aquatic life zone Marine and freshwater portions of the biosphere. Examples include freshwater life zones (such as lakes and streams) and ocean or marine life zones (such as estuaries, coastlines, coral reefs, and the deep ocean). 

aquifer Porous, water-saturated layers of sand, gravel, or bedrock that can yield an economically significant amount of water. 

common law Body of unwritten rules and principles derived from thousands of past legal decisions. It is based on evaluation of what is reasonable behavior in attempting to balance competing social interests. Compare statutory law. 

desalination Purification of salt water or brackish (slightly salty) water by removal of dissolved salts.

drainage basin See watershed. 

drought Condition in which an area does not get enough water because of (1) lower-thannormal precipitation or (2) higher-than-normal temperatures that increase evaporation. 

floodplain Flat valley floor next to a stream channel. For legal purposes, the term often applies to any low area that has the potential for flooding, including certain coastal areas. 

groundwater Water that sinks into the soil and is stored in slowly flowing and slowly renewed underground reservoirs called aquifers; underground water in the zone of saturation, below the water table. Compare runoff, surface water. 

Hydrologic cycle Biogeochemical cycle that collects, purifies, and distributes the earth's fixed supply of water from the environment to living organisms and then back to the environment. 

Natural recharge Natural replenishment of an aquifer by precipitation, which percolates downward through soil and rock. See recharge area.

permeability Degree to which underground rock and soil pores are interconnected and thus a measure of the degree to which water can flow freely from one pore to another. Compare porosity.

porosity Percentage of space in rock or soil occupied by voids, whether the voids are isolated or connected. Compare permeability.

Prior appropriation Legal principle by which the first user of water from a stream establishes a legal right to continued use of the amount originally withdrawn. Compare riparian rights.

recharge area Any area of land allowing water to pass through it and into an aquifer. See aquifer, natural recharge. 

reliable runoff Surface runoff of water that generally can be counted on as a stable source of water from year to year. See runoff.

Saltwater intrusion Movement of salt water into freshwater aquifers in coastal and inland areas as groundwater is withdrawn faster than it is recharged by precipitation. 

stream Flowing body of surface water. Examples are creeks and rivers.

subsidence Slow or rapid sinking of part of the earth's crust that is not slope related.

surface runoff Water flowing off the land into bodies of surface water. See reliable runoff. 

water table Upper surface of the zone of saturation, in which all available pores in the soil and rock in the earth's crust are filled with water. 

watershed Land area that delivers water, sediment, and dissolved substances via small streams to a major stream (river).

zone of Zone in soil that is not saturated with water and lies above the water table.

aeration water table, zone of saturation.