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V

VACUUM DRYING—Removal of liquid material from a solution or mixture under reduced air pressure, which results in drying at a lower temperature than is required at full pressure.

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.

VALE—A valley, often coursed by a stream; a dale.

VALLEY—An area of land that is lower than the land on either side of it. Also see U-Shaped Valleys and V-Shaped Valleys.

VALLEY FILL—Alluvium or other material occupying areas below mountain slopes.

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 BARRIER—A continuous plastic membrane which surrounds the entire thermal envelope of a house and prevents moisture penetration into the wall cavity. Also referred to as a Vapor Retarder.

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 PLUMES—Flue gases visible because they contain water droplets.

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

VAPOR TRAIL—A visible trail of streaks of condensed water vapor or ice crystals sometimes forming in the wake of an aircraft. Also referred to as Contrail.

VARIABLE—(Statistics) A series of comparable observations or characteristics of a phenomenon taken as a single set of data; a listing of specific characteristics of a population or a number of observations taken over a specific period of time which may reasonably be expected to vary from observation to observation.

VARIANCE (2)—(Statistics) A measure of the spread or dispersion of a variable about its Mean or Arithmetic Mean value. The variance is calculated by taking the sum of the squares of the deviations, that is, the sum of the difference between the observed value and the series mean value, and dividing by the sample size (number of observations). The variance for a large data set (the population variance) is calculated as:

s2 [sigma] = (xi — x)2/n [sigma] = (xi — x)2/n

where:

xi is an individual observation;
x is the mean of all observations; and
n is the number of observations. xi is an individual observation;
x is the mean of all observations; and
n is the number of observations.

For smaller data sets (typically less than 50) the sample variance (s2) is calculated by replacing n with n—1 in this equation. The positive square root of the variance is called the Standard Deviation. Both the variance and the standard deviation are non-negative, by definition.

VARVE—(Geology) (1) A layer or series of layers of sediment deposited in a body of still water in one year. (2) A regular, annual layer of silt or clay deposited in a glacier-fed lake within one year's time. (3) A pair of layers of alternately finer and coarser silt or clay believed to comprise an annual cycle of deposition in a body of still water.

VASCULAR PLANT—Any of various plants, such as the ferns and seed-bearing plants, in which the phloem transports sugar and the xylem transports water and salts.

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.

VEGETATIVE CONTROLS—Non-Point Source (NPS) Pollution control practices that involve vegetative cover to reduce erosion and minimize loss of pollutants.

VELOCIMETER—A device for measuring the speed of sound in water.

VELOCITY, AVERAGE INTERSTITIAL ()—The average rate of ground-water flow in interstices, expressed as the product of Hydraulic Conductivity and Hydraulic Gradient divided by the Effective Porosity. It is synonymous with Average Linear Ground-Water Velocity or Effective Velocity.

VELOCITY HEAD—Energy contained by fluid because of its velocity; usually expressed in feet of fluid (foot-pounds per pound).

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.

VENA CONTRACTA—The minimum cross section of a jet of fluid discharging from an orifice or over a weir.

VENT—An air release valve or stand used to release air trapped at high points in a pipeline.

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 EFFECT—The increase in the velocity of a fluid stream as it passes through a constriction in a channel, pipe, or duct. Calculated by the Continuity Equation, or

Q = VA Q = VA

where Q is the volumetric flow rate, A is the Area of flow, and V is the fluid velocity. Because Q does not change, as A gets smaller then V must increase.

VENTURI FLUME—A calibrated measuring flume having a contracted throat section which produces a differential head that can be related to discharge.

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.

VENTURI SCRUBBERS—Air pollution control devices that use water to remove particulate matter from emissions.

VENTURI TUBE—A closed conduit that gradually contracts to a throat, causing a pressure head by which the velocity through the throat may be determined.

VERGLAS—A thin coating of ice, as on rock.

VERNAL POOLS—(1) Wetlands that occur in shallow basins that are generally underlain by an impervious subsoil layer (e.g., a clay pan or hard pan) or bedr