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