Soil Conductance
Coarse-textured soils (e.g., sands and gravels) generally have extremely low soil hydraulic conductivities at suctions >0.1 MPa. Fayer et al. (1992) reported unsaturated hydraulic conductivities of gravel and sand as a function of suction head. Their data show that there is a point where the unsaturated conductivity of these two materials is equal. At a matric suction value of 0.006 MPa for gravel and 0.1 MPa for sand, the unsaturated hydraulic conductivity is 2 x 10-16 m s-1. A simple Darcy-type flow analysis can be used to relate pressure-plate drainage flux to the product of the unsaturated soil hydraulic conductivity and the total head gradient. If the Fayer et al. (1992) gravel and sand are placed on a pressure plate and a 1.5-MPa pressure gradient applied across a 1-cm-high sample 5 cm in diameter, we compute that it would take more than 10 yr to detect flow of 0.1 mL of water when the material has been dried to suctions above 0.006 MPa for the gravel and above 0.1 MPa for the sand. In practice, tests of coarse-textured soils on 15-bar pressure plates may never result in a true equilibrium, since the transmission rate of water through the soil is so slow that equilibrium cannot be achieved in a reasonable time. The effect of reduced soil conductance is to increase equilibrium time, with the net effect of samples appearing wetter than if they had truly equilibrated.
Soil Shrinkage
Most cohesive soils shrink when they dry. Shrinkage can cause the soil to pull away from the plate, increasing contact resistance and delaying or stopping outflow. The effect of soil shrinkage is to increase equilibrium time, with the net effect of having samples appear wetter than if they were properly equilibrated. Townend et al. (2001) suggested that swelling clays in the field are generally subjected to an overburden load but gave no guidance as to what weight should be applied to the samples to replicate the overburden load and to keep the sample in intimate contact with the plate. The pressure-membrane apparatus is equipped with a rubber diaphragm that can be inflated to hold the sample to the membrane as the soil dries (Richards 1965). Such a flexible device can simulate overburden pressure and at the same time maintain plate contact during drying. However, pressure plates are not equipped with such devices. To deal with this problem, Klute (1986) recommended that a load (e.g., 700-g weight) be placed on top of each soil sample. In contrast, Topp et al. (1993) recommended that soil samples be embedded in a thin layer of kaolinite clay to ensure that contact be maintained during drying. It is not clear which, if either, of these methods is successful in reducing the impact of soil shrinkage, simulating overburden pressures, and minimizing contact problems. To our knowledge, there has been no independent check of the matric suction measured on samples where the weight loading has been applied or when kaolinite slurry has been added to the plate.
Plate Conductance
Plate conductance is generally thought to be nonlimiting to final equilibrium. Richards (1965) suggested that plate conductances (or hydraulic conductivities) are not critical "except for retentivity measurements at low suction values." In such cases, plate resistance controls the flow and limits drainage rates. For this reason, it is desirable at low suctions to use plates that have the highest possible conductance since higher conductance gives appreciably faster results (i.e., equilibrium is achieved sooner). In contrast, if a 15-bar plate is used for soils at low suction (i.e., 0.1 MPa), equilibrium might be possible, but much longer times would be required than if a 1-bar plate were used. Table 1 lists rated plate conductances for commercial pressure plates. While a ceramic plate with a relatively high conductance (or high hydraulic conductivity, say greater than 1 x 10-7 m s-1) may not limit final equilibrium, it is also true that ceramic plates can change conductance with time. This can be caused by pore plugging with inorganic (soil) colloids or organic materials (e.g., microbial slime). Pore plugging is more common than normally assumed. Unless care is taken in wetting soil samples, some slaking and dispersion occurs. For this reason, Klute (1986) has recommended the use of deaerated 0.005 M CaSO4 as the wetting fluid for neutral to alkaline soils. Even with care taken to minimize clay dispersion, conditions are often favorable for microbial growth, and pore plugging can occur with time, especially when the samples are allowed to equilibrate for long periods of time (e.g., weeks or months). The effect of lower plate conductance is similar to that of reduced soil conductance and similar to the effects of soil shrinkage. Samples run on plates with low conductance will appear wetter than if they were properly equilibrated.
Plate Leakage
It is possible that plates can develop leaks with time (Richards, 1965). It is generally recommended to check for leaks before starting tests by applying air to the outflow line while the pressure plate is submerged in water. Significant leaks also can be observed during soil testing by keeping the outflow line submerged in a tube partially filled with water. Very slow leaks (less than a few bubbles a minute) are often indistinguishable from the degassing of the water and air that passes through the plate, particularly for 15-bar plates at full pressure (Richards, 1965). Placing wet toweling in the pressure chamber can minimize the effect of a slow leak (Klute, 1986). Plate leakage generally causes the sample to desiccate, thus appearing drier than if the sample were properly equilibrated and no leak had occurred. Of the four test conditions related to equilibrium, plate leakage is probably the most easily detected and controlled.
To assess the effects of sample texture and thickness on pressure-plate equilibrium, we ran a series of numerical experiments. In addition, we ran a series of physical experiments that evaluated the effects of static loading and the addition of kaolinite slurry on pressure-plate equilibrium.
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NUMERICAL EXPERIMENTS
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A two-phase (unsaturated flow) version of the finite-difference code, STOMP (Subsurface Transport Over Multiple Phases) (White and Oostrom, 1996), was used to test the effects of soil conductance, plate conductance, and soil and plate thickness on soil water contents of pressure-plate samples. Table 2 lists the hydraulic parameter inputs required for the modeling. The entry pressure he was assumed to be 1.2 times the maximum allowable pressure for each plate. Parameter 
= 0.3 was assumed for all the plates. Using these parameters, the hydraulic conductivities of the plates were constant and equal to the Ksat values as described in Table 1. A plugged (degraded) ceramic plate with low conductance (listed in Table 2 as 15 Bar-DG) was assigned a Ksat value of 1 x 10-13 m s-1.
To test the effects of soil and plate thickness on the results, we ran three basic cases.- Type 1: soil thickness = 10 mm; plate thickness = 5 mm
- Type 2: soil thickness = 10 mm; plate thickness = 0.5 mm
- Type 3: soil thickness = 2 mm; plate thickness = 5 mm
Type 1 cases dominated the numerical testing because the specified thickness of soil and plate for Type 1 are common for most pressure-plate soil samples and ceramic plates. Type 2 was used to assess the impact of higher plate conductances for pressure membrane–type systems, and Type 3 cases were run to assess the impact of thinner soil samples on the equilibrium water content results.
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LABORATORY EXPERIMENTS
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Vantage sand (clayey-skeletal, smectitic, mesic Lithic Argixeroll), Ritzville silt loam (coarse-silty, mixed, superactive, mesic Calcidic Haploxeroll), and Red Bluff clay (fine, kaolinitic, thermic Ultic Palexeralf) were tested on 15-bar ceramic plates. The plates were obtained from Soilmoisture Equipment Corp. (Goleta, CA). Disturbed soil samples were placed on the pressure plates after the manner described by Klute (1986). Both large and small samples were tested. The smaller samples were held in rings with 4.4-cm i.d. and 1.5 cm high, and the larger samples were held in rings with 4.7-cm i.d. and 3.0 cm high. All of the samples were loaded with a 700-g lead weight (Klute, 1986) to simulate the confining pressure of near-surface soils and to assist in keeping the soil in contact with the plate during drying. The soil samples were wetted from below and allowed to saturate overnight. The pressure chamber was sealed and pressure applied until the pressure difference across the plate was 1.5 MPa (15 bar). The samples were held at this pressure for periods ranging from 3 to 10 d. For some of the tests, a thin layer of kaolinite was applied to the plate before the placement of the sample. Five replicates were run for each of the soils on each 15-bar pressure plate for each test.
Total soil water suction was measured using a commercial peltier psychrometer chamber (SC-10, Decagon Devices, Pullman, WA). Small samples from each replicate were placed into the psychrometer sample cups. A packing cone was used to ensure proper sample configuration for the psychrometer. The SC-10 peltier psychrometer measures the wet-bulb depression of the water vapor in equilibrium with the soil sample, which in turn is related to the total soil water suction through the use of the Kelvin equation (Rawlins and Campbell, 1986).
A simple expression for the total matric suction is:
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where ST is the total soil water suction (MPa), R is the gas constant, T is the Kelvin temperature, M is the molecular mass of water, and Aw is the water activity (relative humidity) of water vapor equilibrated with the soil water. The value of R/M is 0.461 MPa K-1. The psychrometer was calibrated using appropriate salt solutions of known relative humidity and was found to be reliable in estimating the total water suction with an error of 0.05 MPa or less in the range from 0.1 to 2.5 MPa. It was assumed that the matric suction was equal to the total soil water suction; thus, the osmotic suction, while not measured, was assumed to be negligible. We have found for these soils that this is a reasonable approximation. We also reasoned that any error due to neglecting the osmotic component would tend to make the actual error larger than reported. For example, if the measured suction were found to be 0.5 MPa for a sample equilibrated at 1.5 MPa, the apparent error would be 1.0 MPa (the difference between the measured suction and the applied pressure). If, however, the osmotic suction were not zero, but actually 0.2 MPa, and the measured total suction was 0.5 MPa, then the matric suction would be 0.3 Mpa, and the actual error would be 1.2 MPa. Based on this analysis, all errors in matric suction estimated using the SC-10 peltier psychrometer are underestimates of the true error, the difference being the osmotic component. We feel confident that the psychrometer data represent the minimum error in soil matric suction for the samples tested.
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RESULTS AND DISCUSSION
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Numerical Experiments
Results of the numerical experiments with two soils, a sand and silt loam, are reported in Fig. 2 through 7. The simulations show that the Type 1 (1-cm-thick) sandy soil does not equilibrate at any of the applied pressures in <100 d (Fig. 2 and 3). The lack of equilibrium is the result of low unsaturated hydraulic conductivity. Only when the soil hydraulic conductivity remains relatively high (e.g., for the silt loam soil) is equilibrium achieved for the 1- and 5-bar plate tests (see Fig. 4 and 5). Note, however, for the 15-Bar-DG (degraded) plate that equilibrium is never achieved with either soil (Fig. 3 through 6). This is the result of the very low plate hydraulic conductivity. These results suggest that if 15-bar plates do degrade (plug) with time, then the water-content values will be in error, and the resulting data will always show water contents that are too wet. It should also be noted that reducing the sample and plate thicknesses for the sand improved the approach to equilibrium (Fig. 6 and 7) but did not completely eliminate the nonequilibrium for reasonable times (<100 d). The Type 3 (2-mm-thick) sandy soil equilibrated, but it took nearly 100 d (Fig. 7). This suggests that for coarse soils, pressure plates will yield data that are wetter than true (equilibrium) soil matric suction values.
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Fig. 2. Time course of soil water matric suction for sand-equilibrated at 0.03 MPa (0.3 bar) on various pressure plates as indicated for (a), the 5-mm height (sample midpoint) and (b) the soil–plate interface.
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Fig. 7. Time course of soil water matric suction for thin soil sample of sand equilibrated at 0.03 MPa (0.3 bar) on various pressure plates for (a), the 1-mm height (sample midpoint) and (b)the soil–plate interface.
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Fig. 3. Time course of soil water matric suction for sand equilibrated at 0.5 MPa (5 bar) on various pressure plates as indicated for (a), the 5-mm height (sample midpoint) and (b)the soil–plate interface.
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Fig. 4. Time course of soil water matric suction for silt loam equilibrated at 0.03 MPa (0.3 bar) on various pressure plates as indicated for (a), the 5-mm height (sample midpoint) and (b) the soil–plate interface.
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Fig. 5. Time course of soil water matric suction for silt loam equilibrated at 0.5 MPa (5 bar) on various pressure plates as indicated for (a), the 5-mm height (sample midpoint) and (b) the soil–plate interface.
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Fig. 6. Time course of soil water matric suction for sand equilibrated at 0.03 MPa (0.3 bar) on various thin plates of various conductivities for (a), the 5-mm height (sample midpoint) and (b) the soil–plate interface.
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Laboratory Experiments
Tables 2 and 3 show the results of applying 1.5 MPa to soil samples on 15-bar pressure plates for various lengths of time (3–10 d). The laboratory results show that the soils did not equilibrate with an applied pressure of 1.5 MPa on 15-bar pressure plates for any of the soils for any of the test periods. All samples were tested using the prescribed method of loading the individual sample with a 700-g weight. In spite of this treatment, equilibration was not achieved. In general, the longer the samples were left on the plate, the higher the total soil water suction values, but under no test conditions did the soil water suction values exceed 1.0 MPa. When the sample height was increased from 1.5 to 3 cm, soil water suction values were reduced, indicating a soil conductance effect. The addition of kaolinite slurry, designed to improve plate contact, was inconsistent in its effect (Table 4). The suction values were lower for the 1.5-cm-high sand and silt samples but slightly higher for the clay samples. The 3-cm samples treated with kaolinite slurry and run for 9 d had consistently higher suction values than similar samples run without the kaolinite slurry for less time. However, the 3-cm-high samples all exhibited lower suction values than any of the 1.5-cm-high samples, with or without the kaolinite slurry treatment.
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Table 3. Total soil water suction (MPa) for soil samples pressurized at 1.5 Mpa on 15-bar pressure plates for 3, 5, and 10 d, without and with a kaolinite slurry contact.
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Table 4. Total soil water suction for 3-cm-high soil samples pressurized at 1.5 MPa on 15-bar pressure plates for 5 d and 9 d, respectively. Samples tested for 9 d had kaolinite slurry applied to the plate.
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In contrast to our modeling results and those of Campbell and Mulla (1990), the sand exhibited the highest suction values compared with the other two soils for all but the 5-d test with the 3-cm-high samples. It is possible that the hydraulic properties of the Vantage sand were unique, and the sand behaved more like a finer-textured soil. However, it is more likely that there may have been water loss in transferring samples from the pressure plate to the peltier psychrometer. Small (<10-g) samples were transferred from the pressure plate to the psychrometer cups, and in spite of care in transfer, there is a high probability that water was lost during transfer. The impact of water loss during transfer is greatest on the sand since the water content is less and the suction–water content curve is steepest. For example, the loss of 0.5 volume percent in sands can cause the suction values to increase more than 0.3 to 0.5 MPa. In contrast, a similar loss of water from the silt and clay would have less impact on the final suction values. For this reason, we believe that the results show a combined effect of the nonequilibrium (due to hydraulic constraints of soil and plate conductances), which are, in part, offset by sample drying. The data suggest that the absolute error in measured soil water suctions is underestimated by the psychrometer measurements. As indicated above, the addition of the osmotic component will also add to the suction value. Soil drying during transfer and osmotic suction effects cause the reported suction values to be higher than if these effects were not there. Finally, there are potential effects on the equilibrium suction values due to structural sample disturbance for samples taken from the pressure plate and placed in the psychrometers. However, Campbell and Gardner (1971) demonstrate that these effects are negligibly small for suction values much above 0.1 MPa, so for these tests, they were not considered significant. The overall assessment is that the psychrometer data provided a high estimate of the actual suction values for the soils equilibrated on the pressure plates.
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CONCLUSIONS
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Equilibrium on pressure plates is affected by both soil and plate hydraulic properties. For coarse-textured soils (sands and gravels), it appears that equilibrium cannot be achieved in a reasonable time (days to weeks) because of the very low soil water conductivities that occur at low suctions (e.g., hydraulic conductivities <1 x 10-14 m s-1 at <0.1 MPa). In spite of the nonequilibrium error, water contents measured for sands and gravels on 15-bar pressure plates are generally only slightly wetter than if they had been fully equilibrated with the applied pressure. This is because the water retention curves for coarse soils are generally very steep. For finer soils, equilibrium on pressure plates can be achieved, but generally only on plates that have relatively high conductivities (>1 x 10-10 m s-1). The numerical modeling indicated that 15-bar pressure plates did not equilibrate within 20 d for either of the two soils tested (sand and silt loam). This is apparently due to the low conductivity of the plate.
Measurement of the soil water suction using a peltier-psychrometer technique indicated that soil samples equilibrated on 15-bar pressure plates did not equilibrate with the applied pressure (1.5MPa), even when samples were loaded with 700-g weights to simulate confining pressures and ensure good contact with the plate. In addition, samples that were placed on kaolinite slurry also showed suction values that were lower than the applied pressure (1.5 MPa). For the sand, silt, and clay soils, neither loading the samples with a weight nor use of kaolinite slurry to improve soil–plate contact allowed the samples to equilibrate with the applied pressure during the testing period (up to 10 d). For samples equilibrated on 15-bar pressure plates, it appears that equilibrium is not attained in a reasonable time. Independent checks of the matric suction by use of psychrometry will help assess the disequilibria error. However, care must be taken with the psychrometric measurements to minimize water loss in transferring samples from pressure plates for analysis in the psychrometer. Based on these observations, alternative methods to pressure plates should be sought for determining 15-bar water contents. Among those that hold some promise are vapor equilibrium methods, which can indirectly measure the matric suctions above 0.5 MPa.
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REFERENCES
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Reevaluation of the Multistep Outflow Method for Determining Unsaturated Hydraulic Conductivity
Vadose Zone J.,
August 1, 2003;
2(3):
409 - 415.
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S. I. Hwang and S. E. Powers
Using Particle-Size Distribution Models to Estimate Soil Hydraulic Properties
Soil Sci. Soc. Am. J.,
July 1, 2003;
67(4):
1103 - 1112.
[Abstract]
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ABSTRACT
INTRODUCTION
