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. 
 
Aquifers
 
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.

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.

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.

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.

Noun

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

Noun

an underground layer of rock or earth which holds groundwater.

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.

Noun

line separating geographical areas.

Noun

deep, narrow valley with steep sides.

Noun

steep wall of rock, earth, or ice.

Noun

agricultural produce.

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.

Noun

water found in an aquifer.

Noun

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

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.

Noun

watering land, usually for agriculture, by artificial means.

land use
Noun

range of purposes people put to the earth.

marble
Noun

type of metamorphic rock.

Noun

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

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.

Noun

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

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.

Noun

overflow of fluid from a farm or industrial factory.

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.

Noun

period of the year distinguished by special climatic conditions.

Noun

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

Noun

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

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.

Noun

land permanently saturated with water and sometimes covered with it.

topography
Noun

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

Noun

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

vadose zone
Noun

area above an region's water table.

Noun

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

Noun

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

zone of saturation
Noun

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