Water Flow In Unsaturated Soils Research Articles

Water Flow in Unsaturated Soils in Microgravity Environment

In this study, two types of soils with varying soil water potentials were used for evaluating the effect of gravity on water flow through unsaturated soils. Experiments were conducted in both 1- and 0-g environments. Water content distributions were evaluated as a function of distance from the source of water intake and time. The experimental results indicated that the capillary potential and the advective forces due to interfacial tension gradients are overshadowed by the gravitational potential in a 1-g environment. The fast water movement in the 0-g condition is attributed to the capillary potential as well as to the advective forces that developed. In addition, microstructural changes have contributed to water flow in the 0-g condition. Depending on soil type, the magnitude of such an effect (i.e., water movement) varies from three- to four-fold. To analyze the experimental results, a one-dimensional model, based on Darcy’s law and the conservation of mass equations, was developed and solved numerical...

ECOUL: an interactive computer tool to study hydraulic behavior of swelling and rigid soils

ECOUL is an interactive, didactic software package which simulates vertical water flow in unsaturated soils. End-users are given an easily-used tool to predict the evolution of the soil water profile, with a large range of possible boundary conditions, through a classical numerical solution scheme for the Richards equation. Soils must be characterized by water retention curves and hydraulic conductivity curves, the form of which can be chosen among different analytical expressions from the literature. When the parameters are unknown, an inverse method is provided to estimate them from available experimental flow data. A significant original feature of the software is to include recent algorithms extending the water flow model to deal with deforming porous media: widespread swelling soils, the volume of which varies as a function of water content, must be described by a third hydraulic characteristic property, the deformation curve. Again, estimation of the parameters by means of inverse procedures and visualization facilities enable exploration, understanding and then prediction of soil hydraulic behavior under various experimental conditions.

Dynamic Effect in the Capillary Pressure–Saturation Relationship and its Impacts on Unsaturated Flow

Capillary pressure plays a central role in the description of water flow in unsaturated soils. While capillarity is ubiquitous in unsaturated analyses, the theoretical basis and practical implications of capillarity in soils remain poorly understood. In most traditional treatments of capillary pressure, it is defined as the difference between pressures of phases, in this case air and water, and is assumed to be a function of saturation. Recent theories have indicated that capillary pressure should be given a more general thermodynamic definition, and its functional dependence should be generalized to include dynamic effects. Experimental evidence has slowly accumulated in the past decades to support a more general description of capillary pressure that includes dynamic effects. A review of these experiments shows that the coefficient arising in the theoretical analysis can be estimated from the reported data. The calculated values range from 104 to 107 kg (m s)−1 In addition, recently developed pore‐scale models that simulate interface dynamics within a network of pores can also be used to estimate the appropriate dynamic coefficients. Analyses of experiments reported in the literature, and of simulations based on pore‐scale models, indicate a range of dynamic coefficients that spans about three orders of magnitude. To examine whether these coefficients have any practical effects on larger‐scale problems, continuum‐scale simulators may be constructed in which the dynamic effects are included. These simulators may then be run to determine the range of coefficients for which discernable effects occur. Results from such simulations indicate that measured values of dynamic coefficients are within one order of magnitude of those values that produce significant effects in field simulations. This indicates that dynamic effects may be important for some field situations, and numerical simulators for unsaturated flow should generally include the additional term(s) associated with dynamic capillary pressure.

Dynamic Effect in the Capillary Pressure-Saturation Relationship and its Impacts on Unsaturated Flow

Capillary pressure plays a central role in the description of water flow in unsaturated soils. While capillarity is ubiquitous in unsaturated analyses, the theoretical basis and practical implications of capillarity in soils remain poorly understood. In most traditional treatments of capillary pressure, it is defined as the difference between pressures of phases, in this case air and water, and is assumed to be a function of saturation. Recent theories have indicated that capillary pressure should be given a more general thermodynamic definition, and its functional dependence should be generalized to include dynamic effects. Experimental evidence has slowly accumulated in the past decades to support a more general description of capillary pressure that includes dynamic effects. A review of these experiments shows that the coefficient arising in the theoretical analysis can be estimated from the reported data. The calculated values range from 104 to 107 kg (m s)−1 In addition, recently developed pore-scale models that simulate interface dynamics within a network of pores can also be used to estimate the appropriate dynamic coefficients. Analyses of experiments reported in the literature, and of simulations based on pore-scale models, indicate a range of dynamic coefficients that spans about three orders of magnitude. To examine whether these coefficients have any practical effects on larger-scale problems, continuum-scale simulators may be constructed in which the dynamic effects are included. These simulators may then be run to determine the range of coefficients for which discernable effects occur. Results from such simulations indicate that measured values of dynamic coefficients are within one order of magnitude of those values that produce significant effects in field simulations. This indicates that dynamic effects may be important for some field situations, and numerical simulators for unsaturated flow should generally include the additional term(s) associated with dynamic capillary pressure.

Influence of a Nonionic Surfactant on the Water Retention Properties of Unsaturated Soils

Surfactants are widely used in household products, industrial processes and as adjuvants to improve the delivery and effectiveness of agrochemicals. Due to their amphiphilic nature, surfactants tend to accumulate at gas‐liquid and solid‐liquid interfaces, and thus, have the potential to influence water flow and retention in unsaturated soils. The objective of this study was to investigate the effects of a nonionic surfactant, Triton X‐100, on the interfacial properties and capillary pressure‐water content relationships of F‐70 Ottawa sand and Appling soil. In the presence of surfactant, soil water contents decreased incrementally as the surfactant concentration was increased from 0 g L−1 up to the critical micelle concentration (CMC) of Triton X‐100 (0.15 g L−1). Over the same surfactant concentration range, the surface tension of water decreased from 7.2 × 10−2 J m−2 to 3.2 × 10−2 J m−2 while solid‐liquid contact angle decreased from 40° to 10°. No further changes in interfacial properties or soil water characteristics were observed at surfactant concentrations above the CMC. The experimental results were used to develop and evaluate alternative scaling approaches to describe concentration dependent changes in soil water characteristics based on the van Genuchten model. A scaling factor that incorporated both surface tension and content angle relationships provided accurate predictions of soil water retention curves over a range of surfactant concentrations. A simplified form of the scaling factor also was developed, on the basis of a single fitting parameter without the need for surface tension and contact angle data. Although further validation of the simplified scaling factor will be required, this approach offers an efficient means to describe the effects of concentration dependent changes in interfacial properties on soil water characteristics.

MATERIAL COORDINATE FORMULATION AND USE IN ONE-DIMENSIONAL SOIL SOLUTE AND WATER FLOW

Description of one-dimensional water flow in saturated swelling soils is facilitated by using a space-like coordinate based on the distribution of solids; a water-based space-like coordinate facilitates analysis of solute movement during non-steady water flow in unsaturated soils. These space-like material coordinates are simply defined and measured for many practical situations. Furthermore, their mathematical formulations are similar and result in similar non-linear diffusion equations that may be solved for important initial and boundary conditions. The approach may be extended to solute transport during non-steady flow of water in a swelling soil. This paper illustrates the similarity of methods of formulating flow equations, it discusses the benefits of the use of such coordinates, and it comments on the need for dimensional consistency between coordinates and intrinsic soil properties.

Chemical reaction and Co-60 retardation in unsteady, unsaturated soil water flow: the effect of clay content

The ability of the regolith to adsorb and retard radioactive and other noxious chemicals is important when selecting repository sites. Because of its surface charge and great specific surface, the clay in soil contributes significantly to this retardation. This paper illustrates a novel analysis of unsteady water and solute flow in unsaturated soil that quantifies these effects. Analysis and experiments suggest that the use of a clay-based space-like coordinate permits us to generalise results to account for variation in clay content and soil structure across many materials. Results imply that clay content, mineralogy, and charge permit quantitative material classification according to their ability to retard cationic wastes. The approach appears to offer significant economies in selecting materials where adsorption and retardation of cation pollutants is desired.

Numerical experiments to simulate vertical vapor and liquid water transport in unsaturated non-rigid porous media

We present numerical simulations using a model named VALTRAUDE (VApor and Liquid TRansport in Unsaturated DEformable media). It was developed to simulate thermal induced heat and vapor transport and liquid water flow in unsaturated soil as affected by both hydraulic and thermal gradients. Both of the flow phenomena are coupled using the Philip and de Vries [Philip, J.R., de Vries, D.A., 1957. Moisture movement in porous materials under temperature gradients. Trans. Am. Geophys. Union 38 (2), 222–231.] theory. VALTRAUDE is able to consider the porous medium to be non-rigid. We therefore compare numerical simulations of water flow in rigid and non-rigid soil columns under isothermal and non-isothermal conditions. Simulation results indicate that the isothermal steady-state equilibrium pressure head distribution in a soil column under non-isothermal conditions, when the soil column is sealed at the top and at the bottom, so that water content remains constant, is replaced by a dynamic flow equilibrium, where flow of liquid water is balanced by flow of vapor. Thus, in a state of non-isothermal equilibrium, the hydraulic gradient is not zero, and the matric potential at the top of the soil has lower values than under isothermal conditions. Comparing rigid with non-rigid soils, in the case of dry initial conditions and non-isothermal boundary conditions we found smaller flow rates and therefore less negative values of matric potential at the top of the soil. With wet initial conditions, there is nearly no difference between rigid and non-rigid soil.

Efficient method for simulating gravity‐dominated water flow in unsaturated soils

Numerical convergence difficulties associated with nonlinearity in the hydraulic property functions are often encountered when simulating gravity‐dominated water flow in unsaturated soils using Richards' equation. A new approach for approximating the gravity term in Richard's equation is developed. Tests on a wide variety of soils show it is superior to existing Picard and Newton approaches. It is more reliable and efficient and allows larger time steps to be used.

Water Flow in Unsaturated Soil Below Turfgrass Observations and LEACHM (within EXPRES) Predictions

In cropped soils, water sustains the plants, affects the transport of nutrients within the root zone, and controls the leaching of nutrients and chemicals to ground water. The objectives of this study were (i) to investigate the effects of turfgrass on water flow in sandy loam soil during the growing season using field lysimeters, and (ii) to test the abilities of the models EXPRES and LEACHN with free‐drainage and lysimeter bottom‐boundary conditions, respectively, to simulate water movement in the lysimeters. Twelve field lysimeters were packed with a three‐horizon profile, topped with Kentucky bluegrass (Poa pratensis L.) sod, and monitored for 2 yr. Saturated hydraulic conductivity, measured on cores, was much greater and more variable for turf than soil. The moisture‐retention curve for turf also had a much steeper drop in water content at low applied negative head than soil. The lysimeters became very dry during the summer, and only drained during the spring and autumn. The model EXPRES generally predicted water flow well, but had some difficulty with water redistribution during the drying periods (gravity drainage and evapotranspiration). In general, with the free‐drainage bottom‐boundary condition, EXPRES predicted more drainage and less drying during the summer than was observed. Under conditions of little to no irrigation, the free‐drainage condition over‐predicted and the lysimeter condition under‐predicted the total amount of measured drainage. Model predictions of drainage under heavily irrigated conditions were similar for both bottom‐boundary conditions.

Radionuclide Movement during Unsteady, Unsaturated Soil Water Flow

ABSTRACTAbsorption of aqueous solution by soil appears to result in piston-like displacement of the soil solution originally present by the invading solution. Diffusive redistribution of non-reactive solutes, relative to the water, occurs about that front when the solutions are of different chemical constitution. Adsorption of solute by the soil solid retards the solute front relative to the piston- front; it also reduces the effective diffusivity of the solute relative to the water. Such behavior tests prediction of radionuclide transport and retardation during non-steady water flow in unsaturated soil based on measured distribution coefficients.Experiments, where initially uniform soil columns absorbed tritiated water containing 60Co2+ from a source at constant water potential, illustrate these effects. Columns were sampled at significantly different elapsed times and the transient water content, water soluble salts, tritium and cobalt profiles determined. Theory anticipates, and experiment confirms, self- similarity of these profiles when data is graphed in terms of distance divided by the square root of time. Water, and soil solid-based, material coordinates facilitate data analysis; measurement of absorption isotherms is also discussed.

Potentials and limitations of modelling vertical distributions of root water uptake of an Austrian pine forest on a sandy soil

Root water uptake patterns are often studied with simulation models of the unsaturated soil water flow, as they are difficult to measure directly. Calibration of these models is not straightforward and causes uncertainties in simulated uptake distributions. In this paper we study how uncertainties in the calibration of the SWIF model affect uncertainty intervals in simulated uptake patterns of an Austrian pine stand (Pinus nigra var. nigra) on a sandy soil. After calibrating and validating SWIF with a large data set of more than 125 000 measured soil water contents over a three year period, uncertainty ranges in simulated soil water dynamics and root water uptake distributions were estimated with a Monte Carlo analysis. In general, uncertainties in root uptake patterns were small (typically <2 10−4 m3 m−3 day−1) and were higher for trees with a shallow rooting system (0·8 m) than for trees with a deep rooting system (2·5 m). Uncertainties arose mainly from uncertainties in simulated soil water fluxes and from variations in the reduction of uptake during periods of drought. Uncertainties in soil water contents were far higher (typically 0·01 m3 m−3) than uncertainties in uptake, illustrating that uncertainties in uptake parameters and those in the distribution of water uptake hardly affect the modelling of soil water dynamics. Root water uptake models should therefore be validated against measured uptake distributions, which can be determined on sandy soils during dry periods with a high water use when soil fluxes are negligible to uptake. Copyright © 2000 John Wiley & Sons, Ltd.

An unsaturated soil water flow problem and its numerical simulation

A numerical model for the unsaturated flow equation with moisture content as prognostic variable is established in order to simulate liquid moisture flow in an unsaturated zone with homogeneous soil, and different initial and boundary conditions. For an infiltration or evaporation problem, its numerical solution by using a finite difference method is very sensitive to its upper boundary condition and the related soil parameters, and using a traditional finite element method usually yields oscillatory non-physics profiles. However, we obtain a nonoscillatory solution and evade a non-physics solution for the problem by using the mass-lumped finite element method. This kind of boundary conditions is handled very well. Numerical simulations for certain soils show that the numerical scheme can be used in simulation of liquid moisture flow for infiltration, evaporation, re-distribution and their alternate appearances. It can be also applied to a high-resolution land surface model.

General Model for Unsaturated Hydraulic Conductivity for Soils with Lognormal Pore‐Size Distribution

AbstractDescribing water flow in unsaturated soils requires knowledge of the unsaturated hydraulic conductivity. This study was conducted to develop a general conductivity model for soils with lognormal pore‐size distribution based on the Mualem‐Dagan pore‐scale model. The derived model has three parameters other than parameters for a retention model: the parameters α and β, both of which are related to the soil pore tortuosity, and the parameter γ to describe how to evaluate the effective pore radius. The proposed model was applied to observed retention and conductivity data sets for 200 soils. Results showed that both α and β should be treated as fitted parameters for accurate descriptions of conductivity, whereas estimation results were insensitive to γ as long as α and β were optimized. Consequently, a simplified form of the general conductivity model was suggested which uses the constant γ value of unity. Based on the simplified general conductivity model, two predictive methods (AB and FN) were developed. The method AB uses the constant α and β values to minimize the average prediction error. The method FN uses an empirically derived relationship between β and the dimensionless parameter of the retention model, and the constant α value to minimize the average prediction error. Both methods reduced the average prediction error more than 77% compared with the Burdine and Mualem predictive models. The method FN provided slightly better predictions than the method AB, reducing the average prediction error by 28%.

A vanishing diffusion process in unsaturated soils

A model for simulating water flow and air flow in unsaturated soils is presented herein. Drainage from a one-dimensional soil column is specifically investigated. A mathematical analysis of the equations reveals that, due to a vanishing diffusion process, the gradient of the water degree of saturation is infinite at the water table. Moreover, this discontinuity propagates at a velocity that satisfies a local non-linear equation involving only the properties of the material. A numerical simulation of this problem serves to confirm the results obtained.

Modelling evaporation from a drained and rewetted peatland

Evaporation from a cutover raised bog in The Netherlands was modelled using a detailed, physically based evaporation model for heterogeneous vegetation and unsaturated soil water flow “SWAPS”. The model enables a quantification of the role of heterogeneity on evaporation. Micro-meteorological measurements above a vegetation of Molinia caerulea (purple moor grass) in the cutover raised bog “Engbertsdijksvenen” are used for calibration and validation of the model. In the present study, it is investigated how small scale heterogeneity affects the prediction of evaporation. Therefore, two hypothetical variants, which represent two possible types of small scale heterogeneity in peatlands, are evaluated: (1) peatland in which drainage has resulted in a vegetation cover of 50% purple moor grass and 50% birch, (2) peatland in which after rewetting the vegetation consists of 50% purple moor grass and 50% Sphagnum mosses. The results show that characteristics of the evaporation from purple moor grass in the two variants are dissimilar. The evaporation from purple moor grass, forming the lower components of a vegetation stand (variant 1), can be accurately predicted via radiation based models. In case purple moor grass is the exposed part of the vegetation (variant 2), such models are not likely to predict the evaporation from purple moor grass correctly. An exposed, aerodynamically rough vegetation requires a model that accounts for the vapour pressure deficit as well.

Effect of Roots on Water Flow in Unsaturated Soils

In recent years, beneficial effects of plants on the remediation of large volumes of slightly contaminated soils have been addressed by various researchers. Water is the main agent moving soluble contaminants in the soil profile, and an important one in transferring contaminants to root surfaces and subsequently to the shoot. Although there are a number of models for water flow in cropped soil, the parameters of the models, which often include rooting density, root permeability, and root water potential, are chosen by trial and error such that an overall model fits the data. Such methods can be tiresome and impractical. Therefore, the purpose of this research is to study the root water interaction of crops by developing a simple predictive technique that is usable in a wide range of applications, such as in-situ plant remediation, for contaminated soil near the surface zone. The simulation model is developed by incorporating a time-specific root distribution model into a vertical unsaturated soil water fl...

Finite element methods for modeling water flow in variably saturated porous media: Numerical oscillation and mass‐distributed schemes

The finite element method is an attractive numerical method for modeling water flow in variably saturated porous media due to its flexibility in dealing with complicated geometries. It is well known that the conventional mass‐distributed finite element method suffers from numerical oscillations at the wetting front, especially for very dry initial conditions. Routinely, mass‐lumped procedures are used to eliminate them. This paper proposes a physical interpretation of the finite element method applied to the water flow problem. With the finite element method, mass conservation is applied at the element level. The water storage and the flux within each element are split into several components in the function space, each of which corresponds to one component of the boundary flux of the element. However, even though physical laws are correctly applied at the element level, it is shown that the traditional mass‐distributed scheme can still generate an incorrect neighboring node response due to the highly nonlinear properties of water flow in unsaturated soil and cause numerical oscillation. We propose two new mass‐distributed schemes which are free of the numerical oscillation, and reduce the smearing near the wetting front, at slightly increased CPU time.

Constant Rate Rainfall Infiltration in a Field of Variable α

Spatial variability of hydraulic parameters is an important factor in prediction of water flow and solute transport in unsaturated soils. An analytical solution for one‐dimensional constant rate rainfall infiltration in a homogeneous soil is incorporated into a model of a spatially variable field. The model is a composite of vertically homogeneous soil columns that differ in their hydraulic properties. In this study, horizontal variation in infiltration profiles is due to variation in the inverse macroscopic capillary length (α) of each column. The sill in travel time variance of wetting profiles with depth for the homogeneous column confirms the notion of a long‐time traveling wave of fixed shape. Variation in α slightly enhances the spread of wetting profiles and delays formation of the traveling wave. Identification of the mean α value is more critical than inclusion of variability in α for wetting profile simulations under the examined conditions.

Modeling water flow in cropped soils: Water uptake by plant roots

The Richards' equation is used as the basic paradigm for water flow in unsaturated soils. An empirical sink term is included to represent the uptake of water by the plant roots. This sink term incorporates a concept termed Evaporation Front which represents the drying upper layers and the subsequent ineffectual roots in this zone during surface evaporation. The Richards' equation is solved numerically by expressing the model in finite-difference form using the Crank-Nicolson method. The resulting water profiles are compared with field data.