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Soil water is understood to be the equilibrium solution in the soil; pure water refers to the chemically pure compound H20. Water in soil is subject to several force fields originating from the presence of the soil solid phase, the dissolved salts, the action of external gas pressure, and the gravitational field. These effects may be quantitatively expressed by assigning an individual component potential to each (below and Table 4). The sum of these potentials is called the total potential of soil water. total potential of soil water (potentiel total de l'eau du sol) The amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water from a pool of pure water, at a specified eievation and at atmospheric pressure, to the soil water at the point under consideration. The total potential of soil water consists of:

  • osmotic potential
    • The amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water from a pool of pure water, at a specified elevation and at atmospheric pressure, to a pool of water identical in composition with the soil water at the point under consideration, but in all other respects being identical with the reference pool.
  • gravitational potential
    • The amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water, identical in composition with the soil water, from a pool at a specified elevation and at atmospheric pressure, to a similar pool at the elevation of the point under consideration.
  • matric potential (capillary potential)
    • The amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water, identical in composition with the soil water, from a pool at the elevation and the external gas pressure of the point under consideration, to the soil water.
  • gas pressure potential
    • This potential component is to be considered only when external gas pressure differs from atmospheric pressure as in a pressure membrane apparatus. A specific term and definition are not given.
  • water pressure, soil tension
    • The pressure (positive or negative), in relation to the external gas pressure on the soil water, to which a solution identical in composition with the soil water must be subjected in order to be in equilibrium through a porous permeable wall with the soil water. It may be identified with the matric potential defined above.
  • Osmotic pressure
    • The pressure to which a pool of water, identical in composition with the soil water, must be subjected in order to be in equilibrium, through a semipermeable membrane, with a pool of pure water (semipermeable means permeable only to water). It may be identified with the osmotic potential defined above.
  • total pressure
    • The pressure (positive or negative), in relation to the external gas pressure on the soil water, to which a pool of pure water must be subjected in order to be in equilibrium through a semipermeable membrane with the soil water. Total pressure is therefore equal to the sum of soil water pressure and osmotic pressure. Total pressure may also be derived from the measurement of the partial pressure of the water vapor in equilibrium with the soil water. It may be identified with the total potential defined above when gravitational and external gas pressure potentials can be neglected.
  • hydraulic head
    • The elevation with respect to a specified reference level at which water stands in a piezometer connected to the point in question in the soil. Its definition can be extended to soil above the water table if the piezometer is replaced by a tensiometer. The hydraulic head in systems under atmospheric pressure may be identified with a potential expressed in terms of the height of a water column. More specifically, it can be identified with the sum of gravitational and matric potentials and may be called the hydraulic potential.
  • water content
    • The amount of water lost from the soil when it is dried to constant weight at 105oC; expressed either as the weight of water per unit weight of dry soil or as the volume of water per unit bulk volume of soil. The relationship between water content and soil water pressure is called the soil moisture retention curve, or sometimes the soil water (moisture) characteristic. Depending upon whether the curve is determined with decreasing or increasing water content, it is a desorption or adsorption curve respectively.
  • differential water capacity
    • The absolute value of the rate of change of water content with soil water pressure. The water capacity at a given water content will depend on the particular desorption or adsorption curve used. Distinction should be made between volumetric and specific water capacity.

Experimentally it has been established that generally the flow of a fluid in a porous medium can be described by Darcy's law, which states that the flux of fluid is proportional to the driving force. In viscous flow of water in soils, the driving force equals the negative gradient of the hydraulic potential.

  • hydraulic conductivity
    • The proportionality factor in Darcy's law as applied to the viscous flow of water in soil, that is, the flux of water per unit gradient of the hydraulic potential. if conditions require that the viscosity of the fluid be separated from the conductivity of the medium, it is convenient to define the permeability (or intrinsic permeability) of the soil as the conductivity (expressed in g­¹ cm ³ sec) multiplied by the viscosity (in poises). For the purpose of solving the partial differential equation of the nonsteady-state flow in unsaturated soil it is often convenient to introduce a variable called the soil water diffusivity.
  • soil water diffusivity
    • The hydraulic conductivity divided by the differential water capacity (using consistent units), or the flux of water per unit gradient of moisture content in the absence of other force fields.