Jirka Šimůnek

Review and comparison of models for describing non-equilibrium and preferential flow and transport in the vadose zone

Abstract In this paper, we review various approaches for modeling preferential and non-equilibrium flow and transport in the vadose zone. Existing approaches differ in terms of their underlying assumptions and complexity. They range from relatively simplistic models to more complex physically based dual-porosity, dual-permeability, and multi-region type models. A relatively simple dual-porosity flow model results when the Richards equation is combined with composite (double-hump type) equations for the hydraulic properties to account for both soil textural (matrix) and soil structural (fractures, macropores, peds) effects on flow. The simplest non-equilibrium flow model, a single-porosity model which distinguishes between actual and equilibrium water contents, is based on a formulation by Ross and Smettem [Soil Sci. Soc. Am. J. 64 (2000) 1926] that requires only one additional parameter to account for non-equilibrium. A more complex dual-porosity, mobile–immobile water flow model results when the Richards or kinematic wave equations are used for flow in the fractures, and immobile water is assumed to exist in the matrix. We also discuss various dual-permeability models, including the formulation of Gerke and van Genuchten [Water Resour. Res. 29 (1993a) 305] and the kinematic wave approach as used in the MACRO model of Jarvis [Technical Description and Sample Simulations, Department of Soil Science, Swedish University of Agricultural Science, Uppsala, Sweden (1994) 51]. Both of these models invoke terms accounting for the exchange of water and solutes between the matrix and the fractures. Advantages and disadvantages of the different models are discussed, and the need for inter-code comparison is stressed, especially against field data that are sufficiently comprehensive to allow calibration/validation of the more complex models and to distinguish between alternative modeling concepts. Several examples and comparisons of equilibrium and various non-equilibrium flow and transport models are also provided.

A review of model applications for structured soils: (b) Pesticide transport.

The past decade has seen considerable progress in the development of models simulating pesticide transport in structured soils subject to preferential flow (PF). Most PF pesticide transport models are based on the two-region concept and usually assume one (vertical) dimensional flow and transport. Stochastic parameter sets are sometimes used to account for the effects of spatial variability at the field scale. In the past decade, PF pesticide models were also coupled with Geographical Information Systems (GIS) and groundwater flow models for application at the catchment and larger regional scales. A review of PF pesticide model applications reveals that the principal difficulty of their application is still the appropriate parameterization of PF and pesticide processes. Experimental solution strategies involve improving measurement techniques and experimental designs. Model strategies aim at enhancing process descriptions, studying parameter sensitivity, uncertainty, inverse parameter identification, model calibration, and effects of spatial variability, as well as generating model emulators and databases. Model comparison studies demonstrated that, after calibration, PF pesticide models clearly outperform chromatographic models for structured soils. Considering nonlinear and kinetic sorption reactions further enhanced the pesticide transport description. However, inverse techniques combined with typically available experimental data are often limited in their ability to simultaneously identify parameters for describing PF, sorption, degradation and other processes. On the other hand, the predictive capacity of uncalibrated PF pesticide models currently allows at best an approximate (order-of-magnitude) estimation of concentrations. Moreover, models should target the entire soil-plant-atmosphere system, including often neglected above-ground processes such as pesticide volatilization, interception, sorption to plant residues, root uptake, and losses by runoff. The conclusions compile progress, problems, and future research choices for modelling pesticide displacement in structured soils.

Mechanisms and Pathways of Trace Element Mobility in Soils

Publisher Summary The importance of trace elements (TEs) in soils depends largely on their fraction that has immediate biological function, that is, the fraction of the total soil burden that is soluble, mobile, and bio-available. The nature and extent of mobility and bioavailability underlines the integrity and sustainability of a particular environment and in particular, the role of TEs in the functioning and wellbeing of an ecological endpoint. The chapter discusses the basic mechanisms in the solubility and mobility of the TEs in the soil, including their movement in the soil profile, the entire vadose zone and the eventual leaching to the ground water. The mechanism leads to the several transport pathways in soil responsible for disseminating TEs in the form of gaseous, aqueous, colloids, and particulate matter. The chapter discusses the most pertinent factors influencing the partitioning and movement of TEs, and finally illustrates transport modeling of the most environmentally important TEs and their applications typified by field case studies, with an emphasis on transport modeling in the vadose zone. TE transport pathways include diffusion and dispersion, preferential flow, colloidal transport, soluble metal complexes, leaching and runoff, and volatilization. There are basic physical, chemical, and biological processes that control mobility of TEs in soils. The processes that sequester TEs can be grossly termed sorption, which to a large extent, determines the partitioning between the solid and solution phase. Factors like soil pH, chemical speciation, soil organic matter, fertilizers and soil amendments, redox potential, and clay content and soil structure affecting trace element mobility and transport are discussed.

Simultaneous estimation of soil hydraulic and solute transport parameters from transient infiltration experiments

Abstract Estimation of soil hydraulic and solute transport parameters is important to provide input parameters for numerical models simulating transient water flow and solute transport in the vadose zone. The Levenberg–Marquardt optimization algorithm in combination with the HYDRUS-1D numerical code was used to inversely estimate unsaturated soil-hydraulic and solute transport parameters from transient matric pressure head, apparent electrical conductivity, and effluent flux measurements. A 30 cm long soil column with an internal diameter of 5 cm was used for infiltration experiments in a coarse-textured soil. Infiltration experiments were carried out with both increasing and decreasing solute concentrations following a sudden increase in the infiltration rate. Matric pressure heads and solute concentrations were measured using automated mini-tensiometers and four-electrode sensors, respectively. The simultaneous estimation results were compared with independently measured soil water retention, unsaturated hydraulic conductivity, and solute dispersion data obtained from steady-state water flow experiments. The optimized values corresponded well with those measured independently within the range of experimental data. The information contained in the apparent electrical conductivity (which integrates information about both water flow and solute transport) proved to be very useful for the simultaneous estimation of soil hydraulic and solute transport parameters.

Stochastic Fusion of Information for Characterizing and Monitoring the Vadose Zone

are still often raised regarding parameter identifiability and their uniqueness for particular methods. Inverse problems for hydrological processes in the vadose zone While various laboratory and field methods for evaluare often perceived as being ill posed and intractable. Consequently, ating soil hydraulic properties are relatively well estabsolutions to the inverse problems are frequently subject to skepticism. In this paper, we examine the necessary and sufficient conditions for lished, several major problems remain. Most laboratory the inverse problems to be well posed and discuss difficulties associ- methods are applied to samples ranging from 100 to ated with solving the inverse problems. We subsequently explain the about 500 cm 3 . The scale of field methods generally does need for a stochastic conceptualization of inverse problems of the not extend beyond a plot of 1 m 2 and depths of one to vadose zone. Principles of geostatistically based inverse approaches, several meters. There is an urgent need to develop methwhich rely on stochastic concepts, are then illustrated, including cok- ods that characterize hydraulic properties of the vadose riging, a sequential linear estimator, and a successive linear estimator. zone on a much larger scale. Recently developed geoWe then discuss applications involved in the approaches to classical physical methods such as electrical resistivity tomogravadose zone inversion problems (using observed pressure heads, mois

Impact of varying soil structure on transport processes in different diagnostic horizons of three soil types

When soil structure varies in different soil types and the horizons of these soil types, it has a significant impact on water flow and contaminant transport in soils. This paper focuses on the effect of soil structure variations on the transport of pesticides in the soil above the water table. Transport of a pesticide (chlorotoluron) initially applied on soil columns taken from various horizons of three different soil types (Haplic Luvisol, Greyic Phaeozem and Haplic Cambisol) was studied using two scenarios of ponding infiltration. The highest infiltration rate and pesticide mobility were observed for the Bt(1) horizon of Haplic Luvisol that exhibited a well-developed prismatic structure. The lowest infiltration rate was measured for the Bw horizon of Haplic Cambisol, which had a poorly developed soil structure and a low fraction of large capillary pores and gravitational pores. Water infiltration rates were reduced during the experiments by a soil structure breakdown, swelling of clay and/or air entrapped in soil samples. The largest soil structure breakdown and infiltration decrease was observed for the Ap horizon of Haplic Luvisol due to the low aggregate stability of the initially well-aggregated soil. Single-porosity and dual-permeability (with matrix and macropore domains) flow models in HYDRUS-1D were used to estimate soil hydraulic parameters via numerical inversion using data from the first infiltration experiment. A fraction of the macropore domain in the dual-permeability model was estimated using the micro-morphological images. Final soil hydraulic parameters determined using the single-porosity and dual-permeability models were subsequently used to optimize solute transport parameters. To improve numerical inversion results, the two-site sorption model was also applied. Although structural changes observed during the experiment affected water flow and solute transport, the dual-permeability model together with the two-site sorption model proved to be able to approximate experimental data.

Multicomponent solute transport in soil lysimeters irrigated with waters of different quality

[1] A variety of analytical and numerical models have been developed during the past several decades to predict water and solute transfer processes between the soil surface and the groundwater table. While many models quantifying solute transport in soils usually consider only one solute and severely simplify various chemical interactions, others such as the geochemical module of HYDRUS-1D consider multiple solutes and their mutual interactions. In this study we use HYDRUS-1D to analyze water flow and solute transport in three soil lysimeters (1.2 m 2 � 1 m) irrigated during the summer months with waters of different quality that were used to evaluate salinization and alkalization hazards. The soil monoliths were constructed in a Eutric Fluvisol in Alentejo, Portugal. The electrical conductivity (EC) of irrigation water varied between 0.4 and 3.2 dS m � 1 , and the sodium adsorption ratio (SAR) varied between 1 and 6 (mmol(c) L � 1 ) 0.5 , while maintaining a ratio of Ca:Mg equal to 1:2. The soil monoliths were subjected to regular rainfall and leaching during the rest of the year. Water contents and fluxes, concentrations of individual ions (Na + ,C a 2+ , and Mg 2+ ), electrical conductivity of

Subsurface Water Distribution from Drip Irrigation Described by Moment Analyses

Moment analysis techniques are used to describe spatial and temporal subsurface wetting patterns resulting from drip emitters. The water added is considered a ‘‘plume’’ with the zeroth moment representing the total volume of water applied. The first moments lead to the location of the center of the plume, and the second moments relate to the amount of spreading about the mean position. We tested this approach with numerically generated data for infiltration from surface and buried line and point sources in three contrasting soils. Ellipses (in two dimensions) or ellipsoids (in three dimensions) can be depicted about the center of the plume. Any fraction of water added can be related to a ‘‘probability’’ curve relating the size of the ellipse (or ellipsoid) that contains that amount of water. Remarkably, the probability curves are identical for all times and all of the contrasting soils. The consistency of the probability relationships can be exploited to pinpoint the extent of subsurface water for any fraction of the volume added. The new method can be immediately applied to the vital question of how many sensors are needed and where to install them to capture the overall water distribution under drip irrigation. For example, better agreement with the ‘‘exact’’ solution occurs with increasing the number of observation points from 6 to 9 and no significant improvement when increasing from 9 to 16. The method can also be applied to parameter estimation of soil hydraulic properties, which we uniquely reproduced for generated data. DESIGNING drip irrigation systems involves selection of an appropriate combination of emitter discharge rate and spacing between emitters for any given set of soil, crop, and climatic conditions, as well as understanding the wetted zone pattern around the emitter (Bresler, 1978; Lubana andNarda, 2001).Water distribution is affected by many factors, including soil hydraulic characteristics, initial conditions, emitter discharge rate, application frequency, root characteristics, evaporation, and transpiration. A traditional way to visualize spatial and temporal soil water distributions includes determination of the water content at points around the emitter and drawing contours between these points. Practical information includes estimation of the position and shape of thewetted volume (Dasberg andOr, 1999). Previous investigators have used descriptions of the extent of wetting, including the surface wetted diameter, wetted depth, and wetted volume (Ben-Asher et al., 1986; Schwartzman and Zur, 1986; Angelakis et al., 1993; Chu, 1994; Zur, 1996; Dasberg and Or, 1999; Hammami et al., 2002; Thorburn et al., 2003; Cook et al., 2003). The volume of the wetted soil represents the amount of soil water stored in the root zone. The domain of interest should be consistent with the anticipated depth of the root system, while its width is associated with the spacing between emitters and lines (Zur, 1996). A comprehensive method of characterizing spatial– temporal distributions is through moment analyses. This approach is widely used to describe solute transport in the vadose zone (e.g., Barry and Sposito, 1990; Toride and Leij, 1996; Srivastava et al., 2002). With respect to water, Yeh et al. (2005) and Ye et al. (2005) calculated the zeroth, first, and second moments of a three-dimensional water content plume and defined an ellipsoid that described the average shape and orientation of the plume for each observation period. This led to snapshots of the observed water content plume under transient flow conditions, which was used to derive a three-dimensional effective hydraulic conductivity tensor. The objective of this study was to implement moment analyses to describe subsurface water distribution resulting from drip irrigation and to test this approach with numerically generated data. This includes infiltration for both line and point sources in contrasting soils. The considerable advantages gained by the moment analyses over alternatives is detailed.

Numerical investigation of irrigation scheduling based on soil water status

Improving the sustainability of irrigation systems requires the optimization of operational parameters such as irrigation threshold and irrigation amount. Numerical modeling is a fast and accurate means to optimize such operational parameters. However, little work has been carried out to investigate the relationship between irrigation scheduling, irrigation threshold, and irrigation amount. Herein, we compare the results of HYDRUS 2D/3D simulations with experimental data from triggered drip irrigation, and optimize operational parameters. Two field experiments were conducted, one on loamy sand soil and one on sandy loam soil, to evaluate the overall effects of different potential transpiration rates and irrigation management strategies, on the triggered irrigation system. In both experiments, irrigation was controlled by a closed loop irrigation system linked to tensiometers. Collected experimental data were analyzed and compared with HYDRUS 2D/3D simulations. A system-dependant boundary condition, which initiates irrigation whenever the matric head at a predetermined location drops below a certain threshold, was implemented into the code. The experimental model was used to evaluate collected experimental data, and then to optimize the operational parameters for two hypothetical soils. The results show that HYDRUS 2D/3D predictions of irrigation events and matric heads are in good agreement with experimental data, and that the code can be used to optimize irrigation thresholds and water amounts applied in an irrigation episode to increase the efficiency of water use.

Sensitivity analysis of physical and chemical properties affecting field-scale cadmium transport in a heterogeneous soil profile

Field-scale transport of reactive solutes depends on spatially variable physical and chemical soil properties. The quantitative importance of physical and chemical parameters required for the prediction of the field-scale solute flux is generally unknown. A sensitivity analysis is presented that ranks the importance of spatially variable water flow and solute transport parameters affecting field-scale cadmium flux in a layered sandy soil. In a Monte-Carlo simulation approach, partial rank correlation coefficients were calculated between model parameters and cadmium flux concentrations at various time steps. Data on the heterogeneity of flow and transport parameters were obtained from a 180 m-long and 1 m-deep Spodosol transect. Each soil layer was described in terms of probability density functions of five model parameters: two shape parameters of van Genuchten’s water retention curve, saturated hydraulic conductivity, dispersivity and soil ‐ water distribution coefficient. The results showed that the cadmium flux concentrations at the bottom of the soil profile were most sensitive to the cadmium deposition rate and the soil‐ water distribution coefficient of all soil horizons. The maximum cadmium flux concentrations were also affected by variations in hydraulic conductivity of the humic topsoil horizons. Variations in shape parameters of the water retention curve did not significantly affect the field-scale cadmium flux. Variations in the dispersivity of the subsoil significantly influenced the early time cadmium concentrations. Monte-Carlo simulations involving non-linear sorption showed that cadmium flux concentrations were dominated by variations in the sorption constant and in the exponent of the Freundlich isotherm. q 2002 Elsevier Science B.V. All rights reserved.