Glossary “Water Management”

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G

GABION — A wire cage, usually rectangular, filled with cobbles and used as a component for water control structures or for channel and bank protection.

GAGE, or Gauge — (1) An instrument used to measure magnitude or position; gages may be used to measure the elevation of a water surface, the velocity of flowing water, the pressure of water, the amount of intensity of precipitation, the depth of snowfall, etc. (2) The act or operation of registering or measuring magnitude or position. (3) The operation, including both field and office work, of measuring the discharge of a stream of water in a waterway.

GAGE HEIGHT — The height of the water surface above the gage datum (reference level). Gage height is often used interchangeably with the more general term, Stage, although Gage Height is more appropriate when used with a gage reading.

GAGE ROD — A measuring device that shows the water level in the reservoir.

GAGING STATION — A particular site on a stream, canal, lake, or reservoir where systematic observations of Gage Height or discharge are obtained.

GAINING STREAM — A stream or reach of a stream, the flow of which is being increased by the inflow of ground water seepage or from springs in, or alongside, the channel.

GALLERY — (1) A passageway within the body of a dam or abutment; hence the terms "grouting gallery," "inspection gallery," and "drainage gallery." (2) A long and rather narrow hall; hence the following terms for a power plant: "valve gallery," "transformer gallery," and "busbar gallery."

GATE — (1) (Irrigation) Structure or device for controlling the rate of water flow into or from a canal, ditch, or pipe. (2) (Dam) A device in which a leaf or member is moved across the waterway from an external position to control or stop the flow. The following types of gates apply to dams and other such structures:
[1] Bulkhead Gate — A gate used either for temporary closure of a channel or conduit to empty it for inspection or maintenance or for closure against flowing water when the head differential is small, e.g., a diversion tunnel closure. Although a bulkhead gate is usually opened and closed under nearly balanced pressures, it nevertheless may be capable of withstanding a high pressure differential when in the closed position.
[2] Crest Gate (Spillway Gate) — A gate on the crest of a spillway to control overflow or reservoir water level.
[3] Emergency Gate — A standby or reserve gate used only when the normal means of water control is not available.
[4] Fixed Wheel Gate (Fixed Roller Gate, Fixed Axle Gate) — A gate having wheels or rollers mounted on the end posts of the gate. The wheels bear against rails fixed in side grooves or gate guides.
[5] Flap Gate — A gate hinged along one edge usually either the top or bottom edge. Examples of bottom-hinged flap gates are tilting gates and fish belly gates, so-called due to their shape in cross section.
[6] Flood Gate — A gate to control flood release from a reservoir.
[7] Guard Gate (Guard Valve) — A gate or valve that operates fully open or closed. It may function as a secondary device for shutting off the flow of water in case the primary closure device becomes inoperable, but is usually operated under balanced pressure, no-flow conditions.
[8] Outlet Gate — A gate controlling the outflow of water from a reservoir.
[9] Radial Gate (Tainter Gate) — A gate with a curved upstream plate and radial arms hinged to piers or other supporting structures.
[10] Regulating Gate (Regulating Valve) — A gate or valve that operates under full pressure and flow conditions to throttle and vary the rate of discharge.
[11] Slide Gate (Sluice Gate) — A gate that can be opened or closed by sliding it in supporting guides.

GATED PIPE — (Irrigation) Portable pipe with small gates installed along one side for distributing water to corrugations or furrows.

GENERATOR — A machine that changes water power, steam power, or other kinds of mechanical energy into electricity.

GEOGRAPHIC INFORMATION SYSTEM (GIS) — A computer information system that can input, store, manipulate, analyze, and display geographically referenced data to support the decision-making processes of an organization. A map based on a database or databases. System plots locations of information on maps using latitude and longitude.

GEOHYDROLOGY — A term which denotes the branch of Hydrology relating to subsurface or subterranean waters; that is, to all waters below the surface.

GEOMORPHOLOGY (Geomorphic) — That branch of both physiography and geology that deals with the form of the earth, the general configuration of its surface, and the changes that take place in the evolution of land forms. The term usually applies to the origins and dynamic morphology (changing structure and form) of the earth's land surfaces, but it can also include the morphology of the sea floor and the analysis of extraterrestrial terrains. Sometimes included in the field of physical geography, geomorphology is really the geological aspect of the visible landscape.

GEOMORPHOLOGY, HISTORICAL — Historical geomorphology represents one branch of Geomorphology which provides the means to analyze the long-term change in landforms through the concept of cyclic change. The concepts evolved at the turn of the 20th century and were put forward by the American geologist William Morris Davis. The theory stated that every landform could be analyzed in terms of structure, process, and stage. Structure and process are treated by the science of geomorphology. However, the concept of stage introduced the element of time, and is subject to a far greater degree of interpretation. As postulated by Davis, every landform underwent development through a predictable, cyclic sequence: i.e., youth, maturity, and old age. Historical geomorphology relies on various chronological analyses, notably those provided by stratigraphic studies of the last 2 million years, known as the Quaternary Period. The relative chronology usually may be worked out by observation of stratigraphic relationships, with the time intervals involved established more precisely by dating methods such as historical records, radiocarbon analysis, tree-ring counting (Dendrochronology), and paleomagnetic studies. By applying such methods to stratigraphic data, a quantitative chronology of events is constructed that provides a means for calculating long-term rates of change.

GEOMORPHOLOGY, PROCESS — The second branch of Geomorphology, process geomorphology analyzes contemporary dynamic processes at work in landscapes. The mechanisms involved are weathering and erosion and combine processes that are in some respects destructive and in others constructive. The bedrock and soil provide the passive material, whereas the climatic regime and crustal dynamics together provide the principal active variables.

GEOTHERMAL — Terrestrial heat, usually associated with water as around hot springs.

GEOTHERMAL ENERGY — The heat energy available in the earth's subsurface, extracted from three basic sources: (1) steam; (2) hot water; and (3) hot rocks or near surface intrusions of volcanic molten rock. The normal thermal gradient of the earth's crust is such that the temperature in a deep well or mine typically increases by about 1°F (0.56°C) for each 100 feet of depth.

GEOTHERMICS — The science pertaining to the earth's interior heat. Its main practical application is in finding natural concentrations of hot water, the source of Geothermal Energy, for use in electric power generation and direct heat applications such as space heating and industrial drying processes. Heat is produced within the crust and upper mantle of the earth primarily by decay of radioactive elements. This geothermal energy is transferred to the earth's surface by diffusion and by convection movement of magma (molten rock) and deep-lying circulating water. Surface hydrothermal manifestations include hot springs, geysers, and Fumaroles.

GEYSER — A periodic thermal spring that results from the expansive force of super heated steam. Also, a special type of thermal spring which intermittently ejects a column of water and steam into the air with considerable force.

GIGAWATT HOUR (GWh) — One billion Watt-hours (Wh).

GLACIER — A huge mass of ice, formed on land by the compaction and recrystallization of snow, that moves very slowly downslope or outward due to its own weight.

GLACIER MEAL — Finely ground rock particles produced by glacial abrasion.

GLACIOLOGY — Collectively, the branches of science concerned with the causes and modes of ice accumulation and with ice action, on the earth's surface. Specifically, the branch of geology which studies the effects of glacial epochs, glaciation, and ice in modifying the earth's surface and in affecting the life and distribution of plants and animals.

GLADE — An open, spacious Wetland, as in the Everglades.

GRADE — (Hydraulics) The slope of a stream bed.

GRADED STREAM — A stream in which, over a period of years, the slope is delicately adjusted to provide, with available discharge and with prevailing channel characteristics, just the velocity required for transportation of the sediment load supplied from the drainage basin. Also, a stream in which most irregularities, such as waterfalls and cascades, are absent. Streams tend to cut their channels lower at a very slow rate after they become graded.

GRADE STABILIZATION STRUCTURE — A structure for the purpose of stabilizing the grade of a gully or other watercourse, thereby preventing further head-cutting or lowering of the channel grade.

GRADIENT — Degree of incline; slope of a stream bed. The vertical distance that water falls while traveling a horizontal distance downstream.

GRADUALLY VARIED FLOW — (Hydraulics) Non-uniform flow in which depth of flow changes gradually through a reach. Typical of normal natural valley and channel flow, which can be either steady or unsteady flows.

GRASSED WATERWAY OR OUTLET — A natural or constructed waterway, usually broad and shallow and covered with erosion-resistant grasses, suitable to resist potential damages resulting from runoff.

GRAVEL — A mixture composed primarily of rock fragments 2 mm (0.08 inch) to 7.6 cm (3 inches) in diameter. Usually contains much sand.

GRAVEL ENVELOPE — In well construction, a several-inch thickness of uniform gravel poured into the annular space between the well casing and the drilled hole.

GRAVITATIONAL HEAD — Component of total Hydraulic Head related to the position of a given mass of water relative to an arbitrary datum.

GRAVITATIONAL WATER — Water that moves into, through, or out of a soil or rock mass under the influence of gravity.

GRAVITY DAM — A dam constructed of concrete and/or masonry that relies on its weight for stability.

GRAVITY FLOW — The downhill flow of water through a system of pipes, generated by the force of gravity.

GRAVITY IRRIGATION — (1) Irrigation in which the water is not pumped but flows and is distributed by gravity, includes sprinkler systems when gravity furnishes the desired head (pressure). (2) Irrigation method that applies irrigation water to fields by letting it flow from a higher level supply canal through ditches or furrows to fields at a lower level.

GRAYWATER — Waste water from a household or small commercial establishment which specifically excludes water from a toilet, kitchen sink, dishwasher, or water used for washing diapers.

GREYWATER — Wastewater from clothes washing machines, showers, bathtubs, hand washing, lavatories and sinks that are not used for disposal of chemicals or chemical-biological ingredients.

GROSS RESERVOIR CAPACITY — The total amount of storage capacity available in a reservoir for all purposes, from the streambed to the normal maximum operating level. It does not include surcharge (water temporarily stored above the elevation of the top of the spillway), but does include dead (or inactive) storage.

GROSS DUTY OF WATER — (Irrigation) The irrigation water diverted at the intake of a canal system, usually expressed in depth on the irrigable area under the system.

GROSS WATER REQUIREMENT (FARM) — The Farm Delivery Requirement plus the seepage losses in the canal system from the headworks to the farm unit plus the waste of water due to poor operation.

GROSS WATER YIELD — The available water runoff, both surface and subsurface, prior to use by man's activities, use by phreatophytes, or evaporation from free water surfaces.

GROUND — (1) The solid surface of the earth. (2) The floor of a body of water, especially the sea.

GROUND WATER, also Groundwater — (1) Water that flows or seeps downward and saturates soil or rock, supplying springs and wells. The upper level of the saturate zone is called the Water Table. (2) Water stored underground in rock crevices and in the pores of geologic materials that make up the earth's crust. Ground water lies under the surface in the ground's Zone of Saturation.

GROUND WATER BARRIER — Rock, clay, or other natural or artificial materials with a relatively low permeability that occurs (or is placed) below ground surface, where it impedes the movement of ground water and thus causes a pronounced difference in the heads on opposite sides of the barrier.

GROUND WATER BASIN — A ground-water reservoir together with all the overlying land surface and the underlying aquifers that contribute water to the reservoir. In some cases, the boundaries of successively deeper aquifers may differ in a way that creates difficulty in defining the limits of the basin. A ground-water basin could be separated from adjacent basins by geologic boundaries or by hydrologic boundaries.

GROUND WATER, CONFINED — Ground water under pressure significantly greater than atmospheric, with its upper limit the bottom of a bed with hydraulic conductivity distinctly lower than that of the material in which the confined water occurs.

GROUND WATER DISCHARGE — (1) The flow of water from the Zone of Saturation. (2) (Water Quality) Ground water entering near coastal waters which has been contaminated by landfill leachate, deep well injection of hazardous wastes, septic tanks, etc.

GROUND WATER DISPOSAL — Refers to wastewater that is disposed of through the ground either by injection or seepage. This includes the following discharge methods: absorption beds, injection wells, drain fields, percolation ponds, rapid infiltration basins, and spray fields (land application). Land application systems (reuse systems) are considered a groundwater disposal method as the wastewater used to irrigate turf or crops is generally intended to filter down through the soil.

GROUND WATER DIVIDE — A line on a water table on either side of which the water table slopes downward. It is analogous to a drainage divide between two drainage basins on a land surface. It is also the line of highest Hydraulic Head in the water table or Potentiometric Surface.

GROUND WATER FLOW — The movement of water through openings in sediment and rock that occurs in the Zone of Saturation.

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.

GROUND WATER, FREE — Unconfined ground water whose upper boundary is a free water table.

GROUND WATER HYDRAULICS — The study of the movement of water, especially water under pressure and water's movement through various soil medium.

GROUND WATER HYDROLOGY — The branch of Hydrology that deals with ground water; its occurrence and movements, its replenishment and depletion, the properties of rocks that control ground water movement and storage, and the methods of investigation and utilization of ground water. Also referred to as Ground Water Hydraulics, although this term pertains more to the study of the motion of water.

GROUND WATER LAW — The common law doctrine of Riparian Rights and the doctrine of prior appropriation (Appropriative Rights) as applied to ground water. See Appropriation Doctrine and Riparian Doctrine.

GROUND WATER MINING — The withdrawal of water from an aquifer in excess of recharge which, if continued over time, would eventually cause the underground supply to be exhausted or the water table could drop below economically feasible pumping lifts.

GROUND WATER MOUND — Raised area in a water table or other Potentiometric Surface, created by Ground Water Recharge.

GROUND WATER OUTFLOW — That part of the discharge from a drainage basin that occurs through the ground water. The term "underflow" is often used to describe the ground water outflow that takes place in valley alluvium (instead of the surface channel) and thus is not measured at a gaging station.

GROUND WATER OVERDRAFT — The condition of a ground water basin in which the amount of water withdrawn by pumping exceeds the amount of water that recharges the basin over a period of years during which water supply conditions approximate average. Sometimes used interchangeably with Ground Water Mining.

GROUND WATER, PERCHED — Ground water that is separated from the main body of ground water by an impermeable (unsaturated) layer.

GROUND WATER PLUME — A volume of contaminated groundwater that extends downward and outward from a specific source; the shape and movement of the mass of the contaminated water is affected by the local geology, materials present in the plume, and the flow characteristics of the area groundwater.

GROUND WATER PRIME SUPPLY — The long-term average annual percolation to the major ground water basins from precipitation falling on the land and from flows in rivers and streams. Also includes recharge from local sources that have been enhanced by construction of spreading ground or other means. Recharge of imported and reclaimed water is not included nor is recharge using applied irrigation water.

GROUND WATER RECHARGE — Inflow of water to a ground water reservoir (Zone of Saturation) from the surface. Infiltration of precipitation and its movement to the water table is one form of natural recharge. Also, the volume of water added by this process.

GROUND WATER REGISTRATION — A statement made by a well owner registering the Beneficial Use of ground water.

GROUND WATER RESERVOIR — An aquifer or aquifer system in which ground water is stored. The water may be introduced into the aquifer by artificial or natural means.

GROUND WATER RESERVOIR STORAGE — The amount of water in storage within the defined limit of the aquifer.

GROUND WATER RUNOFF — A portion of runoff which has passed into the ground, has become ground water, and has been discharged into a stream channel as spring or seepage water.

GROUND WATER STORAGE — The storage of water in ground water reservoirs.

GROUND WATER STORAGE CAPACITY — The space or voids contained in a given volume of soil and rock deposits. Also, the reservoir space contained in a given volume of deposits. Under optimum conditions of use, the usable ground water storage capacity volume of water that can be alternately extracted and replaced in the deposit, within specified economic limitations.

GROUND WATER SYSTEM — All the components of subsurface materials that relate to water, including Aquifers (confined and unconfined), Zones of Saturation, and Water Tables.

GROUND WATER TABLE — The upper surface of the Zone of Saturation for underground water. It is an irregular surface with a slope or shape determined by the quantity of ground water and the permeability of the earth materials. In general, it is highest beneath hills and lowest beneath valleys. Also referred to as the Water Table.

GROUND WATER, UNCONFINED — Water in an aquifer that has a water table.

GROUND WATER UNDER THE DIRECT INFLUENCE (UDI) OF SURFACE WATER — Any water beneath the surface of the ground with: (1) a significant occurrence of insects or other microorganisms, algae, or large-diameter Pathogens; or (2) significant and relatively rapid shifts in water characteristics such as turbidity, temperature, conductivity, or pH which closely correlate to climatological or surface water conditions. Under direct influence conditions are determined for individual sources in accordance with criteria established by the state.

GROUND WATER VELOCITY — The rate of water movement through openings in rock or sediment.

GROWING SEASON — (1) The period and/or number of days between the last freeze in the spring and the first frost in the fall for the freeze threshold temperature of the crop or other designated temperature threshold. (2) Also, the average number of days exceeding 32°F (0°C).

GROWTH MANAGEMENT PROGRAM — A program comprised of several techniques to coordinate public and private decisions about the location and timing of development in order to best utilize environmental and physical resources.

H

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.