Experimental Retention Curves Research Articles

Parameter estimation of soil hydraulic characteristics by inverse modeling of the analytical equation for unsaturated subsurface water flow

AbstractIn drip-irrigated systems, the understanding of the soil wetting pattern is essential in defining the area effectively irrigated, the spacing between the emitters and their installation depth, and the irrigation rate. Thus, this study aims to estimate soil hydraulic characteristics through inverse modeling of an analytical equation used in wetting bulb simulation based on soil moisture measurements obtained in the field. The parameters of the Gardner model, which describes the unsaturated soil hydraulic conductivity, and the van Genuchten model that describes the soil moisture retention curve, were estimated by inverse modeling techniques. The following options were considered: (A) estimating parameter β while considering the other parameters, Ko, θr, θs, α, n, and m, as known and obtained experimentally; (B) estimating parameters Ko and β while considering the experimental retention curve as known; (C) estimating parameters Ko, β, α, n, and m while considering the values of the θr and θs volumetric moisture as known; (D) estimating all the parameters of Gardner and van Genuchten models (Ko, θr, θs, α, n, and m). The results indicate that option D showed better concordance between the estimated and observed moisture values. Thus, the inverse modeling of the analytical equation is an important tool for irrigation design and management.

Theoretical modelling of retention curve for Livet-Gavet loam

Different approaches have been used for modeling retention curves. The experimental correlation was first proposed [1-3]. The physical modelling of unsaturated soils is used for this study. The shape of the retention curve is a consequence of physical assumptions. The paper presents a theoretical model based on elastic spherical particle arrangement. Firstly, a uniform model is presented with a single diameter of soil particle. The second step extends the use of the model to graded soils. The model uses only physical parameters easy to measure. The model is compared with the experimental retention curve of two different samples, the Livet-Gavet loam (1.61m-3mm) as paste and the Gavet sandy loam compacted with 85 falls per layer of Proctor weight. It shows its ability to model the experimental curves. It is of great interest for engineers as it uses only physical parameters. It gives a direct determination of the retention curve along the wetting path and along the drying path. It shows also the importance of adsorbed water to describe the retention curve.

Nonstationarity of the electrical resistivity and soil moisture relationship in a heterogeneous soil system: a case study

Abstract. Understanding the role of vegetation in the interface between the atmosphere and groundwater is the most decisive key in analyzing the processes involved in water transfer. The main effect of vegetation is its root water uptake, which significantly modifies the processes involved in water transfer in the vadose zone. This paper focuses on mapping temporal and spatial changes in soil moisture using electrical resistivity tomography (ERT). The main objective is to assess how electrical resistivity (ER) is useful for mapping water distribution along a heterogeneous toposequence crossed by a hedgerow. Ten ERT were performed over the studied period for a 28 m long toposequence and compared to matric potential and groundwater level measurements. Soil volumetric water content (VWC) was predicted with two methods: (i) from ER using the Waxman and Smits model (ii) and from matric potential using an experimental retention curve fitted by a Van Genuchten model. Probability density functions (PDFs) of our set of data show that the largest change in mean ER and matric potential was observed in the topsoil layer. We then analyzed the consistency between ER and point measurements in this layer by extracting the arrays at the junction of ER grids and point measurements. PDFs of ER maps at each monitoring time (from T01 to T10) were also calculated to select the most contrasting distributions, corresponding to the wettest (T06) and driest states (T10). Results of ER were consistent with matric-potential measurements, with two different behaviors for locations inside and outside the root zone. A consistent correlation between VWC values from the Waxman and Smits model and those obtained from the retention curve was observed outside the root zone. The heterogeneous soil system inside the root zone shows a different pattern in this relationship. A shift in the relationship between ER and soil moisture for the locations outside and inside the root zone highlights the nonstationarity between wet and dry periods inside the root zone. The equivocal behavior of this relationship shows the limitation of using ER to predict soil moisture in a heterogeneous soil system. Such systems were actually related to the high hedgerow root density and also to a particular topographical context (ditch and bank) that is encountered in Brittany and throughout northwestern Europe.

Lattice Boltzmann modelling of liquid distribution in unsaturated granular media

We use capillary condensation simulated by a multiphase Lattice Boltzmann model as a means to generate homogeneous distributions of liquid clusters in 2D granular media. Liquid droplets condense from the vapour phase between and on the grains, and they transform into capillary bonds and liquid clusters as thermodynamic equilibrium is approached. As the amount of condensed liquid is increased, liquid clusters of increasing connectivity are formed and the distribution of liquid undergoes topological transitions until the whole pore space is filled by the liquid. We investigate the cluster statistics and local grain environments. From extensive simulations, we also obtain the mean Laplace pressure as a function of the amount of liquid, which is found to be quite similar to the well-known experimental retention curve in soil mechanics. The tensile stress carried by the grains increases as a function of the amount of condensed liquid up to a peak in the funicular state beyond which the stress falls off as a result of pressure drop inside the merging clusters.

A theoretical retention model for unsaturated uniform and graded soils

Modelling the relation between the degree of saturation and the suction (ie retention curve) is an important challenge for geotechnical engineering. It has a huge influence on the behavior of large soil constructions as levees, embankments, road earthworks. We present here a theoretical model of retention curve which considers physical relations of unsaturated soils. With this approach, there is no need to assume particular shapes of the retention curves, which are a consequence of the physical assumptions. The present study is focused on a theoretical model based on elastic spherical particle arrangement. As a first step a uniform model is presented with a single diameter of soil particle. A second step extends the use of the model to graded soils. The model uses only 5 physical parameters. It is compared with the experimental retention curve of two different samples of glass uniform particles and two different graded soils, a graded glass sample and the Livet-Gavet loam. It shows its ability to model the experimental curves and a better agreement than the former theory of Brooks and Corey (1966). This current publication is funded by the French National Project «Terredurable» (ANR 2011),.

Simultaneous Determination of Water Retention Curve and Unsaturated Hydraulic Conductivity of Substrates Using a Steady-state Laboratory Method

For effective irrigation and fertilization management, the knowledge of substrate hydraulic properties is essential. In this study, a steady-state laboratory method was used to determine simultaneously the water retention curve, θ(h), and unsaturated hydraulic conductivity as a function of volumetric water content, K(θ), and water pressure head, K(h), of five substrates used widely in horticulture. The substrates examined were pure peat, 75/25 peat/perlite, 50/50 peat/perlite, 50/50 coir/perlite, and pure perlite. The experimental retention curve results showed that in the case of peat and its mixtures with perlite, there is a hysteresis between drying and wetting branches of the retention curve. Whereas in the case of coir/perlite and perlite, the phenomenon of hysteresis was less pronounced. The increase of perlite proportion in the peat/perlite mixtures led to a decrease of total porosity and water-holding capacity and an increase of air space. Study of the K(θ) and K(h) experimental data showed that the hysteresis phenomenon of K(θ) was negligible compared with the K(h) data for all substrates examined. Within a narrow range of water pressure head (0 to –70 cm H2O) that occurs between two successive irrigations, a sharp decrease of the unsaturated hydraulic conductivity was observed. The comparison of the K(θ) experimental data between the peat-based substrate mixtures and the coir-based substrate mixture showed that for water contents lower than 0.40 m3·m−3, the hydraulic conductivity of the 50/50 coir/perlite mixture was greater. The comparison between experimental water retention curves and predictions using Brooks-Corey and van Genuchten models showed a high correlation (0.992 ≤ R2 ≤ 1) for both models for all substrates examined. On the other hand, in the case of unsaturated hydraulic conductivity, the comparison showed a relatively good correlation (0.951 ≤ R2 ≤ 0.981) for the van Genuchten-Mualem model for all substrates used except perlite and a significant deviation (0.436 ≤ R2 ≤ 0.872) for the Brooks-Corey model for all substrates used.

Two-Factor Model of Soil Suction from Capillarity, Shrinkage, Adsorbed Film, and Intra-aggregate Structure~!2009-11-06~!2010-02-08~!2010-04-06~!

The objective of this work is to derive the soil water retention from the soil structure without curve-fitting and only using the physical parameters found irrespective of an experimental retention curve. Two key points underlie the work: (i) the soil suction at drying coincides with that of the soil intra-aggregate matrix and contributive clay; and (ii) both the soil suction and volume shrinkage at drying depend on the same soil water content. In addition the two following results are used: (i) the available two-factor (capillarity and shrinkage) model of clay suction enables one to connect a clay suction and clay water content using the clay matrix structure; and (ii) the recent reference shrinkage curve model based on the concepts of intra-aggregate soil structure permits one to connect the soil water content at shrinkage with the water content of the contributive clay. With that the available two-factor model was essentially modified and, in particular, the effect of adsorbed water film was taken into account. The developed model includes the following input parameters: the solid density, relative volume of contributive-clay solids, relative volume of contributive clay in the oven-dried state, soil clay content, aggregate/intra-aggregate mass ratio, and specific volume of lacunar pores in the aggregates at maximum swelling. The validation of the model is based on available data of water retention and the above input parameters for six soils. A promising agreement between the predicted and observed water retention curves was found.

Simulation of Soil Moisture Profiles Using K(h) from Coupling Experimental Retention Curves and One-Step Outflow Data

In the present work, the effect on the soil moisture profiles development and the cumulative infiltration when three different equations for calculating the diffusivity versus volumetric water content, D(θ) function, from one-step outflow experimental data are used, is investigated. These D(θ) functions are coupled with moisture retention curves θ(h) (h being the soil matric potential) obtained independently for the determination of the hydraulic conductivity function K(θ) or K(h). The so obtained hydraulic conductivity function, together with θ(h), were employed in solving Richards equation numerically under constant flux conditions in one dimensional vertical infiltration process. Two different porous materials were used for this investigation. It is shown that the three different equations used for the prediction of K(h) have no significant effect on the shape of the moisture profiles for the sand mixture and that a Green–Ampt advancement of the wetting front is observed. For the case of sandy clay soil there are some noticeable differences in the moisture profiles and their shape is comparatively more effusive. Also, one could mention that for both porous media the time of incipient ponding (T) differentiates among the three equations used.

Thermodynamics of ternary systems in reverse osmosis

Solute retention curves in binary systems are rather well predicted by the Spiegler-Kedem theory. It is shown that this theory is easily generalized to ternary systems if small Péclet numbers are involved. The thermodynamical description is extended with a special parameter due to the solute-solute interaction. Theoretical and experimental retention curves are presented and discussed.

The Transport of Cations in Soil Columns at Different Pore Velocities

AbstractThe transport of Na+ and Mg2+ in Ca2+‐saturated Yolo loam soil column under saturated steady state flow of different pore velocities was studied. The cation solution was maintained at a concentration such that a linear adsorption isotherm described the cation adsorption. Experimental retention curves were compared to the theoretical solutions obtained from a linear equilibrium model. At low relative concentrations, the equilibrium adsorption of Na+ was significantly less than that of Mg2+. At high pore velocities, the agreement of the experimental retention curves with that of the theoretical calculation was good. At low pore velocity, the agreement was poor. The retention volumes of Na+ and Mg2+ were smaller than those theoretically calculated and those obtained experimentally at high pore velocity. Extensive tailing of Mg2+ observed in the experimental data was not described by the theoretical model.