Water Movement In Unsaturated Soils Research Articles

Characterizing water transfer in compacted soils in the context of energy storage, contribution of magnetic resonance imaging (MRI)

The concept of energy geostructures consists in enhancing existing civil engineering structures from an energy point of view thanks to the insertion of heat exchanger tubes. In order to consider the use of compacted soil embankments for heat storage [1], a good knowledge of water movement is required. Indeed, the increase in temperature of the exchangers leads to a migration of liquid/vapour water to the surface layers [2]. A loss of moisture may alter the thermal capacity of the soil [1] and therefore the system energy efficiency. To determine unsaturated hydraulic properties, the Instantaneous Profile Method (IPM) consists in inducing transient flow in a long cylindrical sample of soil and then measuring the resulting water content and/or pore water pressure profiles at various time intervals [3]. The unsaturated hydraulic conductivity (ku) is then computed using Darcy’s law.
 The direct evaporation technique allows the measurement of the suction profile in a soil column submitted to drying [4] using filter papers whose water retention curve is known. This method is quite reliable but requires long equilibrium time. Thus, the tests need several weeks and do not give accurate results at the early stages of drying, as the soil state is close to saturation. The analysis of the results when the soil retention capacity is low is therefore complicated, especially when the flow is coupled with temperature variations.
 In this study, this classical method was compared to a faster alternate method using nuclear magnetic resonance imaging (MRI) that allows for non-intrusive, continuous and small-scale monitoring of the effect of thermo-hydraulic solicitations on water transfer within the sample. The use of MRI for monitoring the drying/wetting front progression appeared in 1970 [5]. Simpson et al. [6] presented the successive studies that have made it possible to develop a method for monitoring the movement of water within a sample. These measurements can be 1D, 2D and 3D.
 The tests were performed on a sandy lean clay sampled in the Paris region in France. The soil was dried, pulverized, and sieved through a 2 mm sieve before experiments. The compaction state was selected to optimize the heat storage capacity [4] with a water content of 16.3% and a dry density of 1.79 Mg.m-3. Ten cylindrical samples of 70 mm in diameter and 20 mm in height were compacted and superimposed with introducing filter paper disks between them. The regular weighting of each sample and each filter paper during three months allowed the derivation of the ku curve by the IPM method.
 For the MRI method, 9 samples of 36 mm in diameter and 10 mm in height were compacted at the initial state then brought to different degrees of saturation. The measurement of the water proton relaxation time using a Bruker Biospec 24/40 superconducting magnet (100 MHz), allowed the definition of a calibration curve which connects MRI signal to the water content of the samples (Figure 1b). A Single Point Imaging pulse sequence [7] was used to obtain the signal intensity (encoding time 75 µs < t < 200 µs). In an image acquired with the SPI method, the intensity of each voxel (S) is :
 (1) Whereis the magnetization in a pixel and is the transverse relaxation time.
 The calibration curve is shown in Figure 1b. Then, a larger sample of 30mm in height was compacted and exposed to an air flow at 20°C and 30% relative humidity through using a remote humid air generation system. 27 measurements points were monitored with an interval of 1.1 mm along the sample. Four examples of the recorded profiles are shown in Figure 1c. The evolution of the water content inside the sample is then calculated using the calibration curve to interpret the water migration in the samples submitted to humidity- and temperature-controlled air (Figure 1a). The pre-established retention curve of the soil will be used to calculate its matric suction. At each depth, the flow velocity and the hydraulic gradient will be calculated from the water content profiles and suction profiles respectively in order to determine the unsaturated hydraulic conductivity. 
 These experimental developments allow the non-intrusive and continuous quantification of water movement in unsaturated soils during the application of coupled thermo-hydraulic solicitations. The understanding of these fundamental parameters is a key step for studying thermo-hydromechanical behavior of compacted soils considered for heat storage. The results provide insight into the feasibility of these structures in terms of their long-term energy efficiency and stability.

Isotopic evidence for soil water sources and reciprocal movement in a semi-arid degraded wetland: Implications for wetland restoration

Understanding water dynamics is a prerequisite for the restoration of degraded ecosystems in arid and semiarid regions. In this study, we carried out δD and δ18O analyses of precipitation, unsaturated soil water, overland flow, surface runoff, and groundwater samples from a seasonally flooded wetland in the Momoge National Nature Reserve of the Songnen Plain, Northeast China, to identify the water sources and understand the mechanisms of unsaturated soil water movement. Unsaturated soil water content (W/W%) at every 20 cm along with a soil profile (0–100 cm) was collected during the growing season, and the HYDRUS-1D model was used to simulate temporal-spatial variations. The results showed that the local meteoric water line (δD=5.90δ18O-7.34, R2=0.95) had a smaller slope and intercept than the global meteoric water line because of strong evaporation at our study site under semi-arid climate. The groundwater was partly recharged by local precipitation via overland flow and unsaturated soil water infiltration. Unsaturated soil water was sourced from both precipitation and groundwater with variations at different depths. The upper soil layer at 0–15 cm was mainly sourced from limited precipitation, while the groundwater could move up to a 25 cm layer during the dry period. The unsaturated soil water content increased with soil depth in the top 40 cm, decreased at depths of 40 to 80 cm, and increased again at depths of 80 to 100 cm. The HYDRUS-1D model could simulate the unsaturated soil water dynamics well in the upper (0–40 cm) and lower (80–100 cm) sections, but poorly for depths of 40–80 cm due to the upward and downward flow. The bidirectional unsaturated soil water movement highlights the importance of capillary groundwater for wetland plants with similar climatic or hydrogeological conditions.

A new quasi-3-D model with a dual iterative coupling scheme for simulating unsaturated-saturated water flow and solute transport at a regional scale

Soil water in an unsaturated zone is the major source of groundwater recharge, and likewise soil contaminants are the major source of groundwater contamination. It is thus essential to jointly consider soil water movement and solute transport together with groundwater movement and solute transport, which is particularly important when addressing regional-scale problems. This study developed a new quasi-3-D model for simulating regional-scale saturated–unsaturated water movement and solute transport in arid and semi-arid agricultural areas. The unsaturated soil water movement and solute transport processes are simulated by a 1-D soil water and solute transport model referred to as UBMOD, and the saturated groundwater movement and solute transport are simulated by MODFLOW and MT3DMS. In the quasi-3-D model, the soil water movement and groundwater movement modules are iteratively coupled and the soil solute transport and groundwater solute transport modules are iteratively couple. This is referred to as a dual iterative coupling scheme. Strength and weakness of the quasi-3-D model and the coupling scheme were evaluated with two synthetic cases and a real-world application. Results indicate satisfactory performance of the model, which demonstrates the reliability of the dual iterative coupling scheme. Besides, the impacts of spatial and temporal discretization and stress period on model stability and accuracy has been evaluated. And the model is more computationally efficient than the fully numerical methods based on Richards’ equation when the problem is a 2D/3D problem with a scale larger than 10 m. Moreover, the application to the Hetao Irrigation District indicates that the developed model is effective and efficient for the real-world regional-scale agricultural problems. Above all, it is concluded that the new model is more suitable than existing models for simulating regional-scale unsaturated–saturated soil water movement and solute transport in arid and semi-arid agricultural areas.

Methodology to simulate unsaturated zone hydrology in Storm Water Management Model (SWMM) for green infrastructure design and evaluation.

Hydrologic models such as the USEPA Stormwater Management Model (SWMM) are commonly used to assess the design and performance of green infrastructure (GI). To accurately represent GI performance models used in design need to be able to address both the hydrology/hydraulics of the catchment and the GI unsaturated (vadose) zone hydrology. While hydrologic models, such as SWMM, address the need for catchment hydrology/hydraulics, they often simplify the unsaturated zone hydrology. This paper presents a methodology utilizing existing components of SWMM to represent unsaturated zone hydrology in an accessible format that does not require adjustments to the SWMM source code. The methodology simulated the unsaturated soil water movement by considering flow caused by differences of soil matric head and flow caused by gravity between soil layers with finite depth/length. The flow flux related to the soil matric head is a function of soil water diffusivity (D) and the soil moisture gradient, where D can be represented by a pump curve in SWMM. The flow flux related to gravity was controlled by unsaturated hydraulic conductivity (K) only and was also simulated by a pump. The methodology was compared to another variably saturated model, HYDRUS, with theoretical soils (with single layers of sand, loam, silt, and clay, as well as dual-layer scenarios). Field data was used to compare the methodology to HYDRUS and the SWMM LID (Low Impact Development) module. In all comparisons the presented methodology and HYDRUS delivered similar results for the vadose zone response to a storm event, while the LID module of SWMM exhibited slower water movement. The results showed that under natural conditions, the approximation of the presented methodology yielded satisfactory results to simulate flow through the unsaturated vadose zone.

Estimating the macroscopic capillary length from Beerkan infiltration experiments and its impact on saturated soil hydraulic conductivity predictions

The macroscopic capillary length, λc, is a fundamental soil parameter expressing the relative importance of the capillary over gravity forces during water movement in unsaturated soil. In this investigation, we propose a simple field method for estimating λc using only a single-ring infiltration experiment of the Beerkan type and measurements of initial and saturated soil water contents. We assumed that the intercept of the linear regression fitted to the steady-state portion of the experimental infiltration curve could be used as a reliable predictor of λc. This hypothesis was validated by assessing the proposed calculation approach using both analytical and field data. The analytical validation demonstrated that the proposed method was able to provide reliable λc estimates over a wide range of soil textural characteristics and initial soil water contents. The field testing was performed on a large database including 433 Beerkan infiltration experiments, with the 99% of the experiments yielding realistic λc values. The generated λc values were then used in conjunction with four different methods for estimating saturated soil hydraulic conductivity, Ks. Estimated Ks values were close to those generated by a reference method, with relative error < 25% in nearly all cases. By comparison, assuming constant or soil-dependent λc values caused relative errors in Ks of up to 600%. Altogether, the proposed method constitutes an easy solution for estimating λc, which can improve our ability to estimate Ks in the field.

Discontinuous finite volume element method of two-dimensional unsaturated soil water movement problem

In this paper, a numerical approximation method for the two-dimensional unsaturated soil water movement problem is established by using the discontinuous finite volume method. We prove the optimal error estimate for the fully discrete format. Finally, the reliability of the method is verified by numerical experiments. This method is not only simple to calculate, but also stable and reliable.

Soil water characteristic curves for laterite soil at different water contents and methods as lining system

Preventing groundwater contamination from the most common method of solid waste disposal technique i.e. landfilling is by proving hydraulic barriers. Accordingly, water movement in unsaturated soil is important in the analysis of leachate migration in soil material used as hydraulic barrier in waste containment systems. A full-scale experiment would be prohibitively costly and time consuming. The only feasible recourse therefore is to construct a model, which reasonably portray the behaviour of the full-scale system and simulate the relevant physical parameters and describes the overall significant characteristics of the transport phenomena. This paper presents the trends and patterns of soil water characteristic curves (SWCC) in terms of volumetric water content versus soil suction developed for compacted laterite soil specimens using data from pressure plate tests. Specimens were prepared at three different water contents corresponding to dry of optimum, optimum, and wet of optimum conditions. Models suggested by Brooks and Corey (BC), van Genuchten (VG), and Fredlund and Xing (FX) were used to obtain curve fitting parameters using the program “SWRC Fit” which performed nonlinear fitting of soil water retention curves. The SWRC Fit can simultaneously calculate the fitting parameters of these models with Root Mean Square Error values and draw the fitting curves. By comparing the results of the models using SWRC Fit, the model that best fits the laterite soil investigated will be chosen to be used for further analysis. The results show that the BC model represents the soil water retention curves better than the VG and FX models when the soil has distinct air entry suction. On the other hand, the VG and FX models can fit most soil water retention curves very well when discussion on the pore-size distribution is desired.

Ridge Basis Functions Collocation Method for Soil Water Movement Equations Under Infiltration with Two Point-source Emitters

Numerical simulation is an important method to research the soil y water infiltration under two point-source emitters. In order to improve the simulation precision and the practicability of numerical solution, a meshfree method based on ridge basis functions collocation method (RBFCM) is developed for solving 2D water movement equations in unsaturated soil under infiltration. The ridge basis functions are used as approximating functions to construct the method for the numerical solution of the water movement equations. The iterative method is used in solving the nonlinear matrix equation derived from water movement equations based on RBFCM. Compared with the traditional finite difference method and finite element method, the new method reduces the calculation time and calculation error. The numerical results of realistic water movement in unsaturated soil under infiltration meet the demands of actual problems well.

The Contributions of Lewis Fry Richardson to Drainage Theory, Soil Physics, and the Soil-Plant-Atmosphere Continuum

Lewis Fry Richardson (1881-1953) was an English polymath, who made important contributions to many fields, including numerical weather prediction, finite difference solutions of partial differential equations, turbulent flow and diffusion, fractals, and the cause and evolution of conflicts. In 1922 he published the book `Weather Prediction by Numerical Process'. He did the research for this book under difficult circumstances just before, during, and just after World War I. The book received a lot of attention initially, but methods like those proposed in it were not successfully implemented until the invention of fast digital computers around 1950. To model heat and mass transfer in the atmosphere, Richardson did much original work on turbulent flow and defined what is now known as the Richardson number. His technique for improving the convergence of a finite difference calculation is known as Richardson extrapolation, and was used by John Philip in his 1957 semi-analytical solution of the Richards equation for water movement in unsaturated soil. With the first draft of his book just finished, from mid-1916 the pacifist Richardson drove an ambulance in France during the later part of World War I. But even then, In his spare time, he did weather related experiments and computations, rewrote the first draft of his book, and started a new scientific career in the study of conflicts, again using existing and inventing new mathematical tools. Posthumously, most of Richardson$'$s contributions in various fields received considerable attention. But important exceptions are his contributions to soil science and hydrology, on which we focus in this paper. His first published papers in 1908 concerned the numerical solution of the free surface problem of unconfined flow of water in saturated soil, arising in the design of required ditch spacing for draining peat. He developed and used a graphical method to solve this problem. This and other practical problems stimulated his interest in numerical methods and soon led him to the challenge of numerical weather prediction. Richardson formulated an elaborate model for transport processes in the atmosphere. For the lower boundary of his atmospheric model he needed to understand the movement of liquid

Hermite Radial Basis Collocation Method for Unsaturated Soil Water Movement Equation

Due to the nonlinear diffusion term, it is hard to use the collocation method to solve the unsaturated soil water movement equation directly. In this paper, a nonmesh Hermite collocation method with radial basis functions was proposed to solve the nonlinear unsaturated soil water movement equation with the Neumann boundary condition. By preprocessing the nonlinear diffusion term and using the Hermite radial basis function to deal with the Neumann boundary, the phenomenon that the collocation method cannot be used directly is avoided. The numerical results of unsaturated soil moisture movement with Neumann boundary conditions on the regular and nonregular regions show that the new method improved the accuracy significantly, which can be used to solve the low precision problem for the traditional collocation method when simulating the Neumann boundary condition problem. Moreover, the effectiveness and reliability of the algorithm are proved by the one-dimensional and two-dimensional engineering problem of soil water infiltration in arid area. It can be applied to engineering problems.

Geometric and fractal analysis of dynamic cracking patterns subjected to wetting-drying cycles

Desiccation cracks detrimentally affect the hydraulic and mechanical properties of soils. Soil cracking dynamic subjected to wetting-drying (W-D) cycles aids in the understanding of soil shrinking-swelling mechanism and the regularity of water movement in unsaturated soils with variable solid phase. In the present paper, we experimentally investigate the geometric and fractal features of dynamic cracking patterns with the soils undergoing four W-D cycles. Crack patterns were recorded by a digital camera mounted above the soil specimen. Digital image processing and morphological algorithms were employed to yield the geometric and fractal indexes that depict crack patterns (incl. crack ratio, length, mean width, similarity degree, box-counting fractal and multifractal dimensions). The analysis of dynamic crack patterns suggest an inconsistency exhibited in the crack morphology towards the drying (DR) process and backwards the wetting (WT) process, which could be possibly attributed to the different suction stress between soil particles during moisture desorption and sorption, closely analogous to the hysteresis phenomenon of soil-water characteristic curve. The gradient of crack propagating and dissolving related to water content reaches different peak values during an individual W-D cycle. High repeatability is found across the stabilized crack patterns in the four drying cycles and the cracking degree reaches equilibrium with the increasing number of W-D cycles. The crack patterns in the subsequent drying processes are significantly dominated by the first cracking. The similarity degree between the first and respectively the second, third, fourth crack patterns decreases with the increasing W-D cycles, suggesting the soil cracked in the previously cracking vicinity, and saturated wetting/submerge hardly healed the failure zones that caused the repeatable cracking. Attempt of fractal and multifractal methods applied to the crack networks corroborates the applicability of the scaling method to the crack patterns. The analysis indicates that soil cracks progress in hierarchical networks in the first drying cycle, while the gradational crack segments nearly simultaneously emerged or vanished in the subsequent drying or wetting cycles. The study reveals the discrepancy of crack morphology between the crack formation and dissolution during W-D cycles, and demonstrates the repeatability degree and hierarchy of cracks across W-D cycles, which would aid in quantifying physical processes in cracked soil shrinking/swelling and modelling of water transport in unsaturated soils of dynamic structure.

A MASS LUMPING AND DISTRIBUTING FINITE ELEMENT ALGORITHM FOR MODELING FLOW IN VARIABLY SATURATED POROUS MEDIA

The Richards equation for water movement in unsaturated soil is highly nonlinear partial differential equations which are not solvable analytically unless unrealistic and oversimplifying assumptions are made regarding the attributes, dynamics, and properties of the physical systems. Therefore, conventionally, numerical solutions are the only feasible procedures to model flow in partially saturated porous media. The standard Finite element numerical technique is usually coupled with an Euler time discretizations scheme. Except for the fully explicit forward method, any other Euler time-marching algorithm generates nonlinear algebraic equations which should be solved using iterative procedures such as Newton and Picard iterations. In this study, lumped mass and distributed mass in the frame of Picard and Newton iterative techniques were evaluated to determine the most efficient method to solve the Richards equation with finite element model. The accuracy and computational efficiency of the scheme and of the Picard and Newton models are assessed for three test problems simulating one-dimensional flow processes in unsaturated porous media. Results demonstrated that, the conventional mass distributed finite element method suffers from numerical oscillations at the wetting front, especially for very dry initial conditions. Even though small mesh sizes are applied for all the test problems, it is shown that the traditional mass-distributed scheme can still generate an incorrect response due to the highly nonlinear properties of water flow in unsaturated soil and cause numerical oscillation. On the other hand, non oscillatory solutions are obtained and non-physics solutions for these problems are evaded by using the mass-lumped finite element method.

Incorporating Temperature Effects in Soil-Water Characteristic Curves

The study of water movement in unsaturated soils under non isothermal conditions requires understanding the soil water characteristic curves (SWCC) as a function of temperature. Physical processes such as water permeability, and shear strength of unsaturated soils are a function of the SWCC. These curves are generally assumed to be a unique relationship for a soil for all practical purposes. However, factors such as temperature and volume change can affect this uniqueness. The aim of this paper is to present a rational approach to describe the effect of temperature on the SWCC under isothermal conditions. A simple equation based on the relationship between temperature and the capillary rise is applied to find the air entry value at different temperatures when its value is known at a reference temperature. The same equation can be applied to the residual water content term of the SWCC. An experimental procedure for two different coarse-grained soils in the low suction range (i.e., 0–30 kPa) at three different temperatures (4, 20, and \(49\,^{\circ }\hbox {C}\)) were conducted to validate the proposed approach. The comparison between experimental results, published SWCC data from literature and the proposed equation indicate that the proposed method can be used to successfully predict the measured SWCC with respect to temperature.

Experimental Determination of Hydraulic Conductivity at Unsaturated Soil Column

Hydraulic Conductivity (K) is an important hydraulic parameter as it affects the environment by controlling infiltration, irrigation rate, and consequently the water movement through the ground. In order to determine Hydraulic Conductivity in a soil column during unsaturated flow, experiments were performed in the laboratory. A sandy (S) soil sample of known Hydraulic Conductivity at saturation (KS) was placed uniformly in a transparent column. Using a pump, water was applied at the surface of the soil column in certain supplies (Qi), while soil moisture (θ) was measured using TDR probes. At the same time, soil pore pressure (h) was measured using pressure transducers. The cumulative volume of the outgoing water (V) of the column was measured. Experimental data were fitted by Van Genuhten's Hydraulic Conductivity model. The results of the above experimental procedure constitute useful tools for the simulation of water movement in unsaturated soils and can be the outset for further research.

Estimating hysteresis in the soil water retention curve from monolith experiments

Water movement in unsaturated soils is commonly affected by hysteresis, a phenomenon often ignored as to keep the mathematical description of water flow and solute transport simple. In this study, the importance of hysteresis was quantified at lysimeter scale on two Belgian soils, a loamy and a sandy loam soil. Undisturbed soil monoliths (79cm diameter by 105cm height) were collected in each soil and the flow studied for various boundary conditions. Each monolith was equipped with different measuring devices to monitor online soil water content, pressure head, and outlet water fluxes. Four unsaturated steady state flow experiments were conducted on each monolith. Hydraulic properties were obtained by inverting the governing flow equation of Richards, using a global multilevel coordinate search inversion algorithm. Measured soil water contents, pressure heads, and outlet water fluxes were used in the objective function. First soil hydraulic parameters in the retention and hydraulic conductivity functions were estimated neglecting hysteresis and compared with the functions obtained using approaches considering hysteresis in their analysis. Hysteresis was incorporated in the retention curve using the simple empirical model proposed by Scott et al. (1983) and using the universal conceptual model of Mualem (1977). Hysteresis in both soils was characterized by a relative large difference between the main drying and wetting soil water retention curves. However, results showed that the effect of hysteresis was different on various components of water flow in the two studied soils. The Scott's model considerably improved pressure head estimates in the loamy soil and Mualem's model somewhat improved the soil water content estimates in the sandy loam soil. Outlet water fluxes in both soils were less sensitive to hysteresis. In general, the simple hysteretic model of Scott was more successful in studying hysteresis in both studied soils.

Derivation of Soil‐Specific Streaming Potential Electrical Parameters from Hydrodynamic Characteristics of Partially Saturated Soils

Water movement in unsaturated soils gives rise to measurable electrical potential differences that are related to the flow direction and volumetric fluxes, as well as to the soil properties themselves. Laboratory and field data suggest that these so‐called streaming potentials may be several orders of magnitudes larger than theoretical predictions that only consider the influence of the relative permeability and electrical conductivity on the self potential (SP) data. Recent work has improved predictions somewhat by considering how the volumetric excess charge in the pore space scales with the inverse of water saturation. We present a new theoretical approach that uses the flux‐averaged excess charge, not the volumetric excess charge, to predict streaming potentials. We present relationships for how this effective excess charge varies with water saturation for typical soil properties using either the water retention or the relative permeability function. We find large differences between soil types and the predictions based on the relative permeability function display the best agreement with field data. The new relationships better explain laboratory data than previous work and allow us to predict the recorded magnitudes of the streaming potentials following a rainfall event in sandy loam, whereas previous models predict values that are three orders of magnitude too small. We suggest that the strong signals in unsaturated media can be used to gain information about fluxes (including very small ones related to film flow), but also to constrain the relative permeability function, the water retention curve, and the relative electrical conductivity function.

Finite Element Numerical Simulation of Two-Dimensional Unsaturated Soil Water Movement Problems

According to the fundamental theory of soil water dynamics, the mathematical model of 2-D unsaturated soils water movement with variable is established under the condition of furrow irrigation or underground channel seepage irrigation,and is calculated by the finite element numerical simulation method .The numerical simulation shows that the calculated results of numerical simulation is in good agreement with the measured date of experiment.

A robust and energy-conserving model of freezing variably-saturated soil

Abstract. Phenomena involving frozen soil or rock are important in many natural systems and, as a consequence, there is a great interest in the modeling of their behavior. Few models exist that describe this process for both saturated and unsaturated soil and in conditions of freezing and thawing, as the energy equation shows strongly non-linear characteristics and is often difficult to handle with normal methods of iterative integration. Therefore in this paper we propose a method for solving the energy equation in freezing soil. The solver is linked with the solution of Richards equation, and is able to approximate water movement in unsaturated soils and near the liquid-solid phase transition. A globally-convergent Newton method has been implemented to achieve robust convergence of this scheme. The method is tested by comparison with an analytical solution to the Stefan problem and by comparison with experimental data derived from the literature.

The Effect of Various Soil Hydraulic Property Estimates on Soil Moisture Simulations

Models of water movement in unsaturated soils require accurate representations of the soil moisture retention and hydraulic conductivity curves; however, commonly used laboratory methods and pedotransfer functions (PTFs) are rarely verified against field conditions. In this study, we investigated the effects of using soil hydraulic property information obtained from different measurement and estimation techniques on one‐dimensional model predictions of soil moisture content. Pairs of time domain reflectometry waveguides and tensiometers were installed at two depths in the side of a soil pit face to obtain in situ measurements. Undisturbed soil samples were taken near the instruments and subjected to particle size analysis, multistep outflow (MSO), and falling‐head permeability tests to obtain estimates of the soil moisture retention curves. Three scaling methods were then applied to improve the fit of the various estimates to the field data. We found that soil hydraulic property estimates obtained from inverse methods lead to the best simulations of soil moisture dynamics, and that laboratory MSO tests or commonly used PTFs perform poorly. These laboratory and PTF estimates can be dramatically improved, however, by simply constraining the range of possible moisture contents to the minimum and maximum measured in the field. It appears that this method of scaling PTF results can be used to obtain soil hydraulic property inputs of sufficient accuracy for plot‐scale modeling efforts without requiring expensive laboratory or in situ tests.

Estimating falling rate evaporation from finite soil columns

Estimating evaporation is very important in many fields including agriculture and geo-environmental engineering. For example evaporation has a definitive role in the performance of soil covers widely used in mining and landfill applications for environmental protection. The ability to estimate evaporation from different soils without performing detailed experiments or modeling could be very useful especially during the initial stages of design. Gardner and Hillel developed an equation for calculating evaporation from finite soil columns involving the soil moisture diffusivity. The capability of the equation to predict falling rate evaporation from finite coarse sand and fine sand was studied. The studied cases were also modeled using the finite element model SoilCover. The results from the model and the equation were compared to experimental data and found to agree reasonably. The agreement indicates that the analytical solution could be used to estimate falling rate evaporation from finite columns of coarse sand and fine sand. It can facilitate the estimate of actual evaporation from soils especially in the design of engineered covers. The necessary data required are soil–water characteristic curve, the hydraulic conductivity-suction function and initial water content. The analytical method relies heavily on the soil diffusivity; hence the concepts of diffusivity and its role in water movement in unsaturated soil are also discussed.