Multistep Outflow Research Articles

A framework for estimating soil water characteristic curve and hydraulic conductivity function of permeable reactive media

The unsaturated behavior of permeable reactive barriers (PRB) is a critical component in predicting the removal efficiency through the adsorption of contaminants. This study investigates the framework to estimate the soil water characteristic curve (SWCC) and hydraulic conductivity function (HCF) for iron oxide-coated sand (IOCS) and zeolite, which are common materials used in PRBs. A multistep outflow (MSO) experiment was performed and the results of the MSO experiment were used to optimize associated parameters in Kosugi's SWCC and HCF. In addition, three scenarios of optimization analysis were investigated to evaluate the best-fitting model for estimating SWCC and HCF. The low root mean square error (RMSE) of fitted parameters indicates the Kosugi model well described the observed suction profiles in MSO experiments. In addition, the lowest RMSE and coefficient of variation suggested the inclusion of the additional parameter β provided the best estimation of the three materials (clean sand, IOCS, and zeolite). The physically reasonable estimation of SWCC and HCF of the three materials from the optimized parameters suggests the proposed framework is a reasonable model for the unsaturated behavior of PRBs.

Relationship between hydraulic properties and material features in a heterogeneous vadose zone of a vulnerable limestone aquifer

AbstractLimestone aquifers constitute important drinking water resources but are vulnerable to groundwater contaminations. They are also known to be often heterogeneous. However, the hydraulic properties of their entire vadose zone and the relationship of these hydraulic properties with their mineralogical and geochemical characteristics remain rarely studied. The hydraulic properties of soft (soil, powdery limestone, and calcareous sand) and hard (limestone rock) materials sampled throughout the entire vadose zone profile of the Beauce limestone aquifer (France) were determined with the multistep outflow method applied using a triaxial system. Physical, mineralogical, and geochemical analysis brought valuable information about factors contributing to the heterogeneity of the hydraulic properties and helped understanding water flow pathways within the vadose zone. The hydraulic properties of the soft materials were strongly related to their physical properties but also to the proportion and the nature of clay minerals. The massive rock presented a few thin microfissures where calcite was replaced by phyllosilicates, thus increasing its water retention capacity. The low hydraulic conductivity of the massive rock could explain the occurrence of perched water tables in the vadose zone. The weathered rock displayed fissures and vugs and was also characterized by the presence of phyllosilicates. The development of a secondary porosity and footprints of water–rock interactions and mass transfers highlighted the transformation of the limestone rock into a more permeable material. This study also pointed out the need to characterize the impact of natural fractures observed at the core scale on the preferential water flow at the field scale.

Variability of topsoil hydraulic conductivity along the hillslope transects delineated in four areas strongly affected by soil erosion

Abstract Soil hydraulic conductivities of topsoils were studied at 5 points of the hillslope transects delineated at 4 geomorphologically diverse areas, where the original soil types (Chernozem, Luvisol and two Cambisols) were due to erosion transformed into different soil unites. Hydraulic conductivities of saturated soils and for a pressure head of –2 cm were measured directly in the field using a Guelph permeameter (K s,GP ) and mini disk tension infiltrometer (K h=– 2 ,MDI ), and in the laboratory using a multistep outflow method (K s,MSO , K h= – 2 ,MSO ). While K s,GP ≈ K s,MSO in the Chernozem and Cambisol (sandy loam) regions, and K s,GP < K s,MSO in the Luvisol and Cambisol (loam) regions. The K s values obtained using different methods showed different trends along the hillslope transects. The K h= – 2 values obtained using different methods showed similar trends along the transects in the Chernozem and Luvisol regions. These trends could be explained by the position within the transects (i.e., different stages of erosion/accumulation processes). No relationships were found between the K h=– 2 values in the Cambisol regions. The pressure head at an inflection point of the a soil-water retention curve was the main parameter, which appeared to associate (negative correlation) with K h=– 2 and K s,MSO in the Chernozem and Luvisol regions.

Pinhole Multistep Centrifuge Outflow Method for Estimating Unsaturated Hydraulic Properties with Small Volume Soil Samples

If soil hydraulic conductivity or water holding capacity could be measured with a small volume of samples, it would benefit international fields where researchers can only carry a limited amount of soils out of particular regions. We performed a pinhole multistep centrifuge outflow method on three types of soil, which included granite decomposed soil (Masa soil), volcanic ash soil (Andisol soil), and alluvial clayey soil (paddy soil). The experiment was conducted using 2 mL and 15 mL centrifuge tubes in which pinholes were created on the top and bottom for air intrusion and outflow, respectively. Water content was measured at 5, 15, and 30 min after applying the centrifuge to examine the equilibrium time. The results showed that pinhole drainage worked well for outflow, and 15 or 30 min was sufficient to obtain data for each step. Compared with equilibrium data, the retention curve was successfully optimized. Although the curve shape was similar, unsaturated hydraulic conductivities deviated largely, which implied that Ks caused convergence issues. When Ks was set as a measured constant, the unsaturated hydraulic properties converged well and gave excellent results. This method can provide soil hydraulic properties of regions where soil sampling is limited and lacks soil data.

Inverse modeling towards parameter estimation of the nonlinear soil hydraulic functions using developed multistep outflow procedure

Modeling and monitoring vadose zone processes have a key role to effectively identify and solve important emerging issues in hydrology and related disciplines. However, major gaps and challenges remain concerning the further conceptualization of the zone complexities and parameterization of relevant models. Consequently, the main focus of this investigation was to establish a modern developed version of the multistep outflow experiments for the vadose zone characterization and modeling. Besides, soil hydraulic properties (K-θ-h) were determined by the Mualem/Van Genuchten model based on the inverse solution of the Richards equation for the four loamy textured soils. The adjustable parameters were estimated with the least-square approach through the implementation of the generalized reduced gradient (GRG) algorithm. Then, the inverse manner was evaluated by comparing optimized hydraulic functions with those measured independently using an equilibrium condition. The results revealed the optimized attributes complied well with those determined using equilibrium analysis and steady-state situations. The reliable estimation of the parameters to inversely optimize the K-θ-h functions was possible only by measuring the cumulative outflow as a function of time. The simultaneous optimization of the unsaturated hydraulic functions carried out from a single transient experiment. This protocol appears well suited for vadose zone simulation. Furthermore, along with the tensiometer elimination and no independent measurements of Ks and θs, the developed version is generally more cost-effective, faster and more convenient than the conventional method. Finally, we conclude the combination of proposed procedure with GRG algorithm can provide a reasonable procedure to inversely estimate the hydraulic parameters.

Simulation of vadose zone flow processes via inverse modeling of modified multistep outflow for fine‐grained soils

AbstractCharacterization and measurement of the hydraulic properties of unsaturated porous media is still a challenge in natural environments, although exact knowledge of the soil's hydraulic properties [unsaturated soil hydraulic conductivity (K), volumetric water content (θ), soil matric head (h)] is crucial for solving many soil, hydrological, and environmental issues. The main purpose of this study was to establish a modified multistep outflow technique that facilitates laboratory operations and reduces time and costs. We also focused on inverse parameter optimization of the Mualem–van Genuchten (MVG) model with laboratory‐provided data for three fine‐grained soils. This modified version was assessed by inverse modeling of the MVG model and the generalized reduced gradient algorithm. The results represent the best estimate of the adjusted parameters and the very low uncertainty associated with the fitted values. The soil hydraulic functions were simulated well up to pressures of 1,500 kPa and their uncertainties were extremely low. The R2 values were 88 to 97 and 81 to 96% for the soil water characteristics and the unsaturated hydraulic conductivity functions, respectively. The simultaneous optimization of the nonlinear hydraulic functions from a single transient flow experiment agreed well with their observed data for each soil examined. Besides, by eliminating the use of tensiometers and having no independent measurements of saturated soil hydraulic conductivity and saturated volumetric water content, the modified version was faster and easier than the conventional method. The proposed protocol appears to be suited for the vadose zone flow simulation of fine‐grained soils.

Effect of Changing Unit Weight and Decomposition on Unsaturated Hydraulics of Municipal Solid Waste in Bioreactor Landfills

The effects of compression and decomposition on the water retention curve (WRC) and unsaturated hydraulic conductivity (Kθ) of municipal solid waste (MSW) were evaluated in the laboratory and field. Laboratory measurements of the WRC and Kθ were made using the multistep outflow (MSO) method. Numerical inversion of data collected during the Deer Track Bioreactor Experiment (DTBE) was used to estimate field-scale unsaturated hydraulic properties. For the MSW tested in this study, data indicated that increasing dry unit weight resulted in an increase of the air-entry suction. Increasing levels of decomposition had the opposite effect, reducing air-entry suction for MSW at constant dry unit weight. Multiple mechanisms likely resulted in the observed changes in air-entry suction, including reduction in pore size during compression and changes in overall MSW hydrophobicity during decomposition. At field-scale, air-entry suction tended to remain constant as MSW compressed and decomposed simultaneously indicating that the two processes, which had opposite effects in the laboratory, also may have opposing effects the field. For fresh MSW, van Genuchten’s n, which is inversely related to the variance of the pore size of a porous medium, ranged between 1.3 and 2.5 and decreased as the dry unit weight increased in laboratory-scale tests. For decomposed MSW, n ranged from 1.1 to 1.3 and did not vary systematically with dry unit weight. This indicated that for degraded MSW, the variance of the pore-size distribution did not change substantially with increasing dry unit weight. The value of Kθ decreased with increasing dry unit weight for MSW in this study, consistent with observations for saturated hydraulic conductivity in MSW. However, Kθ tended to increase in laboratory tests with increasing levels of decomposition at constant dry unit weight, consistent with observed increases in void ratio. At field scale, observed changes in Kθ varied consistently with changes in saturated hydraulic conductivity as decomposition and densification occurred simultaneously.

Determination of soil-water retention curve from a transient air-water two-phase outflow experiment

Knowledge of soil-water retention curve (SWRC) is essential in modeling water and air movement in soils. A new method based on air-water two-phase flow inverse solution was proposed to rapidly determine the SWRC from a transient outflow experiment. The experiment was designed to produce coupled air and water flow in soil pores, and the outflow evolution was measured during the transient processes. Air-water two-phase flow simulations combined with grid search parameter estimation algorithm were developed to determine the optimal parameters in the van Genuchten model by minimizing the difference between predicted and measured outflow data. The proposed method was validated against the transient outflow experiment and a multistep outflow test reported in the literature. The results showed that the cumulative outflow predicted from the optimal parameters matched well with the measured data. The good agreements concluded that the proposed method was reliable and feasible to determine the SWRC for unsaturated soils.

Error correction of water evaporation loss on the unsaturated hydraulic conductivity

Water evaporation loss occurs during multistep outflow experiments from the soil surface and the outflow vessel. The main focus of this paper is to introduce an improved pressure plate instrument that can correct for the error of water evaporation loss on the unsaturated hydraulic conductivity. Experiments using silicon micropowder (SMP) and Guangxi Guiping clay (GGC) were conducted with an improved instrument. The results showed that the measured value of the water content will be overestimated and the unsaturated hydraulic conductivity will be underestimated without considering the water evaporation loss.

Multi-step outflow and evaporation experiments – Gaining large undisturbed samples and comparison of the two methods

The aim of the study was the comparison of the soil hydraulic properties determined based on two different experimental methods of four large and macroporous soil samples. An undisturbed cylindrical soil column (h = 60 cm and d = 30 cm) was obtained at a natural slope and quartered horizontally in the laboratory. The four sub-samples were saturated and subsequently drained using the extended multi-step outflow method (XMSO). Retention curves and unsaturated hydraulic conductivity curves were derived via inverse modeling based on the measured drainage and corresponding pore water pressure. After resaturation the evaporation method (EVA) was used on the same four samples. The samples were exposed to evaporation and the effective water contents and the average pore water pressures were recorded over time to determine soil hydraulic functions.While both methods yielded similar results in the medium moisture range, the advantage of the XMSO-method is its higher information content with respect to the hydraulic conductivity close to water saturation whereas the advantage of the EVA-method is its fast execution. Moreover, the EVA-method is easier to handle and has significantly lower computational requirements than the XMSO-method.Furthermore, since the four soil samples represent a soil profile in the field, a depth dependent characterizing of the soil profile was possible and showed that the soil properties near saturation are greatly dominated by the ratio of macropores in the sample and that a decrease in the macropore ratio with soil depth was clearly reflected in the hydraulic functions.

Effect of hysteresis on hydraulic properties of soils under multiple drying and wetting cycles

The importance of the hydraulic properties of unsaturated soil for geotechnical designs has been well understood. Many studies on the soil water retention curve (SWRC) and the relative coefficient of permeability under drying and wetting cycles have been performed. However, most of the previous investigations of hydraulic properties are limited to a single drying and wetting cycle or are based on multiple drying–wetting cycles with limited soil experiments. This might be because the measurement of both the SWRC and the coefficient of permeability of unsaturated soil is complex and time consuming. In this study, the hydraulic properties of silicon micropowder (SMP) and Guangxi Guiping clay (GGC) were studied for the same soil specimen under five cycles of drying and wetting in multistep outflow experiments. Significant hysteresis of SWRCs was observed under the first drying and wetting cycle for both soils. However, the hysteresis effect decreased as the number of drying and wetting cycles increased. Furthermore, the coefficient of permeability was estimated using the van Genuchten–Mualem (VGM) model. Results also show that hysteresis plays an important role in the coefficient of permeability for both soils subjected to multiple drying and wetting cycles.

A Method for Directly Measuring the Hydraulic Conductivity of Unsaturated Soil

Abstract Numerous methods, such as the Gardner outflow analysis method and the van Genuchten-Mualem prediction model, have been developed to estimate the hydraulic conductivity of unsaturated soil in multistep outflow experiments. In the Gardner outflow analysis method, the hydraulic conductivity is estimated based on the diffusivity calculated from the line fitted to the water outflow. In contrast, in the van Genuchten-Mualem prediction model, it is predicted via the curve-fitting parameters based on the measured soil water retention curve (SWRC). However, a major drawback of these two approaches is that variation of hydraulic conductivity during each step of the outflow process cannot be represented. In this paper, an improved pressure plate instrument with an evaporation compensation system is developed to measure the transient water content curve (TWCC) of the soil in multi-step outflow experiments. The SWRCs of silicon micro-powder (SMP) and Guangxi Guiping clay (GGC) are derived from this transient curve. Therefore, a direct measurement method is presented to describe the change of hydraulic conductivity during each step. Comparison with the Gardner outflow analysis method and the van Genuchten-Mualem prediction model indicates that our approach shows better agreement with test data for these two types of soils.

Unsaturated hydraulic behaviour of a permeable pavement: Laboratory investigation and numerical analysis by using the HYDRUS-2D model

An adequate hydrological description of water flow in permeable pavement systems relies heavily on the knowledge of the unsaturated hydraulic properties of the construction materials. Although several modeling tools and many laboratory methods already exist in the literature to determine the hydraulic properties of soils, the importance of an accurate materials hydraulic description of the permeable pavement system, is increasingly recognized in the fields of urban hydrology. Thus, the aim of this study is to propose techniques/procedures on how to interpret water flow through the construction system using the HYDRUS model. The overall analysis includes experimental and mathematical procedures for model calibration and validation to assess the suitability of the HYDRUS-2D model to interpret the hydraulic behaviour of a lab-scale permeable pavement system. The system consists of three porous materials: a wear layer of porous concrete blocks, a bedding layers of fine gravel, and a sub-base layer of coarse gravel. The water regime in this system, i.e. outflow at the bottom and water contents in the middle of the bedding layer, was monitored during ten irrigation events of various durations and intensities. The hydraulic properties of porous concrete blocks and fine gravel described by the van Genuchten functions were measured using the clay tank and the multistep outflow experiments, respectively. Coarse gravel properties were set at literature values. In addition, some of the parameters (Ks of the concrete blocks layer, and α, n and Ks of the bedding layer) were optimized with the HYDRUS-2D model from water fluxes and soil water contents measured during irrigation events. The measured and modeled hydrographs were compared using the Nash-Sutcliffe efficiency (NSE) index (varied between 0.95 and 0.99) while the coefficient of determination R2 was used to assess the measured water content versus the modelled water content in the bedding layer (R2=0.81÷0.87). The parameters were validated using the remaining sets of measurements resulting in NSE values greater than 0.90 (0.91÷0.99) and R2 between 0.63 and 0.91. Results have confirmed the applicability of HYDRUS-2D to describe correctly the hydraulic behaviour of the lab-scale system.

Effects of Bentonite, Hydrogel and Biochar Amendments on Soil Hydraulic Properties from Saturation to Oven Dryness

Effects of hydrogel, bentonite, and biochar as soil amendments on soil hydraulic properties and improving water availability from saturation to oven dryness were investigated. Soils were mixed with hydrogel (0.10%, 0.25%, and 0.50%), bentonite (0.5%, 1.0%, and 2.5%), and biochar (1.0%, 2.5%, and 5.0%) as soil amendments (weight:weight). Three methods (extended multistep outflow (XMSO), evaporation (EVA), and WP4 dewpoint potentiometer) were used to measure soil hydraulic properties from saturation to oven dryness. The cumulative XMSO results were more uniform across all the applied pressure steps for the amended soils. The EVA exhibited a shorter linear decrease during the first evaporation stage and a lower evaporation rate during the second evaporation stage. The WP4 results also exhibited that soil amendments increased the soil water content of the amended soils at low matric potentials. The results of soil water retention curves revealed that the unamended soil retained less water at any matric potential compared to the amended soils. Soil hydraulic conductivity decreased with increasing amount of soil amendments. The saturated hydraulic conductivity was higher for the unamended soil than the soils amended with 2.5% bentonite, 0.50% hydrogel, and 5.0% biochar by 11, 3, and 18 times, respectively. These results suggested that soil amendments improved soil water retentivity, which confirmed the appropriateness of these soil amendments for potential use in sandy soil improvements. However, field experiments and economical perception studies should be considered for further investigation.

The impact of the permanent grass cover or conventional tillage on hydraulic properties of Haplic Cambisol developed on paragneiss substrate

AbstractThis study is focused on the comparison of soil structure and soil hydraulic properties of a Haplic Cambisol on paragneiss under two different land managements. Soil samples were taken from all diagnostic horizons (A, Bw and C) of the soil profile under the permanent grass cover (grassland) and under the conventional tillage (arable land). Basic soil properties were measured. Aggregate stability was assessed using the WSA index. Soil composition was evaluated using micromorphological images. Tension disk infiltrometers with two diameters of 2.22 and 10.25 cm (and applied pressure head of −2 cm) and Guelph permeameter were used to measure unsaturated and saturated hydraulic conductivities, respectively. Soil hydraulic properties were measured in the laboratory using the multistep outflow experiment, which was performed on the undisturbed 100 cm

Determination of Hydraulic Properties of Unsaturated Soils Based on Nonequilibrium Multistep Outflow Experiments

AbstractIn this paper, a new procedure for rapidly determining the hydraulic properties of unsaturated soils is developed. The proposed procedure is used to characterize the evolution of the instant degree of saturation in an unsaturated soil during a multistep outflow experiment. By introducing the concept of capillary relaxation, a theoretical model is developed and adopted to describe the dynamic hydraulic properties of unsaturated soils under constant-volume conditions. A series of nonequilibrium multistep outflow experiments are performed on various types of unsaturated soils. It is shown that the evolution of the instant degree of saturation or the outflow mass generally follows an exponential law. Both experimental results and theoretical simulations illustrate that flow instability and water blockage can occur in the soil during multistep outflow experiments. The theoretical predictions are compared to the experimental results, showing that the proposed procedure can be effectively used to determi...

Effect of the soil water dynamics on nitrous oxide emissions

Microbial N2O production may occur via denitrification, a reaction which is influenced by the soil's water content. This study aimed to test the effect of soil water dynamics on N2O production and transport. The treatments consisted of two levels of soil bulk density (BD), negative pressure head applied on the soil cylinder (H), and saturated hydraulic conductivity of the ceramic plate placed at the bottom of the soil sample (K). We controlled the water status of repacked soil samples during two wetting-drying cycles, by using a multistep outflow system. The matric potential, outflow, N2O, and CO2 fluxes were recorded over time. A brief N2O peak occurred at the beginning of soil drying: N2O produced and entrapped in the soil during the wetting phase was released during soil drying with the increase in soil gas diffusion. Similar peaks dynamics were observed for CO2, implying that a physical process was involved. A relationship was observed to occur with maximum N2O fluxes increasing exponentially with cumulative drainage. Indeed, during drying, high N2O fluxes were measured when the air-entry potential was reached, i.e., when gas pathways were available for fast N2O transport in the gas phase. Then, maximum and cumulative N2O fluxes were highest for low BD and fast water flow during drying. Samples with the highest BD had smaller pore sizes, leading to low outflows at a given negative pressure head, and giving more time for further reduction of N2O to N2. We ranked the importance of the parameters controlling cumulative N2O fluxes: H>BD>K.

A comparison of two Bayesian approaches for uncertainty quantification

Statistical calibration of model parameters conditioned on observations is performed in a Bayesian framework by evaluating the joint posterior probability density function (pdf) of the parameters. The posterior pdf is very often inferred by sampling the parameters with Markov Chain Monte Carlo (MCMC) algorithms. Recently, an alternative technique to calculate the so-called Maximal Conditional Posterior Distribution (MCPD) appeared. This technique infers the individual probability distribution of a given parameter under the condition that the other parameters of the model are optimal. Whereas the MCMC approach samples probable draws of the parameters, the MCPD samples the most probable draws when one of the parameters is set at various prescribed values. In this study, the results of a user-friendly MCMC sampler called DREAM(ZS) and those of the MCPD sampler are compared. The differences between the two approaches are highlighted before running a comparison inferring two analytical distributions with collinearity and multimodality. Then, the performances of both samplers are compared on an artificial multistep outflow experiment from which the soil hydraulic parameters are inferred. The results show that parameter and predictive uncertainties can be accurately assessed with both the MCMC and MCPD approaches.

Using X‐ray Computed Tomography to Describe the Dynamics of Nitrous Oxide Emissions during Soil Drying

Water in soil is known to be a key factor for controlling N2O emissions because N2O is mainly produced by denitrification in anoxic environments. In this study, we proposed a methodology to image the water and soil structure of a soil sample with X‐ray computed tomography while controlling the hydric state and monitoring N2O fluxes. We used a multistep outflow system to apply two wetting–drying cycles to an undisturbed soil. The soil core was scanned with coarse‐resolution X‐ray computed tomography, one time during wetting and several times during drying, to measure quantitative and qualitative indicators of the pore network. Nitrous oxide emissions were higher during the first (C1) than during the second (C2) wetting–drying cycle for both the wetting and the drying phases. Fluxes increased quickly after the beginning of the drying phase to reach a peak after 5 h. Differences in the intensity of N2O emissions between the two cycles were attributed to differences in the water saturation, air‐phase connectivity, and relative gas diffusion coefficient, which led to more or less N2O production, consumption, and entrapment in the soil. The speed of the N2O emissions at the beginning of the drying phase depended on the rate of increase of the air‐filled pore volume and connectivity, and was especially well described by the estimated relative gas diffusion coefficient. Parameters of the soil structure were not able to explain completely the intensity of N2O emissions during drying; N2O production and consumption factors were also involved.

Modeling dynamic non-equilibrium water flow observations under various boundary conditions

Summary Experimental evidence of variably-saturated flow in soils often shows a non-equilibrium between water content and water potential. Such phenomena, summarized by the term “dynamic effects” often occur in laboratory experiments specifically designed for the identification of soil hydraulic properties which are a mandatory input for water flow simulations at the field scale using the Richards equation. Depending on boundary conditions, dynamic non-equilibrium leads either to a relaxation of pressure head for constant water content or vice versa. Diamantopoulos et al. (2012) presented a model which describes dynamic effects in the case of multistep outflow experiments by assuming that the total water content is portioned into two fractions: one fraction which is in instantaneous equilibrium with the pressure head, and a second fraction for which the equilibration of the water content with the pressure is described by a first-order kinetics which leads to a temporal shift in water content at constant pressure head. In this work we applied this non-equilibrium model to four different observations of dynamic non-equilibrium effects presented in the literature and demonstrate that it successfully describes (1) non-equilibrium in water absorption experiments which results in a non-uniqueness of the diffusivity vs water content function, (2) the flow-rate dependence of soil hydraulic properties, (3) dynamic effects in multistep outflow and (4) dynamic effects in multistep flux experiments. We conclude the article by a discussion of possible macroscopic explanations, the consideration of dynamic effects for modeling water flow at the field scale, the uniqueness of the estimated non-equilibrium parameters among different experiments, and further theoretical considerations concerning dynamic effects.