Some of this precipitation is captured by tree canopies and evaporates again into the atmosphere. The precipitation that hits the ground becomes runoff, which can accumulate and freeze into snow caps or glaciers. It can also infiltrate the ground and accumulate, eventually storing in aquifers. An aquifer is a large deposit of groundwater that can be extracted and used. This runoff also comes from snowmelt, which occurs when the sun and climate changes melt snow and ice. Finally, some of this runoff makes it way back into lakes and oceans, where it is again evaporated by the sun. You can learn more about the water cycle in How the Earth Works.
Water that falls to the ground and stays in the soil ends up evaporating and retiring to the atmosphere. But groundwater, which is the major source of our drinking water, can accumulate in aquifers over thousands of years. Unconfined aquifers have the water table, or the surface where water pressure equals atmospheric pressure, as their upper boundaries. Confined aquifers often lie below unconfined aquifers and have a layer of rock or other materials as their upper boundaries.
In the United States, the oldest groundwater, known as fossil water, is contained in the Ogallala Aquifer. Lying below about 175,000 square miles (450,000 square kilometers) of eight states in the Great Plains, the Ogallala Aquifer stores about 2,900 million acre-feet (3,600 million kilometers cubed) of water [source: High Plains/Ogallala Aquifer]. The Ogallala Aquifer was formed between 2 and 6 million years ago, when the Rocky Mountain chain was forming. Because the climate of the Great Plains is arid, water in the aquifer is being used faster than it can be recharged. That's why some scientists refer to using fossil water aquifers as water mining.
One aquifer important to note is the Ogallala Aquifer, or High Plains Aquifer, located in the United States Great Plains region. This is the world's largest known aquifer with an approximate area of 174,000 square miles (450,600 square kilometers) and runs from southern South Dakota through parts of Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and northern Texas. It is considered an unconfined aquifer and though it is large in area, much of the aquifer is shallow.
The Ogallala Aquifer was formed about 10 million years ago when water flowed onto the highly permeable sand and gravel of the plains from retreating glaciers and streams from the nearby Rocky Mountains. Because of changes due to erosion and the lack of glacial meltwater, today the Ogallala Aquifer is no longer being recharged by the Rockies.
Because precipitation in the region is only around 12-24 inches (30-60 cm) per year, this heavily agricultural region relies on water from the Ogallala to maintain crop production but also support municipal and industrial development. Since the aquifer was first tapped for irrigation in 1911, its use has increased dramatically. As a result, its water table has dropped and has not been naturally replenished due to the altered stream flow in the Rockies and lack of precipitation. The drop is most prominent in northern Texas because the thickness there is least, but it is also a problem in parts of Oklahoma and Kansas.
Recognizing the problems associated with a dropping water table such as collapsing aquifers, the resultant damage to infrastructure, and the loss of a water source in a normally dry region, portions of Nebraska and Texas have invested in groundwater recharge to allow the Ogallala Aquifer to remain useful for the area. Recovery of aquifers is a long process though and the full impact of such plans is not yet fully known. Current irrigation practices in the region though could use up about half of the Ogallala's water within the next decade.
Stumble It!
Tweet This
Bookmark on Delicious
Share on Facebook
Share on MySpace