Knowledge Base

HABITAT — The native environment or specific surroundings where a plant or animal naturally grows or lives. The surroundings include physical factors such as temperature, moisture, and light together with biological factors such as the presence of food or predator organisms. The term can be employed to define surroundings on almost any scale from marine habitat, which encompasses the oceans, to microhabitat in a hair follicle of the skin.

HABITAT INDICATOR — A physical attribute of the environment measured to characterize conditions necessary to support an organism, population, or community in the absence of pollutants, e.g., salinity of estuarine waters or substrate type in streams or lakes.

HARD WATER — Water which forms a precipitate with soap due to the presence of calcium, magnesium, or ferrous ions in solution.

HARVESTED RAINWATER — The rain that falls on a roof or yard and is channeled by gutters or channels to a storage tank. The first wash of water on a roof is usually discarded and the subsequent rainfall is captured for use if the system is being used for potable water.

HAYSTACK — A vertical standing wave in turbulent river waters.

HAZARD RANKING SYSTEM (HRS) — A method for ranking hazardous waste disposal sites for possible placement on the National Priorities List (Superfund List), as provided for by the Comprehensive, Environmental Response, Compensation, and Liability Act (CERCLA). The ranking uses information gathered by the preliminary assessment and site inspection and the listing site inspection. The need for remedial action is scored on the basis of potential harm to human health resulting from: (1) releases into groundwater, surface water, or the atmosphere; (2) fire and explosion; and/or (3) direct contact with hazardous materials. The HRS evaluation assigns an overall numerical value to each site, which determines its priority for cleanup. Also see Hazardous Substance and Hazardous Substances Superfund.

HEAD — Difference in elevation between intake and discharge points for a liquid. In geology, most commonly of interest in connection with the movement of underground water.

HEAD DITCH — The water supply ditch at the head end of an irrigated field.

HEADER — (1) A pipe that serves as a central connection for two or more smaller pipes. (2) A raised tank or hopper that maintains a constant pressure or supply to a system, especially the small tank that supplies water to a central heating system.

HEADGATE — The gate that controls water flow into irrigation canals and ditches. A watermaster regulates the headgates during water distribution and posts headgate notices declaring official regulations. Headgate also refers to a diversion structure which controls the flow rate from a conveyance system (canals and laterals) into the farm conveyance system.

HEADLAND — (1) A point of land, usually high and with a sheer drop, extending out into a body of water; a promontory. (2) The unplowed land at the end of a plowed furrow.

HEAD LOSS — (1) The decrease in total head caused by friction. (2) The effect of obstructions, such as narrow bridge openings or buildings, that limit the area through which water must flow, raising the surface of the water upstream from the obstruction.

HEADRACE — A channel that carries water to a water wheel or turbine; a forebay.

HEAD, STATIC — The height above a standard datum of the surface of a column of water (or other liquid) that can be supported by the static pressure at a given point. The static head is the sum of the Elevation Head and the Pressure Head.

HEAD, TOTAL — The sum of the Elevation Head (distance of a point above datum), the Pressure Head (the height of a column of liquid that can be supported by static pressure only at the point), and the Velocity Head (the height to which the liquid can be raised by its own kinetic energy.

HEADWARD EROSION — Erosion which occurs in the upstream end of the valley of a stream, causing it to lengthen its course in that direction.

HEADWATER(S) — (1) The source and upper reaches of a stream; also the upper reaches of a reservoir. (2) The water upstream from a structure or point on a stream. (3) The small streams that come together to form a river. Also may be thought of as any and all parts of a river basin except the mainstream river and main tributaries.

HEADWORKS — The diversion structures at the head of a conduit.

HEAT BUDGET, ANNUAL (of a Lake) — The amount of heat necessary to raise the water from the minimum winter temperature to the maximum summer temperature.

HEAVY METALS — Metals having a specific gravity of 5.0 or greater; generally toxic in relatively low concentrations to plant and animal life and tend to accumulate in the food chain. Examples include lead, mercury, cadmium, chromium, and arsenic.

HECTARE — (Abbreviation ha) A metric unit of area equal to 100 Ares (2.471 acres) and equivalent to 10,000 square meters (107,639 square feet).

HIGH WATER (HW) — (1) High tide. (2) The state of a body of water that has reached its highest level.

HIGH WATER MARK (HWM) — A mark indicating the highest level reached by a body of water.

HORSEPOWER (HP) — A unit of power, numerically equal to a rate of 33,000 foot-pounds of work per minute (or 550 foot-pounds per second), used in stating the power of an engine or any other prime mover, or in estimating the power required to drive machinery, or the like. The term horsepower was originated by Boulton and Watt to state the power of their steam engines. In a practical test it was found that the average horse could work constantly at a rate of 22,000 foot-pounds per minute. This was increased by one half (50 percent) in making this arbitrary, and now universal, unit of power. Electrical Horsepower is horsepower calculated from electric units whereby 746 watts of electrical energy is equivalent to one horsepower.

HORSEPOWER, ELECTRICAL — Horsepower calculated from electric units whereby 746 watts of electrical energy is equivalent to one horsepower.

HUMAN ECOLOGY — (1) A branch of sociology dealing particularly with the spatial and temporal interrelationships between humans and their economic, social, and political organization; (2) The ecology of human communities and populations, especially as concerned with preservation of environmental quality (as of air or water) through proper application of conservation and civil engineering practices.

HUMAN ENVIRONMENT — Natural and physical environment and the relationship of people with that environment including physical, biological, cultural, social, and economic factors in a given area.

HUMID — Containing or characterized by perceptible moisture. Usually refers to the atmosphere.

HUMIDITY — The degree of moisture in the air.

HUSBANDRY — (Agriculture) The act or practice of cultivating crops and breeding and raising livestock. Also, the application of scientific principles to agriculture, especially to animal breeding. (Ecology) The careful management or conservation of resources.

HYDRANT — A discharge pipe with a valve and spout at which water may be drawn from a water main (as for fighting fires) called also fireplug; (2) Faucet.

HYDRAULIC — (1) Of, involving, moved by, or operated by a fluid, especially water, under pressure. (2) Able to set and harden under water, as Portland cement. (3) Of or relating to hydraulics.

HYDRAULIC BARRIER — Modifications to a ground-water flow system that restrict or impede movement of water and contaminants. Also, a barrier developed in the Estuary by the release of fresh water from upstream reservoirs to prevent intrusion of sea water into the body of fresh water.

HYDRAULIC CONDUCTIVITY — Simply, a coefficient of proportionality describing the rate at which water can move through an aquifer or other permeable medium. The density and kinematic viscosity of the water must be considered in determining hydraulic conductivity. More specifically, the volume of water at the existing kinematic viscosity that will move, in unit time, under a unit Hydraulic Gradient through a unit area measured at right angles to the direction of flow, assuming the medium is isotropic and the fluid is homogeneous. In the Standard International System, the units are cubic meters per day per square meter of medium (m3/day/m2) or m/day (for unit measures).

HYDRAULIC CONDUCTIVITY, EFFECTIVE — The rate of water flow through a porous medium that contains more than one fluid (such as water and air in the unsaturated zone), which should be specified in terms of both the fluid type and content and the existing pressure.

HYDRAULIC EARTHFILL DAM — An embankment built up from waterborne clay, sand, and gravel carried through a pipe or flume.

HYDRAULIC FRACTURING — Any technique involving the pumping of fluid under high pressure into an oil or gas formation to create fissures and openings in the reservoir rock and increase the flow of oil or gas.

HYDRAULIC GRADE LINE (HGL) — A line whose plotted ordinate position represents the sum of pressure head plus elevation head for the various positions along a given fluid flow path, such as a pipeline or ground-water streamline.

HYDRAULIC GRADIENT (I) — The gradient or slope of a water table or Piezometric Surface in the direction of the greatest slope, generally expressed in feet per mile or feet per feet. Specifically, the change in static head per unit of distance in a given direction, generally the direction of the maximum rate of decrease in head. The difference in hydraulic heads (h1 - h2), divided by the distance (L) along the flowpath, or, expressed in percentage terms:

I = (h1 - h2) / L X 100 I = (h1 - h2) / L X 100

A hydraulic gradient of 100 percent means a one foot drop in head in one foot of flow distance.

HYDRAULIC GRADIENT PIVOT POINT — A location along the water surface in a canal reach where the water level remains essentially constant during changes in flow.

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.

HYDRAULIC JUMP — The rapid change in the depth of flow from a low stage to a high stage, resulting in an abrupt rise of water surface.

HYDRAULIC LOADING — (Water Quality) For a sand filter wastewater treatment unit, the volume of wastewater applied to the surface of the filtering medium per time period. The loading is often expressed in gallons per day per square foot (gpd/ft2), or cubic meters per square meter per day (m3/m2d).

HYDRAULIC MINING — Mining by washing sand and dirt away with water, leaving the desired mineral.

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 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.

HYDRAULIC RAM — A device which uses the energy of falling water to force a small portion of the water to a height greater than the source. A water pump in which the downward flow of naturally running water is intermittently halted by a valve so that the flow is forced upward through an open pipe into a reservoir.

HYDRAULICS — (1) The study of liquids, particularly water, under all conditions of rest and motion. (2) The branch of physics having to do with the mechanical properties of water and other liquids in motion and with the application of these properties in engineering.

HYDRAULIC TRANSIENT — (1) Interim stage when a flow changes from one steady-state condition to another steady-state condition because of a sudden acceleration or deceleration of flow. (2) A wave or pressure change propagated through a canal or pipeline during unsteady flow.

HYDRO — The prefix denoting water or hydrogen.

HYDROCOMPACTION — The settling and hardening of land due to application of large amounts of water for irrigation.

HYDRODYNAMIC DISPERSION — (1) Spreading (at the macroscopic level) of the solute front during transport resulting from both mechanical dispersion and molecular diffusion. (2) The process by which ground water containing a solute is diluted with uncontaminated ground water as it moves through an aquifer. Also see Dispersion Coefficient.

HYDRODYNAMIC LOADS — Forces imposed on structures by floodwaters due other impacts of moving water on the upstream side of the structure, drag along its sides, and eddies or negative pressures on its downstream side.

HYDRODYNAMICS — The branch of science that deals with the dynamics of fluids, especially incompressible fluids, in motion.

HYDROELECTRIC — Having to do with production of electricity by water power from falling water.

HYDROELECTRIC PLANT (CONVENTIONAL) — A hydroelectric power plant which utilizes streamflow only once as the water passes downstream; electric power plant in which the energy of falling water is used to spin a turbine generator to produce electricity.

HYDROELECTRIC PLANT (PUMPED STORAGE) — A hydroelectric power plant which generates electric power during peak load periods by using water pumped into a storage reservoir during off-peak periods.

HYDROELECTRIC POWER — Power (hydroelectricity) produced using water power as a source of energy. Electrical energy generated by means of a power generator coupled to a turbine through which water passes.

HYDROELECTRIC POWER WATER USE — The use of water in the generation of electricity at plants where the turbine generators are driven by falling water. This constitutes an Instream Use of water and is a nonconsumptive use of water.

HYDROELECTRICITY — Electric energy production by water powered turbine generators.

HYDROGEOLOGIC — Those factors that deal with subsurface waters and related geologic aspects of surface waters.

HYDROGEOLOGIC PARAMETERS — Numerical parameters that describe the hydrogeologic characteristics of an aquifer such as Porosity, Permeability, and Transmissivity.

HYDROGEOLOGIC UNIT — Any soil or rock unit or zone that because of its hydraulic properties has a distinct influence on the storage or movement of ground water.

HYDROGEOLOGICAL CYCLE — The natural process recycling water from the atmosphere down to (and through) the earth and back to the atmosphere again. Also see Hydrologic Cycle.

HYDROGEOLOGY — The part of geology concerned with the functions of water in modifying the earth, especially by erosion and deposition; geology of ground water, with particular emphasis on the chemistry and movement of water.

HYDROGEOMORPHIC UNIT — A land form characterized by a specific origin, geomorphic setting, water source, and hydrodynamic.

HYDROGRAPH — A graphic representation or plot of changes in the flow of water or in the elevation of water level plotted against time. A graph showing stage, flow, velocity, or other hydraulic properties of water with respect to time for a particular point on a stream. Hydrographs of wells show the changes in water levels during the period of observation.

HYDROGRAPHIC STUDY AREA — An area of hydrological and climatological similarity so subdivided for study purposes.

HYDROGRAPHIC 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.

HYDROGRAPHY — The study, description, and mapping of oceans, lakes, and rivers, especially with reference to their navigational and commercial uses.

HYDROLOGIC BALANCE — An accounting of all water inflows to, water outflows from, and changes in water storage within a hydrologic unit over a specified period of time.

HYDROLOGIC BASIN — The complete drainage area upstream from a given point on a stream.

HYDROLOGIC BENCHMARK — A hydrologic unit, such as a basin or a ground-water body, that because of its expected freedom from the effects of man, has been designated as a benchmark. Data from such basins may provide a standard with which data from less independent basins can be compared so that changes wrought by man's interference can be distinguished from changes caused by variations in the natural regimen.

HYDROLOGIC BUDGET — An accounting of the inflow, outflow, and storage in a hydrologic unit, such as a drainage basin, aquifer, soil zone, lake, reservoir, or irrigation project.

HYDROLOGIC CONDITION — The runoff potential of a particular cropping practice. A crop under good hydrologic condition will have a higher infiltration rate and lower runoff potential than one under poor conditions.

HYDROLOGIC CYCLE — The circuit of water movement from the atmosphere to the earth and return to the atmosphere through various stages or processes such as precipitation, interception, runoff, infiltration, percolation, storage, evaporation, and transportation. Also referred to as the Water Cycle and Hydrogeologic Cycle.

HYDROLOGIC EQUATION — The water inventory equation: Inflow = [Outflow + Change in Storage], which balances the Hydrologic Budget and 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 MODEL — Mathematical formulations that simulate hydrologic phenomenon considered as processes or as systems.

HYDROLOGIC REGION — A study area, consisting of one or more planning subareas, used to analyze water use and hydrologic conditions. Typically such areas are based on Watersheds.

HYDROLOGIC SOIL GROUPS — The classification of soils by their reference to the intake rate of infiltration of water, which is influenced by texture, organic matter content, stability of the soil aggregates, and soil horizon development.

HYDROLOGIC STUDY APPROACH — The study of a project's water distribution based upon a hydrological balance, where inflow (diversion into project) is balanced with outflow (precautionary drawdowns, crop consumptive use, deep seepage, surface return flows, and undefined "losses").

HYDROLOGIC UNIT — (1) A geographic area representing part or all of a surface drainage basin or distinct hydrologic feature. (2) A classification of soils concerning water infiltration characteristics used in hydrologic analyses.

HYDROLOGY — The science of waters of the earth, their occurrence, distribution, and circulation; their physical and chemical properties; and their reaction with the environment, including living beings.

HYDROLYSIS — The splitting (lysis) of a compound by a reaction with water. Examples are the reaction of salts with water to produce solutions which are not neutral, and the reaction of an ester with water.

HYDROMANCY — Divination by the observation of water.

HYDROMECHANICS — The branch of physics having to do with the laws governing the motion and equilibrium of fluids.

HYDROMETEOROLOGY — The science of the application of meteorology to hydrologic problems; the branch of meteorology that deals with the occurrence, motion, and changes of the state of atmospheric water. The combination of snowpack measurements and climatic forecasts to predict runoff.

HYDROMETER — An instrument used to determine specific gravity, especially a sealed, graduated tube, weighted at one end, that sinks in a fluid to a depth used as a measure of the fluid's specific density.

HYDROMETRIC NETWORK — Network of stations at which measurement of hydrological parameters is performed.

HYDROPOWER — Power (e.g., electrical energy) produced by falling water; the utilization of the energy available in falling water for the generation of electricity.

HYDROSCOPE — An optical device used for viewing objects far below the surface of water.

HYDROSPHERE — (1) The water on or surrounding the surface of the globe, as distinguished from those of the Lithosphere and the Atmosphere. (2) The region that 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. Together, the waters of the Hydrosphere, Atmosphere, Lithosphere, and Biosphere, constitutes the earth's Ecosphere.

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 LOADS — Forces imposed on a flooded structure due to the weight of the water.

HYDROSTATIC PRESSURE — The pressure in a fluid in equilibrium which is due solely to the weight of fluid above.

HYDROSTATICS — The branch of physics that deals with fluids at rest and under pressure.