Unsaturated Conductivity Research Articles

Spatial Variability of Soil Hydraulic Properties in a Forested Hillslope

Spatial variability of soil hydraulic properties was measured in a forested hillslope and analyzed by applying combined water-retention-hydraulic-conductivity models (the LN and VG models) and a power function model for soil hydraulic conductivity (the Leibenzon model). Results showed that the pore-tortuosity parameterl in the LN and VG models should be treated as a fitted parameter for accurate descriptions of the unsaturated conductivity. The simultaneous optimization of both retention and conductivity curves is preferable to obtain appropriate descriptions of hydraulic properties when both retention and conductivity data are available for the parameter estimation. The Leibenzon model produced slightly poorer estimates than the LN and VG models. The exponent parameter in the Leibenzon model exhibited a spatial variation with the standard deviation of 2.38. The spatial variability of hydraulic properties was analyzed based on the spatial variation in parameters of the LN model obtained by the simultaneous optimization procedure. The parameter ψm, which has a positive correlation with median pore-radius, was generally small at the crest and upper slope locations and large at mid-slope to footslope locations. Except for the crest, surface soils had larger ψm values than the subsurface soils, suggesting a well-developed crumb structure in surface horizons of forest soils. For most soils, σ was greater than I, indicating a relatively large width of pore-size distribution. Saturated hydraulic conductivity (Ks) was generally small at crest and upper slope locations, and large at mid-slope to footslope locations. The largerKs values were attributable to largerψm values.

Near-saturated Hydraulic Conductivity and Water Retention in Coarse Podzol Profiles

There is little information available about the hydraulic conductivity of coarse podzol profiles where the vertical variation of the structure is considerable. The near - saturated conductivity of two coarse podzol profiles was measured by tension infiltrometer. The extrapolated saturated conductivity was 106.9 cm h- 1 in the very coarse sand and 16.7 cm h- 1 in the medium coarse sand parent soil, and decreased steadily upward, being between 0.7 and 1.3 cm h-1 in the two top horizons of these profiles. The unsaturated conductivities measured by the instantaneous profile method were in good accordance with the results of the tension infiltrometer, but the measurement range of the instantaneous profile method was rather narrow. The near - saturated hydraulic conductivity and water retention changed in both profiles in a systematic way, which agrees with the differences in matrix scale of structure as reflected in the measured soil textural and pedogenic properties.

Suitability of models for the estimation of soil hydraulic parameters

This paper deals with the applicability and validation of three predictive models for the estimation of soil hydraulic parameters (soil water characteristic and unsaturated hydraulic conductivity) by using routinely available soil data. Comparisons were made of the models of Campbell [Campbell, G.S., 1974. A simple model for determining unsaturated conductivity from moisture retention data. Soil Sci. 117, 311–314; Campbell, G.S., 1985. Soil Physics with BASIC. Elsevier, New York]; regression equations of Vereecken et al. [Vereecken, H., Feyen, J., Maes, J., Darius, P., 1989. Estimating the soil moisture retention characteristic from texture, bulk density, and carbon content. Soil Sci. 148, 389–403; Vereecken, H., Maes, J., Feyen, J., 1990. Estimating unsaturated hydraulic conductivity from easily measured soil properties. Soil Sci. 149, 1–12] and the model of Gregson, Hector, McGowan (GHM model) [Gregson, K., Hector, D.J., McGowan, M., 1987. A one-parameter model for the soil water characteristic. J. Soil Sci. 38, 483–486.]. The predictions of the above models were evaluated based on measured and calculated data from six German soils. Regression analysis showed that the GHM model, based on a single paired measurement of water content vs. matric head, produced noticeably better results than the other two models, which require input of textural properties of the soils. It is recommended that hydraulic parameters be estimated from paired measurements of water content vs. matric head rather than from textural properties of soils.

Maximum-likelihood estimation of unsaturated hydraulic parameters

Maximum-likelihood estimators properly represent measurement error, thus provide a statistically sound basis for evaluating the adequacy of a model fit and for finding the multivariate parameter confidence region. We demonstrate the advantages of using maximum-likelihood estimators rather than simple least-squares estimators for the problem of finding unsaturated hydraulic parameters. Inversion of outflow data given independent retention data can be treated by an extension to a Bayesian estimator. As an example, we apply the methodology to retention and transient unsaturated outflow observations, both obtained on the same medium sand sample. We found the van Genuchten expression to be adequate for the retention data, as the best fit was within measurement error. The Cramer–Rao confidence bound described the true parameter uncertainty approximately. The Mualem–van Genuchten expression was, however, inadequate for our outflow observations, suggesting that the parameters (α, n) may not always be equivalent in describing both retention and unsaturated conductivity.

MEASUREMENT OF UNSATURATED SOIL HYDRAULIC CONDUCTIVITIES USING A CERAMIC CUP TENSIOMETER

The objective of this study was to develop and evaluate a simple field method to determine unsaturated hydraulic conductivities using measurements of water flux into a tensiometer. The tensiometer consists of a ceramic cup glued to one end of a piece of plastic tubing. A suction is first applied to the inside of the tensiometer, which is closed to the atmosphere. The reduced pressure in the tensiometer causes water to flow into the tensiometer from the soil. As the water flows into the tensiometer, the volume of air in the tensiometer decreases, and pressure increases. The rate of water flow into the tensiometer, water flux, is calculated from the measured pressures using a form of the ideal gas equation, PV = Constant, and its full differential, PdV/dt+VdP/dt = 0, where P is the measured pressure, and V is volume. The water flux is obtained from the change in volume with time, -dV/dt. The parameters for the unsaturated conductivity equation are determined by using a two-dimensional finite element soil model (2DSOIL) coupled with a Marquardt-Levenberg algorithm to fit calculated fluxes to measured ones. For comparison purposes, unsaturated hydraulic conductivities were also determined for the same soil within 25-cm-diameter rings from measured water contents and matric potentials during drainage for two locations. Fitted and measured fluxes agreed well. Unsaturated hydraulic conductivities obtained from the tensiometer inflow data, however, were much less than unsaturated hydraulic conductivities measured during drainage. We attributed the differences to anisotropy and scale effects although clogging of the tensiometer pores by fine soil material could also be a contributing factor. The method is relatively quick, uses inexpensive materials, provides consistent results and is not limited greatly by the conductivity of the cup.

Agrophysical assessment of black shrinking soils of Tunisia

A new approach for assessment of soil physical conditions is based on forecasts of the soil water and air regimes. This approach uses the water retention curve and unsaturated conductivity of each soil layer to calculate soil regimes and estimate the possibilities of occurrence of periods of water and air deficit under standard boundary conditions. This approach has the advantages that it assesses these regimes, which govern crop yield, and considers properties of whole soil profiles. These advantages are manifested in the assessment of black shrinking soils.

Error Analysis of an Evaporation Method for Determining Hydrodynamic Properties in Unsaturated Soil

AbstractSolving soil unsaturated flow problems requires knowledge of the water retention θ(h) and unsaturated hydraulic conductivity K(θ) relationships. The purpose of this study was to investigate more thoroughly the properties and accuracy of an evaporation method described by Wind (1969) for determining θ(h) and K(θ) from laboratory cores. Evaporation from a vertical column of soil was first simulated using numerical solution of Richards equation for a given set of soil hydraulic properties. The simulated data were then used to evaluate the ability of Wind's method to provide estimations of the retention and unsaturated conductivity curves when measurement errors were taken into account. The main sources of error were (i) errors due to the position of the tensiometers in the sample, (ii) errors due to the calibration of the transducers used for the pressure head measurements, and (iii) errors due to layering in the soil column. The estimated water retention curves were sensitive only to soil layering. On the other hand, small uncertainties in the position of the tensiometers (1 or 2 mm) and in the calibration curve of the transducers for the pressure head measurements (1–5%) had a great influence on the estimated hydraulic conductivity curves. A correction procedure was proposed and was satisfactory when errors of position in the tensiometers were taken into account. Results also showed that temperature corrections related to viscosity of liquid water were large. Finally, this method gives poor estimates of hydraulic conductivities when raw tensiometric data are corrupted with small errors in their position or calibration.

An Experimental Approach for the Characterisation of Rigid Porous Media and Unsaturated Conductivity Relations

A laboratory procedure for the characterisation of rigid, porous media is developed and tested. The overall objective of the characterisation is to provide a basis for the choice of a relevant unsaturated conductivity relation. With the proposed procedure, the porous materials are characterised through hydraulic conductivity measurements, saturation-pressure measurements, porosity measurements and air entry pressure measurements. The measurements were conducted using water and a selected non-aqueous phase liquid (n-decane), and two types of homogeneous, rigid porous media of different origin (fritted glass samples with guaranteed homogeneity by the manufacturer, and chalk). The laboratory method successfully characterised the fritted glass samples as homogeneous on basis of calculated tortuosity values, measured bubble pressures and measured pore-size density curves. The contact angle between the liquid and the solid had negligible impact when comparing the drainage of water with the drainage of n-decane, and also when comparingn -decane drainage in dry porous media withn -decane drainage in media where a film of water separated then -decane from the pore wall. Also, the differences in saturated conductivities between n-decane and water in both samples of chalk and fritted glass were well predicted on basis of differences in density and viscosity of the liquids. The laboratory procedure was furthermore used for characterisation of the pore space available for non-aqueous liquid flow in partially water-saturated porous media. Then -decane conductivities in the fritted glass samples partially saturated with water were considerably lower than the n-decane conductivities in the dry fritted glass samples.

Implications of Spatial Variability of Soil Physical Properties for Simulation of Evaporation at the Field Scale

Soil water retention curves in eight soil profiles on a sandy soil were determined by field measurements of soil water content and soil water matric potential made during 19 days of continuous drying using time‐domain‐reflectometry and tensiometers. Evaporation from the different profiles was simulated using the retention curves and estimated unsaturated conductivity functions, meteorological data as driving variables, and measured soil water matric potentials at 40 cm depths as the lower boundary condition. Good agreement was obtained between simulated and measured evaporation from different locations in the field. Areal mean evaporation obtained as the arithmetic mean of individual simulations was 2% smaller than the evaporation obtained using mean physical properties in one simulation. This difference was small compared to the variation in simulated evaporation between individual locations (62–142% of the meanevaporation).

Multi-domain model for pore-size dependent transport of solutes in soils

A multi-domain model for the transport of chemicals in soils is developed. The solute flux is related to the microscopic water flux, which is modelled using concepts to estimate the hydraulic conductivity of porous media. The pore space of the soil is divided into an arbitrarily large number of domains each representing an equivalent pore radius. The domains are arranged on a structural coordinate, perpendicular to the direction of mean water flow. Transport in the flow direction takes place in each domain by convection and diffusion with pore-size specific velocities. Solute mixing between the domains is simulated as convective-dispersive transport along the structural coordinate. The model is solved numerically for one-dimensional steady-state water flux under unit-gradient conditions. Required input parameters are the unsaturated conductivity function of a soil and a pore interaction coefficient which characterizes the solute exchange between the pore domains. Simulations show a gradual change from convection dominated transport (isolated tube model) to convective-dispersive transport. The length scale where this change takes place depends on the lateral mixing intensity, pore-size distribution of the medium, and saturation degree.

Evaluation of Van Genuchten–Mualem Relationships to Estimate Unsaturated Hydraulic Conductivity at Low Water Contents

Predicting contaminant migration within the vadose zone, for performance or risk assessment, requires estimates of unsaturated hydraulic conductivity for field soils. Hydraulic conductivities, K, were experimentally determined as a function of volumetric moisture content, θ, for Hanford sediments. The steady state head control method and an ultracentrifuge method were used to measure K(θ) in the laboratory for 22 soil samples. The van Genuchten model was used to fit mathematical functions to the laboratory‐measured moisture retention data. Unsaturated conductivities estimated by the van Genuchten–Mualem predictive model, using the fitted moisture retention curve and measured saturated hydraulic conductivity, Ks, were compared to those obtained by a scaled‐predictive method that uses a single K(θ) measurement as a match point near the dry regime. In general, the measured K values and those predicted from van Genuchten–Mualem relationships showed considerable disagreement. This suggests that the use of laboratory‐measured Ks results in an inadequate characterization of K(θ) for the desired range of moisture content. Deviations between the measured and predicted K were particularly severe at relatively low moisture contents; for some samples, there were differences in excess of 2 orders of magnitude at low θ. However, use of the same moisture retention curve‐fitting parameters and a single steady state head control‐based K(θ) measurement near the dry regime resulted in considerable improvement. In fact, for the coarse‐textured soils considered in this study, results indicate that a K∥θ) measurement near the dry regime must be used to obtain reliable estimates of unsaturated K at low θ. The study provided important insight on application of two different experimental techniques of measuring unsaturated conductivities.

The relationship between structure and the hydraulic conductivity of soil

SummaryA random fractal matrix comprising a hierarchical aggregation of primary structural elements is used to capture the characteristics of a heterogeneous soil structure with a tortuous pore space. The influence of heterogeneity of both the solid matrix and the pore space, as well as the shape of the pore boundary, on the saturated and unsaturated hydraulic conductivity is studied. For such random structures, the fractal (Hausdorff) dimension alone is not enough to characterize the structure from the point of view of fluid flow and additional characterizations are introduced. The porosity, ρp, of the primary elements has a critical value, ρc. With probability 1, both the saturated and unsaturated conductivities are found to be dependent as a power law on the length scale, L, at which the measurement is made when ρp>ρc. When ρp<ρc, only the unsaturated conductivity is scaling in length scale, while the saturated conductivity becomes dominated, with probability close to 1, by the conductivity of the largest connecting pores in the structure, i.e. preferential pathways. The relationships between the parameters of the power laws and structure are derived and are found to depend on the fractal (Hausdorff) and spectral dimensions of the solid matrix, denoted dm and respectively. A discussion of the importance of these results for the interpretation and extrapolation of measurements is presented, and the implications for variability and predictability of the hydraulic properties of soil is discussed.

Soil containing rock fragments: effects on infiltration

Infiltration is a very complex process. Rock fragments affect infiltration by their presence in the soil matrix and on the soil surface. Procedures for quantifying soil and surface rock fragments are reviewed. Within the soil the physical properties and hydraulic properties that affect infiltration are shown to be influenced by rock fragments. Physical properties discussed are bulk density, porosity and water content. Hydraulic properties that affect infiltration are water retention, saturated conductivity and unsaturated conductivity. Rock fragments on the soil surface have been reported to both increase or decrease infiltration rates and amounts.

Comparison of Three Field Methods to Characterize Apparent Macropore Conductivity

AbstractThe objective of this study was to investigate some field‐oriented methods designed to characterize the flow rates of both the soil matrix and macropores near saturation. An apparent macropore conductivity was obtained by the difference between saturated and unsaturated hydraulic conductivities at −0.6 kPa of soil water pressure. Saturated conductivities were measured in double‐ring infiltrometers with tensiometers. Soil matrix conductivities were calculated from measurements of soil water pressures and drainage rate during redistribution and were also measured with a thin sand‐cement crust. Another set of data for saturated and unsaturated conductivities was obtained from unconfined ponded measurements in 76.2‐mm‐diam. rings and a tension infiltrometer. Unsaturated conductivities and calculated apparent macropore conductivities from the crust and redistribution methods were comparable. Unsaturated conductivities and apparent macropore conductivities from the tension infiltrometer and unconfined ponded measurements were greater than those from the infiltration‐redistribution method. We concluded that apparent macropore conductivity for the profile can be characterized by using ponded infiltration rates and matrix conductivities near saturation determined from redistribution data. This method accounts for the continuity of the macropore flow system in subsurface layers and involves a larger area for macropore infiltration measurement.

Fingered flow in two dimensions: 2. Predicting finger moisture profile

An exact integral solution for the moisture profile in growing fingers in sandy soil is derived from Richards' equation. The solution provides moisture content along a finger as a function of position and time and provides applicable results, including the calculation of the asymptotic matric potential of a growing finger and a method of obtaining the unsaturated conductivity in a single experiment. The solution is verified experimentally through comparison with measurements of matric potential and moisture content using high‐speed tensiometers.

Statistical analysis of spatial variability in unsaturated flow parameters

Core scale estimates of soil parameters of the Gardner‐Russo and van Gemachten models of the hydraulic conductivity and water retention functions were obtained for 417 undisturbed soil cores taken from a wall of a trench (20 m long, 2.5 m deep), using a procedure based on inverse problem methodology. These estimates were used to evaluate the first two statistical moments of the underlying random space functions (RSFs), using the restricted maximum likelihood estimation procedure, coupled with the weighted least squares procedure, to estimate parameters of models of the covariance and the drift functions of the pertinent RSFs. The fitted models were used to evaluate the mean and covariance functions of the hydraulic conductivity and water rentention functions for given water saturations. Covariance functions of log‐saturated conductivity (log Ks) and the “shape” parameters of the Gardner‐Russo and van Genuchten models exhibited statistical anisotropy characterized by aspect ratios that vary between 3 to 4. Correlation scales of log Ks were larger than those of the “shape” parameters. Consequently, the product of the variance of log unsaturated conductivity and its correlation scale remained essentially invariant for a considerable range of water saturation. The implications of these results regarding stochastic modeling of transport in heterogeneous porous formations and possible applications of the results of this study are discussed briefly.

Fiberglass Wicks for Sampling of Water and Solutes in the Vadose Zone

AbstractSampling solute concentrations in the vadose zone provides an early‐warning system for groundwater pollution. Various sampling devices are available. This study evaluates the functionality of fiberglass wicks in a soil solution sampler. Fiberglass wicks act as a hanging water column, drawing water from the undisturbed field soil without external application of suction. Flow characteristics of wicks were observed in the laboratory by miscible‐displacement tests using Br and an organic dye. The matric potential in the wick as a function of flow rate was measured. Fiberglass wicks behaved like a porous medium that effectively could apply a suction to the soil while only minimally retarding the organic dye. The relationship between matric potential, moisture content, and unsaturated conductivity was determined for several wicks. Based on these relationships, a set of curves was produced depicting travel time of nonadsorbed chemicals and the matric potential in the wick as a function of flow rate. These curves can be used for selecting optimum wick length and diameter for a given sampler configuration.

Upscaling of hydraulic conductivity in partially saturated heterogeneous porous formation

This paper describes a procedure to upscale unsaturated hydraulic conductivity from the scale at which it is measured to a scale more amenable to actual flow simulations in partially saturated heterogeneous porous formations. In the approach adopted here the block conductivity is defined as the ratio of volume‐averaged flux to the volume‐averaged head gradient. The “point” values of the underlying formation properties (characterized by the saturated conductivity Ks and the pore size distribution parameter α) are viewed as realizations of independent random spatial functions. The general approach is applied to a partially saturated, heterogeneous, three‐dimensional statistically anisotropic formation, with statistical isotropy in the horizontal plane and arbitrarily oriented stratification characterized by a finite aspect ratio, for the case where the mean gradient exists only in the vertical direction. The procedure is illustrated by providing a second‐order approximation of the mean (in terms of the components of the block conductivity tensor) and a first‐order approximation of the covariance function of the block conductivity, which depend on the mean capillary pressure head H, the distribution of the “point” values of the underlying log unsaturated conductivity, and the block size. The results of this study allow assessment of the minimal domain size for which the concept of effective properties is appropriate and assessment of the maximal size of conductivity cells which preserve the heterogeneous structure of the porous formation with respect to simulation of flow and transport in the vadose zone. The application of the results to unconditional or conditional simulation of unsaturated flow and transport is straightforward, since knowledge of the mean and the covariance of the block unsaturated conductivity may lead to a relatively simple algorithm for generating its values on a grid at any level of aggregation desired.

Prediction of unsaturated hydraulic conductivity changes of a loamy soil in different salt solutions by using the equivalent salt solutions concept

Equivalent salt solutions series have been previously defined as solutions with combinations of sodium absorption ratio (SAR) and electrolyte concentration (E,) producing the same extent of clay swelling in a given soil. These equivalent salt solutions series values have yielded satisfactory predictions of changes in saturated hydraulic conductivity, with changes in salt solution composition and concentrations. In the present study, previously published data on changes in saturated and unsaturated hydraulic conductivities of Gilat soil in salt solutions of cationic ratio 0-50 (mmol dm-3)1/2 and electrolyte concentration 2-50 (m.e. dm-3) were used to compare the equivalent salt solution series values for hydraulic conductivities at different water contents. The equivalent salt solution series causing a given change in saturated hydraulic conductivity of a loamy Gilat soil were derived. These equivalent salt solution values were used to predict the unsaturated hydraulic conductivities of this soil at low water contents. Predictions of unsaturated conductivity at relative water contents (�) ranging from 0.80 to 0.20 agreed closely with the measured values. Coefficients log a1 and b1 for Gilat soil, in the equation log Ec = log a1+b1 log SAR, relating the Ec and SAR values of each equivalent salt solutions series were determined at � values between 1.00 and 0.20. The relationship between log a1 and bl was similar at all water contents, in agreement with the equivalent salt solutions concept. Therefore, equivalent salt solution parameters derived from saturated hydraulic conductivity measurements could be used to predict changes in unsaturated conductivities and hence flow rates of saline water under specified boundary conditions.

BIMODAL POROSITY AND UNSATURATED HYDRAULIC CONDUCTIVITY

Soil water characteristic curves determined on undisturbed soil samples in the laboratory were used for the calculation of the unsaturated conductivity function K(h). This computed K(h) was compared to the unsaturated conductivity data measured directly in the field. When the soil water characteristic curve was smoothed according to the van Genuchten procedure and K(h) was evaluated according to Mualem's approach, the coincidence of results of both procedures was poor. When a cubic spline was used for the water retention data, the derived pore size distribution curve showed two peaks. With the assumption on a bimodal porous system, K(h) was computed for each of the subsystem of pores. The function K(h) of the whole porous system is then composed of two parts, one valid close to saturation, the second for medium wetness of the soil. At the intersection of both components of K(h) a singularity exists. The agreement between the K(h) so computed and the directly measured K data has been improved. The hypothesis on the existence of the bimodal porosity within the range of the capillary soil porous system has been therefore confirmed together with the proof on the practical importance of both subsystems when they are detected in the soil.