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Laboratory Tests of a Theory of Fingering during Infiltration into Layered Soils
Abstract
When the conductivity of a coarse-textured sublayer at its effective water-entry suction is greater than the rate of transmission through a finer-textured top layer, the flow velocity increases across the interlayer plane. A spatially-distributed flow field in the top layer will tend to constrict, forming spatially separated streams (fingers) in the sublayer. Ponded infiltration experiments were conducted in air-dry sands. The hydraulic properties were determined by equilibrium capillary rise, parameter estimation, and horizontal absorption experiments. Measurements of flux and suction at the inter-layer plane revealed that the effective water-entry suction is characteristic of the predominant pore size in the sublayer and can be predicted from its median particle size (r2 = 0.99). Predictions regarding the onset of fingering and the wetted fractional volume of fingers were validated. -from Authors
... One major objective of this work is to experimentally examine how a 0 varies with soil types. Other forms of relationships between f and flow parameters of soils have also been proposed by scholars (Baker and Hillel, 1990;Glass et al., 1989b). In Glass et al. (1989b) an empirical relationship was proposed based on fitting of experimental data where: ...
... This relationship is similar to Eq. (4) with a 0 being 0.5 except that there is a small intercept of f at zero flux. Baker and Hillel (1990) related f to the water flux q s and the unsaturated conductivity of the soil: ...
... The first two groups of experiments were conducted in initially dry sands, while the third group of experiments were conducted continuously with increasing water flux by removing layers of the expansive sponge on top of the experimental sands. Table 2 Experimental results of Baker and Hillel (1990) and Glass et al. (1989b). fingers, whereas in the other individual experiments the fingers formed in initially dry sands. ...
... This prevents water from entering the lower layer, thus causing local accumulation of water at the interface (henceforth, simply ponding), a deceleration in the undisturbed advancement of the wetting front, horizontal diversion of water, and a delay in the expected travel time of water. Hill and Parlange (1972), Baker and Hillel (1990), Hillel and Baker (1988), Stormont and Morris (1998), Stormont and Anderson (1999), and Khire et al. (2000) discuss this process in soils, whereas Wakahama (1963), Jordan (1995), Waldner et al. (2004), Peitzsch et al. (2008), and Mitterer et al. (2011) report some examples for layered snowpack. According to the results by Stormont and Anderson (1999) in soils, water will enter the underlying coarser layer when ψ at the interface decreases to ψ WE ; at this suction, the coarser soil layer firstly becomes conductive (Stormont and Morris, 1998;Khire et al., 2000). ...
... A decrease in ψ during ponding is caused by the fact that θ and ψ are related by a hysteretic relation called the water retention curve (WRC) (Daanen and Nieber, 2009;Yamaguchi et al., 2010;Adachi et al., 2012;Yamaguchi et al., 2012). In soils, Hillel and Baker (1988) and Baker and Hillel (1990) note also that, after reaching ψ WE , subsequent flow in the coarser layer will be marked by fingers if, in steady conditions, the hydraulic conductivity of the lower layer at ψ WE is greater than the flux through the top layer q (due to mass conservation). Thus, ponding of water above a capillary barrier is prone to subsequent flow instability, namely, to the development of preferential channels. ...
... These water input rates are a compromise between the need for exploring the properties of capillary barriers over a broad range of W , expected melt rates in natural conditions (DeWalle and Rango, 2011), and operational constraints (specifically the expected duration of the tests). Because the saturated conductivity of snow is rather high compared with the chosen input rates, most existing instability criteria (Saffman and Taylor, 1958;Baker and Hillel, 1990;de Rooij, 2000;DiCarlo, 2013) will predict unstable flow in these conditions. Accordingly, Katsushima et al. (2013) have already observed preferential infiltration in snow with average g S between 0.421 and 1.439 mm for different water input rates (21.7 mm h −1 ≤ W ≤ 205.5 mm h −1 ). ...
Data of liquid water flow around a capillary barrier in snow are still limited. To gain insight into this process, we carried out observations of dyed water infiltration in layered snow at 0°C during cold laboratory experiments. We considered three different finer-over-coarser textures and three different water input rates. By means of visual inspection, horizontal sectioning, and measurements of liquid water content (LWC), capillary barriers and associated preferential flow were characterized. The flow dynamics of each sample were also simulated solving the Richards equation within the 1-D multi-layer physically based snow cover model SNOWPACK. Results revealed that capillary barriers and preferential flow are relevant processes ruling the speed of water infiltration in stratified snow. Both are marked by a high degree of spatial variability at centimeter scale and complex 3-D patterns. During unsteady percolation of water, observed peaks in bulk volumetric LWC at the interface reached ~ 33–36 vol% when the upper layer was composed by fine snow (grain size smaller than 0.5 mm). However, LWC might locally be greater due to the observed heterogeneity in the process. Spatial variability in water transmission increases with grain size, whereas we did not observe a systematic dependency on water input rate for samples containing fine snow. The comparison between observed and simulated LWC profiles revealed that the implementation of the Richards equation reproduces the existence of a capillary barrier for all observed cases and yields a good agreement with observed peaks in LWC at the interface between layers.
... Water in unsaturated porous media does not flow through the largest pores but rather through the smallest pores possible, owing to the fact that a lower pressure is required for water to flow into a small pore than a large pore. As capillary forces hold water in the fine top layer in fine-on-coarse systems, water pressure builds up until it exceeds the air entry value required to flow into the larger pores in the layer underneath (Baker and Hillel 1990;Steenhuis et al. 1991). Flow below capillary barriers can be highly unstable, typically inducing preferential finger flow even in fully homogeneously textured and structured media (Baker and Hillel 1990;Steenhuis et al. 1991). ...
... As capillary forces hold water in the fine top layer in fine-on-coarse systems, water pressure builds up until it exceeds the air entry value required to flow into the larger pores in the layer underneath (Baker and Hillel 1990;Steenhuis et al. 1991). Flow below capillary barriers can be highly unstable, typically inducing preferential finger flow even in fully homogeneously textured and structured media (Baker and Hillel 1990;Steenhuis et al. 1991). This ash-induced preferential flow can explain the irregular soil-wetting patterns found by Bodí et al. (2012) under ash layers, and cause preferential leaching of infiltrated water to deeper layers (Ritsema and Dekker 1995), increasing base flow. ...
... Although capillary barrier effects thereby may explain the occurrence of unsaturated flow in ash-on-coarse soil systems, they cannot explain saturated flow through such ash layers without taking soil water repellency into account. Infiltration into wettable soils, which have negative air entry pressures (Baker and Hillel 1990), will already occur when ash is unsaturated. This is in contrast to water-repellent soils, which have positive air entry pressures (Wang et al. 2000) and therefore require ponding (and thus saturation) to allow water infiltration. ...
Ash plays an important role in controlling runoff and erosion processes after wildfire and has frequently been hypothesised to clog soil pores and reduce infiltration. Yet evidence for clogging is incomplete, as research has focussed on identifying the presence of ash in soil; the actual flow processes remain unknown. We conducted laboratory infiltration experiments coupled with microscope observations in pure sands, saturated hydraulic conductivity analysis, and interaction energy calculations, to test whether ash can clog pores (i.e. block pores such that infiltration is hampered and ponding occurs). Although results confirmed previous observations of ash washing into pores, clogging was not observed in the pure sands tested, nor were conditions found for which this does occur. Clogging by means of strong attachment of ash to sand was deemed unlikely given the negative surface charge of the two materials. Ponding due to washing in of ash was also considered improbable given the high saturated conductivity of pure ash and ash–sand mixtures. This first mechanistic step towards analysing ash transport and attachment processes in field soils therefore suggests that pore clogging by ash is unlikely to occur in sands. Discussion is provided on other mechanisms by which ash can affect post-fire hydrology.
... This preferential flow, often resembling fingering, becomes notably visible when transitioning from a fine-grained layer to an underlying coarse-grained layer, as observed in systems like the capillary barrier [42]. This phenomenon tends to occur when the coefficient of permeability at the water entry value of the coarse-grained layer exceeds the infiltration flux from the fine-grained layer [32,43]. ...
... These curves also experience changes when subjected to repeated wetting and drying processes. Using appropriate soil properties, such as SWCCs, in numerical analyses is crucial to modeling soil behavior accurately under unsaturated conditions [38][39][40][41][42][43][44][45][46][47][48][49][50]. Back analyses were performed to calibrate the soil water characteristics and hydraulic conductivity curves based on the results of the 1D infiltration tests. ...
The capillary barrier is a type of soil cover system commonly used in various geotechnical applications, such as limiting infiltration for slopes or landfills or providing cover for solid waste. It serves to prevent the movement of water through the soil layers by utilizing contrasting particle sizes. This paper focuses on investigating the effect of the granular layer on the performance of a three-layer cover with a capillary barrier, integrating the granular layer within clayey sand. The investigation involved one-dimensional infiltration tests utilizing four uniform granular soils with varying grain sizes. These tests were instrumental in calibrating soil water characteristic curves and hydraulic conductivity curves via back analysis. Subsequently, numerical analyses were conducted using a 15 m long model for each of the four distinct cover types. The results indicated that the fine gravel significantly improved the barrier performance beyond one-dimensional tests, owing to its high permeability and the influence of the slope. After the capillary barrier failure, the intermediate layers transitioned into efficient drainage layers, particularly in the gravel layer with the highest lateral drainage capacity. Clayey sand at the bottom delayed percolation, thereby supporting the conversion of the intermediate layer into an effective drainage component. Overall, the multi-layer system showed superior percolation performance compared to the clayey sand cover lacking a granular layer.
... where is the saturated soil-water conductivity and the water flux imposed by the first (fine) soil layer. It is a fundamental contribution of Hillel (1987); Hillel and Baker (1988); and Baker and Hillel (1990) to have pointed out that Baker and Hillel (1990) to have pointed out that Eq. (2.40) is not constrained enough and that should have a much lower value. ...
... where is the saturated soil-water conductivity and the water flux imposed by the first (fine) soil layer. It is a fundamental contribution of Hillel (1987); Hillel and Baker (1988); and Baker and Hillel (1990) to have pointed out that Baker and Hillel (1990) to have pointed out that Eq. (2.40) is not constrained enough and that should have a much lower value. ...
For many years, first as a student and later as a teacher, we have observed graduate students in ecology and other environmental sciences who had been required as undergraduates to take calculus courses. Those courses have often emphasized how to prove theorems about the beautiful, logical structure of calculus, but have neglected applications. Most of the time, the students have come out of such courses with little or no appreciation of how to apply calculus in their own work. Based on these observations, we developed a course designed in part to re-teach calculus as an everyday tool in ecology and other environmental sciences. we emphasized derivations—working with story problems (sometimes quite complex ones)—in this book.
Its basic purpose is to describe various types of mathematical structures and how they can be apphed in environmental science.
Thus, linear and non-linear algebraic equations, derivatives, integrals, ordinary and partial differential equations are the basic kinds of structures, or types of mathematical models, discussed. For each, the discussion follows a pattern something like this:
1. An example of the type of structure, as apphed to environmental science, specialy contamination of the environment is given.
2. Next, a description of the structure is presented.
3. Usually, this is followed by other examples of how the structure arises in environmental science and contamination in environmental, specialy contamination soil.
4. The analytic methods of solving and learning from the structure are discussed.
5. Numerical methods for use when the going gets too rough analytically are described.
This book is not an introduction to calculus—it assumes that its readers will already have been introduced to the basic ideas of differential and integral calculus.
So far as we know, the combination of materials provided in the book is unique, but we believe it forms the basis for a useful and interesting course. In general, none of the material goes beyond what might be taught in a junior-level math or engineering course, but because the book covers ground from several such courses, the present material is appropriately taught at the graduate level.
Obviously then, parts of the material treated here could be selected for use in an undergraduate course.
In addition to its use as a text for a course, the material here should provide an interesting source for environmental scientists and managers to review forgotten math, and to learn some that is new.
The study of chemical transport in soils is important for a number of reasons. Some chemicals are important as they are required for soil and plant health (e.g. Micronutrients). Other chemicals may be highly toxic, particularly if they are present in high concentrations. A chemical becomes a Contaminate if its concentration exceeds some prescribed water quality standard, or if a beneficial water use has been impaired, and if the cause is induced by human activity. The study of the fate of chemicals and chemical Contamination in soil is vital for sustaining agricultural productivity and land utility.
The geological media between the land surface and the regional water table below is called the unsaturated zone or vadose zone (Stephens, 1996). The word “vadose” is derived from the Latin word vadosus meaning shallow (Looney and Falta, 2000a). In accord with its definition and meaning, the vadose zone includes the crop root layer, the intermediate zone between the root layer and the capillary fringe above the saturated water table. This zone therefore plays an integral role in the global hydrological cycle controlling surface water infiltration, runoff and evaporation and hence the availability of soil water and nutrients to plants. Initial investigations of this zone were focused on water availability to crops and optimal management of the root zone. However, in recent years much more attention has focused on chemical transport in and through this zone as a result of increased use of agrochemicals such as fertilizers and pesticides and increased demands to store and dispose of industrial and municipal wastes such as sewage. This zone is typically the first subsurface environment to encounter surface applied agrochemicals and contaminants and hence all surface and subsurface chemical concentrations and subsequent environmental impacts are inextricably linked to the physical, biological and chemical dynamics including sorption-desorption, volatilization, photolysis and degradation.
Our current understanding of physical and chemical processes in the vadose zone results largely from more than 70 years of mathematical modeling of variably saturated flow using Richards’ equation coupled with the Fickian-based convection-dispersion equation for solute transport. Analytical and numerical solutions of these classical equations are widely used to study and predict water flow and solute transport for specific laboratory and field experiments and to extrapolate these results for other experiments in different soils, crops and climatic conditions. However, many recent studies have demonstrated that the assumptions implicitly adopted in the Richards’ and convective- dispersion equations are limiting the scope and application of solutions to these equations for many agricultural and forestry management strategies. The spread of solute and Contaminates in soils is complicated by non-random spatial and temporal variations of physical, chemical and biological components of soils. One manifestation of spatial and temporal heterogeneity in soils is the phenomenon of preferential flow, a general term used to describe a variety of physical and chemical non-equilibrium flow processes.
In this book we describing the contamination of the environment and its types and we will talk extensively about the contamination of soil and its sources and that affect impact on humans, animals and plants and how to control soil contaminates, all this topices explained in chapter one .
In chapter two we describing the spread of Contamination through soils are discussed by using special models of that cause. The spread of Contamination in soils is controlled by the flow of water and, in most cases, is described by the convective-dispersive equation. First, we consider cases when the water velocity is assumed constant. Effects of boundary conditions, chemical reactions, adsorption and species competition are described in this case. Then two other cases are discussed; (1) when hydrology controls solute transport and (2) when the convective-dispersive equation is less important. In the former case, erosion due to raindrop impact and the transport of Contaminates adsorbed on fine particles is discussed. In the latter case, preferential flows, which can be linked to either structural voids in the soil (e.g. Macropores, cracks, etc.) or to flow instability are considered. Mathematical expressions describing these cases are presented. In the final section of this book we present a discussion of cases when Richards’ equation controls water movement. When Richards’ equation is used, it is difficult to analyze solute transport due to the strongly nonlinear nature of the equation. However, a few exact analytical solutions have been obtained recently and are presented here.
... To simulate preferential flow, the model relies on a parameterization of the water entry suction, a threshold suction value allowing water to enter a dry porous medium (Baker & Hillel, 1990): If the water content in a voxel was smaller than the residual water content, suction did not follow a water retention curve but was assumed equal to water entry suction (see Hirashima et al., 2014). The water entry suction in snow has been first parameterized by Katsushima et al. (2013) and Hirashima et al. (2014) starting from the equation proposed for soils by Baker and Hillel (1990). ...
... To simulate preferential flow, the model relies on a parameterization of the water entry suction, a threshold suction value allowing water to enter a dry porous medium (Baker & Hillel, 1990): If the water content in a voxel was smaller than the residual water content, suction did not follow a water retention curve but was assumed equal to water entry suction (see Hirashima et al., 2014). The water entry suction in snow has been first parameterized by Katsushima et al. (2013) and Hirashima et al. (2014) starting from the equation proposed for soils by Baker and Hillel (1990). The experiments used by Katsushima et al. (2013) and Hirashima et al. (2014) were, however, performed using a relatively high density and grain size compared to those by Avanzi et al. (2017) (density ≥ 386 kg m −3 and grain size ≥ 0.230 mm) and may not be valid for the type of snow considered here. ...
In order to explain the poorly understood transition between preferential and matrix flow in snow, we compared observations from a cold‐laboratory experiment with predictions from a multi‐dimensional water transport snow model. We found a good agreement between the modeled and observed evolution of grain size distributions if two or three dimensions are considered by the model, which validates existing theories of snow grain growth. Furthermore, the model reproduced the spatial migration of preferential flow paths with time and a progressive homogenization of snow wetness and structure only if grain growth was simulated. Spatially varying grain growth thus drives the transition from a preferential flow to a matrix flow regime. This transition is faster when grain size and density are lower, or infiltration rates are higher. This explains why preferential flow is more persistent in firn than in snow.
... The general condition that instability requires is where is the saturated soil-water conductivity and the water flux imposed by the first (fine) soil layer. It is a fundamental contribution of Hillel (1987); Hillel and Baker (1988); and Baker and Hillel (1990) to have pointed out that Baker and Hillel (1990) to have pointed out that Eq. (2.40) is not constrained enough and that should have a much lower value. ...
... The general condition that instability requires is where is the saturated soil-water conductivity and the water flux imposed by the first (fine) soil layer. It is a fundamental contribution of Hillel (1987); Hillel and Baker (1988); and Baker and Hillel (1990) to have pointed out that Baker and Hillel (1990) to have pointed out that Eq. (2.40) is not constrained enough and that should have a much lower value. ...
... where ܭ ௦ is the saturated soil-water conductivity and ࣫ the water flux imposed by the first (fine) soil layer. It is a fundamental contribution of Hillel (1987); Hillel and Baker (1988); and Baker and Hillel (1990) to have pointed out that Baker and Hillel (1990) to have pointed out that Eq. (2.40) is not constrained enough and that ܭ ௦ should have a much lower value. ...
... where ܭ ௦ is the saturated soil-water conductivity and ࣫ the water flux imposed by the first (fine) soil layer. It is a fundamental contribution of Hillel (1987); Hillel and Baker (1988); and Baker and Hillel (1990) to have pointed out that Baker and Hillel (1990) to have pointed out that Eq. (2.40) is not constrained enough and that ܭ ௦ should have a much lower value. ...
Preface
For many years, first as a student and later as a teacher, we have observed graduate students in ecology and other environmental sciences who had been required as undergraduates to take calculus courses. Those courses have often emphasized how to prove theorems about the beautiful, logical structure of calculus, but have neglected applications. Most of the time, the students have come out of such courses with little or no appreciation of how to apply calculus in their own work. Based on these observations, we developed a course designed in part to re-teach calculus as an everyday tool in ecology and other environmental sciences. we emphasized derivations—working with story problems (sometimes quite complex ones)—in this book.
... The maximum value of probe 3 was observed at around 45 min, with a fluctuating range between 45 min and 75 min. It began to decrease after 75 min and remained at 3.3 × 10 −9 mmol/cm 3 until the end of the simulation [36][37][38]. The salt content of the surface soil was stable at both the beginning and end of the experiment, but it was higher at the end. ...
With the expansion of reserve cultivated land resources in coastal saline–alkali areas, the problem of soil salinization is becoming more and more prominent. In order to reveal the influence of different soil media and contact modes on soil water movement, a two-domain Hydrus-3D model was established to verify its performance in heterogeneous soil layers, and the characteristics of water, salt, and wet peak transport of surface soil and sandy soil under horizontal contact and inclined contact conditions were analyzed through experiments and simulations. The measured data show that in horizontal contact mode, probe 3 and probe 2 are close to the interface of the two layers of soil, and their maximum values are measured in about 60 min. The time difference between probe 1 and probe 2 is about 15 min. In the inclined contact mode, probe 4 in the topsoil reached 45% in 10 min and remained stable; the peak lag time of probes 3 and 2 was 10 min, and the peak lag time of probes 2 and 1 was 15 min; the water in the surface soil gradually increases and then stabilizes; and the water in the sand soil is similar to the normal curve. The salt characteristics in the surface soil are similar to the normal curve, while the salt characteristics in the sandy soil gradually increase and then stabilize. The simulation results show that the water content in the topsoil is more than 40%, and the maximum water content in the center of the sand is only 36.9%, which is roughly the same as the experimental results. The results showed that the Hydrus-3D model had a good simulation effect on the groundwater salt transport of heterogeneous soil under two contact methods. The RMSE value and E value are close to 0 and 1, respectively, indicating that the simulation has good feasibility and can be applied to the simulation of water and salt transport processes under different contact modes of soil media.
... The term "water-entry pressure" represents the static capillary pressure needed to initiate wetting-front propagation. The capillary water-entry pressure for a wetting front at a certain velocity is termed dynamic capillary water-entry pressure ( ) (Baker & Hillel, 1990;Geiger & Durnford, 2000;Glass et al., 1989). Following Brindt and Wallach (2020), the Weitz et al. (1987) model that relates and the wetting-front propagation velocity is used to relate and : ...
Unstable flow in homogeneous dry soils, including saturation overshoot, is associated with a nonzero soil‐water contact angle (CA); 1D and 2D models for such flow, based on the moving‐boundary concept, were recently developed, solved, and verified for a constant high CA. However, in many natural soils rendered water‐repellent by natural organic matter, the CA decreases with time to a value that enables water infiltration. Thus, a mathematical model that includes the effect of time‐dependent CA on water‐content distribution and flow in the soil profile is developed in this study. This model, which also uses the moving‐boundary approach, simulates the effect of time‐dependent CA on unstable infiltration patterns. Comparison with a constant CA sheds light on the time‐dependent CA's influence on the aforementioned parameters. The 1D simulations indicate that a higher rate of CA decrease induces a higher wetting‐front velocity and shorter saturation‐overshoot length than a constant CA. However, due to flux imbalance at the wetting front for specific decreasing CA rates, the wetting‐front velocity first increases, and then decreases to an equilibrium value. The 2D simulations show that a time‐dependent CA significantly reduces water‐content accumulation at the finger tip. Moreover, a faster rate of decreasing CA results in a broader and longer plume shape, the latter being more pronounced. Effects of incoming flux at the soil surface and initial time‐dependent CA are also detailed for 1D and 2D flow. This theoretical study demonstrates that a time‐dependent CA significantly influences the formation of saturation overshoot and further impacts unstable flow generation.
... This model is basically an amelioration of the cylindrical model with the introduction of varying section determined similar to the model of sinusoidally constricted capillary channels of Beresnev et al. (2009).Basing on 50 percolation threshold theory (Broadbent and Hammersley, 1957;Ghanbarian et al., 2013;Hunt, 2004), it was demonstrated that two representative radii giving access to the main part of the porous network, namely a threshold radius value for the pore access and a threshold radius for the pore size Hammecker et al. (2004) could describe the dynamics of water infiltration in sedimentary rocks and soils. The threshold value can also be assimilated to the water entry pressure, necessary to initiate infiltration (Baker, 1990;Geiger and Durnford, 2000;Glass et al., 1989). Therefore this model is not aimed at taking into 55 account explicitly tortuosity and lateral interconnectivity, but at simulating the effect of water repellency on infiltration into an ensemble of capillary tubes. ...
Infiltration into water repellent soils has been widely observed, quantified and documented. The modelling of water infiltration into water repellent soils is more rarely taken into account explicitly. In this study, we modelled water infiltration into water repellent soils considering explicitly the contact angle, with the geometrical pore model proposed and validated previously. The applied microscopical approach showed good agreement with macroscopical models and with experimental data. We firstly investigated the case of contact angles lower than 90°, for the cylindrical pore and pearl necklace (PN) models. The cumulative infiltrations were numerically generated versus contact angle and for different pore radii. Then, the modelled infiltration curves were fitted to the two-terms Philip equation and parameters S and A were evaluated versus contact angle. As predicted sorptivity S decreased with increasing contact angle, and the constant infiltration rate A increased with contact angle for both models. Then, the modelled data were fitted to the numerical solution of the Richards equation to derive the equivalent hydraulic parameters assuming van Genuchten model. The results showed that the contact angle decreased the saturated hydraulic conductivity and increased the parameter α. Lastly, our model was used to investigate strong water repellency with contact angles higher than 90°. Cumulative infiltration and related Philip parameters, S and A, were evaluated versus water pressure head at surface h0 and contact angles (between 90° and 96°). Our model may be used to predict water infiltration into water repellent soils for both moderate and strong water repellency, including fingering features
... The working principle of horizontal CBSs and the phenomenon of water breakthrough has been studied by several authors, both experimentally [2,40,[47][48][49] and numerically [12,34,35,42]. Similarly, many authors analysed the behaviour of sloping CBSs and their ability to divert rainwater laterally, theoretically [32,37,38], experimentally [1,11,25,39,50] or numerically [13,21,24,46]. ...
The most common cause of slope instability is intense or sustained rainfall, which may induce reduction in soil suction, and thus, shear strength. Capillary barrier systems (CBSs) can be used to prevent rainwater infiltration into the underlying soil and thus, prevent slope instability. The application of CBSs for prevention of slope instability was studied by means of advanced 2D thermo-hydraulic finite element simulations and limit analyses. The roles of materials and thickness of the CBS, slope height and weather conditions were investigated. Climatic conditions of dry and warm (Cagliari, Italy) and wet and cool (London, UK) European areas were simulated. Sloping CBSs having the finer layer made of finer-grained materials, such as silty sand, were proven to be more effective in regions with warm and dry climates (with occasional intense rainfall events), because their key working mechanism is water storage, whereas sloping CBSs having the finer layer made of slightly coarser-grained materials, such as fine sand, are effective under a wider range of climatic conditions, because their key working mechanism is lateral water diversion. The effectiveness of CBSs was found to decrease with increasing slope height. However, two solutions were proven to be effective at widening the range of applicability of CBSs to higher slopes: multi-layered CBSs and multiple drains. All the CBSs analysed were proven to be effective at preventing rainfall-induced slope instability.
... Moreover, the CB system has been used as a final capping system after the closure of nuclear waste facilities [9]. Some researchers have also considered the application of the CB system for the maintenance of slope stability in soil structures since it can prevent rainwater infiltration into engineered slopes [10][11][12][13][14][15][16][17][18][19]. Meanwhile, many laboratory CB model tests have been performed to examine and improve the water-shielding performance of the CB system under the influence of several factors (e.g., material properties and layer thickness) [15,[20][21][22][23][24]. ...
Capillary barrier (CB) systems consisting of a fine-grained soil layer placed over a coarse-grained soil layer can generally provide a water-shielding effect, increasing the slope stability of soil structures during rainfall. In order to improve the water-shielding performance of CB systems, laboratory model tests have been previously conducted under various conditions; notably, large-scale model tests are especially required. The inefficiency in increasing the production time of CB models until now explains their high cost. In this paper, we propose a laboratory small-scale CB (SSCB) model test for a quick and efficient evaluation of the function of a CB system. In this model test, differently from previous studies, a side drainage flow in the direction of the inclined sand layer was set as the no-flow condition; moreover, the laboratory SSCB model tests were performed by considering three rainfall intensities (i.e., 20, 50, and 100 mm/h) under the lateral no-flow condition. The results showed that the larger the rainfall intensity, the shorter the diversion length was of the CB system. To evaluate the effectiveness of the SSCB model test proposed in this study, the diversion length was estimated by an empirical equation under the lateral flow condition based on hydraulic conductivity functions and the soil water characteristic curves of sand and gravel and then compared to the results of the SSCB model tests. It was hence demonstrated that the water-shielding performance of the CB system can be efficiently evaluated through SSCB model tests under the lateral no-flow condition, rather than through large-scale model tests.
... The necessity of groundwater protectability assessments was acknowl edged and implemented in several countries -France, Czechoslovakia, Germany, USA, USSR, etc. in 1960-1990(Aller et al., 1987Rosen, 1994;Rundquist et al., 1991). ...
... The necessity of groundwater protectability assessments was acknowl edged and implemented in several countries -France, Czechoslovakia, Germany, USA, USSR, etc. in 1960-1990(Aller et al., 1987Rosen, 1994;Rundquist et al., 1991). ...
... Table 1 summarizes soil physical properties. The contrast in soil textures between silt loam and medium sand is expected to create the required capillary barrier effects (Hillel and Baker 1988;Baker and Hillel 1990;Al-Harrasi 2017). Figure 4 illustrates the capillary rise in a column (diameter = 2.54 cm) of medium sand, with a zero hydrostatic pressure at the bottom. ...
New water-conserving irrigation technologies are vital in arid countries. We investigated the effects of (i) soil substrates made of Smart Capillary Barrier Wick (SCB-W), consisting of silt loam blocks surrounded by sand-sheathes and irrigated with a sand wick cylinder (WC) as compared to a control (homogenous soil irrigated by the same wick system, HW), (ii) WC diameters (2.54 cm vs. 1.27 cm), and (iii) 2-cm sand mulch layer on soil–water dynamics during wetting–drying cycles. Field experiments with pots and HYDRUS (2D/3D) modeling were performed in two consecutive phases (with and without sand mulch). Analysis of variance at p < 0.05 was used to assess significant differences in measured water contents, θ, between the two substrates. For the wetting/drying cycles, the modeled and measured θ are in satisfactory/tolerable agreement, as documented by the model evaluation criteria, which are within acceptable ranges (the root mean squared error, RMSE 0.01–0.06; Nash–Sutcliffe coefficient, NSE 0.51–0.97, and Willmott index, d = 0.97–1). SCB-W wets the soil substrate about two times faster than HW during the wetting cycles (p < 0.05). Reducing the WC diameter prolonged the wetting time by 1 and 2 days for SCB-W and HW, respectively, the same trend of two times faster wetting of SCB-W compared to HW was maintained. SCB-W showed higher θ storage (by 44.3–52.4%) at the bottom part of the composite than HW (p < 0.05). The sand mulch layer reduced evaporation and resulted in 20 and 38.9% higher θ during the drying cycle for both the bottom and top sensors, respectively, in both substrates (p < 0.05). SCB-W could improve water conservation in home gardens.
... Hence, the term "water-entry pressure" represents the static capillary pressure needed to initiate wetting-front propagation. The capillary water-entry pressure for a wetting front at a certain velocity is termed dynamic capillary water-entry pressure, ψ wed (Baker & Hillel, 1990;Geiger & Durnford, 2000;Glass et al., 1989). Weitz et al. (1987) model is used to relate ψ wed and the wetting-front propagation velocity: ...
The moving‐boundary approach, which has been successfully used to model stable and unstable 1‐D flow in initially dry soils of various contact angles (Brindt & Wallach, 2017 https://doi.org/10.1002/2016WR019252), was extended here for 2‐D flow. The wetting front is the plume perimeter that is partly formed by the capillary driving force, the remaining part by the combined capillary and gravity driving forces. The moving‐boundary approach overcomes the limitation of the Richards equation for describing gravity‐driven unstable flow with nonmonotonic water‐content distribution. According to this approach, the 2‐D flow domain is divided into two subdomains with a sharp change in fluid saturation between them—the wetting front (moving boundary). The 2‐D Richards equation was solved for the subdomain behind the wetting front for a given flux boundary condition at the soil surface, while the location of the other boundary, for which a no‐flux condition is imposed, was part of the solution. The moving‐boundary solution was used after verification to demonstrate the synergistic effect of contact angle and incoming flux on flow stability and its associated plume shapes. The contact angle that hinders spontaneous invasion of the dry pores decreases the water‐entry capillary pressure, ψwe, while the flux‐dependent dynamic water‐entry value, ψwed, is even lower, both inducing water accumulation behind the wetting front (saturation overshoot). This innovative physically based model for the 2‐D unsaturated flow problem for an initially dry soil of zero and nonzero contact angle using the moving‐boundary approach fulfills several criteria raised by researchers to adequately describe gravity‐driven unstable flow.
... At the estimated water-entry pressure of À0.11 m, the unsaturated hydraulic conductivity (K we ) of the tailings sand is greater than the K s of the unweathered (as-built) LFH, which represents the maximum possible rate of water delivery to the interface. Due to the difference in hydraulic conductivity, the flow face must constrict and occupy only a fraction of the cross-sectional area of the porous media (Hillel and Baker, 1988;Baker and Hillel, 1990), resulting in flow breaking into unstable, isolated and elongated lobes known as fingers. Theoretically, this means that prior to the weathering of LFH at the inter- face all percolation through the tailings sand must occur as finger flow. ...
Mine reclamation landscapes typically comprise layers of mine waste materials such as tailings sands, capped with a cover soil. In addition to the arrangement and placement of these materials, their hydraulic properties govern the performance of the built system. Soil evolution due to freeze-thaw cycling can result in dramatically altered soil hydraulic properties compared to the as-built material. Therefore, prediction of present and future hydrologic behaviour relies on understanding the nature and magnitude of this change and the elapsed time associated with stabilization. This research quantifies the transient hydraulic properties of mine reclamation materials at a constructed upland within a reclaimed watershed, and models the effect of this evolution on the partitioning of soil moisture between evaporation and groundwater recharge. Soil moisture dynamics were simulated using HYDRUS-1D for the ice-free period two, three, and five years after construction. A capillary barrier between the fine-grained cover soil and coarse-grained tailings sand regulated percolation past the interface. Soil evolution of the cover soil was responsible for an increase in saturated hydraulic conductivity by an order of magnitude, decrease in air-entry pressure by a factor of 4, and decrease in the van Genuchten n parameter by a factor of 2. The altered soil hydraulic properties associated with the weathered cover soil ultimately resulted in a 64% increase in groundwater recharge as a consequence of the capillary barrier weakening. The cover soil exhibited minor spatial heterogeneity in soil hydraulic properties, and did not contribute substantial uncertainty to the estimates of groundwater recharge and evaporation. Cover soil thickness exerted a strong influence on the partitioning of soil moisture. Reclaimed uplands will provide the most recharge to downgradient ecosystems in the period following the completion of soil evolution (~4 years) but preceding substantial vegetation development.
... Gravity-driven fingering in unsaturated porous media has been observed experimentally and simulated numerically in many studies (e.g., Baker and Hillel, 1990;Wang et al., 2004;DiCarlo, 2007;Chapwanya and Stockie, 2010). Comprehensive reviews of theories, models, and experiments have been performed by DiCarlo (2013) and Xiong (2014). ...
Core Ideas
We provide detailed analysis of various forms of the dynamic capillarity coefficient.
We focus on the region of saturation overshoot at larger saturation.
The results are evaluated with the aid of experimental observations.
Gravity‐driven fingering has been observed during downward infiltration of water into dry sand. Moreover, the water saturation profile within each finger is non‐monotonic, with a saturation overshoot at the finger tip. As reported in the literature, these effects can be simulated by an extended form of the Richards equation, where a dynamic capillarity term is included. The coefficient of proportionality is called the dynamic capillarity coefficient. The dynamic capillarity coefficient may depend on saturation. However, there is no consensus on the form of this dependence. We provide a detailed traveling wave analysis of four distinctly different functional forms of the dynamic capillarity coefficient. In some forms, the coefficient increases with increasing saturation, and in some forms, it decreases. For each form, we have found an explicit expression for the maximum value of saturation in the overshoot region. In current formulations of dynamic capillarity, if the value of the capillarity coefficient is large, the value of saturation in the overshoot region may exceed unity, which is obviously nonphysical. So, we have been able to ensure boundedness of saturation regardless of the value of the dynamic capillarity coefficient by extending the capillary pressure–saturation relationship. Finally, we provide a qualitative comparison of the results of traveling wave analysis with experimental observations.
... Hillel and Baker (1988) gave a descriptive theory of fingering in layered soils, which was possibly due to the restriction of the water supply from the top layer. According to Baker and Hillel (1990), the finger flow velocity increased across the interlayer plane when the sublayer conductivity was greater Core Ideas ...
Core Ideas
The texture of the top layer controlled infiltration more than that of the sublayer.
Finger flow was more uniform in soils with gravel but irregular in layered water‐repellent soils.
Changes in cumulative infiltration and finger length were well correlated width wetting area.
Finger flow in water‐repellent (WR) soils significantly influences the transport of water and solutes in the soil, but the mechanics of finger flow occurrence in layered WR soils is not clear. Soil chamber infiltration experiments with a total of 20 treatments, including five different WR levels with four layer combinations, i.e., clay or sandy loam overlying sand or heavy gravel, were conducted to reveal the mechanics of finger flow occurrence in layered WR soils. The variations of the finger flow dynamics and infiltration parameters were investigated. The results showed: (i) the temporal variations of cumulative infiltration (CI) decreased with the increase of the WR level so that CI was generally larger when the top layer was sandy loam rather than clay loam and therefore the top layer soil texture controlled CI more than the sublayer; (ii) for the wettable treatments, finger flow was clearly and uniformly generated in layered soils with a sublayer of heavy gravel rather than sand, but for WR layered treatments, fingers developed irregularly with the WR levels and finger length, width, and velocity varied with the WR levels; (iii) there were good power function or linear correlations between CI and cumulative wetting area, and between CI and finger length; and (iv) water content in the top layer was higher than in the sublayer and generally decreased with the increase of WR level. Finger flow development in layered WR soils was generally irregular and showed a large degree of complexity.
... Preferential flow refers to the movement of water in addition to the solutes in a nonuniform flow and is independent from common conditions that characterize soils in various locations (Andreini and Steenhuis, 1990;Baveye et al., 1998;Dekker and Ritsema, 1996;Doerr et al., 2007Doerr et al., , 2006. The occurrence of finger flows, one form of preferential flow that frequently occurs in homogeneous porous media, may appear due to (i) entrapment of air during the movement of the wetting front, (ii) flow through water-repellent soils, (iii) the process of continuous non-ponding infiltration, and (iv) flow through layered materials having different textures (Baker and Hillel, 1990;Diment and Watson, 1985;Du et al., 2001;Glass et al., 1991Glass et al., , 1990Glass et al., , 1989aHill and Parlage, 1972;Hill and Baker, 1988;Parlange and Hill, 1976;Philip, 1975a,b;Raats, 1973;Selker et al., 1992;Tamai et al., 1987;Hendrickx, 1996, 2001). Surface tension and contact angle are the two interfacial parameters most commonly examined in the infiltration process (Yuan and Lee, 2013). ...
In the hydrological cycle, the infiltration process is a critical component in the distribution of water into the soil and in the groundwater system. The nonlinear dynamics of the soil infiltration process yield preferential flow which affects the water distribution in soil. Preferential flow is influenced by the interactions between water, soil, plants, and microorganisms. Although the relationship among the plant roots, their rhizodeposits and water transport in soil has been the subject of extensive study, the effect of microbial exudates has been studied in only a few cases. Here the authors investigated the influence of two artificial microbial exudates–catechol and riboflavin–on the infiltration process, particularly unstable fingered flow, one form of preferential flow. Flow experiments investigating the effects of types and concentrations of microbial exudates on unstable fingered flow were conducted in a two-dimensional tank that was filled with ASTM C778 graded silica sand. The light transmission method (LTM) which is based on capturing the light intensity transmitted through a sand-water system and then converting it into degree of water saturation was used to visualize and characterize the flow of water in porous media as well as to image and measure the spatial and temporal distribution of water in porous media. Flow patterns, vertical and horizontal profiles of the degree of water saturation of the fingers, as well as measurements of the fingers dimension (width), number, and velocity were determined using the light transmission method. Interfacial experiments exploring the influence of microbial exudates on the wettability behavior of water were performed by measuring the contact angle and the interfacial tension of the (solid)-gas-microbial exudate solution systems. Unstable wetting front generating fingered flow was observed in all infiltration experiments. The experimental results showed that the microbial exudate addition affected the infiltration process, as the measurements of the degree of saturation profiles and widths of the fingers differed from those of the control NaCl solution. These differences may be due to an improved water holding capacity in the presence of the microbial exudates. The lowest catechol solution concentration (10 μM) produced the largest finger width (9.69 cm) among the tested catechol solution concentrations and all the other solutions including the control solution (7.24 cm). Moreover, the wettability of the medium for the catechol solution increased with an increase in concentration. The highest riboflavin solution concentration (1000 μM) generated the highest finger width (7.75 cm) among the tested riboflavin solution concentrations. However, the wettability of the medium for the riboflavin solution decreased with an increase in concentration. Our study demonstrated that the microbial exudates which are biochemical compounds produced and released by microbes in the environment are capable of influencing the soil infiltration process. The results of this study also demonstrated that the influence of the contact angle expressed as (cosθ)1/2 should be integrated in the scaling of the finger dimension, i.e., finger width, when the Miller and Miller (1956) scaling theory is applied for the hydrodynamic scaling in porous media.
... This was precisely what we could produce from ponded water experiments in this study. In contrast, the authors in [28] have observed the fingering when there was no slope to the coarse texture layer in their lab experiment. Walter et al. (2000) [29] also investigated the effect of slope and infiltration rate. ...
... Hill andproposed the conceptual model of " finger flow " to describe the water movement in laminated soils and found that the infiltration rate is zero in the interface between the soils of fine and coarse qualities. Hill andBaker (1988Baker ( , 1990) obtained the relationships between the diameter of coarse soil particles and soil absorbability based on experiments to analyze the mechanism of finger flow.Li et al. (2012)quantified the influence of interlayer location on infiltration in layered soil.Fan et al. (2012)employed a two-dimensional numerical model to the simulate infiltration process of layered soil. These studies proposed the efforts related to the mechanism, impact factors, and modeling involved in the regular layered soil water infiltration. ...
The infiltration process is a critical link between surface water and groundwater. In this research, a specific device to observe infiltration processes in homogeneous and heterogeneous soils with triangular and inverted triangular profiles was designed, and the Green-Ampt model was employed for the process simulation. The results indicate that (1) the wetting front in coarse texture soils transports faster than in fine texture soils; (2) for the homogeneous case, the wetting front in triangularshaped soils transports faster than the inverted triangular type, but the triangular-shaped soils show a lower infiltration rate; (3) in the initial step, the wetting front in triangular-shaped soils shows higher transport speed, but depicts lower speed with increase in the time; (4) both the wetting front and infiltration rate show a significant exponential relation with the time. From these findings, an empirical model was developed which agrees well with the observed data and provides a useful method for this field of soil research.
... Depending on the temporal rainfall distribution and rainfall intensity, this can happen very quickly in, for example, the Rocky Mountains, which receive regular rainfall (Ebel, 2012;) as opposed to Southern California where annual precipitation is much lower. Findings demonstrating the unlikeliness of an effect of pore clogging on infiltration in sandy soils have shifted the research focus in the Rockies to the effects of differences between hydraulic conductivity of ash and underlying layers depending on texture, delayed water release from ash as a result of capillary forces, and lateral flow through ash and soil on top of a water repellent soil (Baker & Hillel, 1990;Bodí, Doerr, Cerdà, & Mataix-Solera, 2012;Schroth, Istok, & Selker, 1998). ...
Wildfires can impact streamflow by modifying net precipitation, infiltration, evapotranspiration, snowmelt, and hillslope runoff pathways. Regional differences in fire trends and post-wildfire streamflow responses across the conterminous U.S. (CONUS) have spurred concerns about the impact on streamflow in forests that serve as water resource areas. This is notably the case for the western U.S., where fire activity and burn severity have increased in conjunction with climate change and increased forest density due to human fire suppression. In this review, we discuss the effects of wildfire on hydrological processes with a special focus on regional differences in post-wildfire streamflow responses in forests. Post-wildfire peak flows and annual water yields are generally higher in regions with a Mediterranean or semi-arid climate (Southern California and the Southwest) compared to the highlands (Rocky Mountains and the Pacific Northwest), where fire-induced changes in hydraulic connectivity along the hillslope results in the delivery of more water, more rapidly to streams. No clear streamflow response patterns have been identified in the humid subtropical southeastern U.S., where most fires are prescribed fires with a low burn severity, and more research is needed in that region. Improved assessment of post-wildfire streamflow relies on quantitative spatial knowledge of landscape variables such as pre-storm soil moisture, burn severity and correlations with soil surface sealing, water repellency and ash deposition. The latest studies furthermore emphasize that understanding the effects of hydrological processes on post-wildfire dynamic hydraulic connectivity, notably at the hillslope and watershed scales, and the relationship between overlapping disturbances including those other than wildfire, is necessary for the development of risk assessment tools.
... The high outflow in Column Tests B and C can be attributed to the change in texture of coarse RCA with oil and wax. The oil or wax coated solid particles in the coarse RCA layer might have helped in generating a preferential flow which occurred in the form of fingering flow (Baker and Hillel 1990;Miyazaki 1992). The fingering flow could happen due to the nonexistence of capillary forces among solid particles. ...
The infiltration characteristics of two different hydrophobic materials were investigated in this study. The materials used in this study were fine and coarse recycled concrete aggregates (RCA). One-dimensional (1D) vertical infiltration column tests were carried out on fine RCA over coarse RCA, fine RCA over coarse RCA with oil, and fine RCA over coarse RCA with wax. The column apparatus were instrumented with a tensiometer-transducer system, data acquisition system, soil moisture sensor, and electronic weighing balance. The comprehensive instruments were used to measure pore-water pressure, water content, and outflow rate of water instantaneously and automatically. The experimental results indicated that the water flowed faster within the layer of coarse RCA with oil and wax as compared to that within the layer of coarse RCA without oil and wax. This could be attributed to the hydrophobicity of each solid particle of coarse RCA that was coated with oil and wax.
... Small-scale heterogeneity has also shown its effect on flow and solute transport in the unsaturated zone (Hangen et al., 2004). At small scales, preferential flow can result from the passage of fluid from fine-grained material to coarser material (Parlange and Hill, 1976;Baker and Hillel, 1990). This phenomenon was observed in materials without obvious physical heterogeneity (Ghodrati and Jury, 1992;Jury et al., 2003;Wang et al., 2003;Cho et al., 2005). ...
... The fraction (f) of biochar intraporosity that is accessible to water depends on the biochar surface chemistry, biochar internal pore size distribution, and biochar pore connectivity. If the biochar surface is hydrophilic (h < 90°) then water entry pressure is negative (Baker and Hillel, 1990). If these pores are connected and larger than a water molecule, water can enter the pores and f = 1, meaning the pores can all be filled with water. ...
The amendment of soil with biochar can sequester carbon and alter hydrologic properties by changing physical and chemical characteristics of soil. To understand the effect of biochar amendment on soil hydrology, we measured the hydraulic conductivity (K) of biochar–sand mixtures as well as dissolved organic carbon (DOC) in leachate. Specifically, we assessed the effects of biochar concentration and particle size on K and amount of DOC in the soil leachate. To better understand how physical properties influenced K, we also measured the skeletal density of biochars and sand, and the bulk density, the water saturation, and the porosity of biochar–sand mixtures. Our model soil was sand (0.251–0.853 mm) with biochar rates from 2 to 10 wt% (g biochar/g total soil × 100%). As biochar (<0.853 mm) concentration increased from 0 to 10 wt%, K decreased by 72 ± 3%.
... Small-scale heterogeneity has also shown its effect on flow and solute transport in the unsaturated zone Hangen et al. 2004). At small scales, preferential flow can result from the passage of fluid from fine-grained material to coarser material (Parlange and Hill 1976;Baker and Hillel 1990). This phenomenon was observed in materials without obvious physical heterogeneity (Ghodrati et Jury 1992;Jury et al. 2003;Wang et al. 2003;Cho et al. 2005). ...
Waste rock piles producing acid mine drainage (AMD) are partially saturated systems involving multiphase (gas and liquid) flow and coupled transfer processes. Their internal structure and heterogeneous properties are inherited from their wide-ranging material grain sizes, their modes of deposition, and the underlying topography. This paper aims at assessing the effect of physical heterogeneity and anisotropy of waste rock piles on the physical processes involved in the generation of AMD. Generic waste rock pile conditions were represented with the numerical simulator TOUGH AMD based on those found at the Doyon mine waste rock pile (Canada). Models included four randomly distributed material types (coarse, intermediate, fine and very fine-grained). The term ‘‘randomly’’ as used in this study means that the vertical profile and spatial distribution of materials in waste rock piles (internal structure) defy stratigraphy principles applicable to natural sediments (superposition and continuity). The materials have different permeability and capillary properties, covering the typical range of materials found in waste rock piles. Anisotropy with a larger horizontal than vertical permeability was used to represent the effect of pile construction by benches, while the construction by end-dumping was presumed to induce a higher vertical than horizontal permeability. Results show that infiltrated precipitation preferentially flows in fine-grained materials, which remain almost saturated, whereas gas flows preferentially through the most permeable coarse materials, which have higher volumetric gas saturation. Anisotropy, which depends on pile construction methods, often controls global gas flow paths. Construction by benches favours lateral air entry close to the pile slope, whereas end-dumping leads to air entry from the surface to the interior of the pile by secondary gas convection cells. These results can be useful to construct and rehabilitate waste rock piles to minimize AMD, while controlling gas flow and oxygen supply.
This study presents the field performance of a five-layer composite cover to mitigate acid mine drainage in legacy sulfide tailings in northern Ontario, Canada. Installed in 2008, this cover comprised sand, clay, geosynthetic clay liner, sand, and waste rock layers. To evaluate the effectiveness of the cover in reducing water and oxygen ingress, groundwater and vadose zone hydrological characterization, stable water isotope analysis, pore-gas measurements, oxygen flux calculations, and variably saturated flow modelling were conducted. Results indicate that the clay layer stayed nearly saturated in the spring, fall, and winter, but temporary desiccation occurred during the summer. Compared to uncovered tailings, the cover significantly lowered diffusive oxygen flux. In the summer, fall, and winter, the capillary barrier effect of the cover functioned effectively and inhibited percolation. Atmospheric pore-gas oxygen concentrations at one out of three monitoring locations indicate potential cover imperfections that enabled oxygen transport into the tailings. In the spring and early summer, snowmelt infiltration resulted in percolation that compromised the capillary barrier effect, as well as lateral drainage. The resulting increase in water saturation in the cover limited oxygen transport. Despite potential cover imperfections, this composite cover reduced oxygen and water ingress a decade after installation.
A capillary barrier is a widely used drainage system in recent years. It has been in practice since the end of the 1950s, and research over the past 60 years has provided valuable insight into its safe and economical design. Although capillary barrier application is not an old and traditional technique, it is possible to see its primitive application on earthen roofs of traditional buildings. One of these applications is in the earthen roofs of historical and protected houses in Kemaliye district in eastern Turkey. In this study, the capillary barrier behaviour of the earthen roof system of historical Kemaliye houses, which has existed for centuries, has been experimentally investigated. In this context, soil index properties and water characteristic curves were determined by conducting laboratory experiments on samples taken from the soil roof of a traditional Kemaliye house. In order to examine the capillary barrier behaviour of the roof in precipitation, 1D model infiltration experiments were carried out. In the light of the findings obtained from these experiments, it was concluded that the earthen roofs of traditional Kemaliye houses show varied capillary barrier behaviour depending on the initial conditions of the layers.
A capillary barrier system is the control technique of water flow adopted in waste landfills. A capillary barrier system constitutes a finer soil overlying a coarser one. The infiltration water is retained by the suction of the upper fine soil layer, and the infiltration water into the lower layer is halted. When the amount of infiltration water increases, the water across the interface between the finer soil layer and the coarser one infiltrates. When the constituent materials for the capillary barrier structure are selected, they are empirically selected based on their grain size characteristics, and their hydraulic properties are not considered. In particular, the performance of capillary barriers is dominated by the hydraulic property of the fine-grained soil that is the constituent material, and research has been conducted focusing on those characteristics.
This study focused on the hydraulic properties of the constituent materials for the capillary barrier, and the diversion capacity with different combinations of earth materials is evaluated. From the experimental results, the breakthrough mechanism of the capillary barrier known from the previous studies is verified. The diversion capacities are evaluated using the index that considers the hydraulic properties of the constituent materials, and these results imply that the smaller the index, the greater the diversion capacities.
Small-particle interlayers (lenticles) show some characteristic hydraulic properties and can affect the movement of unsaturated water. In this study, we developed a novel online capillary-water-absorption monitoring device and conducted three groups of comparison tests to simulate lenticle positions and thicknesses with respect to the capillary rise. The results show that the characteristic wetting front exhibits a fast rise in the early stage, a slow rise in the middle stage, and stability in the later stage. The motion of the capillary water in the lenticle is mainly transversal, with the upward curve being “flat,” and the longer is “flat,” the longer is the time needed for the water to move. The interlayer can form a capillary stagnation zone with moisture content close to saturation. The high interlayer may form a discontinuous corrugated capillary zone. Thus, when the wetting front reaches the “coarse-grain (lower)-fine-grain (upper)” interface, the “anti-capillary barrier effect” results in more moisture in the upper layer. Thus, when the wetting front of the capillary water reaches the “fine-grain (upper)-coarse-grain (lower)” interface, the “capillary barrier effect” causes the moisture content of the upper tailings to decreases sharply because of the horizontal movement of the water in the fine medium. It is clear that the presence of lenticles can retard the rise of capillary water by storing water.
Fingerlike partial flow, or fingering flow can occur during vertical downward infiltration with sharp wetting front under the conditions of wetting front instability. To clarify the physical features of fingering flow, in particular, the change of the water pressure at the interlayer plane with increasing finger length and the degree of saturation within fingers, experiments of infiltration into fine over coarse textured layered glass beads were conducted. It was confirmed that distinct fingers occurred and grew in the sublayer when it was initially dry and the criterion for wetting front instability was satisfied. On the other hand, it was observed that when the sublayer was initially wet, the wetted region in the sublayer formed a wavy pattern rather than a fingerlike one. The water pressure at the interlayer plane decreased with the increase of finger length after it reached a maximum value at the onset of water entry into the sublayer when typical fingering occurred under initially dry sublayer conditions. The change of the flux through the top layer was corresponded to the change of the water pressure at the interlayer plane which consisted of one of boundary conditions for percolation in the top layer. Furthermore, it was estimated that it was not saturated within fingers but was under unsaturated condition with entrapped air.
This work enhances our understanding of catchment‐scale N budgets by demonstrating the modification and application of a simple method for direct in situ measurements of vadose zone nitrate leaching and attenuation. We developed a soil passive flux meter (SPFM) to measure solute leaching based on a modified design of ion‐exchange resin columns, and we tested the design in numerical simulations, laboratory experiments, plot‐scale field experiments, and a catchment‐scale field deployment. Our design minimized flow divergence around the resin column to attain nearly 100% capture of surface applied tracers in plot‐ and catchment‐scale deployments. We found that mixing resin with native soil and extending the column height 10 cm above the resin layer minimized divergence of soil water around the column, resulting in a field‐measured convergence factor (χ) of 1.3 that was consistent with numerical simulations. For catchment‐scale testing, SPFMs were used at nine sites in three dominant land uses (crop, pasture, and turf) with known N inputs in two deployments, one during the 4‐mo wet season and an additional set during the 8‐mo dry season, to obtain integral annual measures of soil nitrate fluxes. In situ measured nitrate leaching determined from the SPFMs was positively correlated with known N inputs (R² = 0.55, p < 0.05) and attenuation averaged 67% (± 24% SD) of inputs across all sites. Although N inputs explain a large portion of the variability, our results emphasize the importance of both inter‐ and intra‐land use variability in landscape‐scale N budgets.
Core Ideas
We developed a soil passive flux meter (SPFM) to measure solute leaching in situ.
Our design minimized flow divergence around the resin column (nearly 100% capture).
The SPFMs provided robust and accurate measurements of soil nitrate leaching.
The SPFMs, together with N input data, enabled assessment of nitrate attenuation.
Afin d'étudier l'intégration des écoulements préférentiels à travers les macropores observés dans les sols fins compactés, 3 approches de modélisation différentes sont développées. La première est celle d'un milieu homogène dont les propriétés hydrodynamiques sont décrites par la formulation de Van Genuchten & Mualem. La deuxième est basée sur le concept de la double porosité où la matrice du sol et les macropores sont traités comme deux domaines d'écoulement présentant des propriétés hydrauliques différentes, et un terme d'échange décrit leur interaction. La troisième suppose que la distribution dimensionnelle de la porosité du sol et par conséquent, les courbes de rétention et de conductivité hydraulique suivent des lois bimodales également décrites par la formulation de Van Genuchten & Mualem. Pour chacune de ces approches, un modèle direct simulant l'infiltration verticale et une procédure d'identification des paramètres par la méthode inverse sont présentés. Les résultats des simulations d'essais d'infiltration au laboratoire sont confrontés aux données expérimentales. Il en ressort que le concept de la double porosité, bien qu'il fournisse une description plus réaliste du processus d'infiltration dans les sols macroporeux, montre ses limites d'applicabilité étant donné que l'estimation de ses paramètres s'avère complexe. Les approches plus pragmatiques, basées sur l'intégration de l'effet hydraulique des macropores dans les propriétés hydrodynamiques du sol à travers les lois unimodale et bimodale, se sont montrées plus intéressantes. Les résultats de l'estimation des paramètres pour ces 2 approches, suggèrent que la distribution dimensionnelle des pores dans les sols fins compactés, varie entre les lois unimodale et bimodale et ce, en fonction de la teneur en eau et du mode de compactage.
The heterogeneous movement of liquid water through the snowpack during
precipitation and snowmelt leads to complex liquid water distributions that
are important for avalanche and runoff forecasting. We reproduced the
formation of capillary barriers and the development of preferential flow
through snow using a three-dimensional water transport model, which was then
validated using laboratory experiments of liquid water infiltration into
layered, initially dry snow. Three-dimensional simulations assumed the same
column shape and size, grain size, snow density, and water input rate as the
laboratory experiments. Model evaluation focused on the timing of water
movement, thickness of the upper layer affected by ponding, water content
profiles and wet snow fraction. Simulation results showed that the model
reconstructs relevant features of capillary barriers, including ponding in
the upper layer, preferential infiltration far from the interface, and the
timing of liquid water arrival at the snow base. In contrast, the area of
preferential flow paths was usually underestimated and consequently the
averaged water content in areas characterized by preferential flow paths was
also underestimated. Improving the representation of preferential
infiltration into initially dry snow is necessary to reproduce the transition
from a dry-snow-dominant condition to a wet-snow-dominant one, especially in
long-period simulations.
Geoscientific research was performed on South American table mountains (tepuis) and in their sandstone cave systems. To explain speleogenesis in these poorly soluble rocks, two theories were introduced: a) arenization theory implying selective weathering of quartz along grain boundaries and releasing of sand grains, b) selective lithification theory implying cementation by descending silica-bearing fluid flow. The latter theory presumes that the descending fluid flow becomes unstable on the interface between two layers with different porosity and splits to separate flow channels (so-called “finger flow”). The arenites outside these channels remain uncemented.
To verify the latter theory, small-scale modelling was performed, using layered sands and sodium-silicate solution. Fine to medium sand was used (0.08–0.5 mm), along with a coarse sand fraction (0.5–1.5 mm). The sands were layered and compacted in a transparent plastic boxes. Three liters of sodium-silicate solution (so-called water glass) were left to drip for several hours to the top of the sediment. The fine-grained layers were perfectly laterally impregnated, whereas the descending fluid flows split to “fingers” in the coarse-grained layers due their higher hydraulic conductivity. This small-scale laboratory simulation mimics the real diagenesis by descending silica-bearing fluids and matches the real phenomena observed on the tepuis. The resulting cemented constructions closely mimic many geomorphological features observed on tepuis and inside their caves, e.g. “finger-flow” pillars, overhangs, imperfectly formed (aborted) pillars in forms of hummocks hanging from ceilings, locally also thicker central pillars that originated by merging of smaller fluid-flow channels. The modelling showed that selective lithification theory can explain most of the geomorphological aspects related to the speleogenesis in tepuis.
The van Genuchten model (1980) is widely used for the description of the Soil-Water Characteristic Curve (SWCC) of a variety of soils. This study uses the Genetic Programming (GP) for the presentation of equations estimating the van Genuchten (vG) model fitting parameters for unsaturated clean sand soils. Moreover, this study uses the data derived from the valid dataset of Benson et al. [Benson, C.H., et al. "Estimating van Genuchten parameters a and n for clean sands from particle size distribution data", ASCE GSP, pp. 234-235 (2014).], including 95 measured SWCCs in both drying and wetting phases. The data on the particle size distributions includes the fine-grain percentage (fines %), d60, d10, besides the residual and saturated volumetric water contents (θr and θs), as the GP model inputs of the set of terminals. As for the model outputs of the set of terminals, the fitting parameters for the vG model include a and n. The functions used in the GP training are 'plus', 'minus', and 'times' taken from the MATLAB default functions; 'mydivide' proposed by Silva [Silva, S. "GPLAB: A genetic programming toolbox for MATLAB", Available at http://gplab.sourceforge.net, Coimbra, Portugal (2007)]; and some other new power functions included in this study. Accordingly, new equations are presented for the estimation of vG model fitting parameters for both forms of wetting and drying. Finally, to evaluate the accuracy of the proposed estimation equations, the GP results are evaluated and veriffed in different procedures.
An arrangement of fine-grained soil overlying coarse grained soil along a sloping contact can, under appropriate circumstances, hal infiltrating water away from the coarser material. Such an arrangement is called a capillary barrier. The water halted by a capillary barrier flows downward above the contact. This type of flow is named a funneled flow. The amount of funneled flow increases in the downward direction as additional infiltration is halted by the barrier. Sufficiently far downward, the funneled flow wets the contact to the point that an amount of water equal to the infiltration penetrates vertically into the coarse soil mostly in the form of a fingering flow. The lateral distance at such a point is defined as the diversion capacity of an inclined capillary barrier.
Diversion capacities of the inclined capillary barriers were measured experimentally by using a rain fall simulator under which one of two types of transparent Hele-Shaw models, a small model or a large model, filled with a Hyojun sand layer over a coarse glass beads layer was placed. Since both of the unsaturated funneled flow model proposed by Ross and the saturated funneled flow model proposed by Kung are two dimensional models, they were modified into three dimensional models by using a catchment factor. Soil physical parameters which depend on the bulk densities of materials were corrected by using non-similar media scaling technique. These modifications significantly improved the ability of predictions of the measured diversion capacities.
The Richards equation is unsuccessful at describing gravity-driven unstable flow with nonmonotonic water content distribution. This shortcoming is resolved in the current study by introducing the moving-boundary approach. Following this approach, the flow domain is divided into two subdomains with a sharp change in fluid saturation between them (moving boundary). The upper subdomain consists of water and air, whose relationship varies with space and time following the imposed boundary condition at the soil surface calculated by the Richards equation. The lower subdomain consists of an initially dry soil that remains constant. The location of the boundary between the two subdomains is part of the solution, rendering the problem nonlinear. The moving boundary solution was used after verification to demonstrate the effect of contact angle, soil characteristic curves and incoming flux on the dynamic water-entry pressure of the soil, which depends on the soil's wettability, incoming flux at the soil surface and the wetting front's propagation rate. Lower soil wettability hinders spontaneous invasion of the dry pores and, together with a higher input flux, induces water accumulation behind the wetting front (saturation overshoot). The wetting front starts to propagate once the pressure building up behind it exceeds the dynamic water-entry pressure. To conclude, the physically-based novel moving-boundary approach for solving stable and gravity-driven unstable flow in soils was developed and verified. It supports the conjecture that saturation overshoot is a prerequisite for gravity-driven fingering. This article is protected by copyright. All rights reserved.
Core Ideas
Soil crusting and surface sealing often adversely affect agricultural yields.
A modified Green–Ampt model was evaluated for predicting infiltration of crusted soil.
The model provided a good estimate of soil crusting's influence on infiltration.
Crusting frequently occurs on farmland soil of the Loess Plateau in China and often adversely affects agricultural yields. Therefore, a study was conducted to evaluate the occurrence of soil crusting and its influence on infiltration in a Loess Plateau silt loam soil. Simulated rainfall events were conducted at two intensities (0.67 and 1.33 mm min ⁻¹ ) on four slopes (5, 9, 18, and 29%) under two soil conditions (crusted and uncrusted). Infiltration was evaluated as the difference between precipitation applied and water lost to runoff. A modified Green–Ampt model adjusted to account for soil crusting was also evaluated for predicting infiltration of crusted soil. Mean soil crust thickness was 3.33 and 3.94 mm under rainfall intensities of 0.67 and 1.33 mm min ⁻¹ , respectively. Slope had no effect on crust formation. In all instances, cumulative infiltration was lower in crusted soil than in uncrusted soil. The time to runoff decreased from 17.98 min in uncrusted soil to 4.64 min in crusted soil under a rainfall intensity of 0.67 mm min ⁻¹ . Time to runoff also decreased from 16.25 min in uncrusted soil to 4.54 min in crusted soil at a rainfall intensity of 1.33 mm min ⁻¹ . The modified Green–Ampt model provided a good estimate of soil crusting's influence on infiltration (coefficient of determination range 0.85–0.97). This suggests that the Green–Ampt model could be used as a tool for predicting soil crusting's influence on infiltration and runoff from soils of the Loess Plateau.
It is generally asserted that the adoption of site-specific production techniques benefits soil and water quality while increasing or maintaining farm profitability. To evaluate this assumption, a 30 ha research site located in Beltsville, Maryland, has been characterized and instrumented to determine field-scale fluxes of agricultural chemicals from four 4 ha watersheds each managed under different production practices. EM-38 data was linked to the geo-statistical analysis of >40 km of ground-penetrating radar (GPR) data to produce contour maps showing the extent, orientation, and depth of subsurface layers governing groundwater movement as well as their spatial correlation. These maps were used to identify optimal sites for the installation of various hydrologic instruments, including 256 real-time soil moisture sensors. Soil moisture data confirmed the usefulness of GPR profile images to locate buried lenses and identify subsurface convergent flow pathways.
Please view the pdf by using the Full Text (PDF) link under 'View' to the left. Copyright © 1999. . Copyright © 1999 by the American Society of Agronomy, Inc. Crop Science Society of America, Inc. Soil Science Society of America, Inc. 5585 Guilford Rd., Madison, WI 53711 USA
Our industrial society generates increasing quantities of waste products, some of which can be highly toxic and persistent. Among these are domestic, municipal, industrial, and agricultural wastes which may include pathogenic agents, heavy metals, fertilizer residues, radionuclides, and synthetic compounds of low degradability — all of which threaten over time to poison significant segments of our environment. In recent decades, we have witnessed a growing tendency towards land disposal of wastes, in the expectation that the soil’s legendary capacity to filter, retain, buffer, and degrade many components will immobilize pollutants or otherwise render them harmless. In many cases the soil can indeed serve as a recycling and purifying medium (a “living filter”). In too many other cases, however, its powers are exceeded and the soil becomes merely a way station from which pollutants can continue to migrate and invade other realms of the environment via plants, surface waters, or groundwater.
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