an introduction to hydrology and essential terminology

Hydrology is a branch of the scientific and engineering discipline that deals with the occurrence, distribution, movement and properties of water above and below the land surfaces of the earth. It deals with the relations and interactions of water with the environment, including biota. Hydrological studies allow for the planning, design and realization of water management measures for prospections, quantification, exploitation and efficient utilization of water resources in quality and quantity.


Key Terms:

Aquifer: an underground layer of water-bearing permeable rock, rock fractures or unconsolidated materials (gravel, sand, silt) from which water can be extracted using a water well.

Capillary rise: the ability of a liquid to flow in narrow spaces without the assistance of and in opposition to external forces like gravity.

Catena: a sequence of soils down a slope, created by the balances of processes, such as precipitation, infiltration and runoff.

Condensation: the change of water from its gaseous form (water vapor) into liquid water.  It generally occurs in the atmosphere when warm air rises, cools and loses its capacity to hold water vapor.  The excess water vapor condenses to form cloud droplets.

Depression storage (capacity): the ability of a particular area of land to retain water in pits and depressions, thus preventing it from flowing.

Effective precipitation: the amount of precipitation that is actually added to and stored in the soil (Amount of rain {mm} – 5) x .75).

Evaporation: the process by which water or other liquids change from liquids to a gas vapor; a type of vaporization of a liquid that occurs from the surface of a liquid into a gaseous phase that is not saturated with the evaporating substance.  The process requires heat from the sun.

Evapotranspiration (ET): the sum of evaporation and plant transpiration from the Earth’s land and ocean surface to the atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies.

Evection: the movement of water through the air.

Groundwater: the water present beneath the earth’s surface in soil pore spaces and in the fractures of rock formations.

Infiltration: one of the 6 processes that make up the water cycle; the rainwater soaks into the ground, through the soil and rock layers.

Interception: precipitation that does not reach the soil, but rather is intercepted by the leaves and branches of plants and the forest floor.

Interflow: water that travels laterally or horizontally through the zone(s) of aeration during or immediately following a precipitation event and discharges into another body of water.

Groundwater recharge (deep drainage/deep percolation): an area where water moves downward from surface water to groundwater.

Groundwater table: the surface of the groundwater exposed to an atmospheric pressure beneath the surface of the saturated zone.

Saturated zone: an area of an aquifer, below the water table, in which relatively all pores and fractures are saturated with water.

Sublimation: the transition of a substance directly from the solid to the gas phase without passing through the intermediate liquid phase.

Surface runoff: precipitation that cannot be absorbed by the soil because the soil is already saturated that flows into another body of water; precipitation > infiltration.

Transpiration: the process by which moisture is carried through plants from roots to small pores on the underside of leaves where it changes to vapor and is released into the atmosphere.