Soil Matrix Domain Research Articles

Application of Leak 2D to Describe Preferential Water Flow in a Soil Containing Artificial Macropores

Leak 2D is a new two-dimensional dual permeability mathematical model for the simulation of the preferential flow in the vadose zone. In this model, water flow in the soil matrix domain is described by the two-dimensional h-based Richards’ equation. Water flow in the fracture domain is estimated using the kinematic wave approach. Richards’ equation is solved by a combination of the alternating direction implicit (A.D.I.) method and the Douglas and Jones predictor−corrector method. The kinematic wave equation is solved explicitly. In the present paper, Leak 2D is calibrated and validated with data obtained in a Hele–Shaw apparatus filled with sand. Preferential flow is achieved by inserting four artificial macropores of various sizes into the soil. Six irrigations of various intensities and durations were used for the calibration and validation process. The water content at various depths was recorded by five sensors that were inserted into the soil. A comparison of the simulated water content with the measured profiles shows that Leak 2D can sufficiently describe preferential flow into the unsaturated zone of the soil, even under extreme irrigation conditions.

Revealing subsurface processes in the Uhlířská catchment through combined modelling of unsaturated and saturated flow

AbstractA novel transferable modelling approach was applied to reveal subsurface flow processes and thus complement, verify, and enhance the conceptual knowledge of the small (1.78 km2) mountainous experimental catchment Uhlířská in the northern part of the Czech Republic. We used the S1D model to simulate flow and tracer movement in the vadose zone. MODFLOW‐2000, MODPATH and MT3DS models were used to determine residence time and flow trajectories in the saturated zone. The isotopic tracers 3H and 3H/3He were used to calibrate the groundwater model in the deeper aquifer layers. The modelling approach was established to enhance our understanding and quantification of interactions between the hillslope (82% of the area) and wetland (18% of the area), as well as the aquifer beneath the catchment. Numerical methods corroborated the estimated groundwater residence times and provided a catchment water balance in support of process knowledge. The combined modelling yielded the following water balance for the period 1961–2014: 1220 mm annual precipitation amount, 456 mm of percolation through the soil matrix domain on hillslopes, 534 mm of water through the hillslope preferential domain, and 22 mm of water through the matrix of the riparian wetland. This partitioning resulted in an area‐weighted average annual groundwater recharge of 430 mm. The water residence time on the hillslopes did not exceed 1 year, and was about 10 years for the saturated zone, with 20% of water older than 100 years in the deepest part of the aquifer. The conceptual knowledge of subsurface hydrological processes combined with numerical modelling, computation of water balance and isotope‐supported calibration is considered innovative, particularly the 3H/3He method to determine water residence times of young groundwater in the saturated zone. Lessons learned and recommendations are presented to apply the knowledge from the Uhlířská in the broad context of small research catchments.

Simulating preferential soil water flow and tracer transport using the Lagrangian Soil Water and Solute Transport Model

Abstract. We propose an alternative model concept to represent rainfall-driven soil water dynamics and especially preferential water flow and solute transport in the vadose zone. Our LAST-Model (Lagrangian Soil Water and Solute Transport) is based on a Lagrangian perspective of the movement of water particles (Zehe and Jackisch, 2016) carrying a solute mass through the subsurface which is separated into a soil matrix domain and a preferential flow domain. The preferential flow domain relies on observable field data like the average number of macropores of a given diameter, their hydraulic properties and their vertical length distribution. These data may be derived either from field observations or by inverse modelling using tracer data. Parameterization of the soil matrix domain requires soil hydraulic functions which determine the parameters of the water particle movement and particularly the distribution of flow velocities in different pore sizes. Infiltration into the matrix and the macropores depends on their respective moisture state, and subsequently macropores are gradually filled. Macropores and matrix interact through diffusive mixing of water and solutes between the two flow domains, which again depends on their water content and matric potential at the considered depths. The LAST-Model is evaluated using tracer profiles and macropore data obtained at four different study sites in the Weiherbach catchment in southern Germany and additionally compared against simulations using HYDRUS 1-D as a benchmark model. While both models show qual performance at two matrix-flow-dominated sites, simulations with LAST are in better accordance with the fingerprints of preferential flow at the two other sites compared to HYDRUS 1-D. These findings generally corroborate the feasibility of the model concept and particularly the implemented representation of macropore flow and macropore–matrix exchange. We thus conclude that the LAST-Model approach provides a useful and alternative framework for (a) simulating rainfall-driven soil water and solute dynamics and fingerprints of preferential flow as well as (b) linking model approaches and field experiments. We also suggest that the Lagrangian perspective offers promising opportunities to quantify water ages and to evaluate travel and residence times of water and solutes by a simple age tagging of particles entering and leaving the model domain.

Effects of Anisotropy of Preferential flow on the Hydrology and Stability of Landslides

Infiltration is one of the most important landslides triggering mechanisms and it is controlled by the hydraulic characteristics of the soil, which depend on degree of saturation, existence of preferential flow paths and anisotropy. In order to account for preferential flow that can have place in macro-pores and fissures, it is common to represent the soil matrix by means of the superimposition of two different domains: a soil matrix domain, which mainly accounts for the flow in the porous matrix, and preferential flow domain representing the flow through macro-pores and fissures. There have been recent investigations on the influences of preferential flow on slope stability; however, the combined effects of anisotropy and preferential flow on infiltration processes and on rainfall induced landslide mechanisms have not been studied yet, at our knowledge. Aiming at better understanding the effects that anisotropy combined with preferential flow has on the infiltration process, we investigated the stability of a hillslope using a numerical modelling approach. Results indicate that anisotropy affects the slope stability and its failure area.

Sampling Position under No-Tillage System Affects the Results of Soil Physical Properties

Understanding the spatial behavior of soil physical properties under no-tillage system (NT) is required for the adoption and maintenance of a sustainable soil management system. The aims of this study were to quantify soil bulk density (BD), porosity in the soil macropore domain (PORp) and in the soil matrix domain (PORm), air capacity in the soil matrix (ACm), field capacity (FC), and soil water storage capacity (FC/TP) in the row (R), interrow (IR), and intermediate position between R and IR (designated IP) in the 0.0-0.10 and 0.10-0.20 m soil layers under NT; and to verify if these soil properties have systematic variation in sampling positions related to rows and interrows of corn. Soil sampling was carried out in transect perpendicular to the corn rows in which 40 sampling points were selected at each position (R, IR, IP) and in each soil layer, obtaining undisturbed samples to determine the aforementioned soil physical properties. The influence of sampling position on systematic variation of soil physical properties was evaluated by spectral analysis. In the 0.0-0.1 m layer, tilling the crop rows at the time of planting led to differences in BD, PORp, ACm, FC and FC/TP only in the R position. In the R position, the FC/TP ratio was considered close to ideal (0.66), indicating good water and air availability at this sampling position. The R position also showed BD values lower than the critical bulk density that restricts root growth, suggesting good soil physical conditions for seed germination and plant establishment. Spectral analysis indicated that there was systematic variation in soil physical properties evaluated in the 0.0-0.1 m layer, except for PORm. These results indicated that the soil physical properties evaluated in the 0.0-0.1 m layer were associated with soil position in the rows and interrows of corn. Thus, proper assessment of soil physical properties under NT must take into consideration the sampling positions and previous location of crop rows and interrows.

Transport of iodide in structured soil under spring barley during irrigation experiment analyzed using dual-continuum model

Transport of radioactive iodide 131I− in a black clay loam soil under spring barley in an early ontogenesis phase was monitored during controlled field irrigation experiment. It was found that iodide bound in the soil matrix could be mobilized by the surface leaching enhanced by mechanical impact of water drops and transported below the root zone of crops via soil cracks. The iodide transport through structured soil profile was simulated by the one-dimensional dual-continuum model, which assumes the existence of two inter-connected flow domains: the soil matrix domain and the preferential flow domain. The model predicted relatively deep percolation of iodide within a short time, in a good agreement with the observed vertical iodide distribution in soil. The dual-continuum approach proved to be an adequate tool for evaluation of field irrigation experiments conducted in structured soils.

Macroscopic Representation of the Interface between Flow Domains in Structured Soil

This study attempts to quantify the interface between flow domains in macroscopic dual-porosity flow and transport models. For a structured soil, this interface could be related to the surfaces of soil aggregates and biopores that may serve as preferential flow paths. Mass exchange with the soil matrix domain across this interface requires including the combined effect of various structural shapes and sizes, which has not yet been properly defined. The objective was to test an approach for independent soil structure quantification by characterizing domain interfaces based on surface area/volume ratios using two soil samples with contrasting structures. X-ray computed tomography data from undisturbed soil cores from a medical scanner were analyzed. For a range of density threshold values, surface areas were identified on a voxel basis. The resulting functions were found to be characteristic for the subsoil sample with cylindrical pinhole pores as well as for the aggregated topsoil sample. The results suggest that the interdomain interface area could be used for deriving mass exchange coefficients and a macroscopic characteristic related to visual inspection of the soil structure. The approach, representing an upscaling of individual structural features to the macroscopic scale, may improve the applicability of two-domain models by finding a way to obtain relatively simple estimates of the geometry coefficient from qualitative soil structure information given in soil protocols. Although this analysis did not allow determination of a well-defined interface between porous domains, it enabled narrowing of the range of possible values in a physically plausible region. Still more effort is required to identify specific types of structural surfaces among other features.

Surface Boundary Conditions in Two‐Dimensional Dual‐Permeability Modeling of Tile Drain Bromide Leaching

The simulation of preferential flow in structured soil using dual‐porosity models requires separate sets of the hydraulic, transport, and mass transfer parameters and of the boundary conditions. Analyses of tracer experiments with two‐domain models are limited by constraints in specifying separate boundary conditions (BCs) for each pore domain. The appropriate boundary conditions for dual‐permeability models have not been systematically studied or addressed in experiments. The objective of this study was to numerically evaluate the effects on Br leaching from a tile‐drained field of different surface boundary conditions for a two‐dimensional dual‐permeability model. For the previously described Br tracer experiment at the Bokhorst site (Germany), effects of irrigation intensities, flux‐ and resident‐type solute BCs, and Br application domain on drain discharge and Br effluent concentrations were compared. The two‐dimensional dual‐permeability numerical model divides the soil into soil matrix (SM) and preferential flow (PF) domains. In case of ponding at the surface of the SM domain, water is redistributed toward the PF domain surface. The combination of detailed irrigation record, flux‐type solute BCs, and solute application to the PF domain resulted in the largest Br leaching. In contrast, the lowest Br leaching was predicted for averaged irrigation rates, flux‐type solute BCs and Br addition to the SM domain. For flux‐type (third‐type) solute BC application to both pore domains, enhanced Br leaching was obtained due to surface redistribution effects. Detailed irrigation patterns with realistic intensities predicted higher Br leaching than averaged intensities. In addition to surface effects, the temporal availability of Br in the preferential pathways seemed to control Br leaching patterns. The results suggest that the formulation of the upper BCs strongly affects two‐dimensional dual‐permeability Br leaching predictions. Proper experimental consideration of domain‐specific BCs may help improve descriptions of preferential flow.

Two‐Dimensional Dual‐Permeability Analyses of a Bromide Tracer Experiment on a Tile‐Drained Field

Preferential flow has been hypothesized as an important factor for chemical leaching from tile‐drained agricultural fields with structured soils originating from glacial till sediments. Previous studies showed that one‐dimensional single‐porosity models (1D‐SPM) failed and that one‐dimensional dual‐permeability models (1D‐DPERM) were limited in explaining both Br leaching and residual Br distribution, although tile water outflow peaks could somehow be reproduced. The objective of this paper was to analyze the tile outflow and leaching patterns using a two‐dimensional (2D)‐DPERM and a standard 2D‐SPM for comparison. Flow and transport were simulated in a 2D vertical cross‐section of 5.9 m length and 2 m depth using previously tested parameters. Simulated drainage rates and Br‐effluent concentrations were made comparable with collector data from a field experiment by weighing results for irrigated and nonirrigated plots according to their area fractions. The 2D‐DPERM simulations for surface application of Br in dissolved form in both domains overestimated the observed initial outflow concentration peaks, in contrast to closer approximation of observations assuming Br application in the soil matrix domain only. The simulated 2D mass transfer rate distribution showed most intensive exchange between domains near the water table and in the topsoil. Results from the 2D‐DPERM analyses suggest that conditions at the soil surface, near the water table, and of the field‐scale mixing are significantly affecting leaching patterns, in addition to local nonequilibrium effects. Here, the description of preferential flow toward tile drain could be strongly improved with the 2D‐DPERM compared with the 2D‐SPM. Further improvements remain challenging with respect to DPERM numerical modeling and field experimentation, with special attention toward soil structure and soil surface conditions.

Simulated cadmium transport in macroporous soil during heavy rainstorm using dual-permeability approach

Numerical modelling is used to analyze the transport of cadmium in response to an extreme rainfall event. The cadmium transport through the soil profile was simulated by the one-dimensional dual-permeability model, which assumes the existence of two mutually communicating domains: the soil matrix domain and the preferential flow domain. The model is based on Richards’ equation for water flow and advection-dispersion equation for solute transport. A modified batch technique allowed us to consider domain specific sorption, i.e. each of the domains has its own distribution coefficient. The dual-permeability model predicts that the cadmium can be transported substantially below the root zone after the storm. On the other hand, classical single permeability approach predicted that almost all applied cadmium stays retained near the soil surface.

Dual-permeability modeling of preferential bromide leaching from a tile-drained glacial till agricultural field

A tile-drained agricultural field can be regarded as a ‘field-scale’ lysimeter that may be used to study soil water and chemical transport under relatively natural conditions. Tile discharge and effluent bromide concentrations measured in a previous field tracer experiment for a structured clayey loam at Bokhorst, Northern Germany, indicated strong preferential flow. Simulation using single domain HYDRUS numerical flow and transport model could nevertheless explain water outflow, however, completely failed to describe tile-drain leaching patterns of the conservative tracer. The objective of this paper was to analyze whether the nonequilibrium-type dual-permeability model concept could better capture soil structure related principle mechanisms of preferential leaching in the unsaturated soil at that study site. The dual-permeability model (DUAL) describes for soil matrix and fracture pore systems Darcian flow with coupled Richards' equations and convective–dispersive (CD) solute transport with coupled CD equations. The hydraulic parameters of the dual-permeability model were obtained from standard soil hydraulic measurements by adopting a bimodal fitting procedure, whereas transport parameters were inferred from soil column tracer experiments and geometrical transfer term parameters were derived using qualitative soil structure descriptions. The hydraulic conductivity K a in the inter-domain water transfer term and the effective diffusion coefficient D a in the solute mass transfer term were calibrated by comparing simulated with measured tile flow and effluent concentrations. The DUAL approach described water flow similarly well as the single-domain model. Bromide concentrations in the tile effluent could be approximated with DUAL when decreasing the K a and D a values by three orders of magnitude compared with the values of the soil matrix domain. The dual-permeability approach seems to reflect nonequilibrium transport mechanisms at this structured soil since it not only predicts concentration peaks in initial leaching phases during early time events of the experiment but also the observed reverse reaction of decreased effluent concentrations in initial phases of leaching events during later times of the experiment. The latter occurs when bromide-free infiltrating water is diluting bromide concentrations in the fracture domain such that less concentrated water is preferentially draining from tiles in initial phases of infiltration. Bromide effluent concentrations gradually increase in later phases since vertical flow velocities decrease while ‘diffusive’ solute transfer from the soil matrix continues. Mass transfer restrictions (e.g. due to soil aggregate coatings) may effectively be controlling preferential leaching at this site.