Knowledge Base

DAILY FLOOD PEAK — The maximum mean daily discharge occurring in a stream during a given flood event.

DAILY TEMPERATURE RANGE — The difference between the highest and lowest temperatures recorded on a particular day.

DAM — A structure of earth, rock, or concrete designed to form a basin and hold water back to make a pond, lake, or reservoir. A barrier built, usually across a watercourse, for impounding or diverting the flow of water. General types of dams include:
[1] Arch Dam — Curved masonry or concrete dam, convex in shape upstream, that depends on arch action for its stability; the load or water pressure is transferred by the arch to the Abutments.
[2] Buttress Dam — A dam consisting of a watertight upstream face supported at intervals on the downstream side by a series of buttresses.
[3] Cofferdam — A temporary watertight enclosure that is pumped dry to expose the bottom of a body of water so that construction, as of piers, a dam, and bridge footings, may be undertaken. A "diversion cofferdam" prevents all downstream flow by diverting the flow of a river into a pipe, channel, or tunnel.
[4] Crib Dam — A barrier or form of Gravity Dam constructed of timber forming bays, boxes, cribs, crossed timbers, gabions or cells that are filled with earth, stone or heavy material.
[5] Embankment Dam — A dam structure constructed of fill material, usually earth or rock, placed with sloping sides and usually with a length greater than its height. Types of embankment dams include: Earthfill or Earth Dam — A dam in which more than 50 percent of the total volume is formed of compacted fine-grained material obtained from a borrow area (i.e., excavation pit); Fill Dam — Any dam constructed of excavated natural materials or of industrial waste materials; Homogeneous Earthfill Dam — A dam constructed of similar earth material throughout, except for the possible inclusion of internal drains or drainage blankets; distinguished from a Zoned Earthfill Dam; Hydraulic Fill Dam — A dam constructed of materials, often dredged, that are conveyed and placed by suspension in flowing water; Rockfill Dam — A dam in which more than 50 percent of the total volume is comprised of compacted or dumped pervious natural or crushed rock; Rolled Fill Dam — A dam of earth or rock in which the material is placed in layers and compacted by using rollers or rolling equipment; and Zoned Embankment Dam — A dam which is composed of zones of selected materials having different degrees of porosity, permeability, and density.
[6] Gravity Dam — A dam constructed of concrete and/or masonry that relies on its weight for stability.
[7] Masonry Dam — A dam constructed mainly of stone, brick, or concrete blocks that may or may not be joined with mortar. A dam having only a masonry facing should not be referred to as a masonry dam.
[8] Weir — A dam in a river to stop and raise the water, for the purpose of conducting it to a mill, forming a fishpond, or the like. When uncontrolled, the weir is termed a fixed-crest weir. Other types of weirs include broad-crested, sharp-crested, drowned, and submerged.

DAMAGE-FREQUENCY CURVE — A graph showing the flood damages and their probabilities of occurrence. The total area under the curve represents the annual damage.

DAMAGES PREVENTED — The difference between the amount of damages without a particular water project and the damages with the project in place.

DARCY'S LAW — An empirically derived equation for the flow of fluids through porous media. It is based on the assumption that flow is laminar and inertia can be neglected, and states that velocity of flow is directly proportional to Hydraulic Gradient. For groundwater, this is equivalent to the velocity being equal to the product of the hydraulic gradient and the effective subsoil conductivity or permeability.

DATA — In its strictest sense, data may be defined only as the raw numbers (or descriptions, in the case of qualitative data), either in Time-Series format (data covering observations over specific periods of time), Cross-Sectional format (data consisting of a number of observations taken at a specific point in time or about a specific event or phenomenon), or a combination of these two. Also see Information.

DATA BANK — A well-defined collection of data, usually of the same general type, which can be accessed by a computer and may readily be used for further analysis, presentation, and forecasting. Also referred to as a Data Base.

DATA, CROSS-SECTIONAL — (Statistics) Data which describe the activities or behavior of individual persons, firms, or other units at a given point in time.

DATA MANAGEMENT — The act, process, or means by which data is managed. This may include the compilation, storage, safe-guarding, listing, organization, extraction, retrieval, manipulation, and dissemination of data. In its strictest sense, data may be defined only as the raw numbers for numeric or quantitative data (or descriptions, in the case of qualitative data), either in time-series format (data covering observations over specific periods of time), cross-sectional format (data consisting of a number of observations taken at a specific point in time or about a specific event or phenomenon without regard to its behavior over time), or a combination of these two. Information, on the other hand, deals more specifically with the manipulation, re-organization, analysis, graphing, charting, and presentation of data for specific management and decision-making purposes. Also see Information Management.

DATA, PRIMARY — Typically, data acquired by direct interaction, such as direct observation through measurements, tabulation, or surveys. Contrast with Secondary Data.

DATA, SECONDARY — Typically, data acquired from published sources as opposed to data acquired from direct observation or measurement such as a survey. Contrast with Primary Data.

DATA, TIME-SERIES — (Statistics) Data which describe the movement of a variable over time, e.g., monthly, quarterly, annually.

DEAD END — The end of a water main which is not connected to other parts of the distribution system.

DEAD STORAGE — The volume of water in a reservoir stored below the lowest outlet or operating level.

DEBRIS BASINS — Storage for sediment and floating material provided by a dam with spillway above channel grade, by excavation below grade, or both. Water retention is not an intended function of the structure.

DEBRIS DAM — A barrier built across a stream channel to retain rock, sand, gravel, silt, or other material.

DEBRIS FLOW — A moving mass of rock fragments, soil, and mud with more than one-half of the material being larger than sand size.

DEBRIS GUARD — A screen or grate at the intake of a channel, drainage, or pump structure for the purpose of stopping debris.

DECLARED UNDERGROUND WATER BASIN — An area of a state designated in some states by their respective State Engineers to be underlain by a ground water source having reasonably ascertainable boundaries. By such a designation, the State Engineer assumes jurisdiction over the appropriation and use of ground water from the source. May not be applicable in states which already claim regulatory rights over both surface and ground waters.

DECREED RIGHTS (WATER) — Water rights determined by court decree.

DEDICATED NATURAL FLOW — River flows dedicated to environmental use.

DEDICATIONS (Water) — A controversial water rights policy that involves a trade-off in which a user can begin pumping groundwater in exchange for a guarantee to buy and retire a like amount of surface water in the future. Critics of the policy argue that dedications are often difficult to enforce and can lead to overuse of groundwater when a user fails to fulfill on the guarantee.

DEEP WELL — A well whose pumping head is too great to permit use of a suction pump.

DEEP-WELL DISPOSAL — Transfer of liquid wastewater to underground strata; usually limited to biologically or chemically stable wastes.

DEEP-WELL INJECTION — Deposition of raw or treated, filtered hazardous waste by pumping it into deep wells, where it is contained in the pores of permeable subsurface rock.

DEGRADATION (River Beds or Stream Channels) — The general lowering of the streambed by erosive processes, such as scouring by flowing water. The removal of channel bed materials and downcutting of natural stream channels. Such erosion may initiate degradation of tributary channels, causing damage similar to that due to gully erosion and valley trenching.

DELIVERY — (Irrigation) The release of water from turnouts to water users.

DELIVERY BOX — An irrigation structure for diverting water from a canal to a farm unit, often including measuring devices.

DELIVERY/RELEASE — The amount of water delivered to the point of use and the amount released after use; the difference between these amounts is usually the same as the Consumptive Use.

DELIVERY SYSTEM — A system which conveys water from a single source, such as a storage reservoir, to a number of individual points of use. The delivery system is a common classification. It is associated with irrigation, municipal and industrial use, and fish and wildlife canal systems.

DELTA — (1) An alluvial deposit made of rock particles (sediment and debris) dropped by a stream as it enters a body of water. (2) A plain underlain by an assemblage of sediments that accumulate where a stream flows into a body of standing water where its velocity and transporting power are suddenly reduced. Originally so named because many deltas are roughly triangular in plan, like the Greek letter delta), with the apex pointing upstream.

DEMAND (Water) — Maximum water use under a specified condition.

DEMAND DELIVERY — A method of irrigation water delivery whereby the project delivers water to the headgate upon farm irrigator demand; usually is associated with high head (cfs) delivery rates. Unrestricted use of the available water supply with limitations only on maximum flow rate and total allotment.

DEMAND MANAGEMENT ALTERNATIVES — Water management programs that reduce the demand for water, such as water conservation, drought rationing, rate incentive programs, public awareness and education, drought landscaping, etc.

DEMERSAL — (1) Dwelling at or near the bottom of a body of water, such as demersal fish. (2) Sinking to or deposited near the bottom of a body of water, such as demersal fish eggs.

DEMINERALIZATION, also Demineralize — The act or treatment process that removes dissolved minerals or mineral salts from a liquid, such as water.

DEMINERALIZED WATER — Water which has been passed through a mixed-bed ion exchanger to remove soluble ionic impurities. Nonelectrolytes and Colloids are not removed from water so treated.

DEPLETION — The water consumed within a service area or no longer available as a source of supply; that part of a withdrawal that has been evaporated, transpired, incorporated into crops or products, consumed by man or livestock, or otherwise removed. For agriculture and wetlands, it is the Evapotranspiration of Applied Water (ETAW) (and Evapotranspiration (ET) of flooded wetlands) plus irrecoverable losses. For urban water use, it is the ETAW (water applied to landscaping or home gardens), sewage effluent that flows to a salt sink, and incidental ET losses. For instream use, it is the amount of dedicated flow that proceeds to a salt sink and is not available for reuse.

DEPLETION (GROUND WATER) — The withdrawal of water from a ground water source at a rate greater than its rate of recharge, usually over an extended period of several years.

DEPLETION (STREAMFLOW) — The amount of water that flows into a valley, or onto a particular land area, minus the water that flows out of the valley or off from the particular land area.

DEPLETION (WATER) — That portion of the water supply that is consumptively used.

DEPLETION CURVE — (Hydraulics) A graphical representation of water depletion from storage-stream channels, surface soil, and groundwater. A depletion curve can be drawn for base flow, direct runoff, or total flow.

DEPOSITION — The accumulation of material dropped because of a slackening movement of the transporting medium, e.g., water or wind. Also, the transition of a substance from the vapor phase directly to the solid phase, without passing through an intermediate liquid phase, also referred to as Sublimation.

DEPRESSION STORAGE — Water contained in natural depressions in the land surface, such as puddles.

DEPTH, often Depths — A deep art of place, as the ocean depths.

DEPTH FINDER — An instrument used to measure the depth of water, especially by radar or ultrasound.

DEPTH OF RUNOFF — The total runoff from a drainage basin divided by its area. For convenience in comparing runoff with precipitation, depth of runoff is usually expressed in inches during a given period of time over the drainage area expressed in inches per square mile.

DESALINATION, or Desalinization — To remove salts and other chemicals, as from sea water or soil, for example. Usually used with respect to the salt contained in water. Also referred to as Desalting.

DESALTING — The term used to refer to any process by which the dissolved solids content of saline water or seawater is reduced.

DESIGN FLOOD — The flood magnitude selected for use as a criterion in designing flood control works. The largest flood that a given project is designed to pass safely. In dam design and construction, the reservoir inflow-outflow hydrograph used to estimate the spillway discharge capacity requirements and corresponding maximum surcharge elevation in the reservoir.

DESIGN FLOW — The average flow of wastewater that a treatment facility is built to process efficiently, commonly expressed in millions of gallons per day (MGD).

DESIGN RUNOFF RATE — In irrigation, the maximum runoff rate expected over a given period of time.

DESIGNATED FLOODWAY — The channel of a stream and the portion of the adjoining floodplain designated by a regulatory agency to be kept free of further development to provide for unobstructed passage of flood flows.

DESIGNATED GROUNDWATER BASIN — A basin where permitted ground water rights approach or exceed the estimated average annual recharge and the water resources are being depleted or require additional administration. Under such conditions, a state's water officials will so designate a groundwater basin and, in the interest of public welfare, declare Preferred Uses (e.g., municipal and industrial, domestic, agriculture, etc.).

DESIGNATED USES — Those water uses identified in state water quality standards that must be achieved and maintained as required under the Clean Water Act (CWA). Such uses may include cold water fisheries, public water supply, irrigation, recreation, minimum stream flows, etc.

DESIGNATED WATERSHEDS — Watershed areas that have been set aside as sources of municipal water or other similar purposes would be included in this category. Other uses are either modified or excluded.

DETENTION DAM — A dam constructed for the purpose of temporary storage of streamflow or surface runoff and for releasing the stored water at controlled rates.

DETENTION BASIN — A relatively small storage lagoon for slowing stormwater runoff, generally filled with water for only a short period of time after a heavy rainfall. Also see Retention Basin.

DETENTION FACILITY — A surface water runoff storage facility that is normally dry but is designed to hold (detain) surface water temporarily during and immediately after a runoff event. Examples of detentional facilities are: natural swales provided with crosswise earthen berms to serve as control structures, constructed or natural surface depressions, subsurface tanks or reservoirs, rooftop storage, and infiltration or filtration basins. Also see Retention Facility.

DETENTION STORAGE — The volume of water, other than depression storage, existing on the land surface as flowing water which has not yet reached the channel.

DETENTION STRUCTURE (DAM) — A structure constructed for the temporary storage of floodflows where the opening for release is of a fixed capacity and not manually operated.

DETENTION TIME — (1) The theoretical calculated time required for a small amount of water to pass through a tank at a given rate of flow. (2) The actual time that a small amount of water is in a settling basin, flocculating basin, or rapid-mix chamber. (3) In storage reservoirs, the length of time water will be held before being used.

DETERGENT — Synthetic washing agent that helps to remove dirt and oil. Some contain compounds which kill useful bacteria and encourage algae growth when they are in wastewater that reaches receiving waters.

DEW — The droplets of water condensed from air, usually at night, onto cool surfaces.

DEWATER, and Dewatering — (1) To remove water from a waste produce or streambed, for example. (2) The extraction of a portion of the water present in sludge or slurry, producing a dewatered product which is easier to handle. (3) (Mining) The removal of ground water in conjunction with mining operations, particularly open-pit mining when the excavation has penetrated below the ground-water table. Such operations may include extensive ground-water removal and, if extensive enough and if not re-injected into the groundwater, these discharges may alter surface water (stream) flows and lead to the creation of lakes and wetland areas. As such water removals only last so long as the mine is in operation, eventually surface water impacts, if present, will be eliminated, consequently jeopardizing surface water uses, such as irrigation, livestock, wildlife, or riparian habitat that may have become dependent upon the continuation of these temporary flows. Also, when the mine dewatering operations cease, the remaining open pit will eventually begin to fill up with ground water, resulting in significantly increased evaporation from ground water reservoirs.

DEW POINT — The temperature at which a gas or vapor condenses to form a liquid; the point at which dew begins to form.

DIFFUSION — The movement of a substance from an area of high concentration to an area of low concentration. Turbulent diffusion results from atmospheric motions diffusing water, vapor, heat, and other gaseous components by exchanging parcels called eddies between regions in space in apparent random fashion.

DIFFUSION COEFFICIENT — (1) The rate at which solutes are transported at the microscopic level due to variations in the solute concentrations within the fluid phases. (2) The rate of dispersion of a chemical caused by the kinetic activity of the ionic or molecular constituents. Also referred to as the Coefficient of Molecular Diffusion. See Molecular Diffusion.

DIFFUSIVITY, SOIL WATER — The hydraulic conductivity divided by the differential water capacity, or the flux of water per unit gradient of moisture content in the absence of other force fields.

DIGESTER — (Water Quality) In a Wastewater Treatment Plant, a closed tank that decreases the volume of solids and stabilizes raw sludge by bacterial action.

DIRECT DISCHARGER — A municipal or industrial facility which introduces pollution through a defined conveyance or system such as outlet pipes; a point source.

DIRECT FILTRATION — (Water Quality) A method of treating water which consists of the addition of coagulant chemicals, flash mixing, coagulation, minimal flocculation, and filtration. Sedimentation is not used in this process.

DIRECT PRECIPITATION — Water that falls directly into a lake or stream without passing through any land phase of the runoff cycle.

DIRECT RUNOFF — The runoff entering stream channels most immediately after rainfall or snowmelt. It consists of surface runoff plus interflow and forms the bulk of the Hydrograph of a flood. Direct runoff plus Base Runoff compose the entire flood hydrograph.

DIRECT WATER USES — Uses of water that are apparent, for example, washing, bathing, cooking, etc.

DISASTER AREA — An area that officially qualifies for emergency governmental aid as a result of a catastrophe, such as an earthquake or a flood.

DISCHARGE (HYDROLOGIC) — In its simplest concept, discharge means outflow and is used as a measure of the rate at which a volume of water passes a given point. Therefore, the use of this term is not restricted as to course or location, and it can be used to describe the flow of water from a pipe or a drainage basin. With reference to groundwater, the process by which groundwater leaves the Zone of Saturation via Evaporation, Evapotranspiration, or by flow to the surface through springs and seeps. The data in the reports of the U.S. Geological Survey (USGS) on surface water represent the total fluids measured. Thus, the terms discharge, streamflow, and runoff represent water with the solids dissolved in it and the sediment mixed with it. Of these terms, discharge is the most comprehensive. The discharge of drainage basins is distinguished as follows:
[1] Yield — The total water runout or "water crop" and includes runoff plus underflow;
[2] Runoff — That part of water yield that appears in streams; and
[3] Streamflow — The actual flow in streams, whether or not subject to regulation or underflow.
Each of these terms can be reported in total volumes (e.g., acre-feet) or time-related rates of flow (e.g., cubic feet per second or acre-feet per year).

DISCHARGE AREA — (1) An area in which ground water is discharged to the land surface, surface water, or atmosphere. (2) An area in which there are upward components of hydraulic head in the aquifer. Ground water is flowing toward the surface in a discharge area and may escape as a spring, seep, or base flow, or by evaporation and transpiration.

DISCHARGE, AVERAGE — The arithmetic average of the annual discharges for all complete water years of record whether or not they are consecutive. The term average is generally reserved for average of record and mean is used for averages of shorter periods; namely, daily mean discharge.

DISCHARGE COEFFICIENT — (Hydraulics) The ratio of actual rate of flow to the theoretical rate of flow through orifices, weirs, or other hydraulic structures.

DISCHARGE CURVE — A curve that expresses the relation between the discharge of a stream or open conduit at a given location and the stage or elevation of the liquid surface at or near that location.

DISCHARGE FORMULA — (Hydraulics) A formula used to calculate the rate of flow of fluid in a conduit or through an opening. For a steady flow discharge,
Q = A V
where Q is the rate of flow, A is the cross-sectional area, and V is the mean velocity. Common units are cubic feet per second.

DISCHARGE MEASUREMENT — Total discharge is equal to the cross-sectional area of the water in a channel or pipe times its average velocity.

DISCHARGE PERIOD — The period of time during which effluent is discharged.

DISCHARGE PERMIT — A permit issued by the state to discharge effluent into waters of the state.

DISCHARGE POINT — A location at which effluent is released into a receiving stream or body of water.

DISCHARGE PROBABILITY RELATIONSHIP — A graph of annual instantaneous peak discharge (or other hydrologic quantity) on the vertical axis, versus probability and/or recurrence interval on the horizontal axis. The graph provides a means of estimating the flow that will be reached or exceeded in a given year at a specified probability, or a means of estimating the probability that a specified discharge will be reached or exceeded in a given year.

DISCHARGE, SEDIMENT — The rate at which sediment passes a section of a stream or the quantity of sediment, as measured by dry weight or by volume, that is discharged in a given time.

DISCHARGE VELOCITY — An apparent velocity, calculated by Darcy's Law, which represents the flow rate at which water would move through an aquifer if the aquifer were an open conduit.

DISTRIBUTARY — A diverging stream which does not return to the main stream, but discharges into another stream or the ocean. Also refers to conduits that take water from a main canal for delivery to a farm.

DISTRIBUTARY CHANNEL (or Stream) — A river branch that flows away from a main stream and does not rejoin it. Characteristic of Deltas and Alluvial Fans.

DISTRIBUTION (of Water) — The management of water which allows water users to receive the amount of water to which they are entitled by law and as supply permits.

DISTRIBUTION COEFFICIENT — The quantity of a solute absorbed per unit weight of a solid divided by the quantity dissolved in water per unit volume of water.

DISTRIBUTION GRAPH (DISTRIBUTION HYDROGRAPH) — A Unit Hydrograph of direct runoff modified to show the portion of the volume of runoff that occurs during successive equal units of time.

DISTRIBUTION SYSTEM — (Irrigation) (1) System of ditches and their appurtenances which convey irrigation water from the main canal to the farm units; diverse water from the main canal-side turnout to individual water users or to other smaller distribution systems. (2) Any system that distributes water within a farm.

DISTRIBUTION UNIFORMITY (DU) — (1) Generally, a term used to describe how evenly water is applied on a field and therefore a practical method for measuring the performance of an irrigation system. The concept of distribution uniformity constitutes one of the limiting factors on a system's Irrigation Efficiency (I.E.). (2) Also, a ratio used to measure the infiltration of irrigation water through a given soil profile. More specifically, the ratio of the average low-quarter depth of irrigation to the average depth of irrigation, for the entire farm field, expressed as a percent. Typically, a DU of between 80 and 90 percent is considered very good.

DIVERSION — The transfer of water from a stream, lake, aquifer, or other source of water by a canal, pipe, well, or other conduit to another watercourse or to the land, as in the case of an irrigation system. Also, a turning aside or alteration of the natural course of a flow of water, normally considered physically to leave the natural channel. In some states, this can be a consumptive use direct from a stream, such as by livestock watering. In other states, a diversion must consist of such actions as taking water through a canal or conduit.

DIVERSION CHANNEL — (1) An artificial channel constructed around a town or other point of high potential flood damages to divert floodwater from the main channel to minimize flood damages. (2) A channel carrying water from a diversion dam.

DIVERSION DAM (and DIKE) — A barrier built to divert part or all of the water from a stream into a different course. The diversion dam is commonly constructed on a natural river channel and is designed to check or elevate the water level for diversion into a main canal system.

DIVERSION RATE — A rate of water flow (cfs) diverted into a canal or through a farm headgate.

DIVERSITY INDEX — A numerical expression of the evenness of distribution of aquatic organisms. Several different formulae are in current use for its calculations.

DIVERTIBLE WATER SUPPLY — Includes that amount of water consumptively used and that water which returns to the river system. Since return flow becomes available for subsequent diversion and reuse, the total divertible supply is greater than the available supply.

DIVIDE — An imaginary line indicating the limits of a subbasin, subwatershed, or watershed; the boundary line along a topographic ridge or high point which separates two adjacent drainage basins.

DOMESTIC CONSUMPTION — Water used for household purposes such as washing, food preparation, toilets and showers. It is the quantity, or quantity per capita (person), of water consumed in a municipality or district for domestic uses or purposes during a given period. It sometimes encompasses all uses, including the quantity wasted, lost, or otherwise unaccounted for.

DOMESTIC SEWAGE — Wastewater and solid waste that is characteristic of the flow from toilets, sinks, showers, and tubs in a household. Also referred to as Domestic Waste.

DOMESTIC WASTEWATER FACILITY — Refers to those facilities that receive or dispose of wastewater derived principally from residential dwellings, business or commercial buildings, institutions, and the like. May also include some wastewater derived from industrial facilities. Also referred to as Municipal Wastewater Facility.

DOMESTIC WATER — Water supplied to individual dwellings and other land uses which is suitable for drinking.

DOMESTIC WATER USE — Water used normally for residential purposes, including household use, personal hygiene, drinking, washing clothes and dishes, flushing toilets, watering of domestic animals, and outside uses such as car washing, swimming pools, and for lawns, gardens, trees and shrubs. The water may be obtained from a public supply or may be self supplied. Also referred to as Residential Water Use.

DOUBLE CROPPING — The practice of producing two or more crops consecutively on the same parcel of land during a 12-month period.

DOWNSTREAM — In the direction of the current of a stream.

DOWNSTREAM CONTROL — (Irrigation) Control structure adjustments which are based on information from downstream; the required information is measured by a sensor located downstream or based on the downstream water schedule established by the Watermaster.

DOWNSTREAM SLOPE (of a Dam) — The slope or face of the dam away from the reservoir water, which, for Embankment Dams, requires some form of protection such as grass to protect it from the erosive effects of rain and surface flows.

DOWNSTREAM TOE OF DAM — The junction of the downstream face of a dam with the ground surface. For and Embankment Dam the junction of the upstream face with the ground surface is the upstream toe.

DRAIN — (1) To draw of (a liquid) by a gradual process. (2) A buried pipe or other conduit (closed drain) for the conveyance of surplus groundwater. (3) A ditch (open drain) for carrying off surplus surface water or groundwater. (4) A system to control water tables near the ground surface to maintain levels at or below specified depths.

DRAINAGE — (1) The removal of excess surface water or groundwater from land by means of surface or subsurface drains. (2) Improving the productivity of agricultural land by removing excess water from the soil by such means as ditches or subsurface drainage tiles (pipes). (3) Soil characteristics that affect natural drainage.

DRAINAGE AREA (of a Stream at a Specified Location) — That area, measured in a horizontal plane, enclosed by a topographic (drainage) divide from which direct surface runoff from precipitation normally drains by gravity into the stream above the specified point.

DRAINAGE BASIN — Part of the surface of the earth that is occupied by a drainage system, which consists of a surface stream or a body of impounded surface water together with all tributary surface streams and bodies of impounded surface water. The term is used synonymously with Watershed, River Basin, or Catchment.

DRAINAGE CLASS, SOILS — The relative terms used to describe natural drainage and corresponding types of soils are as follows:
[1] Excessive — Excessively drained soils are commonly very porous and rapidly permeable, and have low water-holding capacity;
[2] Somewhat Excessive — Somewhat excessively drained soils are also very permeable and are free from mottling throughout their profile;
[3] Good — Well drained soils that are nearly free of mottling and are commonly of intermediate texture;
[4] Moderately Good — Moderately well drained soils that commonly have a slowly permeable layer in or immediately beneath the solum. They have uniform color in the surface layers and upper subsoil, and mottling in the lower subsoils and substrata;
[5] Somewhat Poor — Somewhat poorly drained soils are wet for significant periods, but not all the time. They commonly have a slowly permeable layer in their profile, a high water table, additions through seepage, or a combination of these conditions;
[6] Poor — Poorly drained soils are wet for long periods of time. They are light gray and generally are mottled from the surface downward, although mottling may be absent or nearly so in some soils.

DRAINAGE COEFFICIENT — Design rate at which water is to be removed from a drainage area.

DRAINAGE DENSITY — (1) The relative density of natural drainage channels in a given area, obtained by dividing the total length of the stream channels by the area. (2) The length of all channels above those of a specified Stream Order per unit of Drainage Area.

DRAINAGE DISTRICT — A special purpose district created under state law to finance, construct, operate, and maintain a drainage system involving a group of land holdings.

DRAINAGE DIVIDE — The line of highest elevations which separates adjoining drainage basins.

DRAINAGE FIELD DITCH — A shallow graded ditch for collecting excess water within a field, usually constructed with flat side slopes for ease of crossing.

DRAINAGE FLOODING — Ponding of water at or near the point where it fell due to improper or limited drainage.

DRAINAGE LATERAL — A side ditch or conduit which contributes water to a drainage main.

DRAINAGE LAYER (or Blanket) — (Dam) A layer of permeable material in a dam to relieve pore pressure or to facilitate drainage of fill material.

DRAINAGE MAIN — A natural or artificial ditch or conduit for moving water off the land.

DRAINAGE WATER — The water which has been collected by a drainage system. It may come from surface water or from water passing through the soil. It may be of a quality suitable for reuse or it may be of no further economic use.

DRAINAGE WELL — (Irrigation) A vertical opening to a permeable substation into which surface and subsurface water is channeled. A well drilled to carry excess water off agricultural fields. Because they act as a funnel from the surface to the groundwater below, drainage wells can contribute to groundwater pollution.

DRAIN FIELD — A network of buried piping or tubing where the fluid is discharged to the ground through seepage. Most common use is with septic tanks, but can also be used for domestic or industrial wastewater disposal after other treatment methods.

DRAINPIPE — A pipe for carrying off water or sewage.

DRAINS (of a Dam) — A vertical well or borehole, usually downstream of impervious cores, grout curtains, or cutoffs, designed to collect and direct seepage through or under a dam to reduce uplift pressure under or within the dam. A line of such wells forms a drainage curtain. Also referred to as Relief Wells.

DRAW — To cause to flow forth as a pump drawing water.

DRAWDOWN — (1) The act, process, or result of depleting, as a liquid or body of water as in the lowering of the water surface level due to release of water from a reservoir. (2) The magnitude of lowering of the surface of a body of water or of its piezometric surface as a result of withdrawal of the release of water therefrom. (3) The decline of water below the static level during pumping. (4) (Water Table) The lowering of the elevation of the Groundwater Table, usually from pumping wells, but can occur naturally during periods of prolonged drought. At the well, it is the vertical distance between the static and the pumping level.

DRIP — To fall in drops; to shed drops; to ooze or become saturated with or as if with liquid.

DRIP IRRIGATION — A planned irrigation system in which water is applied directly to the Root Zone of plants by means of applicators (orifices, emitters, porous tubing, perforated pipe, etc.) operated under low pressure. The applicators may be placed on or below the surface of the ground. Also see Irrigation Systems.

DRIP (TRICKLE) SOIL ABSORPTION SYSTEM — A shallow slow rate pressure-dosed system used for land application of treated wastewater, particularly under soil conditions unsuitable for normal septic tanks and gravity-fed soil absorption systems. In agriculture, drip soil irrigation systems irrigate crops by means of a network of shallow underground pipes fed by a pump. Such a system conserves water used in crop irrigation by applying it at a controlled rate in the root zone, minimizing evaporation and percolation losses. In the drip soil absorption system, the filtered effluent is delivered via supply lines to a subsurface drip field consisting of parallel rows of polyethylene tubing, known as dripper lines. Emitters are installed along these tubes to uniformly distribute and control the flow of effluent. The key to the effective operation of drip soil absorption systems is the slow and controlled rate at which it applies effluent over a large surface area, allowing relatively shallow placement of the dripper lines and long-term use without risk of saturating soils. This allows such systems to be effectively used for subsurface irrigation of trees, shrubs, and gardens in arid regions.

DROP — The quantity of fluid which falls in one spherical mass; a liquid globule; often, a teardrop, raindrop, dewdrop, etc. The size of a drop varies with the specific gravity and viscosity of the liquid and also with the conditions under which it is formed.

DROP-INLET SPILLWAY — Overfall structure in which the water drops through a vertical riser connected to a discharge conduit.

DROP SPILLWAY — An overfall structure in which water drops over a vertical wall onto a protected apron at a lower elevation.

DROPLET — A small airborne liquid particle that is larger than liquid aerosol and therefore settles out of the atmosphere relatively quickly.

DROPPER — A small tube with a suction bulb at one end for drawing in a liquid and releasing it in drops.

DROPS — Structures to reduce or control water velocity within an irrigation ditch or canal by lowering the water abruptly from one level to a lower level.

DROP SPILLWAY — An overfall structure in which water drops over a vertical wall onto a protected apron at a lower elevation.

DROP STRUCTURE — A structure for dropping water to a lower level and dissipating its surplus energy. A drop may be vertical or inclined.

DROUGHT — There is no universally accepted quantitative definition of drought. Generally, the term is applied to periods of less than average or normal precipitation over a certain period of time sufficiently prolonged to cause a serious hydrological imbalance resulting in biological losses (impact flora and fauna ecosystems) and/or economic losses (affecting man). In a less precise sense, it can also signify nature's failure to fulfill the water wants and needs of man.

DROUGHT CONDITION — Hydrologic conditions during a defined Drought period during which rainfall and runoff are much less than average.

DROUGHT PERIOD — The period of time over which Drought Conditions exist.

DROUGHT RESERVE WATER — Generally, water reserved in upstream reservoirs for release for downstream purposes, e.g., municipal and industrial, agriculture, recreational, etc. Often provisions will be made such that drought reserve water will convert to Fish Credit Water if snowpack water content or runoff is deemed sufficient by a stipulated date.

DROUGHT YEAR SUPPLY — The average annual supply of a water development system during a defined Drought Period. For dedicated natural flow, it is the average flows or levels for specific drought water years for specific streams or bodies of water, or it is the Environmental Flows as required under specific agreements, water rights, court decisions, and congressional directives.

DROWN — (1) To kill by submerging and suffocating in water or another liquid. (2) To drench thoroughly or cover with or as if with a liquid.

DRY DAM — A dam that has an outlet positioned so that essentially all stored water will be drained from the reservoir by gravity. The reservoir will normally be dry. Permanent storage is not involved, and the detention reservoir can be used for other purposes (farming, grazing, recreation) between flood periods.

DRY FARMING — A type of farming practiced in arid areas without irrigation by planting drought-resistant crops and maintaining a fine surface tilth or mulch that protects the natural moisture of the soil from evaporation. Also referred to as Dryland Farming.

DRY HYDRANTS — A siphon buried beneath the water line that enables fire crews to draw, or "draft" water from ponds or other bodies of water located nearby. Such devices are used in more remote locations and are typically used only for interim purposes until a more consistent supply may be obtained to fight a fire.

DRYLAND FARMING — The practice of crop production without irrigation in semiarid regions usually by using moisture-conserving farming techniques.

DUTY (of Water) — The total volume of irrigation water required for irrigation in order to mature a particular type of crop. In stating the duty, the crop, and usually the location of the land in question, as well as the type of soil, should be specified. It also includes consumptive use, evaporation and seepage from on-farm ditches and canals, and the water that is eventually returned to streams by percolation and surface runoff.