Erik Braudeau

Physics of the soil medium organization part 1: thermodynamic formulation of the pedostructure water retention and shrinkage curves

The equations used in soil physics to characterize the hydro-physical properties of the soil medium cannot be other than empirical since they do not take into account the multi-scale functional organization of the soil medium that is described in Pedology. To allow researching the correct formulation of the physical equations describing the soil medium organization and properties, a new paradigm of hydrostructural pedology is being developed. This paradigm is to establish the conceptual link between the classical Pedology and the soil-water physics (hydrostructural characterization and modeling of the soil medium). The paradigm requires the exclusive use of the concept of Structural Representative Elementary Volume (SREV) instead of the classical Representative Elementary Volume (REV) in any physical modeling of the hydrostructural behavior of the soil medium and of the links with the biotic or abiotic processes evolving within it. This article presents the development of the physical equations of the shrinkage curve and the soil water retention curve from the thermodynamic point of view according to the new paradigm. The new equations were tested and the theory validated using data of simultaneous measurement of both curves on a cylindrical soil sample (pedostructure). Implications of these results on the physical modeling in agro-environmental sciences are discussed.

A framework for soil-water modeling using the pedostructure and Structural Representative Elementary Volume (SREV) concepts

Current soil water models do not take into account the internal organization of the soil medium and consequently ignore the physical interaction between the water film at the surface of solids that form the soil structure and the structure itself. In this sense, current models deal empirically with the physical soil properties, which are all generated from this soil water and soil structure interaction. As a result, the thermodynamic state of the soil water medium, which constitutes the local physical conditions of development for all biological and geochemical processes within the soil medium, is still not well defined and characterized. This situation limits modeling and coupling the different processes in the soil medium since they all thermodynamically linked to the soil water cycle. The objective of this article is to present a complete framework for characterizing and modeling the internal soil organization and its hydrostructural properties resulting from interaction of its structure with the soil water dynamics. The paper builds on the pedostructure concept, which allowed the integration of the soil structure into equations of water equilibrium and movement in soils. The paper completes the earlier framework by introducing notions of soil-water thermodynamics that were developed in application to the concept of the Structural Representative Elementary Volume (SREV). Simulation of drainage after infiltration in the Yolo loam soil profile, as compared to measured moisture profile using the measured soil characteristic parameters, showed a high degree of agreement. This new modeling framework opens up new prospects in coupling agro-environmental models with the soil medium, recognizing that the soil organization, hydro-structural, and thermodynamic properties are the foundation for such coupling.

Soil water thermodynamic to unify water retention curve by pressure plates and tensiometer

The pressure plate method is a standard method for measuring the pF curves, also called soil water retention curves, in a large soil moisture range from saturation to a dry state corresponding to a tension pressure of near 1500 kPa. However, the pressure plate can only provide discrete water retention curves represented by a dozen measured points. In contrast, the measurement of the soil water retention curves by tensiometer is direct and continuous, but limited to the range of the tensiometer reading: from saturation to near 70-80 kPa. The two methods stem from two very different concepts of measurement and the compatibility of both methods has never been demonstrated. The recently established thermodynamic formulation of the pedostructure water retention curve, will allow the compatibility of the two curves to be studied, both theoretically and experimentally. This constitutes the object of the present article. We found that the pressure plate method provides accurate measurement points of the pedostructure water retention curve h(W), conceptually the same as that accurately measured by the tensiometer. However, contrarily to what is usually thought, h is not equal to the applied air pressure on the sample, but rather, is proportional to its logarithm, in agreement with the thermodynamic theory developed in the article. The pF curve and soil water retention curve, as well as their methods of measurement are unified in a same physical theory. It is the theory of the soil medium organization (pedostructure) and its interaction with water. We show also how the hydrostructural parameters of the theoretical curve equation can be estimated from any measured curve, whatever the method of measurement. An application example using published pF curves is given.

Improving TDR Measurements through Accounting for Soil Shrinkage Properties

Time domain reflectometry (TDR) is a widely used tool for indirect measurement of soil moisture content. Empirical formulation is used to link the apparent dielectric constant of soil to the volumetric moisture content (Topp’s Equation: Topp et al., 1980) or gravimetric moisture content as a function of soil bulk density (Siddiqui and Drnevich Equation: Siddiqui and Drnevich, 1995). This paper introduces a methodology to account for soil volume change by integrating the true bulk density of the soil into the measurements using the soil Shrinkage Curve (specific volume (cm3/g) versus gravimetric water content). Thus, bulk density becomes a non-constant parameter that can be calculated as a function of the soil water content in the Siddiqui and Drnevich Equation. Experimental evidence demonstrates accounting for soil shrinkage improves the accuracy of TDR measured moisture contents and allows for estimating the shrinkage curve. Direct water content calculation for the Chalmers soil was compared to water contents from TDR readings with and without shrinkage corrections; those with shrinkage corrections showed significantly improved accuracy in TDR-determined soil moisture.

Hydrostructural Pedology, New Scientific Discipline Allowing for Physical Modelling of ‘Green Water’ Dynamics in the Soil-Plant-Atmosphere System

Using a new paradigm of soil characterization and modeling in agro environmental sciences, named hydrostructural pedology, we were able to show that the “green water” concept of agronomists corresponds exactly to the pedostructural water concept which was physically defined in this paradigm. The water in the pedostructure of soils is composed of two types of water, named micro and macro, nested one in the other. They are differentiated by their chemical potential related to their position in the pedostructure: inside primary aggregates or outside of them in the interpedal space. A fundamental physics of the pedostructural water could be developed within this new paradigm. Finally, the soil medium can now be considered as the location in which the free water (named also blue water), coming from surface (rainfall, irrigation, etc.) and going down by gravity through the macro pore space of the soil, is partially absorbed by the pedostructure, and becomes Mini-review Article Braudeau et al.; JAERI, 15(3): xxx-xxx, 2018; Article no.JAERI.43822 2 then the ‘green water’ of the soil. Soil green water is, in fact, the soil water reserve available to plant roots and subsequently transpired by the plants into the canopy. The soil-water model Kamel ® , built according to this new paradigm, is the only model able to physically simulate the opposite dynamic cycles of these two kinds of water (blue and green) within the soil-plantatmosphere system, their exchanges and equilibrium states according to time, at each depth of the pedon. Important implications about strategy of soil-water characterization, mapping and modeling are given for sustainable development and management of agricultural zones.

Soil Water: Functions in Pedostructure

There are several models of soil water that consider the soil medium as an active site for chemical, physical, and biological processes with a bimodal porous medium, micro- and macropore systems. Few of these models consider the soil medium as a structured medium with aggregates. However, none considered the swelling-shrinkage properties of these aggregates and the resulting hydrostructural properties of the soil medium, which significantly contribute to the formulation of transfer functions and their parameterization. Thus, soil dynamics literature describes soil properties independently from the aggregated organization of soils and their structural dynamic with water.

Integrating Large-Scale Hydrologic Modeling into Student Learning and Decision Making

This paper examines how the incorporation of the concept of scaling as well as large-scale processes into student curriculum impact student learning and their decision making capabilities. With the wide range of scales of hydrological processes, spanning about eight orders of magnitude in space and time, defining large-scale hydrological modeling is critical. To achieve this goal, learning material is prepared to introduce the concept of scaling, provide hydrologic modeling case studies, and test for students’ enhanced knowledge and improved decision making skills. The material are designed to accommodate different time allocations, levels (undergraduate vs. graduate), and students’ technical backgrounds. An outcome-based evaluation procedure is used to measure the effectiveness of the use of large-scale hydrologic modeling in enhancing student learning and decision making capabilities. Preliminary results show that introducing the concept of scaling and its application using large scale computer models enhances student learning and their decision making skills. Students’ level of confidence in answering the pre- and post- tests also increases after the introduction of the scaling concept and following a computer model exercise.

Assessing Soil Residual Stresses and Their Impact on Cracking Behavior

Residual stresses develop in soils as they shrink or swell. Assessing those stresses has always been an active area of research in soil science. Shrinkage induced cracking is a phenomenon that develops in soils as result of residual tensile stresses generation in soil structure. This paper presents a method to assess residual effective stresses in soil in an attempt to link the residual stress to soil dynamic properties as a function to water content. Relating stresses to soil water content would contribute to the understanding of various stress-related phenomena, an application of which is to physically characterize shrinkage induced cracking behavior of soils. The objective of this paper is to assess soil shrinkage-induced stresses and to study the impact of these stresses on crack behavior of soil during shrinkage. The Restrained Ring Method is introduced as an approach to measure, as function of soil water content, (1) the effective residual stresses in soils, (2) soil tensile stresses and (3) soil effective modulus of elasticity, Eeff. An analytical solution using elasticity theory was applied to estimate those parameters using experimental results. A silty clay loam soil was tested and results for the soil effective residual stresses, its tensile stresses, and its effective modulus of elasticity, Eeff were provided. Obtained results were compared to ranges from the literature and showed good correlation.

Development and Evaluation of Functional Soil Mapping Unit Using Pedostructure Hierarchy Concept

Accurate soil characterization is crucial for understanding soil-water interactions and allowing for better on-farm agricultural and environmental management. Current soil characterization methods lack quantitative attributes that integrate the soil mapping units with environmental and agronomical models. In this research we proposed a methodology to physically characterize the soil water medium using quantitative parameters. We incorporated the continuously measured soil water potential curve and the soil shrinkage curve to extract the physically based pedostructural (PS) parameters needed for characterizing the soil water medium. We present a methodology to generate and define functional soil mapping units that possess physical and quantitative parameters. We followed a four step general system hierarchical approach overlaying study area map, landform map, and SSURGO map. The fourth step was to validate the generated functional soil units using discriminate analysis performed on the PS parameters extracted for every soil mapping unit. Results of this study show that surface horizon (Ap) have higher PS parameter values than subsurface horizon Bt. The PS concept showed high capabilities in defining the soil mapping units, as concluded from the discriminate analysis that was performed.

KamelSoil®: A model for soil characterization from basic soil textural properties

A new conceptual and functional model of the soil-water medium organization, in which the internal structure of the soil horizon, named the pedostructure, is made up of swelling aggregates in a hierarchy of sizes, was recently presented. This representation leads to define a new paradigm for modeling the physical interaction between the soil structure and the water at the level of the process and for the macroscopic characterization of soil physical properties at the field scale. A computer model of the hydro-structural functioning of a pedon, named Kamel®, was built in the framework of this paradigm. Accordingly, the hydrostructural input parameters of Kamel® are those of physically-based equations that describe the hydraulic functionality of the pedostructure, namely: 1) the shrinkage curve, 2) the soil water potential curve, 3) the conductivity curve, and 4) the swelling dynamic curve. These parameters have a physical meaning and can be extracted precisely from the measurement of the characteristic curves in laboratory. The objective of the paper is i) to present the basic principles of Kamel® along with the state variables and functional parameters used, allowing to calculate the state variables at each depth of the pedon and to integrate this information at the field scale level; and ii) to present “KamelSoil®”, a software that translates the traditional soil characteristics into the required hydrostructural parameters of Kamel®. Therefore Kamel® can theoretically work for all soil types, at high degree of accuracy when the characteristic curves are measured or, at least, with the same approximations made by the usual soil-water models using empirical parameters and pedotransfer functions.