Precipitation that doesn't run off into rivers or is not absorbed into the ground or used by humans ends up in an aquifer. An aquifer is an underground "lake." Just as above-ground water has a definite surface, so do subterranean aquifers. This surface is called a water table. An aquifer is replenished by precipitation seeping from above. A water table affects geological and environmental features on land surface, provides a lifeline for cities and agriculture, and can be detected remotely through seismic analysis.
Concept Illustration
Take a transparent glass and fill it with small pebbles and rocks. Then pour water up to a certain level. You should be able to see a water surface below which the rocks are soaked, and above which the rocks may be wet but still have air pockets between them. Such a "lake surface" in rock is a water table. While this gets the point across, there is an important limitation in the rocks-and-water-cup analogy: constant water table level. In reality, a water table is irregular, and rises and falls independently of human or plant use. Different rock and geological formations have different porosity, allowing for rise or fall of the water table. This pebbles-in-a-glass example by no means conveys the phenomenon's complexity.
Replenishment
Precipitation that does not run off into rivers and streams usually soaks into the ground. Note that this may make the ground saturated with water for a short period of time, but that doesn't correspond to the water table. Whatever does not evaporate eventually seeps into an aquifer. Since subterranean water rests on impermeable rock formations, precipitation raises the water table level.
Use
Many places away from visible water sources thrive due to a relatively shallow water table. Of course, the party can't last forever. If aquifer depletion exceeds the replenishment rate, the water table sinks farther underground. The increased difficulty of getting to water, combined with increased cost and competition, can put significant strain on communities. For example, a USGS educational site chronicles the causes and effects of lowered water table on communities through the United States.
Environmental Effect
A shallow water table allows vegetation to take root to a greater extent as more water is available for plant roots. By contrast, a deep water table leaves little water close enough to ground surface for plant life survival. A deepening water table can therefore facilitate desertification as decreased plant life and underground root density allow for faster erosion of fertile topsoil.
Detection
Resource limitations prohibit digging wells to extreme depths in hope of finding water. An efficient technique for water table detection is vibration analysis. Controlled explosions result in seismic waves propagating through rocks. Just as light is reflected and refracted by the interplay of substances and incident angles, seismic waves differ in their velocity depending on what they encounter. Since water propagates sound much faster than air, sudden increase in wave velocity at certain points corresponds to travel through an aqueous substance. Rock formations also affect seismic propagation, but computerized analysis can differentiate between rock, water table, and background noise. A Stanford Dept. of Energy project poster describes seismic water table detection.
Tags: water table, water table, into ground, into rivers, plant life, Precipitation that, rock water
