https://www.eawag.ch/fileadmin/Domain1/Abteilungen/siam/software/swat/thur.pdf

Modelling hydrology and water quality in the pre-alpine/alpine Thur watershed using SWAT

A one-dimensional dual-porosity model has been developed for the purpose of studying variably saturated water flow and solute transport in structured soils or fractured rocks. The model involves two overlaying continua at the macroscopic level: a macropore or fracture pore system and a less permeable matrix pore system. Water in both pore systems is assumed to be mobile. Variably saturated water flow in the matrix as well as in the fracture pore system is described with the Richards' equation, and solute transport is described with the convection-dispersion equation. Transfer of water and solutes between the two pore regions is simulated by means of first-order rate equations. The mass transfer term for solute transport includes both convective and diffusive components. The formulation leads to two coupled systems of nonlinear partial differential equations which were solved numerically using the Galerkin finite element method. Simulation results demonstrate the complicated nature of solute leaching in structured, unsaturated porous media during transient water flow. Sensitivity studies show the importance of having accurate estimates of the hydraulic conductivity near the surface of soil aggregates or rock matrix blocks. The proposed model is capable of simulating preferential flow situations using parameters which can be related to physical and chemical properties of the medium.

A dual-porosity model for simulating the preferential movement of water and solutes in structured porous media

This review discusses the causes and consequences of 'non-equilibrium' water flow and solute transport in large structural pores or macropores (root and earthworm channels, fissures and interaggregate voids). The experimental evidence suggests that pores larger than c. 0.3 mm in equivalent cylindrical diameter allow rapid non-equilibrium flow. Apart from their large size and continuity, this is also due to the presence of impermeable linings and coatings that restrict lateral mass exchange. Macropores also represent microsites in soil that are more biologically active, and often more chemically reactive than the bulk soil. However, sorption retardation during transport through such pores is weaker than in the bulk soil, due to their small surface areas and significant kinetic effects, especially in larger macropores. The potential for non-equilibrium water flow and solute transport at any site depends on the nature of the macropore network, which is determined by the factors of structure formation and degradation, including the abundance and activity of soil biota such as earthworms, soil properties (e.g. clay content), site factors (e.g. slope position, drying intensity, vegetation) and management (e.g. cropping, tillage, traffic). A conceptual model is proposed that summarizes these effects of site factors on the inherent potential for non-equilibrium water flow and solute transport in macropores. Initial and boundary conditions determine the extent to which this potential is realized. High rain intensities clearly increase the strength of non-equilibrium flow in macropores, but the effects of initial water content seem complex, due to the confounding effects of soil shrinkage and water repellency. The impacts of macropore flow on water quality are most significant for relatively immobile solutes that are foreign to the soil and whose effects on ecosystem and human health are pronounced even at small leached fractions (e.g. pesticides). The review concludes with a discussion of topics where process understanding is still lacking, and also suggests some potential applications of the considerable knowledge that has accumulated in recent decades.

A review of non-equilibrium water flow and solute transport in soil macropores: principles, controlling factors and consequences for water quality

A continental-scale hydrology and water quality model for Europe: Calibration and uncertainty of a high-resolution large-scale SWAT model

Mathematical models have become indispensable tools for studying vadose zone flow and transport processes. We reviewed the history of development, the main processes involved, and selected applications of HYDRUS and related models and software packages developed collaboratively by several groups in the United States, the Czech Republic, Israel, Belgium, and the Netherlands. Our main focus was on modeling tools developed jointly by the U.S. Salinity Laboratory of the USDA, Agricultural Research Service, and the University of California, Riverside. This collaboration during the past three decades has resulted in the development of a large number of numerical [e.g., SWMS_2D, HYDRUS-1D, HYDRUS-2D, HYDRUS (2D/3D), and HP1] as well as analytical (e.g., CXTFIT and STANMOD) computer tools for analyzing water flow and solute transport processes in soils and groundwater. The research also produced additional programs and databases (e.g., RETC, Rosetta, and UNSODA) for quantifying unsaturated soil hydraulic properties. All of the modeling tools, with the exception of HYDRUS-2D and HYDRUS (2D/3D), are in the public domain and can be downloaded freely from several websites.

Development and Applications of the HYDRUS and STANMOD Software Packages and Related Codes

Variably saturated water flow in a dual-porosity medium may be described using two separate flow equations which are coupled by means of a sink source term Γw, to account for the transfer of water between the macropore (or fracture) and soil (or rock) matrix pore systems. In this study we propose a first-order rate expression for Γw, which assumes that water transfer is proportional to the difference in pressure head between the two pore systems. A general expression for the transfer coefficient αw was derived using Laplace transforms of the linearized horizontal flow equation. The value of αw could be related to the size and shape of the matrix blocks (or soil aggregates) and to the hydraulic conductivity Ka of the matrix at the fracture/matrix interface. The transfer term Γw, was evaluated by comparing simulation results with those obtained with equivalent one- and two-dimensional single-porosity flow models. Accurate results were obtained when Ka was evaluated using a simple arithmetic average of the interface conductivities associated with the fracture and matrix pressure heads. Results improved when an empirical scaling coefficient γw was included in αw. A single value of 0.4 for γw was found to be applicable, irrespective of the hydraulic properties or the initial pressure head of the simulated system.

Evaluation of a first-order water transfer term for variably saturated dual-porosity flow models

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 parametersKs and ain 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 saturatedflow 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 waterflow 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

/pdf/estimating-unsaturated-soil-hydraulic-properties-from-5aswx8y7r4.pdf

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

[1] Considerable research suggests that colloid deposition in porous media is frequently not consistent with filtration theory predictions under unfavorable attachment conditions. Filtration theory was developed from an analysis of colloid attachment to the solid-water interface of a single spherical grain collector and therefore does not include the potential influence of pore structure, grain-grain junctions, and surface roughness on straining deposition. This work highlights recent experimental evidence that indicates that straining can play an important role in colloid deposition under unfavorable attachment conditions and may explain many of the reported limitations of filtration theory. This conclusion is based upon pore size distribution information, size exclusion, time- and concentration-dependent deposition behavior, colloid size distribution information, hyperexponential deposition profiles, the dependence of deposition on colloid and porous medium size, batch release rates, micromodel observations, and deposition at textural interfaces. The implications of straining in unsaturated and heterogeneous systems are also discussed, as well as the potential influence of system solution chemistry and hydrodynamics. The inability of attachment theory predictions to describe experimental colloid transport data under unfavorable conditions is demonstrated. Specific tests to identify the occurrence and/or absence of straining and attachment are proposed.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005WR004791

Significance of straining in colloid deposition: Evidence and implications

This paper provides a simple way to convert Brooks-Corey (BC) parameters to van Genuchten (vG) parameters and vice versa, for use primarily in situations where saturated conditions are likely to be encountered. Essential in this conversion is the preservation of the maximum value of a physical characteristic, the "effective capillary drive" H,, (Morel-Seytoux and Khanji, 1974), defined with a good approximation for a soil water and air system as H,., = Jg k,, dh,, where k,, is relative permeability (or conductivity) to water and II, is capillary pressure (head), a positive quantity. With this conversion, infiltration calculations are essentially insensitive to the model used to represent the soil hydraulic properties. It is strictly a matter of convenience for the user which expression is used. On the other hand, the paper shows that other equivalences may lead to great variations in predictions of infiltration capacity. Consequently, the choice of the proper equivalence to use in calculations for rainfall-runoff modeling or for low-level radioactive waste disposal design is a serious matter.

M. T. van Genuchten

Papers

A dual-porosity model for simulating the preferential movement of water and solutes in structured porous media

Evaluation of a first-order water transfer term for variably saturated dual-porosity flow models

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

Significance of straining in colloid deposition: Evidence and implications

Parameter Equivalence for the Brooks-Corey and Van Genuchten Soil Characteristics: Preserving the Effective Capillary Drive