Soil Unsaturated Hydraulic Conductivity Research Articles

IN-SITU CHARACTERIZATION OF UNSATURATED SOIL HYDRAULIC PROPERTIES OF A LATERITE SOIL PROFILE IN COASTAL KARNATAKA

ABSTRACT Unsaturated laterite soil formations play an important hydrologic role in the West Coast region of India. However, very little information on the hydraulic/hydrologic properties of lateritic soils exists in literature. The present study was taken up to characterize the unsaturated soil water retention curves and hydraulic conductivity function of a sandy lateritic soil profile located in the coastal district of Dakshina Kannada, Karnataka State. This was achieved by conducting a field internal drainage test involving measurements of soil moisture contents and soil water pressures at various depths within a 1.5 m deep soil profile over a period of 39 days. Soil moisture contents were measured using a sophisticated Time Domain Reflectrometry (TDR) probe and soil water suction pressures were measured using tensiometers. The water retention curve for each depth layer was characterized by fitting the popular van Genucthen model to observed moisture contents and suction pressures. Depth-wise differences in the shapes of the water retention curves were evident. Observed temporal variations in moisture content and suction pressures during internal drainage were analyzed using well-established procedures to calculate unsaturated hydraulic conductivity values. Results indicated more or less homogeneous depth-wise drainage behaviour and a single relationship based on van Genucthen's model was sufficient to describe the hydraulic conductivity variations with moisture content for the entire profile. Computed hydraulic conductivities showed extreme sensitivity to soil moisture contents and varied from a low value of 0.47 cm/d at moisture content of 0.208 cm3/cm3 to as high as 134.82 cm/d at 0.261 cm3/cm13. Results of this study can provide useful inputs in applications of mathematical models to describe water flow and solute transport in unsaturated lateritic soil media. This may lead to better understanding of the role of the unsaturated soil zone in the hydrologic cycle in the West Coast region and thereby lead to more efficient designs of water conservation structures.

Soil hydraulic properties near saturation, an improved conductivity model

The hydraulic properties near saturation can change dramatically due to the presence of macropores that are usually difficult to handle in traditional pore size models. The purpose of this study is to establish a data set on hydraulic conductivity near saturation, test the predictive capability of commonly used hydraulic conductivity models and give suggestions for improved models. Water retention and near saturated and saturated hydraulic conductivity were measured for a variety of 81 top and subsoils. The hydraulic conductivity models by van Genuchten [ van Genuchten, 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44, 892–898.] (vGM) and Brooks and Corey, modified by Jarvis [ Jarvis, 1991. MACRO—A Model of Water Movement and Solute Transport in Macroporous Soils. Swedish University of Agricultural Sciences. Department of Soil Sciences. Reports and Dissertations 9.] were optimised to describe the unsaturated hydraulic conductivity in the range measured. Different optimisation procedures were tested. Using the measured saturated hydraulic conductivity in the vGM model tends to overestimate the unsaturated hydraulic conductivity. Optimising a matching factor ( k 0) improved the fit considerably whereas optimising the l-parameter in the vGM model improved the fit only slightly. The vGM was improved with an empirical scaling function to account for the rapid increase in conductivity near saturation. Using the improved models, it was possible to describe both the saturated and the unsaturated hydraulic conductivity better than a previously published model by Jarvis. The pore size boundary of the macropores was found at a capillary pressure of −4 hPa corresponding to a circular pore diameter of 750 μm.

Improved soil hydraulic conductivity function based on specific liquid–vapour interfacial area around the soil particles

Improvement in the prediction of soil hydraulic conductivity function is a major concern in soil physics. Zand-Parsa and Sepaskhah (2004) [Zand-Parsa, Sh., Sepaskhah, A.R., 2004. Soil hydraulic conductivity function based on specific liquid–vapour interfacial area around the soil particles. Geoderma 119, 143–157] proposed a new method for the prediction of soil hydraulic conductivity function [ K( h) or K( θ) (where h and θ are soil water pressure head and water content, respectively] based on specific liquid–vapour interfacial area around the soil particles (SLVIA). In the present paper, the SLVIA method was improved as follows: (i) a more straightforward and efficient numerical technique for the prediction of K( θ) function was proposed instead of the analytical method in Zand-Parsa and Sepaskhah; (ii) saturated soil hydraulic conductivity ( K s) was predicted by assumption of a linear relationship between K( θ) and θ for very small θ ranges near saturated soil water content; (iii) the value of soil tortuosity (ratio of actual path length to the straight path length of flow) factor ( Tr) was estimated by using the Newton–Raphson method, by minimizing the difference between predicted and measured saturated soil hydraulic conductivity. The K( θ) curve was predicted by soil water characteristic curve and measured saturated soil hydraulic conductivity. Soil water characteristic curve was predicted by the van Genuchten method with four required soil parameters [ α, n, residual soil water content ( θ r), and saturated soil water content ( θ sat) [van Genuchten, M.Th., 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soil. Soil Sci. Soc. Am. J. 44, 892–898]. A computer program (UNSATK model) written in VISUAL BASIC was used to predict the K( h) or K( θ) curve. The utility of the UNSATK model was tested for three soil types (sandy, loamy, and clayey textures) selected from the literature. The uniqueness of model outputs for each selected soil was checked by different input Tr values. Model predictions yielded reasonable agreement with measured soil hydraulic conductivity data and compared favorably with the widely applied Mualem–van Genuchen method.

Field-saturated hydraulic conductivity of unsaturated soils from falling-head well tests

The field-saturated hydraulic conductivity ( K fs) finds use in many agricultural, environmental and geotechnical investigations and designs. It is commonly derived from falling-head tests in a well, but its derivation is sometimes based on assumptions that may not always hold. The aim of this study was to develop graphs for falling head percolation tests for 340 and 60 mm diameter wells—centred in two separate cylindrical soil tanks—for use in deriving K fs of a fine sand placed in the tanks. The graphs were obtained from results from the finite element program SEEP/W that was used to model percolation from the tank wells into sand, fine sand and silt of known K fs; the soils were all initially field-saturated, and the tests were carried out under (i) field-saturated (by spray irrigation) and (ii) draining conditions. For the modelled field-saturated tests on the three soils, the graphs of fall of head in the well, Δ H w, against the parameter K fs × Δ t, where Δ t is the time taken for the particular fall Δ H w to occur, were coincident. This indicated that for a given measured fall, Δ H w, for a particular head of water, H w, in a well, K fs could be calculated by dividing the parameter K fs × Δ t, by the observed time, Δ t, for the fall, Δ H w, to occur. For the modelled tests on the three draining soils, there was another inverse relationship between Δ H w and the parameter K fs × Δ t for small falls in well heads after start-up. There was good agreement between the K fs value of a fine sand measured in a cylindrical constant-head laboratory permeameter and that obtained using the graphs and the experimental results from both the field-saturated fine sand, and the same fine sand in draining mode at small falls of head in the well. This suggests that falling head tests on non-irrigated soils—initially field-saturated—could be used to determine K fs from early time falls in well heads. Since the time parameter greatly increases with increasing well size and decreasing pressure heads in the percolation well, it is quicker to carry out falling-head percolation tests in smaller test wells and at higher heads in the well. The procedure employed for obtaining K fs for the soils within the K fs range examined could be extended to the field.

Mechanical behaviour of unsaturated marine sediments: experimental and theoretical approaches

The COSTA target areas exhibit different slope failure events, which reflect different triggering mechanisms including gas hydrates dissociation. Gas hydrates stability law depends on temperature, pore pressure, gas chemistry, and pore water salinity. Any change in the equilibrium parameters may convert the hydrate to gas plus water, causing significant weakening of the sediment, and generating a rise of pore pressure. A significant consequence of the hydrate melting is the gas production, which alters significantly the behaviour and mechanical properties of the marine sediments and could be very hazardous when the sediment is unloaded in undrained conditions. The behaviour of the unsaturated marine sediment has to be accurately identified in order to quantify the geological risks associated to the hydrate dissociation [see, for instance, Sultan, N., Cochonat, P., Foucher, J.P., Mienert, J., this volume. Effect of gas hydrates melting on seafloor slope instability]. Thus, in this work, five tests were carried out on synthetic marine sediments in order to identify the gas effect on the hydro-mechanical behaviour of unsaturated marine sediments. Based on the experimental results obtained from this study, on previous constitutive models [Alonso, E.E., Gens, A., Josa A., 1990. A constitutive model for partially saturated soils. Géotechnique 40(3), 405–430; Gens, A., Alonso, E.E., 1992. A framework for the behaviour of unsaturated expansive clays. Can. Geotech. J. 29, 1013–1032] and on the relationship of Van Genuchten [Van Genuchten, M.Th., 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44, 892–898], a new mathematical formulation was proposed to simulate the behaviour of the unsaturated soil. Qualitative comparison between experimental results and model predictions shows the capacity of the proposed model to reasonably reproduce the essential features of the hydro-mechanical behaviour of marine unsaturated sediment. Experimental and simulation results show that unloading (tidal cycles, erosion, natural slope instabilities, excavation) a submarine unsaturated sediment slope in shallow water is much more hazardous than unloading the same submarine slope in deep water.

Calculation of unsaturated hydraulic conductivity using a fractal model for the pore-size distribution

The hydraulic conductivity of unsaturated soils with constant volume is a function of the saturation degree or matric suction, and can be obtained based on currently available procedures. However, each procedure has its limitations and consequently cares should be taken in the selection of a proper procedure. Fractal approach seems to be a potentially useful tool to describe hierarchical systems and is suitable to model the structure and hydraulic properties of porous media. In this paper, the soil–water characteristic curve and hydraulic conductivity function of unsaturated soils were derived from the fractal model for the pore-size distribution, and were expressed by only two independently physical parameters, the fractal dimension and the air-entry value. The predictions of the proposed soil–water characteristic curve and unsaturated hydraulic conductivity were in good accord with the published experimental data. Comparisons between the experimental results and the predictions of both the fractal model and the van Genuchten–Mualem model were performed, and it was found that the predictions of the fractal model were better than that of the van G–M model.

Soil physical quality: Part III: Unsaturated hydraulic conductivity and general conclusions about S-theory

The index of soil physical quality, S, which was proposed in Part I and which was applied to the problems of tillage in Part II is applied in Part III to the unsaturated hydraulic conductivity of soil. S is equal to the slope of the soil water retention curve at its inflection point. This curve must be plotted as the logarithm (to base e) of the water potential against the gravimetric water content (kg kg −1). It is suggested that S is a measure of the micro-structural porosity of the soil. It is shown through an approximate theory, through simulations and through experimental results that the value of S at the inflection point is related to the unsaturated hydraulic conductivity of soil at the inflection point. It is proposed that the inflection point can be used as a “matching point” in studies of unsaturated hydraulic conductivity. Pedo-transfer functions are used to explore the predicted effects of soil texture class and bulk density on the values of the unsaturated hydraulic conductivity at the inflection point. The conclusions from this series of three papers are summarized and as a result, it is recommended that S be used as an index of soil physical quality that enables different soils and the effects of different management treatments and conditions to be compared directly. The use of S for prediction of a range of soil physical properties is summarized and is called S-theory.

A Tensorial Connectivity–Tortuosity Concept to Describe the Unsaturated Hydraulic Properties of Anisotropic Soils

The anisotropy in unsaturated hydraulic conductivity is saturation dependent. Yet, there are few options for modeling this phenomenon in natural soils. A tensorial connectivity–tortuosity (TCT) concept is proposed to describe the unsaturated soil hydraulic conductivity. The TCT concept assumes that soil pore connectivity and/or tortuosity are anisotropic and can be described using a tensor. Saturation‐dependent anisotropy can be easily invoked in common models of relative permeability by incorporating the connectivity tensor. Synthetic Miller‐similar soils having hypothetical anisotropy are defined by allowing the saturated hydraulic conductivity to have different correlation range for different directions of flow. The TCT concept was tested using the synthetic soils with four levels of heterogeneity and four levels of anisotropy. The results show that the soil water retention curves were independent of flow direction but dependent on soil heterogeneity, while the connectivity–tortuosity coefficient is a function of both soil heterogeneity and anisotropy. The TCT model can accurately describe the unsaturated hydraulic functions of anisotropic soils and can be easily combined with commonly used relative permeability functions for use in numerical solutions of the flow equation.

Estimation of unsaturated hydraulic conductivity using soil suction measurements obtained by an insertion tensiometer

To estimate the unsaturated soil hydraulic conductivity of a silty soil, an insertion tensiometer has been used for measuring its suction corresponding to different water contents. These suction values have been used for developing the soil-water characteristic curve (SWCC). The obtained SWCC has been compared with the trends predicted by various fits available in the literature. Further, with the help of the obtained SWCC, the unsaturated soil hydraulic conductivity has been estimated. The study demonstrates the usefulness of insertion tensiometers for measuring soil suction and for estimating its hydraulic conductivity.Key words: silty soil, suction, insertion tensiometer, soil-water characteristic curve, unsaturated soil hydraulic conductivity.

A Tensorial Connectivity–Tortuosity Concept to Describe the Unsaturated Hydraulic Properties of Anisotropic Soils

The anisotropy in unsaturated hydraulic conductivity is saturation dependent. Yet, there are few options for modeling this phenomenon in natural soils. A tensorial connectivity–tortuosity (TCT) concept is proposed to describe the unsaturated soil hydraulic conductivity. The TCT concept assumes that soil pore connectivity and/or tortuosity are anisotropic and can be described using a tensor. Saturation-dependent anisotropy can be easily invoked in common models of relative permeability by incorporating the connectivity tensor. Synthetic Miller-similar soils having hypothetical anisotropy are defined by allowing the saturated hydraulic conductivity to have different correlation range for different directions of flow. The TCT concept was tested using the synthetic soils with four levels of heterogeneity and four levels of anisotropy. The results show that the soil water retention curves were independent of flow direction but dependent on soil heterogeneity, while the connectivity–tortuosity coefficient is a function of both soil heterogeneity and anisotropy. The TCT model can accurately describe the unsaturated hydraulic functions of anisotropic soils and can be easily combined with commonly used relative permeability functions for use in numerical solutions of the flow equation.

Condutividade hidráulica de um Latossolo Roxo, não saturado, sob diferentes sistemas de uso e manejo

O objetivo deste trabalho foi avaliar as alterações na condutividade hidráulica de um Latossolo Roxo ácrico (Typic Acrorthox) do município de Guaira, Região Norte do Estado de São Paulo, submetido ao plantio direto sem irrigação e ao plantio direto irrigado comparando com o solo de mata, condição natural. Determinou-se a condutividade hidráulica do solo (K), no campo, em função do conteúdo de água (teta ), pelo método do perfil instantâneo, avaliando-se a redistribuição da água no perfil do solo, por meio de tensiômetros instalados de 0,1 em 0,1m até 1m de profundidade. Para determinar a função K(teta ) também em laboratório, amostras de solo com estrutura preservada, de 1,15m de comprimento e 0,2m de diâmetro, foram coletadas nos três manejos. Esses monólitos de solo foram instrumentados com tensiômetros e submetidos a variados fluxos estacionários descendentes de água, para determinação da função, pela equação de Darcy-Buckingham. Os resultados demonstraram que a estrutura do solo foi afetada pelos manejos até a profundidade de 0,4m, com maior intensidade no solo sob plantio direto irrigado, provocando redução da condutividade hidráulica do solo não saturado. O movimento da água no solo não saturado também foi afetado pelo aumento da densidade, especificamente pela alteração na distribuição do diâmetro dos poros, demonstrando ainda que, com a metodologia adotada (campo e monólito) o solo, na condição natural, apresenta duas fases distintas com relação à condução de água, uma próxima à saturação e outra a partir da tensão de 40kPa, pelo que as equações de K(teta ) não podem ser extrapoladas para fora da faixa do conteúdo de água em que elas foram obtidas.

Inverse simulation of non-steady-state evaporation using nonequilibrium water retention data: a case study

The aim of this investigation was to check the capability of the Mualem/van Genuchten model to predict unsaturated soil hydraulic conductivity values from water retention data in the absence of spatial variability and effects of macropores. Furthermore, different techniques of parameter estimation were tested. Soil cores of 250 cm 3 were filled with a repacked silty loam soil. After measuring the saturated hydraulic conductivity and equilibrium soil water retention, the soil cores were subject to non-steady-state evaporation (bottom sealed). Pressure head and soil mass were recorded every 60 min. Based upon the van Genuchten/Mualem model and the Darcy–Buckingham equation, an inverse simulation was carried out. As a first option, the parameters θ r, θ s, α, and n were fitted to water retention data. The remaining parameters L (coefficient of tortuosity) and K s were fitted to pressure head versus time recordings during evaporation. Equilibrium soil water retention data yielded a poor fit. The observed pressure head values during evaporation, h( t), could not be described sufficiently by using hydraulic functions determined from equilibrium retention values. A good fit was obtained in case nonequilibrium water retention measurements from the same experiment were used. Obviously, no equilibrium between pressure head and water content was established during evaporation under atmospheric conditions leading to an evaporation rate of about 3 mm day −1. As a second option, all the parameters were fitted simultaneously to both nonequilibrium soil water content data and tensiometer readings during evaporation. Simultaneous fitting yielded slightly decreased errors of simulated h( t) values and slightly increased errors of calculated soil water content. Another simulation model was used to calculate vapor flow in soil during evaporation. Results indicate that vapor flow may be significant in the range h<−500 cm. Before this range was reached, vapor flow could be neglected. Based on the results, we would like to recommend measuring transient water retention under conditions similar to those which are to be predicted in the field. Not only because of hysteresis but also because of transient effects, water retention characteristics of infiltration and evaporation should be distinguished.

Estimation of hydraulic conductivity of unsaturated soils using a geotechnical centrifuge

A saturated silty soil sample is centrifuged in a geotechnical centrifuge to create an unsaturated state. The change in water content of the soil sample is recorded at different points along the length of the sample to obtain the water-content profile, which is then used to obtain the unsaturated hydraulic conductivity of the soil sample. These hydraulic conductivity values are compared with those obtained and reported by previous researchers by conducting accelerated falling-head tests on this soil sample in a centrifuge. The study demonstrates the use of centrifugation techniques to obtain hydraulic conductivities of unsaturated soils.Key words: silty soil, saturated soil, unsaturated soil, hydraulic conductivity, centrifuge testing.

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Three‐region Campbell Model for Unsaturated Hydraulic Conductivity in Undisturbed Soils

A three‐region Campbell (TRC) type model for predicting undisturbed soil unsaturated hydraulic conductivity from water retention is presented. The model assumes that hydraulic conductivity follows separate Campbell functions within the macropore (matric head ψ &gt; −10 cm H2O), the mesopore (−10 ≥ ψ &gt; −350 cm H2O), and the micropore (ψ ≤ −350 cm H2O) regions, and that soil water retention and two reference‐point values of hydraulic conductivity are known. Conductivity and retention data for 100 undisturbed soils from the UNSODA database and 68 soils from a Danish database were used for model development. Conductivity for both highly structured (three‐region) and weakly structured (two‐region) soils mostly followed a piecewise linear function (with slope β) in a Log(conductivity) −Log(water content) plot, supporting the TRC model concept. A unique relationship between the Campbell soil‐water retention parameter, b, and the unsaturated conductivity parameter, β, was found valid for both meso‐ and micropore regions. It was shown that the values of b in the mesopore and micropore regions are not correlated, making the use of single‐region expressions (e.g., the Mualem–van Genuchten type models) questionable and suggests that a multiregion model with noncorrelated retention parameters between pore regions, such as the TRC model, may provide a conceptually more correct description of hydraulic conductivity. The TRC model yielded improved conductivity predictions in loamy and clayey soils whereas predictions for sandy soils were comparable to the single‐region Campbell and van Genuchten models. TRC model predictions compared well with independent data for three differently textured soil profiles.

Three-region Campbell Model for Unsaturated Hydraulic Conductivity in Undisturbed Soils

A three-region Campbell (TRC) type model for predicting undisturbed soil unsaturated hydraulic conductivity from water retention is presented. The model assumes that hydraulic conductivity follows separate Campbell functions within the macropore (matric head ψ > −10 cm H2O), the mesopore (−10 ≥ ψ > −350 cm H2O), and the micropore (ψ ≤ −350 cm H2O) regions, and that soil water retention and two reference-point values of hydraulic conductivity are known. Conductivity and retention data for 100 undisturbed soils from the UNSODA database and 68 soils from a Danish database were used for model development. Conductivity for both highly structured (three-region) and weakly structured (two-region) soils mostly followed a piecewise linear function (with slope β) in a Log(conductivity) −Log(water content) plot, supporting the TRC model concept. A unique relationship between the Campbell soil-water retention parameter, b, and the unsaturated conductivity parameter, β, was found valid for both meso- and micropore regions. It was shown that the values of b in the mesopore and micropore regions are not correlated, making the use of single-region expressions (e.g., the Mualem–van Genuchten type models) questionable and suggests that a multiregion model with noncorrelated retention parameters between pore regions, such as the TRC model, may provide a conceptually more correct description of hydraulic conductivity. The TRC model yielded improved conductivity predictions in loamy and clayey soils whereas predictions for sandy soils were comparable to the single-region Campbell and van Genuchten models. TRC model predictions compared well with independent data for three differently textured soil profiles.

Application of a Geotechnical Centrifuge for Estimation of Unsaturated Soil Hydraulic Conductivity

Abstract Contamination of the groundwater regime and subsurface soils is one of the major problems that has arisen due to rapid industrialization. Spread of contaminants in the unsaturated soil strata, mainly close to the ground surface and lying above the water table, depends on the hydraulic conductivity of the soil. As such, estimation of unsaturated soil hydraulic conductivity becomes very important. However, it is quite difficult to estimate the same in a reasonable amount of time, due to the very low hydraulic conductivity of unsaturated soil. The potential of a geotechnical centrifuge for modeling various geoenvironmental engineering problems being very well established, an effort has been made to obtain the unsaturated state of a silty soil, starting from its saturated state, by centrifugation. The soil water characteristic curves obtained have been used for determining the unsaturated soil hydranlic conductivity.

A generalised relationship between soil electrical and thermal resistivities

Due to rapid industrialization and urbanization there is an increasing demand for the utilization of soils as a conducting and insulating material. Thermal and electrical resistivities of soils are of great importance in view of the subsurface transmission of high power currents wherein it is essential to estimate the resistance offered by the soil in dissipating the heat generated due to transmission of the current. Realising this an effort has been made in this paper to develop a generalised relationship, which can be used, particularly by practising engineers, to estimate either soil electrical resistivity or soil thermal resistivity if one of the two is known. Such a relationship would also be helpful in estimating some fundamental soil properties (viz. unsaturated soil hydraulic conductivity, liquefaction potential, moisture content) as enunciated by other researchers.

Measured and Modeled Unsaturated Hydraulic Conductivity of a Walla Walla Silt Loam

There are numerous methods of estimating unsaturated soil hydraulic conductivity [K(θ)], ranging from direct measurement in the laboratory or field, to models that use only basic soil data, e.g., texture or water release curves (WRC). We evaluated K(θ) for a Walla Walla silt loam (coarse‐silty, mixed, superactive, mesic Typic Haploxeroll) using two field methods and the Mualem–van Genuchten model (MVG). Field methods were the internal drainage method (ID) and the Klaij and Vachaud method (MKV), which was modified to include hydraulic head (H), and used 20 yr of data from a dryland field experiment. Four approaches to estimate WRC were compared for the MKV, and two approaches to estimate parameters of the MVG. For water contents &gt;0.20 m3 m−3, the MKV gave K values that were two orders of magnitude less than those obtained from internal drainage experiments conducted under wetter conditions. When MVG parameters were predicted by the Rosetta model, which uses pedotransfer functions, K values approached those of the MVK. However, when model parameters were estimated from internal drainage and saturated hydraulic conductivity (Ksat) data, K(θ) values were closer to those of the internal drainage experiments. The study reaffirms the difficulty of reconciling K(θ) determined by different methodologies, and of extrapolating to values of θ outside those measured. It also demonstrates the sensitivity of the MVG and pedotransfer functions to different sources of input values. Advantages and disadvantages of the different approaches to determining K(θ) are discussed.