Abstracts

Ankeny, M.D., Ahmed, M., et al. 1991, 'Simple Field Method for Determining Unsaturated Hydraulic Conductivity', Soil Science Society of America Journal, vol. 55, pp. 467-470.

A new method is proposed for determining in situ unsaturated hydraulic conductivities from unsaturated infiltration measurements made at several tensions on the same infiltration surface. Wooding's equation for steady-state unconfined infiltration rates is used in calculating hydraulic conductivities. Hydraulic conductivities calculated with the new method are consistent with unit gradient laboratory measurements of saturated and unsaturated hydraulic conductivity. This simple field method is potentially valuable because it is faster than unit gradient laboratory methods, and it is less disruptive of pore continuity than other field infiltration techniques.

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Ankeny, M.D., Kaspar, T.C., et al. 1988, 'Design for an Automated Tension Infiltrometer', Soil Science Society of America Journal, vol. 52, pp. 893-896.

An automated tension infiltrometer useful for a range of water tensions from 0.02 to 0.50 m and for infiltration rates of 1 x 10^-8 to 5 x 10^-4 m s^-1 is described. Infiltration rates are calculated from the change in water height in a Mariotte column. Water height is automatically measured by using the difference in tension between two pressure transducers, one installed at the top, and the other at the bottom, of the Mariotte column. Precision of water height measurement using two transducers (SD = 2.2 mm) is increased as compared with height measurements made with only one transducer (SD = 6.2 mm). With only one transducer, much of the measurement error is due to tension fluctuations or "noise" caused by bubbling in the Mariotte column. The new design also incorporates several features used or suggested by other workers. A bubble tower is used for tension regulation, and interchangeable Mariotte columns of different diameters are used to match column volume with expected cumulative infiltration. Advantages of the new device include (i) quick and accurate tension control at low tensions, (ii) improved measurement precision at low flow rates, and (iii) automatic measurement and data collection, which increases measurement speed and eliminates bubbling-induced variability.

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Bagarello, V. and Iovino, M. 2003, 'Field Testing Parameter Sensitivity of the Two-term Infiltration Equation using Differentiated Linearization', Vadose Zone Journal, vol. 2, pp. 358-367.

Knowledge of the hydraulic conductivity of the vadose zone is important in many agronomic, engineering and environmental areas. Transient tension infiltrometer experiments can be used to estimate the hydraulic conductivity, K₀, corresponding to a given pressure head by a transient single-test (TST) method that uses the coefficients C₁ and C₂ of the two-term infiltration equation. A differentialized linearization (DL) method was previously proposed to estimate these coefficients when a layer of contact material is used for the experiment. A field test of the DL and TST methods was conducted on a sandy loam and a clay soil. Eliminating the early-time influence of the contact layer was easy when the sorptivity of the contact material was 10 to 12 times higher than the soil sorptivity. In other cases a transition zone, which complicated application of the DL method, appeared between the decreasing and increasing portions of the data set. Therefore, applicability of the DL method required large differences in capillary forces between the contact material and the soil. Estimates of K₀ varied by up to 650% with the duration of the experiment and <50% with the time interval between readings at the water reservoir. Sensitivity of K₀ to the experiment duration was particularly remarkable for the sandy loam soil for soil durations. Considereing a minimum duration of the experiment of approximately 1 h caused estimates of K₀ to vary by a maximum of 40% with the duration of the experiment.

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Bodhinayake, W., Si, B.C., et al. 2004, 'Determination of Hydraulic Properties in Sloping Landscapes from Tension and Double-Ring Infiltrometers', Vadose Zone Journal, vol. 3, pp. 964-970.

The majority of landscapes, natural or cultivated, are nonlevel. However, specifically designed instruments are not available for estimation of soil hydraulic properties in sloping landscapes. The objective of this study is to examine if tension and double-ring infiltrometers are suitable for determination of soil hydraulic properties on sloping soil surfaces. A field experiment was conducted in a silt loam soil (Typic Haplustolls) in Saskatchewan, Canada to explore the usefulness of tension and double-ring infiltrometers for the determination of soil hydraulic properties in sloping landscapes. Soil surfaces were created to represent four treatments, 0 (level), 7, 15, and 20% slopes. For each treatment, water infiltration rates were measured using a double-ring infiltrometer and a tension infiltrometer at -3, -6, -10, -13, -17, and -22 cm water pressure heads. In addition, three-dimensional computer simulation studies were performed for a tension infiltrometer with various disc diameters and water pressure heads for different surface slopes. Steady-state infiltration rate, field-saturated hydraulic conductivity, unsaturated hydraulic conductivity as a function of water pressure head, macroscopic capillary length parameter, and water-conducting macro- and mesoporosity were compared for different surface slopes. These parameters were not significantly different (p < 0.05) between level and sloping lands. Experimental and numerical results of this study suggest that both tension and double-ring infiltrometers are suitable for characterization of surface soil hydraulic properties in landscapes with slopes up to 20%.

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Casey, F.X.M. and Derby, N.E. 2002, 'Improved Design for an Automated Tension Infiltrometer', Soil Science Society of America Journal, vol. 66, pp. 64-67.

Automated measurements of water infiltration rates are commonly done using two gage transducers to measure water level changes in the reservoir of an infiltrometer. Previous studies have evaluated and described in detail infiltrometers automated with two gage-transducers and have shown that measurement precision and accuracy of soil hydraulic properties are improved. A previous study has also suggested the use of a single differential transducer to automate an infiltrometer to eliminate measurement error associated with air bubbles in the infiltrometer reservoir. In this study, the automation of a tension infiltrometer using a differential transducer was developed, evaluated and applied. A single differential transducer was installed at the bottom of an infiltrometer reservoir and the other end was connected by tubing to the head-space in the reservoir. Calibration of the reservoir height measurements vs. transducer voltage output was simplified over previous methods. Measurements were also done to demonstrate the use of the single differential transducer set-up to obtain field measurements of unsaturated hydraulic conductivity and sorptivity. Unlike previous methods though, this method does not allow for the determination of the imposed potential at the soil surface unless adapttions are made.

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Fuentes, J.P., Flury, M., et al. 2004, 'Hydraulic Properties in a Silt Loam Soil under Natural Prairie, Conventional Till, and No-Till', Soil Science Society of America Journal, vol. 68, pp. 1679-1688.

Tillage in the Palouse region of Washington State over the past 100 yr has influenced the soil physical and biological properties. In particular, hydraulic properties are significantly affected by soil cultivation. The objectives of this study were to assess the temporal patterns of soil hydraulic properties under three management systems,natural prairie (NP), conventional till (CT), and no-till (NT), and to compare hydraulic properties between these three systems. Saturated and near-saturated hydraulic conductivities (up to -15 cm-H₂O hydraulic head), and soil water retention curves were determined using intact soil cores taken from the top 10 cm of soil. Soils were sampled at six different times during a period of 1.5 yr from a NP, a long-term (>100 yr) CT, and a 27-yr-old NT system. The NP represented the original soil and natural vegetation of the area. Significant temporal variation in hydraulic conductivity was found. Temporal variation was most evident in the NP soil, where organic matter content was twice as large as under the CT and NT soils. Hydraulic conductivities in the NP were about one order of magnitude larger than in the cultivated soils. In NT, saturated hydraulic conductivities in the top 5 cm of soils were significantly larger than in CT. No-till and CT soils had similar near-saturated hydraulic conductivities, indicating that even 27 yr of continuous NT could not restore the original hydraulic properties of the soil. Restoration of original hydraulic properties in cultivated former prairie soils may take considerably longer.

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Logsdon, S.D. and Jaynes, D.B. 1993, 'Methodology for Determining Hydraulic Conductivity with Tension Infiltrometers', Soil Science Society of America Journal, vol. 57, pp. 1426-1431.

Tension infiltrometers have become a valuable tool for understanding infiltration in macropores and the soil matrix, but methodology varies. Our objective was to compare tension infiltrometer techniques and calculation procedures for determining unsaturated hydraulic conductivity, K(h), as a function of soil water pressure head (h). Field tension infiltrometer measurements were run to determine K(h) from: (i) steady-state infiltration into an excavated one-dimensional column, (ii) calculated sorptivity and measured change in soil water content for steady-state three-dimensional infiltration into dry soil, (iii) steadystate three-dimensional infiltration with two infiltrometer base sizes, and (iv) steady-state infiltration for three negative heads at the same location using two different calculation schemes. For one scheme, a nonlinear regression method was used to fit α [a constant relating in (K) and h] and K(0) from measured infiltration across three negative heads. The fitted α and K(0) were then used to calculate K(h) at each negative pressure head. Calculated K(h) by the nonlinear regression method from three-dimensional infiltration measurements were 105% of measured one-dimensional rates (from excavated columns), closer than any other method of calculation. More importantly, this method did not result in calculated K(h) less than zero or larger than three-dimensional infiltration rates, as some calculation procedures did. The method did not depend on determinations of sorptivity or on initial or final soil water content.

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Perroux, K.M. and White, I. 1988, 'Designs for Disc Permeameters', Soil Science Society of America Journal, vol. 52, pp. 1205-1215.

Disc permeameters are designed to measure hydraulic properties of field soils containing macropores and preferential flow paths and are particularly useful in soil management studies. We present here designs for disc permeameters for both positive and negative water supply heads. The effects of the water supply membrane and soil contact material on permeameter performance are examined using approximate quasi-analytic solutions to the flow equation. This analysis provides approximate criteria for the selection of membrane and soil contact materials. Limitations to performance caused by restricted air entry are considered and design criteria are given also. We present in situ tests of the disc permeameter for the early stages of one-dimensional infiltration and an example of the deterministic variation of sorptivity of a field soil with supply potential. Finally, we use ponded and unsaturated sorptivities measured in situ with disc permeameters to find the saturated hydraulic conductivity and flow-weighted mean characteristic pore dimension of a field soil.

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Rachman, A., Anderson, S.H., et al. 2004, 'Influence of Stiff-Stemmed Grass Hedge Systems on Infiltration', Soil Science Society of America Journal, vol. 68, pp. 2000-2006.

The ability of grass hedge systems to reduce runoff is critical to their effectiveness in controlling soil erosion. The reduction in runoff depends on the infiltration properties of soil managed with hedges. The objective of this study was to evaluate the effects of stiff-stemmed grass hedges on infiltration. The experiment was conducted on a site, which had been managed with switchgrass (Panicum virgatum L.) hedges for 10 yr at the USDA-ARS research station near Treynor, IA. The predominant soil was Monona silt loam (fine-silty, mixed, superactive, mesic Typic Hapludolls). Ponded infiltration measurements were used to determine field-saturated hydraulic conductivity (K_fs). Three positions were sampled: within grass hedges, within a deposition zone 0.5 m upslope from grass hedges, and within a row crop zone 7 m upslope from the hedges in soybean (Glycine max) production during 2001 and corn (Zea mays L.) production during 2002. A tension infiltrometer was used to measure infiltration at three selected tensions (50, 100, and 150 mm) in the grass hedge and row crop positions. The physically based Parlange, the Green and Ampt, and the empirically based Kostiakov infiltration models fit the measured data well (r² = 0.99�1.00). The K_fs within the grass hedge position was more than seven times greater than in the row crop position and 24 times greater than in the deposition position. The infiltration rate at 50- and 100-mm tension in the grass hedge position was significantly larger (P < 0.01) than in the row crop position; values at 150-mm tension were not significantly different. The K_fs was found to be similar in magnitude to laboratory measured saturated hydraulic conductivity (K_sat) treated with bentonite to eliminate by-pass flow. Grass hedges were found to enhance water infiltration compared with conventional row crop management.

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Ramos, T.B., Gonçalves, M.C., et al. 2006, 'Estimation of Soil Hydraulic Properties from Numerical Inversion of Tension Disk Infiltrometer Data', Vadose Zone Journal, vol. 5, pp. 684-696.

Many applications involving variably saturated flow and transport require estimates of the unsaturated soil hydraulic properties. Numerical inversion of cumulative infiltration data during transient flow, complemented with initial or final soil water content data, is an increasingly popular approach for estimating the hydraulic curves. In this study, we compared Mualem-van Genuchten (MVG) soil hydraulic parameters obtained from direct laboratory and in situ unsaturated hydraulic conductivity measurements with estimates using numerical inversion of tension infiltration data of four coarse- to medium-textured soils in Alentejo (Portugal). The laboratory methods used were suction tables, pressure plates, and the evaporation method as applied to undisturbed soil samples collected from the surface horizons of four different soil profiles. Field measurements were taken with a tension disk infiltrometer using consecutive supply pressure heads of -15, -6, -3, and 0 cm. Six MVG parameters (residual soil water content [Θ_r], saturated soil water content [Θ_s], empirical shape factors α, η, and ι, and saturated hydraulic conductivity [K_s]) were estimated from the field data by numerical inversion using the HYDRUS-2D software package, and compared with values estimated from the laboratory data. Macroporosity was also determined. The laboratory- and field-measured water retention curves were found to agree closely for most experiments as reflected by relatively high values of the coefficient of determination, the modified coefficient of efficiency, and the modified index of agreement (always >0.9949, 0.8412, and 0.8931, respectively). The unsaturated hydraulic conductivity curves were predicted less accurately, although good estimates of K_s were obtained.

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Reynolds, W.D. and Elrick, D.E. 1991, 'Determination of Hydraulic Conductivity using a Tension Infiltrometer', Soil Science Society of America Journal, vol. 55, pp. 633-639.

A new procedure is presented for in situ determination of saturated and near-saturated hydraulic conductivity [K_fs and K(Ψ), respectively] from a sequence of steady infiltration measurements made at everal tensions on a single infiltration surface. The method applies to tension infiltration from either a surface disk or from within a ring inserted a small distance into the soil. The analysis employs a modification of Wooding's solution for infiltration from a shallow pond, combined with numerically determined shape factors that account for the interaction effects between flow geometry and soil properties. The method is found, via numerical simulations, to have an overall accuracy within about ±7%, regardless of whether the predicted K(Ψ) function is flat with an indistinct air-entry value, steep with a distinct air-entry value, or very steep with no air-entry value. Some important practical features of the method are that it does not require measurement of the often-difficult square-root-of-time infiltration behavior, no special considerations are required regarding the thickness of the contact layer, it minimizes the effect of local spatial heterogeneity by using only one infiltration surface per set of K(Ψ) measurements, it avoids the use of potentially unstable simultaneous-equations solution procedures, and it can be applied to tension infiltration from both disk and ring infiltrometers.

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Wang, D., Yates, S.R., et al. 1998, 'Determining Soil Hydraulic Properties using Tension Infiltrometers, Time Domain Reflectometry, and Tensiometers', Soil Science Society of America Journal, vol. 62, pp. 318-325.

Tension infiltrometers have become a popular instrument for field determination of soil hydraulic properties. To develop and test different models for parameter estimation based on tension infiltrometer measurement, we obtained simultaneous measurements of transient tension infiltration rate, soil water content, and tension using small time domain reflectometry (TDR) probes and tensiometers installed at fixed locations relative to the infiltrometer disk. Infiltration was made with 10- and 20-cm-diam. disks under 1 and 5 cm of water supply tensions. The soil is an Arlington fine sandy loam (coarseloamy, mixed, thermic Haplic Durixeralf). Wooding's steady-state approximate solution for water flow from a surface circular pond was used to estimate the saturated hydraulic conductivity (K_s) and an empirical parameter (α_G) used in Gardner's exponential hydraulic conductivity function. These two parameters (i.e., K_s and α_G) were then independently estimated using an integral form of the steady-state Darcy-Buckingham flux law. A sorptivity method was also proposed as an alternative to Wooding's steady-state approach. Calculated K_s and α_G with the Darcy-Buckingham flux law method was in good agreement with estimates using Wooding's steady-state approximation. The sorptivity method produced K_s estimates that were statistically similar to those obtained with Wooding's method. The K(h) inferred from measured Θ(h) underestimated the conductivity close to saturation compared with estimates obtained from the infiltrometer measurements.

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