GLOSSARY

Acre-foot - An amount of water covering one square acre to a depth of one foot. One acre-foot is equal to 325,851 gallons, enough water to fill a football field one foot deep or supply all the needs of an average four-person American family for one year.

Aquifer - A subterranean geologic basin, composed of unconsolidated materials such as sand and gravel, or consolidated rock such as sandstone and fractured limestone. Aquifers are permeable enough to store, transmit, and yield groundwater in usable quantities.

Cone of depression - A cone-shaped lowering of the water table around a pumped well.

Confined aquifer - An aquifer with a nearly impermeable upper boundary that helps confine it from other water sources. Unconfined aquifers, by contrast, have direct contact with the water table and thus are generally more available to recharge and more susceptible to contamination than confined aquifers are.

Drawdown - A decrease in the water level of an aquifer or well as the result of pumping.

Federal trust duty - A longstanding doctrine in federal common law that describes the duty the U.S. government owes Native Americans. Under this doctrine, the government must serve as a fiduciary to tribes and tribal lands, must act in the tribes’ best interest, and must interpret all treaties it makes with tribes in the light most favorable to them.

Groundwater - That portion of water beneath the surface of the earth that can be collected through wells, tunnels, or drainage galleries, or that flows naturally to the earth’s surface via seeps and springs.

Overburden - Earth, rock, and other materials that cover a natural deposit of coal.

Potentiometric head - A measure of an aquifer’s water pressure, as reflected by the height to which its water will climb when tapped by a well.

Recharge - The addition of surface water or precipitation to an aquifer. An aquifer’s rate of recharge depends on the availability of water, the physical characteristics of soil and rock that the water must pass through, and the ability of the aquifer to accept the water that arrives.

Safe yield - Historically, a standard of groundwater use based on the difference between an aquifer’s annual rates of recharge and discharge: that is, the volume of water an aquifer takes in minus the volume of water it loses through outflow into springs and washes and through other natural processes. If users pump ground-water at a rate that exceeds this difference, they have exceeded the aquifer’s safe yield.

Water budget - Method of allocating water resources among competing uses.

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