Unsaturated Hydraulic Properties Of Soil Research Articles

Determining the Unsaturated Hydraulic Properties of Cohesionless Soils: An Integrated Experimental and Inverse Modeling Approach

Abstract A simple, inexpensive and quick method for measuring the van Genuchten soil water retention curve (SWRC) model parameters for cohesionless soil is presented. The proposed method is an integration of laboratory testing and numerical modeling. The experimental testing consists of multistage bottom-top water imbibition of a dry sand column while recording pore pressure at the inlet. The experiment is then simulated numerically using Hydrus 1D, and the van Genuchten SWRC model parameters are determined through inverse modeling to match the recorded pore pressures. The soil column flushing includes multiple flow stoppages and resumptions, all before permeating one pore volume of water. The flow stoppage results in a time-dependent redistribution of moisture near the waterfront that is controlled by the unsaturated hydraulic properties of the soil and is reflected by changes in the pore pressure measurements. The uniqueness of this method comes from the simple experimental setup and the multistage flushing that can reduce the testing time by a factor of 30 compared with other methods. The results from testing three different sands with different grain size distributions are presented. The new method generated consistent results from multiple specimens tested at a range of flow rates for the three sands. The hypothesis behind moisture redistribution during flow stoppage was confirmed through a combination of experimental and numerical observations. The van Genuchten SWRC parameters obtained from the new method were comparable to those obtained using the conventional hanging column test for Sands B and C and available data in the literature. Sand A could not be tested using the hanging column test because of its large particle size, which demonstrates the wide range of soils that can be tested using the new proposed method.

Factorial kriging analysis leverages soil physical properties and exhaustive data to predict distinguished zones of hydraulic properties

Measurement of unsaturated hydraulic properties of soil is a time-consuming endeavor which limits the scale of studies. A stratified sampling scheme could enable more efficient use of samples but some information about the strata is needed a priori. The aim of this study is to define zones of soil hydraulic properties using remotely sensed and soil physical inputs. Samples were collected from 50 locations at 4 depth intervals in a 20.8 ha field located along the margin of the Venice Lagoon, Italy, and characterized by paleo-channel structures and highly heterogeneous soils. Water retention curves (WRC) and unsaturated hydraulic conductivity curves (UHC) were determined via inversion of measurements taken in the lab using the Wind method. Factorial kriging analysis (FKA) was applied separately to hydraulic parameters and soil physical properties and the 4 depths treated as independent samples. The mapped principal components (PCs) were used in a fuzzy-c means algorithm to define zones of like properties. To examine the physical significance of these zones, curve parameters and hydraulic curves were investigated. Zonation was able to distinguish between θs (saturated water content), n (shape parameter) and α (inverse of air entry), while θr (residual water content) and Ks (saturated conductivity) were not statistically different between the groups. For curve comparisons, WRCs were found to be significantly different between zones at all tensions while effective saturation curves (Se) differ for the majority of tensions (except at 28 cm), and UHCs did not differ. The spatial relevance of the zones was examined by overlaying hydraulic zones with zones defined using the FKA and fuzzy-c means approach from soil physical properties such as texture and bulk density. The hydraulic zones overlaid the physical property zones with an areal accuracy ranging from 46.66% to 92.41%. The similarity between these zone sets suggests that there is a potential to predict hydraulic zones from zones defined from soil physical properties. This work illustrates the potential to incorporate geospatial statistics in a stratified sampling scheme for hydraulic properties.

Extended multistep outflow method for the accurate determination of soil hydraulic properties near water saturation

Inverse modeling of multistep outflow (MSO) experiments is an established and fast method to determine unsaturated hydraulic properties of soils. A disadvantage of the method is its low sensitivity with respect to the hydraulic conductivity function at saturation, which makes the respective estimation very uncertain. Thus, the use of independently measured values for the saturated hydraulic conductivity, Ks, is generally recommended. This involves disadvantages, namely, the effort to conduct additional experiments and the general problems associated with the combination of data from different experimental sources. To overcome this problem, we propose to combine percolation and outflow in one experiment. This extended multistep outflow experiment (XMSO) starts with a completely water‐saturated soil column, on top of which a small amount of water is ponding. The first experimental phase is a saturated percolation under falling‐head conditions. After ponding ceases, the experiment continues as a standard MSO experiment with an unsaturated drainage process. The XMSO experiment is evaluated by inverse modeling, using measurements of cumulative outflow and tensiometric pressure head data. Our analysis of synthetic and real data demonstrates that XMSO yields very accurate estimates of saturated and near‐saturated hydraulic properties, even for soils with structured pore systems. Furthermore, saturated hydraulic conductivities of the supporting porous plate and the soil can be simultaneously determined with great accuracy. We conclude that the XMSO experimental design solves the problem of the identification of near‐saturated hydraulic properties of soil samples and reduces the estimation uncertainty of Ks to a minimum.

Evaporation Method for Measuring Unsaturated Hydraulic Properties of Soils: Extending the Measurement Range

The evaporation method is frequently used for simultaneous determination of soil water retention and hydraulic conductivity relationships. Tension is measured at two depths within a short soil column as water evaporates from its surface. Water content and flux are determined by weighing the column. Tensions, water contents, and fluxes are used to derive the water retention curve and the unsaturated hydraulic conductivity function. The measurement range of the conventional procedure is limited on the wet end by the inability of pressure transducers to accurately register very small tension differences. Hence, the resulting calculated hydraulic gradient in the vertical direction is associated with large uncertainties. On the dry end, water cavitation in the tensiometer, which typically occurs around 70 to 90 kPa, is the limitation. We present here a new design based on improved tensiometers that resist cavitation to much higher tensions, some reaching values as high as 435 kPa. On the wet end, data from a simple steady‐state method were used to supplement the evaporation method. On the dry end, applying the new tensiometers enabled the quantification of hydraulic functions up to 293 kPa average tension. Experimental results and soil water simulation affirmed the validity of the linearization assumption, even on the dry end when nonlinear tension–depth profiles emerge. The application of evaporation functions as a supplement for frequent weighing reduces costs for the equipment and increases the effectiveness of the method. Their validity for deriving fluxes was confirmed for the extended range, too. Results are presented for soil samples of different textures (sand, loam, silt, clay, and peat), various origins, and various dry bulk densities.

Development of a Large-Scale Infiltration Tank for Determination of the Hydraulic Properties of Expansive Clays

Abstract A large-scale infiltration tank was developed to study the water transfer in compacted expansive clay. Volumetric water content sensors were buried in a soil column for water content monitoring during infiltration. In addition to water content, soil suction and temperature at various locations and the heave at the soil surface were also monitored. Emphasis was put in minimizing the effect of sensors installation on water transfer and soil deformation. The results obtained for 338 days of infiltration were presented in terms of changes in suction, volumetric water content, temperature, and the soil heave. Based on the recorded data, the performance and limitation of different suction and volumetric water content sensors and the adopted test procedure were analyzed. The recorded data on soil suction and volumetric water content were finally analyzed for determining the unsaturated hydraulic properties of soil, such as the water retention curve and the unsaturated hydraulic conductivity. Note also that the results constitute useful data for further physical analysis or numerical models’ calibration.

Estimating Soil Hydraulic Parameters of a Field Drainage Experiment Using Inverse Techniques

Accurate assessment of water flow and contaminant transport in unsaturated porous media at the field scale is often hindered by difficulties associated with obtaining reliable estimates of soil hydraulic properties. The unsteady drainage‐flux method is one of the commonly used methods to measure in situ unsaturated hydraulic properties of soils. However, the properties obtained by this method using instantaneous profile data analysis may not be the best estimation of actual values of hydraulic properties. We present an improved analysis of the data from drainage experiments using inverse modeling, which uses nonlinear regression methods to estimate hydraulic parameters. Parameter identifiability is evaluated through sensitivity and uniqueness analyses. We used the combination of the inverse modeling program, UCODE, with the flow simulator, STOMP, for inverse modeling. Applying the inverse method to a field drainage experiment in sandy soil showed that all the van Genuchten (1980) hydraulic parameters could be estimated uniquely when both water content (θ) and pressure head (h) data were used. The parameter estimates by inverse technique using both θ and h data simulated the flow better than the parameter values obtained by the conventional instantaneous‐profile analysis method. After the spatial and temporal sensitivities were analyzed, a more rational experimental design was recommended.

Estimating Soil Hydraulic Parameters of a Field Drainage Experiment Using Inverse Techniques

Accurate assessment of water flow and contaminant transport in unsaturated porous media at the field scale is often hindered by difficulties associated with obtaining reliable estimates of soil hydraulic properties. The unsteady drainage-flux method is one of the commonly used methods to measure in situ unsaturated hydraulic properties of soils. However, the properties obtained by this method using instantaneous profile data analysis may not be the best estimation of actual values of hydraulic properties. We present an improved analysis of the data from drainage experiments using inverse modeling, which uses nonlinear regression methods to estimate hydraulic parameters. Parameter identifiability is evaluated through sensitivity and uniqueness analyses. We used the combination of the inverse modeling program, UCODE, with the flow simulator, STOMP, for inverse modeling. Applying the inverse method to a field drainage experiment in sandy soil showed that all the van Genuchten (1980) hydraulic parameters could be estimated uniquely when both water content (theta) and pressure head (h) data were used. The parameter estimates by inverse technique using both theta and h data simulated the flow better than the parameter values obtained by the conventional instantaneous-profile analysis method. After the spatial and temporal sensitivities were analyzed, a more rational experimental design was recommended.

Comparative study of the in-situ and laboratory estimated and measured unsaturated hydraulic properties of soil under different conditions

Comparative study of the in-situ and laboratory estimated and measured unsaturated hydraulic properties of soil under different conditions

Estimating Unsaturated Soil Hydraulic Properties from Tension Disc Infiltrometer Data by Numerical Inversion

Tension disc infiltrometers are becoming increasingly popular devices for in situ measurement of the unsaturated hydraulic properties of soil. Tension infiltration data are generally used to evaluate the parameters Ks and α in Gardner's exponential model of the unsaturated hydraulic conductivity. Either two measurements using different disc diameters or measurements with a single disc but using multiple pressure heads are then used. In this paper we describe a parameter estimation procedure which combines the Levenberg‐Marquardt nonlinear parameter optimization method involving weighted least squares, with a quasi‐three‐dimensional numerical model which solves the variably saturated flow equation. By numerical inversion of Richards' equation the unknown parameters in van Genuchten's model of the unsaturated soil‐hydraulic properties are estimated from observed cumulative infiltration data during transient water flow. Additional measurements of the pressure head or water content, as well as a penalty function for constraining the unknown parameters to remain in some feasible region (Bayesian estimation), can be optionally included into the parameter estimation procedure. The problem of optimal sampling design, that is, selecting the best points in space and time for making measurements, is addressed by studying the sensitivity of the objective function to changes in the optimized hydraulic parameters. We calculate objective functions based on available cumulative infiltration, pressure head, and water content measurements and also on several combinations of these data. The behavior of the objective function in three‐dimensional parameter space is evaluated by means of a series of two‐dimensional response surfaces. The utility of the parameter estimation procedure is demonstrated using numerically generated data. The sensitivity of the procedure to different initial estimates of the model parameters is also discussed.

Unsaturated Hydraulic Properties from Upward Flow into Soil Cores

Common methods for determining unsaturated hydraulic properties of soils are time consuming and measurements are restricted to relatively moist soil conditions. This study developed a transient upward infiltration procedure for estimating soil hydraulic properties. The procedure involves pumping a carefully controlled water flux into the bottom of an initially dry soil core. As the unsaturated core slowly absorbs water, soil water potentials are measured with tensiometers, and, optionally, soil water contents are measured with time domain reflectometry. No porous plates are required for this unsaturated flow procedure because a flux boundary is used. Applied fluxes of approximately 0.76 cm d -1 for 60 h to 7.6 cm diam. by 7.6 cm high cores increased the water content from an initial dry state (approximately -7500 kPa) to almost saturation. A Levenberg-Marquardt inverse parameter estimation procedure was combined with a one-dimensional numerical model of the experimental flow system to estimate the soil hydraulic properties from the collected water potential data using five different methods. Parameters in the van Genuchten water retention relationship and the van Genuchten-Mualem or the Campbell hydraulic conductivity relationships were simultaneously estimated. The optional water content data was combined with the water potential data to provide initial estimates of water retention parameters. The best method (lowest RMSE) that estimated all parameters in the water retention relationship [φ m (θ)] and the hydraulic conductivity relationship [K(θ)] produced similar hydraulic property relationships for four identically packed cores of a loamy fine sand.

The application of TDR in laboratory column experiments

An automated TDR system was used together with an automated tensiometer system in laboratory column experiments for the measurement of unsaturated hydraulic properties of soils. The laboratory experiments were a stationary flux (sprinkling infiltrometer) carried out with different fluxes, and a transient (evaporation) method. Simultaneous measurements of water content and pressure head give the water retention characteristic and enable the application of the instantaneous profile analysis for calculating the hydraulic conductivity. This direct analysis is compared with the traditional analysis methods normally used for the laboratory experiments. The application of automated measurement techniques and the use of TDR increases the speed of the measurements, generates more data points and requires less assumptions in the data analysis. Moreover, TDR measurements give insight in the flow process, including effects of hysteresis and heterogeneity within a sample. Performing the sprinkling infiltrometer and evaporation experiments on the same sample enables the determination of the unsaturated hydraulic conductivity at pressure heads ranging from −0.01 m to −6 m.

Simple Field Method for Determining Unsaturated Hydraulic Conductivity

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. R OF SOIL WATER INFILTRATION and SUbSUrface water movement are important to researchers developing soil management practices to minimize potential groundwater contamination from land applied chemicals. A simple and rapid field technique of determining field unsaturated hydraulic conductivity would be useful in achieving this objective. Field and laboratory techniques for measurement of unsaturated hydraulic properties of soil were described by Green et al. (1986) and by Klute and Dirksen (1986), respectively. Solution of unsaturated flow problems generally requires experimental determination of the relationship between hydraulic conductivity and water potential or water content. Field methods used to obtain these relationships include the instantaneous profile method, steady-flux methods (with sprinkler irrigation or artificial crusts), sorptivity measurements, and use of tension infiltrometers (Clothier and White, 1981; Ankeny et al., 1988, 1989; Elrick et al., 1988a; White and Perroux, 1987, 1989; Smettem and Clothier, 1989). Because instantaneous profile and steady-flux techniques require laborious installation of tensiometers or neutron probe access tubes, sample numbers and the extensiveness of a site characterization can be limited. Sorptivity is an unsaturated soil parameter sometimes measured in the field (Green et al., 1986). Although sorptivity measurements are fast and simple, these measurements usually require that initial water content be known. White and Perroux (1989) have proposed a laboratory method for calculating unsaturated hydraulic conductivity from sorptivity measurements. Their method, however, requires air drying the sample between measurements at different tensions, which increases experimental time and may cause wetting/drying effects on soil structure. The Guelph infiltrometer (Soilmoisture Equipment Corp., Santa Barbara, CA) compares infiltration rates for difM.D. Ankeny, Daniel B. Stephens & Assoc., 4415 Hawkins NE, Albuquerque, NM 87109; M. Ahmed, Bangladesh Univ. of Engineering and Technology, Dhaka-1000, Bangladesh; T.C. Kaspar, National Soil Tilth Lab., Ames, IA 50011; and R. Horton, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011. Joint contribution from USDA-ARS and Iowa State Univ. Journal Paper no. J-13716 of the Iowa Agric. and Home Economics Exp. Stn. Projects no. 2659 and 2715. Received 6 Nov. 1989. *Corresponding author. Published in Soil Sci. Soc. Am. J. 55:467-470 (1991). ferent radii surface disks and does not require driving a ring. Different soil surface areas, however, are being compared, which may introduce spatial variability associated with the different soil surfaces. A field method to measure in situ hydraulic conductivity at low water tensions is needed for studies of macroporosity and water flow in agricultural soils. The desired criteria for such a method are: 1. Only steady-state infiltration rate measurements are needed. Knowledge of the initial water potential or content should not be required. 2. Soil pore structure should not be disturbed by driving a ring into soil to obtain one-dimensional flow. This way, larger pores are not truncated or collapsed and infiltration through larger pores is less likely to be underestimated. 3. Measurements should be taken on the same soil surface. Measurements taken by using different radii (e.g., Elrick et al. 1988a) are more dependent on the assumption of soil homogeneity. 4. Calculation of hydraulic conductivities should be straightforward. We present a simple scheme for determination of in situ hydraulic conductivity that meets these criteria.

Using Texture and Other Soil Properties to Predict the Unsaturated Soil Hydraulic Functions

AbstractThe unsaturated hydraulic properties of soils are important but difficult to measure. Rather than measuring the hydraulic properties directly, we followed a different approach by fitting analytical expressions for the soil water retention and hydraulic conductivity functions to experimental data for a wide range of soils in the Netherlands. Analysis of the data shows the flexibility of the analytical expressions and also gives insight into how the different model parameters affect the calculated unsaturated hydraulic functions. Regression analyses are used to relate the estimated model parameters to more easily measured soil properties, such as bulk density and percentages silt, clay and organic matter. After calibration, the relations are used to predict the hydraulic functions of soils for which only the more easily measured soil properties are available. Accuracy of the predictions is analyzed in terms of functional criteria which are relevant to practical management problems. The predictive regression models are useful for estimating the unsaturated soil hydraulic properties of large areas of land, but need improvement for application to specific sites.