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.