Hydrology Terminology Glossary PDF

HYDROLOGY - terminology accumulative precipitation gauge Precipitation gauge used at stations which are visited only at long-spaced time intervals active basin area Basin area excluding undrained parts. actual evaporation Quantity of water evaporated from an open water surface or from t...

HYDROLOGY - terminology accumulative precipitation gauge Precipitation gauge used at stations which are visited only at long-spaced time intervals active basin area Basin area excluding undrained parts. actual evaporation Quantity of water evaporated from an open water surface or from the ground. actual evapotranspiration Sum of the quantities of water evaporated from the soil and the plants when the ground is at its natural moisture content advection Process of transfer of air-mass properties by the velocity field of the atmosphere altitude Vertical distance of a level, a point or an object con- sidered as a point, measured from mean sea-level. annual exceedence series Series of values of independent events, such as floods, occurring above a base value selected so that the number of values in the series is equal to the number of years of the record annual flood - 1) highest peak discharge in a water year - (2) flood which has been equalled or exceeded once each year on average annual flow Total volume of water that flows during a year, usually referring to the outflow of a drainage area or river basin annual maximum series Extreme-value series with largest annual values annual minimum series Extreme-value series with smallest annual values annual runoff Total volume of water that flows during a year, usually referring to the outflow of a drainage area or river basin annual series Hydrological series the terms of which represent some characteristic or element for each of a series of years annual storage - volume of water that can be stored in a reservoir to cope, partly or completely, with seasonal variations in inflow and demand annual storage capacity Volume of water that can be stored in a reservoir to cope, partly or completely, with seasonal variations in inflow and demand antecedent precipitation index Weighted summation of past daily precipitation amounts, used as an index of soil moisture. The weight given each day's precipitation is usually assumed to be an exponential or reciprocal function of time, with the most recent precipitation receiving the greatest weight applied hydrology That branch of hydrology which refers to its application to fields connected with water-resources development and management area-elevation curve Curve showing what part of the area of a river basin is situated above an indicated elevation areal precipitation Precipitation in a specific area expressed as the average depth of liquid water over this area areal reduction factor (of rainfall) Ratio of a mean areal rainfall for a given duration and given return period to a mean point rainfall for the same duration and same return period in the same area arithmetic mean (1) sum of a set of values x1, x2,..., xn divided by their number, n. It is often denoted by a bar, e.g. x (the word "arithmetic" is frequently omitted; where a "mean" is mentioned, the arithmetic mean is to be understood). (2) the first moment (about the origin) of a frequency distribution, f(x). automatic station Station at which instruments make and either transmit or record observations automatically, the conversion to code form, if required, being made either directly or at an editing station. average In ordinary usage, the average is often understood to refer to the arithmetic mean. Generally, it purports to represent or to summarize the relevant features of a set of values, e.g. the median and the mode. In a more limited sense, an average compounds all the values of the set, e.g. the arithmetic or geometric means. average velocity Discharge divided by the cross-sectional area at right angles to the direction of flow or, for a vertical, area of a velocity curve divided by the depth average year Year for which the observed hydrological or meteoological quantity approximately equals the long term average of that quantity bankfull discharge Discharge conveyed in a water course without overtopping the banks bankfull stage Stage at which a stream just overflows its natural banks base-width (of a flood hydrograph) Time interval between the beginning and the end of the direct runoff produced by a storm basin Drainage area of a stream, river or lake basin response Manner in which a basin reacts to a meteorological event or sequence of events bubble gauge; bubble gage (A) Gauge using a gas-purged system to measure water level calibration Experimental determination of the relationship between the quantity to be measured and the indication of the instrument, device or process which measures it. catchment Area having a common outlet for its surface runoff. catchment area Area having a common outlet for its surface runoff. catchment response Manner in which a basin reacts to a meteorological event or sequence of events channel frequency Number of stream segments of all orders within a given basin, divided by its area channel network Arrangement of natural or man-made drainage channels within an area clear overflow weir Weir the flow of which is not submerged by the tail water climate Synthesis of weather conditions in a given area, characterized by long-term statistics (mean values, variances, probabilities of extreme values, etc.) of the meteorological elements in that area climatic change Significant change observed in the climate of a region between two reference periods climatic year Continuous twelve-month period during which a complete annual climatic cycle occurs, and which is selected to provide a more meaningful comparison of meteorological data climatological station for hydrological purposes Climatological station set up in a drainage basin specifically to augment the existing climatological network in order to meet hydrological requirements. coefficient of determination Square of the correlation coefficient between a dependent variate and one or more independent variates; it expresses the proportion of the variance of the dependent variate given by that of the independent variates. coefficient of variation Statistical parameter describing the change of a stochastic variable in time or space, expressed as the ratio of the standard deviation to the mean. (Dimensionless) compound weir Weir containing two or more sections, which may be of different types and/or dimensions. (ISO/772) conceptual hydrological model Simplified mathematical representation of some or all of the processes in the hydrological cycle by a set of hydrological concepts expressed in mathematical notations and linked together in a time and space sequence corresponding to that occurring in nature. Hydrological conceptual models are used for simulation of the behaviour of a basin conditional probability If E and F are events occurring according to a probability distribution, the probability of E given the occurrence of F is the conditional probability of E usually written as Pr(E/F). confidence interval Interval which includes the true value with a prescribed probability and which is a function of the statistics of the sample confidence level Probability that the confidence interval includes the true value confidence limits Values which form the lower and upper limits to the confidence interval constant-rate dilution gauging Determination of flow characteristics (velocity or discharge) by means of a constant rate of injection of a tracer of known concentration and measurement of the tracer concentration, when good mixing is obtained, at some point downstream continental hydrology That branch of hydrology which refers to hydrological processes in continental areas, stress being laid on the continental phase of the water cycle continuity equation Equation describing the conservation of mass of flowing matter (e.g. water) convective precipitation Precipitation caused by convective motion in the atmosphere correlation Interdependence or relationship between two measurable variates correlation coefficient Measure of the interdependence between two variates. The product-moment correlation coefficient is the covariance of the variates divided by the product of their standard deviations crest gauge; crest gage (A) Device used to record the highest water stage cumulative precipitation gage Precipitation gauge used at stations which are visited only at long-spaced time intervals cup-type current meter Current meter whose rotor is composed of a wheel fitted with cups and turning on a vertical axis current meter Instrument for measuring the velocity of water at a point. Traditionally the term refers to instruments with cups or propellers cyclonic precipitation Precipitation caused by the activity of an atmospheric depression  data bank Comprehensive set of related data files for a specific application, usually on a direct access storage device data base Comprehensive set of related data files for a specific application, usually on a direct access storage device data collection system Coordinated system for collecting observations from a hydrological network and the transmission of the observations to a data-processing facility. data processing Handling of observational data until they are in a form ready to be used for a specific purpose. datum level Horizontal surface used as a reference to which elevations are related. decay rate Rate of reduction of the concentration of a quantity, e.g. a pollutant, or of the activity of a radio-active isotope, expressed by its half-life, i.e. the time needed to lose half of its activity. depletion curve Curve showing a decreasing rate of runoff or discharge, caused by depletion. depression storage Volume of water which is required to fill small depressions to their overflow levels. depth (total) of runoff Runoff volume from a drainage basin, divided by its area, in a specified time. depth-area curve Curve showing the relationship between average rainfall depth and the area over which it occurs for a given storm duration. depth-area-duration analysis Analysis of areal distribution of precipitation from a storm, usually made graphically, using depth-area curves for various storm durations. depth-duration curve Curve showing the relationship between average rainfall depth or intensity in a given area and the storm duration. design flood Flood hydrograph or instantaneous peak discharge adopted for the design of a hydraulic structure or river control taking into account economic and hydrological factors. design storm Rainfall amount and distribution adopted over a given drainage area, used in determining the design flood. detention storage That part of precipitation which is temporarily stored en route to or in the stream system, during or shortly after rainfall. Detention storage includes surface and channel detention but does not include depression storage. deterministic hydrology Method of analysis of hydrological processes, using a deterministic approach to investigate the responses of hydrological systems in terms of various parameters. deterministic system A system is said to be deterministic if its response at any time due to a given input is uniquely determined. dilution gauging; dilution gaging (A) Method of determining the discharge of a stream by measuring the degree of dilution by the flowing water of an added tracer solution. direct runoff That part of surface runoff which reaches the catchment outlet shortly after the rain starts. Its volume is equal to rainfall excess. Some procedures for its derivation include prompt subsurface runoff but all exclude base flow. discharge Volume of water flowing through a river (or channel) cross-section in unit time. discharge coefficient Ratio of the observed or actual discharge to the theoretically computed discharge. discharge hydrograph Graph showing the variation in time of some hydrological data such as stage, discharge, velocity, sediment load, etc. (hydrograph is mostly used for stage or discharge). distribution graph Unit hydrograph modified to show the fraction of the volume of runoff that occurs during successive units of time. double mass curve Plot of successive accumulated values of one variable against the contemporaneous accumulated values of another variable. drainage area Area having a common outlet for its surface runoff. drainage density Total channel-segment lengths, accumulated for all orders within a drainage area, divided by the area. drainage divide Summit or boundary line separating adjacent drainage basins. drought Prolonged absence or marked deficiency of precipitation. drought index Computed value which is related to some of the cumulative effects of a prolonged and abnormal moisture deficiency. An index of hydrological drought corresponding to levels below the mean in streams, lakes, reservoirs, and the like. However, an index of agricultural drought must relate to the cumulative effects of either an absolute or an abnormal transpiration deficit. dry year Year of drought during which precipitation or stream flow is significantly less than usual. duration curve Graph representing the time during which the value of a given parameter, e.g. water level, is equalled or exceeded, regardless of continuity in time. dynamic hydrology Method of analysis of hydrological processes, using a deterministic approach to investigate the responses of hydrological systems in terms of various parameters. effective rainfall (1) That part of rainfall which contributes to runoff. In some procedures the prompt subsurface runoff is entirely excluded from direct runoff and then effective rainfall is equal to rainfall excess. (2) In agriculture: that portion of the rainfall which remains in the soil and contributes to the growth of crops. elevation Vertical distance of a level, a point or an object considered as a point, measured from mean sea-level. empirical flood formula Formula expressing peak discharge as a function of catchment area and other factors. engineering hydrology That branch of applied hydrology which deals with hydrological information intended for engineering applications, e.g. planning, designing, operating and maintaining engineering measures and structures. envelope curve Smooth curve which represents the boundary within which all or most of the known data points are contained. enveloping curve Smooth curve which represents the boundary within which all or most of the known data points are contained. ephemeral stream Stream which flows only in direct response to precipitation or to the flow of an intermittent spring. estuary That generally broad portion of a stream near its outlet. evaporation (of water) (1) Emission of water vapour by a free surface at a temperature below the boiling point. (2) Amount of water evaporated. evaporimeter Instrument for measuring the amount of water evaporated into the atmosphere during a given time interval. evapotranspiration Quantity of water transferred from the soil to the atmosphere by evaporation and plant transpiration. expected value Best approximation of the true value which may be the mean of several or of many computations, estimates or measurements. experimental basin Basin in which natural conditions are deliberately modified and in which the effects of these modifications on the hydrological cycle are studied. experimental watershed (AU) Basin in which natural conditions are deliberately modified and in which the effects of these modifications on the hydrological cycle are studied. extreme rainfall Amount of precipitation that is the statistical (physical) upper limit for a given duration over a particular basin. extreme value distribution Probability distribution of the largest (smallest) observation in a sample.. Commonly used extreme value distributions are FrŽchet, Gumbel and Weibull. extreme value series Hydrological series which includes the largest or smallest values, with each value selected from an equal time interval in the record. falling limb Part of a hydrograph in which the discharge is decreasing from a peak. fall-stage-discharge relation Family of curves which expresses the relationship between water surface slope, stage and discharge in an open channel in a given reach subject to variable backwater. (ISO/772) flash flood Flood of short duration with a relatively high peak discharge. flat-V weir Long-base weir with a triangular or other shape of longitudinal profile and a transverse symmetrical V-shaped crest having small side-slopes float Any natural or man-made body which is supported and partly or fully immersed in water, its vertical motion indicating the changes in water level or its horizontal movement indicating the velocity of water at the surface or at various depths. float gauge; float gage (A) Gauge consisting essentially of a float which rides on the water surface and rises or falls with it, its movement being transmitted to a recording or indicating device. flood (1) Rise, usually brief, in the water level in a stream to a peak from which the water level recedes at a slower rate. (2) Relatively high flow as measured by stage height or discharge. (3) Rising tide. flood control Protection of land areas from overflow, or minimization of damage caused by flooding. flood crest Highest (peak) elevation of the water level during a flood in a channel. flood forecasting Estimation of stage, discharge, time of occurrence, and duration of a flood, especially of peak discharge, at a specified point on a stream, resulting from precipitation and/or snowmelt. flood frequency Number of times a flood above a given discharge or stage is likely to occur over a given number of years. flood hydrograph Graph showing the variation in time of some hydrological data such as stage, discharge, velocity, sediment load, etc. (hydrograph is mostly used for stage or discharge). flood marks Natural marks left on a structure or objects indicating the maximum stage of floods. flood plain Nearly level land along a stream flooded only when the streamflow exceeds the water carrying capacity of the channel. flood probability Probability of a flood of a given stage or discharge being equalled or exceeded in a given year. flood protection Protection of land areas from overflow, or minimization of damage caused by flooding. flood routing Technique used to compute the movement and change of shape of a flood wave moving through a river reach or a reservoir. flood warning Advance notice that a flood may occur in the near future at a certain station or in a certain river basin. flood wave Rise in streamflow to a maximum crest, and its subsequent recession, caused by a period of precipitation, snow melt, dam failure or hydroelectric plant releases. flood-control reservoir Reservoir used only for the temporary storage of flood water which is then released as rapidly as channel conditions downstream permit. flooded area Area covered by water when streamflow exceeds the carrying capacity of a channel or as a consequence of damming a river downstream. flooding (1) Overflowing by water of the normal confines of a stream or other body of water, or accumulation of water by drainage over areas which are not normally submerged. (2) Controlled spreading of water for irrigation. flow-duration curve Curve showing the percentage of time during which the flow of a stream is equal to or greater than given amounts, regardless of chronological order. flowmeter Instrument for measuring the rate of flow in a conduit or open channel. flume Man-made channel with clearly specified shape and dimensions which may be used for the measurement of discharge. forecast Definite statement or statistical estimate of the occurrence of a future event. forecast (warning) lead time Interval of time between the issuing of a forecast (warning) and the expected occurrence of the forecast element. free weir Weir the flow of which is not submerged by the tail water frequency analysis Procedure involved in interpreting a past record of hydrological events in terms of future probabilities of occurrence, e.g. estimates of frequencies of floods, droughts, storages, rainfall, water quality, waves. frequency curve Curve relating the possible values of a variate, e.g. the value of a specific hydrological event, to the frequency of its occurrence. frequency distribution Specification of the way in which the frequencies of members of a population are distributed according to the values of the variates which they exhibit. fresh/salt-water interface Surface separating a body of fresh water and one of brackish or salt water, taken somewhere within the transition zone between the two fluids. freshwater Naturally occurring water having a low concentration of salts, or generally accepted as suitable for abstraction and treatment to produce potable water. frontal precipitation Precipitation caused by the expansion of air on ascent along or near a frontal surface. full meander One curved portion of a sinuous or winding stream channel, consisting of two consecutive loops, one turning clockwise and the other anticlockwise. gauge datum; gage datum ( Vertical distance of the zero of a gauge referred to a certain datum level. gauge zero; gage zero Elevation at which a water level gauge zero is set. gauging section; gaging section Cross-section of an open channel in which measurements of depth and velocity are made. gauging site; gaging site Location on a stream where measurements of water level and discharge are regularly made. gradex method Method which allows the substitution of a frequency distribution for floods by a distribution for rainfalls (for a given interval of time). It is based on the exponential behaviour of frequency distribution curves for low frequencies (log-log law of representation) and assumes storms large enough to saturate the soil. headwaters Streams from sources of a river. histogram Univariate frequency diagram with rectangles proportional in area to the class frequency, erected on a horizontal axis with width equal to the class interval. historical data Hydrological and meteorological data of events which occurred in the past. hydrograph Graph showing the variation in time of some hydrological data such as stage, discharge, velocity, sediment load, etc. (hydrograph is mostly used for stage or discharge). hydrographical network Aggregate of rivers and other permanent or temporary watercourses, and also lakes and reservoirs, over any given area. hydrography Science dealing with the description and measurement of open bodies of water, e.g. oceans, seas, streams, rivers, lakes, reservoirs, etc. In particular, charting the open bodies of water for navigational purposes. hydrological cycle Succession of stages through which water passes from the atmosphere to the earth and returns to the atmosphere: evaporation from the land or sea or inland water, condensation to form clouds, precipitation, accumulation in the soil or in bodies of water, and re-evaporation. hydrological drought Period of abnormally dry weather sufficiently prolonged to give rise to a shortage of water as evidenced by below normal streamflow and lake levels and/or the depletion of soil moisture and a lowering of groundwater levels. hydrological element Some hydrological phenomenon such as water level (stage) , discharge or precipitation. hydrological network Aggregate of hydrological stations and observing posts situated within any given area (river basin, administrative region) in such a way as to provide the means of studying the hydrological regime. hydrological observation Direct measurement or evaluation of one or more hydrological elements, such as stage, discharge, water temperature, etc. hydrological observing station Place where hydrological observations or climatological observations for hydrological purposes are made. hydrological regime Variations in the state and characteristics of a water body which are regularly repeated in time and space and which pass through phases, e.g. seasonal. hydrological station for specific purposes Hydrological station established for the observations of a specific element or elements, for the investigation of hydrological phenomena. hydrological year Continuous 12-month period selected in such a way that overall changes in storage are minimal so that carryover is reduced to a minimum. hydrology (1) Science that deals with the waters above and below the land surfaces of the Earth, their occurrence, circulation and distribution, both in time and space, their biological, chemical and physical properties, their reaction with their environment, including their relation to living beings. (2) Science that deals with the processes governing the depletion and replenishment of the water resources of the land areas of the Earth, and treats the various phases of the hydrological cycle. hydrometeorology Study of the atmospheric and land phases of the hydrological cycle, with emphasis on the interrelationships involved. hydrometric network Aggregate of hydrological stations and observing posts situated within any given area (river basin, administrative region) in such a way as to provide the means of studying the hydrological regime. hydrometric station Station at which data on water in rivers, lakes or reservoirs are obtained on one or more of the following elements: stage, streamflow, sediment transport and deposition, water temperature and other physical properties of water, characteristics of ice cover and chemical properties of water. hydrometry Science of the measurement and analysis of water including methods, techniques and instrumentation used in hydrology. hydrosphere That part of the Earth covered by water and ice. hyetograph (1) Map or chart displaying temporal or areal distribution of precipitation. (2) Graph displaying the intensity of precipitation versus time. hypsometric curve Curve showing what part of the area of a river basin is situated above an indicated elevation. index of determination Index equal to one minus the coefficient of determination. instantaneous unit hydrograph Unit hydrograph resulting from unit amount of effective precipitation applied to a drainage basin in an infinitesimally short time. integral curve Curve of an accumulated quantity versus time. integration method Method of estimating quantities such as the mean velocity or mean sediment concentration in a vertical, involving the raising and lowering of an instrument at a constant rate over the entire depth of the vertical. intensity-duration curve Curve showing the relationship between average rainfall depth or intensity in a given area and the storm duration. intensity-duration-frequency curve Curve showing the probability of various short-period rainfall rates for various durations of precipitation at a given location. Often a family of curves is shown, each depicting a specific occurrence frequency or return period in years. interflow (1) That portion of the precipitation which has not passed down to the water table, but is discharged from the area as subsurface flow into stream channels. (2) Flow of water from ephemeral zones of saturation. It moves through the upper strata of a formation at a rate much in excess of normal base-flow seepage. intermittent stream Stream which flows only in direct response to precipitation or to the flow of an intermittent spring. isochrone map Map or chart of a drainage basin in which a series of lines (isochrones) gives the times of travel of water originating on each isochrone to reach the outlet of the basin. isohyet Line joining the points where the amount of precipitation, in a given period, is the same. kurtosis Flatness of a frequency distribution or probability curve. lag-time Time from centre of mass of rainfall to centre of mass of runoff, or to the peak of runoff. lateral inflow Inflow of water to a river, lake or reservoir along any reach from the part of the catchment adjacent to the reach. limnigraph Instrument recording water levels in the course of time. limnology Science that deals with the study of lakes and open reservoirs, including hydrological phenomena, emphasizing the analysis of the environment. log-normal distribution Transformed normal probability distribution in which the variate is replaced by its logarithm. loop rating curve Double-valued part of the rating curve in which the higher values of the discharge apply when the river is rising and the lower values when the river is falling. lower reach part of a stream channel in the lower region of a drainage basin lysimeter (see also evapotranspirometer) vessel containing local soil placed with its top flush with the ground surface for the study of several phases of the hydrological cycle, e.g. infiltration, runoff, evapotranspiration, soluble constituents removed in drainage, etc. mass curve curve of an accumulated quantity versus time. maximum instantaneous discharge (syn. maximum instantaneous discharge; peak flow) maximum instantaneous discharge of a given hydrograph maximum possible flood (see also standard project flood) greatest flood to be expected, assuming complete coincidence of all factors that would produce the heaviest rainfall and maximum runoff. mean deviation measure of dispersion derived from the arithmetic mean of the deviations of calculations, estimates or observations from some central value (e.g. the mean or the median) , the deviations being taken without reference to algebraic sign. mean monthly discharge (see also multi-annual mean ) arithmetic mean of all the individual monthly mean discharges for a named month in a period of record mean velocity (syn. average velocity) Discharge divided by the cross-sectional area at right angles to the direction of flow or, for a vertical, area of a velocity curve divided by the depth. meander (syn. full meander) One curved portion of a sinuous or winding stream channel, consisting of two consecutive loops, one turning clockwise and the other anticlockwise mean-section method (see also mid-section method ; velocity-area method ; velocity-contour method ) Method for computing the discharge of a stream by dividing the cross-section by adjacent verticals at an equal distance. The total discharge is the product of that distance by the sum of the products of the mean of the depths of two adjacent verticals by the mean of the average velocities over those two verticals. measuring flume Man-made channel with clearly specified shape and dimensions which may be used for the measurement of discharge. measuring section (syn gauging section; gaging section) cross-section of an open channel in which measurements of depth and velocity are made measuring weir Device or structure across a stream which causes a free or submerged fall and is used for measuring discharge. median For a continuous frequency distribution, the value of the variate which divides the total frequency into two equal halves. For n discrete data, the middle value of the ranked data if n is odd, or the mean of the two central values if n is even. medium-term (extended) hydrological forecast Forecast of the future value of an element of the regime of a water body for a period ending between two and ten days from the issue of the forecast. meteorological (weather) radar Method, system or technique, including equipment components, for using beamed, repeated and timed electromagnetic radiation to detect, locate and/or quantify objects, to measure altitude and to acquire a terrain image. meteorology Science of the atmosphere minimum annual discharge (syn. minimum annual flow) Least value of discharge in a stream during a hydrological year minimum annual flow (syn. minimum annual discharge) Least value of discharge in a stream during a hydrological year mode In a distribution of a discrete variate, the variate which occurs most frequently. In a distribution of a continuous variate, the variate with maximum probability density. model (see also simulation) Representation in any form of an object, process or system. In hydrology, a model is in most cases a mathematical representation of a basin, a water system, a series of data, etc. model calibration (syn. model fitting) Adjustment of the parameters of a model, either on the basis of physical considerations or by mathematical optimization, so that the agreement between the observed data and estimated output of the model is as good as possible model fitting (syn. model calibration) Adjustment of the parameters of a model, either on the basis of physical considerations or by mathematical optimization, so that the agreement between the observed data and estimated output of the model is as good as possible moment (see also standard deviation ; variance ) In general, a moment is the mean value of a power of a variate. For a distribution of a variate, moments may be taken about the origin or about a given fixed value if the latter is the mean, the moments are central moments monitoring (syn. surveillance) Continuous or frequent standardized measurement and observation of the environment, often used for warning and control moving boat method  Method of measuring discharge from a boat by traversing the stream along the measuring section whilst continuously measuring velocity, depth and distance travelled multi-annual mean (see also average year ; mean monthly discharge ; normals ) Arithmetic mean of the occurrences, over a number of years, of a hydrological quantity associated with a given date or period within the year. e.g. Multi-annual mean monthly discharge for April is the arithmetic mean of all April monthly mean discharges for a number of years. (In contrast, monthly mean discharge (m3 s-1) is the volume (m3) of streamflow in a month divided by the number of seconds in that month.) multi-annual storage (syn. multi-annual storage capacity) Volume of water that can be stored in a reservoir to cope, partly or completely, with variations in inflow and demand over more than one year multi-annual storage capacity (syn multi-annual storage) Volume of water that can be stored in a reservoir to cope, partly or completely, with variations in inflow and demand over more than one year multiple correlation (see also multiple regression) Interdependence of more than two variables multiple correlation coefficient (see also correlation coefficient ; covariance ) First product-moment correlation between the actual values of the "dependent" variate in multiple regression and the values as given by the regression equation multiple regression (see also multiple correlation) Regression of a "dependent" variate on more than one "independent" variate Muskingum method Approximate hydrological method of flood routing, based on the equation of continuity for a reach and a storage equation expressing the linear dependence of the water volume in the reach on the weighted inflow and outflow discharges natural flow Flow in a stream as would occur under natural conditions. normal distribution (syn. Gaussian distribution) Mathematically defined, symmetrical, bell-shaped, continuous probability distribution traditionally assumed to represent random errors normal hydrological value Mean value of a hydrological characteristic taken over a period selected by consensus such that the mean over any longer period does not significantly differ from the value obtained. normals (see also average year ) Period averages of meteorological/hydrometeorological elements calculated over a uniform and relatively long period covering at least thirty consecutive years. n-year event (see also flood frequency ; return period) Magnitude of a hydrological event, the return period of which is n years. optimal design System design based on the selection or combination of all pertinent variables so as to maximize some objective function (such as net benefits) with the requirements of the design criteria. orographic precipitation Precipitation caused by the ascent of moist air over orographic barriers. outflow Flow of water out of a stream, lake, reservoir, container, basin, aquifer system, etc. outlet (see also outfall ) Opening through which water flows out or is extracted from a reservoir or stream outliers Small number of members of a sample so far separated in value from the remainder that they give rise to the question whether they are not from a different population, or that the sampling technique is at faultň overbank flow channel (syn flood plain, bottom land) Nearly level land along a stream flooded only when the streamflow exceeds the water carrying capacity of the channel. overland flow Flow of water over the ground before it enters a definite channel parameter Coefficient in some hydrological model, formula or other relationship which may be adjusted to apply the general model, etc. to particular cases. For example fitting a rainfall-runoff model to a particular catchment will involve determining parameter values appropriate to that catchment parametric hydrology (syn. deterministic hydrology ; analytical hydrology; dynamic hydrology) Method of analysis of hydrological processes, using a deterministic approach to investigate the responses of hydrological systems in terms of various parameters. partial duration series (syn. peaks-over-threshold series; see also annual exceedence series ) Series of values of independent events, such as floods, occurring above a base value selected, without regard to the number within a given period. peak discharge (syn. maximum instantaneous discharge; peak flow) Maximum instantaneous discharge of a given hydrograph peak flow (syn. peak discharge) Maximum instantaneous discharge of a given hydrograph peaks-over-threshold series (syn. peaks-over-threshold series; see also annual exceedence series ) Series of values of independent events, such as floods, occurring above a base value selected, without regard to the number within a given period peak-to-mean flow ratio Ratio of the peak discharge of a flow hydrograph, or a unit hydrograph, to its average discharge Pearson distribution Group of probability distributions of varying skewness and other properties which were proposed by Karl Pearson and which are sometimes fitted to hydrological data. persistence Tendency of a hydrological process or time-series to recur or continue. There is a positive correlation between successive values when arranged in the order of their occurrence. pluviometer (syn. raingauge; rain gage) Instrument for measuring the depth of water from precipitation at a point PMP - probable (possible) maximum precipitation (syn. extreme rainfall) Amount of precipitation that is the statistical (physical) upper limit for a given duration over a particular basin point data Observations at a geographical site, e.g. the site of a rain gauge or a stream-gauging station point gauge; point gage (A) (see also hook gauge) Adjustable water-level gauge consisting of a pointed metallic rod pointing downwards point precipitation (see also areal precipitation ) Precipitation at a particular site polder A mostly low-lying area artificially protected from surrounding water and within which the water table can be controlled potamology (see also surface water hydrology) That branch of hydrology dealing with surface streams and their regime. It includes fluvial dynamics and all the phenomena related to erosion of and sedimentation on the stream bed. potential evaporation (syn. evaporative capacity) Quantity of water vapour which could be emitted by a surface of pure water in the existing conditions potential evapotranspiration Maximum quantity of water capable of being evaporated in a given climate from a continuous stretch of vegetation covering the whole ground and well supplied with water. It thus includes evaporation from the soil and transpiration from the vegetation of a specified region in a given time interval, expressed as depth precipitation (1) liquid or solid products of the condensation of water vapour falling from clouds or deposited from air on the ground. (2) amount of precipitation (as defined under (1) ) on a unit of horizontal surface per unit time precipitation gauge; precipitation gage General term for any device that measures the amount of precipitation principally a raingauge or snow gauge precipitation intensity (see also rainfall intensity ) Amount of precipitation collected in unit time interval precipitation station Station at which observations of precipitation only are made primary station (syn. principal hydrometric station. base station; permanent station) Hydrometric station at which one or a number of elements are observed for a period of many years taking into account the significance of such elements in relation to the physical environment. Such a station is usually equipped with recording instruments. probabilistic process Process in which the probability of occurrence of the variables is taken into account, whereas their sequence of occurrence is ignored. probability Basic statistical concept either expressing in some way a "degree of belief" or taken as a limiting relative frequency of occurrence in an infinite series. probability distribution Distribution giving the probability of a value of a variate as a function of the variate. probability of forecast The chance of occurrence of a quantified assessment (in real time) of future events. probability paper Graph paper designed in such a way that the cumulative probability of a theoretical distribution plots as a straight line, e.g. normal probability paper, log-normal probability paper, extreme value probability paper. probable (possible) maximum precipitation (PMP) Amount of precipitation that is the statistical (physical) upper limit for a given duration over a particular basin. propeller-type current meter Current meter the rotor of which is a propeller rotating around an axis parallel to the flow. psychrometer Instrument used for measuring the water-vapour content of the atmosphere. It consists of two thermometers, one of which (the dry bulb) is an ordinary glass thermometer, while the other (wet bulb) has its bulb covered with a jacket of clean muslin which is saturated with distilled water prior to an observation. pulse dilution gauging Determination of flow characteristics (velocity or discharge) by means of an instantaneous injection of a given quantity of a tracer and measurement of the pulse of tracer concentration, when good mixing is obtained, at some point downstream. pumping test Pumping of water from a well at one or more selected discharge rates, during which piezometric levels are measured regularly at the pumped well and at nearby observation wells. The data are used for determining the aquifer parameters in the vicinity of the pumped well. quantile Class of (n-1) partition values of a variate which divide the total frequency of a population or a sample into a given number, n, of equal proportions. Hence quartiles (n = 4) , deciles (n = 10) , etc. quantitative precipitation forecast (QPF) Forecast of precipitation amount. radar (-rainfall) equation Equation defining the maximum range to be anticipated from a particular radar system in terms of the design parameters incorporated with it and the effective target echo area. rain Precipitation of liquid water, either in the form of drops of more than 0.5 mm diameter, or of smaller, widely scattered drops. rain gauge; rain gage Instrument for measuring the depth of water from precipitation at a point. rain recorder Instrument which includes an arrangement for the time recording of the depth of water from precipitation. rain shadow Region, situated on the lee side of a mountain, or mountain range, where the rainfall is much less than on the windward side. rainfall (depth) Supply of liquid water falling from the atmosphere (expressed as depth of water on a horizontal surface). rainfall distribution Manner in which the depth of rainfall varies in space and time. rainfall excess That part of rainfall which contributes directly to surface runoff. rainfall intensity Rate at which rainfall occurs, expressed in units of depth per unit of time. rainfall intensity pattern Distribution of rainfall rate, in time, during a storm. rainfall intensity return period Average time interval in years between the occurrence of rainfall of a given intensity and that of an equal or greater intensity. rainfall maximization Procedure used to derive probable (possible) maximum precipitation. rain-gauge shield; rain-gage shield Procedure used to derive probable (possible) maximum precipitation. random error Deviation of a computed, estimated or measured value from the true value, the deviation behaving like a variate in the sense that any particular value occurs as though chosen at random from a probability distribution of such errors. random process Generally synonymous with the more usual and preferable ""stochastic process"". Sometimes employed to denote a process in which the movement from one state to the next is determined by a variable which is independent of the initial and final state (a pure random process). rate of flow Volume of water flowing through a river (or channel) cross-section in unit time. rating curve (rating table) Curve showing the relation between stage and discharge of a stream at a hydrometic station. If digitized, it is a rating table. rational method Formula expressing the estimated peak rate of storm runoff as the product of the catchment area, a peak rate of rainfall, and a runoff coefficient. recession Period of decreasing discharge as indicated by the falling limb of a hydrograph starting from the peak. recording rain gauge; recording rain gage Instrument which includes an arrangement for the time recording of the depth of water from precipitation. recurrence interval Long-term average interval of time or number of years within which an event will be equalled or exceeded, e.g. flood peak discharge. regional analysis Extension of the results of the analysis of point data to a region. regression coefficient Coefficient of an independent variate in a regression equation. For a first degree linear regression, the slope of the regression line. regression curve, syn. regression line Diagrammatic exposition of a regression equation, generally referring to a linear regression. remote sensing Measurement or acquisition of information on some property of an object or phenomenon by a recording device that is not in physical or intimate contact with the object or phenomenon under study. (MRS) representative basin (1) Basin permitting the study of the hydrological cycle in a characteristic natural region by the simultaneous observation of climatic and hydrometric data. (2) Basin where intensive hydrological studies are conducted under relatively unchanged conditions. (3) Basin in which hydrological stations are installed to make simultaneous hydrometeorological and hydrometric observations so that the measurements would represent a broad area in lieu of making measurements on all basins in a given region. reservoir routing Technique used to compute the movement and change of shape of a flood wave moving through a river reach or a reservoir. response function Mathematical representation of the behaviour of a (hydrological) system due to a specified input. retention That part of the precipitation falling on a drainage area which does not escape as surface streamflow during a given period. return flow Any flow which returns to a stream channel or to the groundwater after use. return period Long-term average interval of time or number of years within which an event will be equalled or exceeded, e.g. flood peak discharge. rising limb Part of a hydrograph in which the discharge is increasing towards a peak. risk (1) Potential realization of unwanted consequences of an event, a function of the probability and the value of the consequence. (2) Also sometimes used as the probability of occurrence of an event of a given or greater magnitude within a specified period of time. river Large stream which serves as the natural drainage channel for a drainage basin. river basin Area having a common outlet for its surface runoff. river bed Lowest part of a river valley shaped by the flow of water and along which most of the sediment and runoff moves in interflood periods. river mouth Place of discharge of a river into a sea or a lake. runoff That part of precipitation that appears as streamflow. runoff coefficient (1) Ratio of runoff depth to precipitation depth. (2) Coefficient used in the rational method formula (dimensionless). salt-dilution gauging Method of determining the discharge of a stream by measuring the degree of dilution by the flowing water of an added tracer solution. sampling distribution Distribution of a statistic in all possible samples which can be chosen according to a specified sampling scheme, generally involving random selection and usually samples of the same size. S-curve Hydrograph that would result from an infinite series of sequential excess rainfall increments of unit rate (e.g. 1 cm in T hours) , by adding a series of T-hour unit hydrographs, each lagged T hours with respect to the preceding one. sediment concentration Ratio of weight of dry solids to the weight of a water/sediment sample. sediment discharge Discharge of sediments of a stream at a given cross section. serial correlation Correlation between members of a sample of a time (or space) series and those members lagging behind or leading by a fixed interval in time (or space). significance An effect is said to be significant if the hypothesis that the effect is not present is rejected on the basis of statistical analysis (significance tests). simulation Reproduction or representation of an event or sequence of related events in a model such as that of a water resources system. skewness Asymmetry in a frequency or probability distribution. spillway design flood (1) Flood used for the design of a dam spillway. (2) Maximum flood flow that could be passed without damage or serious threat to the stability of engineering structures. staff gauge; staff gage Graduated scale used to indicate the level of the water surface in a stream channel, reservoir, lake, etc. stage Vertical distance of the water surface of a stream, lake, reservoir (or groundwater observation well) relative to a gauge datum. stage gauge; stage gage Graduated scale used to indicate the level of the water surface in a stream channel, reservoir, lake, etc. stage hydrograph Graph showing the variation in time of some hydrological data such as stage, discharge, velocity, sediment load, etc. (hydrograph is mostly used for stage or discharge). stage-discharge relation Relation between stage and discharge at a river cross section and which may be expressed as a curve, table or equation(s). standard deviation Widely used measure of dispersion of a frequency distribution or of a set of values the positive square root of the variance. standard error Positive square root of the variance of the sampling distribution of a statistic. standard error of estimate Expression for the standard deviation of the observed values about a regression line, i.e. an estimate of the variation likely to be encountered in making predictions from the regression equation. standard project flood Discharge that may be expected from the most severe combination of meteorological and hydrological conditions that are considered reasonably characteristic of the geographical region involved, excluding extremely rare combinations. standardized variate Value of a variate reduced to standardized form (zero mean, unit variance) by subtracting the population or sample mean and then dividing by the population or sample standard deviation. stationary process A stochastic process is said to be strictly stationary if its probability distribution is completely independent of time. It is said to be stationary in the wide sense (or to the second order) if the mean and variance are independent of time. The statistics of samples drawn from a stationary process may vary due to sampling variability but not due to their size or their position in the population. station-year method Method based on a combination of records from several independent stations, and their treatment as a single record, the length of which is equal to the sum of the individual records (used in frequency analysis). statistical hydrology Hydrological processes and phenomena which are described and analysed by the methods of probability theory. stochastic hydrology Hydrological processes and phenomena which are described and analysed by the methods of probability theory. stochastic process One which produces variates, x(s) , where s assumes consecutive values in a certain range, t. In most cases, s and t refer to time but they may also refer to space. storage equation Equation expressing the balance between the inflows, outflows and change of storage in any water body over a period of time. storage precipitation gauge Precipitation gauge used at stations which are visited only at long-spaced time intervals. storm (1) Wind with a speed between 23 and 26 m/s (2) Heavy fall of rain, snow or hail, whether accompanied by wind or not, associated with a separable meteoro- logical event. storm duration Period between the beginning and end of precipitation, either total or above a threshold value. storm runoff That part of surface runoff which reaches the catchment outlet shortly after the rain starts. Its volume is equal to rainfall excess. Some procedures for its derivation include prompt subsurface runoff but all exclude base flow. storm transposition Transposition of an observed storm (depth-area-duration pattern) from the basin area of its occurrence to a project-basin or area within the same region of meteorological homogeneity. storm-intensity pattern Distribution of rainfall rate, in time, during a storm. stream (1) Body of water, generally flowing in a natural surface channel. (2) Water flowing in an open or closed conduit, a jet of water issuing from an orifice, or a body of flowing groundwater. stream routing Technique used to compute the movement and change of shape of a flood wave moving through a river reach or a reservoir. streamflow General term for water flowing in a stream or river channel. structural flood mitigation Structural system for reduction of the effects of floods using physical solutions, e.g. reservoirs, levees, dredging, diversions, flood-proofing, etc. surface detention That part of the rain which remains on the ground surface during rain and either runs off, or infiltrates after the rain ends, not including depression storage. surface flow That part of the precipitation which flows on the ground surface. surface retention That part of precipitation which does not appear either as infiltration or as surface runoff during the period of precipitation or immediately thereafter. It includes interception by vegetal cover, depression storage, and evaporation during precipitation, but does not include surface detention. surface runoff That part of the precipitation which flows on the ground surface. surface water Water which flows over or is stored on the ground surface. surface water hydrology That branch of hydrology which deals with hydrological Phenomena and processes which occur on the Earth's surface, emphasizing overland flows. synthetic (unit) hydrograph (Unit) hydrograph developed on the basis of estimation of coefficients related to various physical features of a catchment. systematic error Error which is in some sense biased, i.e. has a distribution with a non-zero mean. Thiessen polygon Polygons formed by the perpendicular bisectors of the straight lines joining adjacent rainfall stations. throughfall Part of rainfall that reaches the ground directly through the vegetative canopy, through intershrub spaces in the canopy and as drip from the leaves, twigs and stems. time of concentration Period of time required for storm runoff to flow to the outlet from the point of a drainage basin having the longest travel time. time series Set of observations, in order, taken at successive points of time, commonly at a fixed interval. tolerance limit Limiting lower or upper value specified for a quantitative characteristic. totalizer Precipitation gauge used at stations which are visited only at long-spaced time intervals. totalizer precipitation gauge Precipitation gauge used at stations which are visited only at long-spaced time intervals. tracer dilution gauging Method of determining the discharge of a stream by measuring the degree of dilution by the flowing water of an added tracer solution. travel time Time elapsing between the passage of a water parcel or packet between a given point and another point downstream, e.g. a flood wave, down an open channel. trend Long-term smooth movement in a time series. triangular-profile weir Long-base weir having a longitudinal triangular profile. (ISO/772) tributary Watercourse flowing into a larger watercourse or into a lake. true value Value which characterizes a quantity in the conditions which exist at the moment when that quantity is observed. It is an ideal value which could be known only if all causes of error were eliminated. twin-gauge station; twin-gage station (A) Gauging station at which two water level gauges define a reach for measurement of water-surface slopes as an essential parameter for establishing a stage-discharge relation. (ISO/772) unit hydrograph Hydrograph of direct runoff resulting from a unit amount of effective rainfall generated uniformly over a watershed during a specified duration. urban hydrology That branch of hydrology which deals with the hydrology of urban and metropolitan areas, where substantial portions consist of nearly impervious surfaces and artificial land relief, emphasizing the effect of urban developments. variable Quantity which may take any one of a specified set of values a variable in the mathematical sense. variance Second central moment of a frequency distribution.. For a set of values the sum of the squares of the deviations from the mean divided by the number of values. variate Quantity which may take any of the values of a specified set with a specified relative frequency or probability; to be regarded as defined, not merely by a set of permissible values like an ordinary mathematical variable but by an associated frequency (probability) function. velocity curve Curve of distribution of measured point velocity usually over a vertical line in the cross section of a stream or current. velocity-area method Method of measuring the discharge of streams by determining the velocity of the flowing water at a number of points over the cross section, measuring depths over the area of the cross section, and summing products of mean velocities by elemental areas. velocity-contour method Determination of the discharge of a stream by measuring the elemental areas between successive isovels and summing the products of each area by the mean of its boundary velocities. V-notch weir Contracted sharp-crested measuring weir notch with sides that form an angle with its apex pointing downward. water Liquid phase of a chemical compound consisting of approximately two parts by weight of hydrogen and 16 parts by weight of oxygen. In nature it contains small amounts of heavy water, gases and solids (mainly salts) in solution. water balance Inventory of water based on the principle that during a certain time interval, the total water gain to a given catchment area or body of water must equal the total water loss plus the net change in storage in the catchment or body of water. water body Mass of water distinct from other masses of water. water cycle Succession of stages through which water passes from the atmosphere to the earth and returns to the atmosphere: evaporation from the land or sea or inland water, condensation to form clouds, precipitation, accumulation in the soil or in bodies of water, and re-evaporation. water level Elevation of the free-water surface of a body of water relative to a datum level. water loss (1) In a water balance: sum of water lost from a given land area during any specific time by transpiration from vegetation (agricultural crops or native vegetation) and building of plant tissue, by evaporation from water surfaces, soil moisture and snow, and by interception. (2) In irrigation: the losses made up of seepage and evaporation from canals and ditches, water wasted from the canal system, excess water drained from land surfaces, and water lost from the soil by deep water resources assessment Determination of the sources, extent, dependability and quality of water resources for their utilization and control. water sampler Device used to obtain a sample of water, either discretely or continuously, for the purpose of examination of various defined characteristics. (ISO/6107) water year Continuous 12-month period selected in such a way that overall changes in storage are minimal so that carryover is reduced to a minimum. watercourse Natural or man-made channel through or along which water may flow. water-level recorder Instrument recording water levels in the course of time. watershed Summit or boundary line separating adjacent drainage basins. watershed divide Summit or boundary line separating adjacent drainage basins. watershed management Controlled use of drainage basins in accordance with predetermined objectives. weighted mean Mean or average of quantities to which have been attached a series of weights in order to make proper allowance for their relative importance. (For example, the areal rainfall computed by the Thiessen polygon method.) zonality of hydrological phenomena Changes of hydrological characteristics mainly with altitude, latitude and local conditions.