Alluvium a general term for detrital deposits made by streams on river beds, flood plains, and alluvial fans, especially a deposit of silt or silty clay laid down during a time of flood.

Anisotropy The condition of having different properties in different directions.  Properties that are dependent upon direction are said to be anisotropic.

Aquifer 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.

Area of influence the area throughout which water levels decline measurably because of discharge from a well.  Theoretically, the effects of pumping extend to the boundary of the aquifer.

Baseflow That part of stream discharge derived from groundwater seeping into the stream.

Basin The drainage area of a stream, includes both surface and ground water drainage.

Clay a detrital mineral particle having a diameter less than 1/256 mm (4 microns).

Evapotranspiration That portion of the precipitation returned to the air through evaporation and transpiration.

Fluvial of or pertaining to rivers, produced by the action of a stream or river.

Formation a formation is a regionally extensive mappable geologic unit with reasonably constant lithology; an aquifer may comprise one or more formations or may consist of particular high-conductivity beds within a formation.

Glacial pertaining to the activities of glaciers, or to the features or materials produced thereby.

Gravel rounded fragments with diameters in the range 4.76 to 76 mm (3 in.), more loosely, an unconsolidated natural accumulation of rounded rock fragments, mostly of particles larger than sand (diameter greater than 2 mm), such as boulders, cobbles, pebbles, granules, or any combination of these.

Ground water That part of the subsurface water that is in the zone of saturation.

Ground water divide a ridge in the water table, from which ground water moves away in both directions.

Hydraulic conductivity The hydraulic conductivity of a 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.

Hydrologic cycle The constant circulation of water from the sea through the atmosphere, to the land, and its eventual return to the atmosphere by way of transpiration and evaporation from the land and evaporation fro the sea.

Potentiometric gradient The slope of the potentiometric surface, which is an imaginary surface representing the total head of ground water and defined by the level to which water will rise in a well.  The water table is a particular potentiometric surface.

Residence time The average length of time a substance spends in given reservoir that is at a steady stare with respect to the processes that add and remove the substance to and from the reservoir. Residence time is calculated as the ratio of the reservoir size to the rate of inflow or outflow (which are equal at steady state).

Sand a detrital particle smaller than a granule and larger than a silt grain, having a diameter in the range of 1/16 to 2 mm.

Silt A detrital particle finer than fine sand and coarser than clay, commonly in the range 1/16 to 1/256 mm.

Soil moisture The water contained in the unsaturated zone.

Specific capacity The rate of discharge of water from the well divided by drawdown of water level within the well.  Specific capacity decreases with duration of pumping.

Specific yield The ratio of volume of water yielded from water-bearing material by gravity drainage, as occurs when the water table declines, to the volume of water-bearing material.

Stratified drift glaciofluvial, glaciolacustrine, or glaciomarine drift, consisting of sorted and layered material deposited by a meltwater stream or settled from suspension in a body of quiet water adjoining a glacier.

Streamflow depletion The reduction of streamflow caused by pumping due to either induced infiltration or the capture of ground water that would have reached the stream if the pumping had not diverted it to the well.

Till Unstratified drift, deposited directly by a glacier without reworking by meltwater, consisting of a mixture of clay, silt, sand, gravel, and boulders ranging widely in shape and size.

Transmissivity The rate at which water at the prevailing kinematic viscosity is transmitted through a unit width of the aquifer under a unit hydraulic gradient.

Unconsolidated material A sediment that is loosely arranged or unstratified or whose particles are not cemented together, occurring either at the surface or at depth.

Zone of saturation a subsurface zone in which all the interstices are filled with water under pressure greater than that of the atmosphere.  The water table is the top of the zone of saturation.