VACUUM FILTRATION — (Water Quality) A process by which liquid is extracted from a sludge. The filtrate is forced through a filtering material by means of a vacuum.

VADOSE — Of, relating to, or being water that is located in the Zone of Aeration in the earth's crust above the ground water level.

VADOSE ZONE — The subsurface zone between the water table (Zone of Saturation) and the land surface where some of the spaces between the soil particles are filled with air. Also referred to as the Unsaturated Zone or, less frequently, the Zone of Aeration.

VADOSE WATER — Water occurring in the Unsaturated Zone (Vadose Zone) between the land surface and the water table.

VALLEY — An area of land that is lower than the land on either side of it.

VALVE — A device fitted to a pipeline or orifice in which the closure member is either rotated or moved transversely or longitudinally in the waterway so as to control or stop the flow.

VAPOR — The gaseous state of a substance which under ordinary conditions exists as a liquid or solid.

VAPOR BLANKET — The layer of air which overlies a body of water and, due to its proximity to the water, has a water vapor content higher than that of the surrounding atmosphere.

VAPOR FLOW — The gaseous flow of water vapor in soils from a moist or warm zone of higher potential to a drier or colder zone of lower potential.

VAPORIZATION — The change of a substance from a liquid or solid state to the gaseous state.

VAPOR PRESSURE — The partial pressure of water vapor in the atmosphere.

VEGETATION MANAGEMENT — The practice of manipulating the species mix, age, fuel load, and distribution of wildland plant communities within a prescribed management area. It includes prescribed burning, grazing, chemical applications, biomass harvesting, and any other economically feasible methods of enhancing, retarding, or removing the above-ground parts of plants.

VELOCITY OF A STREAM — Rate of motion of a stream measured in terms of the distance its water travels in a unit of time, usually in feet per second.

VENTURI — A short tube with a constricted throat used to determine fluid pressures and velocities by measurement of differential pressures generated at the throat as a fluid traverses the tube.

VENTURI METER — A meter, developed by Clemens Herschel, for measuring flow of water or other fluids through closed conduits or pipes. It consists of a venturi tube and one of several forms of flow registering devices.

VERTICAL SEPARATION DISTANCE — (Water Quality) A phrase used to describe the distance between the bottom of a sewage septic system's drain field and the underlying water table. The separation distance allows Pathogens (disease-causing bacteria, viruses, or protozoa) in the effluent to be removed by the soil before it comes in contact with the groundwater. Many different factors can affect pathogen removal and directly impact the separation distance needed for removal, including temperature, seasonal high groundwater tables, groundwater mounding, and soil type.

VERTICAL-VELOCITY CURVE — A curve showing how the down-gradient velocity varies with depth along a vertical depth-observation line in a surface stream.

VIRGIN FLOW — The streamflow which exists or would exist if man had not modified the conditions on or along the stream or in the drainage basin.