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| Titel |
Can flow velocity distribution at a pore-scale be quantified by a celerity-saturation curve? |
| VerfasserIn |
Wei Shao, Ye Su, Thom Bogaard, Mark Bakker, Huub Savenije |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250109676
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| Publikation (Nr.) |
 EGU/EGU2015-9611.pdf |
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| Zusammenfassung |
| The macroscopic subsurface hydrological behaviour, such as surface water infiltration,
volumetric water flow in a hillslope, groundwater pressure propagation, or tracer transport,
are intimately related with the variability of microscopic flow velocity in the soil porous
medium. The subsurface flow equations, expressed by a continuum approach, conceptualize
the uniform flow in a representative elementary volume (REV), in which the volumetric flow
velocity and average pore velocity are two common variables of water flow velocity.
Even though a combination of the continuity equation with Darcian flow velocity
is able to quantify the volumetric flow, such a continuum approach is unable to
represent the variability of flow velocity at pore-scale. As result of the homogeneity
assumption in the subsurface flow equations, the pore-scale heterogeneity cannot be fully
represented.
Celerity describes the speed of a perturbation-induced propagation of flow or
pressure wave. The physical meaning of celerity differs in saturated and unsaturated
condition, and such difference can lead to confusion. Specifically, for saturated flow, the
celerity indicates pressure transmission, while, for unsaturated flow, the celerity
transmits a disturbance through water flow. If a soil is in an equilibrium state (steady
condition), even a ‘tiny disturbance’ of water actuates both water flow and pressure
propagation following the path of minimum resistance. Under a perturbation analysis,
the celerity, therefore, represents the maximum pore water velocity among all the
water-filled pores that contribute to the water flow. Consequently, the relationship
between celerity and effective soil saturation reveals a distribution of pore water
velocities.
A theoretical study was performed to analyse and quantify the hydraulic behaviour of
natural soils with a special emphasis on the difference between pore water flow velocity and
pressure propagation. The Mualem-Van Genuchten and Brooks-Corey constitutive
relationships were used to describe the non-linear hydraulic conductivity of the soils. The
analysis manifests that under full saturated conditions, a small fraction of the pores (with
larger size and lower tortuosity) can conduct a large amount of volumetric water flow, while,
under near-saturated condition, the celerity can be significantly larger than the Darcian
velocity or average pore water velocity. If the soil saturation is below a certain threshold, pore
water velocity and its variability are rather small. Solute transport in a variable saturated soil
is controlled by both process of diffusion (driven by concentration gradient) and convection
(related with distribution of flow velocities). Therefore, a variable pore water velocity
induces a bimodal behaviour of the mass transport that is often observed in tracer
experiments. We will present the results of our analysis and focus on pore water
velocities derived from celerity – effective saturation plots and discuss whether this
could be considered as a universal phenomenon in the subsurface flow system. |
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