Preface

This book aims to define as completely as possible the subject matter and main points of this new discipline, hydrostructural pedology, as theorized now for the first time: the underlying concepts, the purpose and role that it has to play within the agro-environmental sciences. It is divided into two parts:

• - a theoretical part, where the systemic approach applied to the soil is presented, showing how this leads to the thermodynamic formulation of water in the soil's organized medium and to the systemic modeling of soil-water-coupling in natural or anthropic organizations;
• - a methodological part, dedicated to determining the hydrostructural characteristics of a pedostructure1, characteristic parameters of equilibrium state equations and the hydrostructural functioning of soil.

Below we give a brief overview of the key points in the emergence of this discipline up to the development of a physical theory of soil-water; one might indeed wonder and ask why only now, in 2015, is the theory presented as new and complete?

Realizing the need for a change in the paradigm to quantitatively describe water-soil structure interactions

Only after it became possible to continuously and accurately measure the shrinkage curve [BRA 88] did the hydrostructural properties of the soil medium organization become accessible in laboratory-based experimental studies and their physical modeling possible, in particular those of the pedostructure - the first hydrofunctional level of a soil horizon. Considering the shrinkage curve as proof of the interaction between water and the soil structure has logically led us to conceive another paradigm for soil characterization, which involved adopting another system of descriptive soil variables than that currently used, to allow us to take into consideration the hierarchical organization of the soil medium. Being one of the few to possess a measuring instrument (retractometer), we were effectively the only lab at IRD to work on the shrinkage curve produced by the water-soil structure interaction, and thus, to perceive the need for a change in paradigm. A new paradigm of the characterization and modeling of the soil hydrostructural functioning was finally theorized, in a close relationship with the systemic approach, which consequently needed revision and clarification of its principles in order to be applied to pedology and the description of natural organizations. This is explained in sections 3.1 and 3.2 of the book.

Formalization of equations in the new paradigm and completion of the theory of physical and systemic modeling of the water-soil coupling

However, measuring a shrinkage curve alone, i.e. without the other soil moisture characteristic curve, the water retention curve, did not allow for determining of the validity of the equations of the water-soil interaction theory occurring in the new paradigm. We still lacked a laboratory apparatus that could continuously and simultaneously measure the two characteristic curves of soil moisture, namely the soil shrinkage curve and the soil-water retention curve, in order to be able to finalize the theory. This device, TypoSoil®, was built in 2012 at the soil hydrophysics laboratory at IRD in Bondy [BEL 13] in collaboration with Valorhiz. It was tested at Qatar Environment and Energy Research Institute (QEERI) - Qatar Foundation in 2013 [BRA 13]. The use of the TypoSoil® has effectively allowed exact thermodynamic formulation of state equations of the pedostructure: the soil shrinkage curve and the water retention curve written in the new systemic paradigm where the concept of a thermodynamic system, with regard to soil structure, was established. These major scientific advances in soil-water thermodynamics are explained in sections 3.3 and 3.4.

The pedostructure and pedostructural water, "green water" of the soil: new objects of study and research in agro-eco-environmental sciences

Hydrostructural pedology integrates classical pedology, the descriptive science of soil's internal organizations throughout the soil profile and the horizontal organization of soil types (pedological cover), with the physics of soil-water in a single, physical, systemic model - hydrofunctioning of soil organizations at different levels of hydrofunctional scale.

Water is, in fact, omnipresent in the environment. Water not only plays a pivotal role in the formation of the hierarchal organization of soil's hydrofunctional units (relief units, geomorphological units, soil units, pedon, horizons, aggregates, and primary peds), but also controls the activities and equilibriums within these units. Water is, therefore, omnipresent in this ecosystem: in the air above the vegetal cover, in the plant that leads the soil-water back into the atmosphere, and in the soil that receives rainwater, part of, which is stored and reserved to the plant and part of, which percolates deeply by gravity to supply groundwater. These two water cycles do not have the same function and must be distinguished from one another in the soil. In fact, the natural exchange between the two types of water "gravitational" (rain, irrigation) that travels downwards and "thermodynamic" (absorbed and retained by soil clays) that the plants introduce into the upwards water cycle of the soil-plant-atmosphere system of the critical zone. Agronomists have given the name "green water" to this thermodynamic water stored in the soil and then released back into the atmosphere through the plant. Unfortunately, the distinction between the two water cycles within the soil medium, and thus the mechanical exchange between the two, are impossible to formulate with the current paradigm of soil physics that is based on the REV principle: Representative Elementary Volume. Current soil-water models are generally based on this principle to deal with the soil structure and are not capable of identifying the green water of the soil; they continue to use the former "water reserve" concepts that are estimated empirically (calculated between two standard fixed retention pressures).

Knowing how to quantify this water and its dynamics in the soil has always been a challenge of great importance in agriculture: it is linked to the water demands of the plant, survival conditions of an ecosystem, resilience in case of climate change, etc. It is only in the new paradigm, where the modeling and the hydrostructural characterization of the soil takes into account the soil structure and its internal organization into aggregates, that this challenge has been overcome. We have recently been able to identify the "green water" of soil to the water of the pedostructure (or pedostructural water) and thus to model the thermodynamic and hydrostructural properties of this green water using physically established equations (non-empirical). Therefore, we can say that the only soil-water model that takes into account the pedostructure and its hydrostructural properties, including pedostructural water, is the Kamel® model elaborated in the new paradigm of hydrostructural modeling of soil mentioned above.

A radical stance: the natural organization can only be known after its transformation into a system (organized) by the systemic approach

Here we explain why an understanding of the activity mechanisms in the natural environment, or of natural objects such as soils and their physical modeling, requires one to take a clear stance from the beginning with regard to the distinction between organization and system. The systemic approach serves to transform the organization into an organized system available to man to understand its internal functioning, external activity, management, use, etc. We follow the footsteps of Bertalanffy and his companions, who founded the general systems theory (1932-1950), in particular by readjusting the work of the contemporary systemician J.L. Le Moigne [LEM 94] to the issue of the physical modeling of the soil organization functioning with water. A new adaptation has put back the theory of systemic modeling in Cartesian logic, abandoning "the four precepts of the new discourse of the method" proposed by Le Moigne, as the basis for his theory, and replacing them by the four precepts of Descartes that he had refuted "radically".

The discovery of the correct thermodynamic formulation of the soil-water retention curve, and consequently that of the shrinkage curve, would not have been possible without adopting the systemic "Cartesian" approach and that of the new concept proposed in this approach, the SREV: Structural Representative Elementary Volume. The SREV is the base concept of the new paradigm of the physics of water in the organized soil medium. It will replace what lies at the base of the current paradigm of soil-water physics, namely, the REV or Representative Elementary Volume, a concept which was posed as fundamental hypothesis in the physics of continuous porous mediums. Each of the two paradigms has its own system of well-defined descriptive variables of the soil medium, but both are exclusive of one another. Thus, there is a change in paradigm when we assume that the SREV replaces REV, because the system of descriptive variables radically changes. With the SREV hypothesis, we use the set of systemic variables, allowing us to write equations describing the physical processes, and not only, as is...