GLOSSARY
The following text books were used for the glossary: Glossary of Geology (1980) and
International Glossary of Hydrology (1998), IHP/OHP-Reports, Koblenz, 243 pp.
Age: → Water age
Apparent age: Age determined by any method without consideration of all necessary
corrections and conventions.
Applied hydrology: Branch of hydrology which applies to its application to the filed
connected with water-resources development and management.
Aquiclude: A body of relatively impermeable rock that is capable of absorbing water slowly
but functions as an upper or lower boundary of an aquifer and does not transmit
groundwater rapidly enough to supply a well or spring.
Aquifer: A body of rock that contains sufficient saturated permeable material to conduct
groundwater and to yield economically significant quantities of groundwater to wells and
springs (→ permeability, Sect.1.1).
Aquitard: A confining bed that retards but does not prevent the flow of water from an
adjacent aquifer (leaky confining bed). It does not readily yield water to wells or springs,
but may serve as a storage unit for groundwater (→ Sect.1.1).
Artesian aquifer: A confined aquifer containing groundwater under pressure, high enough to
cause outflow in a well or spring.
Artesian spring: A spring discharging confined water from an underlying aquifer through a
fissure or other opening in the confining bed that overlies the aquifer (→ Sect.3.1.4.1).
Artesian water: In the traditional sense groundwater with a piezometric pressure high
enough to cause outflow from a well or spring (→ confined groundwater).
Artesian well: A well tapping a confined or artesian aquifer in which the static water level
stands above the surface of the ground or at least of the level of the shallow groundwater
(→ confined aquifer).
Artesian recharge: Recharge of groundwater at a rate greater than natural, resulting from the
activities of man, by means of dug basins, drilled wells, or spreading of water across the
land surface.
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Glossary
Base flow: The part of the discharge, which enters a stream channel from groundwater. This
is the runoff observed during long periods when no precipitation or snowmelt occurs.
Boundary conditions: Set of conditions to be satisfied by the solution of a differential
equation at the boundary (including fluid boundary) of the region in which the solution is
sought.
Brackish water: Groundwater with a total salt concentration of 1000 to 10 000 mg/L.
Capillary fringe: The zone immediately above the water table, where water is drawn upward
by capillary attraction.
Catchment area: The entire area between the recharge and the discharge area
Coefficient of filtration: → hydraulic conductivity (Sect.3.1.1)
Cone of depression: → Pumping cone. Depression, in the shape of a cone with convex
upwards limits, of the piezometric surface which defines the area of influence of a well.
Conceptional hydrological model: Simplified mathematical representation of some or all of
the processes in the hydrological cycle by a set of hydrological concepts expressed in
mathematical notations and linked together in a time and space sequence corresponding
to that occurring in nature. Hydrological conceptional models are used for simulation of
the behaviour of a basin (Sect.3.1.2 and 3.1.3; Volume VI).
Confined aquifer: An aquifer bounded above and below by impermeable beds or by beds of
distinctly lower permeability than that of the aquifer itself; an aquifer containing confined
groundwater.
Confined groundwater: Groundwater under a pressure significantly greater than that of the
atmosphere. If tapped by a borehole, the water level in the borehole rises so that it
corresponds to the pressure height at the point where it was tapped.
Confined groundwater surface: The upper surface of the groundwater body is under a
pressure higher than that of the atmosphere. Identical with the upper surface of a confined
aquifer. If tapped by a borehole, groundwater rises to the piezometric surface.
Confining bed: The formation overlying or underlying a much more permeable aquifer. →
aquiclude, aquitard
Connate water: Water entrapped in the interstices of a sedimentary rock at the time of its
deposition that has been out of contact with the atmosphere for at least an appreciable
part of a geologic period.
Darcy law: → Sect.3.1.1
Diffusion: Process of spreading of a solute as a result of the thermal movement of the
molecules of this solute. → Sect.3.1.2
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Glossary
Direct runoff: Flow of water entering stream channels promptly. It includes surface flows
and interflow and is used where interflow cannot be separated in hydrological analysis.
This part of surface runoff which reaches the catchment outlet shortly after the rain starts.
Its volume is equal to rainfall excess. Some procedures for its derivation include prompt
subsurface runoff but all exclude base flow.
Distance velocity: (syn. tracer velocity) – Velocity of a tracer defined by the quotient of
distance and time. → filter velocity (Sect.3.1.1).
Drawdown: The distance by which the water level in a well is lowered by the withdrawal of
water.
Effective porosity: The percentage of the total volume of a given mass of soil or rock that
consists of interconnecting interstices through which water or other fluids can travel. →
Sect.1.3.1
Field capacity: amount of water held in a soil after gravitational water has drained away.
→ Sect.1.3.2
Filter velocity: mass transport velocity in volume unit per time and area units calculated by
the Darcy law. → Sect.3.1.1
Fossil water: Water recharged in an ancient geological period and not participating in the
contemporary hydrological cycle.
Free water table: The unconfined groundwater level at which the pressure is equal to the
atmospheric pressure.
Freshwater: Naturally occurring water having a concentration of salt < 1000 mg/L.
Gravitational water: (syn. mobile water) – Water in the unsaturated zone which moves
under the influence of gravitity. → Sect.1.3.1
Groundwater: (a) Subsurface water that is in the zone of saturation. It includes underground
streams. (b) Loosely all subsurface water (excluding internal water) as distinct from
surface water.
Groundwater dating: Determination of the time between the recharge of the groundwater
and its sampling. → Sect.5.2.2
Groundwater divide: Line on a water table or piezometric surface on either side of which the
groundwater flow diverges.
Groundwater level: (syn. groundwater table) – The elevation, at a certain location and time,
of the phreatic or piezometric surface of an aquifer.
Groundwater level: Elevation, at a certain location and time, of the water table or
piezometric surface of an aquifer.
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Glossary
Groundwater mining: Withdrawal from an aquifer containing fossil groundwater
(Sect.3.2.1).
Groundwater overexploitation: Withdrawal of a groundwater reservoir in excess of the
average rate of replenishment.
Groundwater recharge → Recharge
Groundwater table: (syn. groundwater level, phreatic surface) – The surface within the zone
of saturation of an unconfined aquifer along which the pressure is atmospheric.
Head, total: The sum of the elevation head, the pressure head, and the velocity head at a
given point in an aquifer.
Histogram: Univariate frequency diagram with rectangles proportional to the area to the class
frequency, erected on a horizontal axis with width equal to the class interval.
Hydraulic conductivity (K): (syn. permeability coefficient) – A coefficient of
proportionality describing the rate at which water can move through a permeable
medium. The density and kinematic viscosity must be considered in determining the
hydraulic conductivity. It has a dimension of velocity. → Sect.1.3.3
Hydrological cycle: Succession of stages through which water passes from the atmosphere to
the earth and returns to the atmosphere: evaporation from the land or sea or inland water,
condensation to form clouds, precipitation, accumulation in the soil or in bodies of water,
and re-evaporation. → Volume I
Hydrological regime: Variations in the state and characteristics of a water body which are
regularly repeated in time and space and which pass through phases, e.g. seasonal. →
Sect.1.4.2
Hydrosphere: That part of the earth covered by water and ice. → Volume I
Intermediate zone: That part of the unsaturated zone below the root zone and above the
capillary fringe. → Sect.5.1
Infiltration coefficient: Ratio of infiltration to rainfall.
Interflow: That portion of the precipitation which has not passed down to the water table, but
is discharged from the unsaturated zone by lateral movement during and immediately
after a precipitation event. The water moving as interflow discharges directly into a
stream channel or lake.
Intermittent spring: Spring, which discharge occurs only during certain periods and ceases
at other periods. → Sect.3.1.4.1
Juvenile water: A term applied to water that is known to have been derived directly from
magma and that is thought to have come to the Earth’s surface for the first time.
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Glossary
Karst: Limestone and dolomite areas that possess a topography peculiar to and dependent
upon underground solution and the diversion of surface waters to underground routes. →
Sect.1.4.2.4
Leaky aquifer: Aquifer overlain and/or underlain by a relatively thin semi-pervious layer,
through which flow into or out of the aquifer can take place.
Magmatic water: Water brought to the earth's surface from great depths by the upwards
movement of intrusive igneous rocks. → Chapter 6
Modern water: The groundwater directly recharged after 1963/64 labeled by higher activities
of nuclear-bomb-produced tritium and radiocarbon. → Sect.5.2.2.1 and 5.2.2.2
Moisture content or soil humidity: The weighted moisture content is expressed by the ratio
of the mass of water Mw to the total mass or to the mass of the solid Mt. The volumetric
humidity or volumetric water content is the ratio of the volume of water Vw to the total
volume of the sample Vt. → Sect.5.1.1
Observation well: Well used for measuring the static head of groundwater, and specially to
observe the frequency and magnitude of changes in head or another physical or chemical
parameter. → Sect.3.1.4.2
Perched groundwater: Unsaturated groundwater separated from an underlying main body of
groundwater by an unsaturated zone (Sect.5.1).
Permeability: The property or capacity of a porous rock, sediment, or soil for transmitting a
fluid; it is a measure of the relative ease of fluid flow under unequal pressure. The
expressions “permeable” and “impermeable” have a relative meaning. An intercalation of
the same permeability between layers of lesser permeability can act as an aquifer while
between more permeable layers; it can act as an aquiclude. → Sect.1.3.3
Phreatic surface: The surface between the zone of saturation and the zone of aeration at
which the pressure is equal to that of the atmosphere.
Phreatic water: A term that originally was applied only to water that occurs in the upper part
of the zone of saturation under water-table conditions, but has come to be applied to all
water in the zone of saturation, thus making it an exact synonym of groundwater.
Piezometer: A pipe sealed along its length, open to water flow at the bottom and open to the
atmosphere at the top, that is used to measure the piezometric head at the point of an
aquifer to which it reaches.
Piezometric head: (a) The elevation to which water of a given aquifer will rise in a
piezometer. (b) The sum of the elevation and the pressure head in a liquid expressed in
units of height.
Piezometric surface: (syn. potentiometric surface) – The surface to which water would rise
in piezometers reaching into the same aquifer. In general, the piezometric surface is not
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Glossary
horizontal and plane but curved and it reflects the distribution of the hydraulic potential,
i.e. of the mechanical energy of water within the aquifer at the depth reached by the
piezometers. → Sect.1.3.4
Playa: Lake bed found in arid or desert regions in the lower part of a enclosed valley whose
drainage is centripetal or inward. The lake is usually dry, except after heavy rains storms,
when it may be covered by a thin sheet of water which quickly disappears through
evaporation and/or infiltration.
Porosity: The total porosity is defined by the ratio of the total volume of water Vw and gas Vg
available for water to the total volume of the sample Vt. This parameter may vary in case
of refraction or expansion of the solid matrix (clays, gypsum). The effective porosity is
defined by Vw / Vt. → Sect.1.3.1
Potential evaporation: Quantity of water valour which could be emitted by a surface of pure
water in the existing conditions.
Potentiometric surface: → Piezometric surface
Pumping cone: The area around a discharging well where the hydraulic head in the aquifer
has been lowered by pumping. → cone of depression
Pumping test: (syn. aquifer test) – A test made by pumping a well for a period of time and
observing the change in hydraulic head in the aquifer. A pumping test may be used to
determine the capacity of the well and the hydraulic characteristics of the aquifer.
Recharge: Direct recharge is related to the seepage of rainwater through the unsaturated
zone into the aquifer. Indirect recharge is the recharge via rivers, lakes and other
superficial water resources. Artificial recharge is the seepage of e.g. river water into a
superficial sand bed where it recharge an exploited aquifer.
Residence time: Period during which water or a substance remains in a component part of the
hydrological cycle. → Sect.3.1.2
Return flow: Any flow returns to a stream channel or to the groundwater after use.
Root zone: The zone from the land surface to the depth penetrated by plant roots. The root
zone may contain part or all of the unsaturated zone, depending upon the depth of the
roots and the thickness of the unsaturated zone (Sect.5.1).
Runoff: The total amount of water flowing in a stream. It includes overland flow, return flow,
interflow and base flow.
Safe yield: Amount of water (in general, the long-term average amount) which can be
withdrawn from a groundwater basin or surface water system without causing undesirable
results.
Salt water: Water of which salt concentration exceeds 10 g/L.
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Glossary
Saturated zone: −> Zone of saturation.
Secondary porosity: The porosity developed in a rock after its deposition or emplacement,
through such processes as solution or fracturing.
Storage coefficient: (syn. storativity) – Volume of water an aquifer releases from or takes
into storage per unit surface area of the aquifer per unit change of head.
Subsurface water: (syn. subterranean water, underground water, groundwater) – Water in the
lithosphere in solid, liquid, or gaseous form. It includes all water beneath the land surface
and beneath the bodies of surface water.
Time series: Set of observations, in order, taken at successive points of time, commonly at a
fixed interval.
Tracer velocity: → Distance velocity, Sect. 3.1.1
Transmissivity: The rate at which water of prevailing density and viscosity is transmitted
through a unit width of an aquifer or confining bed under a unit hydraulic gradient. It is a
function of properties of the liquid, the porous media, and the thickness of the porous
media. → Sect.1.3.3
Turnover rate: Ratio of the annual average groundwater recharge, expressed in volume, to
the average groundwater storage of an aquifer. → Sect.3.1.2
Turnover time: Time required for supplying a volume equal to the total water reserve in a
surface or groundwater reservoir, at the average rate of natural inflow or replenishment.
→ Sect.3.1.2
Unconfined aquifer: An aquifer containing unconfined groundwater. → phreatic water
Unconfined groundwater: Groundwater that has a free water table, i.e. water not confined
under pressure beneath impermeable rocks. → phreatic water
Unconfined groundwater level: The groundwater level at which the pressure is equal to the
atmospheric pressure, i.e. the surface of groundwater in an unconfined aquifer. →
phreatic water
Unsaturated zone: The zone between the land surface and the water table. It includes the
root zone, intermediate zone, and capillary fringe. The pore spaces contain water at less
than atmospheric pressure, as well as air and other gases. Saturated bodies, such as
perched groundwater, may exist in the unsaturated zone. Also called zone of aeration and
vadose zone. → zone of aeration, Sect.5.1
Vadose water: any water that occurs in the unsaturated zone. → Sect.5.1
Velocity: → Distance velocity
Zone of aeration: (syn. unsaturated zone) – A subsurface zone containing water under
pressure less than that of the atmosphere. It bears water held by capillary forces and air or
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Glossary
gases generally under atmospheric pressure. This zone is limited above by the land
surface and below by the surface of the zone of saturation, i.e. the water table. The zone
is subdivided into the belt of soil water, the intermediate belt and the capillary fringe. →
Sect. 5.1
Zone of saturation: A subsurface zone in which all the interstices are filled with water under
pressure greater than that of the atmosphere. → Sect. 5.2.
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