Science of Water : Concepts and Applications

Glossary 409
Headwaters The source and upper part of a stream.
Hydraulic conductivity The capacity of a rock to transmit water. It is expressed as 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 fl ow.
Hydraulic gradient The change of hydraulic head per unit of distance in a given direction.
Hydrogen bonding The term used to describe the weak but effective attraction that occurs
between polar covalent molecules.
Hydrograph Graph showing variation of water elevation, velocity, streamfl ow, or other property
of water with respect to time.
Hydrologic cycle Literally the water–earth cycle. The movement of water in all three physical
forms through the various environmental mediums (air, water, biota, and soil).
Hydrology The science that deals with water as it occurs in the atmosphere, on the surface of
the ground, and underground.
Hydrostatic pressure The pressure exerted by the water at any given point in a body of water
at rest.
Hygroscopic A substance that readily absorbs moisture.
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Impermeability The incapacity of a rock to transmit a fl uid.
Index of biotic integrity (IBI) An aggregated number, or index, based on several attributes or
metrics of a fi sh community that provides an assessment of biological conditions.
Indicator sites Stream sampling sites located at outlets of drainage basins with relatively homogeneous
land use and physiographic conditions; most indicator-site basins have drainage
areas ranging from 20 to 200 square miles.
Infi ltration The downward movement of water from the atmosphere into soil or porous rock.
Infl uent Water fl owing into a reservoir, basin, or treatment plant.
Inorganic Containing no carbon; matter other than plant or animal.
Inorganic chemical A chemical substance of mineral origin not having carbon in its molecular
structure.
Inorganic soil Soil with less than 20% organic matter in the upper 16 in.
Ionic bond The attractive forces between oppositely charged ions—for example, the forces
between the sodium and chloride ions in a sodium chloride crystal.
Instantaneous discharge The volume of water that passes a point at a particular instant of time.
Instream use Water use taking place within the stream channel for such purposes as hydroelectric
power generation, navigation, water-quality improvement, fi sh propagation, and
recreation. Sometimes called nonwithdrawal use or in-channel use.
Intermittent stream A stream that fl ows only when it receives water from rainfall runoff or
springs, or from some surface source such as melting snow.
Internal drainage Surface drainage whereby the water does not reach the ocean, such as drainage
toward the lowermost or central part of an interior basin or closed depression.
Intertidal Alternately fl ooded and exposed by tides.
Intolerant organisms Organisms that are not adaptable to human alterations to the environment
and thus decline in numbers where alterations occur.
Invertebrate An animal having no backbone or spinal column.
Ion A positively or negatively charged atom or group of atoms.
Irrigation Controlled application of water to arable land to supply requirements of crops not
satisfi ed by rainfall.
Irrigation return fl ow The part of irrigation applied to the surface that is not consumed
by evapotranspiration or uptake by plants and that migrates to an aquifer or surfacewater
body.