• A water table describes the boundary between water-saturated ground and unsaturated ground. Below the water table, rocks and soil are full of water. Pockets of water existing below the water table are called aquifers.
    An area's water table can fluctuate as water seeps downward from the surface. It filters through soil, sediment, and rocks. This water includes precipitation, such as rain and snow. Irrigation from crops and other plants may also contribute to a rising water table.
    This seeping process is called saturation. Sediment or rocks that are full of water are saturated. The water table sits on top of what experts call the zone of saturation, or phreatic zone. The area above the water table is called the vadose zone.
    Unlike the tables you'd find in your house, a water table usually isn't flat, or horizontal. Water tables often (but not always) follow the topography, or upward and downward tilts, of the land above them. 
    Sometimes, a water table runs intersects with the land surface. A spring or an oasis might be the water table intersecting with the surface. A canyon, cliff, or sloping hillside may expose an underground river or lake sitting at the area's water table.
    In addition to topography, water tables are influenced by many factors, including geology, weather, ground cover, and land use.
    Geology is often responsible for how much water filters below the zone of saturation, making the water table easy to measure. Light, porous rocks can hold more water than heavy, dense rocks. An area underlain with pumice, a very light and porous rock, is more likely to hold a fuller aquifer and provide a clearer measurement for a water table. The water table of an area underlain with hard granite or marble may be much more difficult to assess.
    Water tables are also influenced by weather. They will be usually be higher in rainy seasons or in the early spring, as snowmelt filters below the zone of saturation.
    Ground cover can contribute to an area's water table. The spongy, absorbent vegetation in swamps, for instance, are saturated at least part of every year. Water tables in swamps are nearly level or even higher than the surface.
    Land use can also influence an area's water table. Urban areas often have impervious surfaces, such as parking lots, for instance. Impervious surfaces prevent water from seeping into the ground below. Instead of entering the area's zone of saturation, water becomes runoff. The water table dips. 
    Water tables are useful tools for measuring aquifers, saturated areas beneath the water table. Aquifers are used to extract water for people, plants and every organism living on the surface of the Earth. 
    Some water tables are dropping very quickly, as people drain aquifers for industry, agriculture, and private use. Scientists call this process "aquifer depletion." 
    In regions such as North Africa, people are using the water in aquifers faster than it can be replaced by rain or snow. People and businesses in North Africa are not using more water than people in other areas, but their aquifers, beneath the Sahara Desert, are much shallower than aquifers in North America or Australia. Parts of North Africa are experiencing aquifer depletion.
    Even the enormous aquifers in North America can be threatened with aquifer depletion. The Oglalla Aquifer stretches more than 450,000 square kilometers (174,000 square miles) through parts of the U.S. states of South Dakota, Wyoming, Nebraska, Colorado, Kansas, New Mexico, Oklahoma, and Texas. The Oglalla Aquifer holds more than 3,000 cubic kilometers (2.4 billion acre-feet) of groundwater
    The Oglalla Aquifer is one the most important source of water for irrigation, drinking, industry, and hygiene in the U.S. However, aquifer depletion became a threat in the 20th century, as industrial agriculture and development drained the aquifer faster than it could naturally replenish itself. 
    Although the water table varies throughout the Oglalla Aquifer, it is generally 15 to 90 meters (50 to 300 feet) below the land surface. Industrial agriculture and development in the 1940s and 1950s contributed to lowering the water table by more than a meter (3.5 feet) year. In parts of the Texas Panhandle, where the water table was lowest, the aquifer was nearly drained.
    Improved irrigation practices have slowed the rate of aquifer depletion, and some water tables in the Oglalla Aquifer have risen.
    water table
    During a drought, the water table is low.

    Fossil Water Tables
    Water that has been stored in aquifers for thousands of years is called fossil water. Fossil water is often considered a non-renewable resource, because it cannot be replenished by precipitation. Extracting fossil water permanently lowers an area's water table.

    Well, Well, Well
    Water wells are simply holes dug below the water table. Wells can be dug by hand if the water table is relatively close to the surface, or may require machinery if the water table is hundreds of meters deep. Water can be pulled out of a well by hand (in a bucket on a rope or chain) or by more high-tech equipment like pumps.

    Tidal Tables
    Some oceanic islands' water tables are determined by the tides. On these islands, freshwater seeps down to intersect with pockets of seawater that collect in porous soil. The denser seawater stays beneath the freshwater, causing the water table to rises and fall with the tides.

  • Term Part of Speech Definition Encyclopedic Entry
    agriculture Noun

    the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching).

    Encyclopedic Entry: agriculture
    aquifer Noun

    an underground layer of rock or earth which holds groundwater.

    Encyclopedic Entry: aquifer
    aquifer depletion Noun

    process by which people pump more water out of aquifers than can be replaced by rain or snow.

    assess Verb

    to evaluate or determine the amount of.

    boundary Noun

    line separating geographical areas.

    Encyclopedic Entry: boundary
    canyon Noun

    deep, narrow valley with steep sides.

    Encyclopedic Entry: canyon
    cliff Noun

    steep wall of rock, earth, or ice.

    Encyclopedic Entry: cliff
    crop Noun

    agricultural produce.

    Encyclopedic Entry: crop
    dense Adjective

    having parts or molecules that are packed closely together.

    filter Verb

    to remove particles from a substance by passing the substance through a screen or other material that catches larger particles and lets the rest of the substance pass through.

    fluctuate Verb

    to constantly change back and forth.

    geology Noun

    study of the physical history of the Earth, its composition, its structure, and the processes that form and change it.

    granite Noun

    type of hard, igneous rock.

    granite Noun

    type of hard, igneous rock.

    ground cover Noun

    grasses, shrubs, and other vegetation covering a surface of the Earth.

    groundwater Noun

    water found in an aquifer.

    Encyclopedic Entry: groundwater
    hill Noun

    land that rises above its surroundings and has a rounded summit, usually less than 300 meters (1,000 feet).

    Encyclopedic Entry: hill
    hygiene Noun

    science and methods of keeping clean and healthy.

    impervious surface Noun

    boundary that does not allow water to penetrate it.

    industry Noun

    activity that produces goods and services.

    intersect Verb

    to cross paths with.

    irrigation Noun

    watering land, usually for agriculture, by artificial means.

    Encyclopedic Entry: irrigation
    land use Noun

    range of purposes people put to the earth.

    marble Noun

    type of metamorphic rock.

    oasis Noun

    area made fertile by a source of fresh water in an otherwise arid region.

    Encyclopedic Entry: oasis
    phreatic zone Noun

    area below a water table, where groundwater saturates the rocks and soil. Also called a zone of saturation.

    plant Noun

    organism that produces its own food through photosynthesis and whose cells have walls.

    porous Adjective

    full of tiny holes, or able to be permeated by water.

    precipitation Noun

    all forms in which water falls to Earth from the atmosphere.

    Encyclopedic Entry: precipitation
    pumice Noun

    type of igneous rock with many pores.

    rainy season Noun

    time of year when most of the rain in a region falls.

    replenish Verb

    to supply or refill.

    rock Noun

    natural substance composed of solid mineral matter.

    runoff Noun

    overflow of fluid from a farm or industrial factory.

    Encyclopedic Entry: runoff
    saturate Verb

    to fill one substance with as much of another substance as it can take.

    saturation Noun

    process by which one substance, such as sand, is filled with another substance, such as water.

    season Noun

    period of the year distinguished by special climatic conditions.

    Encyclopedic Entry: season
    sediment Noun

    solid material transported and deposited by water, ice, and wind.

    Encyclopedic Entry: sediment
    sediment Noun

    solid material transported and deposited by water, ice, and wind.

    Encyclopedic Entry: sediment
    seep Verb

    to slowly flow through a border.

    soil Noun

    top layer of the Earth's surface where plants can grow.

    spring Noun

    small flow of water flowing naturally from an underground water source.

    swamp Noun

    land permanently saturated with water and sometimes covered with it.

    Encyclopedic Entry: swamp
    topography Noun

    study of the shape of the surface features of an area.

    urban area Noun

    developed, densely populated area where most inhabitants have nonagricultural jobs.

    Encyclopedic Entry: urban area
    vadose zone Noun

    area above an region's water table.

    water table Noun

    underground area where the Earth's surface is saturated with water. Also called water level.

    Encyclopedic Entry: water table
    weather Noun

    state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.

    Encyclopedic Entry: weather
    zone of saturation Noun

    area below a water table, where groundwater saturates the rocks and soil.