Soil Unsaturated Hydraulic Conductivity Research Articles

The use of hyperbolic and Asaoka’s methods to estimate the moisture diffusivity and the hydraulic conductivity of unsaturated soils from tests to determine the SWRC

The equation governing the unsaturated transient flow in a soil sample when subjected to suction (ψ) at its base and that of the classical 1-D consolidation are exactly alike (equation of diffusion). The former can be arrived at by Richards’ equation, being in this case the moisture diffusivity (D), instead of the coefficient of consolidation (cv), the governing parameter. D need not be constant, but rather a function of the volumetric water content, D=D(θ), and is defined as the ratio of the hydraulic conductivity, k(θ), over the specific water capacity, C(θ)=dθ/dψ, i.e., the slope of the SWRC. The hyperbolic method has been used for several geotechnical purposes and, most importantly, as an alternative to Asaoka’s method for predicting the final settlement and cv of soft soils undergoing consolidation, improved by preloading. This paper shows that both methods prove to be very useful as a means of obtaining D(θ) and k(θ) at a certain range of θ, provided that a reduced number of water contents at known elapsed times are determined over the medium stage of this transient flow. It is addressed in the paper both by theoretical grounds and on the light of experimental data of 4 soils.

Wetted front advance under point-source field dripper system for estimation of unsaturated hydraulic conductivity function

Unsaturated soil hydraulic conductivity function is often required for designing a drip irrigation system. Point-source field dripper methods are available for quick measurement of saturated hydraulic conductivity (Ks) and scaling parameters of the soil. However, steady-state saturated front radii is difficult to differentiate compared to the wetted front radii, which is easy and precise. Field dripper methods were used for estimation of Ks and scale parameters using saturated front radii for four different soils viz ., cultivated tilled and no-tilled normal and sodic soils. There exists a good consistency among the values obtained by using saturated as well as wetted front radii. Use of wetted front radii could be useful for measurement of Ks and scale parameter of the soil with more accuracy.

A method for estimating soil water characteristic curve with limited experimental data

Because of the difficulty to reach high suction at the residual state of unsaturated soils, the saturation-suction experimental data for soil water characteristic curve (SWCC) is often limited and incomplete, which will lead to unreliable estimation of mechanical and hydraulic conductivity of unsaturated soils. However, no study has been published on how to obtain a reliable SWCC for unsaturated soils with limited experimental data. In response to this situation, an innovative method was proposed based on the analysis of shape characteristics of SWCC with three identified critical points in it. 78 experimental datasets from existing literature were compiled to identify the mathematical relationship among three critical points in the SWCC. The proposed method was evaluated by experimental datasets from this study and existing literature and proved to have a satisfying performance in predicting a complete SWCC for a wide range of soils, from the coarse-grained to the fine-grained. Overall, the research results present an effective and capable method for estimating a more reliable SWCC when experimental data is limited.

New pedotransfer function (“CRC”) for the prediction of unsaturated soil hydraulic conductivity using soil water retention data

New pedotransfer function (“CRC”) for the prediction of unsaturated soil hydraulic conductivity using soil water retention data

Estimation of Unsaturated Hydraulic Conductivity of Granular Soils from Particle Size Parameters

Estimation of unsaturated hydraulic conductivity could benefit many engineering or research problems such as water flow in the vadose zone, unsaturated seepage and capillary barriers for underground waste isolation. The unsaturated hydraulic conductivity of a soil is related to its saturated hydraulic conductivity value as well as its water retention behaviour. By following the first author’s previous work, the saturated hydraulic conductivity and water retention curve (WRC) of sandy soils can be estimated from their basic gradation parameters. In this paper, we further suggest the applicable range of the estimation method is for soils with d10 > 0.02mm and Cu < 20, in which d10 is the grain diameter corresponding to 10% passing and Cu is the coefficient of uniformity (Cu=d60d10). The estimation method is also modified to consider the porosity variation effect. Then the proposed method is applied to predict unsaturated hydraulic conductivity properties of different sandy soils and also compared with laboratory and field test results. The comparison shows that the newly developed estimation method, which predicts the relative permeability of unsaturated sands from basic grain size parameters and porosity, generally has a fair agreement with measured data. It also indicates that the air-entry value is mainly relative to the mean grain size and porosity value change from the intrinsic value. The rate of permeability decline with suction is mainly associated with grain size polydispersity.

The Scale Effect of Double-Ring Infiltration and Soil Infiltration Zoning in a Semi-Arid Steppe

The double-ring infiltrometer is widely used to measure soil unsaturated hydraulic conductivity in the field. The scale effect of the inner and outer ring size (especially the inner one) affects the measurement results. In the semi-arid steppe, where water is scarce and transportation is inadequate, studying the scale effect caused by the inner-ring diameter of the infiltrometer can reduce the test consumption on the premise of ensuring the test accuracy. In this paper, a total of 190 double-ring infiltration tests with different inner-ring diameters (15, 20, 25, 30, and 40 cm) and 0.33 times outer buffer index were carried out at 38 sites with five soil types in the Xilin river basin, China. Results showed that: (1) When comparing the simulated parameters of six infiltration models, parameters increased with the increase of the infiltrometer inner diameter, but the trend gradually slowed down, indicating that the increase of the infiltrometer inner diameter would weaken the influence of the infiltrometer scale effect. However, the infiltrometer with an inner diameter of 40 cm is not enough to completely overcome the scale effect. (2) Through principal component analysis, the infiltration process is mainly affected by the particle size and the initial moisture content. (3) The soil infiltration map based on infiltration tests was more practical than the soil type map, which can provide a theoretical basis for ecological and soil restoration in the future.

Development of an aid tool for evaluation of unsaturated soils permeability

PurposePrediction models for the unsaturated permeability proposed in the literature are numerous. However, a model may give a good result for a sample of a given soil when it may give a bad result for another sample belonging to the same type of soil. This showed that the choice of a model to complete the permeability curve in the unsaturated state is complex. To facilitate such studies, this paper aims to present a help system capable of defining the mathematical model to the user that best represents the permeability of the soil.Design/methodology/approachThe authors have detailed the difficulties in determining the correct value of kuns from a thorough bibliographic study. To develop this idea, the authors took real examples, to which they applied mathematical models and then compared their results with those of the bibliographic study. Knowledge structuring in the form of classes, rules and functions. Implementation of the data in generator of help system Kappa-pc. validation of results.FindingsAn aid tool was developed for the evaluation of unsaturated soils permeability using Brooks and Corey (1964) and Leong and Rahardjo (1997) models, which are known for their effectiveness and ease of application. This system will also evaluate these two methods using estimation models of saturated permeability [Dane and Pocket (1992), Terzaghi (1981) and laboratory data]. This system allows the evaluation of unsaturated permeability by the aforementioned two models, makes comparison between these two models, classifies them and proposes the model presenting the best result.Originality/valueThis aid system is able to compare results of different models of prediction of the hydraulic conductivity of unsaturated soils according to several criteria (suction, degree of saturation, plasticity index, models of estimation of the permeability to the soil, saturated state, particle size, etc.). It can also deduce the model that best adapts to a given soil. This aid system will be of great use for geotechnical engineers and researchers in the field.

Capillary pressure–saturation curves of thin hydrophilic fibrous layers: effects of overburden pressure, number of layers, and multiple imbibition–drainage cycles

Unsaturated fluid flow in thin porous media depends on hydraulic properties, such as the capillary pressure, P c, as a function of saturation, S. We measured this relationship for two different types of compressible thin hydrophilic fibrous layers under varying conditions. Among other factors, we changed the number of layers and the overburden pressure (i.e. the confined solid pressure applied on top of the sample) imposed on one layer or a stack of layers. Applying an overburden pressure drastically affected the [Formula: see text] curves. However, increasing the number of fibrous layers had little impact on the capillary pressure–saturation curves. We also investigated the effect of multiple imbibition–drainage cycles on the [Formula: see text] data. Measured data points were used to find general expressions for the [Formula: see text] relationships of compressible thin porous media. Existing quasi-empirical correlations used in vadose zone hydrology, notably expressions by van Genuchten (Van Genuchten MTh. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci Soc Am J 1980; 44: 892-898) and Durner (Durner W. Hydraulic conductivity estimation for soils with heterogeneous pore structure. Water Resour Res 1994; 32: 211–223) for single- and dual-porosity media, respectively, were employed to fit the measured data points.

Characteristics of an in-situ unsaturated hydraulic conductivity of soil

ABSTRACT A soil hydraulic conductivity (K) indicates the ability of a soil to conduct water and is often assumed to be in one-to-one correspondence with matric head (h) or volumetric water content (θ) for mathematically analyzing field soil water regimes. However, to what extent this assumption represents K in a field is rarely questioned, and how h-K and θ-K relations actually behave under natural field conditions is seldom investigated sufficiently. Therefore, this study aimed at clarifying actual behaviors of h-K and θ-K relations under natural field conditions (in-situ K), and evaluated the degree to which the in-situ K(h) diverted from its equivalent h-K relations that were measured through drainage processes starting from water-saturated conditions (primary-drying K(h)). The in-situ K values were obtained in the range of 4×10-8 cm s-1 < K < 8×10-6 cm s-1 with -55 cm < h < -5 cm during a four-years monitoring in a field. And the variation among the observed in-situ K values for a given h was one to two orders of magnitude, implying that the amount and pattern of water-conducting pores for a given h had changed frequently through years of wetting-drying cycles of the field soil, and that the application of a single-valued K(h) function to quantifying in-situ soil water flow can cause errors in these orders of magnitude. Also the in-situ K was 10 to 100 times lower than the primary-drying K in any h. The in-situ K fluctuated even for a given θ. Thus, it was suggested that neither an h-K relation nor a θ-K relation can sufficiently work for understanding a field soil water regime if it is regarded as merely a single-valued function. And more knowledge about the temporal variability in in-situ K is required for understanding soil water regimes in a field.

Approaches for estimating unsaturated soil hydraulic conductivities at various bulk densities with the extended Mualem-van Genuchten model

The Mualem-van Genuchten model has been widely used for estimating unsaturated soil hydraulic conductivity (Ku) from measured saturated hydraulic conductivity (Ks) and fitted water retention curve (WRC) parameters. Soil bulk density (ρb) variations affect the accuracy of Ku estimates. In this study, we extend the Mualem-van Genuchten model to account for the ρb effect with ρb-related WRC and Ks models. We apply two functions (A and B) that relate the van Genuchten WRC model to ρb and two models (1 and 2) that estimate Ks with various ρb. By combining the ρb-related WRC functions and Ks models, we develop four integrated approaches (i.e., A1, A2, B1, and B2) for estimating Ku at various ρb. Ku measurements made on five soils with various textures and ρb are used to evaluate the accuracy of the four approaches. The results show that all approaches produce reasonable Ku estimates, with average root mean square errors (RMSEs) less than 0.35 (expressed in dimensionless unit because logarithmic Ku values are used). Approach A2, with an average RMSE of 0.25, agrees better with Ku measurements than does Approach A1 that has an average RMSE of 0.28. This is because Model 2 accounts for the WRC shape effect near saturation. Approaches A1 and A2 give more accurate Ku estimates than do Approaches B1 and B2 which both have average RMSEs of 0.35, because Function A performs better in estimating WRCs than does Function B. The proposed approaches could be incorporated into simulation models for improved prediction of water, solute, and gas transport in soils.

Modelling the unsaturated hydraulic conductivity of a sandy loam soil using Gaussian process regression

Unsaturated soil hydraulic conductivity is a main parameter in agricultural and environmental studies, necessary for predicting and managing water and solute transport in soils. This parameter is difficult to measure in agricultural fields; thus, a simple and practical estimation method would be preferable, and quantitative methods (analytical and numerical) to predict the field parameters should be developed. Field experiments were conducted to collect water quality data to model the unsaturated hydraulic conductivity of a sandy loam soil. A mini disk infiltrometer (MDI) was used to measure soil infiltration rate. Input variables included electrical conductivity and the sodium adsorption ratio of irrigation water. Suction rate (pressure head), soil bulk density, and soil moisture content acted as inputs, with unsaturated soil hydraulic conductivity as output. The performance of Gaussian process regression (GPR) was analysed, with multiple linear regression (LR) and multi-layer perceptron (MLP) models used for comparison. Three performance criteria were compared: correlation coefficient (r), root mean square error (RMSE), and mean absolute error (MAE). The simulations employed the Waikato environment for knowledge analysis (WEKA) open source tool. The results indicate that the GPR with Pearson VII function-based universal kernel (PUK kernel), cache size 250007, Omega 1.0 and Sigma 1.0 performs better than other kernels when evaluating test split data, with a correlation coefficient of 0.9646. The RMSEs for GPR (PUK kernel), MLP, and LR were 1.16 × 10−04, 1.87 × 10−04, and 2.22 × 10−04 cm·s−1, respectively. Predictive data mining algorithms (DMA) enable an estimate of unknown values based on patterns in a database. Therefore, the present methodology can be put to use in predictive tools to manage water and solute transport in soils, as the GPR model provides much greater accuracy than the LR and MLP models in predicting the unsaturated hydraulic conductivity of a sandy loam soil.

A Fractal Model to Interpret Porosity‐Dependent Hydraulic Properties for Unsaturated Soils

This paper presents a simple fractal model to quantify the effects of initial porosity on the soil‐water retention curve and hydraulic conductivity of unsaturated soils. In the proposed conceptual model, the change of maximum pore radius, which largely determines the change of the air‐entry value, is directly related to the fractal dimension of pore volume (D) and porosity change. The hydraulic properties of unsaturated soils are then governed by the maximum pore radius, the fractal dimension of pore volume (D), and the fractal dimension of drainable pore volume (Dd ≤ D). The new fractal model removes the empirical fitting parameters that have no physical meaning from existing models for porosity‐dependent water retention and hydraulic behaviour and employs parameters of fractal dimensions that are intrinsic to the nature of the fractal porous materials. The proposed model is then validated against experimental data from the literature on soil‐water retention behaviour and unsaturated conductivity.

Determining in-situ unsaturated soil hydraulic conductivity at a fine depth scale with heat pulse and water potential sensors

Unsaturated hydraulic conductivity (K) of surface soil changes substantially with space and time, and it is of great importance for many ecological, agricultural, and hydrological applications. In general, K is measured in the laboratory, or more commonly, predicted using soil water retention curve and saturated hydraulic conductivity. In the field, K can be determined through infiltration experiments. However, none of these approaches are capable of continuously monitoring K in-situ at fine depth scales. In this study, we propose and investigate an approach to continuously estimate fine depth-scale K dynamics under field conditions. Evaporation rate and change in water storage in a near-surface soil layer are measured with the heat pulse method. Then, water flux density at the lower boundary of the soil layer is estimated from evaporation rate, change in water storage, and rainfall or irrigation rate using a simple water balance approach. Finally, K values at different soil depths are derived using the Buckingham-Darcy equation from water flux densities and measured water potential gradients. A field experiment is performed to evaluate the performance of the proposed approach. K values at 2-, 4-, 7.5-, and 12.5-cm depths are estimated with the new approach. The results show that in-situ K estimates vary with time following changes in soil water content, and the K-water content relationship changes with depth due to the difference in bulk density. In-situ estimated K-matric potential curves agree well with those measured in the laboratory. In-situ K estimates also show good agreement with the Mualem-van Genuchten model predictions, with an average root mean square error in log10 (K, mm h−1) of 0.54 and an average bias of 0.17. The new approach provides reasonable in-situ K estimates and has potential to reveal the influences of natural soil conditions on hydraulic properties as they change with depth and time.

Comparison of seven water retention functions used for modelling soil hydraulic conductivity due to film flow

AbstractThe unsaturated soil hydraulic conductivity accounting for film flow is important for understanding soil hydrological and biological processes, especially in arid and semi‐arid regions. Recently, a theoretically based hydraulic conductivity model was developed to describe the hydraulic conductivity as a function of water content. We have used this model to compare seven soil water retention functions commonly used for predicting soil hydraulic conductivity due to film flow. A total of 30 soils, varying in basic properties, were selected from the Unsaturated Soil Hydraulic Database to evaluate the seven functions. The Webb method was applied to identify the critical soil matric potential (hc) below which thin film flow controls water movement. Soil hydraulic conductivity measurements at matric potential below hc were then used for curve fitting according to the seven functions. Slight differences were observed among the functions in predicting soil hydraulic conductivity due to film flow. Six of the seven functions in combination with the hydraulic conductivity model described the hydraulic conductivity due to film flow well, according to the terms of the coefficient of efficiency. The relatively poor performance of the one exception was due to the fact that the linear shape of the function made it less flexible at low matric potentials. In addition, the effect of textural class on its performance was substantial, showing a poorer fit for the sand soil compared with the loam and clay soils. These findings have important applications related to soil and water resources conservation especially in arid and semi‐arid regions.

Improved evaporation method for the measurement of the hydraulic conductivity of unsaturated soil in the wet range

The evaporation method (EM) is an effective technique for measuring soil hydraulic properties. However, the EM generally fails to measure hydraulic conductivity curves (K–h; K and h are the hydraulic conductivity and the pressure head, respectively) in the wet (low suction) range when the hydraulic gradient within soil samples exceeds the minimum accuracy of the pressure transducer. In this study, we developed a new experimental device that uses the EM to perform conductivity measurements in the wet range. By making two major improvements, we successfully expanded the measurable range of K–h to cover the wet range, enabling accurate and effective measurements. Firstly, the accuracy of the hydraulic gradient measurement was remarkably improved by the use of a differential pressure transducer. Assuming the minimum evaporation speed (0.18 cm/day at the center of soil sample), the estimated measurable range of K–h is up to 27 cm/day for any type of soil. That is, our device has expanded the range of K–h measurable by the EM by more than two orders of magnitude on the wet end. Secondly, our device can execute the saturated/unsaturated steady-state infiltration method (SSIM/USIM) and the EM on the same sample. This diminishes not only time and cost but also errors between different methods, ensuring good accuracy and representativeness. The range of K–h measurable by the USIM was determined to be more than 21 cm/day, which overlaps with the range measurable by the EM. As a result, our device can obtain consecutive K–h curves from saturation to the medium pressure range (−500 < h ⩽ 0 cm) for any type of soil.

Prediction of unsaturated hydraulic conductivity using fuzzy logic and artificial neural network

Knowledge of hydraulic properties is necessary for hydrological studies, artificial recharge of the aquifer, watershed management and agriculture system. The major objective of this study was to develop a fuzzy logic and artificial neural network (ANN) based models for estimating the unsaturated hydraulic conductivity of soil (Ku). A mini disk infiltrometer, being handy used for determining infiltration characteristics. In this study mini Disk Infiltrometer (Decagon Devices, Inc.) at a suction head varying from 1 to 6 cm was used for determining Ku of sandy soil. All the measurements have been done on predetermined initial condition of different proportions of rice husk ash and fly ash mixed with sand. For modeling randomely selected 70% data was applied for training and residual 30% for the test. Comparison of results show that the prediction with ANN approach works well with correlation of coefficient value of 0.8662 (root mean square error 4.5607 cm/h).

Effect of hydraulic conductivity of unsaturated soil on the earth dam performance

In this paper, the finite element method is uzed to solve the governing equations of flow through earth dams. The computer program Geo-Slope is used in the analysis through its sub-program named SEEP/W. A case study is considered to be Al-Adhaim dam which consists of zoned embankment with a total length of 3.1 km. The dam in its actual design is analyzed. Then, an attempt is made to study the seepage in unsaturated zone of the dam through studying the effect of several parameters including the effect of changing the unsaturated hydraulic conductivity with the degree of saturation of the core soil and changing of water level in the reservoir. A procedure is proposed to define the hydraulic conductivity function from the soil water characteristic curve which is measured by the filter paper method. Fitting methods are applied through the program SoilVision. A parametric study was carried out and different parameters were changed to study their effects on the behavior of partially saturated soil. The study reveals that the rate of flow is decreased by about 20 - 27% when the degree of saturation of the core material is decreased from 100% to 50% at water level 115.75 m, while the exit gradient of flow is decreased by about 13 -15%. This decrease in flow rate becomes 13-15% and 8-9.5% when the reservoir water level is 131.5 m and 143.5 m, respectively, while the exit gradient of flow is increased by about 23-29.5% and 29-29.5% when the reservoir water level is 131.5 m and 143.5 m, respectively. When the state of soil changes from fully saturated S= 100% to partially saturated S= 90%, a rapid increase in head gradient and pore water takes place at the embankment base for different water levels in the reservoir. This decrease plateaus out on further decrease in the degree of saturation.

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.

Prediction of unsaturated hydraulic conductivity using adaptive neuro- fuzzy inference system (ANFIS)

This paper aims to predict the unsaturated hydraulic conductivity of soil using Adaptive Neuro- fuzzy inference system (ANFIS), Multi-Linear Regression (MLR), and artificial neural network (ANN). Laboratory experiments carried out on 46 samples of sand, rice husk ash and fly ash (FA) mixture. Out of 46 data-set for modeling of unsaturated hydraulic conductivity 32 random data used for training and remaining 14 to the test. The results suggest improved performance by Gaussian membership function than triangular and generalized bell-shaped membership-based ANFIS. MLR is better than ANN and Gaussian membership function-based ANFIS for unsaturated hydraulic conductivity.

Impacts of thinning of a Mediterranean oak forest on soil properties influencing water infiltration

AbstractIn Mediterranean ecosystems, special attention needs to be paid to forest–water relationships due to water scarcity. In this context, Adaptive Forest Management (AFM) has the objective to establish how forest resources have to be managed with regards to the efficient use of water, which needs maintaining healthy soil properties even after disturbance. The main objective of this investigation was to understand the effect of one of the AFM methods, namely forest thinning, on soil hydraulic properties. At this aim, soil hydraulic characterization was performed on two contiguous Mediterranean oak forest plots, one of them thinned to reduce the forest density from 861 to 414 tree per ha. Three years after the intervention, thinning had not affected soil water permeability of the studied plots. Both ponding and tension infiltration runs yielded not significantly different saturated,Ks, and unsaturated,K−20, hydraulic conductivity values at the thinned and control plots. Therefore, thinning had no an adverse effect on vertical water fluxes at the soil surface. MeanKsvalues estimated with the ponded ring infiltrometer were two orders of magnitude higher thanK−20values estimated with the minidisk infiltrometer, revealing probably soil structure with macropores and fractures. The input of hydrophobic organic matter, as a consequence of the addition of plant residues after the thinning treatment, resulted in slight differences in terms of both water drop penetration time, WDPT, and the index of water repellency,R, between thinned and control plots. Soil water repellency only affected unsaturated soil hydraulic conductivity measurements. Moreover,K−20values showed a negative correlation with both WDPT andR, whereasKsvalues did not, revealing that the soil hydrophobic behavior has no impact on saturated hydraulic conductivity.