Van Genuchten-Mualem Research Articles

Inverse estimation of soil hydraulic properties and water repellency following artificially induced drought stress

Abstract Global climate change is projected to continue and result in prolonged and more intense droughts, which can increase soil water repellency (SWR). To be able to estimate the consequences of SWR on vadose zone hydrology, it is important to determine soil hydraulic properties (SHP). Sequential modeling using HYDRUS (2D/3D) was performed on an experimental field site with artificially imposed drought scenarios (moderately M and severely S stressed) and a control plot. First, inverse modeling was performed for SHP estimation based on water and ethanol infiltration experimental data, followed by model validation on one selected irrigation event. Finally, hillslope modeling was performed to assess water balance for 2014. Results suggest that prolonged dry periods can increase soil water repellency. Inverse modeling was successfully performed for infiltrating liquids, water and ethanol, with R2 and model efficiency (E) values both > 0.9. SHP derived from the ethanol measurements showed large differences in van Genuchten-Mualem (VGM) parameters for the M and S plots compared to water infiltration experiments. SWR resulted in large saturated hydraulic conductivity (Ks ) decrease on the M and S scenarios. After validation of SHP on water content measurements during a selected irrigation event, one year simulations (2014) showed that water repellency increases surface runoff in non-structured soils at hillslopes.

Further tests of the HYPROP evaporation method for estimating the unsaturated soil hydraulic properties

Abstract Many soil, hydrologic and environmental applications require information about the unsaturated soil hydraulic properties. The evaporation method has long been used for estimating the drying branches of the soil hydraulic functions. An increasingly popular version of the evaporation method is the semi-automated HYPROP© measurement system (HMS) commercialized by Decagon Devices (Pullman, WA) and UMS AG (München, Germany). Several studies were previously carried out to test the HMS methodology by using the Richards equation and the van-Genuchten-Mualem (VG) or Kosugi-Mualem soil hydraulic functions to obtain synthetic data for use in the HMS analysis, and then to compare results against the original hydraulic properties. Using HYDRUS-1D, we carried out independent tests of the HYPROP system as applied to the VG functions for a broad range of soil textures. Our results closely agreed with previous findings. Accurate estimates were especially obtained for the soil water retention curve and its parameters, at least over the range of available retention measurements. We also successfully tested a dual-porosity soil, as well as an extremely coarse medium with a very high van Genuchten n value. The latter case gave excellent results for water retention, but failed for the hydraulic conductivity. In many cases, especially for soils with intermediate and high n values, an independent estimate of the saturated hydraulic conductivity should be obtained. Overall, the HMS methodology performed extremely well and as such constitutes a much-needed addition to current soil hydraulic measurement techniques.

Simulating vertical flow wetlands using filter media with different grain sizes with the HYDRUS Wetland Module

Abstract In this study, the simulation results of four vertical flow wetland systems using the HYDRUS Wetland Module are presented. The four wetland systems comprise three single-stage pilot scale systems and one full-scale two-stage system. The main difference between these systems is the filter media used, referred to as fine media, i.e., sand with a grain size distribution 0.063–4 mm, or coarse media, i.e., sand with grain size distributions between 1–4 mm, respectively. The water-flow simulation of each system is carried out using the single porosity van Genuchten-Mualem model. A good match between measured and simulated volumetric effluent flow rates could be achieved for all wetland systems. For reactive transport simulations, the CW2D biokinetic model was applied. First, simulations were run using the standard CW2D parameter set. For some systems, adjustments of the parameter set were needed in order to avoid unlimited bacteria growth. To better fit measured COD, NH4-N, and NO3-N effluent concentrations, adjustments of few parameters of the standard parameter set were required. The results show that for the VF wetlands with fine sand, no adjustments of the CW2D standard parameter set were needed, while for systems with coarser filter media as the main layer, the standard parameter set had to be adjusted to match simulated and measured effluent concentrations.

Inverse Modeling of Soil Hydraulic Parameters Based on a Hybrid of Vector-Evaluated Genetic Algorithm and Particle Swarm Optimization

The accurate estimation of soil hydraulic parameters (θs, α, n, and Ks) of the van Genuchten–Mualem model has attracted considerable attention. In this study, we proposed a new two-step inversion method, which first estimated the hydraulic parameter θs using objective function by the final water content, and subsequently estimated the soil hydraulic parameters α, n, and Ks, using a vector-evaluated genetic algorithm and particle swarm optimization (VEGA-PSO) method based on objective functions by cumulative infiltration and infiltration rate. The parameters were inversely estimated for four types of soils (sand, loam, silt, and clay) under an in silico experiment simulating the tension disc infiltration at three initial water content levels. The results indicated that the method is excellent and robust. Because the objective function had multilocal minima in a tiny range near the true values, inverse estimation of the hydraulic parameters was difficult; however, the estimated soil water retention curves and hydraulic conductivity curves were nearly identical to the true curves. In addition, the proposed method was able to estimate the hydraulic parameters accurately despite substantial measurement errors in initial water content, final water content, and cumulative infiltration, proving that the method was feasible and practical for field application.

A Simple and Improved Model for Describing Soil Hydraulic Properties from Saturation to Oven Dryness

Core Ideas Soil hydraulic properties from saturation to oven dryness are needed to simulate soil water movement. A new soil hydraulic model was compared with the classic van Genuchten–Mualem models. The new model with different fitted parameters was more efficient than the van Genuchten–Mualem models. Soil water retention characteristics and hydraulic conductivity from saturation to oven dryness are needed for simulating soil water movement and solute transport, especially in arid and semiarid regions. The aim of this study was to derive a new soil hydraulic model and compare this model with the classic van Genuchten–Mualem (VGM) models (M1). A total of 12 soils with varying basic properties were selected from the Unsaturated Soil Hydraulic Database (UNSODA) for the evaluation of the new model. Five pairs of soil hydraulic models were considered, including M1 and the new model with different parameters fitted (M2–M5). Correlation analysis was conducted to analyze the relationships between the new model parameters and basic soil properties. In addition, the Morris method was applied to quantify the parameter sensitivity of the new model. Results showed that M2 to M5 gave slightly better performance than M1 in predicting soil water retention characteristics, with a decrease of ∼15% in the root mean squared error (RMSE). However, they produced substantially better performance than M1 in estimating soil hydraulic conductivities, with a decrease of ∼40% in the RMSE. The new model with all of the model parameters being fitted (M2) was more efficient than those with one or two model parameters not fitted. However, an over‐parameterization problem was detected with M2. Physically unrealistic parameters (e.g., the saturated soil water content and effective grain diameter) exist in M2. These parameters were found to have important influences on the results of the soil hydraulic properties prediction based on the Morris‐based sensitivity analysis. Overall, the new model with two parameters not fitted (M5) can best describe the soil hydraulic properties from saturation to oven dryness and had the potential to simulate water and solute transport in the vadose zone.

The Effect of Drying around Power Cables on the Vadose Zone Temperature

Core Ideas We show the impact of soil hydraulic properties on its critical temperature. Ambient conditions have a larger impact in some fields than local soil parameters. We introduce a parameter for optimizing cable backfill based on hydraulic properties. The effects of moisture migration away from power cables depend strongly in most soils on whether the soil has reached the so‐called critical conditions (e.g., critical temperature) for a dry‐out. However, research on how to estimate this critical temperature has been limited. We solved numerically a differential equation for the planar case of steady‐state coupled heat and mass transfer in porous media, using the van Genuchten–Mualem model for the hydraulic properties of the medium. The critical isotherm is assessed by the temperature at which the steepest change occurs in the moisture content with temperature. We considered the impact of the properties of the surrounding medium and of the ambient temperature and moisture content on the dry‐out temperature. We found that seasonal variations in ambient conditions in a field in Italy have a greater impact on the critical temperature than the variations in soil properties in that field (under the assumption of a nearly uniform moisture content with depth). However, our analysis shows that, under certain conditions, a change in the van Genuchten parameter n and in a specific combination of parameters that we call a dynamic parameter can have as large an effect on the critical temperature. This suggests a direction in the optimization of backfill materials.

A pore-size classification for peat bogs derived from unsaturated hydraulic properties

Abstract. In ombrotrophic peatlands, the moisture content of the acrotelm (vadoze zone) controls oxygen diffusion rates, redox state, and the turnover of organic matter. Thus, variably saturated flow processes determine whether peatlands act as sinks or sources of atmospheric carbon, and modelling these processes is crucial to assess effects of changed environmental conditions on the future development of these ecosystems. We show that the Richards equation can be used to accurately describe the moisture dynamics under evaporative conditions in variably saturated peat soil, encompassing the transition from the topmost living moss layer to the decomposed peat as part of the vadose zone. Soil hydraulic properties (SHP) were identified by inverse simulation of evaporation experiments on samples from the entire acrotelm. To obtain consistent descriptions of the observations, the traditional van Genuchten–Mualem model was extended to account for non-capillary water storage and flow. We found that the SHP of the uppermost moss layer reflect a pore-size distribution (PSD) that combines three distinct pore systems of the Sphagnum moss. For deeper samples, acrotelm pedogenesis changes the shape of the SHP due to the collapse of inter-plant pores and an infill with smaller particles. This leads to gradually more homogeneous and bi-modal PSDs with increasing depth, which in turn can serve as a proxy for increasing state of pedogenesis in peatlands. From this, we derive a nomenclature and size classification for the pore spaces of Sphagnum mosses and define inter-, intra-, and inner-plant pore spaces, with effective pore diameters of > 300, 300–30, and 30–10 µm, respectively.

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.

Constraining Unsaturated Hydraulic Parameters Using the Latin Hypercube Sampling Method and Coupled Hydrogeophysical Approach

The coupled hydrogeophysical approach has proved to be a valuable tool for improving the use of geoelectrical data for hydrological model parameterization. In the coupled approach, hydrological parameters are directly inferred from geoelectrical measurements in a forward manner to eliminate the uncertainty connected to the independent inversion of electrical resistivity data. Several numerical studies have been conducted to demonstrate the advantages of a coupled approach; however, only a few attempts have been made to apply the coupled approach to actual field data. In this study, we developed a 1D coupled hydrogeophysical code to estimate the van Genuchten–Mualem model parameters, K s, n, θ r and α, from time-lapse vertical electrical sounding data collected during a constant inflow infiltration experiment. van Genuchten–Mualem parameters were sampled using the Latin hypercube sampling method to provide a full coverage of the range of each parameter from their distributions. By applying the coupled approach, vertical electrical sounding data were coupled to hydrological models inferred from van Genuchten–Mualem parameter samples to investigate the feasibility of constraining the hydrological model. The key approaches taken in the study are to (1) integrate electrical resistivity and hydrological data and avoiding data inversion, (2) estimate the total water mass recovery of electrical resistivity data and consider it in van Genuchten–Mualem parameters evaluation and (3) correct the influence of subsurface temperature fluctuations during the infiltration experiment on electrical resistivity data. The results of the study revealed that the coupled hydrogeophysical approach can improve the value of geophysical measurements in hydrological model parameterization. However, the approach cannot overcome the technical limitations of the geoelectrical method associated with resolution and of water mass recovery.

A global data set of soil hydraulic properties and sub-grid variability of soil water retention and hydraulic conductivity curves

Abstract. Agroecosystem models, regional and global climate models, and numerical weather prediction models require adequate parameterization of soil hydraulic properties. These properties are fundamental for describing and predicting water and energy exchange processes at the transition zone between solid earth and atmosphere, and regulate evapotranspiration, infiltration and runoff generation. Hydraulic parameters describing the soil water retention (WRC) and hydraulic conductivity (HCC) curves are typically derived from soil texture via pedotransfer functions (PTFs). Resampling of those parameters for specific model grids is typically performed by different aggregation approaches such a spatial averaging and the use of dominant textural properties or soil classes. These aggregation approaches introduce uncertainty, bias and parameter inconsistencies throughout spatial scales due to nonlinear relationships between hydraulic parameters and soil texture. Therefore, we present a method to scale hydraulic parameters to individual model grids and provide a global data set that overcomes the mentioned problems. The approach is based on Miller–Miller scaling in the relaxed form by Warrick, that fits the parameters of the WRC through all sub-grid WRCs to provide an effective parameterization for the grid cell at model resolution; at the same time it preserves the information of sub-grid variability of the water retention curve by deriving local scaling parameters. Based on the Mualem–van Genuchten approach we also derive the unsaturated hydraulic conductivity from the water retention functions, thereby assuming that the local parameters are also valid for this function. In addition, via the Warrick scaling parameter λ, information on global sub-grid scaling variance is given that enables modellers to improve dynamical downscaling of (regional) climate models or to perturb hydraulic parameters for model ensemble output generation. The present analysis is based on the ROSETTA PTF of Schaap et al. (2001) applied to the SoilGrids1km data set of Hengl et al. (2014). The example data set is provided at a global resolution of 0.25° at https://doi.org/10.1594/PANGAEA.870605.

Hydraulic and transport parameter assessment using column infiltration experiments

Abstract. The quality of statistical calibration of hydraulic and transport soil properties is studied for infiltration experiments in which, over a given period, tracer-contaminated water is injected into an hypothetical column filled with a homogeneous soil. The saturated hydraulic conductivity, the saturated and residual water contents, the Mualem–van Genuchten shape parameters and the longitudinal dispersivity are estimated in a Bayesian framework using the Markov chain Monte Carlo (MCMC) sampler. The impact of the kind of measurement sets (water content, pressure inside the column, cumulative outflow and outlet solute concentration) and that of the solute injection duration is investigated by analyzing the calibrated model parameters and their confidence intervals for different scenarios. The results show that the injection period has a significant effect on the quality of the estimation, in particular, on the posterior uncertainty range of the parameters. All hydraulic and transport parameters of the investigated soil can be well estimated from the experiment using only the outlet concentration and cumulative outflow, which are measured non-intrusively. An improvement of the identifiability of the hydraulic parameters is observed when the pressure data from measurements taken inside the column are also considered in the inversion.

Analysis of the Hysteretic Hydraulic Properties of Unsaturated Soil

Core Ideas We devised a setup to measure hydraulic properties using the multistep flux method. Results showed good agreement with the data obtained with the hanging water column. However, they deviated slightly from independent data obtained with the HYPROP. Knowledge of the unsaturated soil hydraulic properties is essential for modeling water flow and solute transport processes in variably saturated subsurface systems. Various experimental setups have been developed over the years to measure the hydraulic properties. We devised a relatively simple setup for simultaneous measurement of the water retention and unsaturated hydraulic conductivity curves using multistep flux (MSF) and hanging water column (HWC) experiments. Our focus was especially on medium‐ and coarse‐textured sands having relatively narrow particle‐size distributions as reflected by the van Genuchten–Mualem (VGM) hydraulic parameter n. Values of the VGM parameter n for some of the sands we used were as high as 15. We also measured the hydraulic properties using the HYPROP evaporation method. The MSF and HWC results showed excellent agreement, but deviated slightly from independent data obtained with the HYPROP measurement system.

An improved Mualem–Van Genuchten method and its verification using data on Beit Netofa clay

An improved Mualem–Van Genuchten method for estimating soil hydraulic conductivity in the vadose zone from data on filtration coefficient and water-retention capacity of soil is proposed. The approach offered by Kosugi for a functional description of hydraulic conductivity of soil is applied. To calculate the values of hydraulic conductivity by Mualem’s formula, the function of differential water capacity of soil with interpreted parameters has been used instead of the function of integral water capacity of soil, which describes the water-retention capacity of soil. Approximations to functions of soil water-retention capacity and hydraulic conductivity are offered here. On the basis of some concepts on the specificity of the curve that describes soil water-retention capacity, a technique for identification of these parameters is developed. The experimental data from two parts of capillary pressure range, on which the water-retention capacity of soil is measured, are used in the technique. The first part corresponds to the zone of mainly film moisture, where the sorption component of the capillary-sorption forces retaining the water in the soil predominates. The second part includes (a) the zone of the mainly capillary-suspended moisture, where the capillary component of the capillary-sorption forces predominates, and (b) the zone of the capillary-backed moisture. The improved method for estimating relative values of hydraulic conductivity of soil has been verified with the use of measured data for the Beit Netofa Сlay soil. The advantages of this method include (a) the ability to identify parameters of the soil hydrophysical functions using relatively available soil indices and (b) the higher accuracy of calculating the soil hydraulic conductivity in comparison with the original version of the method.

Determination of the saturated film conductivity to improve the EMFX model in describing the soil hydraulic properties over the entire moisture range

Difficulty in measuring hydraulic conductivity, particularly under dry conditions, calls for methods of predicting the conductivity from easily obtained soil properties. As a complement to the recently published EMFX model, a method based on two specific suction conditions is proposed to estimate saturated film conductivity from the soil water retention curve. This method reduces one fitting parameter in the previous EMFX model, making it possible to predict the hydraulic conductivity from the soil water retention curve over the complete moisture range. Model performance is evaluated with published data of soils in a broad texture range from sand to clay. The testing results indicate that 1) the modified EMFX model (namely the EMFX-K model), incorporating both capillary and adsorption forces, provides good agreement with the conductivity data over the entire moisture range; 2) a value of 0.5 for the tortuosity factor in the EMFX-K model as that in the Mualem’s model gives comparable estimation of the relative conductivity associated with the capillary force; and 3) a value of −1.0×10−20J for the Hamaker constant, rather than the commonly used value of −6.0×10−20J, appears to be more appropriate to represent solely the effect of the van der Waals forces and to predict the film conductivity. In comparison with the commonly used van Genuchten-Mualem model, the EMFX-K model significantly improves the prediction of hydraulic conductivity under dry conditions. The sensitivity analysis result suggests that the uncertainty in the film thickness estimation is important in explaining the model underestimation of hydraulic conductivity for the soils with fine texture, in addition to the uncertainties from the measurements and the model structure. High quality data that cover the complete moisture range for a variety of soil textures are required to further test the method.

High resolution modelling of soil moisture patterns with TerrSysMP: A comparison with sensor network data

The prediction of the spatial and temporal variability of land surface states and fluxes with land surface models at high spatial resolution is still a challenge. This study compares simulation results using TerrSysMP including a 3D variably saturated groundwater flow model (ParFlow) coupled to the Community Land Model (CLM) of a 38ha managed grassland head-water catchment in the Eifel (Germany), with soil water content (SWC) measurements from a wireless sensor network, actual evapotranspiration recorded by lysimeters and eddy covariance stations and discharge observations. TerrSysMP was discretized with a 10×10m lateral resolution, variable vertical resolution (0.025–0.575m), and the following parameterization strategies of the subsurface soil hydraulic parameters: (i) completely homogeneous, (ii) homogeneous parameters for different soil horizons, (iii) different parameters for each soil unit and soil horizon and (iv) heterogeneous stochastic realizations. Hydraulic conductivity and Mualem-Van Genuchten parameters in these simulations were sampled from probability density functions, constructed from either (i) soil texture measurements and Rosetta pedotransfer functions (ROS), or (ii) estimated soil hydraulic parameters by 1D inverse modelling using shuffle complex evolution (SCE).The results indicate that the spatial variability of SWC at the scale of a small headwater catchment is dominated by topography and spatially heterogeneous soil hydraulic parameters. The spatial variability of the soil water content thereby increases as a function of heterogeneity of soil hydraulic parameters. For lower levels of complexity, spatial variability of the SWC was underrepresented in particular for the ROS-simulations. Whereas all model simulations were able to reproduce the seasonal evapotranspiration variability, the poor discharge simulations with high model bias are likely related to short-term ET dynamics and the lack of information about bedrock characteristics and an on-site drainage system in the uncalibrated model. In general, simulation performance was better for the SCE setups. The SCE-simulations had a higher inverse air entry parameter resulting in SWC dynamics in better correspondence with data than the ROS simulations during dry periods. This illustrates that small scale measurements of soil hydraulic parameters cannot be transferred to the larger scale and that interpolated 1D inverse parameter estimates result in an acceptable performance for the catchment.

Soil moisture prediction of bare soil profiles using diffuse spectral reflectance information and vadose zone flow modeling

Soil hydraulic property information of the vadose zone is key to quantifying the temporal and spatial variability of soil moisture, and for modeling water flow and contaminant transport processes in the near surface. This study deals with exploring the feasibility of using diffuse soil spectral information in the visible, near-infrared and shortwave infrared range (350–2500 nm) to estimate coarse-scale soil hydraulic parameters and predict soil moisture profiles using a topography-based aggregation scheme in conjunction with a 1D mechanistic water flow model. Three different types of parametric transfer functions (so-called spectrotransfer functions, STFs; pedotransfer functions, PTFs; and spectral pedotransfer functions, SPTFs) were aggregated from the point scale to 1 km2 pixel size. to provide coarse scale estimates of van Genuchten-Mualem (VGM) hydraulic parameters. The coarse scale hydraulic parameters were evaluated by simulating soil water dynamics of the 1 km2 pixels across the Zanjanrood River sub-watershed (ZRS) in northwest Iran. Resultant soil water states were compared with ground-truth measurements and advanced synthetic aperture radar (ASAR) estimates of soil water content. The topography-based aggregation scheme was found to provide effective values of the VGM hydraulic parameters across the ZRS study site. The coarse scale STFs performed best in terms of simulating surface, near-surface and subsurface soil water dynamics, followed by the coarse scale SPTFs and PTFs, which performed similarly. The average simulated soil water contents of the surface layer closely correlated with ASAR estimates during relatively wet periods. Simulated subsurface soil water dynamics matched well with the ground-truth measurements. These findings indicate the feasibility of using spectral data to predict VGM hydraulic parameters and, ultimately, to predict soil water dynamics at the larger scales.

Erratum to “Modeling of Hydrophysical Properties of the Soil as Capillary-Porous Media and Improvement of Mualem-Van Genuchten Method as a Part of Foundation Arrangement Research”

Erratum to “Modeling of Hydrophysical Properties of the Soil as Capillary-Porous Media and Improvement of Mualem-Van Genuchten Method as a Part of Foundation Arrangement Research”

Gas permeability and electrical conductivity of structural concretes: Impact of pore structure and pore saturation

This paper investigates the gas permeability and the electrical conductivity of structural concretes under different pore saturations. The gas permeability was measured by CemBureau device and the electrical conductivity by alternating current method. The pore structure was characterized by mercury intrusion porosimetry (MIP) and gravimetric methods. The Archie's law is used to interpret the tortuosity of the pore structure. The impact of pore saturation is evaluated through the Van Genuchten-Mualem (VGM) and Kozeny's models. The results show that (1) the global correlation between the gas permeability in dried state and the electrical conductivity in saturated sate is weak; (2) both VGM and Kozeny's models can describe the relative permeability/conductivity, but the VGM model gives more consistent exponents for permeability and conductivity; (3) as the pore gas and liquid phases are both percolated, the gas permeability is correlated to the electrical conductivity for arbitrary pore saturation.

Numerical Issues Arising in Determination of Interlayer Conductivities in Layered Unsaturated Soils

AbstractThe one-dimensional (1D) Richards equation is used to model infiltration flow in an unsaturated two-layer soil. Imposing continuities of both flux and pressure head at the interface yields a nonlinear equation for determining interface conductivities. The authors show that multiple solutions of this nonlinear interface equation may exist if the spatial discretization is not fine enough around the interface, in particular as sharp wetting fronts pass through the interface, or for flow across highly dissimilar materials. Three hydraulic models, the Gardner model (G), the Mualem-van Genuchten model (MvG), and the Fredlund-Xing-Leong-Rahardjo model (FXLR), are investigated to demonstrate the nonuniqueness of solutions to the interface problem. For the simplest G model, a full mathematical analysis of the interface problem in terms of two parameters depending on local hydraulic conditions and mesh size is possible. For more advanced models, the interface equation can only be analyzed numerically. In al...

Implications and Application of the Raats Superclass of Soils Equations

Core Ideas The Raats model for the water retention characteristic was analyzed. Expressions for other related soil physical properties are provided. The subclasses of the Raats model are described. The Raats model was successfully applied for different porous media. The optimized parameters did not result in one of the subclasses. According to the Richards equation, the capacity of a soil to hold and conduct water is determined by the water retention and hydraulic conductivity characteristics. Many mathematical relationships have been proposed in the literature to describe these characteristics. Raats introduced a general functional relationship with only four parameters that included as special cases four pre‐1990 models found in the literature, including the well‐known relationships by Mualem–van Genuchten and Brooks and Corey. The aims of this study were (i) to discuss this general functional relationship and its four special cases, (ii) to present expressions for the differential moisture capacity, water diffusivity, and matric flux potential corresponding to the general functional relationship and its special cases, (iii) to discuss methods for determining the parameter elasticity and sensitivity, and (iv) to apply the Raats model to experimental data. Soil water retention and hydraulic conductivity data for 11 soils were used to optimize the values of the four major parameters in the Raats model. In none of the cases did the optimized coefficients indicate that the Raats model approached one of the four submodels that it includes.