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Glossary
This glossary is a collection of definitions from throughout the Technical Reference Manual plus definitions of other pertinent hydrology terms. Many of the definitions herein are from the electronic glossaries available from U.S. Geological Survey and the Bureau of Reclamation.
Additional terms commonly used within USACE Flood Risk Management studies can be found here: Key USACE Flood Risk Management Terms.
A
A14: NOAA Atlas 14. A multi-volume document produced by the NWS Hydrometeorological Design Studies Center that contains precipitation-frequency estimates across the United States. Not all areas are covered by a volume of A14, most notably the Northwest.
ACE: annual chance exceedance
AEP: annual exceedance probability
AMS: annual maximum series. A sample containing the largest observation of some variable from each year.
Annual Flood: The maximum peak discharge in a water year.
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.
AORC: Analysis of Record for Calibration, a high-resolution meteorological dataset created by assimilation of multiple input datasources including NEXRAD radar, satellite precipitation estimation, and gage data.
ARF: area reduction function/factor. A means for generalizing the behavior of area-averaged precipitation based on the point maximum intensity.
ARI: average return interval
Area-Capacity Curve: A graph showing the relation between the surface area of the water in a reservoir and the corresponding volume.
Attenuation: The reduction in the peak of a hydrograph resulting in a more broad, flat hydrograph.
B
Backwater: Water backed up or retarded in its course as compared with its normal or natural condition of flow. In stream gaging, 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.
Balanced Hyetograph: a storm temporal pattern in which any nested duration has the same frequency; e.g. the 1-hour, 3-hour, 6-hour, 12-hour, and 24-hour totals all have 1% AEP.
Bank: The margins of a channel. Banks are called right or left as viewed facing in the direction of the flow.
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.
Bankfull Stage: Maximum stage of a stream before it overflows its banks. Bankfull stage is a hydraulic term, whereas flood stage implies damage. See also flood stage.
Base Discharge: In the US Geological Survey's 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 three peaks a year will be presented. See also partial-duration flood series.
Baseflow: 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.
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. Examples include records of precipitation, streamflow, ground-water, and quality-of-water analyses. Basic hydrologic information is a broader term that includes surveys of the water resources of particular areas and a study of their physical and related economic processes, interrelations and mechanisms.
Basic-Stage Flood Series: See partial duration flood series.
Bifurcation: The point where a stream channel splits into two distinct channels.
Boundary Condition: Known or hypothetical conditions at the boundary of a problem that govern its solution. For example, when solving a routing problem for a given reach, an upstream boundary condition is necessary to determine condition at the downstream boundary.
C
Calibration: Derivation of a set of model parameter values that produces the "best" fit to observed data.
Canopy Interception: Precipitation that falls on, and is stored in the leaf or trunk of vegetation. The term can refer to either the process or a volume.
Channel: An naturally or artificially created open conduit that may convey water. See also watercourse.
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.
Computation Duration: The user-defined time window used in hydrologic modeling.
Computation Interval: The user-defined time step used by a hydrologic model for performing mathematical computations. For example, if the computation interval is 15 minutes and the starting time is 1200, hydrograph ordinates will be computed at 1200, 1215, 1230, 1245, and so on.
Concentration Time: See time of concentration.
Confluence: The point at which two streams converge.
Continuous Model: A model that tracks the periods between precipitation events, as well as the events themselves. Compare event-based model.
Correlation: 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 is more than one independent variable. For gaging station records, the usual variables are the short-term gaging-station record and one or more long-term gaging-station records.
D
DAD: Depth-area-duration, an idealized way of representing the spatial-temporal pattern of a precipitation event
DDF: Depth-duration-frequency, a generalization of precipitation-frequency analysis results
Dendritic: Channel pattern of streams with tributaries that branch to form a tree-like pattern.
Depression Storage: The volume of water contained in natural depressions in the land surface, such as puddles.
Detention Basin: Storage, such as a small unregulated reservoir, which delays the conveyance of water downstream.
Diffusion: Dissipation of the energy associated with a flood wave; results in the attenuation of the flood wave.
Direct Runoff: The runoff entering stream channels promptly after rainfall or snowmelt. Superposed on base runoff, it forms the bulk of the hydrograph of a flood. The terms base runoff and direct runoff are time classifications of runoff. The terms groundwater runoff and surface runoff are classifications according to source. See also surface runoff
Discharge: 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.
Discharge data in US Geological Survey 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 or crop; 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. See also streamflow and runoff.
Discharge Rating Curve: See stage discharge relation.
Distribution Graph: A unit hydrograph of direct runoff modified to show the proportions of the volume of runoff that occurs during successive equal units of time.
Diversion: The taking of water from a stream or other body of water into a canal, pipe, or other conduit.
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.
Drainage Divide: The rim of a drainage basin. See also watershed.
Duration Curve: See flow-duration curve for one type.
E
ET: See evapotranspiration.
Effective Precipitation: That part of the precipitation that produces runoff. Also, a weighted average of current and antecedent precipitation that is "effective" in correlating with runoff.
ERL: equivalent record length. A measure of information content in a regional analysis based on counting the number of independent storms in the regionally-pooled observations. Also referred to as "equivalent independent record length (EIRL)".
Evaporation: 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. See also total evaporation.
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 also tension zone.
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 feet in diameter, 10 inches deep, set up on a timber grillage so that the top rim is about 16 inches from the ground. The water level in the pan during the course of observation is maintained between 2 and 3 inches below the rim.
Evapotranspiration: Water withdrawn from a land area by evaporation from water surfaces and moist soils and plant transpiration.
Event-Based Model: A model that simulates some hydrologic response to a precipitation event. Compare continuous model.
Exceedance Probability: Hydrologically, the probability that an event selected at random will exceed a specified magnitude.
Excess Precipitation: The precipitation in excess of infiltration capacity, evaporation, transpiration, and other losses. Also referred to as effective precipitation.
Excess Rainfall: The volume of rainfall available for direct runoff. It is equal to the total rainfall minus interception, depression storage, and absorption.
F
Falling Limb: The portion of a hydrograph where runoff is decreasing.
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.
Field-Moisture Deficiency: The quantity of water, which would be required to restore the soil moisture to field-moisture capacity.
Flood: 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 gage height or discharge quantity.
Flood Crest: See flood peak.
Flood Event: See flood wave.
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.
Floodplain: 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 highwater; 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.
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.
Flood Routing: The process of progressively determining the timing and shape of a flood wave at successive points along a river.
Flood Stage: The gage height of the lowest bank of the reach in which the gage 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 cause damage in the reach in which the elevation is measured. See also bankfull stage.
Flood Wave: A distinct rise in stage culminating in a crest and followed by recession to lower stages.
Flood-Frequency Curve: 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. Also, a similar graph but with recurrence intervals of floods plotted as abscissa.
Floodway: 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 floodflow of any river or stream.
Flow-Duration Curve: A cumulative frequency curve that shows the percentage of time that specified discharges are equaled or exceeded.
Fluvial Flooding: inundation caused by riverine flooding instead of overland flow or infiltration excess (to contrast pluvial flooding).
G
Gaging Station: A particular site on a stream, canal, lake, or reservoir where systematic observations of gage height or discharge are obtained. See also stream-gaging station.
GEV: the generalized extreme value distribution. It is the probability distribution of IID block maxima, and generalizes the three extreme value distributions (Gumbel, Fréchet, Weibull).
GPA: the generalized Pareto distribution. It is the probability distribution of IID excesses of a sufficiently high threshold. GPD is also sometimes used.
GPD: see GPA
Ground Water: Water in the ground that is in the zone of saturation, from which wells, springs, and groundwater runoff are supplied.
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 gaging station.
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.
H
Heterogeneous/heterogeneity: having different properties (or the degree to which the properties are different). May also be called "inhomogeneous/inhomogeneity."
Homogeneous/homogeneity: having the same properties (or the degree to which the properties are similar)
HUC: hydrologic unit code, a unique numeric identifier of watersheds in the United States
Hydraulic Radius: The flow area divided by the wetted perimeter. The wetted perimeter does not include the free surface.
Hydrograph: A graph showing stage, flow, velocity, or other property of water with respect to time.
Hydrologic Budget: An accounting of the inflow to, outflow from, and storage in, a hydrologic unit, such as a drainage basin, aquifer, soil zone, lake, reservoir, or irrigation project.
Hydrologic Cycle: The continuous process of water movement between the oceans, atmosphere, and land.
Hydrology: The study of water; generally focuses on the distribution of water and interaction with the land surface and underlying soils and rocks.
Hyetograph: Rainfall intensity versus time; often represented by a bar graph.
I
IID: independent and identically distributed
Index Precipitation: An index that can be used to adjust for bias in regional precipitation, often quantified as the expected annual precipitation.
Infiltration: The movement of water from the land surface into the soil.
Infiltration Capacity: The maximum rate at which the soil, when in a given condition, can absorb falling rain or melting snow.
Infiltration Index: An average rate of infiltration, in inches per hour, equal to the average rate of rainfall such that the volume of rain fall at greater rates equals the total direct runoff.
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.
Initial Conditions: The conditions prevailing prior to an event. See also to antecedent conditions.
Interception: The capture of precipitation above the ground surface, for example by vegetation or buildings.
Interflow: Rapid subsurface flow through pipes, macropores, and seepage zones in the soil
Isohyet: Lines of equal rainfall intensity.
Isohyetal Line: A line drawn on a map or chart joining points that receive the same amount of precipitation.
Isopluvial: Greek for "same rainfall", in the precipitation frequency context, a map showing the precipitation depth for the same AEP and duration everywhere
K
Kriging: an interpolation method that relies on Gaussian processes to describe the relationship between variables across dimensions. Typical application is in 2-dimensional spatial statistics. Kriging is a complicated topic and this definition does not do it justice.
L
Lag: Variously defined as time from beginning (or center of mass) of rainfall to peak (or center of mass) of runoff.
Lag Time: The time from the center of mass of excess rainfall to the hydrograph peak. Also referred to as basin lag.
Loss: 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.
L-moment: a descriptor of the shape of a sample or population of data using linear combinations of the values in the dataset
LMRD: L-moment ratio diagram. A plot of L-skewness vs. L-kurtosis that can be used for characterizing sample data and probability distributions.
M
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 also double-mass curve and residual-mass curve.
Maximum Probable Flood: See probable maximum flood.
Meander: The winding of a stream channel.
Model: A physical or mathematical representation of a process that can be used to predict some aspect of the process.
Moisture: Water diffused in the atmosphere or the ground.
N
NARR: North American Regional Reanalysis; an NCEP reanalysis product for North America
NCEP: National Centers for Environmental Prediction
Non-stationarity: a sample that has properties that are not constant across a dimension; e.g. a time series with a trend
NWS: National Weather Service
O
Objective Function: A mathematical expression that allows comparison between a calculated result and a specified goal. In the program, the objective function correlates calculated discharge with observed discharge. The value of the objective function is the basis for calibrating model parameters.
Overland Flow: The flow of rainwater or snowmelt over the land surface toward stream channels. After it enters a stream, it becomes runoff.
P
Parameter: 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.
Parameter Estimation: The selection of a parameter value based on the results of analysis and/or engineering judgement. Analysis techniques include calibration, regional analysis, estimating equations, and physically based methods. See also calibration.
Peak: The highest elevation reached by a flood wave. Also referred to as the crest.
Peak Flow: The point of the hydrograph that has the highest flow.
Peakedness: Describes the rate of rise and fall of a hydrograph.
Percolation: The movement, under hydrostatic pressure, of water through the interstices of a rock or soil.
PDS: partial duration series; also called "peaks over threshold." A sample containing all independent observations of some variable greater than a chosen value.
PF: precipitation-frequency
PFDS: Precipitation Frequency Data Server. NOAA/National Weather Service/Hydrometeorological Design Studies Center source for information related to precipitation frequency analysis. https://hdsc.nws.noaa.gov/hdsc/pfds/
Pluvial flooding: inundation caused by precipitation instead of flowing from a river (in contrast to fluvial flooding). Can be caused by overland flow or infiltration excess.
PMF: probable maximum flood
PMP: probable maximum precipitation
Point-to-area reduction: accounting for the difference between the maximum intensity of rainfall at a point, and the average intensity over a larger area. Synonyms: depth-area-reduction, area reduction factor
POR: period of record
POT: peaks over threshold (see PDS)
Precipitation: 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.
Q
Quasi-Continuous: a hydrologic modeling technique that mimics continuous modeling in an "event mode" by randomly selecting the event date and dependent initial conditions, to capture the full range of variability in hydrologic conditions for design storm modeling. The initial conditions are drawn from a POR continuous hydrologic simulation.
R
Rain: Liquid precipitation.
Rainfall: The quantity of water that falls as rain only. Not synonymous with precipitation.
Rainfall Excess: See excess rainfall.
Rating Curve: The relationship between stage and discharge.
Reach: A segment of a stream channel.
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.
Recurrence Interval: 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.
Region/regionalization: a collection of sites grouped together based on similarity that is used to improve estimates for the properties of extremes in that area. In the SWT method, this is typically one of the NWS climate divisions, which is viewed as meteorologically but not necessarily statistically homogeneous
Regulation: The artificial manipulation of the flow of a stream.
Reservoir: A pond, lake, or basin, either natural or artificial, for the storage, regulation, and control of water.
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 also mass curve.
Retention Basin: Similar to detention basin but water in storage is permanently obstructed from flowing downstream.
Return Period: See recurrence interval.
Rising Limb: Portion of the hydrograph where runoff is increasing.
ROI: region of influence. A regionalization method that treats each station as the nucleus of a number of other, homogeneous stations, that form a region. The process is repeated for every station in the study area.
RRFA: regional rainfall-frequency analysis
Runoff: That part of the precipitation that appears in surface streams. It is the same as streamflow unaffected by artificial diversions, storage, or other works of man in or on the stream channels.
S
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.
SCS Curve Number: An empirically derived relationship between location, soil-type, land use, antecedent moisture conditions and runoff. A SCS curve number is used in many event-based models to establish the initial soil moisture condition, and the infiltration characteristics.
Site: a location where observations of the hydrometeorological variable of interest are taken
Space-for-time substitution: using collections of similar observations of extremes across a geographic extent to increase the effective number of observations of those extremes
Spatial regression: a regression analysis where the predictor(s) and predictand are linked by being co-located in space
Snow: A form of precipitation composed of ice crystals.
Snow Water Equivalent: the height of water if a snow cover is completely melted, on a corresponding horizontal surface area
Soil Moisture Accounting: 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.
Soil Moisture: 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 also field-moisture capacity and field-moisture deficiency.
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.
Soil Water: See soil moisture.
SST: stochastic storm transposition
Stage: The height of a water surface in relation to a datum.
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.
Stage-Discharge 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. See also rating curve.
Stage-Discharge Relation: The relation expressed by the stage-discharge curve.
Stationarity: in typical usage, constant with respect to time. More generally, a process does not display trends in any parameter across any dimension. The mean and variance are constant with respect to time (or space, in the case of spatial statistics.)
Station-year: total number of years of record when aggregated over a number of stations. For example, there are 60 station-years in an area where there are two stations, one with 35 years of record, and one with 25.
Stemflow: Rainfall or snowmelt led to the ground down the trunks or stems of plants.
Storage: Water artificially or naturally impounded in surface or underground reservoirs. The term regulation refers to the action of this storage in modifying downstream streamflow. Also, 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.
Storm: 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.
Storm typing: identifying the causal mechanism behind all rainfall events from a meteorological basis
Stream: 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 gaging, it is applied to the water flowing in any channel, natural or artificial.
Stream Gaging: The process and art of measuring the depths, areas, velocities, and rates of flow in natural or artificial channels.
Streamflow: 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.
Stream Gaging Station: A gaging station where a record of discharge of a stream is obtained. Within the US Geological Survey this term is used only for those gaging stations where a continuous record of discharge is obtained.
Sub-region: a subset of sites within a region that offers some refinement of region characteristics, typically used to improve estimates of spatial variability. In the SWT method these are statistically homogeneous collections of sites within a climate division
Sublimation: The process of transformation directly between a solid and a gas.
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.
Surface Water: Water on the surface of the earth.
T
Tension Zone: In the context of the program, 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 also soil profile.
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.
Time of Rise: The time from the start of rainfall excess to the peak of the hydrograph.
Time to Peak: The time from the center of mass of the rainfall excess to the peak of the hydrograph. See also to lag time.
Tobler's First Law of Geography: “Everything is related to everything else, but near things are more related than distant things.”
Total Evaporation: 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.
Transpiration: 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.
U
Underflow: 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.
Unit Hydrograph: A direct runoff hydrograph produced by one unit of excess precipitation over a specified duration. For example, a one-hour unit hydrograph is the direct runoff from one unit of excess precipitation occurring uniformly over one hour.
V
Vadose Zone: The portion of the soil profile above the saturation zone.
Validation: The calibrated model, without any further parameter modifications, is used to compute outputs which are compared against observed data for independent events that were not considered during model calibration.
W
Water Year: A 12-month period during which hydrologic quantities are measured. In the United States, a water year is defined as October 1 through September 30 and is is designated by the calendar year in which it ends and which includes 9 of the 12 months. Thus, the water year ending on September 30, 1959, is called the 1959 water year.
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.
Watershed: 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.