Unsaturated Soil Hydraulic Properties Research Articles

Closed‐form hydraulic conductivity equations for multimodal unsaturated soil hydraulic properties

AbstractClosed‐form expressions of the hydraulic conductivity function for linearly superposed subretention (multimodal) functions were derived for arbitrary sets of the Brooks and Corey (BC), van Genuchten (VG), and Kosugi (KO) water retention models. The generalized Mualem hydraulic conductivity model was evaluated using the mathematical approach of Priesack and Durner. Three types of modification to the multimodel were also proposed. Firstly, the derived conductivity equations can be simplified when the submodel parameters, for the BC model, for the VG model, and for the KO model have the same (common) value (denoted as CH). Secondly, as in the case of the modified single VG and KO models, a hypothetical air‐entry head near saturation can be introduced for the multimodal VG and KO models to prevent unrealistic reductions in the hydraulic conductivity near saturation when the VG n parameter approaches its lower limit of n = 1. Furthermore, the multimodal hydraulic conductivity functions become a simple sum of conductivity subfunctions when the exponent r is unity (such as for Burdine's model), which leads to independent tortuosity effects for each submodel. The models are illustrated for two soils: a highly aggregated Kumamoto Andisol and a relatively unimodal dune sand. The dual‐(BC, VG, KO) and the VG1BC2 models equally represented the water retention data of the Andisol, with similar hydraulic conductivity curves. The dual‐BC‐CH, dual‐VG‐CH, and VG1BC2‐CH models fitted the water retention data of the dune sand well, with the hydraulic conductivity curves of the dual‐porosity model being similar to those of the Fayer and Simmons (FS) model.

Observations on soil-atmosphere interactions after long-term monitoring at two sample sites subjected to shallow landslides

Soil-atmosphere interaction has implications in different scientific research contexts and is increasingly investigated through field measurements. This paper reports a detailed description of interaction between shallow soil and atmosphere at two test sites in Oltrepò Pavese area (Northern Italy). The two test sites are in the same climatic area but are characterised by different geological features. In fact, the first objective is to compare the behaviour of two different soils, namely a clayey-sandy silt (CL) and a silty clay (CH), under similar meteorological events. Soil-atmosphere interaction is studied on the basis of long-term (about 87 and 42 months for the two test sites, respectively) monitoring data of both volumetric water content and soil water potential, recorded at different depths along two vertical soil profiles in the first two metres from ground level. Field measurements, together with meteorological data such as precipitation and air temperature, allow for clear identification of the seasonal fluctuations of unsaturated soil hydraulic properties. To infer detailed information, the recorded data were processed and relationships between soil water potential and water content were investigated. Different time spans, from several months to a few days, even including single rainy events, are considered to show the hydraulic soil behaviour. The hysteretic cycles of water content with respect to soil water potential and non-equilibrium flow are highlighted. In particular, the measured soil water potential is in the range of 0–800 kPa and of 0–1500 kPa for the CL and CH soil, respectively. At both sites, the observed hysteretic cycles are more frequent in the hot season (summer) than in the cold season (winter) and tend to reduce with depth. The experimental results are compared with the soil water characteristic curves (SWCCs) to assess whether and to what extent the SWCCs are reliable in modelling the hydraulic behaviour of partially saturated soils, under atmospheric forcing, at least in the considered climatic contexts.

Comment on ‘‘A Modular Framework for Modeling Unsaturated Soil Hydraulic Properties Over the Full Moisture Range” by Weber et al.

AbstractWeber et al. (2019) (W19 in the following) present a modular framework for parametrizing soil hydraulic properties. The W19 model framework is almost identical to the model system developed jointly by Peters (2013), https://doi.org/10.1002/wrcr.20548, Iden and Durner (2014), https://doi.org/10.1002/2014wr015937, and Peters (2014), https://doi.org/10.1002/2014wr016107, to which we will refer as PDI ("Peters‐Durner‐Iden") model in the following. The primary goal of this comment is to uncover some inconsistencies of the W19 formulation. In the following, we (i) comment on the statement made by W19 regarding the number of parameters in the original PDI formulation, (ii) critically analyze the use of their function to parameterize the water retention curve (WRC), and (iii) show that, in contrast to the statement in W19, using their formulation of for parametrizing noncapillary hydraulic conductivity leads to systematic deviations from the physically based film‐conductivity model developed by Tokunaga (2009), https://doi.org/10.1029/2009wr007734.

Estimation of soil water retention curves from soil bulk electrical conductivity and water content measurements

Measurement of soil water retention curves (SWRCs) is time consuming, and there is no single laboratory device available to measure a SWRC over an entire range of relevant pressures. The van Genuchten (vG) model is commonly used to characterize the shape of the SWRC. Bulk soil electrical conductivity as a function of water content, σ(θ), has been used to estimate hydraulic properties of unsaturated soils, thus making it possible to relate σ(θ) and SWRC. The saturated and residual water content values, θs and θr, can be estimated from soil bulk density and particle size distribution. In this study, we present an approach to estimate vG parameters m and α from σ measured at saturated and residual soil water contents, as well as σ(θ) values measured at intermediate water contents. A thermo-time domain reflectometry (thermo-TDR) sensor is used to measure σ and θ of the same soil sample volume. SWRCs for three soils (Glassil 530 sand, Tennessee silt loam and Illinois clay loam with bulk densities ranging from 1.52 to 1.67 g cm−3, 1.05 to 1.25 g cm−3 and 1.05 to 1.2 g cm−3, respectively) are estimated from σ(θ) measurements and compared with direct SWRC measurements obtained with a tension table and pressure plate extractors. Additional comparisons are made using data obtained from the literature. The proposed method to estimate SWRCs performs well when compared to direct SWRC measurements (with an average RMSE and an average bias of 0.041 cm3 cm−3 and 0.008 cm3 cm−3, respectively). Results indicate that the new σ(θ) based method accurately estimates SWRCs.

A simple method for predicting the hydraulic properties of unsaturated soils with different void ratios

Reliable estimates of the unsaturated soil hydraulic properties are needed in many research and engineering projects. The void ratio or bulk density is an important factor affecting the unsaturated soil hydraulic properties. The bulk density of one soil can change considerably due to the influences of rain, irrigation, and traffic compaction. It remains a difficult task to accurately describe the hydraulic hysteresis of the unsaturated soil with different void ratios or bulk densities. The aim of this study was to evaluate hydraulic hysteresis behavior and permeability of unsaturated soil with different void ratios or bulk densities based on the normalized soil water retention curves (SWRCs) method. After the normalization, the normalized main drying and wetting SWRCs are independent of the void ratio or bulk density. The hysteresis behavior between the normalized main drying and wetting SWRCs can be described by referring to the existing void ratio independent hysteresis model. Additionally, a simple estimation method to simulate the effects of the void ratio or bulk density on the relative permeability coefficient of unsaturated soils is proposed. The approach proposed here requires no additional parameters. The results show that the proposed hysteresis model can be used to describe the arbitrary main drying, wetting, and scanning curves at arbitrary void ratios over a wide suction range, and the predictions for the relative permeability coefficient with different void ratios by the proposed permeability model can match the experimental data well.

Using inverse analysis to estimate hydraulic properties for unsaturated layered sand

In order to better evaluate the applicability of the inverse analysis method for calculation and evaluation of hydraulic properties of unsaturated soil in more realistic conditions, a transient one – step outflow experiment for layered sands was applied in the desaturation process with the purpose to attain the profiles of suction, saturation and flow rate with time. In this study, the fine sand and medium sand were used with the same thickness of 40cm for each layer. The sand grains were mixed under water and scooped into the plexiglas column (H = 80cm, D = 28cm, wall thickness = 1cm) to prepare a fully saturated sample. For homogeneity within each sand layer, the density of two sands must be controlled during soil column construction. For numerical study, the inverse simulation and one straightforward calculation were carried out to determine the unsaturated hydraulic properties of sands. Unsaturated hydraulic parameters in the van Genuchten model were estimated using soil suction measurements at 10cm intervals and an outflow rate at the bottom of a layered sand column. To reduce the quantity of data for analysis and simulation but still keep enough typical information for the experiment, four data sets of soil suction and saturation at four locations (L2, L4, L5 and L8) were selected out of eight to compile the Soil Water Characteristic Curve. The comparison between predicted unsaturated hydraulic properties and the experimental unsaturated hydraulic properties shows good agreement in the case of the fine sand was overlaid with medium sand. The results concluded that besides the homogeneous sand, the inverse analysis based on the 1-D outflow experiment promises to be a useful method in determining the hydraulic properties for unsaturated heterogeneous sand.

Unsaturated Soil Hydraulic Properties according to Double-Ring Infiltration of Saline Water

Inverse modeling is a relatively complex procedure that allows quick estimation of soil hydraulic properties, yielding parameters for both soil water retention and unsaturated hydraulic conductivity function from a single experiment. The purpose of this research was to evaluate saline water effect on unsaturated hydraulic properties of clay loamy nonsaline-sodic soil layers and estimate these properties inversely from infiltration data using a double-ring infiltrometer.Three levels of water salinity (EC) 1.1, 2.2 and 5.8 dS/m were used for each treatment (T1, T2, and T3, respectively). Soil samples at three depths (0–20, 20–40 and 40–60 cm) were taken to measure Mualem–van Genuchten (MVG) hydraulic parameters (θr, residual water content, θs, saturated water content, α and n, shape parameters, and Ks, lab saturated hydraulic conductivity). Soil hydraulic parameters were estimated with an inverse solution using HYDRUS-1D model and infiltration data. Besides, the most sensitive MVG parameters to infiltration data (n, θs, and Ks) were inversely estimated due to difficulty in estimating several parameters simultaneously. Results showed that salinity had no significant effect on soil water infiltration and soil hydraulic conductivity changes in the specified salinity range according to the variance analyses (ANOVA).The Nash–Sutcliffe coefficient of efficiency (Ce) between simulated (HYDRUS) and measured values of cumulative infiltration data was about 0.998, indicating an excellent match. Soil water retention, as well as unsaturated soil hydraulic conductivity, were optimized with reasonable accuracy when the soil profile was assumed to be homogenous. In addition, simulated soil-water contents were quite similar to the measured values in which the values of Nash–Sutcliffe efficiency (Ce) for T1, T2, and T3 were 0.83, 0.85, and 0.85, respectively. In conclusion, the soil hydraulic parameters can be inversely estimated using the measured infiltration data through the double ring method if the soil is homogeneous.

Development of Hierarchical Ensemble Model and Estimates of Soil Water Retention With Global Coverage

AbstractCorrect quantification of mass and energy exchange processes between land surface and atmosphere requires an accurate description of unsaturated soil hydraulic properties. Soil pedotransfer functions (PTFs) have been widely used to predict soil hydraulic parameters. Here, 13 PTFs were grouped according to input data requirements and evaluated against a well‐documented database (National Cooperative Soil Survey Characterization [NCSS]) covering the continental United States (87.7% of data) and other regions of the globe (12.3% of data). Weighted ensembles were shown to have improved performance over individual PTFs in terms of evaluation criteria. Validation of moisture content estimated from the ensemble models against observations showed promising results. Global maps of soil water retention data from the ensemble models as well as their uncertainty were provided. Our full 13‐member ensemble model provides more accurate estimates than PTFs that are currently being used in Earth system models, which may, therefore, provide improved water fluxes and reduce uncertainty of the estimations.

Estimation of the soil hydraulic properties from field data by solving an inverse problem

Estimating unsaturated soil hydraulic properties to predict water dynamics through a vertical soil profile under the effects of irrigation, drainage and evapotranspiration is imperative for managing soils in the arid regions. The aim of this work was to determine the soil water retention curve and the hydraulic permeability function of a bare soil profile in a Tunisian oasis threatened by salinization. The developed model combines a numerical inversion of the unsaturated flow equation with the BOBYQA optimisation algorithm. The direct model solved the Richards equation using a cell-centred finite difference model. Hydraulic properties were described by van Genuchten-Mualem models. Input data for the inverse problem are the infiltration flow, soil water contents and pressure heads measured during ponded infiltration and internal drainage tests. Numerical simulations of these two tests were performed considering a homogeneous single-layer soil profile but a better fitting between measured and simulated water contents was obtained when the soil profile was divided into five sub-layers. The hysteresis phenomenon was highlighted from the soil water retention and the relative permeability curves corresponding to the ponded infiltration and internal drainage tests.

Effect of Sodicity on Soil–Water Retention and Hydraulic Properties

AbstractThe present study analyses the effect of sodicity on unsaturated soil hydraulic properties. Permeameter and pressure plate experiments were conducted on six texturally different soils at va...

Anisotropy of unsaturated soil hydraulic properties of eroded Luvisol after conversion to hayfield comparing alfalfa and grass plots

The ploughing-induced compaction of the interface between topsoil and subsoil negatively affects the connectivity and continuity of the complex pore system through plough pans as artificial boundary resulting in water-logged conditions. The conversion of arable land into hayfield is an opportunity for breaking up plough pans and recovering pore networks in the long-term. The basic idea of the current study was to investigate the potential pore structure recovery effect by growing either deep-rooting alfalfa or shallow-rooting grass on former conventionally-tilled cropland. The alfalfa and grass plots located in moraine region in Northeast Germany on erosion-affected truncated Albic Luvisol, were sampled nine years after the conversion to hayfield. Undisturbed soil cores (300 cm3) were extracted in vertical (0°) and horizontal directions (90°) from the boundary between Ap and Al-Bt- horizons (about 25−35 cm soil depth). The soil water retention and the hydraulic conductivity under variable-saturated conditions were determined by using a combination of methods including suction plate, pressure chamber, double membrane through-flow, and evaporation experiments. The anisotropy ratio, AR, (horizontal versus vertical) of the unsaturated hydraulic conductivity function, K(Se), was less pronounced for the soil at the alfalfa as compared to that of the grass plot, especially for wide coarse pores (wCP: >−60 hPa). The AR-ratio was differently depending on the pore size for soil from grass and alfalfa plot, and ranged between AR-values of 1 and 7. The pore size class-related matric flow potential, ϕ, derived from K(Se) function was used for comparing the volume fraction of pore size classes with a pore size-related fraction of the capillary potential. The volume fraction of the wide coarse pores was negatively related with ϕ (r2: 0.91). The smaller AR-values for the soil of the alfalfa plot suggest a tendency for better changing the platy structure into a prismatic-platy structure through the more intense rooting in the vertical direction than in the soil of the grass plot.

Determination of soil-water retention curve: an artificial intelligence-based approach

Soil Water Retention Curve (SWRC) is a fundamental relationship in unsaturated soil mechanics, knowledge of which is essential for determining major mechanical and hydraulic properties of unsaturated soils. There are several empirical, semi-empirical and physically-based models which have been proposed to date for estimating SWRC. While the physically-based models which employ the basic soil characteristics such as grain-size and pore-size distributions are regarded superior to purely empirical models, their Achilles’ heel is the several simplifying assumptions based on which these models are constructed, thereby, restricting their applications and influencing their accuracy. Given the complexity of the soil porous structure, one may resort to the new inference techniques rather than mechanistic modelling to find the relationship among soil physical characteristics and the retention properties. Therefore, an alternative approach to purely empirical relationships as well as physically-based and conceptual models for determining SWRC is the use of Artificial Intelligence (AI) based techniques to acquire a relationship for SWRC based on the soil basic properties, especially grain size distribution and porosity. Among AI-based methods, Multi-Gene Genetic Programming (MGGP), often used to establish a close form equation for a complex physical system, offers a suitable alternative to the current approaches. In this study, a database compromising of 437 soils (containing various soil types, namely, sand, clay, silt, loam, silt loam, clay loam, sandy loam, sandy clay loam, silty clay loam, silty clay, and loamy sand soils) was used along with MGGP to establish a relationship among suction, saturation, porosity and grain size distribution. The proposed equation had a reasonable agreement with the experimental data compared to the other grain-based and physically-based models.

Unsaturated Soil Hydraulic Properties Identification using Numerical Inversion and In-Situ Experiments from Mnasra Area, Morocco

In soil physics, accurate estimation of hydrodynamic properties is essential for effective groundwater management. Experimentally, determination of these properties in the laboratory or in-situ conditions is difficult step, long and may involve uncertainties significant for the vast majority practice use. Frequently, inverse methods were established to determine accurate parameters in the field scale based on measured soil moisture and hydraulic conductivity over time and depth. These inversions methods involve hydraulic soil-models using adapted optimization algorithms that ensure a best combination of parameters minimizing a cost function. In this work, direct and inverse optimization approaches has been proposed to estimate the effective hydraulic parameters, based on the temporal description of the soil physical and hydraulic parameters using field water content measurements from located in Mnasra area (northwest of Morocco). The Levenberg Marquardt optimization algorithm coupling with the vadose zone model (VZM), numerical code in Fortran, has been used to inverse estimation of hydraulic parameters based on transients soil water content measurements. These uses have been conducted using temporal measurements of soil moisture in depths: 20, 40, 60 and 80 cm. The estimated properties are in good agreement with those measured in-situ. Overall, it was concluded that this technique can greatly widen the range of hydrological problems admissible for inversion and can be used for many applications in hydrologic engineering, either alone or in conjunction with traditional techniques.

A Modular Framework for Modeling Unsaturated Soil Hydraulic Properties Over the Full Moisture Range

AbstractA generalized modular framework for partitioning soil hydraulic property (SHP) functions into a capillary and a noncapillary part is developed. The full water retention curve (WRC) is modeled as a weighted sum of a parametric capillary saturation function and a new general model for the noncapillary saturation function. This model is directly computed from any selected capillary saturation function. With it, a physically complete, continuous, and flexible representation of the WRC is achieved, ensuring zero water content at oven dryness. In a modular and hierarchical framework, the expressions for the capillary and noncapillary saturation function are used to calculate the respective hydraulic conductivity curves (HCC). This is achieved by adopting Mualem's integral for the capillary part of the HCC only and calculating the noncapillary HCC directly from the new noncapillary saturation function. This leads to consistent descriptions of measured HCC data, including the often observed change in slope beyond −100 cm pressure head. Compared to the classical van Genuchten‐Mualem approach, it requires only one additional model parameter. The SHP framework model describes both WRC and HCC adequately and coherently. We demonstrate the suitability of the SHP framework and versatility by describing measured WRC and HCC data across the full moisture range using soil samples from a wide range of textures and origins. The modular framework was implemented in the soil physics and soil hydrology (spsh) R‐package, available from the Comprehensive R Archive Network. It contains several SHP models, model parameter estimation, and features options for goodness of fit statistics, and model selection.

Effect of hysteresis on hydraulic properties of soils under multiple drying and wetting cycles

The importance of the hydraulic properties of unsaturated soil for geotechnical designs has been well understood. Many studies on the soil water retention curve (SWRC) and the relative coefficient of permeability under drying and wetting cycles have been performed. However, most of the previous investigations of hydraulic properties are limited to a single drying and wetting cycle or are based on multiple drying–wetting cycles with limited soil experiments. This might be because the measurement of both the SWRC and the coefficient of permeability of unsaturated soil is complex and time consuming. In this study, the hydraulic properties of silicon micropowder (SMP) and Guangxi Guiping clay (GGC) were studied for the same soil specimen under five cycles of drying and wetting in multistep outflow experiments. Significant hysteresis of SWRCs was observed under the first drying and wetting cycle for both soils. However, the hysteresis effect decreased as the number of drying and wetting cycles increased. Furthermore, the coefficient of permeability was estimated using the van Genuchten–Mualem (VGM) model. Results also show that hysteresis plays an important role in the coefficient of permeability for both soils subjected to multiple drying and wetting cycles.

Tests on mechanical behavior of unsaturated decomposed granite and its modelling considering finite deformation

Generally speaking, most of the geomaterials in the surface ground are in an unsaturated state. The mechanical and hydraulic properties of unsaturated soil are much more complicated than those of saturated soil. To rationally describe these properties, it is important to couple the stress-strain relation of the unsaturated soil with its water retention characteristics using rational state variables. In this paper, oedometer and triaxial compression tests on decomposed granite under constant-suction and constant degree of saturation conditions were conducted. Based on the test results, a modified constitutive model was proposed to build an incremental relation between the degree of saturation and suction that considers the influence of finite deformation. The modified model was utilized to simulate the corresponding laboratory tests. It is found that the modified constitutive model has satisfactory accuracy to describe the mechanical and hydraulic properties of unsaturated decomposed granite, which verified the reasonability of the assumption adopted in this paper. The test results are also helpful for the understanding of the moisture characteristics of the decomposed granite under constant degree of saturation condition.

A Fractal Model to Interpret Porosity‐Dependent Hydraulic Properties for Unsaturated Soils

This paper presents a simple fractal model to quantify the effects of initial porosity on the soil‐water retention curve and hydraulic conductivity of unsaturated soils. In the proposed conceptual model, the change of maximum pore radius, which largely determines the change of the air‐entry value, is directly related to the fractal dimension of pore volume (D) and porosity change. The hydraulic properties of unsaturated soils are then governed by the maximum pore radius, the fractal dimension of pore volume (D), and the fractal dimension of drainable pore volume (Dd ≤ D). The new fractal model removes the empirical fitting parameters that have no physical meaning from existing models for porosity‐dependent water retention and hydraulic behaviour and employs parameters of fractal dimensions that are intrinsic to the nature of the fractal porous materials. The proposed model is then validated against experimental data from the literature on soil‐water retention behaviour and unsaturated conductivity.

Effects of hysteresis on hydro-mechanical behavior of unsaturated soil

Hysteresis is a common feature in the hydraulic properties of unsaturated soils. At a given matric suction, the volumetric water content on a wetting curve is always lower than that on a drying curve. The hysteresis may affect the mechanical behavior and transient process in unsaturated soils. This study focuses on the hysteresis observed in soil-water retention curve (SWRC) tests in the laboratory and inferred from a semi-empirical equation for numerical implementations. Coupled hydro-mechanical finite element analyses, employing an initial drying, main drying, and main wetting curve as hydraulic properties of the soil, were performed to numerically study the effect of hysteresis on the hydro-mechanical behavior of unsaturated soil subjected to water infiltration. Numerical results show that the unsaturated soil exhibits lower soil strength and matric suction distributions when applying the main wetting curve as compared with other SWRCs. The hysteresis appears to affect the stress-strain behavior of the unsaturated soil during prolonged rainfall. Therefore, to predict the instability of the rainfall-induced soil slope more precisely, it is necessary to apply the wetting curve of the SWRC, which shows the hydraulic characteristics of the slope surface.

Further tests of the HYPROP evaporation method for estimating the unsaturated soil hydraulic properties

Abstract Many soil, hydrologic and environmental applications require information about the unsaturated soil hydraulic properties. The evaporation method has long been used for estimating the drying branches of the soil hydraulic functions. An increasingly popular version of the evaporation method is the semi-automated HYPROP© measurement system (HMS) commercialized by Decagon Devices (Pullman, WA) and UMS AG (München, Germany). Several studies were previously carried out to test the HMS methodology by using the Richards equation and the van-Genuchten-Mualem (VG) or Kosugi-Mualem soil hydraulic functions to obtain synthetic data for use in the HMS analysis, and then to compare results against the original hydraulic properties. Using HYDRUS-1D, we carried out independent tests of the HYPROP system as applied to the VG functions for a broad range of soil textures. Our results closely agreed with previous findings. Accurate estimates were especially obtained for the soil water retention curve and its parameters, at least over the range of available retention measurements. We also successfully tested a dual-porosity soil, as well as an extremely coarse medium with a very high van Genuchten n value. The latter case gave excellent results for water retention, but failed for the hydraulic conductivity. In many cases, especially for soils with intermediate and high n values, an independent estimate of the saturated hydraulic conductivity should be obtained. Overall, the HMS methodology performed extremely well and as such constitutes a much-needed addition to current soil hydraulic measurement techniques.

Fractal-based models for the unsaturated soil hydraulic functions

Reliable estimates of the unsaturated soil hydraulic properties are needed in many research and engineering projects. Fractal based approaches have become popular for describing the hydraulic functions. New models based on fractal geometry are presented to describe the wetting phase relative permeability (unsaturated hydraulic conductivity) of porous media. Analytical expressions were derived using the fractal capillary pressure (water retention) model proposed by Alfaro Soto and Vilar (2006) and the statistical pore size distribution models of Mualem (1976a) and Burdine (1953) for the unsaturated hydraulic conductivity. The newly derived models provide predictions of the relative permeability to water, as well as of the fractal dimension for modeling the pore structure of a soil. Results are compared with previous formulations by Brooks and Corey (1964), van Genuchten (1980) and Kosugi (1996), as well as with experimental water retention and hydraulic conductivity data of four soils having widely different soil textures. The proposed models agreed closely with observed data. The analytical models may be attractive alternative for estimating the unsaturated hydraulic conductivity when no laboratory or field measurements are available.