Definitions of Selected


Ground-Water Terms-


Revisions and Conceptual


Refinements 

By S . W. LOHMAN and others 

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1988 

A report of the Committee on Redefinition 

of Ground-Later Terms-R. R. Bennett, 

R. H. Brown, H. H. Cooper, Jr., WJ. 

Drescher, J. G. Ferris, 14. I. Johnson, 

S. hF. Lohman (chairman beginning June 

1968), C. L. McGuinness, 14 . M. Piper 

(chairman from 1965 until retirement in 

1968), M. I. Rorabaugh, R. AV. Stallman, 

and C. V. Theis 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972 
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary 

GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

First printing 1960


Second printing 1961


Third printing 1966


Fourth printing 1973


Fifth printing 1983


For sale by the Distribution Branch, U.S . Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 
FOREWORD 

This report by an ad hoc Committee on Redefinition of GroundWater Terms contains recommendations for certain changes in 

terms and concepts and more importantly for the use of consistent 

units in ground-water flow equations. The report comes at an appropriate time as communication between hydrologists throughout the 

world is increasing greatly. The recommendations submitted by the 

committee will make future publications of the Water Resources 

Division, Geological Survey, more readily understood by all hydrologists and will promote more uniform usage of terms by Geological 

Survey personnel. 

The terms defined and recommendations in the report shall be 

standard for reports of the Geological Survey. In accordance with 

prevailing Survey practice, however, it is permissible to use different 

terms, if local circumstances or conditions so require, once the standard 

usage has been made clear. 

E. L. Hendricks, 

ChiefHydrologist 
------------

--------

---------

CONTENTS 

page 

Foreword_________________________________________________________ III 

Symbols and dimensions____________________________________________ v 

Introduction______________________________________________________ 1 

Terms____________________________________________________________ 2 

Selected references_________________________________________________ 15 

TABLES 

Page 

TABLE 1 . Relation of units . A. Hydraulic conductivity. B. Transmissivity . 

C. Permeability______________________________________ 18 

2 . Conversion factors for hydraulic units of measure 19 

3. Variation of properties of pure water with temperature__ 20 

4 . Relation between head of water and units of pressure 21 

SYMBOLS AND DIMENSIONS 

Symbol Dimensions Description 

A L2 Area. 

b L Saturated thickness of aquifer . 

cm L Centimeter. 

ft L Foot. 

9 LT-2 Acceleration due t, gravity. 

H L Total head, he +hp+h,. 

h L Static head, h,+hD . 

ha L Elevation head . 

hp L Pressure head. 

hti L Velocity head. 

J ML2 T-2 Joule. 

K LT-1 Hydraulic conductivity. 

K e LT-1 Effective hydraulic conductivity . 

k. L2 Intrinsic permeability . 

kg M Kilogram . 

l L Length. 

m L Meter. in La Milliliter. 
SYMBOLS AND DIMENSIONS 

Symbol Dimtnafons 

n (1) 

n, (1) 

P LZ 

Pm Lz 

Pf LT-1 

P ML-1 T-2 

Pa ML-1 T-2 

Q L3T-1

q LT-1 

(1) 

Sr (1) 

S, L-1 

S (1) 

s L 

T DT-1 

v LT-1 

v; LT-1 

x, y L 

z L 

p 

,u ML-1 T-1 

v L2 T-1 

P ML-3 

L2 T-2 

t __ _ _ _ _ _ _ _ _ _ _ 

1 Dimensionless. 

Description 

Porosity. 

Effective porosity. 

t Coefficient of permeability. 

tMeinzer's coefficient of permeability . 

tField coefficient of permeability . 

Pressure. 

Atmospheric pressure. 

Total discharge, or total flux . 

Specific discharge, or specific flux. 

Storage coefficient . 

Specific retention . 

Specific storage . 

Specific yield . 

Drawdown. 

Transmissivity . 

Velocity. 

Average interstitial velocity . 

Coordinates in horizontal plane. 

Vertical coordinate. 

Nabla: the gradient operator. 

Dynamic viscosity. 

Kinematic viscosity . 

Density. 

Fluid potential. 

_ _ _ _ Dagger preceding term indicates that use of 

term has been discontinued in reports of 

the U.S . Geological Survey. 
DEFINITIONS OF SELECTED


GROUND-WATER TERMS--REVISIONS


AND CONCEPTUAL REFINEMENTS


By S. W. LOHMAN and others 

INTRODUCTION 

For many years there has been a need for redefinition or more 

precise definition of certain ground-water terms used in publications 

by members of the U.S . Geological Survey. Another problem has 

been the expression of the coefficient of permeability (herein redefined 

as hydraulic conductivity) and the coefficient of transmissibility (herein 

redefined as transmissivity) in inconsistent units that included the 

U.S . gallon, the foot, and in some expressions, the mile. Such inconsistent units and the attendant confusing numerical conversion 

factors used in flow equations, such as 527 .7, 264, and 114.6, makes 

it unnecessarily difficult for hydrologists, especially in foreign countries, to follow and use our published results. Because of this it is 

advisable that basic ground-water flow equations in publications by 

members of the Geological Survey contain only the pure dimensionless numbers that result from the derivation of the equations, such 

as 2, 2.30, e, 7r, and 4, and that numerical results having dimensions 

should be expressed in consistent units of measurement. 

If in the solution of problems it is necessary or desirable to use 

inconsistent units, suitable conversion factors should be included so 

that the result is expressed in consistent units of length and time. 

For example, if a discharge rate is given in U.S. gallons per minute, 

conversion factors such as 7.48 gal ft -3 and 1,440 min day-' should 

be included. Many hydrologists in English-speaking countries including the United States are already using consistent units in the 

fps, cgs, or mks systems of measurement. 

To meet the growing need for consistency, J. T. Callahan, then 

acting chief, Ground Water Branch, in a memoradum of October 21, 

1 
2 DEFINITIONS OF SELECTED GROUNDWATER TERMS 

1965, appointed the Committee on Redefinition of Ground-Water 

Terms. After two additions, the committee comprised 

R. R. Bennett S. W. Lohman 

R. H. Brown C. L. McGuinness 

H. H. Cooper, Jr. A. M. Piper (chairman) 

W. J. Drescher M. I . Rorabaugh 

J. G. Ferris R. W. Stallman 

A. I . Johnson C. V. Theis. 

A. M. Piper retired from the Geological Survey in 1968 and was succeeded as chairman on June 28, 1968, by S. W. Lohman. 

The committee wishes to express its appreciation to A. M. Piper for his contribution as its first chairman and regrets that its work 

(the present report) was not completed prior to his retirement . The 

present chairman wishes to express his sincere thanks to all members 

of the committee for their excellent cooperation, and to several colleagues who contributed suggestions to various members of the committee . Finally, the committee wishes to express its deep appreciation

to the late O. E. Meinzer for his pioneer work in the definition of 

ground-water terms (Meinzer, 1923) and to the late C. E. Jacob for 

his untiring promotion of the use of consistent units in all matters 

pertaining to ground-water movement. (See Jacob, 1946a, b.) Meinzer's classic statements relating "Facts, concepts, definitions, and 

terms" (1923, p . 1-2) were an inspiration to the committee. If not 

directly applicable in their established form, terms drawn from other 

physical sciences are defined with a parallel construction for use in 

ground-water hydrology. 

TERMS 

Aquifer 

An aquifer is a formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield

significant quantities of water to wells and springs. 

The term aquifer was defined by O. E. Meinzer from a geological 

concept in which water bodies are classified in accordance with 

stratigraphy or rock types. Meinzer (1945, p. 159) clearly intended 

that an aquifer include the unsaturated part of the permeable unit . Artesian 

Artesian is synonymous with confined. Artesian water and artesian 

water body are equivalent respectively to confined ground water and 

confined water body. 

An artesian well is a well deriving its water from an artesian or 

confined water body. The water level in an artesian well stands above 

the top of the artesian water body it taps . 
TERMS 3 

If the water level in an artesian well stands above the land surface 

the well is a flowing artesian well . If the water level in the well stands 

above the water table, it indicates that the artesian water can and 

probably does discharge to the unconfined water body. Old definitions 

restricting artesian to one of these conditions, in particular Meinzer's 

definition (1923, p. 3S) which specified a head higher than the water 

table, are abandoned . It should be noted also that in discharge areas 

wells having heads higher than the water table, or even flowing wells, 

may exist without confinement of the water body, owing to vertical 

components of gradient in the flow field. 

Capillary fringe 

The capillary fringe is the zone immediately above the water table 

in which all or some of the interstices are filled with water that is 

under less than atmospheric pressure and that is continuous with the 

water below the water table- (Meinzer, 1923, p. 26) . The water is held 

above the water table, by interfacial forces (for example, surface 

tension) . The capillary fringe is typically saturated to some distance 

above its base at the water table ; upward from the saturated part 

only progressively smaller pores are filled and the upper limit is indistinct . In some quantitative studies it is convenient to define the 

upper limit more or less arbitrarily. For instance, this limit may be 

defined as the level at which 50 percent of the pore space, is filled 

with water (R. tip' . Stallman, oral common., May 15, 1969) . 

Some lateral flow generally occurs throughout the capillary fringe, 

but because the effective hydraulic conductivity decreases rapidly 

with moisture content, the lateral flow in the capillary fringe generally is negligible compared with that in the saturated zone, except 

where the capillary fringe and the saturated zone are of comparable 

thickness. 

Capture 

Water withdrawn artificially from an aquifer is derived from a 

decrease in storage in the aquifer, a reduction in the previous discharge 

from the aquifer, an increase in the recharge, or a combination of these 

changes. (See Theis, 1940, p. 277.) The decrease in discharge plus the 

increase in recharge is termed capture. Capture may occur in the form 

of decreases in the ground-water discharge into streams, lakes, and the 

ocean, or from decreases in that component of evapotranspiration 

derived from the saturated zone. After a new artificial withdrawal 

from the aquifer has begun, the head in the aquifer will continue to 

decline until the new withdrawal is balanced by capture . 

475-549 0- 72 - 2 
4 DEFINITIONS OF SELECTED GROUND-WATER TERMS

Conductivity, effective hydraulic, K, [LT-1 ] 

The effective hydraulic conductivity is the rate of flow of water 

through a porous medium that contains more than one fluid, such as 

water and air in the unsaturated zone, and should be specified in 

terms of both the fluid type and content and the existing pressure. 

For a precise definition of hydraulic conductivity for water, see 

"Conductivity, hydraulic." Effective hydraulic conductivity has been 

called cap ilary conductivity by many soil physicists and effective 

permeability by many petroleum engineers. 

Conductivity, hydraulic, K [LT-i1 

Hydraulic conductivity, K, replaces the term t"field coefficient of 

permeability," Pf, introduced by Meinzer and Wenzel, 1942, p. 7), 

which embodies the inconsistent units gallon, foot, and mile. If a 

porous medium is isotropic and the fluid is homogeneous, the hydraulic 

conductivity of the medium is the volume of water at the existing 

kinematic viscosity that will move in unit time under a unit hydraulic 

gradient through a unit area measured at right angles to the direction 

of flow . 

Darcy's law can be expressed as 

-Q-A-_KA 

dl~ 

K, the constant of proportionality in equation 1, is the hydraulic 

conductivity and may be expressed as 

K= dlh/dl . (2) 

Hydraulic conductivity can have any units of LT-1 suitable to the 

problem involved . In data tabulations of the Geological Survey, 

hydraulic conductivity may be expressed in feet per day and, so that 

the work of the Geological Survey may be readily interpreted in other 

countries, also in meters per day. Thus, 

__ ft, 1 K- day ft' (-ft ft-1)-ft day3 

�(4)~

K=-day m2 I m_1} =m day-1 

m 

Hydraulic conductivity is dependent primarily on the nature of the 

pore space, the type of liquid occupying it, and the strength of the 

gravitational field. For comparing the hydraulic conductivities of 

aquifers at different localities that contain water of appreciably 
TERMS 5 

different kinematic viscosity, it is only necessary to relate them by the 

dimensionless ratio of the kinematic viscosities and values of the 

acceleration due to gravity; thus, for the same intrinsic permeability, 

K,=v29i �(5)K2 v192 

Ordinarily, differences in the acceleration due to gravity are negligible ; 

hence, equation 5 is closely approximated by 

K,= v K2 vi 

In anisotropic media the direction of the specific discharge q is not 

generally parallel to that of the gradient (dhldl) of the head. In such 

media the cartesian components of the specific discharge are related 

to those of the gradient by 

6h bh 

-qz=Kxx ax+K~

Oh+Kzz y, 

ay+Kvz - a- q,=K"' ax+K.

-qZ=KZZ ax+Kri ay+KZ, az 

The quantities in the form K,xx , K,, Kzz , and so forth, called conducti~ , ity coefrzcie»ts . are the components of a second-order tensor, generally syminetric . The physical meaning of the conductivity coefficients 

may be visualized from the following examples : K,.,, is a conductivity 

coefficient that contributes a component of flux along the x-axis due 

to the component of head gradient along x ( =ahlax) . K,x is a conductivity coefficient that contributes a component of flux along the 

-axis due to the- component of head gradient. along x(=ahlax) . 

K,,j is a conductivity coefficient that contributes a component of flux 

along the x-axis due to the component, of head gradient along the 

z-axis ( =ahiaz) . 

Confining bed 

Confining bed is a term which will now supplant the terms t"aquielude," t"aquitard," and t"aquifuge" in reports of the Geological 

Survey and is defined as a body of "impermeable" material stratigraphically adjacent to one or more aquifers . In nature, however, its 

hydraulic conductivity may range from nearly zero to some value 
6 DEFINITIONS OF SELECTED GROUND-WATER TERMS 

distinctly lower than that of the aquifer. Its conductivity relative to 

that of the aquifer it confines should be specified or indicated by a 

suitable modifier such as slightly permeable or moderately permeable. 

Flow, steady 

Steady flow occurs when at any point the magnitude and direction 

of the specific discharge are constant in time. (See also "Flow, unsteady .") 

Flow, uniform 

A property is uniform if, at a given instant, it is the same at every 

point. Thus, uniformflow occurs if at every point the specific discharge 

has the same magnitude and direction. 

Flow, unsteady 

Unsteady, or nonsteady, flow occurs when at any point the magnitude or direction of the specific discharge changes with time. (See also 

"Flow, steady" .) 

The word transient is used in reference to the temporary features of 

unsteady flow. Thus, in unsteady flow, the specific discharge, the head, 

and perhaps other factors consist of a steady component plus a 

transient component. 

Fluid potential, 0 [LZT-a] 

Thefluid potential is the mechanical energy per unit mass of a fluid 

at any given point in space and time with respect to an arbitrary state 

and datum. Loss of fluid potential incurred as the fluid moves from a 

region of high potential to one of low potential represents loss of 

mechanical energy which is converted to heat by friction. The fluid 

potential may be expressed (Hubbert, 1940, p. 801) as 

O-gz+I. dP +2 . 

In ground-water movement the kinetic energy term v2 /2 ordinarily is 

negligible ; therefore, for ground-water movement the fluid potential

is closely approximated by 

. (8) O=gz+Jpa dP 

If the expansion and contraction of the fluid due to changes in pressure are unimportant to the problem being considered, the fluid can 

be assumed to be incompressible, and the equation for the potential is 

further approximated by - O-gz+p pa. (9) P 
TERMS 7 

At a given point in a body of liquid, the fluid potential is proportional 

to the head (see "Head, static") ; that is, 

Ground water, confined 

Confined ground water is under pressure significantly greater than 

atmospheric, and its upper limit is the bottom of a bed of distinctly 

lower hydraulic conductivity than that of the material in which the 

confined water occurs . 

Ground water, perched 

Perched ground water is unconfined ground water separated from an 

underlying body of ground water by an unsaturated zone. Its water 

table is a perched water table. It is held up by a perching bed whose 

permeability is so low that water percolating downward through it is 

not able to bring water in the underlying unsaturated zone above 

atmospheric pressure . 

Perched ground water may be either permanent, where recharge is 

frequent enough to maintain a saturated zone above the perching 

bed, or temporary, where intermittent recharge is not great or frequent 

enough to prevent the perched water from disappearing from time to 

time as a result of drainage over the edge of or through the perching 

bed. 

Ground water, unconfined 

Unconfined ground water is water in an aquifer that has a water 

table. 

Head, static, h [L] 

The static head is the height above a standard datum of. the surface 

of a column of water (or other liquid) that can be supported by the 

static pressure at a given point. 

The static head is the sum of the elevation head, he ,- and the pressure 

head, h, ; that is, h=he -}- hD . (See "Head, total".) Under conditions 

to which Darcy's law may be applied, the velocity of ground water 

is so small that the velocity head, hD = v2 /2g, is negligible . Head, 

when used alone, is understood to mean static head. Inspection of 

equation 10 shows that the head is proportional to the fluid potential ; 

therefore, the head is a measure of the potential. 

Head, total, H [L] 

The total head of a liquid at a given point is the sum of three components : (1) elevation head, he, which is equal to the elevation of the 

point above a datum, (2) pressure head, hp, which is the height of a 
8 DEFINITIONS OF SELECTED GROUND-WATER TERMS 

column of static water that can be supported by the static pressure 

at the point, and (3) velocity head, hD, which is the height the kinetic 

energy of the liquid is capable of lifting the liquid . Thus, the total 

head can be expressed as 

V2

H=he+hD+h IP _dp 9(11) .=z+9J_ +

where pa is atmospheric pressure. 

Homogeneity 

Homogeneity is synonymous with uniformity. A material is homogeneous if its hydrologic properties are identical everywhere . Although 

no known aquifer is homogeneous in detail, models based upon the 

assumption of homogeneity have been shown empirically to be valuable 

tools for predicting the approximate relationship between discharge 

and potential in many aquifers . 

Hydraulic diffusivity, T/S or K/S, [L2 T-1 ] 

The hydraulic difusivity is the parameter TIS or K/Se. It is the 

conductivity of the saturated medium when the unit volume of 

water moving is that involved in changing the head a unit amount 

in a unit volume of medium. By analogy with Maxwell's nomenclature in heat conduction theory (thermometric conductivity), it 

may be considered potentiometric conductivity. Similar diffusivities, 

having dimensions L'T-1 , characterize the flow of heat and of electricity by conduction and the movement of a dissolved substance in a 

liquid by diffusion. The parameter arises from the fundamental 

differential equation for liquid flow in a porous medium. 

In any isotropic homogeneous system the time involved for a 

given head change to occur at a particular point in response to a 

greater change in head at another point is inversely proportional to 

the diffusivity. As a common example the cone of depression affects 

moderately distant wells by measurable amounts in a short time in 

confined ground-water bodies for which the diffusivities are commonly

large and only after a longer time in unconfined water bodies for 

which the diffusivities are commonly much smaller. 

Hydraulic gradient [dimensionless] 

The hydraulic gradient is the change in static head per unit of 

distance in a given direction. If not specified, the direction generally 

is understood to be that of the maximum rate of decrease in head. The giv.dient of the head is a mathematical term which refers to the 

vector denoted by '7h or grad h, whose magnitude is equal to the 

maximum rate of change in head and whose direction is that in 
TERMS 9 

which the maximum rate of increase occurs . The hydraulic gradient 

and the gradient of the head are equal but of opposite sign . If the 

density of the water is not uniform, the hydraulic gradient can be 

written as (Hubbert, 1940, p. 815) 

dh-1 do- 1 dp dz 

Tl 9 Ti-pg TI +dl 

Isotropy 

Isotropy is that condition in which all significant properties are 

independent of direction. Although no aquifers are isotropic in detail, 

models based upon the assumption of isotropy have been shown to 

be valuable tools for predicting the approximate relationship between 

discharge and potential in many aquifers . 

Permeability, intrinsic, k [LZ] 

Intrinsic permeability is a measure of the relative ease with which 

a porous medium can transmit a liquid under a potential gradient. 

It is a property of the medium alone and is independent of the nature 

of the liquid and of the force field causing movement. It is a property 

of the medium that is dependent upon the shape and size of the pores. 

Darcy's law may be expressed in several ways (Hubbert, 1940, 

p. 803, 819), such as : 

_ dh__ __K d_ch_- _k_ d_p d_0-_ k_P do-_ _k do 

q--K Tl g dl fc(dl +~ dA dl - P dl (12) 

Then, solving the last version for k, 

k- ,qv (13) do/dl 

From equation 13 it may be deduced that if a porous medium is 

isotropic and the fluid is homogeneous, the intrinsic permeability of 

the medium is the volume of liquid of unit kinematic viscosity that 

will move in unit, time under a unit potential gradient through a 

unit area, measured at right angles to the direction of flow . 

If q is measured in m sec', v in m2 sec1 , 0 in J kg-1 , and l in m, 

the unit for k is in m2. In publications of the Geological Survey, 

k should be expressed in square micrometers, J(IM)2=10-11 m2. Thus 

intrinsic permeabilities of water-bearing materials range from fractions to thousands of (,,M)2,. 

From equation 12, k=K v/g. If k is expressed in (1,m)2[10-12m2I and 

K in m day-', v/g would be expressed in m day; thus, for water at 

60° F, the standard temperature used by Meinzer, for which v=1 .1296 

centistokes (table 3), 
10 DEFTNITIONS OF SELECTED GROUND-WATER TERMS 

_v _ 1 .1296X10 -2 cm2 sec-1 

g (980.665 cm sec-2 ) (102 cm m-1 ) (86,400 sec day -1 ) 

=1 .333X 10-12mday=4

3X10-12 mday, (14) 

whence 

k[10 -12 m2]=3[10 -12m day]K[m day -1 ] 

in which, it will be noted, k is numerically equal to 3 K. For water 

at any other temperature, if v is its kinematic viscosity in centistokes, 

k[(AM)2]=3 [10 -12m day] ] . (15) [1.1296] 9[m day-1 

It is apparent that k and K, in the designated units, differ numerically by only a small factor for most ground waters . 

The darcy, the unit of permeability used in the petroleum industry 

was originally defined (Wyckoff and others, 1934, p. 166, 167) as the 

volume in cubic centimeters of water of 1 centipoise viscosity flowing

in 1 sec through an area of 1 cm' under a pressure gradient of 1 atmosphere per em' per cm . Although not stated in the definition, the 

direction of flow must be horizontal to negate gravitational effects. 

The volume was later changed to 1 ml ; thus the darcy now contains 

the inconsistent units cm, ml, and atmosphere . The darcy has a 

value of 0.987 ()UM)2, which corresponds closely with the unit proposed

for Geological Survey usage. 

Porosity, n [dimensionless] 

The porosity of a rock or soil is its property of containing interstices 

or voids and may be expressed. quantitatively as the ratio of the volume 

of its interstices to its total volume. (See Meinzer, 1923, p. 19 .)

It may be expressed as a decimal fraction or as a percentage. With 

respect to the movement of water only the system of interconnected 

interstices is significant. (See "Porosity, effective.") 

Porosity, effective, n, [dimensionless] 

Effective porosity refers to the amount of interconnected pore space

available for fluid transmission . It is expressed as a percentage of the 

total volume occupied by the interconnecting interstices . Although

effective porosity has been used to mean about the same thing as 

specific yield, such use is discouraged. It may be noted that the present

definition of effective porosity differs from that of Meinzer (1923, 

p. 28) . 
Potentiometric surface 

The potentiometric surface, which replaces the term t "piezometric 

surface," is asurface which represents the static head. As related to an 

aquifer, it is defined by the levels to which water will rise in tightly 

cased wells . Where the head varies appreciably with depth in the 

aquifer, a potentiometric surface is meaningful only if it describes the 

static head along a particular specified surface or stratum in that 

aquifer. More than one potentiometric surface is then required to 

describe the distribution of head. The water table is a particular 

potentiometric surface. 

Pressure, static, p [ML-1 T-2] 

Static pressure is the pressure exerted by the fluid. It is the mean 

normal compressive stress on the surface of a, ;mall sphere around a 

given point. 

The static pressure does not include the dynamic pressure, pv2%2, 

and therefore is distinguished from the total pressure. The velocity of 

ground water ordinarily is so small that the dynamic pressure is 

negligible . Pressure, when used alone, is understood to mean static 

pressure . 

Specific capacity [L2 T-1] 

The specific capacity of a well is the rate of discharge of water 

from the well divided by the drawdown of water level within the well . 

It varies slowly with duration of discharge which should be stated 

when known. If the specific capacity is constant except for the time 

variation, it is roughly proportional to the transmissivity of the 

aquifer. 

The relation between discharge and drawdown is affected by the 

construction of the well, its development, the character of the screen 

or casing perforation, and the velocity andlength of flow up the casing. 

If the well losses are significant, the ratio between discharge and 

drawdown decreases with increasing discharge; it is generally possible 

roughly to separate the effects of the aquifer from those of the well 

by step drawdown tests . In aquifers with large tubular openings the 

ratio between discharge and drawdown may also decrease with increasing discharge because of a departure from laminar flow near the well, 

or in other words, a departure from Darcy's law. 

Specific discharge, or specific flux, q [L?"] 

The spedfrc discharge. or shedfcc #ux. for ground water is the rate 

of discharge of ground water per unit area of the poroits medium 
12 DEFINITIONS OF SELECTED GROUND-WATER TERMS 

measured at right angles to the direction of flow. Specific discharge 

has the dimensions of velocity, as follows 

q=Q/A (16) 

where Q equals total discharge, or totalflux, through area A. 

Specific discharge has sometimes been called the bulk velocity or the 

Darcian velocity. Specific discharge is a precise term (Hubbert, 1940, 

p. 790) and is preferred to terms involving "velocity" because of 

possible confusion with actual velocity through the pores if aqualifying 

term is not constantly repeated. 

Specific retention, Sr [dimensionless] 

The specific retention of a rock or soil is the ratio of (1) the volume 

of water which the rock or soil, after being saturated, will retain against 

the pull of gravity to (2) the volume of the rock or soil. (See Meinzer, 

1923, p. 28-29.) 

Ideally, the definition implies that gravity drainage is complete . 

However, the amount of water held in pores above the water table 

during gravity drainage is dependent upon particle size, distance above 

the water table, time of drainage, and other variables . Lowering of the 

water table anct infiltration occur over such short periods of time that 

gravity drainage is rarely or never complete. Thus the concepts embodied in specific retention do not recognize adequately the highly 

complex set of interacting conditions that regulate moisture retention. 

Nevertheless, specific retention is a useful though approximate measure of the moisture holding capacity of the unsaturated zone in that 

region above the capillary fringe. (See also "Specific yield.") 

Specific yield, Sy [dimensionless] 

The specific yield of a rock or soil is the ratio of (1) the volume of 

water which the rock or soil, of ter being saturated, will yield by gravity 

to (2) the volume of the rock or soil . (See Aleitnzer, 1923. p. 28.) The 

definition implies that gravity drainage is complete. 

In the natural environment, specific yield is generally observed as 

the change that occurs in the amount of water in storage per unit area 

of unconfined aquifer as the result of a unit change in head. Such a 

change in storage is produced by the draining or filling of pore space 

and is therefore dependent upon particle size, rate of change of the 

water table, time, and other variables. Hence, specific yield is only an 

approximate measure of the relation between storage and head in unconfined aquifers. It is equal to porosity minus specific retention. 

Storage, bank [L3] 

The change in storage in an aquifer resulting from a change in stage 

of an adjacent surface-water body is referred to as bank storage. 
TERMS 13 

Storage, specific, S, [L-11 

In problems of three-dimensional transient flow in a compressible 

ground-water body, it is necessary to consider the amount of water 

released from or taken into storage per unit volume of the porous 

medium . The specific storage, Ss , is the volume of water released from 

or taken into storage per unit volume of the porous medium per unit 

change in head . 

Storage coefficient, S [dimensionless] 

The storage coefficient is the volume of water an aquifer releases 

from or takes into storage per unit surface area of the aquifer per 

unit change in head. 

In a confined water body the water derived from storage with 

decline in head comes from expansion of the water and compression 

of the aquifer; similarly, water added to storage with arise in head is 

accommodated partly by compression of the water and partly by 

expansion of the aquifer. In an unconfined water body, the amount 

of water derived from or added to the aquifer by these processes 

generally is negligible compared to that involved in gravity drainage 

or filling of pores ; hence, in an unconfined water body the storage 

coefficient is virtually equal to the specific yield. 

Stream, gaining 

A gaining stream, which replaces the term t "effluent stream," is a 

stream or reach of a stream whose flow is being increased by inflow 

of ground water. 

Stream, losing 

A losing stream, which replaces the term t "influent stream," is a 

stream or reach of a stream that is losing water to the ground. 

Transmissivity, T [La T-1 ] 

Transmissivity is the rate at which water of the prevailing kinem atic 

viscosity is transmitted through a unit width of the aquifer under a 

unit hydraulic gradient . It replaces the term t "coefficient of transmissibility" because by convention it is considered a property of the 

aquifer, which is transmissive, whereas the contained liquid is transmissible. However, though spoken of as a property of the aquifer, it 

embodies also the saturated thickness of the aquifer (b) and the 

properties of the contained liquid . It is equal to an integration of the 

hydraulic conductivities across the saturated part of the aquifer 

perpendicular to the flow paths. 

Velocity, average interstitial, a; [LT-1 l 

Although the specific discharge, q, has the dimensions of a velocity, 

it expresses the average volume rate of flow rather than the particle 
14 DEFINITIONS OF SELECTED GROUNDWATER TERMS 

velocity . In order to determine the average interstitial velocity, vi, it is necessary to know also the effective porosity, ne . (See "Pozosity, effective.") The average interstitial velocity, 

-_Kdh/dl. ne- (17) e 

As the hydraulic gradient and effective porosity are dimensionless 

ratios, v i has the same dimensions as g and K. Even within a homogeneous medium there is a wide range of velocities within the pores

and from one pore to another, and in a nonhomogeneous medium the 

velocities may range over several orders of magnitude. 

Water table 

The water table is that surface in a ground-water body at which 

the water pressure is atmospheric. It is defined by the levels at which 

water stands in wells that penetrate the water body just far enough to hold standing water. In wells which penetrate to greater depths, the water level will stand above or below the water table if an up­ ward or downward component of ground-water flow exists. (See also 

"Ground water, perched.") 

Zone, saturated 

The saturated zone is that part of the earth's crust beneath the 

deepest water table in which all voids, large and small, are ideally filled with water under pressure greater than atmospheric. The 

saturated zone may depart from the ideal in some respects. A rising 

water table may cause entrapment of air in the upper part of the 

zone of saturation, and the lower part may include accumulations of 

other natural fluids . The saturated zone has been called the t phre­ atic zone by some. (See I1leinzer, 1923, p. 5, 22 ; Daubr4e, 1887, p. 19 ; 

Hantush, 1964, p. 283.) 

The foregoing definition is virtually that given by Dleinzer (1923, 

p. 21) . Later, other authors, emphasizing that ordinary interstices 

somewhat. above the water table in porous media are filled with 

capillary water, extended the term "saturated zone" to include this 

water. However, this capillary water cannot be distinguished in the 

field without special instrumentation . Hence, the definition accepting the water table as the top of the saturated zone is standard for 

reports of the Geological Survey . 

Zone, unsaturated 

The unsaturated zorie, which replaces the terms t "zone of aeration" and t "vadose zone," is the zone between the land surface and 

the deepest water table. It includes the capillary fr°inge . Generally, 

water in this zone is under less than atmospheric pressure, and some 
SELECTED REFERENCES 15 

of the voids may contain air or other bases at atmospheric pressure. 

Beneath flooded areas or in perched water bodies the water pressure 

locally may be greater than atmospheric. 

SELECTED REFERENCES 

Daubr6e, A. G., 1887, Les eaux souterraines & 1'6poque actuelle et aux 6poques 

anciennes : v. 1, Paris . 

Hantush, M. S., 1964, Hydraulics of wells, in Chow, Ven Te, ed., Advances in 

hydroscience, v. 1 : New York, Academic Press, Inc., p. 281-442 . 

Hubbert, M. K., 1940, The theory of ground-water motion : Jour. Geology, v. 48, 

no. 8, pt. 1, p. 785-944. 

Jacob, C. E., 1946a, Report of the Subcommittee on Permeability : Am. Geophys . 

Union Trans., v. 27, no. 2, p. 245-256. 

1946b, Notes on the permeability coefficient and its units : Am. Geophys . 

Union Trans., v. 27, no. 2, p. 265-273 . 

Meinzer, O. E., 1923, Outline of ground-water hydrology, with definitions : U.S . 

Geol. Survey Water-Supply Paper 494, 71 p. 

1945, Problems of the perennial yield of artesian aquifers : Econ. Geology, 

v. 40, no. 3, p . 159-163. 

1949, Occurrence, origin, and discharge of ground-water, chap. 10b in 

Meinzer, O. E., ed., Hydrology : New York, McGraw-Hill Book Co., Inc., 

p. 385-443 ; Natl. Research Council, Physics of the earth, v. 9. 

Meinzer, O. E., and Wenzel, L. K., 1942, Movement of ground water and its relation to head, permeability, and storage, in Meinzer, O. E., ed., Hydrology : 

New York, McGraw-Hill Book Co., Inc., p. 444-477 ; Natl. Research Council, Physics of the earth, v. 9. 

Theis, C. V., 1940, The source of water derived from wells : Civil Eng., v. 10, 

no. 5, p. 277-280. 

Wenzel, L. K., 1942, Methods for determining permeability of water-bearing 

materials, with special reference to discharging-well methods, with a section 

on Direct laboratory methods and Bibliography on permeability and laminar 

flow, by V. C. Fishel : U.S . Geol. Survey Water-Supply Paper 887, 192 p. 

Wyckoff, R. D., Botset, H. G., Muskat, Morris, and Reed, D . W., 1934, Measurement of permeability of porous media: Am. Assoc. Petroleum Geologists 

Bull ., v. 18, no. 2, p. 161-190. 
TABLES 1-4

DEFINITIONS OF SELECTED GROUNDWATER TERMS 

TABLE 1 .-Relation of units 

[Equivalent values shown in same horizontal lines . t indicates abandoned term] 

A. Hydraulic conductivity 

Hydraulic conductivity tField coefficient (K) of permeability (Pf) 

Feet per day Meters per day t Gallons per day (ft day -1) (m day-') per square foot (gal day -1 ft -2) 

One 0.305 7.48 

3.28 One 24 .5 

.134 .041 One 

B. Transmiseirily (T) 

Square feet per day SUM meters per t Gallons per day (ft2 day -') day ( m2 day - ') per foot (gal day - ' ft-') 

One 0.0929 7.48 

10.76 One 80.5 

. 134 .0124 One 

C. Permeability 

Intrinsic permeability qla f Coefficient of permeability 

k= qv Darcy=-dpldl+pg dz/dl P Or Pm=-q(at 60°F .) 

d p/dl dh/dl 

[(pm) 2=10-8 cm2] [0.987X 10 - 8 cm2] t[gal day-'ft -2 at 60° F.] 

One 1 .01 18.4 

0.987 One 18.2 

.054 .055 One 
TABLE 2.-Conversion factors for hydraulic units of measure 

[Equivalent values shown in same horizontal lines) 

Cubic feet per second 

(fV sec-') 

Acre-feet per day (acre-ft day­ ') 

Acre-feet per year

(acre-ft yr1) 

Gallons per minute 

(gal min-') 

Gallons per day

(gal day-') Cubic feet per day(fta day -') 

Cubic meters per

day (ma day-9 

Square mile-inch 

per year

(rr'i2-in yr') 

One 1.9835 7. 2397 X 102 4. 4883 X 102 6. 4632X 105 8. 6400X 104 2. 4466 X 103 1. 3574X 101 

5. 0417X 10-1 One 3. 6500X 102 2. 2629X 1C2 3. 2585X 105 4. 3560X 104 1 . 2335X 103 6 .8438 

1 . 3813 X 10-3 2. 7397 X 10-3 One 6. 1996X 10-1 8. 9274 X 102 1 . 1934X 102 3.3794 1. 8750 X 10-2 

2. 2280 X 10-3 4. 4192X 10-3 1.6130 One 1 . 4400X 103 1. 925OX 10 2 5.4510 3. 0244X 10-2 

1.5472X10-6 3.0689X10-6 1 . 1201 X 10 -3 6. 9444 X 10-4 One 1 . 3368X 10-1 3. 7854X 10-3 2. 1003 X 10-5 

1. 1574 X 10-5 2. 2957X 10-5 8. 3793X 10-3 5 . 1948 X 10-3 7.4805 One 2. 8317X 10-2 1 . 5711 X 10-4 

4. 0873 X 10-4 8. 1071 X 10 -4 2. 9591X 10-1 1. 8345 X 10-1 2. 6417X 102 3. 5314X 101 One 5. 5483X 10-3 

7.3668X10-2 1 .4612X10- 1 5. 3333X 101 3. 3065X 101 4. 7613X 104 6. 3649 X 103 1. 8024 X 102 One 
20 DEFINITIONS OF SELECTED GROUND-WATER TERMS 

TABLE 3.-Variation of properties of pure water with temperature 

[`, interpolated] 

Temperature 

°C ° F 

Density at 1 atmosphere I

(gm cm­2) 

Dynamic

viscosity, in 

centipoises 2 (10-2dyne-sec 

em-2) 

Kinematic 

viscosity, in 

centistokes 3 

(10-2 cm2 sec-') 

Surface tension 

against air 4 

(dynecm-9 

Vapor 

pressure s 

(mm Hg) 

0 

7 

3 

4 

5 

32.0 

33.8 

35.6 

37.4 

39.2 

4L 0 

0.999841 

.999900 

.999941 

.999965 

AW3 

.999965 

1 .7938 

1 .7320 

L6740 

L6193 

L5676 

L5188 

1.7941 

1.7322 

1.6741 

1.6194 

1.5676 

1.5189 

75.64 

75.50 

75.35 

75.21 

75. 06 

74.92 

4.579 

4.926 

5. 294 

5. 685 

6.101 

6.543 

6 

7 

8

9 

10 

42.8 

44.6 

46.4 

48. 2 

50.0 

. 999941 

.999902 

.999849 

.999781 

.999700 

L4726 

1.4288 

1.3872 

L 3476 

1.3097 

1.4727 

1.4289 

1.3874 

L 3479 

1.3101 

74.78

74.64 

74.50 

74.36 

74.22 

7.013 

7.513 

8.045 

8. 609 

9.209 

11 

12 

13

14 

15 

51.8 

53.6 

55.4 

57.2 

59.0 

.999605 

.999498 

.999377 

.999244 

.999099 

1 .2735 

1 .2390 

L 2061 

1 .1748 

L 1447 

1.2740 

1.2396 

1. 2069 

1.1757 

L1457 

74.07 

73.93 

73.78 

73.64 

73.49 

9.844 

10.518

11.231 

11.987 

12 .788 

16 

17 

60.8 

62.6 

.998943 

.998774 

1.1156 

1.0875 

1 .1168 

1 .0889 73.34 73.19 13.634 

14. 530 

18 

19 

20 

64.4 

66.2 

6& 0 

.998595 

.998405 

. 998203 

1 .0603 

L 0340 

1.0087 

1 .0618 

1.0357 

1.0105 

73 .05 

72,90

72.75 

15.477 

16.477 

17.535 

21 

22 

23 

24 

25 

69.8 

7L 6 

73.4 

75.2 

77.0 

.997992 

.997770 

.997538 

.997296 

.997044 

.9843

.9608 

.9380

.9161 

.8949 

.9863 

., 9629 

.9403 

.9186 

.8976 

72.59 

7244 

72.28 

7213 

7L 97 

1& 650 

19.827 

21.068 

22.377 

23.756 

26 

27 

28 

29 

30 

78.8 

80.6 

82.4 

84.2 

86.0 

.996783 

.996512 

.996232 

.995944 

.995646 

.8746

.8551 

.8363 

.8181 

.8004 

.8774

.8581 

.8395 

.8214 

.8039 

71.82 

71.66 

71.50 

7L 35 

71.18 

25.209 

26.739 

28.345 

30.043 

31 .824 

31 

32 

33 

34 

35 

87.8 

89.6 

9L4 

93.2 

95.0 

. 995340 

.995025 

.994702 

.994371 

.99403 

.7834 

.7630 

.7511 

.7357 

.7208 

.7871 

.7668 

.7551 

.7399 

.7251 

" 7L 02 

'70.86 

'70.70 

"70.53 

70.38 

33 .695 

35 .663 

37.729 

39 .898 

42.175 

36 

37 

38

39 

40 

96.8 

9& 6 

100.4 

102.2 

104.0 

.99368 

. 99333 

.99296 

.99259

.99221 

.7064 

.6925 

.6791 

.6661 

.6536 

.7109 

.6971 

.6839 

.6711 

.6587 

'70.21 

'70.05 

"69.88 "69.72 

69.56 

44.563 

47.067 

49.692 

52 .442 

55 .324 

41 

42 

43 

44 

45 

105.8

107.6 

109.4 

11L 2 

113.0 

.99183 

.99144 

.99104 

.99063 

.99021 

.6415

.6298 

.6184 

.6075 

.5970 

.6468 

.6352

.6240 

.6132 

.6029 

'69.40 

'69.23 

"69.07 

'6& 90 

6& 74 

58.34 

61.50 

64.80 

68.26 

71.88 

46 

47 

48 

49 

50 

114.8 

116. 6 

118. 4 

120. 2 

122.0 

.98979 

.98937 

.98893 

.98849 

.98804 

.5868 

. 5770 

.5675 

.5582

.5492 

.5929 

. 5832 

.573D 

.5647 

.5558 

"68.57 

0 6& 41 

'6& 24 

'6& 07 

67.91 

75.65 

79.60 

83. 71 

8& 02 

92.51 

51 123.8 .98759 .5405 .5473 0 67.74 97.20 

52 125.6 .98712 .5320 .5389 '67.56 10209 

53 127.4 .98666 .5236 .5307 " 67.39 107.20 

54 129. 2 .98618 .5153 .5225 '67.22 112.51 

55 131.0 .98570 .5072 .5146 67.05 11&04 

56 132.8 0.98522 .4994 . 5m '66.87 123.80 

57 134.6 '.98472 .4918 .4994 '66.70 129.82 

58

69 

60 

136.4 

135.2

140.0 

'.98422

" .98372 

.98321 

.4843 

.4770 

.4699 

.4921 

.4849 

.4779 

'66.53 "66.35 

66.18 

136.08 

142.60 

149.38 

61

62 

14L 8 143.6 '. 98269 '.98217 

.4629 

.4561 . 4711 

.4644 "66. 0 '65.8 

156. 43 

163.77 

See footnotes at end of table. 
TABLES 21 

TABLE 3.Variation of properties of pure water with temperature-Continued 

Temperature Density at Dynamic Kinematic Surface tension Vapor

1 atmosphere 1 viscosity, in viscosity, in against air 4 pressure e 

(gm em-8) eenti ices 2 centistokes 3 (dyne cm-1) (mm Hg)

°C °F (10-3 dyne-see (10-2 cm2 sec-1) cm-3) 

63 145.4 ' .98164 .4495 .4679 '66.7 171.38 

64 147.2 " .98110 .4431 .4516 '66.5 179.31 

65 149.0 .98056 .4368 .4455 $65.3 187.54 

66 150.8 '. 98002 .4306 .4394 " 66.1 19& 09 

67 152.6 = .97947 .4245 .4334 " 64.9 204.96 

68 154.4 ".97891 .4186 .4276 '64.8 214.17 

69 156.2 ' .97835 .4128 .4219 '64.6 223.73 

70 168.0 .97778 .4071 .4164 64.42 233.7 

71 159.8 '.97720 .4016 .4110 " 64.2 243.9 

72 161 .6 '.97663 .3962 .4057 -64.0 264 .6 

73 163.4 - .97604 .3909 .4005 '63.9 266.7 

74 166.2 ' .97546 .3867 .3954 -63.7 277.2 75 167.0 .97486 .3806 .3904 -63.5 289.1 

76 168.8 ' .97426 .3756 .3855 '63.3 301.4 

77 170.6 -.97366 .3708 .3808 -63.1 314.1 

78 172.4 -.97304 .3661 .3762 '63.0 327.3 

79 174.2 ' .97242 .3615 .3718 062.8 341 .0 

80 176.0 .97180 .3570 .3674 62.61 355.1 

81 177.8 ',97118 .3528 .3631 -62.4 369.7 

82 179.6 -.97064 .3483 .3589 -62.2 384.9 

83 181 .4 -.96991 .3440 .3547 -62.0 400.6 

84 183.2 '.96927 .3396 .3504 -61.9 416 .8 85 186.0 .96862 .3357 .3466 '61.7 433.6 

86 186.8 -.96797 .3317 .3427 -61.5 450.9 

87 18& 6 -. 96731 .3278 .3389 -61 .3 468.7 88 190.4 '.96665 .3240 .3352 -61 .1 487.1 89 192.2 - .96698 .3203 .3316 -60.9 506.1 90 194.0 .96531 .3166 .3280 60.75 526.76 

91 195.8 -.96484 .3130 .3245 '60.6 646.05 92 197.6 -. 96396 .3095 .3211 -60.4 56& 99 93 199.4 ".96327 .3061 .3178 -60 .2 588.60 94, 201.2 -.96268 .3027 .3145 . 60.0 610.90 95 203.0 - .96189 .2994 .3113 -59.8 633.90 

96 240.8 -.96119 .2962 .3082 -59.6 657.62- 97 20& 6 -. 96048 .2930 .3051 -59. 5 682.07 206.4 -.95978 .2899 .3020 -59.3 707.27 99 210.2 -.95906 .2869 .2991 '59.1 733.24 100 212 .95835 .2839 .2962 58.85 760.00 

1 Computed from the relative values in the "Absolute Density of Water" table of Weast, R. C ., ed .,1965, Handbook of chemistry and physics [46th ed .] : Cleveland Chem. Rubber Co., p F-4. 3 National Academy of Sciences, 1929, International critical tables of numerical data, physics, chemistry,

and technology: Natl. Acad . Sci., v. 5, p. 10 . 3 Dynamic viscosity divided by density. s National Academyof Sciences, 1929, International critical tables of numerical data, physics, chemistry,

and technology : Natl. Acad . Sci. v. 5, p . 447.

1 Weast R. C ., ed . 1965, Handbwok of chemistry and physics [46th ed .] : Cleveland, Chem. Rubber Co ., "Vapor Pressure of Water Below 100° C." table, D-94. 

TABLE 4.-Relation between head of water and units of pressure I 

[Equivalent values shown in same horizontal lines] 

Ft of water at 4°C Lb In -2 Atmosphares Bars Dyne cm-3 (standard) 

One 4.3352X10-1 '2.9499X10-3 2. 9890X 10-9 2. 9890 X 104 

2.3066 One 6. 8046X 10-9 6. 8947 X 10-9 6. 8947X 10+ 

3. 3899X 101 1 . 4696X 101 One 1.01325 1 . 01325X 105 

3. 3456X 101 1. 4504X 101 9. 8692X 10-1 One 1 . 0000X 106 

3. 3456X 10-5 1. 4504X 10-5 9. 8692 X 10-7 1 . 0000X 10-5 One 

1 Ho an, C. D., ed ., 1955, Handbook of chemistry and physics 137th ed .] : Cleveland, Chem. Rubber 

Publish ng Co ., p. 2873-2874 . 

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