The water table is the depth at which the ground becomes saturated with water. Groundwater flows down through pores in rocks and soil until it reaches the water table. The depth of the water table varies depending on factors such as topography, soil material, and water abstraction. Aquifers are areas of rock or sediment that contain groundwater that can be accessed for human use. Over-extraction of groundwater can cause an overdraft and drop the water table.
The water table, commonly called the water table, is the depth at which the ground becomes saturated, or filled to maximum capacity, with water. When water reaches the surface of the Earth, whether through rain, flood or other means, the water begins to seep or pass through the ground. Groundwater flows down through pores in rocks and soil until it reaches a point where all available spaces are filled. While the term is often used loosely, groundwater technically refers only to water at or below this level. In this way, the water table can be thought of as the upper surface of groundwater.
Soil can be divided into two parts: the unsaturated zone which is above the water table and the saturated zone which is below the water table. Groundwater that passes through the top six to ten feet (1.83-3.05 meters) of the soil nourishes plant roots and is called groundwater. As groundwater travels down past the roots in the unsaturated zone, it becomes known as vedose water. Vadose water descends into the saturated zone through pores, or small holes, in rocks and sediments until it reaches the water table and becomes groundwater.
The depth at which the water table lies can vary greatly, from a few feet in some places, to hundreds or thousands of feet in other places. This variation can depend on several factors including the regional topography or characteristics of the Earth’s surface in a particular area, the type of material the water has to pass through, the seasons, and water abstraction. Aquifers are often shallower under valleys and deeper under hills, because the distance to travel is greater. The rate at which the soil becomes saturated, and therefore the rate at which groundwater rises, also depends on the porosity, or amount of space, in the soil material. Rock, for example, fills in faster than sand because there’s simply less free space to fill.
Seasonal droughts, floods or rainfall can also affect the groundwater level if it is not too far from the soil surface. However, some saturated zones are so deep that the time it takes for water to infiltrate the saturated zone compensates for seasonal changes, leaving the water table unaffected by seasonal variation. In general, infiltration is a slow process, as is the discharge process, where water naturally leaves the aquifer.
An aquifer is an area of rock or sediment that contains groundwater that can be accessed for human use. One example is the Ogallala Aquifer which covers approximately 174,000 square miles (450657.9 square kilometers) in the American Midwest. Unlike a confined aquifer, where groundwater is trapped between impermeable material, the Ogallala is not confined and can still be recharged by snowmelt, precipitation, and other surface water types.
Like many other aquifers, however, the recharge rate in the Ogallala is very slow and the extraction rate is high. Humans abstract or withdraw groundwater water in large volumes for agriculture, residential use, and industry. When the extraction rate exceeds the recharge capacity of the aquifer, it is called an overdraft and causes the water table to drop. The overdraft caused the water table in many places on the Ogallala to drop more than 100 feet, or more than 30.5 meters.
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