More than 99 percent of the earth's water lies beneath the surface, according to the United States Geological Survey, or USGS. Rapidly multiplying human populations place increasing demands on usable water supplies for agricultural and industrial uses. Water supplies both above and beneath the surface are increasingly threatened worldwide. Changes in the water table serve as a warning signal indicating the depletion of this essential resource.
Water Table Definition
The water table is the surface of ground water located in completely saturated sediment. It serves as the defining boundary between layers of sediment containing subsurface water. Above the water table, soil water and vadose water are located in partially saturated sub-surface sediment layers.
Water table depth varies by precipitation levels, density of vegetation root systems and demands placed upon subsurface water supplies by wells and other human activity. When subsurface water supplies deplete at a faster rate than they can be replenished by precipitation and underground water, partially saturated sediment dries out. With continued depletion, groundwater diminishes, the level of the water table drops and drought sometimes results.
Water Tables and Ground Water
Gravity draws subsurface water down toward the center of the earth. Sediment layers containing ground water often extend hundreds of feet below the Earth's surface. At its deepest levels, ground water may be more than 100 years old.
The layer of sediment beneath groundwater known as bedrock absorbs very little water, which makes it nonporous. Because it is nonporous, bedrock works against the force of gravity and causes subsurface water to flow sideways above it. This action, known as recharge, replenishes surface rivers, lakes and streams.
Ground water also replenishes subsurface water reservoirs, called aquifers, that are accessible by drilling wells. Wells must reach ground water levels to successfully yield water. Subsurface water can also become trapped in a layer of porous sediment sandwiched between two nonporous sediment layers, creating a confined aquifer. Wells that tap into confined aquifers sometimes release pressure that causes water to flow to the surface without pumping, which creates an artesian well.
Water Tables and The Layer of Aeration
Plant roots systems draw water from soil water, which is the partially saturated sediment that extends 6 to 10 feet below the earth's surface. Vadose water is located in sediment layers between soil water and ground water. Soil water and vadose water sediment layers combine to form what is known as the layer of aeration. The sediment located in the layer of aeration can vary from very porous sand to dense clay. Although nonporous bedrock sometimes occurs at or near the earth's surface in general, more porous sediment tends to occur closer to the earth's surface.
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