Soil Water Physics Research Articles

Gravitational and capillary soil moisture dynamics for distributed hydrologic models

Abstract. Distributed and continuous catchment models are used to simulate water and energy balance and fluxes across varied topography and landscape. The landscape is discretized into computational plan elements at resolutions of 101–103 m, and soil moisture is the hydrologic state variable. At the local scale, the vertical soil moisture dynamics link hydrologic fluxes and provide continuity in time. In catchment models these local-scale processes are modeled using 1-D soil columns that are discretized into layers that are usually 10−3–10−1 m in thickness. This creates a mismatch between the horizontal and vertical scales. For applications across large domains and in ensemble mode, this treatment can be a limiting factor due to its high computational demand. This study compares continuous multi-year simulations of soil moisture at the local scale using (i) a 1-pixel version of a distributed catchment hydrologic model and (ii) a benchmark detailed soil water physics solver. The distributed model uses a single soil layer with a novel dual-pore structure and employs linear parameterization of infiltration and some other fluxes. The detailed solver uses multiple soil layers and employs nonlinear soil physics relations to model flow in unsaturated soils. Using two sites with different climates (semiarid and sub-humid), it is shown that the efficient parameterization in the distributed model captures the essential dynamics of the detailed solver.

Combined effects of gap creation and deer exclusion on restoration of belowground systems of secondary woodlands: A field experiment in warm-temperate monsoon Asia

Ground-layer vegetation in abandoned woodlands of the Northern Hemisphere is rapidly declining because of overshading and an overabundance of ungulate herbivores; these changes may be affecting belowground ecosystems. An effective countermeasure is to restore ground-layer vegetation by establishing deer exclosures immediately after creating canopy gaps. However, such measures disturb belowground processes (e.g., decelerate decomposition rates and decrease soil water content) and may cause further disintegration of the whole system. To estimate the combined effects of gap creation and exclosure installation, we established a 2×2-factorial experiment (including gap creation, exclosure installation, both methods, and a control) in warm-temperate secondary woodlands. After five years, we studied ground vegetation, litter cover, soil–water physics, abundance of litter-dwelling invertebrates, and litter decomposition in each experimental plot. A significant increase in ground vegetation was found in gap-and-exclosure plots and gap-and-deer plots, although the increase in the latter was limited to approximately half of that in the former; in contrast, no significant increase was found in the control or canopy-and-exclosure plots. The abundance of litter-dwelling invertebrates was significantly reduced by gap creation, although this effect was offset by installation of exclosures in gaps. Fine porosity and water holding capacity of topsoil were reduced by gap creation, but these negative effects were mitigated by exclosures installed in gaps. Meanwhile, coarse porosity and water permeability of topsoil were not affected by gap creation; they tended to be increased by exclosure installation both in gaps and under the canopy. Litter decomposition tended to be delayed in the gap-and-deer plot, although this effect was temporary; exclosure installation slightly increased decomposition rates, but this effect was limited. These results show the possibility of restoring belowground dynamics in abandoned forests by creating small canopy gaps and establishing ungulate exclosures. This mixture of interventions should increase habitat heterogeneity in these forests and help prevent biological homogenization while mitigating long-term negative effects of gap creation on belowground ecosystems.

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.

Pedostructure and Pedoclimate: New Concepts in Soil Water Physics Leading Hydrostructural Pedology at the Heart of Agri-Environmental Disciplines

Pedostructure and Pedoclimate: New Concepts in Soil Water Physics Leading Hydrostructural Pedology at the Heart of Agri-Environmental Disciplines

Modeling the soil system: Bridging the gap between pedology and soil–water physics

The biological and geochemical processes in soil such as organic matter mineralization, microbiological activity, and plant alimentation can be accurately assessed and modeled only with the knowledge of the thermodynamic status of the soil medium where these processes take place. However, current soil water models do not define and characterize the soil structure or the thermodynamic state of the soil water interacting with this structure. This article presents a new paradigm in characterizing and modeling the organized soil medium and the physical properties resulting from this organization. It describes a framework of the modeling approach as a contribution to the General Systems theory. The basic concept of Representative Elementary Volume (REV) in soil physics and hydrology was transformed into the concept of Structure Representative Volume (SREV) which takes into account the hierarchical organization of the structured soil medium. The pedostructure is defined as the SREV of the soil medium and this concept is at the basis of the new paradigm including variables, equations, parameters, and units in soil physics, in a similar way that the REV is at the basis of the continuous porous media mechanics applied to soils. The paradigm allows for a thermodynamic characterization of the structured soil medium with respect to soil water content then bridging the gap between pedology and soil physics. We show that the two points of view (REV and SREV) are complementary and must be used in the scaling of information. This approach leads to a new dimension in soil–water properties characterization that ensures a physically based modeling of processes in soil and the transfer of information from the physical scale of processes (pedostructure or laboratory measurements scale) to the application scale of the other disciplines (modeling and mapping scale).

Historical development of soil-water physics and solute transport in porous media

The science of soil-water physics and contaminant transport in porous media began a little more than a century ago. The first equation to quantify the flow of water is attributed to Darcy. The next major development for unsaturated media was made by Buckingham in 1907. Buckingham quantified the energy state of soil water based on the thermodynamic potential energy. Buckingham then introduced the concept of unsaturated hydraulic conductivity, a function of water content. The water flux as the product of the unsaturated hydraulic conductivity and the total potential gradient has become the accepted Buckingham-Darcy law. Two decades later, Richards applied the continuity equation to Buckingham's equation and obtained a general partial differential equation describing water flow in unsaturated soils. For combined water and solute transport, it had been recognized since the latter half of the 19th century that salts and water do not move uniformly. It wasn't until the middle of the 20th century that scientists began to understand the complex processes of diffusion, dispersion, and convection and to develop mathematical formulations for solute transport. Knowledge on water flow and solute transport processes has expanded greatly since the early part of the 20th century to the present.

Developments in soil–water physics since the mid 1960s

The theory for movement of water in unsaturated soils published by Richards 70 years ago is still an important starting point for the analysis of most soil physical problems. Over the last quarter century, the interest in finding new solutions of the Richards equation by either analytical or numerical methods was intense, particularly with regard to field situations. Experimental methods became more diverse and sophisticated: electromagnetic methods for measuring water content and salinity are now reliable and widely available, inverse methods for inferring the soil physical properties have matured. But during this period, we see also a widening of the scope of soil physics beyond the classical theory of Richards with further studies of various multiphase aspects—particularly, simultaneous movement of water and air, simultaneous transport of heat and moisture, flow of water and transport of solutes in structured soils—, of water movement in soils subject to swelling and shrinkage, and of transport of solutes in unsaturated soils. The latter two subjects became manageable by replacing the traditional spatial descriptions by material descriptions in which, respectively, the solid phase serves as the reference continuum for the water and the water serves as the reference continuum for the solutes. Progress was driven not only by a healthy theoretical, computational, and experimental basis, but also by productive interaction with adjoining disciplines and by challenging societal problems.

A theoretical scheme for evaluating heat and water exchange between the land surface and the atmosphere

The conception of heat and water exchange between the land surface and the atmosphere elaborated in the Laboratory of Soil Water Physics (Institute of Water Problems, Russian Academy of Sciences) is given. In frame of this conception the models of evapotranspiration and its components are described. The values of main parameters of the models are presented (partly they have been obtained during KUREX‐88 and KUREX‐91). The possible ways of the development of methods for (1) the soil water regime evaluation and (2) accounting for land surface spatial inhomogeneity are analyzed. It is shown that these problems can be solved if the techniques of the remote sensing data (on the radiative temperature of the land surface, the net radiation and the percentage cover of vegetation) are available.

A Visual Aid for Teaching Basic Concepts of Soil-Water Physics

T ERRESTRIAL plants depend on the soil for water supply. Thus, plant water relations and plant growth and productivity are tightly linked with water status in the soil. The soil can be viewed as a reservoir from which the plants draw the water but, water behavior in the soil is not simple. The unique physical properties of the soil particles determine the forces that govern soil water holding capacity and their uptake by the plants. This article presents a visual aid that helps teach the physical phenomena related to water status and water movement in the soil. These phenomena are relevant for the studies of plant physiology, ecology, and agriculture. However, many teachers are reluctant to teach them because certain basic concepts related to soil-water physics that operate at the microscopic and molecular levels are difficult to explain to students at the high school and college levels. Most notable among these concepts are matric water potential, diffusion path tortuosity, and void space partitioning between gas and liquid phases. A full mathematical formulation of the concepts in question is, in many cases, beyond the scope of basic courses in plant

The “physics” of soil water physics

Some aspects of the underlying conceptualizations (as opposed to experimental methodologies or mathematical algorithms) in soil water physics are reviewed. Contemporary issues relating to the symmetry properties of the Richards Equation, the status of the energy picture of soil water, and the theoretical description of coupled heat and water flows in soil are raised to exemplify as yet unresolved problems in the “physics” of soil water physics. Four basic questions which formalize these problems are posed as suggestions for future research.

Optimizing the Soil Physical Environment Toward Greater Crop Yields. Proceedings of an Invitational Panel Convened During the International Symposium on Soil-Water Physics and Technology Held at The Hebrew University Faculty of Agriculture in Rehovot, Israel, August 29 to September 5, 1971.Daniel Hillel

<i>Optimizing the Soil Physical Environment Toward Greater Crop Yields. Proceedings of an Invitational Panel Convened During the International Symposium on Soil-Water Physics and Technology Held at The Hebrew University Faculty of Agriculture in Rehovot, Israel, August 29 to September 5, 1971.</i>Daniel Hillel

An international symposium on soil water physics and technology

An international symposium on soil water physics and technology

International Symposium on Soil Water Physics and Technology

An International Symposium on Soil Water Physics and Technology will be held in Rehovot, Israel, August 29–September 5, 1971. The topics will include energy relations of water in soil, infiltration and unsaturated flow phenomena, cycle and balance of water in the field, evapotranspiration, irrigation, and soil water salinity problems. A special panel will be convened on the topic of ’Optimizing the Soil Physical Environment toward Greater Crop Yields.‘The proceedings of the symposium and of the special panel will be published.

An international symposium on soil water physics and technology

An international symposium on soil water physics and technology