Modelling gravity-driven fingering in soils having an intrinsic non-zero contact angle (water repellent soils) using the innovative moving-boundary approach

2020 ◽  
Author(s):  
Rony Wallach ◽  
Naaran Brindt
Keyword(s):  
Contact Angle ◽  
Water Content ◽  
Preferential Flow ◽  
Moving Boundary ◽  
Richards Equation ◽  
Water Repellent ◽  
Wetting Front ◽  
Unstable Flow ◽  
Pressure Distributions ◽  
Gravity Driven

<p>Quantitative and Qualitative description of infiltration into soils in general and initially dry soils in particular those in which the hydraulic properties vary spatial and temporal have been challenging soil physicists and hydrologists. Water repellent soils, whose contact angle is higher than 40° and can even reach values that are greater than 90° (noted as hydrophobic soils) are an example of such challenge cases. Infiltration in these soils takes usually place along preferential flow pathways (noted as gravity-induced fingering), rather than in a laterally uniform moving wetting front. The water content and capillary pressure distributions along these fingers are non-monotonic with water accumulation behind the moving wetting front (noted as saturation overshoot) and a decreasing water content toward the soil surface. Being a parabolic-type partial differential equation, the Richards equation that is commonly used to model flow in soils can't handle such water content/pressure distributions. Many attempts have been made to modify the Richards equation to enable it to model the non-monotonic water content profiles. These attempts that are not based on the measurable soil properties that can highlight the physics that induces the formation of such non-monotonic distribution.  </p><p>A new conceptual modelling approach, noted as the moving-boundary approach, will be presented. This approach overcomes the existing theoretical gaps in the quantitative descriptions that have been suggested for the non-monotonic water content distribution in the gravity-induced fingers. The moving-boundary approach is based on the presumption that non-monotonicity in water content is formed by an intrinsic higher-than-zero contact angle. Note that non-zero contact angle have been rarely incorporated in models used for quantifying infiltration into field soils, in spite of the findings that most soils feature some degree of repellency. The verified moving-boundary solution will be used to demonstrate the synergistic effect of contact angle and incoming flux on the stability of 2D flow and its associated plume shapes. The physically-based moving-boundary approach fulfils several criteria raised by researchers to adequately describe gravity-driven unstable flow.</p><p> </p>

Phase-Field Models of Gravity-Driven Unsaturated Flow in Porous Media

2008 ◽  
Author(s):  
Luis Cueto-Felgueroso ◽  
Ruben Juanes
Keyword(s):  
Porous Media ◽  
Preferential Flow ◽  
Unsaturated Flow ◽  
Flow In Porous Media ◽  
Macroscopic Model ◽  
Wetting Front ◽  
Forming Mechanism ◽  
Gravity Driven ◽  
Pattern Forming ◽  
Models Of Gravity

Existing continuum models of multiphase flow in porous media are unable to explain why preferential flow (fingering) occurs during infiltration into homogeneous, dry soil. We identify a relevant pattern-forming mechanism in the dynamics of the wetting front, and present a macroscopic model that reproduces the experimentally observed features of fingered flows. The proposed model reveals a scaling between local and nonlocal interface phenomena in imbibition, and does not introduce new independent parameters. The predictions based on this model are consistent with experiments and theories of scaling in porous media.

The rate of water entry into dry sand and calculation of the advancing contact angle

Soil Research ◽  
1963 ◽  
Vol 1 (1) ◽  
pp. 9 ◽  
Author(s):  
WW Emerson ◽  
RD Bond
Keyword(s):  
Contact Angle ◽  
Capillary Rise ◽  
Applied Pressure ◽  
Water Entry ◽  
Water Repellent ◽  
Maximum Height ◽  
Wetting Front ◽  
Vertical Column ◽  
Advancing Contact Angle ◽  
Dry Sand

For water entry into a vertical column of dry sand, the height of capillary rise, h, is defined for the present purposes as the maximum height of capillary rise in the sand, below which the moisture content is uniform. Previous experimental results on water entry into dry sand have been reviewed to show the validity and usefulness of this definition. The rise of the wetting front into a vertical column of sand was measured, the rate of rise of the wetting front was plotted against the reciprocal of the height of rise, and 1/h was found by extrapolating the line to zero rate of rise. For water-repellent sand a positive hydrostatic head was applied to the base of the sand to obtain an adequate number of points for the extrapolation. This pressure was adjusted so that the initial rate of advance of water into the sand was about equal to that of water into the ignited sand with no positive applied pressure. The advancing contact angle averaged over the wetted surface area of the sand was then calculated from the ratio of the values of h obtained with sand before and after ignition. The contact angle of a water-repellent sand has been shown to be higher than 90�. This explains the difficulty experienced in the field of wetting these sands. Two remedial measures are suggested: one is to cultivate and mix the soil to give a uniform average contact angle, the other is to cut slots so that a positive hydrostatic pressure can be applied to the deeper patches of high contact angle sand.

scholarly journals Modeling gravity driven unstable flow in a water repellent soil

1999 ◽  
Vol 215 (1-4) ◽  
pp. 202-214 ◽  
Author(s):  
H.V Nguyen ◽  
J.L Nieber ◽  
C.J Ritsema ◽  
L.W Dekker ◽  
T.S Steenhuis
Keyword(s):  
Water Repellent ◽  
Unstable Flow ◽  
Gravity Driven

Numerical simulation of experimental gravity-driven unstable flow in water repellent sand

2000 ◽  
Vol 231-232 ◽  
pp. 295-307 ◽  
Author(s):  
J.L Nieber ◽  
T.W.J Bauters ◽  
T.S Steenhuis ◽  
J.-Y Parlange
Keyword(s):  
Numerical Simulation ◽  
Water Repellent ◽  
Unstable Flow ◽  
Gravity Driven ◽  
Experimental Gravity

Modelling non-equillibrium unsaturated flow in soils during sudden pressure head changes by solving Richards' equation with a Method of lines approach

2021 ◽  
Author(s):  
Robert Mietrach ◽  
Thomas Wöhling ◽  
Niels Schütze
Keyword(s):  
Water Content ◽  
Preferential Flow ◽  
Unsaturated Flow ◽  
Full Range ◽  
Method Of Lines ◽  
Pressure Head ◽  
Richards Equation ◽  
Extended Model ◽  
Solution Methods ◽  
Numerical Robustness

<p>The classical formulation of Richards' equation is relying on a unique functional relationship between water content, conductivity and pressure head. Some phenomena like hystersis effects in the water content during wetting and drying cycles and hydraulic non-equillibrium cannot be accounted for with this formulation. Therefor it has been extended in different ways in the past to be able to include these effects in the simulation. Each modification comes with its own challenges regarding implementation and numerical stability.<br>The Method Of Lines approach to solving the Richards' equation has already be shown to be an efficient and stable alternative to established solution methods, such as low-order finite difference and finite element methods applied to the mixed form of Richards' equation.<br>In this work a slightly modified Method Of Lines approach is used to solve the pressure based 1D Richards' equation. A finite differencing scheme is applied to the spatial derivative and the resulting system of ordinary differential equations is reformulated as differential-algebraic system of equations. The open-source code IDAS from the Sundials suite is used to solve the DAE system. Different extensions to Richards' equation have been incorporated into the model to address the shortcomings mentioned above. These extensions are a model able to simulate preferential flow using a coupled two domain approach, a simple hysteretic model to account for hysteresis in the water retention curve and also two models to either fully or partially calculate hydraulic non-equillibrium effects. To verify the numerical robustness of the extended model, stochastic parameterizations were generated that represent the full range of all soil types. Simulations were carried out using these parameter sets and real-world meteorological boundary conditions at 10 minutes time intervals, that exhibit drastic flux changes and poses numerical challenges for classical solution methods.</p><p>The results show that not only does the extended model converge for all parameterizations, but that numerical robustness and performance is maintained. Where applicable the results have been verified against solutions from the software Hydrus and show good agreement with those.</p>

Pines influence hydrophysical parameters and water flow in a sandy soil

Biologia ◽  
2013 ◽  
Vol 68 (6) ◽  
Author(s):  
Ľubomír Lichner ◽  
Jozef Capuliak ◽  
Natalia Zhukova ◽  
Ladislav Holko ◽  
Henryk Czachor ◽  
...  
Keyword(s):  
Hydraulic Conductivity ◽  
Forest Soil ◽  
Soil Water ◽  
Water Flow ◽  
Sandy Soil ◽  
Preferential Flow ◽  
Water Repellency ◽  
Water Repellent ◽  
Wetting Front ◽  
Soil Water Repellency

AbstractPines, used for sand dune stabilization, can influence the hydrophysical parameters and water flow in an aeolian sandy soil considerably, mainly due to soil water repellency. Two sites, separated by distance of about 20 m, formed the basis of our study. A control soil (“Pure sand“) with limited impact of vegetation or organic matter was formed at 50 cm depth beneath a forest glade area. This was compared to a “Forest soil” in a 30-year old Scots pine (Pinus sylvestris) forest. Most of the hydrophysical parameters were substantially different between the two soil surfaces. The forest soil was substantially more water repellent and had two-times the degree of preferential flow compared to pure sand. Water and ethanol sorptivities, hydraulic conductivity, and saturated hydraulic conductivity were 1%, 84%, 2% and 26% those of the pure sand, respectively. The change in soil hydrophysical parameters due to soil water repellency resulted in preferential flow in the forest soil, emerging during a simulated heavy rain following a long hot, dry period. The wetting front established in pure sand exhibited a form typical of that for stable flow. Such a shape of the wetting front can be expected in the forest soil in spring, when soil water repellency is alleviated substantially.

scholarly journals Unstable Flow in Repellent and Sub-critically Repellent Soils: Theory and Management Implications

2012 ◽  
Author(s):  
Rony Wallach ◽  
Tammo Steenhuis ◽  
Ellen R. Graber ◽  
David DiCarlo ◽  
Yves Parlange
Keyword(s):  
Contact Angle ◽  
Water Distribution ◽  
Preferential Flow ◽  
Soil Surface ◽  
Water Repellency ◽  
Chemical Components ◽  
Unstable Flow ◽  
Management Implications ◽  
Structured Soils ◽  
Preferential Flowpaths

Water repellency causes unstable wetting fronts that result in water moving in preferential flowpaths through homogeneous soils as well in structured soils where macropores enhance the preferential flow pattern. Water repellency is typically associated with extended water ponding on the soil surface, but we have found that repellency is important even before the water ponds. Preferential flow fingers can form under conditions where the contact angle is less than 90o, but greater than 0o. This means that even when the soil is considered wettable (i.e., immediate penetration of water), water distribution in the soil profile can be significantly non-uniform. Our work concentrated on various aspects of this subject, with an emphasis on visualizing water and colloid flow in soil, characterizing mathematically the important processes that affect water distribution, and defining the chemical components that are important for determining contact angle. Five papers have been published to date from this research, and there are a number of papers in various stages of preparation.

scholarly journals Relevance and controls of preferential flow at the landscape scale

2019 ◽  
Author(s):  
Dominic Demand ◽  
Theresa Blume ◽  
Markus Weiler
Keyword(s):  
Soil Moisture ◽  
Water Content ◽  
Soil Water ◽  
Preferential Flow ◽  
Clay Content ◽  
Wetting Front ◽  
Moisture Sensor ◽  
Soil Moisture Sensor ◽  
Landscape Units ◽  
Moisture Profiles

Abstract. The spatial and temporal controls of preferential flow (PF) during infiltration are still not fully understood. Soil moisture sensor networks give the possibility to measure infiltration response in high temporal and spatial resolution. Therefore, we used a large-scale sensor network with 135 soil moisture profiles distributed across a complex catchment. The experimental design covers three major geological regions (Slate, Marl, Sandstone) and two land covers (forest, grassland) in Luxembourg. We analyzed the responses of up to 353 rainfall events for every of the 135 soil moisture profiles. Non-sequential responses within the soil moisture depth-profiles were taken as an indication of PF. For sequential responses wetting front velocities were determined from the observations and compared with predictions by capillary flow. A measured wetting front velocity higher than the capillary prediction was also taken as a proxy for PF. We observed the highest fraction of non-sequential response (NSR) in forests on clay-rich soils (Slate, Marl). Furthermore, these two landscape units showed an increase of NSR with lower initial soil water content and higher maximum rainfall intensity. Wetting front velocities ranged from 6 cm day−1 to 80 640 cm day−1 with a median of 113 cm day−1 across all events and landscape units. The soils in the Marl geology had the highest flow velocities, independent of land cover, especially between 30 and 50 cm depth where the clay content increased. For Marl the median water content change was highest for the deepest soil moisture sensor (50 cm), whereas the other two geologies (Slate, Sandstone) showed a decrease of soil moisture change with depth. This confirms that clay content and vegetation strongly influence infiltration and reinforce preferential flow. Capillary-based soil water flow modelling was unable to predict the observed patterns. This demonstrates the danger of treating especially clay soils in the vadose zone as a low-conductivity layer, as the development of soil structure can dominate over the effect of low-conductive texture.

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