Advancing Wetting Front Research Articles

Effect of surfactant surface and interfacial tension reduction on infiltration into hydrophobic porous media

Surfactant molecules increase the infiltration rate into hydrophobic porous media by lowering the infiltrating water's surface tension and the interfacial tension between hydrophobic surfaces and water molecules. We investigated the relative effect of these rate-limiting processes on the infiltration rate of aqueous surfactant solution into hydrophobic porous media. Two surfactants at various concentrations were applied at a constant pressure head to 1D columns filled with hydrophobic soil, and water was applied at the same pressure head to columns filled with surfactant-pretreated hydrophobic soil. Based on the measured contact angle, surfactant pretreatment significantly reduced the hydrophobic soil's interfacial surface tension, which increased the infiltration rate compared to the direct aqueous surfactant application to the hydrophobic soil. The latter's slower infiltration rate was attributed to the depletion of surfactant molecules due to its adsorption to the hydrophobic molecules near the advancing wetting front, yielding an increase in the surface tension of the infiltrating solution. Surfactant pretreatment increased the opportunity time for surfactant adsorption to the hydrophobic molecules, resulting in interfacial tension reduction and infiltration rate increase. Diffusion-limited surfactant adsorption on the hydrophobic surfaces, leading to reduced interfacial tension between the surface and infiltrating liquid, had a greater impact on limiting infiltration into hydrophobic porous media compared to the reduction in surface tension of the infiltrating liquid due to surfactant presence.

Assessment of Sowing Density Impact on Water Front Advancement and Infiltration in Furrow Irrigation

This research aims to evaluate the impact of durum-wheat sowing density on wetting front advancement and water infiltration rate along the furrow length. In addition, durum wheat yield and furrow irrigation performance under the different sowing densities were assessed. For that purpose, a field experiment was conducted under semi-arid conditions in Tunisia using four durum-wheat sowing densities: SD1 (250 seeds m−2), SD2 (350 seeds m−2), SD3 (450 seeds m−2) and SD0 (bare soil). The results reveal that water front advancement tends to be inversely proportional to sowing density. In fact, under the SD3 treatment, both irrigation duration and applied water volume increased about twofold compared to those recorded under SD0, resulting in an increasing soil infiltration rate. Furthermore, the two-point method performed well in estimating water front advancement, with an R2 value close to 1. Regarding durum wheat yield, values varied between 3.5 and 4.9 t ha−1 with the highest value attributed to the SD3 treatment. Meanwhile, higher irrigation water productivity was recorded under SD1. Considering irrigation performance indicators, the results indicate that distribution uniformity (DU) increases with increasing sowing density. Moreover, the lowest application efficiency values (75%) were recorded under the SD0 and SD3 treatments as compared to 82% for SD2 and 80% for SD1.

Approximate water vertical infiltration extremum path in unsaturated soils

According to the principle of stationary action, water vertically infiltrates unsaturated soils along an infiltration-duration extremum path with linear deviation. The solution of the Euler–Lagrange equation with appropriate boundary conditions yields a linear relationship between the moisture diffusion function and the ratio of the spatial location to the wetting front distance. By integrating the law of mass conservation at the boundary, the temporal evolution of the wetting front is implicitly derived. A linear term is introduced to compensate for the deviation in calculating the position of the wetting front owing to the Taylor series approximation. Based on the representation of the diffusion function in relation to the boundary diffusivity, the spatial location and the position of the wetting front, a direct way to determine hydraulic conductivity is presented. Unlike existing methods, the proposed approach involves fewer hypotheses and is more theoretically robust. Moreover, the material parameters in the proposed approach have a clear physical meaning and can be explicitly determined. Using the Brooks–Corey hydraulic function, the evolution and distribution characteristics of the water content are explicitly determined, while the advancing wetting front is implicitly represented. The results of the proposed solution for different types of soil are in agreement with these numerical and analytical solutions.

Analysis of the Mobilization of an Unsaturated Infinite Natural Slope Due to Strength Reduction

This work analyses the mobilization of infinite natural slopes that are initially partially saturated. Starting from dry conditions (the water table is assumed to be deep, far from the slope surface), the soil shear strength will decrease when the wetting front advances from the surface. First, the geometry of the failure mechanism that can develop because of such strength reduction is determined. Second, after the mobilization causes the progressive reduction in the soil strength, the slide speed reached the minimum strength is determined, obtaining an indicator of the risk derived from failure.

A new method of estimating soil water diffusivity based on variation in the wetting front position and on the principle of stationary action

This paper presents a novel method for the estimation of soil water diffusivity that solves the nongravitational Richards equation using the variation in the boundary of the advancing wetting front. The solution is based on the principle of stationary action. The position of the wetting front is determined from the continuity of water content at the wetting front interface, as the underlying physical principles of this area are well known and thus it can be described using relatively few assumptions. This also eliminates the need for the Boltzmann-transformation, which is commonly required in traditional methods. The Brooks-Corey empirical water diffusion function is used to analytically determine the significant parameters of diffusivity. The soil water diffusivities of 15 soil samples matched well with their measured soil–water diffusivities, thereby confirming the reliability of the method. All of the measurements required for using this method are rapidly and easily obtained, and the experimental apparatus is simple and inexpensive. Compared with other methods, this novel method is more accurate and convenient for use in laboratories for measuring the soil hydraulic properties.

Wetting Front Velocity Determination in Soil Infiltration Processes: An Experimental Sensitivity Analysis

An experimental approach for measuring the wetting front velocity in soils during water infiltration processes is presented. This experimental research is developed to test the sensitivity of the measurement technique in the detection of differences in the dynamics of the wetting front advancement in different soil testing scenarios. Experimental scenarios include undisturbed and artificially compacted soils, square and circular cross-sections of soil samples, water flowing in both directions downward and horizontal, and water infiltration in a block soil sample. The measurement technique is based on wetting front detection employing moisture sensors that measure the soil’s electrical conductivity. The technique was able to detect differences in wetting front in contrasting scenarios. Two-dimensional wetting front advancement experiments carried out on a block sample confirmed the results obtained for the one-dimensional case.

Field‐Scale Subsurface Flow Processes Inferred From Continuous Gravity Monitoring During a Sprinkling Experiment

AbstractField‐scale subsurface flow processes are difficult to observe and monitor. We investigated the value of gravity time series to identify subsurface flow processes by carrying out a sprinkling experiment in the direct vicinity of a superconducting gravimeter. We demonstrate how different water mass distributions in the subsoil affect the gravity signal and show the benefit of using the shape of the gravity response curve to identify different subsurface flow processes. For this purpose, a simple hydro‐gravimetric model was set up to test different scenarios in an optimization approach, including the processes macropore flow, preferential flow, wetting front advancement (WFA), bypass flow and perched water table rise. Besides the gravity observations, electrical resistivity and soil moisture data were used for evaluation. For the study site, the process combination of preferential flow and WFA led to the best correspondence to the observations in a multi‐criteria assessment. We argue that the approach of combining field‐scale sprinkling experiments in combination with gravity monitoring can be transferred to other sites for process identification, and discuss related uncertainties including limitations of the simple model used here. The study stresses the value of advancing terrestrial gravimetry as an integrative and non‐invasive monitoring technique for assessing hydrological states and dynamics.

Reconstruction of a wetting-induced shallow landslide in Shillong, India

The hill slopes of the north eastern region of India often encounters shallow landslide during the rainy season. Forensic study of a failed hill-slope in such areas is useful in improving the understanding of the failure mechanism. An incessant rainfall triggered a shallow landslide in the Umshing Mawkynroh locality of Shillong in North-east India during the rainy season of 2019. In this work, a forensic investigation was carried out by performing a coupled unsteady-state seepage and stability modelling to reconstruct the actual failure surface. The recorded rainfall data, slope geology, and soil hydraulic properties of the compacted laterite soil were used to back-estimate accurately the in situ failure surface. The underlying mechanism responsible for the shallow landslide during the rainfall infiltration was identified from the numerical modelling. The combined effect of wetting front advancement in the initial partial saturation zone and the rise of the perched groundwater table was found to be the mechanism responsible for the failure.

Surfing the capillary wave: Wetting dynamics beneath an impacting drop

Experiments show that prior to impact upon a solid surface, a droplet compresses the air beneath it into a nanometer-scale film. This air film is destroyed by the local initiation of liquid-solid contact, forming capillary bridges. A capillary wave leads the advancing wetting front, limiting its speed.

Wetting-induced collapse behavior associated with infiltration: A case study

Limited information is available in the literature for interpreting the influence of water infiltration process that contributes to in-situ collapse in loess deposits. For this reason, the focus of the present investigation has been directed to studying the collapse deformation with respect to time in Heifangtai, China, where significant wetting-induced collapse deformation is widespread due to multiple years of irrigation activities. A method that is based on easy-to-obtain parameters from constant water content oedometer tests and the wetting soil-water characteristic curve is used for predicting the long-term wetting-induced collapse deformation. The proposed method is validated using a field infiltration test. The wetting-induced collapse behavior in Heifangtai is interpreted using the proposed method. Results of the study suggest that there is a significant difference in total subsidence between the irrigation and non-irrigation areas. The collapse in the irrigation area occurs during the period when the wetting front advances and the groundwater table (GWT) forms and rises until the formation of a new hydraulic equilibrium state, which is significantly different from its initial condition. However, the collapse in the non-irrigation area is mainly related to the GWT variation. The difference in the total subsidence between the irrigation and non-irrigation areas is predominantly associated with the propagation of the wetting front. The proposed method is simple and can be extended in practical applications for reliably estimating the in-situ collapse deformation associated with the water infiltration process.

SOIL CONDITIONER EFFECT ON SOIL WETTING PATTERNS UNDER PULSED DRIP IRRIGATION SYSTEM

Optimal drip irrigation design and management depend on soil wetting patterns where, sandy soil under continuous drip irrigation was the depth of wetting pattern is relatively higher than the width that leads to increase deep percolation beyond root zone. Therefore, the study aim to improve drip irrigation efficiency using soil conditioners with pulse drip irrigation. An empirical mathematical model was used to describe soil wetting patterns. The laboratory experiments were conducted on sand soil using soil conditioners named commercially by Composite that contains mixed materials in the rate of 5:1 from local Aswan Clay and Hydrogel (super absorption polymer, SAP) with pulsed drip irrigation at cycle ratio equal 0.5 for different cycles times P (20, 30, 40 and 60 minutes). And also, an empirical model by Al-Ogaidi, et al., (2016) was used to estimate the wetted radius of dripper at the soil surface and at the depth of the wetting pattern. Results demonstrated that using composite with pulse flow technique in sand soil led to increase both soil moisture distribution, moisture percent, and the wetting front advances in horizontal direction more than vertical direction. Percentage of the surface and maximum wetted radius to wetted depth under dripper were increased. Also, the percentage of the soil surface wetted radius for all treatments compared to continuous flow without soil conditioner was increased. Predicted and observed values were compared well to test model applicability in laboratory conditions on the basis of model performance parameters. The result shows the advantage of soil conditioner with pulsed flow method for reducing the deep percolation of water under the root zone, while obtaining a wide horizontal spread of wetting.

Effects of Loess Cavity on Soil Moisture Movement: Revealed by Soil Column and HYDRUS Model

The purpose of this study is to advance our current understanding of soil moisture storage in subsurface and water infiltration rate in loess soil. Therefore, a set of experiments was conducted on two soil columns filled with silty clay loam, with and without applying cavity technical method. For the soil column applied with loess cavity, the ponding infiltration was simulated using HYDRUS-2D/3D, version 2.x and the simulated results were verified by those of observation. The results show that 1) the loess cavity significantly decreased the infiltration rates when the flux permeated through it (varying from 0.358 to 0.208 cm·min-1) as compared with no cavity soil column (varying from 0.408 to 0.241 cm·min-1); 2) similarly, the total cumulative infiltration and at the termination of wetting front advancement of soil column with cavity were 66 cm and 69 cm lower than that of no cavity soil column (76 and 78 cm), respectively. Consequently, the soil moisture at the subsurface and surrounding the loess cavity was effectively ameliorated; 3) the model was capable of predicting water infiltration processes in the soil column with loess cavity, and the root mean square error of simulated water contents, wetting front advancements, cumulative infiltrations, and infiltration rates were from 0.22 to 3.63 cm3·cm-3, 1.6 to 3.63 cm, 3.44 cm, and 0.026 cm·min-1, respectively. Overall, the findings in this study indicate that loess cavity can effectively increase soil moisture storage at shallow surface and the HYDRUS-2D/3D model is capable of simulating and predicting scenarios to help achieve stable shallow soil surface with loess cavity.

Anomalous water absorption in cement-based materials caused by drying shrinkage induced microcracks

This paper concerns understanding the influence of drying induced microcracking on water absorption by capillary suction. Paste, mortar and concrete samples with different binder type, w/b ratio, thickness, aggregate size, and curing age were tested. Samples were subjected to gentle stepwise drying at 21 °C/93% → 55% RH, or drying at 21 °C/55% RH, 21 °C/0% RH, 50 °C or 105 °C to induce microcracks <100 μm wide. Results show that the presence of microcracks causes cumulative water absorption to scale non-linearly with t. The observed relationship is approximately sigmoidal/S-shaped, with the position of inflection point related to microcracking and the degree of non-linearity increasing with drying severity. A simple fluorescence imaging method was developed to enable continuous monitoring of the advancing wetting front and to study the effect of microcracks. Quantitative image analysis of water penetration produced results consistent with gravimetric measurements.

Design safe distance between a buried pipe and a soil slope

Leakage from pipes buried in a soil slope can lead to failure and scour of the slope. Placing the pipe at a safe distance from the slope crest is a key concern in design. The objectives of the work reported here were to investigate the wetting front advancement in a soil slope due to leakage from buried pipes at variable distances from the slope crest considering different soil densities and boundary conditions of the slope, and to recommend a safe separation distance based on the relation between the factor of safety of the slope and the separation distance. Stability charts for slopes were established for different separation distances and infiltration times, which will serve as a guide to determining a safe separation distance. The safe distance depends on soil permeability, slope geometry and infiltration time. A safe distance, regardless of how long the leakage is expected to last, is recommended as twice the slope height for soil slopes 15 m in height and 35° in inclination compacted to 95% relative compaction under impermeable side and bottom boundary conditions. When a pipe is laid within this distance, the ability to detect and repair pipe leakage in a timely manner becomes an essential means of maintaining slope stability.

Infiltration on sloping terrain and its role on runoff generation and slope stability

A modified Green-and-Ampt model is formulated to quantify infiltration on sloping terrain underlain by homogeneous soil wetted by surficial water application. This paper’s theory for quantifying infiltration relies on the mathematical statement of the coupled partial differential equations (pdes) governing infiltration and runoff. These pdes are solved by employing an explicit finite-difference numerical method that yields the infiltration, the infiltration rate, the depth to the wetting front, the rate of runoff, and the depth of runoff everywhere on the slope during external wetting. Data inputs consist of a water application rate or the rainfall hyetograph of a storm of arbitrary duration, soil hydraulic characteristics and antecedent moisture, and the slope’s hydraulic and geometric characteristics. The presented theory predicts the effect an advancing wetting front has on slope stability with respect to translational sliding. This paper’s theory also develops the 1D pde governing suspended sediment transport and slope degradation caused by runoff influenced by infiltration. Three examples illustrate the application of the developed theory to calculate infiltration and runoff on a slope and their role on the stability of cohesive and cohesionless soils forming sloping terrain.

Water Movement and Finger Flow Characterization in Homogeneous Water‐Repellent Soils

Core Ideas The cumulative infiltration decreased with increasing soil water repellency. Soil water movement in higher water‐repellent levels tended to be more unstable. Finger flow occurred preferentially in the coarser, more water‐repellent soils. Strong correlations of cumulative infiltration and cumulative wetting area were shown. Soil water repellency (SWR) is a widespread property in soils around the world. It influences soil water movement and causes unstable flow. The mechanics of finger flow occurrence in water‐repellent soils are not clearly understood. Slab chamber infiltration experiments consisting of increasing SWR persistence in clay and sandy loam soils were conducted to compare wetting front advancement and mechanics of finger flow development. The temporal variation curves of cumulative infiltration (CI) decreased with an increase in SWR persistence. For wettable soils, the wetting front advanced regularly with time, but for water‐repellent soils, it became unstable and finger flow occurred as the SWR persistence increased. Water movement in soils with higher SWR persistence tended to be more unstable. Water repellency contributed to finger flow development, especially for sandy loam. There were strong correlations among CI, finger length, and the cumulative wetting area. In the strongly, severely, and extremely water‐repellent sandy loam soils, the power function relationship fit better than the linear function. The average soil water content decreased with a higher SWR persistence, which meant that less water was available in the profile. Finger flow development was related to the more severe water‐repellent conditions and tended to be more easily formed in the water‐repellent sandy loam soils than the clay loam soils due to the faster infiltration rate.

Numerical Analysis of Groundwater Ridging Processes Considering Water-Air Flow in a Hillslope.

In this study, a water-air two-phase flow model was employed to investigate the formation, extension, and dissipation of groundwater ridging induced by recharge events in a hypothetical hillslope-riparian zone, considering interactions between the liquid and gas phases in soil voids. The simulation results show that, after a rain begins, the groundwater table near the stream is elevated instantaneously and significantly, thereby generating a pressure gradient driving water toward both the stream (the discharge of groundwater to the stream) and upslope (the extension of groundwater ridging into upslope). Meanwhile, the airflow upslope triggered by the advancing wetting front moves downward gradually. Therefore, the extension of groundwater ridging into upslope and the downward airflow interact within a certain region. After the rain stops, groundwater ridging near the stream declines quickly while the airflow in the lower part of upslope is still moving into the hillslope. Thus, the airflow upslope mitigates the dissipation of groundwater ridging. Additionally, the development of groundwater ridging under different conditions, including rain intensity, intrinsic permeability, capillary fringe height, and initial groundwater table, was analyzed. Changes in intrinsic permeability affect the magnitude of groundwater ridging near the stream, as well as the downward speed of airflow, thereby generating highly complex responses. The capillary fringe is not a controlling factor but an influence factor on the formation of groundwater ridging, which is mainly related to the antecedent moisture. It was demonstrated that groundwater ridging also occurs where an unsaturated zone occurs above the capillary fringe with a subsurface lateral flow.

Investigation on rainwater infiltration into layered shallow covers in pyroclastic soils and its effect on slope stability

In recent years, a number of flowslides and debris flows triggered by rainfall affected a wide mountainous area surrounding the Piana Campana (Southern Italy). The involved slopes are covered with unsaturated air-fall layered pyroclastic deposits, which stability is guaranteed by the contribution of suction to shear strength. To understand the mechanisms of infiltration and to assess the soil suction distribution in such layered deposits, infiltration experiments have been carried out in small-scale slopes. The interpretation of the results has been made with the aid of a 2D mathematical model.The results highlight that the presence of coarse-textured unsaturated pumiceous layers, interbedded between finer ashy layers, can delay the wetting front advancement, thus initially confining the infiltration process within the uppermost finer layer. A diversion of the flow from the vertical towards the slope direction occurs when the soil approaches saturation. However, whereas a high hydraulic gradient establishes across pumices, water infiltration into the deepest layer begins, and part of the water crosses the pumices, before the complete saturation of the uppermost soil profile.

Effects of Airflow Induced by Rainfall Infiltration on Unsaturated Soil Slope Stability

The aim of this study was to investigate the characteristics of airflow induced by rainfall infiltration and their effects on unsaturated soil slope stability using numerical methods. The TOUGH2/EOS3 simulator was used to analyze the features of water–air two-phase flow in a soil slope under rain conditions. The results show that when the rain infiltrates the soil, the air is pushed into the soil by the advancing wetting front, the pore-air pressure in the unsaturated zone increases firstly, then the airflow is reversed, and the pore-air pressure decreases until it is close to atmospheric pressure. The features of water single-phase flow (i.e., the gas phase is neglected) were also simulated using the TOUGH2/EOS9 simulator. When the infiltration of rainwater into the soil slope in the water single-phase flow model is set as same as that in the water–air two-phase flow model, the water saturation and the capillary pressure distributions in the unsaturated zone are probably the same. Slope stability studies were then performed based on the simulated water–air two-phase and water single-phase seepage conditions, respectively. The slope stability analysis on a given slip surface based on the simulated water–air two-phase seepage condition shows that the safety factor decreases during rainfall, reaches the lowest value when rain stops, generates immediately a sudden rise, and then increases slowly to a steady value. The capillary pressure is beneficial to slope stability, while the pore-air pressure is unfavorable to slope stability. The variation tendency of the safety factor is similar to that of the negative total air pressure acting on the slip surface. The slope stability analysis on the given slip surface based on the water single-phase seepage condition shows that the safety factor is approximate to FS1 (is the safety factor for the case that does not take into account the air pressure based on the water–air two-phase seepage condition). Therefore, difference between the safety factor and FS1 based on the water–air two-phase seepage condition can approximately represent the effects of airflow induced by rainfall on unsaturated soil slope stability under the condition that the infiltration of rainwater into the soil slope remains unchanged.

GIS-based three-dimensional slope stability analysis considering rainfall infiltration

Rainfall-induced landslides are a major cause of slope failure in mountainous areas. As rainfall begins to infiltrate a slope the wetting front advances into the soil and reduces its shear strength. Slope failures occur when the reduced shear strength becomes less than the resisting shear strength needed for equilibrium. These areas of instability are usually located near the ground surface where pore-water pressure changes rapidly during infiltration. The wetting front depth in a slope plays an important role in slope stability. In this study a well-known infiltration model, the Green and Ampt model, is integrated into three GIS-based three-dimensional limit equilibrium methods to assess the impact of rainfall on slope stability. This infiltration model can predict the depth of the wetting front during steady and unsteady rainfall. The applied three-dimensional methods are modified according to different positions of the wetting front to reflect the influence of rainfall on slope stability. This approach is capable of calculating safety factors corresponding to individual rainfall events and is also capable of predicting the corresponding failure time. The accuracy of the presented study has been verified by simulating the failure process of a real landslide triggered by a rainstorm.