General Introduction end 

Hydrologic Definitions

By W. B. LANGBEIN and KATHLEEN T. ISERI

Manual of Hydrology: Part 1. General Surf ace-Water 

Techniques

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1541-A

Methods and practices 

of the Geological Survey

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1960
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

First printing 1972 

Second printing 1972 

Third printing 1983

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Picket! Street, Alexandria, VA 22304
CONTENTS

Page 

Introduction______________________________________________________ 1

Hydrologic definitions.._ _________________-_-_____-___-_-_-__--___-- 2

Topical finding list..__--________-___-_____-_____---_-_-_-___--_--._ 22

References. ____________________-_.-___-_____---_-_____-__-_------_ 26
MANUAL OF HYDROLOGY: PART I. GENERAL SURFACE-WATE'

TECHNIQUES

GENERAL INTRODUCTION AND HYDROLOGIC 

DEFINITIONS

By W. B. LANGBEIN and KATHLEEN T. ISERI

INTRODUCTION

Hydrologic techniques, capably applied, are essential for the sound 

development and management of water resources. The population of 

our country is increasing rapidly but water use outpaces the growth in 

population and thus creates demands for water that frequently impinge on the available supply. The mounting demands, the increa sing 

areas of conflict, and the rising rate of flood damage impel a need for 

refined methods of analysis of water problems.

Consider the planning for a water supply for a town or an ii-igation project: How much water is available in the river? How does 

this supply compare with the demand ? Is reservoir storage needed ? 

If so, how much ? Riverflow varies and therefore reservoirs are built 

to store water to tide over from times of excess to times of deficiency. 

The problem is to design the storage reservoir so that the demands 

for water can be supplied despite variations in riverflow. Reservoirs 

that are built too large waste money and water; reservoirs built too 

small cannot do the job expected of them.

Floods pose a different set of hydrologic questions. The height 

of levees and the size of reservoirs needed to restrain floodwater must 

be determined if these structures are to be used to reduce flood damage. 

Flood damage can also be reduced by the proper design of homes and 

factories which are built on the flood plain, often in ignorance of flood 

danger. Highway bridges and culverts are other structures that 

are often damaged by floods. If highways are to cross streams sa fely 

and economically, bridges and culverts must be adequate to spar the 

floods expected within their useful life.

Hydraulic engineers with weighty problems of water supply, floods, 

and pollution control to solve welcome methods that will help them 

use basic data. The solution of water problems begins with water
2 MANUAL OF HYDROLOGY: GENERAL SURFACE-WATER TECHNIQUES

data, of which more than 100,000 station-years of streanflow records 

are available. Collection of basic data, although the essential first 

step, is only part of the task. Water facts provide the necessary 

foundation; they do not in themselves give direct solutions to water 

problems. Sound methods of putting data to practical use in solving 

water problems are also essential. These problems are increasing in 

number and complexity, often at a faster pace than the means for 

solving them.

Another prerequisite to analyzing basic data is that the data be 

arranged in a form adapted to the methods of analysis. For example, 

after the development and publication of methods for deriving and 

using duration curves, there remains the need for presenting the data 

so that duration curves can be readily prepared. Meeting this need 

is part of the plan of the Geological Survey for increasing the usefulness of water data for solving water problems. Thus, tl Q.re are three 

steps prior to analysis: collection of data; development of methods 

of using the data; and processing the data into convenient form. 

This manual discusses the second step: describing hydrologic techniques for the hydraulic engineer.

The general outline of techniques given in engineer;ng textbooks 

is directed to solving specific problems such as water supply, irrigation, and flood control. In contrast, the purpose of this- manual is to 

describe techniques for analyzing basic river data, to give results applicable to many kinds of hydraulic engineering problems concerned 

with riverflow. Two of the chapters discuss methods of increasing 

the reliability of short records, a common need in the many uses of 

streamflow data. Another chapter discusses double-mass curves, and 

another, duration curves, both useful tools in using records of streamflow. A chapter is devoted to flood-frequency curves, a universal tool 

for applying flood data to the solution of flood problems. Special 

problems like flood zoning are included because they afford new opportunities, not yet found in textbooks, for the use of bs sic data.

This manual began with a series of reports prepared for use within 

the Geological Survey during the past 10 years. These have been 

revised and improved for publication in three parts. The first part 

presents methods of general interest, including a list of definitions; the 

second part discusses methods of solving low-flow problems found 

in developing water supply; and the third part discusses flood 

hydrology.

HYDROLOGIC DEFINITIONS

Surface-water hydrology is the study of the origin and processes of 

water in streams and lakes, in nature, and as modified by man. It includes such subjects as infiltration, channel storage, floods and
GENERAL INTRODUCTION AND HYDROLOGIC DEFINITIONS 3

droughts, direct runoff, and base flow. Surface-water hydrology 

shares with meteorology the study of precipitation and evaporation. 

Also, surface-water hydrology shares with geomorphology the study 

of the shape, size, and number of river channels, because river channels 

are formed as a consequence of the rates and quantities of water the}' 

must carry. Some of the tools used in the study and application of 

surface-water hydrology are unit hydrographs, flow-duration curves, 

flood-frequency curves, and correlation, all of which are defined in 

this report.

The definitions in this list are intended to explain the terminology 

in the "Manual of Hydrology" of which this report is the first chapter. 

It is, however, more than a glossary of terms used in the manurl, as 

terms are included from the entire field of surface-water hydrology. 

Terms within some of the definitions are defined in this chapter. 

Where such terms might affect the meaning of the definition, they are 

shown in italics.

Excluded from the list are such terms as "river," "lake," "creek," 

and other names for surface-water features, for which hydrolo^ists 

have not devised better definitions than are in the dictionaries. In 

general, therefore, common dictionary terms such as "anomaly" or 

"abnormal" are not included but there are exceptions where a term, 

like "anabranch," deserves wider usage. Also excluded are (hose 

terms, although often used in surface-water hydrology, that are mrinly 

geologic, hydraulic, statistical, or meteorologic. These terms are defined, as necessary, in separate chapters.

The usefulness of a list of definitions is limited by the extent to 

which the concepts they embody are accepted. To enhance precision 

and to promote agreement, many of the definitions listed were selected 

from research papers or reports on field investigations to which appropriate reference is given. Multiple references are cited where they 

amplify the meaning, and these references can be used as a source of 

additional information on surface-water hydrology. Where acceptable definitions could not be found in published works, substitutes were 

written especially for this chapter. Terms defined in this report are 

grouped by subjects in the section entitled "Topical finding list" to 

facilitate locating the definitions of related terms.

Walter B. Langbein selected or composed the definitions presented 

in the report, and Kathleen T. Iseri arranged the report for publication, verified references, and prepared the topical finding list.
MANUAL OF HYDROLOGY: GENERAL SURFACE-WATER TECHNIQUES

Ablation. The process by which ice 

and snow waste away owing to 

melting and evaporation.

Absorption. The entrance of water 

into the soil or rocks by all natural 

processes. It includes the infiltration of precipitation or snowmelt, 

gravity flow of streams into the 

valley alluvium (see Bank storage) 

into sinkholes or other large openings, and the movement of atmospheric moisture.

Acre-foot. A unit for measuring the 

volume of water, is equal to the 

quantity of water required to cover 

1 acre to a depth of 1 foot and is 

equal to 43,560 cubic feet or 325,851 

gallon^ The term is commonly 

used in measuring volumes of 

Wftit-r nsefi or stored,

Anabranch. A diverging branch of a 

river whicn rwMt-r* Jlie inn in 

stream.

Anabranch

Anchor ice. Ice in the bed of a stream 

or upon a submerged body or structure. (See -also Schaefer, V. J., 

1950, p. 888.)'

Annual flood. The highest peak discharge in a water year.

Annual flood series. A list of annual 

floods.

Antecedent precipitation index. An 

index of moisture stored within a 

drainage basin before a storm. 

(Linsley and others, 1949, p. 414.)

Area-capacity curve. A graph showing 

the relation between the surface 

area of the water in a reservoir 

and the corresponding volume.

Average discharge. In the annual 

series of the Geological Survey's

Average discharge Cortinued

reports on surface-water supply 

the arithmetic avenge of all complete water years of record whether 

or not they are consecutive. Average discharge is no* published for 

less than 5 years of record. The 

term "average" is generally reserved for average of record and 

"mean" is used for averages of 

shorter periods, nairely, daily mean 

discharge.

Backwater. Water backed up or retarded in its course as compared 

with its normal or natural condition of flow. In sfeam gaging, a 

rise in stage produced by a temporary obstruction such as ice or 

weeds, or by the flooding of the 

stream below. The difference between the observed stage and that 

indicated by the stage-discharge relation, is reported as backwater.

Bank. The margins of a channel, 

Banks are called right or left as 

viewed facing in the direction of 

the flow.

Bankfull stage. Stage at which a 

stream first overflows its natural 

banks. (See also Flood stage. 

Bankfull stage is a hydraulic term, 

whereas flood stag? implies damage.)

Bank storage. The water absorbed 

into the banks of a stream channel, 

when the stages rise above the 

water table in the bank formations, 

then returns to t>e channel as 

effluent seepage when the stages 

fall below the water table. (After 

Houk, 1951, p. 179.)

Base discharge (for peak discharge). 

In the Geological Survey's annual 

reports on surface-water supply, 

the discharge above which peak 

discharge data are published. The 

base discharge at each station is 

selected so that an rverage of about 

three peaks a year will be presented. ( See also Partial-duration 

flood series.)
GENERAL INTRODUCTION AND HYDROLOGIC DEFINITIONS

Base flow. See Base runoff.

Base runoff. Sustained or fair weather 

runoff. In most streams, base runoff is composed largely of groundwater effluent. (Langbein and 

others, 1947, p. 6.) The term base 

flow is often used in the same 

sense as base runoff. However, 

the distinction is the same as that 

between streamflow and runoff. 

When the concept in the terms 

base flow and base runoff is that 

of the natural flow in a stream, 

base runoff is the logical term. 

(See also Ground-water runoff and 

Direct runoff.)

Basic hydrologic data. Includes inventories of features of land and 

water that vary only from place 

to place (topographic and geologic 

maps are examples), and records 

of processes that vary with both 

place and time. (Records of precipitation, streamfiow, ground-water, and quality-of-water analyses 

are examples.)

Basic hydrologic information is 

a broader term that includes surveys of the water resources of particular areas and a study of their 

physical and related economic 

processes, interrelations and mechanisms.

Basic-stage flood series. See Partialduration flood series.

Braiding of river channels. Successive division and rejoining (of 

riverflow) with accompanying islands is the important characteristic denoted by the synonymous 

terms, braided or anastomosing 

stream. (Leopold and Wolman, 

1957, p. 40.) A braided stream is 

composed of anabranches.

Catchment area. Sec Drainage basin.

Cfs. Abbreviation of cubic feet per 

second.

Cfs-day. The volume of water represented by a flow of 1 cubic foot 

per second for 24 hours. It equals

Cfs-day Continued

86,400 cubic feet, 1.983471 acrefeet, or 646,317 gallons.

Cfsm (cubic feet per second per snuare 

mile). The average number of 

cubic feet of water per second flowing from each square mile of area 

drained by a stream, assuming 

that the runoff is distributed uniformly in time, and area.

Channel (watercourse). An open conduit either naturally or artiflcally 

created which periodically o- continuously contains moving water, 

or which forms a connecting link 

between two bodies of water. 

River, creek, run, branch, anabranch, and tributary are so^e of 

the terms used to describe natural 

channels. Natural channels may 

be single or braided (see Braiding 

of river channels). Canal and 

floodway are some of the terms 

used to describe artificial channels.

Channel storage. The volume cf water at a given time in the ct finnel 

or over the flood plain of the 

streams in a drainage basin or 

river reach. Channel storrtje is 

great during the progress of r flood 

event. (See Horton, 1935, r. 3.)

Climate. The sum total of the nreteorological elements that characterize 

the average and extreme condition 

of the atmosphere over a lo^g period of time at any one place or 

region of the earth's surface. The 

collective state of the atmosohere 

at a given place or over a given 

area within a specified per'od of 

time. (Landsberg, 1945, p. 928.)

Climatic year. A continuous 12-month 

period during which a complete annual cycle occurs, arbitrarily selected for the presentation of data 

relative to hydrologic or n^teorologic phenomena. The cl'madc 

year is usually designated by the 

calendar year during whict most 

of the 12 months occur. (Sec 

Water year.)
6 MANUAL OF HYDROLOGY: GENERAL SURFACE-WATER TECHNIQUES

Cloudburst. A torrential downpour of 

rain, which by its spottiness and 

relatively high intensify suggests 

the bursting and discharge of a 

whole cloud at once, (Woolley, 

1946, p. ii.)

Concentration time. See Time of concentration.

Concordant flows. Flows at different 

points in a river system, that have 

the same recurrence interval, or 

the same frequency of occurrence. 

It is most often applied to floodliows.

Condensation. The process by which 

water changes from the vapor state 

into the liquid or solid state. It 

is the reverse of evaporation.

Conservation storage. Storage of 

water for later release for useful 

purposes such as municipal water 

supply, power, or irrigation in contrast with storage capacity used 

for flood control.

Consumptive use. The quantity of 

water absorbed by the crop and 

transpired or used directly in the 

building of plant tissue together 

with that evaporated from the 

cropped area. (U.S. Bur. of Reclamation, 1952, p. 3.)

The quantity of water transpired 

and evaporated from a cropped 

area or the normal loss of water 

from the soil by evaporation 

and plant transpiration. (Blaney, 

1951b, p. 190.) (See also Water 

requirement and Blaney, 1951a, 

p. 4.)

The quantity of water discharged 

to the atmosphere or incorporated 

in the products of the process in 

connection with vegetative growth, 

food processing, or an industrial 

process. (MacKichan, 1957, p. 2.)

Consumptive use, net. The consumptive use decreased by the estimated 

contribution by rainfall toward the 

production of irrigated crops. 

(Simons, 1953, p. 12.) (See Effective precipitation (3).) Net conConsumptive use, net Continued

sumptive use is sometimes called 

crop irrigation requirement.

Consumptive waste. The water that 

returns to the atmosphere without 

benefiting man. (Thomas, 1951, 

p. 217.)

Contents. The volume of water in a 

reservoir. Unless otherwise indicated reservoir content is computed 

on the basis of a level pool and does 

not include bank storage.

Control. A natural constriction of the 

channel, a long reach of the channel, a stretch of rap'ds, or an artificial structure downstream from a 

gaging station that determines the 

stage-discharge relation at the 

gage.

A control may be complete or 

partial. A complete control exists 

where the stage-discharge relation at a gaging sta tion is entirely 

independent of fluctuations in stage 

downstream from the control. A 

partial control exists where downstream fluctuations have some effect upon the stage-discharge relation at a gaging stat'on. A control, 

either partial or coir^lete, may also 

be shifting. Most ratural controls 

are shifting to a deg-ee, but a shifting control exists where the stagedischarge relation experiences 

frequent changes owing to impermanent bed or banks.

Correlation. The process of establishing a relation between a variable 

and one or more related variables. 

Correlation is simple if there is 

only one independent variable; multiple, if there is more than one independent variable. For gaging 

station records, the usual variables 

are the short-term gaging-station 

record and one or more long-term 

gaging-station records. (Searcy, 

1960.)

Correlative estimate. A discharge determined by correction. A correlative estimate represents a likely 

value of the discharge for any
GENERAL INTRODUCTION AND HYDROLOGIC DEFINITIONS

Correlative estimate Continued

particular period commonly a 

month according to a specified 

method of analysis. (After Langbein and Hardison, 1955, [no. 826], 

p. 826-8.)

Cryology. Science of ice and snow.

Cubic feet per second. A unit expressing rates of discharge. One cubic 

foot per second is equal to the discharge of a stream of rectangular 

cross section, 1 foot wide and 1 

foot deep, flowing water an average 

velocity of 1 foot per second.

Current meter. An instrument for 

measuring the speed of flowing 

water. The Geological Survey uses 

a rotating cup meter.

a Cue. (6) 

b Contact chambtr 

c Scriw (or honow 

d Tail p«c«

Current meter

Cusec. This abbreviation for cubic foot 

per second, common in the British 

Commonwealth countries (except 

Canada), is not used by the U.S. 

Geological Survey; instead, cfs is 

used.

Cycle. A regularly recurring succession of events such as the cycle of 

the seasons. Use of cycle to describe a group of wet years followed 

or preceded by a group of dry years 

is to be avoided.

Dead storage. The volume in a reservoir below the lowest controllable 

level. (Thomas and Harbeck, 

1956, p. 13.)

Dependable yield, n-years. The minimum supply of a given water development that is available on demand, with the understanding that 

lower yields will occur once in n 

years, on the average. (Paulsen, 

1950, p. 801.)

Depletion. The progressive withdrawal of water from surface- or

Depletion Continued

ground-water reservoirs at s rate 

greater than that of replenish ment. 

(See Recession curve and st~eamflow depletion.)

Depression storage. The volume of 

water contained in natural depressions in the land surface, such 

as puddles. (After Horton, 1935, 

P. 2.)

Direct runoff. The runoff entering 

stream channels promptly after 

rainfall or snowmelt. Superposed 

on base runoff, it forms the fr'lk of 

the hydrograph of a flood.

See also surface runoff. The 

terms base runoff and direct runoff are time classifications of runoff. The terms ground-water runoff and surface runoff are c'assifications according to source.

Discharge. In its simplest concept discharge means outflow; therefore, 

the use of this term is not restricted as to course or location, 

and it can be applied to describe 

the flow of water from a pipe or 

from a drainage basin. If the discharge occurs in some course or 

channel, it is correct to speak of 

the discharge of a canal or of a 

river. It is also correct to sp^ak of 

the discharge of a canal or stream 

into a lake, a stream, or an ocean. 

(See also Streamflow and Runoff.) 

The data in the reports of the 

Geological Survey on surface water 

represent the total fluids mersured. 

Thus, the terms discharge, streamflow, and runoff represent water 

with the solids dissolved in it and 

the sediment mixed with it. Of 

these terms, discharge is th° most 

comprehensive. The discharge of 

drainage basins is distinguished as 

follows:

Yield. Total water rurout or 

crop; includes runoff plus 

underflow.

Runoff. That part of water 

yield that appears in 

streams.
8 MANUAL OF HYDROLOGY: GENERAL SURFACE-WATER TECHNIQUES

Discharge Continued

Streamflow. The actual flow 

in streams, whether or not 

subject to regulation, or 

underflow.

Each of these terms can be reported in total volumes (such as 

acre-feet) or time rates (such as 

cubic feet per second or acre-feet 

per year). The differentiation between runoff as a volume and 

streamflow as a rate is not accepted.

Discharge rating curve. See Stagedischarge relation.

Distribution graph (distribution hydrograph). A unit hydrograph of 

direct runoff modified to show the 

proportions of the volume of runoff that occurs during successive 

equal units of time. (After Hoyt 

and others, 1936, p. 124.)

Diversion. The taking of water from 

a stream or other body of water 

into a canal, pipe, or other conduit.

Double-mass curve. A plot on arithmetic cross-section paper of the 

cumulated values of one variable 

against the cumulated values of another or against the computed values of the same variable for a 

concurrent period of time. (See 

Searcy and Hardison, 1960.)

Drainage area. The drainage area of 

a stream at a specified location 

is that area, measured iu a horizontal plane, which is enclosed by 

a drainage divide. (See [U.S.] 

Federal Inter-Agency River Basin 

Committee, Subcommittee on Hydrology, 1951, p. II.) 1

Drainage basin. A part of the surface 

of the earth that is occupied by a 

drainage system, which consists of 

a surface stream or a body of impounded surface water together 

with all tributary surface streams 

and bodies of impounded surface 

water.

Drainage density. Length of all channels above those of a specified 

stream order per ur; t of drainage 

area.

Drainage divide. The rim of a drainage basin. (See Watershed.)

Drought. A period of deficient precipitation or runoff extending over 

an indefinite numbe" of days, but 

with no set standard by which to 

determine the amour t of deficiency 

needed to constitute a drought. 

Thus, there is no universally accepted quantitative definition of 

drought; generally, each investigator establishes his own definition. 

The following paragraph (Hoyt, 

1936, p. 2) discusses the problem of 

defining a drought:

When in an area that is ordinarily classed as tumid, natural 

vegetation becomes desiccated or 

defoliates unseasonrbly and crops 

fail to mature owing to lack of 

precipitation, or when precipitation 

is insufficient to meet the needs 

of established human activities, 

drought conditions may be said to 

prevail. Although water for irrigation or other uses in arid areas 

is always limited, special shortages in such ar^as are also 

regarded as drought? Unsatisfactory distribution of precipitation 

throughout the year may be as 

effective a factor in causing a 

drought as a shortage in the total 

amount. Temperature and wind 

may also play an important part, 

especially in relation to the damage 

done.

Duration curve. See Flow-duration 

curve for one type.

Effective precipitation (rainfall). 1. 

That part of the precipitation that 

produces runoff. 2. A weighted average of current and antecedent 

precipitation that is "effective" in 

correlating with ruroff. 3. As de1 [U.S.] Federal Inter-Agency River Basin Committee, Subcommittee on Hydrology, 

Inter-Agency coordination of drainage area data, notes on hydrologic activities, Bull. 4, 

November 1951: Washington, U.S. Geol. Survey, 48 p.
GENERAL INTRODUCTION AND HYDROLOGIC DEFINITIONS 9

Effective precipitation Continued

scribed by U.S. Bureau of Reclamation (1952, p. 4), that part of the 

precipitation falling on an irrigated area that is effective in 

meeting the consumptive use 

requirements.

Epilimnion. See thermal stratification.

Evaporation. The process by which 

water is changed from the liquid 

or the solid state into the vapor 

state. In hydrology, evaporation 

is vaporization that takes place at 

a temperature below the boiling 

point.

Evaporation opportunity (relative 

evaporation). The ratio of the 

rate of evaporation from a land or 

water surface in contact with the 

atmosphere, to the evaporativity 

under existing atmospheric conditions. It is the ratio of actual to 

potential rate of evaporation, generally stated as a percentage. 

(Derived from Meinzer, 1923, p. 

14.)

The opportunity for a given rate 

of evaporation to continue is determined by the available moisture 

supply. (Meyer, 1928, p. 244.)

Evaporation pan. An open tank used 

to contain water for measuring the 

amount of evaporation. The U.S. 

Weather Bureau class A pan is 4 

feet in diameter, 10 inches deep, 

set up on a timber grillage so that 

the top rim is about 16 inches 

from the ground. The water level 

in the pan during the course of 

observation is maintained between 

2 and 3 inches below the rim.

Evaporation, total. The sum of water 

lost from a given land area during 

any specific time by transpiration 

from vegetation and building of 

plant tissue; by evaporation from 

water surfaces, moist soil, and 

snow; and by interception. * * * 

It has been variously termed "evaporation," "evaporation from land 

areas," "evapotranspiration," "total

Evaporation, total Continued

loss," "water losses," and "fly off." 

(Lee, 1949, p. 314.)

Evaporativity (potential rate of evaporation). The rate of evaporation 

under the existing atmospheri? conditions from a surface of water 

that is chemically pure anc1 has 

the temperature of the atmosphere. 

(Meinzer, 1923, p. 13.)

Evapotranspiration. Water withdrawn 

from a land area by evaporation 

from water surfaces and moist, soil 

and plant transpiration. It is a 

coined word; probably the fir^t recorded use is on page 296 of the 

Transactions of the American 

Geophysical Union, part 2, 1^34.

Evapotranspiration, potential. See Potential evapotranspiration.

Excessive rainfall. See Rainfall, excessive.

Field capacity. See Field-moisture 

capacity.

Field-moisture capacity. The quantity 

of water which can be permarently 

retained in the soil in opposition 

to the downward pull of gnvity. 

(Horton, 1935, p. 3.)

Field-moisture deficiency. The quantity of water, which would be required to restore the soil moisture 

to field-moisture capacity. (Horton, 1935, p. 3.)

Firn (firn snow). Old snow on the 

top of glaciers, granular and compact but not yet converted into ice. 

It is a transitional stage between 

snow and ice. Also called n<*ve.

Firn line. The highest level to which 

the fresh snow on a glacier'^ surface retreats during the malting 

season. (Matthes, 1949, p. 161.) 

The line separating the accumulation area from the ablation area.

Flood. An overflow or inundatiou that 

comes from a river or other body 

of water (Barrows, 1948, p. 4), 

and causes or threatens damage.

Any relatively high streamflow 

overtopping the natural or artificial 

banks in any reach of a stream.
10 MANUAL OF HYDROLOGY I GENERAL SURFACE-WATER TECHNIQUES

Flood Continued

(Leopold and Maddock, 1954, p. 

249-251.)

A relatively high flow as measured by either gage height or discharge quantity. (Jarvis and 

others, 1936, p. 463.)

A glossary of flood terms is given 

in "The Flood Control Controversy." (Leopold and Maddock, 

1954, p. 249-251.) See Annual 

flood.

Flood-control storage. Storage of 

water in reservoirs to abate flood 

damage. (See Retarding reservoir.)

Flood crest. See Flood peak.

Flood event. See Flood wave.

Flood-frequency curve. 1. A graph 

showing the number of times per 

year on the average, plotted as 

abscissa, that floods of magnitude, 

indicated by the ordinate, are 

equaled or exceeded. 2. A similar 

graph but with recurrence intervals 

of floods plotted as abscissa. (See 

Dalrymple, 1960.)

Flood, maximum probable. The largest 

flood for which there is any reasonable expectancy in this climatic 

era. (Leopold and Maddock, 1954, 

p. 112.)

Flood peak. The highest value of the 

stage or discharge attained by a 

flood; thus, peak stage or peak discharge. Flood crest has nearly the 

same meaning, but since it connotes 

the top of the flood wave, it is properly used only in referring to 

stage thus, crest stage, but not 

crest discharge.

Flood plain. A strip of relatively 

smooth land bordering a stream, 

built of sediment carried by the 

stream and dropped in the slack 

water beyond the influence of the 

swiftest current. It is called a living flood plain if it is overflowed in 

times of highwater; but a fossil 

flood plain if it is beyond the reach 

of the highest flood. (Bryan, 1922, 

p. 88.)

Flood plain Continued

The lowland that borders a river, 

usually dry but subject to flooding. 

(Hoyt and Langbein, 1955, p. 12.)

That land outsid? of a stream 

channel described br the perimeter 

of the maximum probable flood. 

(After White, 1945, p. 44.)

Flood plair

Flood plane. The position occupied by 

the water surface of a stream during a particular flood. Also, 

loosely, the elevatio'1 of the water 

surface at various points along the 

stream during a particular flood.

Flood profile. A graph of elevation of 

the water surface of a river in flood, 

plotted as ordinate, against distance, measured in the downstream 

direction, plotted as abscissa. A 

flood profile may be drawn to show 

elevation at a given time, crests 

during a particular food, or to show 

stages of concordant flows.

Flood routing. The process of determining progressively the timing and 

shape of a flood war? at successive 

points along a river. (See Carter 

and Godfrey, 1960.)

Floods above a base. See Partial-duration flood series.

Flood stage. The gage height of the 

lowest bank of the reach in which 

the gage is situated. The term 

"lowest bank" is, however, not to be 

taken to mean an unusually low 

place or break in th<» natural bank 

through which the water inundates 

an unimportant ard small area. 

(Linsley, 1942, p. 8?.)

The stage at whi°h overflow of 

the natural banks cf a stream begins to cause damage in the reach 

in which the elevation is measured. 

(U.S. Weather Bur.) 

See also Bankfull stage.
GENERAL INTRODUCTION AND HYDROLOGIC DEFINITIONS 11

Flood wave. A distinct rise in stage 

culminating in a crest and followed 

by recession to lower stages.

Floodway. A part of the flood plain, 

otherwise leveed, reserved for emergency diversion of water during 

floods. A part of the flood plain 

which, to facilitate the passage of 

floodwater, is kept clear of encumbrances.

The channel of a river or stream 

and those parts of the flood plains 

adjoining the channel, which are 

reasonably required to carry and 

discharge the floodwater or floodflow of any river or stream (Erbe 

and Flores, 1957, p. 443).

Flood zone. The land bordering a 

stream which is subject to floods of 

about equal frequency; for example, a strip of the flood plain subject to flooding more often than 

once but not as frequently as twice 

in a century. (See White, 1945, 

P. 44.)

Flow-duration curve. A cumulative 

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

(See Searcy, 1959.)

Flow-duration curve

Forest influences. Effects res'ilting 

from the presence of forest or 

brush upon climate, soil water, runoff, streamflow, floods, erosion, and 

soil productivity. (Kittredge, 1948, 

P-l.)

Frazil (frazil ice). A French-Canadian 

term for fine spicular ice, derived 

from the French for cinders which 

this variety of ice most resembles. 

When formed in salt water, it is 

known as lolly ice. It is conrioosed 

of fine particles which, when first 

formed, are colloidal and no4: seen 

in the water in which they are floating. (Barnes, 1928, p. 108; see also 

Schaefer, 1950, p. 888.)

Gage height. The water-surface elevation referred to some arbitrary 

gage datum. Gage height is often 

used interchangeably witl the 

more general term stage although 

gage height is more appropriate 

when used with a reading on a 

gage.

Gaging station. A particular site on a 

stream, canal, lake, or reservoir 

where systematic observations of 

gage height or discharge are obtained. (See also Stream-gaging 

station.)

Glacier. Bodies of land ice that consist of recrystallized snow accumulated on the surface of the {round 

(Matthes, 1949, p. 150), ani that 

move slowly downslope.

Ground water. Water in the ground 

that is in the zone of saturation, 

from which wells, springs, and 

ground-water runoff are supplied. 

(After Meinzer, 1949, p. 38r,)

Ground-water outflow. That part of 

the discharge from a drainage 

basin that occurs through the 

ground water. The term "Tinderflow" is often used to deserve the 

ground-water outflow that takes 

place in valley alluvium (instead 

of the surface channel) and thus is 

not measured at a gaging station.
12 MANUAL OF HYDROLOGY! GENERAL SURFACE-WATEE TECHNIQUES

Ground-water runoff. That part of the 

runoff which has passed into the 

ground, has become ground wat3r, 

and has been discharged int a 

stream channel as spring or seepage water. See also Base runoff 

and Direct runoff.

Guttation. The loss of water in liquid 

form from the uninjured leaf or 

stem of the plant, principally 

through water stomata. (Lee, 

1949, p. 260.)

Heat budget, annual (of a lake). The 

amount of heat necessary to raise 

the water from the minimum temperature of winter to the maximum 

temperature of summer. (Welch, 

1952, p. 65.)

Hydrograph. A graph showing stage, 

flow, velocity, or other property of 

water with respect to time.

a Base runoff

b Direcf runoff

c Recession curve

Hydrograph

Hydrologic budget. An accounting of 

the inflow to, outflow from, and 

storage in, a hydrologic unit, such 

as a drainage basin, aquifer, soil 

zone, lake, reservoir, or irrigation 

project.

Hydrologic cycle. A convenient term 

to denote the circulation of water 

from the sea, through the atmosphere, to the land; and thence, 

with many delays, back to the sea 

by overland and subterranean 

routes, and in part by way of the 

atmosphere; also the many short 

circuits of the water that is returned to the atmosphere without

Hydrologic cycle Continued

reaching the sea. (After Meinzer,

1949, p. 1.) 

Hydrologic equation. The equation

balancing the hydrologic budget. 

Hydrology. The science encompassing

the behavior of water as it occurs

in the atmosphere, on the surface

of the ground, and underground.

(Am. Soc. Civil E"<rineers, 1949,

P. 1.)

The science that relates to the 

water of the earth. (Meinzer, 

1923, p. 9.)

The science treating of the waters of the earth, tl ?ir occurrence, 

distribution, and movements. (Jarvis and others, 1936, p. 464.)

In practice the study of the water of the oceans and the atmosphere is considerei part of the 

sciences of oceanography and meteorology.

Hyetograph. Graphical representation 

of rainfall intensity against time.

Hypolimnion. See Thermal stratification.

Infiltration. The flow of a fluid into 

a substance through pores or small 

openings. It connotes flow into a 

substance in contn distinction to 

the word percolation, which connotes flow through a porous substance. (Horton, 1942, p. 480.) 

See also Schiff rnd Dreibelbis 

(1949, p. 76) and Musgrave (1946, 

p. 726-747).

Infiltration capacity. The maximum 

rate at which the soil, when in a 

given condition, can absorb falling 

rain or melting snow. (After 

Horton, 1935, p. 2.)

Infiltration index. An average rate of 

infiltration, in inches per hour, 

equal to the average rate of rainfall such that the volume of rainfall at greater rates equals the 

total direct runoff. (Langbein and 

others, 1947, p. 11.)

Interception. The process and the 

amount of rain or snow stored on
GENERAL INTRODUCTION AND HYDROLOGIC DEFINITIONS 13

Interception Continued

leaves and branches and eventually 

evaporated back to the air. Interception equals the precipitation on 

the vegetation minus stem flow and 

throughfall (after Hoover, 1953, 

p. 1.)

Irrigated area. The gross farm area 

upon which water is artificially 

applied for the production of crops, 

with no reduction for access roads, 

canals, or farm buildings. (Simons, 1953, p. 8.)

Irrigation. Tr-e controlled application 

of wi^er to arable lands to supply 

v/atrr ,-.-qui regents net satisfied by 

raiufr-n (After Plouk, 1951, p. 1.)

7 irrigation efficiency. The percentage 

of water applied that can be accounted for in soil-moisture increase. (Pillsbury, Compton, and 

Picker, 1944, p. 7.)

Irrigation requirement. The quantity 

of water, exclusive of precipitation, that is required for crop 

production. It includes surface 

evaporation and other economically 

unavoidable wastes. Blaney, 

1951a, p. 4.)

Irrigation, supplemental. See Supplemental irrigation.

Isohyet. See Isohyetal line.

Isohyetal line (isohyet). A line drawn 

on a map or chart joining points 

that receive the same amount of 

precipitation.

Isobyetal lines

Lag. Variously defined as time from 

beginning (or center of mas^) of 

rainfall to peak (or center of 

mass) of runoff. (After Am. Soc. 

Civil Engineers, 1949, p. 106.)

Limnology. That branch of hydrology 

pertaining to the study of lakes.

Long-period variations. Secular when 

a cycle or a change in trend is 

completed within a century: climatic when the period of cl ange 

runs through centuries or a few 

millenia ; geologic when the period 

runs into geological time. (Willett, 1948, p. 806.) (Sec Tr«nd.)

Low-flow frequency curve. A graph 

showing the magnitude and frequency of minimum flows for a 

period of given length. Frequency 

is usually expressed as the average 

interval, in years, between recurrences of an annual minimum flow 

equal to or less than that shovn by 

the magnitude scale.

Lysimeter. Structure containirg a 

mass of soil, and designed to permit the measurement of water 

draining through the soil. (Harrold and Dreibelbis, 1951, p. 3.) 

(See also Kohnke, Dreibelbis and 

Davidson, 1940, p. 1-67.)

Mass curve. A graph of the cumulative values of a hydrologic quantity (such as precipitation or 

runoff), generally as ordinate, 

plotted against time or date as 

abscissa. (See Double-mass 

curve, and Residual-mass cur^e.)

Maximum probable flood. Sec Flood, 

maximum probable.
14 MANUAL OF HYDROLOGY: GENERAL SURFACE-WATEF TECHNIQUES

Meander. The winding of a stream 

channel.

a Meander amplitude 

b Meander belt 

c Meander length

Meanders

Meander amplitude. Distance between 

points of maximum curvature of 

successive meanders of opposite 

phase in a direction normal to the 

general course of the meander belt, 

measured between centerlines of 

channels.

Meander belt. Area between lines 

drawn tangential to the extreme 

limits of fully developed meanders.

Meander breadth. The distance between the lines used to define the 

mean&er "belt.

Meander length. Distance in the general course of the meanders between 

corresponding points of successive 

meanders of the same phase.

Twice the distance between successive points of inflection of the 

meander wave. (Leopold and 

Wolman, 1957, p. 55.)

Meromictic lake. A lake in which some 

water remains partly or wholly unmixed with the main water mass at 

circulation periods is said to be 

meromictic. The process leading

Meromictic lake Continued

to a meromictic s-tate is termed 

meromixis. The perennially stagnant deep layer of a meromictic 

lake is called the monimolimnion. 

The part of a meromictic lake in 

which free circulation can occur is 

called the mirolimnion. The 

boundary between the monimolimnion and the mixolimnion is called 

the chemocline. (Hutchinson, 1957, 

p. 480.)

Moisture. Water diffused in the atmosphere or the ground.

Moisture equivalent. The ratio of (a) 

the weight of water which the soil, 

after saturation, will retain against 

a centrifugal force 1,000 times the 

force of gravity, to (b) the weight 

of the soil when dry. The ratio 

is stated a£ a percentage. (Meinzer, 1923, p. 25 ; see also Briggs and 

McLane, 1907, p. 5 )

Mudflow. A well-mixec1 mass of water 

and alluvium whicl . because of its 

high viscosity and low fluidity as 

compared with water, moves at a 

much slower rate, usually piling 

up and spreading o^er the fan like 

a sheet of wet mortar or concrete. 

(Woolley, 1946, p. 73.)

Normal. A central value (such as 

arithmetic average or median) of 

annual quantities for a 30-year period ending with an even 10-year, 

thus 1921-50; 1931-60, and so forth. 

This definition accords with that 

recommended by th^ Subcommittee 

on Hydrology of th? Federal InterAgency Committee on Water 

Resources.

Overland flow. The flew of rainwater 

or snowmelt over the land surface 

toward stream channels. After it 

enters a stream, it becomes runoff.

Partial-duration flood series. A list of 

all flood peaks that exceed a chosen 

base stage or discharge, regardless 

of the number of peaks occurring 

in a year. (Also crlied basic-stage 

flood series, or floods above a base.)
GENERAL INTRODUCTION AND HYDROLOGIC DEFINITIONS 15

Percolation. The movement, under 

hydrostatic pressure, of water 

through the interstices of a rock 

or soil, except the movement 

through large openings such as 

caves. (Meinzer, 1923, p. 42; see 

also Rorabaugh, 1951, p. 165.

Percolation, deep. In irrigation or 

farming practice, the amount of 

water that passes below the root 

zone of the crop or vegetation. 

(Barrett and Milligan, 1953, p. 24.)

Pondage. Small-scale storage at a 

waterpower plant to equalize daily 

or weekly fluctuations in riverflow 

or to permit irregular hourly use 

of the water for power generation 

to accord with fluctuations in load. 

(After Barrows, 1943, p. 166.)

Pool. A deep reach of a stream. The 

reach of a stream between two 

riffles. Natural streams often consist of a succession of pools and 

riffles.

Pool and riffles

Potential evapotranspiration. Water 

loss that will occur if at no time 

there is a deficiency of water in 

the soil for use of vegetation. 

(Thornthwaite, 1944, p. 687.)

Potential natural water loss. The 

water loss during years when the 

annual precipitation greatly exceeds the average water loss. It 

represents the approximate upper 

limit to water loss under the type 

and density of vegetation native to 

a basin, actual conditions of moisture supply, and other basin characteristics, whereas potential

Potential natural water loss Con.

evapotranspiration represerts the 

hypothetical condition of DO deficiency of water in the soil at any 

time for use of the type ard density of vegetation that would develop. (After Troxell and others, 

1954, pi. 11B.)

Potential rate of evaporation. See 

Evaporativity.

Precipitation. As used in hydrology, 

precipitation is the discharge of 

water, in liquid or solid strte, out 

of the atmosphere, generall" upon 

a land or water surface. It is the 

common process by which atmospheric water becomes surface or 

subsurface water * * *. The term 

"precipitation" is also corimonly 

used to designate the quant'tity of 

water that is precipitated. (Meinzer, 1923, p. 15.)

Precipitation includes rainfall, 

snow, hail, and sleet, and is therefore a more general terr« than 

rainfall.

Rain. Liquid precipitation.

Rainfall. The quantity of water that 

falls as rain only. Not synonymous with precipitation.

Rainfall excess. The volume of rainfall available for direct ruroff. It 

is equal to the total rainfall minus 

interception, depression storage, 

and absorption. (See Ar\ Soc. 

Civil Engineers, 1949, p. 106.)

Rainfall, excessive. Rainfall in which 

the rate of fall is greater than certain adopted limits, chosen with 

regard to the normal precipitation 

(excluding snow) of a given place 

or area. In the U.S. leather 

Bureau, it is defined, for States 

along the southern Atlantic coast 

and the Gulf coast, as rainfall in 

which the depth of precipitation is 

0.90 inch at the end of 30 minuties and 1.50 inches at the end of 

an hour, and for the rest of the 

country as rainfall in wl 'ch the 

depth of precipitation at the end
10 MANUAL OF HYDROLOGY : GENERAL SURFACE-WATER TECHNIQUES

Rainfall, excessive Continued

of each of the same periods is 0.50 

and 0.80 inch, respectively.

Reach. 1. The length of channel uniform with respect to discharge, 

depth, area, and slope. 2. The 

length of a channel for which a 

single gage affords a satisfactory 

measure of the stage and discharge. 

3. The length of a river between 

two gaging stations. 4. More generally, any length of a river.

Recession curve. A hydrograph showing the decreasing rate of runoff 

following a period of rain or snowmelt. Since direct runoff and 

base runoff recede at different 

rates, separate curves, called direct runoff recession curves or base 

runoff recession curves, are generally drawn. The term "depletion curve" in the sense of base 

runoff recession is not recommended.

Recurrence interval (return period). 

The average interval of time within 

which the given flood will be 

equaled or exceeded once. (Am. 

Soc. of Civil Engineers, 1953, p. 

1221.)

Regime. "Regime theory" is a theory 

of the forming of channels in material carried by the streams. As 

used in this sense, the word "regime" applies only to streams that 

make at least part of their boundaries from their transported load 

and part of their transported load 

from their boundaries, carrying out 

the process at different places and 

times in any one stream in a balanced or alternating manner that 

prevents unlimited growth or removal of boundaries. A stream, 

river, or canal of this type is called 

a "regime stream, river, or canal." 

A regime channel is said to be 

"in regime" when it has achieved 

average equilibrium; that is, the 

average values of the quantities 

that constitute regime do not show

Regime Continued

a definite trend over a considerable 

period generally of the order of 

a decade. In unspecialized use 

"regime" and "regiiren" are synonyms. (After Blench, 1957, p. 2.)

Regimen of a stream. The system or 

order characteristic of a stream; 

in other words, its habits with 

respect to velocity and volume, 

form of and changes in channel, 

capacity to transport sediment, 

and amount of material r-'ir-r': -^ 

for transportation. The tern is 

also applied to a stream which has 

reached an equilibrium between 

corrosion and deposition or, in 

other words, to a g-arted stream. 

(Bryan, 1922, p f®.)

Regulation. The ar*'1c'?j ~-?"?7 r:' 

tion of the flow of a str^r-,.

Re-regulating reservoir. A reservoir 

for reducing diurnal fluctuations 

resulting from the operation of an 

upstream reservoir f ~T povs-ar production.

Reservoir. A pond, lal-e, or basin, 

either natural or artificial, for the 

storage, regulation, rnd control of 

water.

Residual-mass curve. A graph of the 

cumulative departures from a 

given reference such as the arithmetic average, generally as ordinate, plotted against time or date, 

as abscissa. (See Mass curve.)

Retarding reservoir. Ungated reservoir for temporary storage of flood 

water. Sometimes ca lied detention 

reservoir.

Return flow. That part of irrigation 

water that is not consumed by 

evapotranspiration and that returns to its source or another body 

of water. The term is also applied 

to the water that is discharged 

from industrial plants. Also 

called return water.

Riffle. A rapid in a stream.

Riparian. Pertaining to the banks of 

a stream.
GENERAL INTRODUCTION AND HYDROLOGIC DEFINITIONS 17

Runoff. That part of the precipitation 

that appears in surface streams. 

It is the same as streamflow unaffected by artificial diversions, 

storage, or other works of man in 

or on the stream channels. Runoff may be classified as follows: 

Classification as to speed of 

appearance after rainfall or snow melting: 

Direct runoff 

Base runoff 

Classification as to source:

Surface runoff (see Overland flow) 

Storm seepage 

Ground-water runoff (see

Stream, gaining) 

Runout. See Water yield. 

Second-foot. Same as cfs. This term 

is no longer used in published reports of the U.S. Geological Survey. 

Sediment. Fragmental material that 

originates from weathering of rocks 

and is transported by, suspended 

in, or deposited by water or air or 

is accumulated in beds by other 

natural agencies. (Colby, Hembree, and Jochens, 1953, p. 24.) 

Sediment discharge. The rate at which 

dry weight of sediment passes a 

section of a stream or is the quantity of sediment, as measured by 

dry weight, or by volume, that is 

discharged in a given time. (Colby, 

Hembree, and Jochens, 1953, p. 24.) 

Seiche. The free oscillation of the bulk 

of water in a lake and the motion 

caused by it on the surface of the 

lake. (Bergsten, 1926, p. 1.) 

Shifting control. See Control. 

Skimming. The diversion of water 

from a stream or conduit by a 

shallow overflow used to avoid diversion of sand, silt, or other debris 

carried as bottom load. 

Snow. A form of precipitation composed of ice crystals. 

Snow course. A line or series of connecting lines along which snow 

samples are taken at regularly 

spaced points. (U.S. Dept. AgriSnow course Continued

culture, Soil Conserv. Service and 

Nevada State Engineer, 1948, p. 2.)

Snow density. Ratio between tb« volume of melt water derived f'om a 

sample of snow and the initial volume of the sample. This is numerically equal to the specific gravity of the snow. (Linsley, Kohler, 

and Paulhus, 1949, p. 127.)

Snowline. The general altitude to 

which the continuous snow cover 

of high mountains retreats ir summer, chiefly controlled by the depth 

of the winter snowfall and by the 

temperature of the summer.

Snowline, temporary. A line sometimes 

drawn on a weather map during 

the winter showing the southern 

limit of the snow cover.

Snow, quality of. The ratio of I eat of 

melting of snow, in calories per 

gram to the 80 calories per gram 

for melting pure ice at 0°C. (Bernard and Wilson, 1941, p., 178-179.) 

(See also Wilson, 1942b, p 553- 

556.)

Percentage by weight which is 

ice (Linsley, Kohler, and Peulhus, 

1949, p. 129).

Soil moisture (Soil water). Water 

diffused in the soil, the upper part 

of the zone of aeration from which 

water is discharged by the transpiration of plants or by soil evaporation. See Field-moisture capacity and Field-moisture deficiency.

Stage. The height of a water snirface 

above an established datum, plane; 

also gage height.

Stage-capacity curve. A graph showing the relation between tl » surface elevation of the wate~ in a 

reservoir, usually plotted as ordinate, against the volume belcw that 

elevation, plotted as abscissa.

Stage-discharge curve (rating curve). 

A graph showing the relation between the gage height, usually 

plotted as ordinate, and the rmount 

of water flowing in a chanr^l, ex-
18 MANUAL OF HYDROLOGY: GENERAL SURFACE-WATER TECHNIQUES

Stage-discharge curve Continued

pressed as volume per unit of time, 

plotted as abscissa.

Stage-discharge relation. The relation expressed by the stage-discharge curve.

Stage, flood. See Flood stage.

Stemflow. Rainfall or snowmelt led 

to the ground down the trunks or 

stems of plants (Hoover, 1953, p, 

1).

Storage. 1. Water artificially impounded in surface or underground 

reservoirs, for future use. The 

term regulation refers to the action 

of this storage in modifying 

sir earn flow. See also Conservation 

storage, Total storage, Dead storage, and Usable storage. 2. Water 

naturally detained in a drainage 

basin, such as ground water, channel storage, and depression storage. The term "drainage basin 

storage" or simply "basin storage" 

is sometimes used to refer collectively to the amount of water in 

natural storage in a drainage basin.

Storage, bank. See Bank storage.

Storage, conservation. See Conservation storage.

Storage, dead. See Dead storage.

Storage, depression. See Depression 

storage.

Storage ratio. The net available storage divided by the mean flow for 1 

year. (Hazen, 1930, p. 1446.) 

(See also Thomas and Harbeck, 

1956, p. 14.)

Storage-required frequency curve. A 

graph showing the frequency with 

which storage equal to or greater 

than selected amounts will be required to maintain selected rates 

of regulated flow.

Storage, total. See Total storage.

Storage, usable. See Usable Storage.

Storm. A disturbance of the ordinary 

average conditions of the atmosphere which, unless specifically 

qualified, may include any or all 

meteorological disturbances, such

Storm Continued

as wind, rain, snow, hail, or thunder.

Stormflow. See Direct runoff. 

Storm seepage. That part of precipitation which inflltretes the surface 

soil, and moves toward the streams 

as ephemeral, shallow, perched 

ground water above the main 

ground-water level. Storm seepage is usually part of the direct 

runoff.

Stream. A general terrr for a body of 

flowing water. In hydrology the 

term is generally applied to the 

water flowing in a natural channel 

as distinct from a canal. More 

generally as in th^ term stream 

gaging, it is applied to the water 

flowing in any charnel, natural or 

artificial.

Streams in nature! channels may 

be classified as follows (after 

Meinzer, 1923, p. 56-58) : 

Relation to time. 

Perennial. One which flows

continuously.

Intermittent or seasonal. One 

which flows only at certain 

times of the year when it receives water from springs or 

from some surface source 

such as me1 ting snow in 

mountainous areas. 

Ephemeral. Ore that flows 

only in direct response to 

precipitation, and whose 

channel is at all times above 

the water table. 

Relation to space.

Continuous. O T e that does not 

have interruptions in space. 

Interrupted. One which contains alternating reaches, 

.that are eithe'- perennial, intermittent, or ephemeral. 

Relation to groun^ water.

Gaining. A str?am or reach of

a stream that receives water

from the zone of saturation.

Losing. A stream or reach of

a stream ttat contributes
GENERAL INTRODUCTION AND HYDROLOGIC DEFINITIONS 19

Stream Continued

water to the zone of saturation.

Insulated. A stream or reach 

of a stream that neither contributes water to the zone of 

saturation nor receives water 

from it. It is separated 

from the zones of saturation 

by an impermeable bed. 

Perched. A perched stream is 

either a losing stream or an 

insulated stream that is separated from the underlying 

ground water by a zone of 

aeration.

Streamflow. The discharge that occurs 

in a natural channel. Although 

the term discharge can be applied 

to the flow of a canal, the word 

streamflow uniquely describes the 

discharge in a surface stream 

course. The term "streamflow" is 

more general than runoff, as 

streamflow may be applied to discharge whether or not it is affected 

by diversion or regulation. 

Streamflow depletion. The amount of 

water that flows into a valley, or 

onto a particular land area, minus 

the water that flows out the valley 

or off from the particular land 

area. (Blaney, 1951a, p. 4.) 

Stream gaging. The process and art of 

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

Corbett and others, 1943.) 

Stream-gaging station. A gaffing station where a record of discharge 

of a stream is obtained. Within 

the Geological Survey this term is 

used only for those gaging stations 

where a continuous record of discharge is obtained.

Stream order. A method of numbering 

streams as part of a drainage basin 

network. The smallest unbranched 

mapped tributary is called first 

order, the stream receiving the 

tributary is called second order, and 

so on. It is usually necessary to

Stream order Continued

specify the scale of the map used. 

A first-order stream on a 1:62,500 

map, may be a third-order stream 

on a 1:12,000 map. (After Leopold 

and Miller, 1956, p. 16.)

Tributaries which have no 

branches are designated as of the 

first order, streams which receive 

only first-order tributaries ere of 

the second order, larger branches 

which receive only first-orde- and 

second-order tributaries are designated third order, and so on, the 

main stream being always of the 

highest order. (Horton, 1932, 

p. 356.)

Stream orders

Submeander. Small meander contained with banks of main channel, associated with relatively low 

discharges.

Subsurface runoff. See Storm seepage.

Supplemental irrigation. Commonly, 

irrigation as carried on in humid 

areas. The term means that the 

irrigation water is supplementary 

to the natural rainfall rathe" than 

being the primary source of moisture as in the arid and serniarid 

West. Supplementary irrigation 

is used generally to prevent retardation of growth during periods of 

drought. (Huffman, 1953, p. 231.)

Supplemental sources. When irrigation water supplies are obtained
20 MANUAL OF HYDROLOGY: GENERAL SURFACE-WATER TECHNIQUES

Supplemental sources Continued

from more than one source, the 

source furnishing the principal 

supply is commonly designated the 

primary source, and the sources 

furnishing the additional supplies, 

the supplemental sources. (Houk,

1951. p. 396.)

Surface runoff. That part of the runoff which travels over the soil surface to the nearest stream channel. 

It is also defined as that part of 

the runoff of a drainage basin that 

has not passed beneath the surface since precipitation. The term 

is misused when applied in the 

sense of direct runoff. See also, 

Runoff, Overland flow, Direct runoff, Ground-water runoff, and Surface water.

Surface water. Water on the surface 

of the earth.

Tank. An artificial reservoir for stock 

water; local in Southwest.

Terrace. A berm or discontinuous segments of a berm, in a valley at 

some height above the fl.ood plain, 

representing a former abandoned 

flood plain of the stream.

Thermal stratification (of a lake). 

Vertical temperature stratification 

that shows the following: The upper layer of the lake, known as 

the epilimnion, in which the water 

temperature -is virtually uniform; 

a stratum next below, known as 

the thermocline, in which there is 

a marked drop in temperature per 

unit of depth; and the lowermost 

region or stratum, known as the 

hypolimnion, in which the temperature from its upper limit to the 

bottom is nearly uniform. (Welch,

1952. p. 51.)

TEMPERATURE 

A B

Thermal stratification. A. Cross section. 

B. Graph showing variation in temperature with depth

Thermocline. See Thermal stratification.

Throughfall. In a vegetated area, the 

precipitation that falls directly to 

the ground or the rainwater or 

snowmelt that drops from twigs 

or leaves. (After Hoover, 1953, 

p. 1.) (See Stemflow.)

Time of concentration. The time required for water to flow from the 

farthest point on th^ watershed to 

the gaging stat'on. (Ramser, 

1927, p. 804.)

Total storage. The vohime of a reservoir below the marimum controllable level including dead storage. 

(Thomas and HarVck, 1956, p. 

13.)

Transpiration. The quantity of water 

absorbed and transpired and used 

directly in the bui"'iing of plant 

tissue, in a specified time. It does 

not include soil evaporation. 

(After Blaney, I951a, p. 4.)

The process by which water vapor escapes from tte living plant, 

principally the leaves, and enters 

the atmosphere. * * * As considered practically, transpiration also 

includes guttation. (Lee, 1949, p. 

260.)

Trend. A statistical term referring to 

the direction or rate of increase 

or decrease in magnitude of the 

individual members of a time series 

of data when randcm fluctuations 

of individual memt^rs are disregarded.

Underflow. The downstream flow of 

water through the permeable deposits that underlie a stream and 

that are more or less limited by 

rocks of low permeability.

Unit hydrograph. The hydrograph of 

direct runoff from a storm uniformly distributed ever the drainage basin during a specified unit of 

time; the hydrograph is reduced 

in vertical scale to correspond to a 

volume of runoff o* 1 inch from 

the drainage basin. (After Am. 

Soc. Civil Engineers 1949, p. 105.)
21

Unit hydrograph Continued

The hydrograph of surface runoff 

(not including ground-water runoff) on a given basin due to an 

effective rain falling for a unit of 

time. (Sherman, 1949, p. 514.) 

(See also Hoyt and others, 1936, 

p. 124.)

Usable storage. The volume normally 

available for release from a reservoir below the stage of the maximum controllable level. (Thomas 

and Harbeck, 1956, p. 13.)

Water balance. See Hydrologic 

budget.

Water content of snow. See Water 

equivalent of snow.

Water crop. See Water yield.

Water equivalent of snow. Amount of 

water that would be obtained if the 

snow should be completely melted. 

Water content may be merely the 

amount of liquid water in the snow 

at the time of observation. (Wilson, 1942a, p. 153-154.)

Water loss. The difference between 

the average precipitation over a 

drainage basin and the water yield 

from the basin for a given period. 

(After Williams and others, 1940, 

p. 3.) The basic concept is that 

water loss is equal to evapotranspiration, that is, water that returns to the atmosphere and thus 

is no longer available for use. 

However, the term is also applied 

to differences between measured 

inflow and outflow even where part 

of the difference may be seepage.

Water requirement. The quantity of 

water, regardless of its source, required by a crop in a given period 

of time, for its nonnal growth 

under field conditions. It includes 

surface evaporation and other 

economically unavoidable wastes. 

(Blaney, 1951a, p. 4.)

Watershed. The divide separating 

one drainage basin from another 

and in the past has been generally 

used to convey this meaning.

Watershed Continued

However, over the years, use of the 

term to signify drainage basin or 

catchment area has come to predominate, although drainage basin 

is preferred. Drainage divide, or 

just divide, is used to denote the 

boundary between one drainage 

area and another. Used alone, the 

term "watershed" is ambiguous 

and should not be used unless the 

intended meaning is made cleer.

Water table. The upper surface of a 

zone of saturation. No water table 

exists where that surface is formed 

by an impermeable body. (Heinzer, 1923, p. 22.)

Water year. In Geological Survey reports dealing with surface-vater 

supply, the 12-month period, October 1 through September 30. The 

water year is designated by the 

calendar year in which it ends- and 

which includes 9 of the 12 months. 

Thus, the year ended September 30, 

1959, is called the "1959 vater 

year."

Water yield (water crop or runout). 

The runoff from the drainage t asin. 

including ground-water outflow 

that appears in the stream plus 

ground-water outflow that bypasses 

the gaging station and leavep the 

basin underground. Water yield 

is the precipitation minus the 

evapotranspiration.

Withdrawal use of water. The vater 

removed from the ground o~ di 

verted from a stream or lake for 

use. (MacKichan, 1957, p. 2.)

Year. See Climatic year; Water year.

Zone of aeration. The zone abov? the 

water table. Water in the zo^e of 

aeration does not flow into a well.

Zone of saturation. The zone in vhich 

the functional permeable rocks are 

saturated with water under hydrostatic pressure. (Meinzer, 1923, p. 

21.) Water in the zone of saturation will flow into a well, and is 

called ground water.
22 MANUAL OF HYDROLOGY: GENERAL SURFACE-WATER TECHNIQUES

TOPICAL FINDING LIST

With terms arranged in alphabetical order as they are in the 

previous section, it is difficult to find the definitions of related terms. 

For convenience in finding such terms, the following list is organized 

by major topics, with related terms grouped. The main headings are 

alphabetized, and the subheadings are arranged approximately in the 

order of their association with the main headings. TT°- terms that 

are grouped with the subheadings are similarly arrange'!. All terms 

defined in this manual are included, and some may appear under more 

than one heading. Page numbers are given for ready reference to 

the definitions.

Atmospheric water

Precipitation, 15; interception, 12; throughfall, 20; stemflow, 18.

Rain, 15; rainfall, 15; rainfall, excessive, 15; cloudburst, 6; rainfall excess, 15; effective precipitation, 8; antecedent precipitation index, 4.

Snow, 17; water equivalent of snow, 21.

Condensation, 6; moisture, 14.

Evaporation, 9; evaporation pan, 9; interception, 12; potential 

rate of evaporation, 15; evaporation opportunity, 9; evaporativity, 9.

Transpiration, 20; guttation, 12.

Evapotranspiration, 9; evaporation, total, 9; water loss, 21; consumptive waste, 6; potential evapotranspiration, 15; potential 

natural water loss, 15.

Climate, 5; drought, 8. 

Channel

Channel, 5; channel storage, 5; stream, 18; stream, perennial, 18; 

stream, intermittent or seasonal, 18; stream, ephemeral, 18; 

stream, continuous, 18; stream, interrupted, 18; discharge, 7; 

anabranch, 4; reach, 16.

Bank, 4; bank storage, 4; riparian, 16.

Stage, 17; gage height, 11; bankfull stage, 4; flood stage, 10; 

backwater, 4.

Regime, 16; regimen of a stream, 16; braiding of river channels, 5; 

pool, 15; riffle, 16.

Meander, 14; meander amplitude, 14; meander belt, 14; meander

breadth, 14; meander length, 14; submeander, 19. 

Floods

Flood, 9; flood wave, 11; flood event, 10; flood peak, 10; flood 

crest, 10; flood stage, 10; bankfull stage, 4; direct runoff, 7; 

flood, maximum probable, 10.
GENERAL INTRODUCTION AND HYDROLOGIC DEFINITIONS 23

Floods Continued

Flood profile, 10; flood plane, 10; concordant flows, 6; flood routing, 10; channel storage, 5; lag, 13; time of concentration, 20.

Flood-frequency curve, 10; annual flood, 4; annual flood series, 4; 

partial-duration flood series, 14; basic-stage flood series, 5; base 

discharge, 4; floods above a base, 10; recurrence interval, 16.

Flood plain, 10; flood zone, 11.

Floodway, 11; flood-control storage, 10; retarding reservoir, 16.

Unit hydrograph, 20; distribution graph, 8; antecedent precipitation index, 4; rainfall excess, 15; lag, 13. 

Geomorphology

Drainage area, 8; drainage basin, 8; catchment area, 5; drainage 

divide, 8; watershed, 21.

Bank, 4; riparian, 16; bankfull stage, 4: flood stage, 10; stream, 

18; stream, perennial, 18; stream, intermittent or seasonal, 18; 

stream, ephemeral, 18; stream, continuous, 18; stream, interrupted, 18; regimen of a stream, 16; stream order, 19.

Channel, 5; braiding of river channels, 5; anabranch, 4; drainage 

density, 8; reach, 16; riffle, 16; pool, 15; control, 6; s*:agedischarge relation, 18.

Meander, 14; meander amplitude, 14; meander belt, 14; mea nder 

breadth, 14; meander length, 14; submeander, 19.

Flood plain, 10; terrace, 20.

Runoff, 17; concordant flows, 6;'lag, 13; depression storage, 7.

Sediment, 17; sediment discharge, 17; mudflow, 14; regime, 16. 

Ground water

Ground water, 11; ground-water outflow, 11; underflow, 20.

Runoff, 17; base runoff, 5; base flow, 5; ground-water runoff, 12; 

storm seepage, 18; recession curve, 16.

Zone of aeration, 21; percolation, 15; percolation, deep, 15; zone 

of saturation, 21; water table, 21.

Stream, gaining, 18; stream, losing, 18; stream, insulatec1 19;

stream, perched, 19. 

Hydrology

Hydrology, 12; hydrologic cycle, 12; hydrologic budget, 12; 

hydrologic equation, 12; water balance, 21; water yield, 21; 

water loss, 21.

Surface water, 20; ground water, 11; precipitation, 15; evaporation, 9; transpiration, 20; limnology, 13.

Basic hydrologic data, 5; water year, 21; climatic year, 5.

Climate, 5; storm, 18; drought, 8; cycle, 7; long-period variations, 13; normal, 14; trend, 20.
24 MANUAL OF HYDROLOGY: GENERAL SURFACE-WATER TECHNIQUES

Lakes

Limnology, 13; surface water, 20; evaporation, 9.

Thermal stratification (epilimnion, hypolimnion, thermocline), 

20; meromictic lake, 14.

Heat budget, annual, 12.

Seiche, 17.

Storage, 18; area-capacity curve, 4; stage-capacity curve, 17;

contents, 6. 

Land surface

Absorption, 4; infiltration, 12; infiltration capacity, 12; infiltration index, 12.

Overland flow, 14; surface runoff, 20; lag, 13.

Depression storage, 7; streamflow depletion, 19. 

Low flow

Base flow, 5; base runoff, 5; ground-water outflow, 11; groundwater runoff, 12; recession curve, 16; dependable yield, n-years, 

7.

Climatic year, 5; drought, 8.

Low-flow frequency curve, 13; recurrence interval, 16; flow-duration curve, 11; storage-required frequency curve, 18. 

Snow, ice, and glaciers

Cryology, 7.

Snow, 17; snowline, 17; snowline, temporary, 17; snow course, 17; 

snow density, 17; snow, quality of, 17; water equivalent of 

snow, 21; water content of snow, 21.

Anchor ice, 4; frazil, 11.

Glacier, 11; ablation, 4; firn, 9; firn line, 9. 

Soil water

Soil moisture, 17; field-moisture capacity, 9; field-moisture deficiency, 9; moisture equivalent, 14; zone of aeration, 21.

Absorption, 4; infiltration, 12; infiltration capacity, 12; infiltration index, 12; percolation, 15; irrigation efficiency, 13.

Storm seepage, 18. 

Stream gaging

Stream gaging, 19; basic hydrologic data, 5.

Gaging station, 11; stream-gaging station, 19; current meter, 7.

Discharge, 7; streamflow, 19; runoff, 17; average discharge, 4.

Stage, 17; gage height, 11.

Stage-discharge curve, 17; discharge rating curve, 8; stage-discharge relation, 18; stage-capacity curve, 17; area-capacity 

curve, 4; control, 6; shifting control, 17.

Reach, 16; backwater, 4.

Drainage area, 8; drainage basin, 8; catchment are^,, 5.
GENERAL INTRODUCTION AND HYDROLOGIC DEFINITIONS 25

Streamflow, runoff, and water yield

Streamflow, 19; discharge, 7; average discharge, 4; normal, 14; 

diversion, 8; regulation, 16; skimming, 17; Streamflow depletion, 

19; stream-gaging station, 19.

Runoff, 17; direct runoff, 7; stormflow, 18; overland flow, 14; 

base runoff, 5; base flow, 5; surface runoff, 20; subsurface runoff, 19; ground-water runoff, 12; storm seepage, 18; depression 

storage, 7; effective precipitation, 8; rainfall excess, 15; time 

of concentration, 20.

Water yield, 21 ; water crop, 21; runout, 17; hydrologic budget, 

12; water balance, 21; hydrologic equation, 12; water loss 21; 

long-period variations, 13; trend, 20; normal, 14.

Drainage basin, 8; drainage area, 8; catchment area, 5; drainage

divide, 8; watershed, 21; stream, 18. 

Tools of analyses, instruments

Basic hydrologic data, 5; average discharge, 4; normal, 14; recurrence interval, 16; climatic year, 5; water year, 21; gaging 

station, 11; current meter, 7; lysimeter, 13.

Stage-discharge curve, 17; stage-capacity curve, 17; storage ratio, 

18.

Flow-duration curve, 11; duration curve, 8; recession curve., 16.

Mass curve, 13; residual-mass curve, 16; double-mass curve, 8.

Low-flow frequency curve, 13; storage-required frequency curve, 

18; recurrence interval, 16.

Flood-frequency curve, 10; annual flood series, 4; partial-duration flood series, 14; recurrence interval, 16; base discharge 

(for peak discharge), 4.

Correlation, 6; correlative estimate, 6.

Unit hydrograph, 20; distribution graph, 8; rainfall excess 15; 

time of concentration, 20; lag, 13.

Hydrograph, 12; hyetograph, 12; isohyetal line, 13; antecedent 

precipitation index, 4; effective precipitation, 8; trend, 20; 

stream order, 19. 

Units and abbreviations

Acre-foot, 4.

Cfs, 5; cubic feet per second, 7; second-foot, 17; cusec, 7.

Cfs-day, 5.

Cfsm (cubic feet per second per square mile), 5. 

Vegetation

Interception, 12; throughfall, 20; stemflow, 18.

Transpiration, 20; evapotranspiration, 9; consumptive use, 6; 

guttation, 12; water loss, 21.
26 MANUAL OP HYDROLOGY: GENERAL SURFACE-WATEF TECHNIQUES

Vegetation Continued

Irrigation, 13; irrigated area, 13; irrigation requirement, 13; 

supplemental irrigation, 19; water requirement, 21.

Soil moisture, 17; field-moisture capacity, 9; field-moisture deficiency, 9; moisture equivalent, 14.

Forest influences, 11. 

Water use

Withdrawal use of water, 21; consumptive use, 6; consumptive 

waste, 6; diversion, 8; skimming, 17.

Irrigation, 13; irrigated area, 13; irrigation efficiency, 13; consumptive use, net, 6; irrigation requirement, 13; supplemental 

irrigation, 19; supplemental sources, 19; return flow, 16; water 

requirement, 21; streamflow depletion, 19; effective precipitation, 8.

Reservoir. 16; storage ratio, 18; regulation, 16; retarding reservoir, 16; re-regulating reservoir, 16; tank, 20.

Storage, 18; pondage, 15; contents, 6; dead storage, 7; conservation storage, 6; flood-control storage, 10; usable storage, 21; 

total storage, 20; storage-required frequency curve, 18; depletion, 7.

DependaM** yield, n-years, 7; water crop, 21; water yield, 21; 

drought, : 

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GENERAL INTRODUCTION AND HYDROLOGIC DEFINITIONS 27

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28 MANUAL OF HYDROLOGY: GENERAL SURFACE-WATER TECHNIQUES

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