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The level of groundwater.
Source: EPA
The water table or phreatic surface is the upper limit of abundant groundwater. In the vadose zone, above the water table, the interstices between particles of earth are filled by air, or by air and water (with the exception of the capillary fringe). Below it, every available space is saturated with water. The definition of the water table is surface where the pressure head is equal to atmospheric pressure (where gauge pressure = 0).
A large amount of water within a body of sand or rock below the water table is called an aquifer. A so-called "perched aquifer" (or perched water table) occurs when the descent of water percolating from above is blocked by a shelf of impermeable rock.
The porous media in which groundwaters occur are the complex geologic materials near the earth surface; hence local details of porosity and permeability are as complex as those materials. Generally, the more productive and useful aquifers are in sedimentary geologic formations, though weathered and fractured crystalline rocks yield smaller volumes of groundwater in many environments. Among the most productive groundwater environments are unconsolidated to poorly cemented alluvial materials that have accumulated as valley-filling sediments in major river valleys and geologically subsiding structural basins.
The water table is that surface beneath which all interconnected pore space in the rock is water-filled or saturated. Groundwater recharge is the process of adding new water to the groundwater body, as through infiltration of precipitation on the land surface. Discharge is the process of water escaping from the groundwater body, as when the water table intersects the land surface, allowing water to flow out from a spring.
In undeveloped regions, or areas with high amounts of precipitation, the water table roughly follows the contour of the overlying land surface, and rises and falls with rainy or dry weather. Springs and oases occur when the water table reaches the surface. Springs commonly form on hillsides, where the earth's slanting surface may "intersect" with the water table. Other, unseen springs are found under rivers and lakes, and account for the sometimes surprisingly well-preserved water levels which occur in times of mild drought.
The practice of drilling wells to extract groundwater is dependent on understanding the water table. Because wells must reach the water table, its depth determines the minimum depth of a viable well, and thus the feasibility of drilling it.
In Australia, because of farming, salt water tables have risen nearer to the surface, endangering plant life and farm economy as the soil grows more salty. These salt water tables were formed millions of years ago when parts of Australia were covered by ocean. As the continent dried, the water became trapped underground.
In areas with a high water table, tunnels and basements are less common due to the risk of flooding.
Source: wikipedia
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