H.320 Standard A widely-used video compression standard that allows a wide variety of videoconferencing systems to communicate. H.320 includes a number of individual recommendations for coding, framing, signaling, and establishing connections.
habitat The environment, usually the natural environment in which a population of plants or animals occurs.
habitat structure The physical structure of a habitat; e.g., the layering of vegetation in a forest or the grain of a coral reef.
hail Precipitation in the form of balls or irregular lumps of ice with a diameter of 5 mm or more, always produced by convective clouds, nearly always cumulonimbus.
half cell A pure metal in contact with a solution of known concentration of its own ion, at a specific temperature, develops a potential that is characteristic and reproducible; when coupled with another half cell, an overall potential develops which is the sum of both half cells.
half-life The time required for a pollutant to be reduced by 50 percent of its original amount.
half-tide level A tidal datum. The arithmetic mean of mean high water and mean low water. Same as mean tide level.
halocline A layer in which the salinity changes significantly (relative to the layers above and below) with depth.
halogen One of the chemical elements chlorine, bromine, fluorine, astatine, or iodine.
halon Bromine-containing compounds, used in fire-fighting, with long atmospheric lifetimes whose breakdown in the stratosphere causes ozone depletion.
halophyte A plant that grows in salty soil or water.
hammermill A high-speed machine that uses hammers and cutters to crush, grind, chip, or shred solid waste.
hanging (ice) dam A mass of ice composed mainly of frazil or broken ice deposited underneath an ice cover in a region of low flow velocity.
hapten A low molecular weight substance that alone can react with its corresponding antibody.
harbor
harbor of refuge A name given to havens along shorelines located between commercial and recreational harbors; designed primarily to be a place of refuge for small craft during storm periods.
hard drive (hard disk) A rigid non-removable disk in a computer and the drive that houses it. Hard disks store more data and can be accessed quicker than floppy disks.
hardhat Hard protective helmet worn as protection against falling material or impact.
hard water Alkaline water containing dissolved salts that interfere with some industrial processes and prevent soap from lathering. Water may be considered hard if it has a hardness greater than the typical hardness of water from a region. Some texts define hard water as water with a hardness of more than 100 mg/L as calcium carbonate.
hardness, water A characteristic of water caused mainly by the salts of calcium and magnesium, such as bicarbonate, carbonate, sulfate, chloride, and nitrate. Excessive hardness in water is undesirable because it causes the formation of soap curds, increased use of soap, deposition of scale in boilers, damage in some industrial processes, and sometimes causing objectionable tastes in drinking water.
hardpan
harmonic analysis The mathematical process by which the observed tide or tidal current at any place is separated into basic harmonic constituents.
harmonic analyzer A machine designed for the resolution of a periodic curve into its harmonic constituents. Now performed by electronic digital computer.
harmonic constants The amplitudes and epochs of the harmonic constituents of the tide or tidal current at any place.
harmonic constituent See constituent.
harmonic function In its simplest form, a quantity that varies as the cosine of an angle that increases uniformly with time. It may be expressed by the formula: y = A cos at in which y is a function of time (t), A is a constant coefficient, and a is the rate of change in the angle at.
harmonic prediction Method of predicting tides and tidal currents by combining the harmonic constituents into a single tide curve. The work is usually performed by electronic digital computer.
harmonic reduction Same as harmonic analysis.
harmonic tide plane Same as Indian spring low water.
hauler A solid waste collection company that offers complete refuse removal service; many also collect recyclables.
Hazard Communication Standard An OSHA regulation that requires chemical manufacturers, suppliers, and importers to assess the hazards of the chemicals that they make, supply, or import and to inform employees, customers, and workers of these hazards through MSDS sheets. See MSDS.
hazard evaluation A component of risk assessment that involves gathering and evaluating data on the types of health injury or disease (e.g., cancer) that may be produced by a chemical and on the conditions of exposure under which injury or disease is produced.
hazard identification Determining if a chemical can cause adverse health effects in humans and what those affects might be.
hazardous air pollutants Air pollutants that are not covered by ambient air quality standards but which, as defined in the Clean Air Act, may reasonably be expected to cause or contribute to irreversible illness or death. Such pollutants include asbestos, beryllium, mercury, benzene, coke oven emissions, radionuclides, and vinyl chloride.
hazardous chemical A designation for any hazardous material requiring an MSDS under OSHA's Hazard Communication Standard. Such substances are capable of producing fires and explosions or adverse health effects like cancer and dermatitis. Hazardous chemicals are distinct from hazardous waste. See Hazardous Waste.
hazardous ranking system The principle screening tool used to evaluate risks to public health and the environment associated with abandoned or uncontrolled hazardous waste sites. The resultant core is the primary factor in deciding if the site should be on the National Priorities List and, if so, what ranking it should have compared to other sites on the list.
hazardous substance 1) Any material that poses a threat to human health and to the environment. Typically hazardous substances are toxic, corrosive, ignitable, explosive, or chemically reactive. 2) Any substance designated by a regulatory agency to be reported if a designated quantity of the substance is spilled in the waters of the United States or otherwise released into the environment.
hazardous waste Unwanted products, possessing at least one of four characteristics (ignitability, corrosivity, reactivity, or toxicity), that can pose a substantial or potential hazard to human health or the environment when improperly managed.
hazardous waste landfill An excavated or engineered site where hazardous waste is deposited and covered.
hazards analysis Procedures used to: identify potential sources of release of hazardous materials from fixed facilities or transportation accidents, determine the vulnerability of a geographical area to a release of hazardous materials, and compare hazards to determine which present greater or lesser risks to a community.
hazards identification Providing information on which facilities have extremely hazardous substances, what those chemicals are, how much there is at each facility, how the chemicals are stored, and whether they are used at high temperatures.
head 1) The vertical distance (in feet) equal to the pressure (in psi) at a specific point. The pressure head is equal to the pressure in psi times 2.31 ft/psi. 2) The difference in water level at either end of a strait, channel, inlet, etc.
head loss The head, pressure, or energy (they are the same) lost by a fluid flowing in a pipe or channel as a result of turbulence caused by the velocity of the fluid and the roughness of the pipe, channel, or restrictions caused by the fittings. Water flowing in a pipe loses head, pressure, or energy as a result of friction losses. Also see energy and friction losses.
header A large pipe to which a series of smaller pipes are connected. Also called a manifold.
head race A channel which directs water to a water wheel; a forebay.
headwater basin A basin at the headwaters of a river. All discharge of the river at this point comes only from within this basin.
head of tide The inland or upstream limit of water affected by the tide. For practical application in the tabulation for computation of tidal datums, head of tide is the inland or upstream point where the mean range becomes less than 0.2 foot. Tidal datums (except for mean water level) are not computed beyond head of tide.
headwater erosion Erosion which occurs in the upstream end of the valley of a stream, causing it to lengthen its course in such a direction.
headwaters Streams at the source of a river.
Headwaters Advisory Program (ADVIS) A program which uses the antecedent precipitation index (API) method of estimating runoff, unit hydrograph theory and stage-discharge ratings to produce hydrologic forecasts for headwater basins.
headwaters advisory table A table developed by a river forecast center for a headwater guidance point. A precomputed matrix of values allows a forecaster to ascertain an anticipated crest or rise on a small river or stream for a variety of rainfall events and soil moisture conditions.
headworks
health A state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity.
health advisory level (HAL) The maximum concentration of a contaminant in water that may be safely consumed over a specific time period. EPA sets HALs for short-term exposures, such as one day and ten days, and longerterm exposures of greater than ten days up to several years, and over a lifetime.
health assessment An evaluation of available data on existing or potential risks to human health imposed by a Superfund site.
health hazard 1) Any condition, device, or practice in a waterworks or its operation that creates, or may create, a danger to the health and well-being of the water consumer. 2) The potential exposure of persons to toxic or disease causing agents.
health hazard evaluation An evaluation by FDA scientists of the threat to health presented by a product, including its labeling and/or promotional materials, that is being recalled or considered for recall.
health indicator A measure that reflects, or indicates, the state of health of persons in a defined population, e.g., the infant mortality rate.
health information system A combination of health statistics from various sources, used to derive information about health status, health care, provision and use of services, and impact on health.
health regulations Regulations which include all primary maximum contaminant levels, treatment technique requirements, and all operational regulations, the violation of which would jeopardize the public health.
hearing The opportunity for a party to present views.
hearing threshold level (HTL) The amount, in decibels, by which the threshold of audibility for an ear differs from a standard audiometric threshold.
heat budget The quantitative listing of all heat inputs, transformations, and outputs of an ecosystem or organism.
heat capacity The quantity of energy that must be supplied to raise the temperature of a substance. For contaminated soils heat capacity is the quantity of energy that must be added to the soil to volatilize organic components. The typical range of heat capacity of soils is relatively narrow, therefore variations are not likely to have a major impact on application of a thermal desorption process.
heat island effect A dome of elevated temperatures over an urban area caused by structural and pavement heat fluxes and pollutant emissions.
heat sensor A device that opens and closes a switch in response to changes in temperature. This device might be a metal contact, a thermocouple which generates a minute electrical current proportional to the difference in heat, or a variable resistor whose value changes in response to changes in temperature. Also called a temperature sensor.
heat transfer
heavy metals Metallic elements with high atomic weights, e.g., mercury, chromium, cadmium, arsenic, and lead. They may be toxic to living things at low concentrations and tend to accumulate in the food chain.
hectare A measure of area in the metric system similar to an acre. One hectare is equal to 10,000 square meters or 2.4711 acres.
helicity A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve.
hematologic Having to do with the blood.
hematopoiesis The production of blood and blood cells; hemopoiesis.
hemoglobin The red, oxygen-carrying protein found in red blood cells.
Henry's Law A thermodynamic relationship which states that in a closed system the concentration of a constituent in the vapor phase and the corresponding equilibrium concentration in the liquid phase are related by a constant. See Henry's Law constant.
Henry's Law constant The ratio of the concentration of a compound in air (or vapor) to the concentration of the compound in water under equilibrium conditions.
hepatic Pertaining to the liver.
hepatitis Hepatitis is an inflammation of the liver usually caused by an acute viral infection. Yellow jaundice is one symptom of hepatitis.
hepatoma A malignant tumor occurring in the liver.
herbaceous Of or relating to any plant, lacking woody tissue, in which the leaves and stems fall to the ground level during freezing or drying weather.
herbicide A compound, usually a manmade organic chemical, used to kill or control plant growth.
herbivore An organism that eats living plants or plant parts (e.g., seeds).
herpetofauna The amphibian and reptile species characteristic of an area.
hertz The number of complete electromagnetic cycles or waves in one second of an electrical or electronic circuit. Also called the frequency of the current; abbreviated Hz.
heterogeneous Varying in structure or composition at different locations in space.
heterotroph A microorganism which is unable to use carbon dioxide as its sole source of carbon and which requires one or more organic compounds to complete metabolism.
heterotrophic Designating or typical of organisms that derive carbon for the manufacture of cell mass from organic matter.
heterotrophic microflora Microorgansims that utilize organic material for energy and growth.
heterotrophic organisms
heterotrophic plate count (HPC) In essence, a total count of heterotrophic microorganisms, usually expressed in number of bacteria per milliliter of sample.
high affinity antibody An antibody with a high affinity for antigen.
high definition TV (HDTV) Regular NTSC signals have 525 lines of resolution. HDTV has 1125 lines of resolution having over five times the video information than that of a conventional NTSC-type TV set. In spite of its obvious advantages, transmission requires extraordinary bandwidth of five times the capacity of a conventional TV signal.
higher low water (HLW) The highest of the low waters of any specified tidal day due to the declination Al effects of the Moon and Sun.
higher plants Flowering plants that reproduce by seeds; phanerogams, not cryptogams.
highest and best use That use of land and natural resources that maximizes the benefit to man while minimizing the long-term effects to nature.
high-level radioactive waste Waste generated in core fuel of a nuclear reactor, found at nuclear reactors or by nuclear fuel reprocessing; a serious threat to anyone who comes near the waste without shielding. See low-level radioactive waste.
high-level nuclear waste facility Plant designed to handle disposal of used nuclear fuel, high-level radioactive waste, and plutonium waste.
high-line jumpers Pipes or hoses connected to fire hydrants and laid on top of the ground to provide emergency water service for an isolated portion of a distribution system.
high-pressure fire system
high-pressure system An area of relative pressure maximum that has diverging winds and a rotation opposite to the earth's rotation. This is clockwise the in Northern Hemisphere and counterclockwise in the Southern Hemisphere. Also known as an anticyclone, it is the opposite of an area of low pressure or a cyclone.
high-rate filter 1) A water tatment or tertiary wastewater treatment filter operated at loading rates above 5 gpd per sq. ft. 2) A biological contactor operated at a high average daily dosing rate, usually between 100 and 1,000 gpd/sq. ft.
high risk community A community located within the vicinity of numerous sites or facilities or other potential sources of environmental exposure/health hazards that may result in high levels of exposure to contaminants or pollutants.
high risk Severe weather is expected to affect more than 10 percent of the area. A high risk is rare, and implies an unusually dangerous situation and usually the possibility of a major severe weather outbreak.
high risk group A group in the community with an elevated risk of disease. See high risk community.
high tide Same as high water.
high-to-low-dose extrapolation The process of prediction of low exposure risks to rodents from the measured high exposure-high risk data.
high water The maximum height reached by a rising tide. The high water is due to the periodic tidal forces and the effects of meteorological, hydrologic, and/or oceanographic conditions. For tidal datum computational purposes, the maximum height is not considered a high water unless it contains a tidal high water.
high water, full and change (HWF&C) Same as establishment of the port.
high water inequality See diurnal inequality.
high water interval (HWI) See lunitidal interval.
high water line The intersection of the land with the water surface at an elevation of high water.
high water mark A line or mark left upon tide flats, beach, or along shore objects indicating the elevation of the intrusion of high water. The mark may be a line of oil or scum on along shore objects, or a more or less continuous deposit of fine shell or debris on the fore shore or berm. This mark is physical evidence of the general height reached by wave run up at recent high waters. It should not be confused with the mean high water line or mean higher high water line.
high wind advisory Announcement issued by the National Weather Service for substained winds exceeding 25 mph (19 knots).
high wind watch/warning A high inland wind watch/warning issued by the National Weather Service when either of the following occurs or are expected to occur in the near term 1) Sustained surface winds (1-minute average) of 40 mph (35 knots) or greater lasting for 1 hour or longer; or 2) Sustained winds or gusts of 58 mph (50 knots) or greater for any duration.
hinge crack Crack caused by significant changes in water level.
histogram A graphic representation of the frequency distribution of a continuous variable. Rectangles are drawn in such a way that their bases lie on a linear scale representing different intervals, and their heights are proportional to the frequencies of the values within each of the intervals.
histology The study of the structure of cells and tissues; usually involves microscopic examination of tissue slices.
holding pond A pond or reservoir, usually made of earth, built to store treated effluent or polluted runoff.
holocoenotic Of or related to a system that is organized so that the total system has properties not present in its individual parts. As this is true with all systems, the term is superfluous, but is often used for emphasis.
holomictic Condition of a lake in which the surface and bottom waters are completely mixed by vertical circulation, even if just occasionally.
homeowner water system Any water system that supplies piped water to a single residence.
homeostasis The inherent stability or self-regulation of a biological system; the ability of such a system to resist external changes.
homeotherms Animals (mammals and birds) that maintain a more or less constant body temperature despite variations in external temperature; warm-blooded animals or endotherms.
home range The area or space of normal activity of an individual animal; sometimes, but not necessarily, defended against intrusion by other individuals. See territory.
hood capture efficiency Ratio of the emissions captured by a hood and directed into a control or disposal device, expressed as a percent of all emissions.
horizon (soil) A layer of soil, approximately parallel to the soil surface with distinct characteristics produced by soil-forming processes.
hose bib A location in the line where a hose is connected.
host Organisms, simple or complex, including man, that are capable of being infected by a specific agent.
host factor An intrinsic factor (age, race, sex, behaviors, etc.) which influences an individual's exposure, susceptibility, or response to a causative agent.
household waste Solid waste, composed of garbage and rubbish, which normally originated in a private home or apartment house. Domestic waste may contain toxic or hazardous waste. Also called domestic waste.
HTH High Test Hypochlorite. Calcium hypochlorite or Ca(OCl)2..
human equivalent dose A dose which, when administered to humans, produces an effect equal to that produced by a dose in animals.
human exposure evaluation A component of risk assessment that involves describing the nature and size of the population exposed to a substance and the magnitude and duration of their exposure. The evaluation could concern past exposures, current exposures, or anticipated exposures.
human health risk The likelihood (or probability) that a given exposure or series of exposures may have or will damage the health of individuals experiencing the exposures.
humic Of, or related to, or derived from the soil or waterborne substance resulting from the partial decay of leaves and other plant material.
humidity The amount of water vapor in the air. Popularly, it is used synonymously with relative humidity.
humidity, relative A measure of the amount of water in the air compared to the amount of water vapor the air has the potential to hold. (Note: the potential of air to hold water changes with air temperature. Therefore, relative humidity can change as air temperature changes without an actual change in the amount of water vapor.)
hummock hillock of broken ice which has been forced upward by pressure.
hummocked ice Ice piled haphazardly one piece over another to form an uneven surface.
humus Organic portion of the soil remaining after prolonged microbial decomposition.
hurricane A tropical cyclone in the Northern Hemisphere with substained winds of at least 74 mph (64 knots) or greater in the North Atlantic Ocean, Caribbean Sea or Gulf of Mexico. These winds blow in a large spiral around a relatively calm center of extremely low pressure known as the eye. Around the rim of the eye, winds may gust to more than 200 miles per hour. The entire storm, which can be up to 340 (550) in diameter, dominates the ocean surface and lower atmosphere over tens of thousands of square miles. Hurricanes draw their energy from the warm surface water of the tropics (usually above 27 Celsius) and latent heat of condensation, which explains why hurricanes dissipate rapidly once they move over cold water or large land masses.
hurricane advisory Notice, issued by the National Hurricane Center, numbered consecutively for each storm, describing the present and forecasted position and intensity. Advisories are issued at six-hour intervals at midnight, 6 a.m., noon, and 6 p.m., Eastern Daylight Time. Bulletins provide additional information. Each message gives the name, eye position, intensity and forecast movement of the storm.
hurricane clips A structural bracing device used on the installation of roofs which reinforce the joints of a house and give a stronger connection of wood to wood roofing trusses than just nails. In many coastal communities, hurricane clips are enforced as a code restriction for new homes.
hurricane damage Losses due to a combination of flood and wind.
hurricane eye The relatively calm area near the center of the storm. In this area, winds are light and the sky is often partly covered by clouds.
hurricane eye landfall When the eye, or physical center of the hurricane, reaches the coastline from the hurricane's approach over water.
hurricane liaison team A team of FEMA, NWS, State and local emergency management officials which respond to the National Hurricane Center prior to the landfall of a hurricane in the Atlantic Ocean, Caribbean Sea or Gulf of Mexico. The primary purpose of the Team is to assist in coordinating the latest advisories from the NHC to the Federal, State and local emergency management agencies.
hurricane local statement A public release prepared by local National Weather Service Field Offices in or near a threatened area giving specific details for its county/parish warning area on 1) weather conditions; 2) evacuation decisions made by local officials and; 3) other precautions necessary to protect life and property.
hurricane path or track Line of movement (propagation) of the eye through an area.
hurricane season The portion of the year having a relatively high incidence of hurricanes. The hurricane season in the Atlantic, Caribbean, and Gulf of Mexico runs from June 1 to November 30.
hurricane/tropical storm probabilities The National Weather Service issues hurricane/tropical storm probabilities in public advisories to realistically assess the threat of a hurricane or tropical storm hitting your community. The probabilities are defined as the chance in percent that the center of the storm will pass within approximately 65 miles of 44 selected locations from Brownsville, Texas, to Eastport, Maine.
hurricane warning A warning added to a hurricane advisory that sustained winds of 74 mph (64 knots) or higher associated with a hurricane are expected in a specified coastal area within 24 hours or less. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force. A warning is used to inform the public and marine interests of the storm's location, intensity, and movement. The NHC chooses a distance of approximately 300 miles.
hurricane watch An announcement added to a hurricane advisory that hurricane conditions pose a possible threat to a specified coastal area within 36 hours. A watch is used to inform the public and marine interests of the storm's location, intensity, and movement.
hydrarch Successions that originate in aquatic habitats, such as lakes and ponds, and progress toward more terrestrial conditions, as in bogs and swamps.
hydrated lime Limestone that has been burned and treated with water under controlled conditions until the calcium oxide portion has been converted to calcium hydroxide (Ca(OH)2). Hydrated lime is quicklime combined with water. CaO + H2O Ca(OH)2. Also see quicklime.
hydraulic current A current in a channel caused by a difference in the surface elevation at the two ends. Such a current may be expected in a strait connecting two bodies of water in which the tides differ in time or range. The current in the East River, New York, connecting Long Island Sound and New York Harbor, is an example.
hydraulic detention time The theoretical time required to displace the contents of a basin, tank, or unit operation at a given discharge rate (volume of tank divided by discharge rate).
hydraulic fill dam A dam constructed of materials, often dredged, that are conveyed and placed by suspension in flowing water.
hydraulic grade line The surface or profile of water flowing in an open channel or a pipe flowing partially full. If a pipe is under pressure, the hydraulic grade line is at the level water would rise in a small vertical tube connected to the pipe. Also see energy grade line.
hydraulic gradient The slope of the hydraulic grade line. In open channel flow, it is the slope of the water surface. In pipes under pressure, it is the slope of the water pressure. In groundwater, it is the measure of the change in groundwater head over a given distance.
hydraulic head 1)The height of the free surface of a body of water above a given point beneath the surface. 2) The height of the water level at the headworks, or an upstream point, of a waterway, and the water surface at a given point downstream. 3)The height of a hydraulic grade line above the center line of a pressure pipe, at a given point. 4) The height above a specific datum (generally mean sea level) that water will rise in a well.
hydraulic loading The volume of water or wastewater applied to a unit operation in a given time.
hydraulic mean depth The right cross-sectional area of a stream of water divided by the length of that part of its periphery in contact with its containing conduit; the ratio of area to wetted perimeter. Also called hydraulic radius.
hydraulic permeability The flow of water through a unit cross-sectional area of soil normal to the direction of flow when the hydraulic gradient is unity.
hydraulic radius The right cross-sectional area of a stream of water divided by the length of that part of its periphery in contact with its containing conduit; the ratio of area to wetted perimeter. Also called hydraulic mean depth.
hydric Characterized or pertaining to conditions of abundant moisture supply.
hydrocarbons Chemical compounds that consist entirely of carbon and hydrogen.
hydrochlorination The application of hypochlorite compounds to water for the purpose of disinfection.
hydrochlorinators Chlorine pumps, chemical feed pumps or devices used to dispense chlorine solutions made from hypochlorites, such as bleach (sodium hypochlorite) or calcium hypochlorite (HTH) into the water being treated.
hydrodynamics The branch of physics that studies the forces and movements of water and other liquids.
hydroelectric power water use The use of water in the generation of electricity at plants where the turbine generators are driven by falling water. Hydroelectric water use is classified as an instream use in this report.
hydrogen sulfide (H2S)A gas, smelling like rotten eggs, emitted during organic decomposition. Also a byproduct of oil refining and burning. Smells like rotten eggs and, in heavy concentration, can kill or cause illness.
hydrogen sulfide detector An instrument similar to an oxygen meter except that it is adjusted to sound an alarm when a particular contaminant level for hydrogen sulfide is reached.
hydrogeologic conditions Conditions stemming from the interaction of groundwater and the surrounding soil and rock.
hydrogeologic cycle The natural process recycling water from the atmosphere down to and through the earth and back to the atmosphere again.
hydrogeology The geology of groundwater with particular emphasis on the chemistry and movement of water.
hydrogeologist A person who studies and works with groundwater.
hydrograph A graph that shows the elevation of groundwater in a well, or surface water level, above a particular datum point, against time.
hydrographic
hydrographic datum A datum used for referencing depths of water and the heights of predicted tides or water level observations. Same as chart datum.
hydrograph separation The process where the storm hydrograph is separated into baseflow and surface runoff components.
hydrograph survey An instrumental survey to measure and determine characteristics of streams and other bodies of water within an area, including such things as location, areal extent, and depth of water in lakes or the ocean; the width, depth, and course of streams; position and elevation of high water marks; location and depth of wells, etc.
hydrological setting The composite description of the regional geology of a specific area that characterizes the stratigraphy, structure, and lithology of the materials and the occurrence and chemistry of the groundwater.
hydrologic budget An accounting of the inflow to, outflow from, and storage in, a hydrologic unit, such as a drainage basin, aquifer, soil zone, lake, reservoir, or irrigation project.
hydrologic cycle The endless circulation of water between the ocean, atmosphere (by evaporation), land (by precipitation), and back to the ocean (by stream and subsurface flow).
hydrologic equation The water inventory equation (inflow = outflow + change in storage) which expresses the basic principle that during a given time interval the total inflow to an area must equal the total outflow plus the net change in storage.
hydrologic focal point The person in a WFO responsible for maintaining the hydrologic program when a service hydrologist is not present.
hydrologic model A conceptual or physically-based procedure for numerically simulating a process or processes which occur in a watershed.
hydrologic service area A geographical area assigned to weather forecast office which embraces one or more rivers.
hydrologic services A general term referring to the operations, products, verbal communications, and related forms of support provided by the NWS for the nation's streams, reservoirs, and other areas affected by surface water.
hydrologic unit A geographical area representing part or all of a surface drainage basin or distinct hydrologic feature such as a reservoir, lake, etc.
hydrology The study of the occurrence, distribution, circulation, and chemistry of water.
hydrolysis The process by which a chemical bond is split to form other chemical products through the intervention of a molecule of water.
hydrometeorologist An individual who has the combined knowledge in the fields of both meteorology and hydrology.
hydrometeorology The interdisciplinary science involving the study and analysis of the interrelationships between the atmospheric and land phases of water as it moves through the hydrologic cycle.
hydrophillic Having a strong affinity for water. The opposite of hydrophobic.
hydrophyte A plant that grows in water or very wet earth.
hydrophobic Having a strong aversion (dislike) for water. The opposite of hydrophillic.
hydropneumatic A water system, usually small, in which a water pump is automatically controlled by the air pressure in a compressed-air tank.
hydrosphere The region which includes all the earth's liquid water, frozen water, floating ice, frozen upper layer of soil, and the small amounts of water vapor in the earth's atmosphere.
hydrostatic head A measure of pressure at a given point in a liquid in terms of the vertical height of a column of the same liquid which would produce the same pressure.
hydrostatic pressure 1) The pressure at a specific elevation exerted by a body of water at rest or 2) In the case of groundwater, the pressure at a specific elevation due to the weight of water at higher levels in the same zone of saturation.
hydroxide A compound formed by the union of a metal or a radical with one or more hydroxyl (OH) groups, as sodium hydroxide (NaOH).
hyetograph A graphical representation of rainfall intensity with respect to time.
hypha A single thread of mycelium or filament.
hyperendemic disease A disease that is constantly present at a high incidence and/or prevalence rate.
hypertext Text with links to other text. Documents written as hypertext contain text that when "clicked on" by the user with a mouse, links to other documents.
hypertext markup language (HTML) Set of programming commands inserted around blocks of text that describe how to display it. HTML commands also display other media (graphics, sound, and video). Documents on the Web are often written in HTML.
hypochlorinator A device used to feed calcium-, sodium-, or lithium hypochlorite as the disinfecting agent.
hypochlorite Chemical compounds containing available chlorine; used for disinfection. They are available as liquids (bleach) or solids (powder, granules, or pellets). Salts of hypochlorous acid.
hypolimnion The lowest layer in a thermally stratified lake or reservoir. This layer consists of colder, more dense water, has a constant temperature, and no mixing occurs.
hypothesis A supposition, arrived at from observation or reflection, that leads to refutable predictions. Any conjecture cast in a form that will allow it to be tested and refuted.
hypothesis, null The first step in testing for statistical significance in which it is assumed that the exposure is not related to disease.
hypothesis, alternative The hypothesis, to be adopted if the null hypothesis proves implausible, in which exposure is associated with disease.