Soils often exhibit a variety of small-scale heterogeneities such as cracks, inter-aggregate (or structural) pores, decayed root channels and other types of macro- and coarse micropores. Due to these local-scale heterogeneities, a non-ideality known as ‘Preferential Flow’ generally occurs, creating localscale non-equilibrium conditions in pressure head and solute concentrations between regions of faster and slower flow. According to Gerke (2006), “preferential flow comprises all phenomena where water and solutes move along certain pathways, while bypassing other volume fractions of the porous soil matrix.” Preferential flow is not only a theoretical challenge, but also has practical significance in that it enhances the leaching of pollutants from the surface to deeper layers and into the saturated groundwater zone. The reason is that the buffer capacity, as well as biological activity, that are found in the organic-rich upper soil horizons and that are vital to the degradation of pollutants, are generally weaker or absent altogether in the deeper layers. Thus the danger of groundwater contamination is increased due to preferential flow. The theoretical foundation for the phenomenon of preferential flow was laid half a century ago by Barenblatt et al. (1960). The theory was initially formulated in terms of the seepage of liquids in fissured rocks. The profound importance of cracks and fissures was highlighted by Freeze and Cherry (1979), who found that “the hydraulic conductivity through crack network is often 10 to 104 times higher than that of the adjacent rock matrix.” The theory was later expanded by soil physicists to apply to porous media containing cracks or macropores. The first step was the introduction of the concept of mobile and immobile water in a porous medium by Gaudet et al. (1977). This was followed by Beven and Germann (1981, 1982), who formulated the theory of flow in soils containing macropores. The fundamental problem of disequilibrium due to macroporosity was discussed by Baveye and Sposito (1984). The theorywas later extended, using numerical approaches, to the concept of dual porosity (Moench, 1984; Arbogast, 1987), and specialized for soils by Gerke and van Genuchten (1993). Nkedi-Kizza et al. (1983) studied ion-exchange and miscible displacement in aggregated soils. Othmer et al. (1991) demonstrated that for soils with bimodal porosity, the numerical simulation improved when both hydraulic functions, soil water retention and unsaturated hydraulic conductivity, were included into separate pore domains of the model. Another advance was the dual permeability concept by Šimůnek et al. (2003). Recent advances in preferential flow research were reviewed by Jarvis (1998) and Gerke (2006). Accounting for themechanismsinvolved,we distinguish the following types of preferential flowat the ‘pedon scale’ (i.e. the scale of larger units of soil particles formed by aggregation): 1. Preferential flowin realmacropores (or non-capillary pores). At least in principle, Richards' equation is not applicable. Instead, the kinematic wave equation or the simple Hagen– Poiseuille equation can be applied. Fluxes are significantly accelerated compared to those in the pores. 2. Preferential flow in inter-aggregate pores, denoted also as interpedal or structural pores. Sometimes inter-aggregate pores are described as macropores, but this terminology neglects the principal difference between non-capillary and capillary pores. Richards' equation is applicable since the pores are in the category of capillary pores. The fluxes are accelerated compared to those in the intrapedal pores of the matrix when infiltration and redistribution are considered, but less accelerated compared to the fluxes in Type 1. Some authors question the applicability of Richards' equation in those coarser micropores. 3. Fingering due to the instability on the wetting front. This occurs most frequently at the interface of a less permeable layer above a more permeable one. In numerical models, apparent fingering can be induced by numerical instabilities. Fluxes are comparable to those of Type 2. 4. Preferential flow due to spatial irregularities or temporal dynamics in soil wettability (or water repellency). This type of preferential flow is often assumed to occur when the initial soilwater content is belowa critical value. Fluxes are comparable to those of Type 2. Local-scale heterogeneities and non-equilibrium preferential flow are obviously expected to be strongly related, but the quantification of this relationship remains a challenge. Given the important effects of preferential flow on water and solute transport, a large body of literature can be found dealing with the description of preferential-flow pore networks and the mechanisms of macropore flow, especially at the pedon scale. The use of morphometric data in combination with soil physical characteristics is recognized as particularly relevant. Recent advances allow the complex geometry of the pore space to be quantified by means of tomography, magnetic resonance and image analysis. New devices and measurement techniques are now available, such as time domain reflectometry and disc permeametry, allowing better observations of preferential flow at the pedon scale. Additionally, insight into the physical influence of local-scale heterogeneities on soil processes can now be gained by appropriately modelling water and solute transport behavior in natural soils with different structures. Accordingly, the body of knowledge on preferential flow available at present, both theoretical and experimental, is extensive. Nevertheless, knowledge gaps still exist. The purpose of this Special Issue is to fill some of these gaps. The papers selected for the Special Issue address a range of issues, from the basic principles governing the generation of preferential flow at the pore scale, to the flow mechanisms and the microscale properties of the porous soil system, and to the problem of upscaling. The papers show that better observations using new devices, and analyzed using innovative analytical and predictive approaches, may bring us toward a better understanding and prediction of preferential flow in structured soils. The Special Issue starts with a comprehensive two-paper overviewon model applications in preferential flow/transport in structured soils by Koehne et al., one paper on water flow and tracer transport, and the other on pesticide transport. These papers describe the present state of the art. Preferential flow in real macropores (or non-capillary pores), and preferential flow in interpedal (inter-aggregate) pores is investigated in two papers, one by Kutílek and Germann, and another by Hincapié and Germann. The limits of applicability of Richards' equation as a function of pore size, and the fluxes involved, are investigated in these papers. Recent mechanistic approaches for interpreting transport in heterogeneous soils are presented in four papers: Coppola, Comegna et al.; Kutílek et al.; Gerke and Badorrek; and Kodešová et al. All of these authors make use of experimental evidence for testing the validity and assessing the pros and cons of the various alternative approaches. In the papers by Coppola, Comegna et al. and by Kutílek et al., the simpler but approximate composite porosity (bimodal porosity) approaches are discussed. Although composite porosity approaches can account for a fast propagation of the wetting front, they implicitly assume instantaneous equilibrium between pore systems. Non-equilibrium flow is accounted for in the paper by Gerke and Badorrek, as well as in that by Kodešová et al., where the more complex double-permeability models are applied. The first paper analyzes the effects of local heterogeneities on preferential flow in a lignitic mine. The second paper investigates the relationships between preferential flow and soil properties at the soil profile scale, as well as the influence of solute characteristics and soil management practices on preferential pore flow. Two papers, one by Sander and Gerke, and the other by Rosenbom et al., present new advances in the understanding of preferential flow that have resulted from the use of novel experimental techniques, as well as two-dimensional modelling that also takes horizontal flow/transport into account. The latter uses a powerful 3D model for variably saturated flow and transport. Finally, in the last paper, Coppola, Basile et al. consider the important applied research issue of upscaling by investigating the influence of physical heterogeneities and preferential pathways on flow and transport processes across spatial scales and related scale-dependent parameterizations in laband field-scale transport experiments. Although the work presented here is far from comprehensive, and although many more questions remain, we hope the reader will enjoy reading this modest volume and will agree that it has advanced the general understanding of preferential flow in a significant way.

Transport in preferential flow domains of the soil porous system: Measurement, interpretation, modelling, and upscaling

COPPOLA, Antonio;
2009

Abstract

Soils often exhibit a variety of small-scale heterogeneities such as cracks, inter-aggregate (or structural) pores, decayed root channels and other types of macro- and coarse micropores. Due to these local-scale heterogeneities, a non-ideality known as ‘Preferential Flow’ generally occurs, creating localscale non-equilibrium conditions in pressure head and solute concentrations between regions of faster and slower flow. According to Gerke (2006), “preferential flow comprises all phenomena where water and solutes move along certain pathways, while bypassing other volume fractions of the porous soil matrix.” Preferential flow is not only a theoretical challenge, but also has practical significance in that it enhances the leaching of pollutants from the surface to deeper layers and into the saturated groundwater zone. The reason is that the buffer capacity, as well as biological activity, that are found in the organic-rich upper soil horizons and that are vital to the degradation of pollutants, are generally weaker or absent altogether in the deeper layers. Thus the danger of groundwater contamination is increased due to preferential flow. The theoretical foundation for the phenomenon of preferential flow was laid half a century ago by Barenblatt et al. (1960). The theory was initially formulated in terms of the seepage of liquids in fissured rocks. The profound importance of cracks and fissures was highlighted by Freeze and Cherry (1979), who found that “the hydraulic conductivity through crack network is often 10 to 104 times higher than that of the adjacent rock matrix.” The theory was later expanded by soil physicists to apply to porous media containing cracks or macropores. The first step was the introduction of the concept of mobile and immobile water in a porous medium by Gaudet et al. (1977). This was followed by Beven and Germann (1981, 1982), who formulated the theory of flow in soils containing macropores. The fundamental problem of disequilibrium due to macroporosity was discussed by Baveye and Sposito (1984). The theorywas later extended, using numerical approaches, to the concept of dual porosity (Moench, 1984; Arbogast, 1987), and specialized for soils by Gerke and van Genuchten (1993). Nkedi-Kizza et al. (1983) studied ion-exchange and miscible displacement in aggregated soils. Othmer et al. (1991) demonstrated that for soils with bimodal porosity, the numerical simulation improved when both hydraulic functions, soil water retention and unsaturated hydraulic conductivity, were included into separate pore domains of the model. Another advance was the dual permeability concept by Šimůnek et al. (2003). Recent advances in preferential flow research were reviewed by Jarvis (1998) and Gerke (2006). Accounting for themechanismsinvolved,we distinguish the following types of preferential flowat the ‘pedon scale’ (i.e. the scale of larger units of soil particles formed by aggregation): 1. Preferential flowin realmacropores (or non-capillary pores). At least in principle, Richards' equation is not applicable. Instead, the kinematic wave equation or the simple Hagen– Poiseuille equation can be applied. Fluxes are significantly accelerated compared to those in the pores. 2. Preferential flow in inter-aggregate pores, denoted also as interpedal or structural pores. Sometimes inter-aggregate pores are described as macropores, but this terminology neglects the principal difference between non-capillary and capillary pores. Richards' equation is applicable since the pores are in the category of capillary pores. The fluxes are accelerated compared to those in the intrapedal pores of the matrix when infiltration and redistribution are considered, but less accelerated compared to the fluxes in Type 1. Some authors question the applicability of Richards' equation in those coarser micropores. 3. Fingering due to the instability on the wetting front. This occurs most frequently at the interface of a less permeable layer above a more permeable one. In numerical models, apparent fingering can be induced by numerical instabilities. Fluxes are comparable to those of Type 2. 4. Preferential flow due to spatial irregularities or temporal dynamics in soil wettability (or water repellency). This type of preferential flow is often assumed to occur when the initial soilwater content is belowa critical value. Fluxes are comparable to those of Type 2. Local-scale heterogeneities and non-equilibrium preferential flow are obviously expected to be strongly related, but the quantification of this relationship remains a challenge. Given the important effects of preferential flow on water and solute transport, a large body of literature can be found dealing with the description of preferential-flow pore networks and the mechanisms of macropore flow, especially at the pedon scale. The use of morphometric data in combination with soil physical characteristics is recognized as particularly relevant. Recent advances allow the complex geometry of the pore space to be quantified by means of tomography, magnetic resonance and image analysis. New devices and measurement techniques are now available, such as time domain reflectometry and disc permeametry, allowing better observations of preferential flow at the pedon scale. Additionally, insight into the physical influence of local-scale heterogeneities on soil processes can now be gained by appropriately modelling water and solute transport behavior in natural soils with different structures. Accordingly, the body of knowledge on preferential flow available at present, both theoretical and experimental, is extensive. Nevertheless, knowledge gaps still exist. The purpose of this Special Issue is to fill some of these gaps. The papers selected for the Special Issue address a range of issues, from the basic principles governing the generation of preferential flow at the pore scale, to the flow mechanisms and the microscale properties of the porous soil system, and to the problem of upscaling. The papers show that better observations using new devices, and analyzed using innovative analytical and predictive approaches, may bring us toward a better understanding and prediction of preferential flow in structured soils. The Special Issue starts with a comprehensive two-paper overviewon model applications in preferential flow/transport in structured soils by Koehne et al., one paper on water flow and tracer transport, and the other on pesticide transport. These papers describe the present state of the art. Preferential flow in real macropores (or non-capillary pores), and preferential flow in interpedal (inter-aggregate) pores is investigated in two papers, one by Kutílek and Germann, and another by Hincapié and Germann. The limits of applicability of Richards' equation as a function of pore size, and the fluxes involved, are investigated in these papers. Recent mechanistic approaches for interpreting transport in heterogeneous soils are presented in four papers: Coppola, Comegna et al.; Kutílek et al.; Gerke and Badorrek; and Kodešová et al. All of these authors make use of experimental evidence for testing the validity and assessing the pros and cons of the various alternative approaches. In the papers by Coppola, Comegna et al. and by Kutílek et al., the simpler but approximate composite porosity (bimodal porosity) approaches are discussed. Although composite porosity approaches can account for a fast propagation of the wetting front, they implicitly assume instantaneous equilibrium between pore systems. Non-equilibrium flow is accounted for in the paper by Gerke and Badorrek, as well as in that by Kodešová et al., where the more complex double-permeability models are applied. The first paper analyzes the effects of local heterogeneities on preferential flow in a lignitic mine. The second paper investigates the relationships between preferential flow and soil properties at the soil profile scale, as well as the influence of solute characteristics and soil management practices on preferential pore flow. Two papers, one by Sander and Gerke, and the other by Rosenbom et al., present new advances in the understanding of preferential flow that have resulted from the use of novel experimental techniques, as well as two-dimensional modelling that also takes horizontal flow/transport into account. The latter uses a powerful 3D model for variably saturated flow and transport. Finally, in the last paper, Coppola, Basile et al. consider the important applied research issue of upscaling by investigating the influence of physical heterogeneities and preferential pathways on flow and transport processes across spatial scales and related scale-dependent parameterizations in laband field-scale transport experiments. Although the work presented here is far from comprehensive, and although many more questions remain, we hope the reader will enjoy reading this modest volume and will agree that it has advanced the general understanding of preferential flow in a significant way.
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