The subject of this article is the dynamics of water in a soil pedostructure sample whose internal environment is subjected to a potential gradient created by the departure of water through surface evaporation. This work refers entirely to the results and conclusions of a fundamental theoretical study focused on the molecular thermodynamic equilibrium of the two aqueous phases of the soil pedostructure. The new concepts and descriptive variables of the hydro-thermodynamic equilibrium state of the soil medium, which have been established at the molecular level of the fluid phases of the pedostructure (water and air) in a previous article, are recalled here in the systemic paradigm of hydrostructural pedology. They allow access to the molecular description of water migration in the soil pedostructure and go beyond the classical mono-scale description of soil water dynamics. We have specially studied and physically defined the flux of water in the soil pedostructure at its three levels of scale of organization: the Eulerian flux at macroscopic level of the total section of the pedostructure sample, the real flux of the two aqueous phases of the pedostructure through the inter primary-aggregates space; and the molecular flux of the mobile aqueous phase in the inter primary-aggregates space. The continuity equation can be written for each of these fluxes at the three scale levels and it is that defines the space/time ratio we need to establish the thermodynamic equation of the water conductivity. The experimental results show a perfect agreement with the theory, validating at the same time the systemic approach which was the framework.

The subject of this article is the dynamics of water in a soil pedostructure sample whose internal environment is subjected to a potential gradient created by the departure of water through surface evaporation. This work refers entirely to the results and conclusions of a fundamental theoretical study focused on the molecular thermodynamic equilibrium of the two aqueous phases of the soil pedostructure. The new concepts and descriptive variables of the hydro-thermodynamic equilibrium state of the soil medium, which have been established at the molecular level of the fluid phases of the pedostructure (water and air) in a previous article, are recalled here in the systemic paradigm of hydrostructural pedology. They allow access to the molecular description of water migration in the soil and go beyond the classical mono-scale description of soil water dynamics. We obtain a hydro-thermodynamic description of the soil′s pedostructure at different hydro-functional scale levels including those relating to the water molecule and its atoms. The experimental results show a perfect agreement with the theory, at the same time validating the systemic approach that was the framework.

Following the recent theorization of the systemic approach of natural organizations such as soils, we give in this article the systemic definition of three fundamental variables of thermodynamics: temperature as the internal energy of molecules constituting a fluid phase “ ; entropy ( ), as the ratio of two organization variables of the phase that are: the occupational volume of molecules and their own volume in the phase of volume ; and the internal molecular chemical potential as the ratio of the temperature of a molecule to its mass . This allowed the following conceptual advances that could not be done using the two principles of thermodynamics: i) establishing the definitional equations of the 3 equivalent forms of the Gibbs free energy of the system, ii) establishing that the general equilibrium criterion of the system is the internal molecular potential rather than the temperature , iii) removing the confusion between internal and external pressures of the system that did not allow to distinguish the two types of energy of a molecule: internal and external , then iv) correcting accordingly the differential equations of thermodynamic potentials as well as the Gibbs-Duhem equation. Application to the soil water air system is given followed by some comments about this new vision of thermodynamic equilibrium modeling

The purpose of this study was to evaluate the use of the pedostructure concept to determine the soil available water capacity, specifically the field capacity (FC). Pedostructure describes the soil aggregate structure and its thermodynamic interaction with water. Specifically, this work compared the calculation of soil water-holding properties based on the pedostructure concept with other standard methods for determining FC and permanent wilting point (PWP). The standard methods evaluated were the FAO texture estimate (FAO method), the Saxton-Rawls pedotransfer functions (PTFs method), and the water content at predefined soil suction (330 and 15,000 hPa) as measured with a pressure plate apparatus (PP method). Additionally, two pedostructure methods were assessed: the thermodynamic water retention curve (TWRC method) and the thermodynamic pedostructure (TPC method). Undisturbed loamy fine sand soil from a field in Millican, Texas, was analyzed at both the Ap and E horizons. The results showed that the estimated water content at FC and PWP for the three standard methods and for the TWRC method were in relative agreement. However, the TPC method used characteristic transition points in the modeled contents of different water pools in the soil aggregate and was higher for the Ap horizon, but in agreement with the other methods for the E horizon. For example, for the Ap horizon of the soil analyzed in this study, the FC estimated with the standard and TWRC methods ranged from 0.073 to 0.150 m(3) H2O m(-3) (soil), while the TPC method estimate was 0.221 m(3) H2O m(-3) (soil). Overall, the different methods showed good agreement in estimating the available water; however, the results also showed some variations in these estimates. It is clear that the TPC method has advantages over the other methods in considering the soil aggregate structure and modeling the soil water content within the aggregate structure. The thermodynamic nature of the TPC method enabled the use of both the soil shrinkage curve and the water retention curve in a weakly structured soil. It is expected that the TPC method would provide more comprehensive advances in understanding the soil water-holding properties of structured soils with higher clay contents.

Soil plays a pivotal role in enhancing global water and food security. Irrigation water constitutes more than 70% of the global water demand. The anticipated demographic increase and changing climate will impose more pressures on the global water and food systems. Therefore, and to achieve the target of “more crop per drop per area”, water management plans must be based on more accurate quantitative and dynamic approaches. It is increasingly obvious that the unique aggregates structure of the soil medium regulates water and nutrient circulations, and consequently defines soil and water health, productivity, and water use efficiency. However, the soil aggregates structure is not currently well considered in the quantification of soil–water holding properties. The authors applied a thermodynamic and soil structure-based approach to quantify soil–water holding properties. Specifically, the paper aims at providing a methodology, based on the pedostructure concept, to quantify field capacity (FC), permanent wilting point (PWP), and plant available water (AW). Pedostructure is a representative aggregates unit of a soil horizon that describes the structural organization of the soil medium. Four types of soil were analyzed considering various soil texture and aggregates structure: loamy fine sand, silt loam, clay loam, and silty clay loam. The calculated values for FC and PWP, based on the proposed pedostructure method, were compared with the recommended values by the standard FAO method and soil suction method. Results showed good agreement between the calculated values of the two methods. The proposed pedostructure method introduces a shift in quantifying the plant available water from a texture-based estimation to a soil aggregates structure-based calculation. Such a shift will enable capturing the changes in soil aggregates structure due to agro-environmental practices and the associated impact of these changes on soil–water holding properties.

The purpose of this study was to evaluate the use of the pedostructure concept to determine the soil available water capacity, specifically the field capacity (FC). Pedostructure describes the soil aggregate structure and its thermodynamic interaction with water. Specifically, this work compared the calculation of soil water-holding properties based on the pedostructure concept with other standard methods for determining FC and permanent wilting point (PWP). The standard methods evaluated were the FAO texture estimate (FAO method), the Saxton-Rawls pedotransfer functions (PTFs method), and the water content at predefined soil suction (330 and 15,000 hPa) as measured with a pressure plate apparatus (PP method). Additionally, two pedostructure methods were assessed: the thermodynamic water retention curve (TWRC method) and the thermodynamic pedostructure (TPC method). Undisturbed loamy fine sand soil from a field in Millican, Texas, was analyzed at both the Ap and E horizons. The results showed that the estimated water content at FC and PWP for the three standard methods and for the TWRC method were in relative agreement. However, the TPC method used characteristic transition points in the modeled contents of different water pools in the soil aggregate and was higher for the Ap horizon, but in agreement with the other methods for the E horizon. For example, for the Ap horizon of the soil analyzed in this study, the FC estimated with the standard and TWRC methods ranged from 0.073 to 0.150 mH2O msoil, while the TPC method estimate was 0.221 mH2O msoil. Overall, the different methods showed good agreement in estimating the available water; however, the results also showed some variations in these estimates. It is clear that the TPC method has advantages over the other methods in considering the soil aggregate structure and modeling the soil water content within the aggregate structure. The thermodynamic nature of the TPC method enabled the use of both the soil shrinkage curve and the water retention curve in a weakly structured soil. It is expected that the TPC method would provide more comprehensive advances in understanding the soil water-holding properties of structured soils with higher clay contents.

Soil aggregates structure (pedostructure) plays a pivotal role in regulating water and nutrient circulation, and consequently defines soil health, productivity, and water use efficiency. However, the soil aggregates structure is not currently considered in the quantification of soil-water holding properties. The authors applied a thermodynamic and soil structure-based approach to quantify soil-water holding properties. The paper provides a methodology, based on pedostructure concept, to quantify field capacity (FC), permanent wilting point (PWP), and available water (AW). The validity of the developed method was tested through application to two types of soil: a loamy fine sand soil and a silt loam soil. The calculated values for FC, PWP, and AW were compared with the FAO recommended values of FC, PWP and AW. For the loamy fine sand, the calculated values were: FC = 0.208 m(3)/m(3), PWP = 0.068 m(3)/m(3), and AW = 0.140 m(3)/m(3) all of which fall within the recommended values of FAO for such a soil type. Similarly, the calculated values for the silt loam were: FC = 0.283 m(3)/m(3), PWP = 0.184 m(3)/m(3), and AW = 0.071 m(3)/m(3) all were in agreement with the FAO recommended ranges for such a soil type. A thermodynamic, structure-based approach for soil water holding properties. Unique solutions for quantifying both field capacity and permanent wilting point. (C) 2018 The Author(s). Published by Elsevier B.V.

Using hydrostructural pedology, a new paradigm of soil characterization and modeling in agro environmental sciences, 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 new paradigm. The water in the pedostructure of soils is composed of two types of water: the micro water and the macro water. These two water pools are nested one in the other within the soil aggregates. They can either be found inside primary aggregates or outside of these aggregates in the interpedal space, depending on their chemical potential in relation to their position in the pedostructure. Using this new paradigm, a fundamental physics of the pedostructural water might be constructed. Finally, the soil medium can now be considered as the location in which the free water, blue water from rainfall or irrigation, and infiltrates by gravity through the macro pore space of the soil, is partially absorbed by the pedostructure, and becomes then the ‘green water’ of the soil. In reality, soil green water is the soil water reserve that is accessible to plant roots and is subsequently transpired into the canopy by the plants. The soil-water model Kamel®, developed based on this new paradigm, is uniquely able to physically simulate the opposite dynamic cycles of these two kinds of water (blue and green) within the soil-plant-atmosphere system, their exchanges and equilibrium states with time, at each depth of the pedon. For the sustainable development and management of agricultural zones, significant implications are offered for the strategy of soil-water characterisation, mapping, and modelling.

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.

Accurate measurement of the two soil moisture characteristic curves, namely, water retention curve (WRC) and soil shrinkage curve (SSC) is fundamental for the physical modeling of hydrostructural processes in vadose zone. This paper is the application part following the theory presented in part I about physics of soil medium organization. Two native Aridisols in the state of Qatar named locally Rodah "räôd´ə" soil and Sabkha "săb′kə" soil were studied. The paper concluded two main results: the first one is about the importance of having continuous and simultaneous measurement of soil water content, water potential and volume change. Such measurement is imperative for accurate and consistent characterization of each of the two moisture characteristic curves, and consequently the hydrostructural properties of the soil medium. The second is about the simplicity, reliability, strength and uniqueness of identifying the characteristic parameters of the two curves. The results also confirmed the validity of the thermodynamic-based equations of the two characteristic curves presented in part I.

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.

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.

This paper presents a new device [TypoSoilTM] for characterizing the hydrostructural properties of the soil medium organization. It intended to simultaneously and continuously measure both the shrinkage [V(W)] and the water potential [h(W)] characteristic curves for 8 cylindrical soil samples (~100 〖"cm" 〗^"3" ) at the same time during evaporation from the saturation to dry state. No other device makes similar measurements that are crucial to research the fundamental equations of the hydrostructural behavior of soils. This device makes part of a complete chain of measurements of the hydrostructural properties of the soil medium including also the soil swelling curve [V(t)] and the unsaturated hydraulic conductivity curve [K(W)]. Twelve soil samples were prepared, and analyzed by TypoSoilTM. These samples included three replicates of reconstituted and undisturbed soil cores of two native soils in the state of Qatar, named locally "Rodah soil" and "Sabkha soil". The obtained results indicated a good procedure for the soil samples preparation, and consistent measurement of the TypoSoilTM. Minor variations were observed among SWCC and SSCC of the three replicates of each soil type showing the trustworthiness of measurement. The results will help us to make an accurate hydro-functional typology of these kinds of soils in arid countries. This information is also needed for understanding and simulating the soil hydraulic behavior under agronomical practices (irrigation), or for planning some remediation techniques of the Sabkha soils. The quality of results confirmed also the thermodynamic theory behind the exploitation of these curves to extract the hydrostructural characteristic parameters.

This study involves a field evaluation of the pedostructure-based model Kamel(R) and comparisons between Kamel(R) and the Hydrus-1D(R) model. Both models are commonly named 1D - soil water models because they simulate the flux of water within the soil, related to climate conditions variation and water uptake by roots. The field site used in the investigation is within the Upper Cedar Creek Watershed (UCCW) in northwestern Indiana, USA. A description of both models has been made, emphasizing on the physical principles on which the pedostructure parameters for Kamel(R) were calculated or estimated using usual pedotransfer functions according to a methodology explained in the article. Sensitivity analysis for the pedostructure parameters of the Kamel(R) model showed that values for Kbs (slope of shrinkage curve for basic water content), VA (specific volume of pedostructure at dry state) and @a"L (hydraulic conductivity coefficient for high range of macropores) were the most sensitive parameters of the soil medium for Kamel(R) model input. The calibration for Kamel(R) and Hydrus-1D(R) was carried out for a 15day time period for determining the non-measured parameters. The calibrated parameters were used for validation of both models for the field site. Both models were found to be in good agreement with measured water content at the 5 and 20cm layers for the field site, but performance of Kamel(R) was superior to Hydrus-1D(R) for deeper layers. However the lack of information about the internal organization of the soil in situ, like thickness of horizons, roots repartition, accepted by Kamel(R), does not permit to make a complete validation of the model. We only show here that Kamel(R) can do as well as Hydrus-1D(R), a more known and experimented soil water model; however, Kamel(R) is more promising in terms of ability to couple dynamics of the internal hydrostructural state of the soil medium to the external climatic conditions and the biological processes in the critical zone.