FACE (of a Dam) — The external surface of a structure, such as the surface of an appurtenance or a dam.

FALL OVERTURN — A physical phenomenon that may take place in a body of water during early autumn. The sequence of events leading to fall overturn include:
[1] The cooling of surface waters;
[2] A density change in surface waters producing convection currents from top to bottom;
[3] The circulation of the total water volume by wind action; and
[4] Eventual vertical temperature equality.
The overturn results in a uniformity of the physical and chemical properties of the entire water body. Also referred to as Fall Turnover.

FALLOW — (1) Allowing cropland, either tilled or untilled, to lie idle during the whole or greater portion of the growing season. (2) Land plowed and tilled and left unplanted.

FALLS — A waterfall or other precipitous descent of water.

FARM DELIVERY REQUIREMENT — The Crop Irrigation Requirement plus farm losses due to evaporation, deep percolation, surface waste, and nonproductive consumption. The losses are measured by the Farm Irrigation Efficiency, which is the percent of farm-delivered water that remains in the root zone and is available for crop growth.

FARM EFFICIENCY — The consumptive Crop Irrigation Requirement (CIR) divided by the farm water delivery.

FARM HEADGATE DELIVERY (DIVERSION) — That amount of water in acre feet (AF) delivered through a farm headgate.

FARM IRRIGATION EFFICIENCY — An expression comparing the amount of water actually required for growing a crop to the amount of irrigation water that is diverted at the farm headgate. Expressed as a percentage on an annual basis.

FARM POND — A water impoundment made by constructing a dam or embankment or by excavating a pit or "dug out".

FARM SURFACE RUNOFF (TAILWATER) — A portion of the Farm Headgate Delivery that flows off the lower portion of the farm or field surface (drain ditch) flow. This is one loss component considered in Farm Irrigation Efficiency.

FARM WASTE AND DEEP PERCOLATION — The amount of irrigation water delivered to the crop area from a canal turnout or ground water pump that is not consumptively used on the crop area. Includes water moving through the root zone to the water table, water intercepted by drainage systems, and surface waste to natural or constructed drainage systems, and non-cropped areas.

FEEDWATER — (Water Quality) Water input into a desalting or water treatment plant.

FERTIGATION — The use of irrigation water as a vehicle for spreading fertilizer on the land.

FERTILIZER — Any organic or inorganic material of natural or synthetic origin that is added to a soil to supply elements essential to plant growth. Various types of fertilizers include acid-forming, blended, bulk-blended, chemical, coated, conditioned, granular, liquid, non-granular, prilled, solution, straight, and suspension.

FETCH — (1) The distance traveled by waves in open water, from their point of origin to the point where they break. (2) The distance the wind blows over water or another homogeneous surface without appreciable change in direction.

FIELD — (1) A broad, level, open expanse of land; a meadow. (2) A cultivated expanse of land, especially one devoted to a particular crop. (3) A portion of land or a geologic formation containing a specified natural resource. (4) A wide, unbroken expanse, as of ice.

FIELD (MOISTURE) CAPACITY — The capacity of soil to hold water. It is measured by the soil scientist as the ratio of the weight of water retained by the soil to the weight of the dry soil.

FIELD DIVERSION — An interception channel near the contour to carry runoff to a waterway. Intervals vary with the precipitation, slope, and cropping.

FIELD-MOISTURE CAPACITY — The quantity of water which can be permanently retained in the soil in opposition to the downward pull of gravity.

FIELD-MOISTURE DEFICIENCY — The quantity of water which would be required to restore the soil moisture to Field-Moisture Capacity.

FIELD PERMEABILITY — Permeability corresponding to the temperature which occurs under field conditions.

FIELD SPRINKLER SYSTEM — A system of closed conduits carrying irrigation water under pressure to orifices designed to distribute the water over a given area.

FILTER — A device used to remove solids from a mixture or to separate materials. A porous material through which a liquid or gas is passed in order to separate the fluid from suspended particular matter. Suspended materials are frequently separated from water using filters.

FILTER BED — A layer of sand or gravel on the bottom of a reservoir or tank, used to filter water or sewage.

FILTER CAKE — (1) The solids or semisolids deposited on a filter as a fluid is moved through it. (2) The remaining solids or semisolids on a filter after the fluid in a material is extracted by a negative pressure.

FILTER FEEDER — An aquatic animal, such as a clam, barnacle, or sponge, that feeds by filtering particulate organic material from water.

FILTER STRIP — A strip or area of vegetation used for removing sediment, organic matter, and other pollutants from runoff and waste water.

FILTER ZONE (of a Dam) — A band or zone of granular material that is incorporated into a dam and is graded (either naturally or by selection) so as to allow seepage to flow across or down the filter without causing the migration of material from zones adjacent to the filter zone.

FILTERABLE — Of particles that are sufficiently small to allow their passage through filters capable of retaining most particles. For example, a filterable virus is one that will pass through a filter that will normally retain bacteria.

FILTRATE — Liquid that has been passed through a filter.

FILTRATION — (1) The process in which suspended matter is removed from a liquid through a medium which is permeable to the liquid but not to the suspended material. (2) (Water Quality) A treatment process, under the control of qualified operators, for removing solid (particulate) matter from water by means of porous media such as sand or a man-made filter; often used to remove particles that contain Pathogens.

FINAL CLARIFIER — (Water Quality) A gravitational settling tank installed as part of some wastewater treatment plants and placed after the biological treatment step. The tank functions to remove suspended solids.

FINISHED WATER — (Water Quality) Water that has completed a purification or treatment process; water that has passed through all the processes in a water treatment plant and is ready to be delivered to consumers. Contrast with Raw Water.

FIRM CAPACITY — For public drinking water supplies, the system delivery capacity with the largest single water well or production unit out of service.

FIRM YIELD — The maximum annual supply of a given water development that is expected to be available on demand, with the understanding that lower yields will occur in accordance with a predetermined schedule or probability.

"FIRST IN TIME, FIRST IN RIGHT" — A phrase indicating that older water rights have priority over more recent rights if there is not enough water to satisfy all rights.

FIXED GROUND WATER — Water held in saturated material within pore spaces so small that it is permanently attached to the walls, or moves so slowly that it is usually not available as a source of water for pumping.

FLATBOAT — A boat with a flat bottom and square ends used for transportation of bulky freight, especially used in shallow waters.

FLAT-WATER — Of or on a level or slow-moving watercourse.

FLOAT — (1) To remain suspended within or on the surface of a fluid without sinking. To cause to remain suspended without sinking or falling. (2) To put into water; launch. (3) To flood (land), as for irrigation.

FLOATER — A Wetland plant that floats on the surface of the water.

FLOC — Generally, a very fine, fluffy mass formed by the aggregation of fine suspended particles, as in a precipitate. In terms of water quality, clumped solids or precipitates formed in sewage by biological or chemical activity.

FLOCCULATE — To aggregate or clump together individual, tiny particles into small clumps or clusters.

FLOCCULATION — (Water Quality) In water and wastewater treatment, the agglomeration or clustering of colloidal and finely divided suspended matter after coagulation by gentle stirring by either mechanical or hydraulic means such that they can be separated from water or sewage.

FLOE — An ice flow. Also a segment that has separated from such an ice mass.

FLOE ICE — Ice usually several feet thick, which has formed on the surface of a body of water and then has broken into pieces and is floating on the water's surface.

FLOOD, or Flood Waters — (1) An overflow of water onto lands that are used or usable by man and not normally covered by water. Floods have two essential characteristics: The inundation of land is temporary; and the land is adjacent to and inundated by overflow from a river, stream, lake, or ocean. (2) As defined, in part, in the Standard Flood Insurance Policy (SFIP): "A general and temporary condition of partial or complete inundation of normally dry land areas from overflow of inland or tidal waters or from the unusual and rapid accumulation or runoff of surface waters from any source."

FLOOD, 100-YEAR — A 100-year flood does not refer to a flood that occurs once every 100 years, but to a flood level with a 1 percent or greater chance of being equaled or exceeded in any given year. Areas between the 100-year and the 500-year flood boundaries are termed Moderate Flood Hazard Areas. The remaining areas are above the 500-year flood level and are termed Minimal Flood Hazard Areas.

FLOOD, ANNUAL — The highest peak discharge in a water year.

FLOOD-BASE DISCHARGE — A value of high flow usually computed during the first 5 years of station operation that, on the average, is exceeded about three times per year.

FLOOD CAPACITY — The flow carried by a stream or floodway at bankfull water level. Also, the storage capacity of the flood pool at a reservoir.

FLOOD CONTROL (STORAGE) — The control of flood waters by the construction of flood storage reservoirs, flood water retaining structures, channel improvements, levees, bypass channels, other engineering works, or vegetative changes.

FLOOD CONTROL POOL — Reservoir volume reserved for flood runoff and then evacuated as soon as possible to keep that volume in readiness for the next flood.

FLOOD CREST — The maximum stage or elevation reached by the waters of a flood at a given location.

FLOOD DAMAGE — The direct and indirect economic loss caused by floods including damage by inundation, erosion, or sediment deposition. Indirect damages may also include emergency costs and business or financial losses. Evaluation may be based on the cost of replacing, repairing, or rehabilitating; or the comparative change in market or sales value; or on the change in income or production caused by flooding.

FLOOD DURATION CURVE — A cumulative frequency curve that shows the percentage of time that specified discharges are equaled or exceeded.

FLOOD FORECASTING — Flood forecasts are primarily the responsibility of the National Weather Service, National Oceanic and Atmospheric Administration (NOAA), and are used to predict flood stages and times and indicate areas subject to flooding.

FLOOD FREQUENCY — A statistical expression or measure of how often a hydrologic event of a given size or magnitude should, on an average, be equaled or exceeded. For example, a 50-year frequency flood (2 percent change of occurrence) should be equaled or exceeded, on the average, once in 50 years.

FLOOD FREQUENCY CURVE — (1) A graph showing the average interval of time within which a flood of a given magnitude will be equaled or exceeded once. (2) A similar graph but plotted with the Recurrence Intervals of floods plotted instead.

FLOODGATE — (1) A gate used to control the flow of a body of water. Also referred to as a Water Gate. (2) Something that restrains a flood or an outpouring.

FLOODING — Temporary inundation of all or part of the floodplain along a well-defined channel or temporary localized inundation occurring when surface water runoff moves via surface flow, swales, channels, and sewers toward well-defined channels. Flooding is not necessarily synonymous with Flooding Problem.

FLOODING PROBLEM — The disruption to community affairs, damage to property and facilities, and the danger to human life and health that occurs when land use is incompatible with the hydrologic-hydraulic system.

FLOOD, INTERMEDIATE REGIONAL — A flood having a one percent probability, or an average frequency of occurrence on the order of once in 100 years, although the flood may occur in any year. The intermediate regional flood is based on statistical analyses of streamflow records available for the watershed and analyses of rainfall and runoff characteristics in the "general region of the watershed."

FLOOD IRRIGATION — The application of irrigation water where the entire surface of the soil is covered by a sheet of water, called Controlled Flooding when water is impounded or the flow directed by border dikes, ridges, or ditches.

FLOOD, MAXIMUM PROBABLE — The greatest flood that may be expected at a place, taking into account all pertinent factors of location, meteorology, hydrology, and terrain.

FLOOD OF RECORD — The highest observed river stage or discharge at a given site during the period of record keeping. May not necessarily be the highest known stage.

FLOOD PEAK — The maximum instantaneous discharge of a flood at a given location. It usually occurs at or near the time of the flood crest, i.e., the maximum stage or elevation reached by the flood flow.

FLOOD PLAIN, also Floodplain — (1) A strip of relatively smooth land bordering a stream, built of sediment carried by the stream and dropped in the slack water beyond the influence of the swiftest current. It is called a Living Flood Plain if it is overflowed in times of high water but a Fossil Flood Plain if it is beyond the reach of the highest flood. (2) The lowland that borders a stream or river, usually dry but subject to flooding. (3) That land outside of a stream channel described by the perimeter of the Maximum Probable Flood.

FLOOD PLANE — The position occupied by the water surface of a stream during a particular flood. Also, loosely, the elevation of the water surface at various points along the stream during a particular flood.

FLOOD PREVENTION — Methods or structural measures used to prevent floods.

FLOOD PROBABILITY — The statistical probability that a flood of a given size will be equaled or exceeded in a given period of time.

FLOOD PROFILE — A graph showing the relationship of water surface elevation to location, the latter generally expressed as distance above mouth for a stream of water flowing in an open channel. It is generally drawn to show surface elevation for the crest of a specific flood, but may be prepared for conditions at a given time or stage.

FLORA — (1) A term used to describe the entire plant species of a specified region or time. (2) The sum total of the kinds of plants in an area at one time. All plant life associated with a given habitat, country, area, or period. Bacteria are considered flora.

FLOW — The rate of water discharged from a source given in volume with respect to time.

FLOWAGE — (1) The act of flowing or overflowing. (2) The state of being flooded; a body of water, such as a lake or reservoir, formed by usually deliberate flooding. (3) An outflow or overflow.

FLOW AUGMENTATION — The addition of water to a stream especially to meet instream flow needs.

FLOW BOUNDARIES — Anything which inhibits ground water flow, such as a ground water divide or an impermeable geologic unit.

FLOW DURATION CURVE — A cumulative frequency curve that shows the percentage of time that specified discharges are equaled or exceeded.

FLOWLINE (STREAMLINE) — (1) The general path that a particle of water follows under laminar flow conditions. (2) The line indicating the direction followed by ground water toward points of discharge. Flow lines are perpendicular to Equipotential Lines.

FLOW METER — A device which allows for measurement of stream flow by measuring velocity in a given cross-sectional area.

(GROUND WATER) FLOW MODEL — (1) A digital computer model that calculates a hydraulic head field for the modeling domain using numerical methods to arrive at an approximate solution to the differential equation of ground-water flow. (2) Any representation, typically using plastic or glass cross-sectional viewing boxes, with representative soil samples, depicting ground-water flows and frequently used for educational purposes.

FLOW, LAMINAR — Flow of water in well-defined flow lines in which the viscous force is predominant; in channels it occurs at a Reynolds Number smaller than 500-2,000 and through porous media at Reynolds Number smaller than 1-10.

FLOW, MODIFIED — That streamflow which would have existed had the works of man in or on the stream channels and in the drainage basin been consistent throughout the period of record. Usually used with an adjective such as "present" or specific year to mean that the flow record was modified to represent the record that would have been obtained had the "present" conditions prevailed throughout the period of record. Modified flow is equal to Virgin Flow minus the amount of Streamflow Depletion occurring at the specified time.

FLOW, NATURAL — The rate of water movement past a specified point on a natural stream from a drainage area which has not been affected by stream diversion, storage, import, export, return flow or change in consumptive use resulting from man's modification of land use. Natural flow rarely occurs in a developed country.

FLOW, NET — A graphical representation of flow lines and Equipotential Lines for two-dimensional, steady-state ground-water flow.

FLOW, OVERLAND — The flow of rainwater or snowmelt over the land surface toward stream channels. Upon entering a stream, it becomes runoff.

FLOW PATH — The subsurface course a water molecule or solute would follow in a given ground-water velocity field.

FLOW RATE — The rate, expressed in gallons or liters-per-hour, at which a fluid escapes from a hole or fissure in a tank. Such measurements are also made of liquid waste, effluent, and surface water movement.

FLOW RESOURCES Versus STOCK RESOURCES — Flow resources are resources that are not permanently expendable under usual circumstances; they are resources which are replaced. They are commonly expressed in annual rates at which they are regenerated. Examples are fresh-water runoff and timber. Stock resources can be permanently expended and whose quantity is usually expressed in absolute amounts rather than in rates. Examples are coal and petroleum deposits.

FLOW, STEADY — A flow in which the magnitude and direction of the specific discharge are constant in time.

FLOW, TURBULENT — A flow in which successive flow particles follow independent path lines, and head loss varies approximately with the square of the velocity. In stream channels it occurs at a Reynolds Number greater than 5,000.

FLOW, UNIFORM — A characteristic of a flow system where specific discharge has the same magnitude and direction at any point.

FLOW VELOCITY — The volume of water flowing through a unit cross-sectional area of an aquifer.

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

FLOWING WELL — An Artesian Well having sufficient head to discharge water above the land surface; a well where the Piezometric Surface lies above the ground surface..

FLOWMETER — A gauge indicating the velocity of wastewater moving through a treatment plant or of any liquid moving through various industrial processes.

FLUID — Having particles which easily move and change their relative position without a separation of the mass, and which easily yield to pressure; capable of flowing; liquid or gaseous.

FLUVIAL — Of or pertaining to rivers and streams; growing or living in streams ponds; produced the action of a river or stream.

FOREBAY — The water behind a dam. A reservoir or pond situated at the intake of a pumping plant or power plant to stabilize water levels; also a storage basin for regulating water for percolation into ground water basins.

FOREBAY RESERVOIR — A reservoir used to regulate the flow of water to a hydroelectric plant; it may also serve other purposes such as recreation.

FORECAST (FORECASTING) — (Statistics) A forecast is a quantitative estimate (or set of estimates) about the likelihood of future events based on past and current information. This "past and current information" is specifically embodied in the structure of the econometric model used to generate the forecasts. By extrapolating the model out beyond the period over which it was estimated, we can use the information contained in it to make forecasts about future events. It is useful to distinguish between two types of forecasting, ex post and ex ante. In an ex post forecasts all values of dependent and independent variables are known with certainty and therefore provides a means of evaluating a forecasting model. Specifically, in an ex post forecast, a model will be estimated using observations excluding those in the ex post period, and then comparisons of the forecasts will be made to these actual values. An ex ante forecast predicts values of the dependent variable beyond the estimation period using values for the explanatory variables which may or may not be known with certainty.

FOREST HYDROLOGY — The study of hydrologic processes as influenced by forest and associated vegetation.

FOREST INFLUENCES — The effects resulting from the presence of forest or brush upon climate, soil water, runoff, streamflow, floods, erosion, and soil productivity.

FOREST LAND — Land which is at least 10 percent occupied by forest trees of any size or formerly having had such tree cover and not currently developed for non-forest use. Lands developed for non-forest use include areas for crops, improved pasture, residential, or administrative areas, improved roads of any width, and adjoining road clearing and power line clearing of any width.

FORFEITED WATER RIGHT — A water right that is no longer valid because of five or more consecutive years of nonuse.

FORFEITURE — The invalidation of a water right because of five or more consecutive years of nonuse.

FOSSIL WATER — Limited subterranean water deposits laid down in past ages but drawn on by modern man.

FOUNDATION (of a Dam) — The natural material on which the dam structure is placed.

FOUNDER — To sink below the water.

FOUNTAIN — (1) An artificially created jet or stream of water; a structure, often decorative, from which a jet or stream of water issues. (2) A spring, especially the source of a stream. (3) A reservoir or chamber containing a supply of liquid that can be siphoned off as needed.

FREE FLOW — (Hydraulics) Flow through or over a structure not affected by submergence or backwater.

FREE-FLOWING — Flowing without artificial restrictions.

FREE-FLOWING STREAM — A stream or a portion of a stream that is unmodified by the works of man or, if modified, still retains its natural scenic qualities and recreational opportunities.

FREE-FLOWING WEIR — A weir that in use has the tailwater lower than the crest of the weir.

FREE-FLOWING WELL — An Artesian Well in which the potentiometric surface is above the land surface.

FREE GROUND WATER — Water in interconnected pore spaces in the Zone of Saturation down to the first impervious barrier, moving under the control of the water table slope.

FREE LIQUIDS — (Water Quality) Liquids capable of migrating from waste and contaminating ground water. Hazardous waste containing free liquids may not be disposed of in landfills.

FREE MOISTURE — Liquid that will drain freely from solid waste by the action of gravity only.

FREE WATER SURFACE (FWS) CONSTRUCTED WETLAND — A type of constructed wetland, a man-made marsh-like area used to treat wastewater. In this type of wetland, the effluent flows through various aquatic plants, with the water level exposed to the air. While this type of wetland is relatively easy to construct, it is not as effective as the Subsurface Flow (SF) Constructed Wetland with respect to associated odors, potential for insect breeding, and risk of public exposure and contact with the water in the system.

FREEZE — (1) To pass from the liquid to the solid state by loss of heat. (2) To acquire a surface of coat of ice from cold.

FREEZING — The change of a liquid into a solid as temperature decreases. For water, the freezing point is 32F (Fahrenheit) or ⁰C (Celsius).

FREEZING POINT — (1) The temperature at which a liquid of specified composition solidifies under a specified pressure. (2) The temperature at which the liquid and solid phases of a substance of specified composition are in equilibrium at atmospheric pressure.

FRENCH DRAIN — An underground passageway for water through the interstices among stones placed loosely in a trench.

FREQUENCY ANALYSIS — A statistical procedure involved in interpreting the past record of a hydrological event to occurrences of that event in the future (e.g., estimates of frequencies of floods, droughts, storage, rainfall, water quality, etc.).

FREQUENCY CURVE — A graphical representation of the frequency of occurrence of specific events.

FREQUENCY DISTRIBUTION — An arrangement of quantities pertaining to a single event, in order of magnitude and frequency of occurrence.

FRESH — (1) Not saline or salty. (2) Free from impurity or pollution.

FRESH-SALT WATER INTERFACE — The region where fresh water and salt water meet.

FRESHWATER — (1) Of, relating to, living in, or consisting of water that is not salty. (2) Water with salinity less than 0.5 (parts per thousand) dissolved salts. (3) Water that contains less than 1,000 milligrams per liter (mg/l) of dissolved solids; generally, more than 500 mg/l of dissolved solids is undesirable for drinking and many industrial uses. (4) (Nautical) Accustomed to sailing on inland waters only.

FRINGE WATER — Water occurring in the Capillary Fringe.

FRONT — (1) Land bordering a lake or river. (2) (Meteorology) A line of separation or interface between air masses of different temperatures or densities.

FRONTAGE — Land adjacent to something, such as a body of water.

FROST — (1) Thin ice crystals in the shape of scales, needles, feathers or fans which are deposited by Sublimation at temperatures of 32°F (0°C) or lower. (2) A temperature low enough to cause freezing. (3) The process of freezing.

FULLY PERMANENT SPRINKLER SYSTEM — An irrigation system usually composed of buried enclosed conduits carrying water under pressure to fixed orifices to distribute water over a given area.

FURROW — A long, narrow, shallow trench made in the ground by a plow for planting and irrigation.

FURROW DAMS — Small earth ridges or rows used to impound water in furrows.

FURROW IRRIGATION — Spreading water by directing it into small channels across the field.

FURROW STREAM — The size of water flow released into the furrow; the size of the stream is adjusted to prevent erosion, limited in amount to the capacity of the furrow, and as needed for the intake rates of the soil involved.