Hydrological Terminology

ACID RAIN

Rain, which in the course of its history has combined with chemical elements or pollutants in the atmosphere and reaches the earth's surface as a weak acid solution.

ACIDITY OF WATER

Amount of acids, given as milli equivalents of a strong base per 1 litre of water, necessary to titrate the sample to a certain PH value.

ACTUAL EVAPOTRANSPIRATION

The real evapotranspiration occurring in a specific situation.

AFFLUENT

Watercourse flowing into a larger watercourse or into a lake.

ALBEDO

Ratio of reflected to incoming radiation, usually given in percent.

ALKALINITY OF WATER

Amount of cations balanced by weak acids, expressed as milli equivalents of neutralized hydrogen ions per litre of water.

ALLUVIAL PLAIN

Plain formed by the deposition of alluvial material eroded from areas of higher elevation.

ALLUVIAL STREAM

An alluvial stream is one whose bed is composed of unconsolidated silt, sand and gravel. The bed is constantly in motion and highly unstable.

ANEMOMETER

Instrument used for the measurement of wind speed and direction.

ANNUAL FLOOD

The highest peak discharge in a water year.

ANTECEDENT PRECIPITATION

The precipitation occurring during some period antecedent to the defined event or some part of the defined event.

ANTECEDENT PRECIPITATION INDEX

A weighed summation of daily precipitation amounts used as an index of soil moisture. The weight given to 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.

ANTICYCLONE

An area of relatively high pressure surrounded by closed isobars, the pressure gradient being directed from the center so that the wind blows spirally outward in a clockwise direction in the northern hemisphere, counter- clockwise in the southern hemisphere.

APPLIED HYDROLOGY

That branch of hydrology which refers to its applications to field connected with water resources development and management.

AQUICLUDE

It is a formation which may contain large volumes of water but does not permit its movement at rates sufficiently high for economical development e.g. clay and shale.

AQUIFER

It is a formation or a geological structure which has good permeability to supply sufficient quantity of water to a well or spring.

AQUIFUGE

It is a formation which has no interconnected openings and hence can not absorb or transmit water. It is neither porous nor permeable.

AQUITARD

It is a formation which has low to medium permeability which is not sufficient to be a source of water to flow on a regional scale from one aquifer to the other due to leakage. Formations having predominance of silt and clay along with kankar form aquitard. Behave as semi-confining layers.

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AREA OF INFLUENCE

The areal extent of the cone of depression is called 'area of influence'.

AREA-ELEVATION-CAPACITY CURVE

Curves showing what part of the area of a watershed is situated above an indicated elevation and what is the capacity of watershed up to that elevation.

AREAL PRECIPITATION

Precipitation in a specific area expressed as the average depth of liquid water over this area.

AREA-VELOCITY METHOD

A method of measuring discharge at a section in a stream based on the continuity principle. Cross-sectional area of the stream is measured and velocity of flow is calculated using some type of instrument, say a current meter. Multiplication of area and velocity gives the discharge at that station.

ARID CLIMATE

A term applied to regions where precipitation is so deficient in quantity or occurs at such times that agriculture is impracticable without irrigation.

ARTESIAN AQUIFER

An artesian aquifer is overlain and underlain by confining layers such that water in these aquifers occurs under pressure, which is more than the atmospheric pressure.

ARTESIAN WELL

A well penetrating an artesian aquifer is called artesian well. If the water level rises above the bottom of the confining bed but remains below Ground surface, then it is called artesian well. If water rises above ground surface, then it is called flowing well.

ARTIFICIAL PRECIPITATION

It means causing precipitation artificially by the introduction of materials like solid carbon dioxide or silver iodide into a non-precipitating cloud. The experiments have not yet become of economic importance.

ARTIFICIAL RECHARGE

Artificial recharge may be defined as augmenting the natural replenishment of ground water storage by some method of construction, spreading of water or by artificially changing natural conditions.

ATMOMETER

Porous, porcelain spheres, cylinders or blocks commonly used by plant physiologists for measuring evaporation because evaporation from their surfaces is considered to be quite representative of that from plants.

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AUXILIARY GAUGE

At those hydrometric stations where variable back-water occurs, it is necessary to utilize fall in a reach for the determination of discharge. An auxiliary gauge is installed some distance d/s with the same datum as that of the principal gauge.

AVAILABLE HEAD

Amount of fall in a stream which is available for hydroelectric power development.

AVALANCHE

A moving mass of debris, snow and ice sliding rapidly down a mountain slope.

AVERAGE ANNUAL FLOOD

A flood equal to the average of the annual floods during the period of record.

AVERAGE ANNUAL RAINFALL

Rainfall equal to the average of the annual rainfalls during the period of record.

BACKWASHING

Reversal of the flow of water under pressure, for example, in a well to free the screen or strainer and the adjacent aquifer of clogging material.

BACKWATER CURVE

Longitudinal profile of the water surface upstream in a stream where the water surface is raised by a natural or artificial obstruction.

BANK STORAGE

Water absorbed and stored in the banks of a stream, lake or reservoir and returned in whole or in part as the level of the surface water body falls.

BAROMETER

An instrument used for measuring pressure of the atmosphere.

BASE FLOW

The sustained or dry-weather flow of streams resulting from the outflow of permanent or perched ground water and from the draining of large lakes and swamps. Also water from glaciers, snow and all other possible sources not resulting from direct runoff.

BASIN

Area drained by a river is called basin of the river.

BASIN LAG

Actually basin lag (also known as lag time) is the time difference between the centroids of the input (rainfall excess) and the output (surface runoff). Physically, it represents the mean time of travel of water particles from all parts of the catchment to the outlet during a given storm.

BASIN RECHARGE

Basin recharge is the difference between precipitation and storm runoff. It is often being called loss because it represents loss to runoff, but the term loss is actually a misnomer from agricultural point of view.

BASIN RESPONSE

Manner in which a basin reacts to a meteorological event or sequence of events.

BED LOAD

Bed load may be defined as the load of bed material in the bed layer where suspension is impossible for fluid dynamic reasons. Sediment grains in the bed layer are not vertically supported by flow but rest on the bed almost continuously while sliding, rolling and jumping along.

BIFURCATION RATIO

The ratio of number of stream segments of a given order to the number of stream segments of the next higher order.

BRACKISH WATER

Water containing salts at a concentration significantly less than that of sea water. The concentration of total dissolved salts is usually in the range 100 10000 mg per liters.

BUBBLE GAUGE

In this gauge, compressed air or gas is made to bleed out at very small rate through an outlet placed at the bottom of the river. A pressure gauge measure gas pressure which is equal to the water column above the outlet. Small change in water surface elevation is felt as change in pressure.

CANOPY INTERCEPTION

Rainfall retained on standing vegetation and evaporated without dripping off or running down the stems or trunks.

CAPILLARY RISE

The maximum height to which water will rise due to capillary forces above the water table.

CAPILLARY WATER

Water held in the soil above the water table by capillary action.

CATCHMENT AREA

The drainage basin of a river is called its catchment area. The catchment area includes all points that lie above the elevation of the gauging station and within the topographic divide that separate adjacent watersheds.

CATCHMENT ORDER

A catchment order is described depending on the stream order at the outlet or gauging station. A stream of any order has two or more tributaries of the next lower order.

CHANNEL

A natural or artificial clearly distinguished waterway which periodically or continuously contains moving water or which forms a connecting link between two bodies of water.

CHANNEL DETENTION

Volume of water which can be temporarily stored in channels during flood periods.

CHANNEL PRECIPITATION

Precipitation which falls directly on the water surfaces of lakes and streams.

CHANNEL ROUTING

The routing of a flood wave in a stream when the only storage is the valley storage.

CHANNEL STORAGE

The quantity of water within the main channel.

CHEMICAL OXYGEN DEMAND

Mass concentration of oxygen equivalent to the amount of a specified oxidants consumed by dissolved or suspended matter when a water sample is treated with that oxidant under defined conditions.

CLASS-A PAN

It is an instrument used for estimation of evaporation. It is a standard par of 1210 mm diameter and 255 mm depth used by US Weather Bureau.

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CLIMATE

The sum total of all atmospheric or meteorological influences principally temperature, moisture, wind, pressure and evaporation which combine to characterise a region and give it individuality by influencing the nature of its land forms, soil, vegetation and land use.

CLIMATE SHELTER

Climate station instruments that must be protected from condensation, precipitation and radiation are house in climate shelters or screens. The typical shelter is white, double-topped, with louvered sides to permit free circulation of air.

CLIMATIC CYCLE

Actual or supposed recurrences of such weather phenomena as wet and dry years, hot and cold years, at more or less regular intervals, in response to long range terrestrial and solar influences.

CLOUD BURST

Rain storm of high intensity and of a relatively short duration usually over a relatively small area.

CLOUD SEEDING

In cloud seeding, the clouds which contain appreciable amount of liquid water under colloidally stable conditions, are made colloidally unstable by the addition of dry ice, silver iodide or other chemical agents so that a certain part of this otherwise unavailable water will reach the ground as precipitation.

COLLUVIUM

Soil which is eroded from sloping land may become lodged at fence rows and vegetated areas or deposited below breaks in slopes in the form of colluvium.

COLORADO SUNKEN PAN

This pan, used to estimate evaporation is 920 mm square and 460 mm deep and buried into the ground within 100 mm of the top with the advantage that radiation and aerodynamic characteristics are similar to those of a lake.

COMPOUND HYDROGRAPH

The hydrograph of an intermittent storm when the flow on account of one substorm continues during the next substorm.

CONCENTRATION CURVE

The rising limb of a hydrograph which represents the increase in discharge due to gradual building up of storage in channels and over the catchment surface.

CONCENTRIC RING INFILTROMETER

This is an instrument used for the measurement of infiltration. It consists of two concentric rings which are inserted into the ground and water is maintained on the soil surface to a common fixed level.

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 the basin.

CONDENSATION NUCLEI

Condensation of water vapour into cloud droplets takes place on certain hygroscopic particles which are commonly called condensation nuclei.

CONE OF DEPRESSION

A downward curve showing the variation of draw down with distance from the well describes a conic shape in three dimensions called cone of depression.

CONFIDENCE LIMITS

Values which form the lower and upper limits to the confidence interval.

CONFINED AQUIFER

A confined aquifer, also known as artesian or pressure aquifer, is an aquifer which is confined between two impervious beds such as aquicludes or aquifuges and in which groundwater is confined under pressure greater than atmosphere.

CONFLUENCE

Joining, or the place of junction, of two or more streams.

CONJUNCTIVE USE

Conjunctive use involves the coordinated and planned operation of both surface water and ground water resources to meet water requirements in a manner whereby water is conserved. The basic difference between the usual surface water development with it's associated ground water development and a conjunctive operation of surface water and ground water resource is that the separate firm yields of the former can be replaced by larger and more economic joint, yields of the later.

CONSUMPTIVE USE

The quantity of water used by vegetative growth of a given area in transpiration or building of plant tissue and that evaporated from the soil or from intercepted precipitation on the area in any specified time. It is expressed in water depth unit or depth-area units per unit area and for specified periods, such as days, months and seasons.

CONTAMINATION

Introduction of any undesirable substance, normally not present, in water, e.g. micro-organisms, chemicals, waste or sewage, which renders the water unfit for its intended use.

CONTROL SECTION

Reach of a stream channel in which there exists a unique stage- discharge relationship.

CONVECTIVE PRECIPITATION

Precipitation resulting from the upward movement of air that is warmer than its surrounding. It is generally of a showery nature with rapid changes of intensities.

CORRELATION COEFFICIENT

Measure of the inter-dependence between two variates.

COVARIANCE

First product moment of two variates about their mean values.

CREST SEGMENT

It is that part of the hydrograph which contains the peak flow. Peak flow occurs when the runoff from various parts of the catchment simultaneously contribute the maximum amount of flow at the basin outlet.

CREST STAGE INDICATOR

This indicator is used to delineate the peak stage of a flood at points other than at a hydrometric station. Such data are valuable in the establishment of flood profiles.

CRITICAL STORM PERIOD

The duration of that storm which causes the greatest peak at a station in a drainage basin.

CROP COEFFICIENT

It is an empirical coefficient used in Blaney-Criddle formula for calculating the potential evapotranspiration for a particular crop. It is different for different crops.

CROSS SECTION

Section of a stream at right angles to the main (average) direction of flow.

CRYOLOGY

The science of ice in all its forms such as snow, ice and hall.

CUP-TYPE CURRENT METER

Current meter whose rotor is composed of a wheel fitted with cups and turning on a vertical axis.

CURRENT METER

It is the most commonly used instrument in hydrometry for measuring the velocity at a point in the flow cross-section of a river.

CYCLONIC PRECIPITATION

The precipitation associated with the passage of depressions of cyclones.

DAM

Barrier constructed across a valley for impounding water or creating a reservoir.

DARCY'S LAW

It states that the rate of flow per unit area of an aquifer is proportional to the gradient of the potential head measured in the direction of flow.

DATA BASE

Comprehensive set of related data files for a specific application, usually on a direct access storage device.

DATA PROCESSING

Handling of observational data until they are in a form ready to be used for a specific purpose.

DEAD STORAGE

Storage volume which can not be released under normal conditions.

DEAD WATER

Water in a state of slow or no circulation, usually leading to an oxygen deficit.

DEEP PERCOLATION

Water which percolates below the root zone and towards a deeper water table.

DEGREE DAY

It is a unit expressing the amount of heat in term of the departure of one degree per day in the daily mean temperature from an adopted reference temperature. The number of degree days for an individual day is the actual departure of the mean temperature from the standard. Standard temperature is usually taken as 0 C to 32 F.

DENSITY CURRENT

It is defined as the gravitational flow of one fluid under another fluid of approximately equal density. Density currents thus separate the turbid water from the clear water and make the turbid water flow along the river bottom in the vicinity of the dam.

DEPENDABLE FLOW

In the flow-duration curve, the ordinate Qp at any percentage probability Pp represents the flow magnitude in an average year that can be expected to be equalled or exceeded Pp % of time is called Pp % dependable flow. For perennial streams, Q is a finite value while for ephemeral stream Q is zero.

DEPLETION CURVE

The depletion curve extends from the point of inflection at the end of the crest segment to the commencement of natural groundwater flow in a hydrograph. It represents the withdrawal of water from the storage built up in the basin during earlier phases of the storm.

DEPRESSION STORAGE

Also called pocket storage, the volume of water usually expressed as depth on the drainage area which is required to fill natural depressions, large or small, to their overflow levels.

DEPTH OF RUNOFF

The total runoff from a drainage area or basin, divided by the area, expressed in either units of depth or units of volume per unit area of the basin.

DEPTH-AREA RELATIONSHIP

It is a relation which is expressed between progressively decreasing average depth of rainfall of a given duration over a progressively increasing area from centre of maximum precipitation of a storm outward to its edges in an exponential fashion.

DEPTH-AREA-DURATION CURVE

A curve which graphically indicates the precipitation amounts for various areas and durations for a particular rainstorm.

DETENTION RESERVOIR

Flood-control reservoir with uncontrolled outlets.

DETERMINISTIC HYDROLOGY

Method of analysis of hydrological processes, using a deterministic approach to investigate the responses of hydrological systems in terms of various parameters.

DEW

Deposit of water drops on objects at or near the ground, produced by the condensation of water vapour from the surrounding clear air.

DIFFUSION WELL

Recharge well that is sunk only into the unsaturated zone distinguished from an injection well.

DISCHARGE COEFFICIENT

Ratio of the observed or actual discharge to the theoretically computed discharge.

DOWNSTREAM

In the direction of the current in a river or stream.

DRAINAGE

Removal of surface water or groundwater from a given area by gravity or by pumping.

DRAINAGE BASIN

The area from which a lake, stream or waterway and reservoir receives surface flow which originates as precipitation.

DRAINAGE COEFFICIENT 

Drainage coefficient is the water depth drained from an area in one day. These coefficients enable the designer to compare various drainage methods.

DRAINAGE DENSITY 

It is defined as the ratio of the total channel length to the total drainage area. A large drainage density creates situation conducive for quick disposal of runoff which is reflected in pronounced peaked discharge.

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DRAWDOWN

The drawdown at a given point is the distance by which water level is lowered.

DRAWDOWN CURVE

A drawdown curve shows the variation of drawdown with distance from the well. In three dimensions, the drawdown curve describes a conic shape known as the cone of depression.

DRIZZLE

A fine sprinkle of numerous water droplets of size less than 0.5 mm and intensity less than 1mm/h. The drops are so small that they appear to float in the air.

DROUGHT

In general an extended period of dry weather or a period of deficient rainfall that may extend over an indefinite number of days. Without any set quantitative standard by which to determine the degree of deficiency needed to constitute a drought. Qualitatively, it may be defined by its effects, as a dry period sufficient in length and severity to cause at least partial crop failure.

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 is agricultural drought must relate to the cumulative effects of either an absolute or an abnormal transpiration deficit.

DRY YEAR

Year of drought in which precipitation or streamflow is significantly less than normal.

DRY-WEATHER FLOW

The flow of water in a stream during the non-rainy season. It is primarily made of water which seeps from the ground. However, water supplied by snowmelt or regulated water released from a storage also become a part of it.

DURATION CURVE

A graph representing the time during which the value of a given parameter e.g. water level, piezometric head, discharge, concentration of dissolved solids, is equalled or exceeded regardless of continuity in time.

ECHO-DEPTH RECORDER

An instrument by which the depth of water is determined by measuring the time taken by a sound signal to travel to the bottom and return.

EFFECTIVE RAINFALL

Part of the rain that appears as runoff in the stream.

EFFLUENT STREAM

A stream or stretch of stream which receives water from groundwater in the zone of saturation. The water surface of such a stream stands at a lower level than the water table or piezometric surface of the ground water body from which it receives water. Also a stream flowing out of another stream or out of a lake.

ELECTROMAGNETIC FLOWMETER

It is an instrument for measuring discharge in a stream. It is based on the principle that an e.m.f. is induced in the conductor (water in the present case) when it cuts a normal magnetic field.

ELONGATION RATIO

It is defined as the ratio of diameter of a circle of the same area as the basin to the maximum basin length. This ratio runs between 0.6 to 1.0 over a wide variety of climatic and geologic types.

EMERGENCY SPILLWAY

Auxiliary spillway used in the event of floods exceeding the capacity of the main spillway.

EMPIRICAL FLOOD FORMULA

Formula expressing peak discharge as a function of catchment area and other factors.

ENERGY BUDGET METHOD 

It is an analytical method for the determination of evaporation. It is a measurement of continuity of flow of energy. Energy available for evaporation is determined by considering the incoming energy, outgoing energy and energy stored in the water body over a known time interval.

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.

ENVELOP CURVES 

  1. A smooth curve which envelops all the plotted points representing maximum recorded flood peaks and volumes for hydrometeorologically comparable areas.
  2. A smooth curve covering either all peak values or all trough values of certain quantities e.g. rainfall, runoff etc. plotted against other factors such as area and time. In general, none of the peak values goes above the curve in former case, called the maximum envelop and non of the minimum points fall below in the later case called the minimum

    envelop.

EPHEMERAL STREAM 

A stream or a portion of a stream which flows only in direct response to precipitation.

EQUILIBRIUM EQUATION 

Equilibrium equation is used to determine the hydraulic conductivity or the transmissivity of a confined aquifer from a pumped well by measuring draw downs in two observation wells at different distances from a well pumped at constant rate.

EROSION

It is defined as the wearing away of land by water, wind, ice or gravity.

EUTROPHIC LAKE

Lake characterized by a great amount of nutrients and biogenic matters and by highly developed phytoplankton is summer.

EVAPORATION

The process by which water is changed from the liquid state to the gaseous state below the boiling point through the transfer of heat energy.

EVAPORATION OPPORTUNITY

Ratio of actual rate of evaporation from land or water surface in contact with the atmosphere to the potential rate of evaporation under existing atmospheric conditions.

EVAPORATION PAN

An experimental tank used to determine the amount of evaporation from the surface of water under measured or observed climatic conditions.

EVAPOTRANSPIRATION 

An instrument for measuring evaporation.

EVAPOTRANSPIRATION 

It is the process by which water moves from the soil to the atmosphere. It consists of transpiration, the movement of water through the plant to the atmosphere & evaporation, the movement of water vapour from soil and vegetative surfaces. Thus the entire surface as well as subsurface water which is released from a basin into the atmosphere by process of evaporation and transpiration is generally known as evapotranspiration.

EXCEEDENCE INTERVAL

The exceedence interval is defined as the average number of years between the occurrence of an event and a greater event.

EXPERIMENTAL BASIN

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 physical upper limit for a given duration over a particular basin.

EXTREME VALUE DISTRIBUTION

Fisher Tippet Type I External distribution applied by Gumbel to the annual maximum flood series and by others to rainfall series.

FAIR-WEATHER RUNOFF

Also called base flow, it is composed of ground water runoff and delayed subsurface runoff.

FIELD CAPACITY

The amount of water held in the soil after the excess gravitational water has drained away and after the rate of downward movement of water has materially decreased. Essentially the same as 'specific retention', a more general term used in studies of ground water which covers all types of strata. Furthermore, field capacity is usually expressed as a percentage of weight while specific retention is generally given as percentage by volume.

FIELD EFFICIENCY

The percentage of the total volume of water delivered to the field that is finally consumed by evapotranspiration.

FIRN LINE

Boundary which, at the Earth's surface, separates zone of accumulation of a glacier from the zone of ablation.

FLASH FLOOD

A flood of short duration and abrupt rise with a relatively high peak rate of flow, usually resulting form a high intensity of rainfall.

FLOAT GAUGE

This is most common type of automatic float operated stage recorder. Float operating in a stilling well is balanced by means of a counter weight over the pulley of a recorder. Displacement of float is traversed on a chart continuously and stage versus time plot is made.

FLOATING PAN

It is US Geological Survey evaporation pan set afloat in a lake with a view to simulate the characteristics of a large body of water. Water level in the pan is kept at the same level as the lake, leaving a ring of 75 mm.

FLOOD

The flow pattern in a stream, constituting a distinct progressive rise culminating in a peak or summit together with the recession that follows the crest.

FLOOD ABATEMENT

Any measure taken outside of stream channel with the effect of educing the crest of flood flows or changing the debris load for a flood event.

FLOOD ABSORPTION

The increase in storage of water in a reservoir, lake, valley or channel resulting in a reduction of streamflow.

FLOOD CONTROL

Flood control means flood damage prevention or reduction together with the protection of economic development and protection of life.

FLOOD DAMAGE

The destruction or impairment, partial or complete, of human and animal lives, property, goods, services, flora and fauna or of health etc., resulting from the action of floods water and the silt and debris they carry. It includes direct and indirect losses.

FLOOD DISCHARGE

It is the discharge passing at a particular site during a flood event.

FLOOD FORECASTING

Prediction 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 or snowmelt so that people could be warned well in advance and life and movable goods could be saved to a large extent.

FLOOD FREQUENCY

  1. The number of times a flood of a given magnitude is likely to be equaled or exceeded over a period of years on the average.
  2. The number of years in which a flood of a given magnitude is likely to be equaled or exceeded once on the average over a period of years.

FLOOD MARKS

The trace of any kind left on the banks or flood plain by a flood which may be used, after the flood, to determine the highest level attained by the water surface during the flood.

FLOOD PLAIN

Land adjoining the channel which is inundated only during floods.

FLOOD PROOFING

Combination of emergency equation and structural adjustments for modifying a given property and thus reducing flood losses.

FLOOD ROUTING

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

FLOOD SERIES

A List of flood events which occurred during a specified period of time.

FLOOD STAGE

The elevation of water surface during a flood relative to a datum, local or national.

FLOOD WAY

The channel of a river or stream and those portions of the flood plains adjoining the channel, which are required to carry and discharge the flood water.

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.

FLOW METER

It is an instrument used for measuring the rate of flow in a conduit or open channel.

FLOW-MASS CURVE

A graph of the cumulative values of discharge of runoff, generally as ordinate, plotted against time as abscissa. The curve has many useful applications such as the determination of reservoir capacity, operations procedure and flood routing.

FOREST HYDROLOGY

It is the science of water related phenomena that are influenced by forest cover. It is an interdisciplinary science, the union of forestry and hydrology.

FORMATION LOSS

It is the head drop required to cause laminar flow through the porous media.

FREEBOARD

Vertical distance between the normal maximum level of the surface of liquid in a conduit, reservoir, tank, canal, etc. and the top of the sides of the retaining structure.

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, storage, rainfall, water quality, waves.

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 (DST).

FRESH WATER

Water neither salty nor bitter to the taste and in general, chemically suitable for human consumption (having a low content in dissolved solids).

FRONTAL PRECIPITATION

Precipitation caused by the expansion of air on ascent along or near a frontal surface.

FROST

A light feathery deposit of ice caused by condensation of water vapour directly in the crystalline form, on terrestrial objects whose temperature is below freezing, the process being the same by which dew is formed, except that the later occurs only when the temperature of the bedewed object is above freezing.

GAINING STREAM

Stream fed by groundwater.

GAUGE

An instrument used for measuring depth of water.

GAUGING SITE

Location on a stream where measurements of water level and discharge are regularly made.

GEOHYDROLOGY

Branch of hydrology related to subsurface and subterranean water.

GLACIER

Body of land ice formed from recrystallised snow accumulated on the ground; may form where annual accretion of snow is greater than ablation by runoff and evaporation. There are two broad classes

  1. Ice streams which form in mountain valleys and move downslope under gravity.
  2. Ice cap which cover large land masses and spread out radially because of great pressures built up by their weight.

GLACIOMETER

An instrument used for measuring glacial motion.

GRAVITATIONAL WATER

Water in the unsaturated zone which moves under the influence of gravity.

GLAZE

Also called freezing rain, it is reported when rain falls into a cold layer of air and freezes when it strikes objects on the ground.

GROUND WATER

Water in a saturated zone of geologic stratum.

GROUND WATER BASIN

It may be defined as a hydrogeologic unit containing one large aquifer or several connected and interrelated aquifers. It implies an area containing a groundwater reservoir capable of furnishing a substantial water supply.

GROUNDWATER RECESSION

Decreasing rate of groundwater discharge to surface water bodies during periods of no recharge, connected to the depletion of ground water storage, and expressed by groundwater recessive curve.

GROUNDWATER RECHARGE

Process by which water is added from outside to the zone of saturation of an aquifer, either directly into a formation, or indirectly by way another formation.

GROUND WATER RUNOFF

That part of the runoff which consists of water that has passed into the earth and has entered the zone of saturation and has later been discharged into a water body.

GULLY

A channel or miniature valley formed as a result of erosion and caused by concentrated but intermittent flow of water usually during or immediately after heavy rains. The channel is deep enough to interior with tillage operations. Gully may be dendritic or branching or linear (long, narrow and of uniform width). Gully may be U, V or W shaped.

GUMBEL'S DISTRIBUTION

It is one of the most widely used probability distribution functions for extreme values in hydrologic and meteorologic studies for prediction of flood peaks, maximum rainfall, maximum wind speed etc.

HAIL

Small, roughly spherical lumps of approximately concentric shells of clear ice and compact snow usually ranging from 5 to 10 mm or more in diameter which fall either separately or agglomerated into larger irregular lumps precipitated during thunder storms.

HARDNESS OF WATER

That property of water, due mainly to bicarbonates, chlorides anulphates of calcium and magnesium, which prevents the production of abundant lather with soap.

HEAD LOSS

Decrease of total head, expressed in units of height, due to energy dissipation.

HEADWATERS

Streams from the sources of a river.

HEAVY WATER

Water enriched in water molecules containing heavier (stable and radioactive) isotopes of hydrogen (Deuterium, Tritium) and Oxygen 18.

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. Data is collected from natural phenomena that can be observed only once and then will not occur again.

HURRICANE

These are very intense low pressure systems with winds in excess of 75 m.p.h. It is an intense cyclone of tropical origin and of relatively small horizontal dimensions (110-300 miles). Storms of this type are called cyclones in India and typhoons in the far east. Hurricanes form where sea surface temperature is above 27 C in general; hurricanes are accompanied by torrential rains.

HYDRAULIC CONDUCTIVITY

It is a constant that serves as a measure of permeability of the porous medium. A medium has a unit hydraulic conductivity if it will transmit in unit time a unit volume of ground water at the prevailing kinematic viscosity through a cross section of unit area measured at right angles to the direction of flow, under a unit hydraulic gradient.

HYDRAULIC GRADIENT

  1. In a closed conduit the slope of the hydraulic grade line.
  2. In open channels: the slope of the water surface.
  3. In porous media: measure of the decrease in head per unit distance in the direction of flow.

HYDROGEOLOGY

That branch of geology relating to effect of water on earth.

HYDROGRAPH

A graph showing the stage, volume of flow, velocity, sediment concentration or sediment discharge or some other feature of flowing water with respect to time at a given place. For example, a graph showing the discharge of a stream as ordinate against time as abscissa is called a discharge hydrograph.

HYDROLOGIC CYCLE

A phenomena relating to circulation of water from the sea, through the atmosphere to the land, and thence, often with many delays, back to the sea or ocean through various stages and processes, for example, precipitation, interception, runoff, infiltration, percolation, ground water storage, evaporation and transpiration. Also the many short circuits of the water that is returned to the atmosphere without reaching the sea.

HYDROLOGIC EQUATION

The water inventory equation (inflow = o0utflow + change in storage) which expresses the basic principle that during a given time interval the total inflow to an area must equal the total outflow plus the net change in storage.

HYDROLOGIC FAILURE

Failures due to improper assessment of hydrological factors such as overtopping and consequent failure of an earthen dam due to inadequate spillway capacity, failure of bridges and culverts due to excess flood flow, inability of a large reservoir to fill up due to overestimation of stream flow etc.

HYDROLOGIC NETWORK

It is a network of stations for measuring hydrologic variables such as rainfall and river stage stations. Adequacy of hydrologic network is dependent upon the character of the drainage basin and the critical need of the regulating system.

HYDROMETEOROLOGY

Study of the atmospheric and land phases of the hydrological cycle, with emphasis on the interrelationships involved.

HYDROLOGY

The applied science concerned with the water of the earth in all its stages their occurrences, distribution and circulation through the unending hydrologic cycle of precipitation, runoff, streamflow, infiltration, storage, evaporation and reprecipitation. It is concerned with the physical, chemical and physiological reactions of water with the rest of the earth and its relation to the life of the earth.

HYDROMETRY

The measurement and analysis of the flow of water as well as the measurement of the specific gravity of water or suspensions of finely divided solids in water.

HYDROMETRY STATION

These are the stations where measurement of discharge in a stream is made at particular points. These must be sited adequately in the catchment area so that the water potential of the area can be assessed as accurately as possible.

HYETOGRAPH

A bar graph of average rainfall, rainfall excess rates or volumes over specified areas during successive units of time during a storm.

HYSTERESIS

The phenomenon that soil moisture tension at a given moisture content depends on the past history of wetting and drying cycles (of soil moisture). The variability of the stage-discharge relation at a gauging station subject to variable slope where, for the same gauge height, the discharge on the rising stage is greater than on the falling stage.

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ISI STANDARD PAN

This pan evaporimeter specified by IS, also known as modified class A pan, consists of a pan 1220 mm in diameter, 255 mm deep and made of copper sheet 0.9 mm thick. The top is fully covered with a hexagonal wire mesh to protect water from birds.

ICE

Solid form of water in nature formed by the freezing of water, the condensation of atmospheric water vapour directly into crystals, the compaction of snow with or without the motion of a glacier or the impregnation of porous snow masses with water which subsequently freezes.

ICE CAP

Perennial cover of ice and snow over an extensive are a of land or sea.

INDUCED RECHARGE

It is water entering the ground from a surface water source as a result of withdrawal of ground water adjacent to the source. Induced recharge can furnish water free of organic matter and pathogenic bacteria.

INFILTRATION

The entrance of water into the soil or other porous material through the interstices or pores of a soil or other porous medium.

INFILTRATION CAPACITY

Maximum rate at which specified soil in given condition can absorb water.

INFILTRATION CAPACITY CURVE

A curve showing what the infiltration rate would be at any period during a specified storm if the rainfall intensity were to equal or exceed the capacity at any instant.

INFILTRATION INDEX

Infiltration index, in general, expresses infiltration at an average rate throughout the storm.

INFILTROMETER

A device by which the rate and amount of water infiltrating into the soil is determined.

INFLECTION POINT

It is a point on the hydrograph which represents start of withdrawal of water from the storage built up in the basin during the earlier phases of the hydrograph. It is the starting point of the recession limb and end point of crest segment. Point of inflection represents the condition of maximum storage.

INFLUENT STREAM

A stream or stretch of stream which contributes water to the zone of saturation. The water surface of such stream stands at a highest level than the water table or piezometric surface of the ground water body to which it contributes water.

INITIAL ABSTRACTION

Maximum amount of rainfall that can be absorbed under specific conditions without producing runoff. Also referred to as initial losses, it is sum of interception and depression storage.

INITIAL DETENTION

That part of precipitation which does not appear either as infiltration or as surface runoff during period of precipitation or immediately thereafter, includes interception by vegetal cover, depression storage and evaporation during precipitation, does not include surface detention.

INSTANTANEOUS UNIT HYDROGRAPH

When the unit duration of the rainfall excess is infinitesimally small, the resulting hydrograph is known as instantaneous unit hydrograph.

INTENSITY-DURATION-FREQUENCY CURVE

The intensity of storms decreases with the increase in storage duration. Further a storm of any given duration will have a larger intensity if its return period is large. The curves showing the interdependence between the intensity, duration and return period are commonly referred to as intensity duration frequency curves.

INTERCEPTION

The process by which precipitation is caught and held by foliage twigs and branches of the trees, shrubs and other vegetation, and lost by evaporation, never reaching the surface of the ground.

INTERCEPTOMETER

Throughfall gauges are called interceptometers, evidently because of their function in estimating canopy interception.

INTERFLOW

A part of the precipitation that infilters into the ground moves laterally through upper crusts of the soil and returns to the surface at some location away from the point of entry into the soil. This component of runoff is known as interflow. The amount of interflow depends on the geological conditions of the catchment.

INTERMITTENT STREAM

Stream which flows during a season. It has limited contribution from the ground water. During the wet season, water table is above the stream bed and there is contribution of base flow to the stream flow. During dry season, water table drops below stream bed and stream dries out.

IRRIGATION REQUIREMENT

Quantity of water, exclusive of precipitation, that is required for optimal crop production.

ISOCHRONE

It is a line on a map of a catchment joining points having equal time of travel to the outlet of the catchment.

ISOHYET

A line drawn on a map passing through places having equal amount of rainfall recorded during the same period at these places (these lines are drawn after giving consideration to the topography of the region).

ISOPLETH

Lines on a map through places having equal depths of evapotranspiration.

ISOTHERM

Lines joining points of equal temperatures.

JUVENILE WATER

It is water derived from magma or molten mass of igneous rocks during their crystallisation or from lava flows in the form of stream. It is water that has come to the earth surface from great depths for the first time.

KARSTIC RIVER

River which originates from a karstic spring or flows in a karstic region that is, in a region having carbonaceous rocks as CaCo3.

LAG TIME

It is the time interval from the centre of mass of rainfall to the centre of mass of hydrograph.

LAKE

An extensive sheet of water, bounded by land, in a hollow of the earth surface. It is an inland body of water which is formed more due to glacial erosion than due to any other agency.

LAND PAN

It is a pan placed on the ground for the purpose of measuring evaporation.

LARGE WATERSHED

A large watershed is one in which the effect of channel flow or basin storage is dominating rather than the effect of overland flow. It has very less sensitivity to high intensity rainfall of short duration.

LATERAL INFLOW

Inflow of water to a river, lake or reservoir along any reach from the part of the catchment adjacent to the reach.

LEACHING REQUIREMENT

Water required for the removal of salts from the upper soil by relatively salt-free water.

LEAKY AQUIFER

Aquifers which are overlain or underlain by semi-permeable strata are referred to as leaky aquifers. Such aquifers are confined in the sense that pumping does not dewater the aquifers, but a significant portion of the yield may be derived by vertical leakage through the confining formations into the aquifers.

LIMNOLOGY

That branch of hydrology relating to water of lakes and ponds.

LINEAR CHANNEL

An imaginary channel in which the rating curve between discharge and area is a straight line such that at any point, the velocity of flow is constant for all discharges, but may vary from point to point along the channel.

LINEAR RESERVOIR

An imaginary reservoir in which the storage S is directly proportional to the outflow Q.

LIVE STORAGE

Volume or cubic capacity of a lake or reservoir between the maximum and minimum operating levels.

LOAD-CARRYING CAPACITY

Maximum sediment quantity per unit time which can be transported by specified flow in a channel.

LOCAL INFLOW

Water entering a stream between two gauging stations.

LOG-NORMAL DISTRIBUTION

This is a transformed normal distribution in which the variate is replaced by its logarithmic value. This is a screw distribution of unlimited range in both directions.

LOG PEARSON TYPE III DISTRIBUTION

This is one of the commonly used frequency distribution functions for the prediction of extreme flood values. In this distribution, the variate is first transformed into logarithmic form and the transformed data is then analysed.

LOSING STREAM

Stream which contributes water to the groundwater by infiltration.

LOST RIVER

The term is applied to a stream that disappears completely underground in a limestone terrain.

LYSIMETER

Tanks with pervious bottom commonly used for determining evapotranspiration of crops and natural vegetation by growing the plants in them and measuring the loss of water necessary to maintain the growth satisfactorily.

MASS CURVE

A curve with values of cumulative rainfall or runoff etc. plotted against time.

MASS TRANSFER METHOD

This is one of the analytical methods for the determination of lake evaporation. This method is based on theories of turbulent mass transfer in boundary layer to calculate the mass water vapour transfer from the surface to the surrounding atmosphere.

MAXIMUM INTENSITY OF FLOOD

Also called momentary flood peak, it is the maximum instantaneous rate of flow during a period.

MAXIMUM KNOWN FLOOD

The highest flood which has occurred within the memory of the inhabitants of a region.

MAXIMUM POSSIBLE FLOOD

It is the greatest flood to be expected assuming complete coincidence of all factors that would produce heaviest rainfall and maximum runoff.

MAXIMUM POSSIBLE PRECIPITATION

The maximum amount of precipitation that can theoretically occur for a certain duration in a drainage area or basin during the present climatic era.

MAXIMUM PROBABLE FLOOD

The extreme flood that is physically possible in a region as a result of severe most combinations, including rare combinations of meteorological and hydrological factors.

MEAN ANNUAL EVAPORATION

The mean value in depth units of evaporation, the period of observation being of adequate duration to secure approximate constancy.

MEAN ANNUAL FLOOD

It is defined as a flood having a recurrence interval of 2.33 years. This is based on Gumbel's distribution which has the property that gives T = 2.33 years for the average of the annual series when N is very large.

MEAN ANNUAL PRECIPITATION

The mean of annual amount of precipitation observed over a period which is sufficiently (say 30 years or more) to produce a fairly constant mean value.

MEAN MONTHLY RUNOFF

The value of the monthly volume of water discharged by the stream draining the area, the period of observation being sufficiently long to secure a fair mean.

MEASURING SECTION

Cross-section of an open channel in which measurements of depth and velocity are made.

MEDIAN

Middle value of the variate which divides frequencies in a distribution into two equal portions.

METEOROLOGY

Science of the atmosphere.

MINIMUM ANNUAL FLOW

The smallest of the annual flows during the period of record.

MODEL

Model with different scale in different directions.

MOISTURE ADJUSTMENT FACTOR

It is the ratio of the maximum total moisture in an atmospheric column of unit cross section in the region to the total moisture in a similar column that occurred during the storm.

MOISTURE TENSION

Under-pressure to which water must be subjected in order to be in hydraulic equilibrium, through a porous permeable wall or membrane, with the water of the soil, usually expressed in cm of water or mm of mercury.

MONSOON

It is a wind system with an annual oscillation, blowing from oceans to continents in summer and in the reverse direction in winter. These oscillations, which are in response to the annual heating and cooling of the underlying surface, are quite general, though the Indian monsoon is most widely known, mainly because of excessive dryness during the winter and the equally excessive rainfall during the summer season.

MOVING AVERAGE ANALYSIS

It is a method of flood routing through a channel. The method involves the concept of wedge and prism storage.

MULTIPLE CORRELATION

Analysis of the interdependence of more than two variables.

MULTI-PURPOSE PROJECT

Project designed, constructed and operated to serve more than purpose, e.g. flood control, hydroelectric power, navigation, irrigation, fisheries, water supply, recreation.

MUSKINGUM METHOD

It is a method of flood routing through a channel. The method involves the concept of wedge and prism storage.

NATURAL CONTROL

Reach of a stream channel where natural conditions exist that the water level upstream a stable index of the discharge.

NET STORM RAIN

Portion of rainfall during a storm which reaches a stream channel as direct runoff.

NIPHER SHIELD

Wind shield for precipitation gauges, shaped like an inverted cone, with base of the cone level with the lip of the gauge.

NON-RECORDING RAINGAUGE

These gauges do not produce a continuous plot of rainfall against time but measure only the total depth of precipitation due to a particular storm.

NON-UNIFORM FLOW

Flow in which the velocity vector is not constant along every streamline.

NORMAL DISTRIBUTION

Mathematically defined, symmetrical, bell-shaped, continuous probability distribution traditionally assumed to represent random errors.

NORMAL RAINFALL

Evan value of rainfall taken over a period of such length that the mean over any longer period does not significantly effect the value obtained. It is used as a standard of comparison. It is generally taken as the average value of rainfall at a particular date, month or year over a specified 30 year period.

NORMAL RATIO METHOD

It is a method of estimating the missing data of precipitation at a station by weighing the precipitation at various neighbouring stations by the ratios of normal annual precipitation. This method is used when the normal annual precipitation of the station having missing record differs from the normal annual precipitation of the stations having known record by more than 10%.

NORTH-EAST MONSOON

It is north easterly flow of air that picks up moisture in the Bay of Bengal and this air mass then strikes the east coast of the southern peninsula (Tamilnadu) and causes rainfall.

N-YEAR FLOOD

A flood which has a probability of being equaled or exceeded once in N years or has one chance in N of occurring in any one year.

OPEN CHANNEL FLOW

Flowing water having its surface exposed to the atmosphere.

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.

OPTIMAL YIELD

Amount of water which can be withdrawn annually from an aquifer or from a basin according to some pre-determined criterion of optimal exploitation.

OROGRAPHIC PRECIPITATION

Precipitation caused by dynamic cooling of air as an air current rises over a mountain barrier.

OUTFALL

Lowest point on the boundary of a drainage system.

OVER BANK FLOW

The portion of stream flows which exceed the carrying capacity of the normal channel and overflow the adjoining flood plains.

OVERDRAFT

Amount of water withdrawn from a water resources system in excess of the safe yield.

OVERLAND FLOW

The flow of water over the land surface towards stream channels before it becomes channelised is called overland flow.

PAN COEFFICIENT

It is the ratio of actual evaporation to the observed evaporation in evaporation pan. Evaporation pans do not give exact evaporation values because

  1. they differ in the heat storing capacity and heat transfer from the sides and bottom
  2. the height of rim in the pan affects wind action on the surface
  3. heat transfer characteristics of the pan material is different from that of the reservoir.

PARTIAL DURATION SERIES

Also called partial series, it is a series of data which are so selected that their magnitude is greater than a certain base value.

PARTIALLY PENETRATING WELL

Well in which the length of water entry is less than the thickness of the saturated aquifer which it penetrates.

PEAK DISCHARGE

Maximum instantaneous discharge of a given hydrograph.

PEARSON DISTRIBUTION

Group of probability distribution of varying skewness and other properties which were proposed by karl pearson and which are sometimes fitted to hydrological data.

PELLICULAR WATER

Water in the zone of aeration is held against gravity most of the time, in capillary interstices and in thin films over surfaces of the grains due to strong molecular attraction of the solid particles at the solid/liquid interface.

PERCENT RUNOFF

The amount of runoff expressed as percentage of total rainfall on a given area.

PERCENT NORMAL METHOD

It is a method of finding average precipitation in mountaineous areas where arithmetic means and Thiessen weights can not be applied accurately. In this method, storm precipitation values at each station can be expressed in percent of its annual normal, and these percentage values are averaged for the basin. The basin normal annual precipitation multiplied by this storm percent value provides an average storm precipitation. Use of this percent normal method reduces the need for a consistent reporting network.

PERCHED WATER TABLE

Sometimes a lens or localised patch of impervious strata can occur inside an unconfined aquifer in such a way that it retains water table above the general water table. Such a water table retained around the impervious material is known as perched water table.

PERCOLATION

It is flow through a porous substance.

PERENNIAL STREAM

A perennial stream is one which always carries some flow. There is considerable amount of ground water flow throughout the year. Even during dry seasons the water table will be above the bed of the stream.

PERMANENT CONTROL

If the stage discharge relationship for a gauging section is constant and does not change with time, the control is said to be permanent control. A majority of streams and rivers, especially non alluvial exhibit permanent control.

PERMANENT WILTING PERCENTAGE

Moisture content of the soil at which the leaves of plants growing in that soil become permanently wilted.

PERMEABILITY

Permeability of a rock or soils defines its ability to transmit a fluid. This is property of the medium only and is independent of fluid properties.

PERMEABILITY COEFFICIENT

It is defined as the rate of discharge per unit cross sectional area of a porous medium under unit hydraulic gradient.

pH

Absolute value of the decimal logarithm of the hydrogen-ion concentration (activity). Used as an indicator of acidity (pH<7) or alkalinity (pH > 7).

PERMEAMETER

It is an instrument for measuring coefficient of permeability of a porous medium.

PHI-INDEX

The index is an average rate of infiltration derived from a time intensity graph of rainfall in such a manner that the volume of rainfall in excess of this rate will equal the volume of storm runoff.

PHYTOMETER

Device for measuring transpiration, consisting of vessel containing soil in which one or more plants are rooted and sealed so that water can escape only by transpiration from plants.

PIEZOMETRIC HEAD

  1. Elevation to which water will rise in a piezometer connected to a point in an aquifer.
  2. Sum of the elevation and pressure head in a liquid, expressed in units of height.

PLOTTING POSITION

The purpose of frequency analysis of an annual series is to obtain a relation between the magnitude of the event and its probability of exceedence. The exceedence probability of the event obtained by the use of an empirical formula is called, plotting position. Various empirical formulae used are California, Hazen, Weibull etc.

POINT RAINFALL

Rainfall at a particular site recorded by a raingauge.

POROSITY

Volume of voids filled with stagnant water, which practically does not participate in the general flow, per unit gross soil volume, voids inclusively.

POTAMOLOGY

That branch of hydrology which pertains to surface streams, the science of rivers.

POTENTIAL EVAPOTRANSPIRATION

The amount of water utilised by plant growth including evaporation from the soil if the soil contains sufficient moisture for plant growth at all times.

PRECIPITABLE WATER

It is the total amount of water vapors in the atmosphere, frequently expressed as depth of precipitable water. This term is a misnomer, since no natural precipitation process removes all the moisture from the air.

PRECIPITATION

The total supply of water derived from the atmosphere in the form of rain, snow, dew, mist, frost, hail, sleet etc. It is usually expressed as depth of liquid water on a horizontal surface in a day, month or year and designated so daily, monthly or annual precipitation.

PRISM STORAGE

It is that portion of the total channel storage during a flood which corresponds to a condition of steady flow that is when inflow and outflow are equal. It is the volume formed by an imaginary plane parallel to the channel bottom drawn at the outflow section water surface.

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 given the probability of a value of a variate as a function of the variate.

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 MAXIMUM FLOOD

It is defined as that flood which is estimated to result, if the most critical combination of severe meteorological and hydrological conditions considered reasonably possible in the region, were to occur.

PROBABLE MAXIMUM PRECIPITATION

It is the theoretically greatest depth of precipitation for a given duration that is physically possible over a particular drainage basin at a particular season.

PROMPT INTERFLOW

It is the interflow with least time lag between infiltration and outflow.

PSYCHROMETER

A hygrometer, or instrument for measuring the aqueous vapour in the atmosphere, consisting essentially of two similar thermometers, the bulb of one being kept wet.

PUMPING TEST

Extraction of water from a well at one or more selected discharge rates, during which piezometric or phreatic 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.

QUICK RETURN FLOW

Also called interflow, it is a part of precipitation which after infiltration moves laterally through upper crusts of the soil and returns to the surface at some location away from the point of entry into the soil.

RADIOACTIVE DATING

Method of age determination based on the property of radioactive decay of isotopes.

RADIOACTIVE TRACER

Radioactive material detectable by its nuclear radiation and suitable for water tracing even at very low concentrations.

RADIUS OF INFLUENCE

Radial extent of the cone of depression is called radius of influence. It is the distance between centre of well and outer periphery of drawdown curve.

RAIN GAUGE

Also called pluviometer, ombrometer and hyetometer, it is an instrument for measuring the quantity of rain that falls at a given place and time.

RAIN SHADOW

A region on the leeward side of a mountain or mountain range where the rainfall is much less than one the windward side.

RAINDROP EVAPORATION

Evaporation from the raindrops in the process of their fall from the atmosphere to the earth.

RAINFALL

The total liquid products of precipitation or condensation from the atmosphere as received and measured in a rain gauge.

RAINFALL DISTRIBUTION COEFFICIENT

The distribution coefficient for any storm is the ratio of the maximum rainfall at any point to the mean rainfall in the basin.

RAINFALL EXCESS

Also called net rainfall or effective rainfall, it is part of the rainfall that appears as runoff in the stream.

RAINFALL INTENSITY

The amount of rainfall occurring in a unit interval of time, generally expressed in mm per hour.

RAINFALL SIMULATOR

It is one type of infiltrometer in which water is applied in the form of natural rain and at rates comparable with natural rainfall. Specially designed nozzles produce raindrops falling from a height of 2 m and are capable of producing various intensities of rainfall.

RANDOM PROCESS

Stochastic process in which the numbers of the time series are independent among themselves.

RATING CURVE

A curve showing the relation between stage and discharge of stream at a given gauging station.

REACH

Length of open channel between two defined cross sections.

RECHARGE

It is a natural or artificial process by which water is added from outside to the zone of saturation of an aquifer, either directly into a formation or indirectly by way of another formation.

RECORDING RAINGAUGE

Recording raingauges produce a continuous plot of rainfall against time and provide valuable data of intensity and duration of rainfall for hydrological analysis of storms. Tipping bucket type raingauge gives intensity of rainfall whereas weighing bucket type raingauge gives mass curve of rainfall.

RECOVERY TEST

Pumping test consisting of the measurement at predetermined time intervals, of the rise of the piezometric level or water table in a pumped well or in the surrounding observation wells after stoppage of pumping.

RECURRENCE INTERVAL

Statistical parameter used in frequency analysis as a measure of most probable time interval between occurrence of a given event and that of an equal or greater event.

REGRESSION ANALYSIS

Statistical method developed to investigate the interdependence or relationship between two or more measurable variates. The most common form of regression analysis is linear regression.

REGRESSION CURVE

The falling limb, after the point of contra flexure, of hydrograph after a flood event. This represents withdrawal of water from storage in the valley, stream channel and the subsurface runoff.

RELATIVE HUMIDITY

At a given pressure and temperature, the percentage ratio of the mole fraction of the water vapour to the mole fraction that the air would have if it were saturated with respect to water at the same pressure and temperature.

REPRESENTATIVE BASIN

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

Body of water, either natural or man-made, used for storage, regulation and control of water resources.

RESERVOIR RELEASE RULES

Rules governing the way in which volumes of water are released from a reservoir in order to meet demand, downstream protection, expected future low flows and other considerations.

RESERVOIR ROUTING

The routing of a flood wave through a reservoir.

RESIDUAL MASS CURVE

Graph of the cumulative departures from a given reference, such as the arithmetic average, versus time or date.

RETARDING RESERVOIR

A reservoir wherein water is stored for a relatively brief period of time, part of it being retained until the stream can safely carry the ordinary flow plus the released water.

RETENTION

That part of the precipitation falling on a drainage area which does not escape as surface stream flow during a given period. It is the difference between total precipitation and total runoff during the period and represents evaporation, transpiration, subsurface leakage, infiltration and when short periods are considered, temporary surface and underground storage on an area. When periods of several years are considered, it approximates consumptive use.

RETENTION CURVE

Graph representing the suction pressure versus the moisture or water content, in an unsaturated soil.

RETURN FLOW

That portion of the water, diverted from a river or stream, which ultimately finds its way back through surface runoff (visible flow) and as percolation or seepage through the bed and banks (invisible flow).

RETURN PERIOD

Also called recurrence interval, it is statistical parameter used in frequency analysis as measure of most probable time interval between occurrence of a given event and that of an equal or greater event.

RISING LIMB

The ascending portion of a hydrograph.

RIVER STAGE RECORDER

These are instruments used for measuring discharge in a river at a particular gauging station. They record the water surface elevation in the river above the datum which is related to the discharge in the stream.

ROOT ZONE

Layer of soil containing plant roots.

RULE CURVE

A curve devised to indicate operation of a reservoir so as to obtain the best results based on past experience and to be applied to future operation with a view to attain best use of the reservoir for its intended purposes.

RUNOFF

It is defined as that portion of precipitation which is not absorbed by the deep strata but finds its way into the streams after meeting the persistent demands of evapotranspiration including interception and other losses. It includes surface runoff received into the channels after rainfall, delayed runoff that enters the streams after passing through portion of the earth and other delayed runoff that has been temporarily detained as snow cover or stored in natural lakes or swamps.

RUNOFF COEFFICIENT

The ratio of runoff to precipitation.

S-CURVE

A graph showing the summation of the ordinates of a series of unit hydrographs spaced at unit rainfall duration interval. It represents the hydrograph of average rate of rainfall excess of the unit duration continued indefinitely.

SAFE YIELD

It is defined as the amount of water which can be withdrawn annually from a ground water basin without producing undesirable effect.

SALINITY

Measure of the concentration of dissolved salts, mainly sodium chloride, in saline water or sea water.

SALT-WATER INTRUSION

Phenomenon occurring when a body of salt water invades a body of fresh water. It can occur either in surface or groundwater bodies.

SATURATION VAPOUR PRESSURE

Maximum possible partial pressure of water vapour in the air or atmosphere at a given temperature.

SEDIMENT LOAD

Also called sediment discharge, it is the quantity of sediment (suspended load and bed load) by volume or dry weight transported in unit time across a stream or channel cross section.

SEDIMENTATION

It is the process involving settling and deposition of sediments in water by gravity.

SEEPAGE

Slow percolation generally associated with flow in an unsaturated medium. Seepage into a body is termed "influent seepage" and that away from a body as "effluent seepage". The difference between percolation and seepage is that the latter is through unsaturated material while the former is through saturated material.

SEMI ARID

A term applied to a zone or climate neither entirely arid nor strictly humid but with a pronounced tendency towards arid character in which certain type of crops can be grown without irrigation.

SEMI-CONFINED AQUIFER

Aquifer overlain and/or underlain by a relatively thin semi-pervious layer, through which flow into or out of the aquifer can take place.

SEMI HUMID

Land or climate, neither entirely arid nor strictly humid, with pronounced tendency towards humid character.

SHIFTING CONTROL

If stage discharge relationship for a gauging section changes with time, it is called shifting control. The relationship can change due to

  1. changing characteristics caused by weed growth, dredging or channel encroachment
  2. aggradation or degradation phenomenon in an alluvial channel
  3. unsteady flow effects of a rapidly changing stage
  4. variable back water effects affecting the gauging station

SHORT-TERM HYDROLOGICAL FORECASTING

Forecast of the future value of an element of the regime of a water body for a period ending up to two days from the issue of the forecast.

SLEET

It is frozen raindrop or transparent grains which form when rain falls through air at subfreezing temperature. In Britain, sleet denotes precipitation of snow and rain simultaneously.

SLOPE-AREA METHOD

It is a very versatile indirect method of discharge estimation which requires

  1. selection of a reach in which cross sectional properties including bed elevations are known at its ends
  2. the value of Manning's N and
  3. water surface elevations at the two end sections.

SMALL WATERSHED

A small watershed may be defined as one that is so small that its sensitivity to high intensity rainfall of short duration and to land use are not suppressed by the channel storage characteristics. By this definition, the size of a small watershed may be found from few acres to 1000 acres, or even upto 50sq. miles. The upper limit of the area depends on the condition at which the above mentioned sensitivity becomes practically lost due to the channel storage effect. Effect of overland flow is a dominating factor than the effect of channel flow.

SNOW

Precipitation from the atmosphere in the form of branched hexagonal crystals or stars, often mixed with simple ice crystals, which fall more or less continuously from a solid cloud sheet. These crystals may fall either separately or in coherent clusters forming snow flakes.

SNOW COARSE

A snow course consists of a series of sampling points, usually not fewer than 10. The points are located along a predetermined geometric pattern at a spacing of 50 to 100 ft. The ends and pivots of the pattern are permanently marked to make certain that the snow is surveyed at the same locations year after year.

SNOW GAUGE

An instrument used to measure the amount of snowfall. Weighing type storage gauges operate in mountainous regions for 1 to 2 months without servicing while some non recording storage gauges are designed to operate for an entire season without attention.

SNOW MELT

It is the transformation of snow into liquid water.

SNOW PACK

Field of naturally packed snow that ordinarily melts slowly and yields water during early summer months.

SOFT WATER

Water without significant hardness.

SOIL CONSERVATION

Soil conservation implies all the effective measures taken to reduce the amount of soil erosion. Erosion resulting from intense rainfall can be reduced by (i) protecting the soil surface from raindrop impact through the use of crop cover or mulch and (ii) reduction of surface runoff by encouraging infiltration through proper selection of cover.

SOIL MOISTURE DEFICIT

The amount of water that should be applied to the soil to cause thorough drainage and is substantially equal to the soil moisture deficit then existing.

SOIL WATER ZONE

This zone lies close to the ground surface in the major root band of the vegetation from which water is lost to the atmosphere by evapotranspiration.

SOUNDING WEIGHT

Current meters are weighed down by lead weights called sounding weights. These weights enable current meters to enable themselves to position in a stable manner at the required location in flowing water.

SOUTH-WEST MONSOON

It is south westerly flow of air which originates in the Indian Ocean, picks up moisture and advances over India in two branches i.e. the Arabian sea branch and the Bay of Bengal branch. The former sets in at the extreme southern part of Kerala and the latter in Assam, almost simultaneously in the first week of June. This is the principle rain causing monsoon in India.

SPECIFIC CAPACITY

Specific capacity of a well is the discharge per unit drawdown and is a measure of the performance of the well. For a given well, it is not constant but decreases with increase in discharge and time.

SPECIFIC RETENTION

It is a ratio, expressed as a percentage, of the volume of water retained in the soil against the force of gravity, after complete saturation, to the volume of the soil.

SPECIFIC YIELD

It is a ratio, expressed as percentage, of the volume of water which is drained by the force of gravity, after complete saturation, to the volume of the soil.

SPILLWAY DESIGN FLOOD

Design flood used for the specific purpose of designing the spillway of a storage structure. This term is frequently used to denote the maximum discharge that can be passed in a hydraulic structure without any damage or serious threat to the stability of the structure.

SPRING

Spring is a natural discharge point where ground water issues from soil or rock in concentrated flow.

STABLE CHANNEL

Channel in which accretion balances scour on the average.

STAGE

The stage of a river is defined as its water surface elevation measured above a datum. This datum can be the mean sea level or any arbitrary datum connected independently to the mean sea level.

STAGE HYDROGRAPH

The stage data, often presented in the form of a plot of stage against chronological time is known as stage hydrograph. A plot of stage versus time.

STAGE-DISCHARGE CURVE

The relationship between the stage in a river and the corresponding discharge at the particular section is represented in the form of a curve called Stage-discharge curve.

STANDARD DEVIATION

Widely used measure of dispersion of a frequency distribution or of a set of values.

STANDARD ERROR

Positive square root of the variance of the sampling distribution of a statistic.

STANDARD PROJECT FLOOD

The flood resulting from the most severe combinations of meteorological and hydrological conditions considered reasonably characteristic of the region. Extremely rare combination of factors are excluded.

STANDARD PROJECT STORM

The standard project storm estimate for a particular drainage area and season for year in which snow melt is not a major consideration should represent the most severe flood producing rainfall depth area duration relationship and isohyetal pattern of any storm that is considered reasonably characteristic of the region in ;which the drainage basin is located, giving consideration to the runoff characteristics and existence of water regulation structure in the basin.

STATION YEAR METHOD

This is a method of extending the length of record for a frequency curve at a station, based on the assumption that records for the same or different periods of records at a number of stations may be considered as a composite record for a single station for a period equal to the total number of years involved. For example, if 50 years of record is available for 100 stations, it might be assumed that this was equivalent to 5000, years at a single station.

STATISTICAL HYDROLOGY

Hydrological processes and phenomena which are described and analyzed by using the methods of probability theory.

STEM FLOW

It is that part of rainfall that, having been intercepted by the canopy, reaches the ground by running down the stems.

STOCHASTIC SYSTEM

It is a system in which the various processes are dependent on chance as well as time. All the hydrologic data is, more or less, stochastic in nature.

STORAGE COEFFICIENT

It is a formation constant of an aquifer which represents the volume of water released by a column of confined aquifer of unit cross sectional area under a unit decrease of piezometric head normal to the area.

STORAGE TIME CONSTANT

It is a coefficient used in the Muskingum equation of channel routing. It is approximately equal to the time of travel of a flood wave through the channel reach. It has the dimensions of time.

STORM

It is a term commonly used for violent atmospheric motion, such as gale, thunderstorm, rainstorm, snowstorm or dust storm.

STORM INTERVAL

The interval of time from the beginning of a rain, through the rain period and the subsequent dry or rainless period to the beginning of the next subsequent rain period.

STORM MAXIMIZATION

Procedure used to derive probable maximum precipitation.

STORM RUNOFF

It is that part of runoff which enters the stream immediately after the precipitation. It includes surface runoff, prompt interflow and channel precipitation. It is also called direct runoff.

STORM SEEPAGE

A part of precipitation that in filters, more literally through upper crust of the soil and returns to the surface at some location away from the point of entry into the soil. This component of runoff is known variously as storm seepage or interflow or throughflow or storm flow.

STORM TRACK

The path traversed by the centre of the storm.

STREAM

A natural channel for conveying water.

STREAM ORDER

It is a classification of river basin reflecting the degree of branching or bifurcation within a basin. A first order stream has no tributary. A second order stream has two or more tributaries of the next lower order. A catchment is described as first, second or higher order depending on the stream order at the outlet.

STREAMFLOW

It is movement of water under the force of gravity through well defined surface channel. It is the total runoff confined in stream channel.

SUBSURFACE FLOW

Any flow below the surface of the ground which may contribute to interflow, base flow or deep percolation.

SURFACE RETENTION

Also known as surface storage or initial detention, it refers to that part of precipitation which does not appear either as infiltration or as surface runoff during the period of infiltration or as surface runoff during the period of precipitation or immediately thereafter. Thus surface retention includes interception by vegetal cover, depression storage and evaporation during precipitation. It does not include water which is temporarily stored en route to the storm system.

SURFACE RUNOFF

The water which reaches the stream by travelling over the soil surface or falls directly into the stream channels.

SURFACE WATER HYDROLOGY

That branch of hydrology which deals with hydrological phenomena and processes which occur on the Earth's surface, emphasizing overland flows.

SUSTAINED RUNOFF

Also known as base flow or fair weather flow, it is composed of ground water runoff and delayed subsurface runoff.

SYNTHETIC UNIT HYDROGRAPH

A unit hydrograph developed on the basis of estimation of coefficients expressing various physical features of a catchment.

TELEMETERING RAINGAUGE

These raingauges are of the recording type and contain electronic units to transmit the data on rainfall to the base station both at regular interval or on interrogation. The tipping bucket type raingauge, being ideally suited, is usually adopted for this purpose. Telemetering gauges are of utmost use in gathering rainfall data from mountainous and generally inaccessible places.

TENSIOMETER

It is a device used for measuring soil water tension. It operates only up to about 0.85 atm. tension.

THIESSEN POLYGON

The points of location of rain gauges on a map are joined by straight lines and their perpendicular bisectors are drawn. The polygon formed around each raingauge station by these perpendiculars is called, after its originator, a Thiessen polygon.

THROUGHFALL

Also known as interflow or storm seepage, it is that part of precipitation which after infiltration moves laterally through upper crusts of the soil and returns to the surface at some location away from the point of entry into the soil.

TIME BASE

Time base of a hydrograph is considered to be the time from which the concentration curve begins until the direct runoff component essentially reaches zero.

TIME OF CONCENTRATION

The time taken by the runoff from the farthest point of the catchment to reach the point under consideration.

TIME-AREA HISTOGRAM

It is a bar plot of inter isochrone area Vs. time. If a rainfall excess of 1 cm occurs instantaneously and uniformly over the catchment area, this time area histogram represents the sequence in which the volume of rainfall will be moved out of the catchment and arrive at the outlet.

TORNADO

A rotary storm, one of the most violent type of storms known, of small diameter, which travels across the country and leaves great devastation along a narrow path. Its chief characteristics are

  1. Under a heavy cumulonimbus cloud there hangs a funnel shaped cloud, which marks the vertex and, as the storm moves along, may or may not touch the earth.
  2. Heavy precipitation and usually hail occur, with thunder. In addition to the thunder, there is a roar attending the tornado cloud when it touches the surface.
  3. The winds blow spirally upward around the axis of a tornado cloud.
  4. The speed of the storm itself in travelling over the earth is comparatively slow, 40 to 65 Km an hour, its path is short, averaging about 480 Km.

TORRENT

A stream of water flowing with great velocity or turbulence, as during a freshet or down a steep incline.

TRANSMISSIBILITY

It is defined as the rate of flow through an aquifer of unit width and total saturation depth under unit hydraulic gradient. It is equal to product of full saturation depth of aquifer and its coefficient of permeability.

TRANSPOSITION OF STORM

It means application of a storm from one area to some other area within the same region of meteorological homogeneity. It requires the determination of whether the particular storm could have occurred in the area to which it is to be transposed.

TEMPERATURE LAPSE RATE

Decrease of the air temperature in degrees Celsius per 100 m of latitude increase.

TIME SERIES

Set of observations, in order, taken at successive points of time, commonly at a fixed interval.

TOTAL DISSOLVED SOLIDS

Total weight of dissolved mineral constituents in water per unit volume (or weight) of water in the sample.

TRAP EFFICIENCY

It is the ability of the reservoir to trap and retain sediment and is expressed as the percent of sediment yield, which is retained in the basin. It increases with the increase in the capacity of the reservoir but it decreases with outflow discharge increase.

TREND

Unidirectional, monotonous (diminishing or increasing) change in the average value of a hydrological variable.

TRIBUTARY

Watercourse flowing into a larger watercourse or into a lake.

TROPICAL CYCLONE

Also called cyclone in India, hurricane in U.S.A. and typhoon in south east Asia, it is a wind system with an intensely storm depression with msl pressures sometimes below 915 mbars. Normal and areal extent is about 100 200 Km in diameter. Isobars are closely spaced and winds are anti-clock wise in northern hemisphere. Rainfall is normally heavy in the entire area occupied by the cyclone.

ULTRASONIC FLOW METER

The device, which estimates the discharge in a stream, depends for its operation upon the Doppler effect of ultrasonic waves passing through water. A transmitter directs a signal towards a receiver some distance upstream. The ultrasonic waves moving upstream are compressed, those returning are attenuated. The magnitude of this effect can be recorded and related to water velocity.

UNCONFINED AQUIFER

Also called non-artesian aquifer, it is a water bearing strata having no confined impermeable overburden. In this aquifer, water table varies in undulating form and in slope depending on areas of recharge, pumpage from wells and permeability.

UNDERFLOW

Movement of water through a pervious stratum under the bed of a river.

UNIT HYDROGRAPH

Hydrograph of storm runoff at a given point on a given stream which will result from an isolated rainfall excess of unit duration occurring over the contributing drainage area and resulting in a unit of runoff.

UNIT RAINFALL DURATION

The duration of runoff producing rainfall or rainfall excess that results in a unit hydrograph.

UNSATURATED ZONE

That portion of the lithosphere in which the interstices are filled partly with air and partly with water, held or suspended by molecular forces.

UNSTABLE CHANNEL

Channel in which stage-discharge relation changes in the course of time.

UPSTREAM

In the direction opposite to the main current.

UNSTEADY FLOW

Flow in which, the velocity changes in magnitude or direction with respect to time.

URBAN HYDROLOGY

It is defined as the interdisciplinary science of water and its interrelationships with urban man. It is the study of hydrological processes both within and outside the urban environment that are affected by urbanisation.

VADOSE WATER

Water in the zone of aeration is called vadose water.

VALLEY STORAGE

  1. The volume below the water surface profile.
  2. The natural storage capacity or volume occupied by a stream in flood after it has over flown its banks. It includes the channel storage and lateral storage.

VIRGIN FLOW

It is the stream flow unaffected by artificial divergence, storage of other works of man in or on the stream channels or in the drainage basin or watershed.

W-INDEX

The average rate of infiltration during the time the rainfall intensity exceeds the infiltration capacity.

WADING ROD

A graduated rod to which a current meter is attached for measuring the velocity in shallow water. This rod measures the depth below water surface at which velocity is being measured.

WARNING STAGE

The river stage at which it is necessary to begin issuing warnings of river forecasts to enable adequate precautionary measures to be taken to avoid damage or inconvenience due to flooding.

WATER BODY

Mass of water distinct from other masses of water.

WATER BUDGET METHOD

It is an analytical method for the determination of evaporation. It is measurement of continuity of flow of water. This holds true for any time interval and applies to any drainage basin and to the earth as a whole. It involves writing the hydrological continuity equation for the reservoir and determining the evaporation from a knowledge or estimation of the variables.

WATER CONSERVATION

Measures introduced to reduce the amount of water used for any purpose and/or to protect it from pollution.

WATER EQUIVALENT OF SNOW

It means depth of water which would result from melting of snow. It depends on the snow density as well as its depth.

WATER LOGGING

Condition of land when the ground water stands at a level that is detrimental to plant growth, as a result of excessive irrigation coupled with inadequate drainage.

WATER RESOURCES

Supply of water in a given area or basin interpreted in terms of availability of surface and underground water.

WATER RESOURCES MANAGEMENT

Planned development, distribution and use of water resources.

WATER TABLE

Water table is the upper surface of the zone of saturation at atmospheric pressure. It is the level at which water stands in a well penetrating the unconfined aquifers.

WATER YEAR

Continuous twelve month period selected for maintaining or presenting records of flow, and/or use of water of any river system.

WATERSHED

The terms drainage basin and watershed are often considered synonymous but strictly speaking, a watershed is the divide separating one drainage basin from another.

WATERSHED LEAKAGE

The geological formation under many drainage basins is such that precipitation falling on one basin finds its way underground through fissures and water bearing strata to an outlet either in a nearby or a remote drainage basin, or directly to the sea. This is called watershed leakage.

WATERSHED MANAGEMENT

Management of watershed to conserve soil and water requires that the land be used within its capabilities and treated according to its need. The objectives are to protect the land against all forms of soil deterioration, to rebuild eroded and depleted soils, to build up soil fertility, to stabilise critical runoff and sediment producing areas, to improve grass lands, woodlands and wild life lands, to conserve water for beneficial use, to provide needed drainage and irrigation and to reduce flood land sediment drainage.

WEDGE STORAGE

It is the wedge like volume formed between the actual water surface profile and the top surface of the prism storage. It is the difference between the total channel storage and the prism storage.

WEIBULL DISTRIBUTION

Fisher-Tippett Type III External distribution used usually in drought studies.

WELL

A water well is a hole or shaft, usually vertical, excavated in the earth for bringing ground water to the earth. Occasionally, wells serve other purposes such as subsurface exploration and observation, artificial recharge and disposal of waste waters.

WELL LOSS

It is the head loss caused by flow through the well screen and flow inside of the well to the pump intake.

WET YEAR

Year in which precipitation or stream flow is significantly above normal.

WILTING COEFFICIENT

The moisture content of the soil expressed as a percentage of the dry weight at the time when the leaves of a plant growing in the soil first undergo a permanent reduction in their moisture content, as a result of the deficiency in the soil moisture supply.

WIRE GAUGE

This is used for determining the stage of a river by lowering a weight to the water surface. It is mostly attached on the bridges and is read by means of a mechanical counter attached to the reel on which wire is wound.

YIELD

Total volume or flow from a drainage basin for a long stipulated period of time, for example annual yield of drainage basin is the mean annual runoff.

ZERO MOISTURE INDEX

The index of moisture when the precipitation is just adequate to supply all the water that would be needed for maximum evaporation and transpiration in the course of a year.

ZONE OF AERATION

That portion of lithosphere in which the interstices are filled partly with water which is held or suspended by molecular forces.

ZONE OF SATURATION

That part of lithosphere in which the pores are completely filled with water and pressure is at or above atmospheric.