Equivalent Conductivity: The equivalent conductivity of a non-homogeneous soil is equal to the flux through that soil divided by the driving force or gradient in total potential across that soil. It could be considered the equivalent or average conductivity for the non-uniform soil.

Flux Density: The flux density of water passing through a soil is the volume of water passing through the soil per unit cross-sectional area (perpendicular to the flow) per unit time. It has units of length per unit time such as mm/sec, mm/hour, or inches/ day. Many times flux and flux density are used interchangeably.

Gravitational Potential: The gravitational potential of water is the amount of work required per unit quantity of water to move a very small amount of water reversibly and isothermally from a pool of pure water at atmospheric pressure at a reference level to another pool of pure water at the elevation of interest. This is simply the amount of work required to lift or lower the water from the reference level. (See also Units of Potential).

Hydraulic Conductivity: The hydraulic conductivity of a soil is a measure of the ease at which water moves through the soil. It can be obtained experimentally by measuring the flux density of water passing through a soil, the difference in total potential and the length of the soil. The conductivity is the proportionality constant which when multiplied by the driving force (or gradient in total potential) causing water to move gives the flux density of water. If the potential is defined in terms of a unit weight of water, then the gradient in total head has no dimensions and the conductivity has units of length per unit time just as the flux density does.

Matric Potential: The matric potential of water in a soil is the amount of work work required per unit quantity of water to move a very small amount of water reversibly and isothermally to the point of interest in the soil from a pool of pure water at atmospheric pressure at the same elevation. This is the amount of work required to move water into a soil from outside of it. Since the elevations are the same, gravity has no impact upon matric potential. Matric potential is another term for pressure potential or pressure head. (See also Units of Potential)

Total Potential: The total potential of water in a soil is the amount of work required per unit quantity of water to move a very small amount of water reversibly and isothermally from a pool of pure water at atmospheric pressure and at a reference level to the point of interest in the soil. This is the sum of the matric potential and the gravitational potential. (See also Units of Potential)

Units of Potential: All definitions of potential refer to work per unit quantity of water. The final units of potential depends upon the unit quantity of water chosen. It is convenient to define potential per unit weight. This means that all types of potential have units of length. This form of potential is often called "head". So we commonly talk of total head, gravitational head, and pressure head or matric head.

Volumetric Water Content: The volume of water in a soil divided by the total volume of the soil (i.e. the sum of the volumes of solids and pores).