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Section 15: Glossary of Hydrology Terms

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Annual Exceedance Probability (AEP)

The probability of exceedance in a given year.

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Annual Flood

The maximum peak discharge in a water year.

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Annual Flood Series

A list of annual floods.

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Antecedent Conditions

Watershed conditions prevailing prior to an event; normally used to characterize basin wetness, e.g., soil moisture. Also referred to as initial conditions or antecedent moisture conditions (AMC).

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Area-Capacity Curve

A graph showing the relation between the surface area of the water in a reservoir and the corresponding volume.

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The reduction in the peak of a hydrograph resulting in a broad, flat hydrograph.

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Water backed up or retarded in its course as compared with its normal or natural condition of flow. In stream gauging, a rise in stage produced by a temporary obstruction such as ice or weeds, or by the flooding of the stream below. The difference between the observed stage and that indicated by the stage-discharge relation is reported as backwater.

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The margins of a channel. Banks are called right or left as viewed facing downstream (in the direction of the flow).

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Bank Storage

The water absorbed into the banks of a stream channel, when the stages rise above the water table in the bank formations, then returns to the channel as effluent seepage when the stages fall below the water table.

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Bankfull Stage

Maximum stage of a stream before it overflows its banks. (see also flood stage.)

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Base Discharge (for peak discharge)

In the USGS annual reports on surface-water supply, the discharge above which peak discharge data are published. The base discharge at each station is selected so that an average of about 3 peaks a year will be presented. (See also partial-duration flood series.)

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The sustained or fair weather flow in a channel due to subsurface runoff. In most streams, baseflow is composed largely of groundwater effluent. Also known as base runoff.

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Basic Hydrologic Data

Includes inventories of features of land and water that vary spatially (topographic and geologic maps are examples), and records of processes that vary with both place and time. (Records of precipitation, streamflow, ground water, and quality-of-water analyses are examples.) Basic hydrologic information is a broad term that includes surveys of the water resources of particular areas and a study of their physical and related economic processes, interrelations, and mechanisms.

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Basic-Stage Flood Series

See partial duration flood series.

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The point where a stream channel splits into two distinct channels.

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Binomial Statistical Distribution

The frequency distribution of the probability of a specified number of successes in an arbitrary number of repeated independent Bernoulli trials. Also called Bernoulli distribution.

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Boundary Condition

Conditions at the boundary of a problem that govern its solution. For example, when solving a routing problem for a given reach, an upstream inflow boundary condition is necessary to solve for the outflow at the downstream end of the reach.

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Derivation of a set of model parameter values that produces the best fit to observed data.

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Precipitation that falls on and is stored in the leaves or trunks of vegetation. The term can refer to either the process or a volume.

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Channel (watercourse)

An open conduit either naturally or artificially created which periodically or continuously contains moving water, or which forms a connecting link between two bodies of water. River, creek, run, branch, anabranch, and tributary are some of the terms used to describe natural channels. Natural channels may be single or braided. Canal and floodway are terms used to describe artificial channels.

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Channel Storage

The volume of water at a given time in the channel or over the flood plain of the streams in a drainage basin or river reach. Channel storage can be large during the progress of a flood event.

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Computation Duration

The user-defined time window used in hydrologic modeling.

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Computation Interval

The user-defined time step used by a hydrologic model for performing mathematical computations. For example, if the computation interval is 15 min. and the starting time is 1200, hydrograph ordinates will be computed at 1200, 1215, 1230, 1245, and so on.

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Concentration Time

See time of concentration.

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The point at which two distinct stream channels converge

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Continuous Model

A model that tracks the periods between precipitation events, as well as the events themselves. See event-based model.

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The process of establishing a relation between a variable and one or more related variables. Correlation is simple if there is only one independent variable and multiple when there are two or more independent variables. For gauging station records, the usual variables are the short-term gauging-station record and one or more long-term gauging-station records

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Channel pattern of streams with tributaries that branch to form a tree-like pattern.

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Depression Storage

The volume of water contained in natural depressions in the land surface, such as puddles.

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Design Flood

The flood that is chosen as the basis for the design of a hydraulic structure.

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Design Storm

Rainfall amount and distribution in time and space used to determine a design flood or design peak discharge

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Detention Basin

Storage facility, such as a small unregulated reservoir, which delays the conveyance of water downstream.

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Dissipation of the energy associated with a flood wave; results in the attenuation of the flood wave.

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Direct Runoff

The runoff entering stream channels promptly after rainfall or snowmelt. Superimposed on base runoff, it forms the bulk of the hydrograph of a flood. See also surface runoff. The terms base runoff and direct runoff are time classifications of runoff. The terms groundwater runoff and surface runoff are classifications according to source.

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The volume of water that passes through a given cross-section per unit time; commonly measured in cubic feet per second (cfs) or cubic meters per second (m3/s). Also referred to as flow.

In its simplest concept discharge means outflow; therefore, the use of this term is not restricted as to course or location, and it can be applied to describe the flow of water from a pipe or from a drainage basin. If the discharge occurs in some course or channel, it is correct to speak of the discharge of a canal or of a river. It is also correct to speak of the discharge of a canal or stream into a lake, a stream, or an ocean. (See also streamflow and runoff.)

Discharge data in USGS reports on surface water represent the total fluids measured. Thus, the terms discharge, streamflow, and runoff represent water with sediment and dissolved solids. Of these terms, discharge is the most comprehensive. The discharge of drainage basins is distinguished as follows:

Yield: Total water runout; includes runoff plus underflow.

Runoff: That part of water yield that appears in streams.

Streamflow: The actual flow in streams, whether or not subject to regulation, or underflow.

Each of these terms can be reported in total volumes or time rates. The differentiation between runoff as a volume and streamflow as a rate is not accepted.

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Discharge Rating Curve

See stage discharge relation.

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Distribution Graph (distribution hydrograph)

A unit hydrograph of direct runoff modified to show the proportions of the volume of runoff that occurs during successive equal units of time.

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The taking of water from a stream or other body of water into a canal, pipe, or other conduit.

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Drainage Area

The drainage area of a stream at a specified location is that area, measured in a horizontal plane, which is enclosed by a drainage divide.

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Drainage Divide

The rim of a drainage basin. (See watershed.)

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Duration Curve

See flow-duration curve for one type.

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See evapotranspiration.

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Effective Precipitation (rainfall)

1. That part of the precipitation that produces runoff.

2. A weighted average of current and antecedent precipitation that is "effective" in correlating with runoff.

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The process by which water is changed from the liquid or the solid state into the vapor state. In hydrology, evaporation is vaporization and sublimation that takes place at a temperature below the boiling point. In a general sense, evaporation is often used interchangeably with evapotranspiration or ET.

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Evaporation Demand

The maximum potential evaporation generally determined using an evaporation pan. For example, if there is sufficient water in the combination of canopy and surface storage, and in the soil profile, the actual evaporation will equal the evaporation demand. A soil-water retention curve describes the relationship between evaporation demand and actual evaporation when the demand is greater than available water. See tension zone.

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Evaporation Pan

An open tank used to contain water for measuring the amount of evaporation. The US National Weather Service class A pan is 4 ft in diameter, 10 in. deep, set up on a timber grillage so that the top rim is about 16 in. from the ground. The water level in the pan during the course of observation is maintained between 2 and 3 in. below the rim.

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Evaporation, Total

The sum of water lost from a given land area during any specific time by transpiration from vegetation and building of plant tissue; by evaporation from water surfaces, moist soil, and snow; and by interception. It has been variously termed evaporation, evaporation from land areas, evapotranspiration, total loss, water losses, and fly off.

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Water withdrawn from a land area by evaporation from water surfaces and moist soils and plant transpiration.

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Event-Based Model

A model that simulates some hydrologic response to a precipitation event. See continuous model.

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Exceedance Probability

Hydrologically, the probability that an event selected at random will exceed a specified magnitude.

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Excess Precipitation

The precipitation in excess of infiltration capacity, evaporation, transpiration, and other losses. Also referred to as effective precipitation.

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Excessive Rainfall

See rainfall, excessive.

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Falling Limb

The portion of a hydrograph where runoff is decreasing.

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Field Capacity

The quantity of water which can be permanently retained in the soil in opposition to the downward pull of gravity. Also known as field-moisture capacity.

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Field-Moisture Deficiency

The quantity of water which would be required to restore the soil moisture to field-moisture capacity.

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An overflow or inundation that comes from a river or other body of water, and causes or threatens damage. Any relatively high streamflow overtopping the natural or artificial banks in any reach of a stream. A relatively high flow as measured by either gauge height or discharge quantity. As it relates to highway drainage design for TxDOT, and for the purposes of this manual, any direct runoff from precipitation; not limited to an out-of-banks event.

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Flood Crest

See flood peak.

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Flood Event

See flood wave.

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Flood Peak

The highest value of the stage or discharge attained by a flood; thus, peak stage or peak discharge. Flood crest has nearly the same meaning, but since it connotes the top of the flood wave, it is properly used only in referring to stage—thus, crest stage, but not crest discharge.

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A strip of relatively flat land bordering a stream, built of sediment carried by the stream and dropped in the slack water beyond the influence of the swiftest current. It is called a living flood plain if it is overflowed in times of high water; but a fossil flood plain if it is beyond the reach of the highest flood. The lowland that borders a river, usually dry but subject to flooding. That land outside of a stream channel described by the perimeter of the maximum probable flood.

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Flood Profile

A graph of elevation of the water surface of a river in flood, plotted as ordinate, against distance, measured in the downstream direction, plotted as abscissa. A flood profile may be drawn to show elevation at a given time, crests during a particular flood, or to show stages of concordant flows.

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Flood Routing

The process of progressively determining the timing and shape of a flood wave at successive points along a river.

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Flood Stage

The gauge height of the lowest bank of the reach in which the gauge is situated. The term "lowest bank" is, however, not to be taken to mean an unusually low place or break in the natural bank through which the water inundates an unimportant and small area. The stage at which overflow of the natural banks of a stream begins to occur. See also bankfull stage.

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Flood Wave

A distinct rise in stage culminating in a crest and followed by recession to lower stages.

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Flood, Maximum Probable

The flood magnitude that may be expected from the most critical combination of meteorologic and hydrologic conditions reasonably possible for a given watershed.

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Flood-Frequency Curve

1. A graph showing the number of times per year on the average, plotted as abscissa, that floods of magnitude, indicated by the ordinate, are equaled or exceeded.

2. A similar graph but with recurrence intervals of floods plotted as abscissa.

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A part of the floodplain otherwise leveed, reserved for emergency diversion of water during floods. A part of the floodplain which, to facilitate the passage of floodwater, is kept clear of encumbrances.

The channel of a river or stream and those parts of the floodplains adjoining the channel which are reasonably required to carry and discharge the floodwater or flood flow of any river or stream.

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Flow-Duration Curve

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

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Gauging Station

A particular site on a stream, canal, lake, or reservoir where systematic observations of gauge height or discharge are obtained. (See also stream-gauging station.)

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Ground Water

Water in the ground that is in the zone of saturation, from which wells, springs, and groundwater runoff are supplied.

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Groundwater 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 groundwater outflow that takes place in valley alluvium (instead of the surface channel) and thus is not measured at a gauging station.

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Groundwater Runoff

That part of the runoff that has passed into the ground, has become ground water, and has been discharged into a stream channel as spring or seepage water. See also base runoff and direct runoff.

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Hydraulic Radius

The flow area of a channel cross section divided by the wetted perimeter. The wetted perimeter does not include the free surface.

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A graph showing stage, flow, velocity, or other property of water with respect to time.

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Hydrologic Budget

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

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Hydrologic Cycle

The continuous process of water movement between the oceans, atmosphere, and land.

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The study of water; generally focuses on the distribution of water and interaction with the land surface and underlying soils and rocks.

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A plot of rainfall intensity versus time; often represented by a bar graph.

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Index Precipitation

An index that can be used to adjust for bias in regional precipitation, often quantified as the expected annual precipitation.

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The movement of water from the land surface into the soil.

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Infiltration Capacity

The maximum rate at which the soil, when in a given condition, can absorb falling rain or melting snow.

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Infiltration Index

An average rate of infiltration, in inches per hour, equal to the average rate of rainfall such that the volume of rainfall at greater rates equals the total direct runoff.

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Inflection Point

Generally refers the point on a hydrograph separating the falling limb from the recession curve; any point on the hydrograph where the curve changes concavity.

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Initial Condition

The conditions prevailing prior to an event. Refer also to antecedent conditions.

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The capture of precipitation above the ground surface (e.g., by vegetation or buildings).

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Isohyetal Line

A line drawn on a map or chart joining points that receive the same amount of precipitation.

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Variously defined as time from beginning (or center of mass) of rainfall to peak (or center of mass) of a runoff hydrograph.

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Lag Time

The time from the center of mass of excess rainfall to the hydrograph peak. Also referred to as basin lag.

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The difference between the volume of rainfall and the volume of runoff. Losses include water absorbed by infiltration, water stored in surface depressions, and water intercepted by vegetation.

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Mass Curve

A graph of the cumulative values of a hydrologic quantity (such as precipitation or runoff), generally as ordinate, plotted against time or date as abscissa. (See double-mass curve and residual-mass curve.)

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Maximum Probable Flood

See flood, maximum probable.

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The winding of a stream channel.

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A physical or mathematical representation of a process that can be used to predict some aspect of the process.

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Water diffused in the atmosphere or the ground.

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Objective Function

A mathematical expression that allows comparison between a calculated result and a specified goal. For model calibration, the objective function compares calculated discharge with observed discharge.

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Overland Flow

The shallow flow of water over the land surface before combining with additional flow to become channel flow.

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A variable, in a general model, whose value is adjusted to make the model specific to a given situation. A numerical measure of the properties of the real-world system.

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Parameter Estimation

The selection of a parameter value based on the results of analysis and/or engineering judgment. Analysis techniques include calibration, regional analysis, estimating equations, and physically based methods. Refer also to calibration.

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Partial-Duration Flood Series

A list of all flood peaks that exceed a chosen base stage or discharge, regardless of the number of peaks occurring in a year. (Also called basic-stage flood series, or floods above a base.)

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Peak Flow

The point of the hydrograph that has the highest flow.

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Peak Stage

The highest elevation reached by a flood wave. Also referred to as the crest.

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The movement, under hydrostatic pressure, of water through the interstices of a rock or soil.

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Probable maximum flood; see flood, probable maximum.

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As used in hydrology, precipitation is the discharge of water, in liquid or solid state, out of the atmosphere, generally upon a land or water surface. It is the common process by which atmospheric water becomes surface or subsurface water. The term precipitation is also commonly used to designate the quantity of water that is precipitated. Precipitation includes rainfall, snow, hail, and sleet, and is therefore a more general term than rainfall.

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Precipitation, Probable Maximum

Theoretically the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location at a certain time of the year.

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Probability of Capacity Exceedance

The likelihood of the design flow rate (or volume of water with specified duration) of a hydraulic structure being exceeded in a given year.

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Probability of Exceedance

The likelihood of a specified flow rate (or volume of water with specified duration) being exceeded in a given year.

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Liquid precipitation.

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The quantity of water that falls as rain only. Not synonymous with precipitation.

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Rainfall Excess

The volume of rainfall available for direct runoff. It is equal to the total rainfall minus interception, depression storage, and absorption.

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Rating Curve

The relationship between stage and discharge.

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A segment of a stream channel.

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Recession Curve

The portion of the hydrograph where runoff is predominantly produced from basin storage (subsurface and small land depressions); it is separated from the falling limb of the hydrograph by an inflection point.

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Recurrence Interval (return period)

The average interval of time within which the given flood will be equaled or exceeded once. When the recurrence interval is expressed in years, it is the reciprocal of the annual exceedance probability (AEP).

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Regulation, Regulated

The artificial manipulation of the flow of a stream.

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A pond, lake, or basin, either natural or artificial, for the storage, regulation, and control of water.

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Residual-Mass Curve

A graph of the cumulative departures from a given reference such as the arithmetic average, generally as ordinate, plotted against time or date, as abscissa. (See mass curve.)

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Retention Basin

Similar to detention basin but water in storage is permanently obstructed from flowing downstream.

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Rising Limb

Portion of the hydrograph where runoff is increasing.

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Precipitation on the ground that is not captured by evaporation, infiltration, interception, or surface storage.

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Saturation Zone

The portion of the soil profile where available water storage is completely filled. The boundary between the vadose zone and the saturation zone is called the water table. Note that under certain periods of infiltration, the uppermost layers of the soil profile can be saturated. See vadose zone.

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NRCS Curve Number

An empirically derived relationship between location, soil-type, land use, antecedent moisture conditions, and runoff. A Natural Resources Conservation Service (NRCS) curve number is used in an event-based model to establish the initial soil moisture condition and the infiltration.

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A form of precipitation composed of ice crystals.

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Soil Moisture Accounting (SMA)

A modeling process that accounts for continuous fluxes to and from the soil profile. Models can be event-based or continuous. When using a continuous simulation, a soil moisture accounting method is used to account for changes in soil moisture between precipitation events.

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Soil Moisture (soil water)

Water diffused in the soil, the upper part of the zone of aeration from which water is discharged by the transpiration of plants or by soil evaporation. See field-moisture capacity and field-moisture deficiency.

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Soil Profile

A description of the uppermost layers of the ground down to bedrock. In a hydrologic context, the portion of the ground subject to infiltration, evaporation, and percolation fluxes.

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The height of a water surface in relation to a datum.

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Stage-Capacity Curve

A graph showing the relation between the surface elevation of the water in a reservoir usually plotted as ordinate, against the volume below that elevation plotted as abscissa.

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Stage-Discharge Curve (rating curve)

A graph showing the relation between the water height, usually plotted as ordinate, and the amount of water flowing in a channel, expressed as volume per unit of time, plotted as abscissa.

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Stage-Discharge Relation

The relation expressed by the stage-discharge curve.

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Rainfall or snowmelt led to the ground down the trunks or stems of plants.

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1. Water artificially or naturally impounded in surface or underground reservoirs. The term regulation refers to the action of this storage in modifying downstream streamflow.

2. Water naturally detained in a drainage basin, such as ground water, channel storage, and depression storage. The term drainage basin storage or simply basin storage is sometimes used to refer collectively to the amount of water in natural storage in a drainage basin.

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A disturbance of the ordinary average conditions of the atmosphere which, unless specifically qualified, may include any or all meteorological disturbances, such as wind, rain, snow, hail, or thunder.

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A general term for a body of flowing water. In hydrology the term is generally applied to the water flowing in a natural channel as distinct from a canal. More generally, as in the term stream gauging, it is applied to the water flowing in any channel, natural or artificial.

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Stream Gauging

The process of measuring the depths, areas, velocities, and rates of flow in natural or artificial channels.

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The discharge that occurs in a natural channel. Although the term discharge can be applied to the flow of a canal, the word streamflow uniquely describes the discharge in a surface stream course. The term streamflow is more general than runoff, as streamflow may be applied to discharge whether or not it is affected by diversion or regulation.

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Stream-Gauging Station

A gauging station where a record of discharge of a stream is obtained. Within the USGS this term is used only for those gauging stations where a continuous record of discharge is obtained.

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The process of transformation directly between a solid and a gas.

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Surface Runoff

That part of the runoff that travels over the soil surface to the nearest stream channel. It is also defined as that part of the runoff of a drainage basin that has not passed beneath the surface since precipitation. The term is misused when applied in the sense of direct runoff. See also runoff, overland flow, direct runoff, groundwater runoff, and surface water.

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Surface Water

Water on the surface of the earth.

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Tension Zone

In the context of HEC-HMS, the portion of the soil profile that will lose water only to evapotranspiration. This designation allows modeling water held in the interstices of the soil. See soil profile.

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Time of Concentration

The travel time from the hydraulically furthermost point in a watershed to the outlet. Also defined as the time from the end of rainfall excess to the inflection point on the recession curve.

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Time of Rise

The time from the start of rainfall excess to the peak of the hydrograph.

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Time to Peak

The time from the center of mass of the rainfall excess to the peak of the hydrograph. Refer also to lag time

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Computer program developed by the NRCS that provides a hydrologic analysis of a watershed under present conditions. Output consists of peaks and/or flood hydrographs. Subarea surface runoff hydrographs are developed from storm rainfall using the dimensionless unit hydrograph, drainage areas, times of concentration, and NRCS runoff curve numbers. Instructions to develop, route, add, store, divert, or divide hydrographs are established to convey floodwater from the headwaters to the watershed outlet.

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Urban Hydrology for Small Watershed—Technical Release 55 published by the NRCS. Technical Release 55 (TR-55) presents simplified procedures to calculate storm runoff volume, peak rate of discharge, hydrographs, and storage volumes required for floodwater reservoirs. These procedures are applicable to small watersheds, especially urbanizing watersheds, in the United States.

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The quantity of water absorbed and transpired and used directly in the building of plant tissue, in a specified time. It does not include soil evaporation. The process by which water vapor escapes from the living plant, principally the leaves, and enters the atmosphere.

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The downstream flow of water through the permeable deposits that underlie a stream and that are more or less limited by rocks of low permeability.

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Unit Hydrograph

A direct runoff hydrograph produced by 1 unit of excess precipitation over a specified duration. For example, a 1-hour unit hydrograph is the direct runoff from one unit of excess precipitation occurring uniformly over one hour.

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Vadose Zone

The portion of the soil profile above the saturation zone.

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Water Year

In USGS reports dealing with surface-water supply, the 12-month period, October 1 through September 30. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 months. Thus, the year ended September 30, 1959, is called the 1959 water year.

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An area characterized by all direct runoff being conveyed to the same outlet. Similar terms include basin, drainage basin, catchment, and catch basin.

A 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.

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A MS Windows-based computer program developed by NRCS. WinTR-55 uses the procedures presented in TR-20 as the driving engine for more accurate analysis of the hydrology of the small watershed system being studied.

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