Hydrology Glossary

A body of permeable rock or sediment that stores and transmits groundwater in usable quantities.

The component of streamflow derived from groundwater discharge, sustaining flow during dry periods.

The zone immediately above the water table where water is drawn upward by capillary forces in soil pores.

An aquifer bounded above and below by impermeable layers (aquitards), in which water is under pressure greater than atmospheric.

A fundamental equation relating groundwater flow rate to hydraulic conductivity, cross-sectional area, and hydraulic gradient.

The volume of water flowing past a point in a stream per unit time, typically expressed in cubic meters per second (m³/s) or cubic feet per second (cfs).

The topographic boundary separating adjacent drainage basins; precipitation falling on either side flows to different outlets.

The sum of evaporation from land and water surfaces plus transpiration from vegetation, returning water to the atmosphere.

The statistical characterization of how often floods of a given magnitude are expected to occur at a particular location.

Water found beneath the Earth's surface in the pores and fractures of rock and sediment, below the water table.

A measure of a porous medium's ability to transmit water, depending on both the medium properties and the fluid properties.

The change in hydraulic head per unit distance in the direction of groundwater flow; it drives the movement of groundwater.

A graph showing discharge versus time at a particular location in a stream or river, used to analyze watershed response to precipitation.

The continuous natural cycle of water moving through evaporation, condensation, precipitation, runoff, infiltration, and storage.

The entry of water from the surface into the soil, governed by soil type, moisture content, surface conditions, and rainfall intensity.

A line on a map connecting points of equal precipitation, used in the isohyetal method of estimating areal rainfall.

An empirical formula for calculating velocity and flow in open channels based on channel roughness, hydraulic radius, and slope.

The downward movement of water through soil and rock under gravity, from the unsaturated zone toward the water table.

The ratio of void space to total volume in a soil or rock, indicating its capacity to hold water.

Water falling to the Earth's surface from the atmosphere in the form of rain, snow, sleet, or hail.

The empirical relationship between water surface elevation (stage) and discharge at a streamflow gauging station.

The process by which water is added to an aquifer, typically through infiltration of precipitation or seepage from surface water bodies.

The average time interval between occurrences of a hydrologic event of a given magnitude; the reciprocal of annual exceedance probability.

Water that flows over the land surface or through shallow subsurface layers to reach streams, as opposed to infiltrating deeply or evaporating.

The upper boundary of the zone of saturation in an unconfined aquifer, where water pressure equals atmospheric pressure.