THE HYDROLOGIC CYCLE
Water plays a vital role in the unique chemistry of the Earth's outer layers. Water saturates the air, falls to the ground as precipitation, moves through a complex system of rivers and streams, and is stored for long periods in underground reservoirs, oceans, and ice. Water shapes the surface of our planet, and it provided the medium in which life began. The combination of process by which water moves from repository to repository above, below and on the Earth's surface is the hydrologic cycle.
The total amount of water on the Earth's surface has stayed roughly the same from very early times. Water first reached the surface during the outgassing of the young, volcano—covered Earth. When the planet's surface temperature finally fell below 100ºC (212 ºF) this water condensed into liquid form and began to fill the ocean basins. Relatively minor process still add and remove water from the Earth. High in the atmosphere, ultraviolet rays from the Sun break up water molecules, freeing hydrogen atoms, which, because of their low mass, may escape into space. At the same time, at converging and diverging plate boundaries and other sites of volcanism, small amounts of new water emitted from the Earth's deep interior. These losses and gains are in rough equilibrium, and in any case both are rather small—by one estimate, no more than one or two Olympic–size swimming pools of water per year. Thus, for all intents and purposes, we can treat the Earth as if it has a fixed amount of water at its surface for billions of years. The water that we have is all there is.
Reservoirs of Water
The Earth boasts several major water repositories. In addition to oceans, lakes and rivers, significant amounts of water are locked up into Earth's polar ice caps and glaciers, bodies of ice that form in regions where snowfall exceeds melting. Ice caps are layers of ice that form at the north and south polar regions of the Earth. Glaciers are large bodies of ice that slowly flow down a slope or valley under the influence of gravity. Approximately 96% of glaciers (by volume) occur in Antarctica and Greenland, while the rest are widely scattered in mountain areas.
All kinds of these places where water occurs tap into the same central supply. During its lifetime, a given molecule of water will cycle through many different kinds of bodies, over and over again.
The hydrologic cycle with which most of us are familiar involves the short–term back–and–forth transfer of water molecules between the oceans and the land. Water evaporates off the surface of the ocean, forms into clouds, falls and then returns to the oceans via rivers and streams. Most terrestrial life depends upon this simple cycle.
A vast part of this cycle remains unseen, however, some of the water that falls on the continents does not immediately return to the ocean. It rather seeps into the Earth as groundwater. There, it goes into large aquifers–bodies that re, in effect underground storage tanks of water. By some estimates, more than 98& of the world's fresh water is groundwater. Water typically percolates into the ground and fills the tiny spaces between grains of sandstone and other porous rock layers. These layers of water–saturated rock are often bounded by impermeable materials such as clay, which keeps the water from seeping away. Humans tap into these aquifers when they drill wells to supply water for cities and agriculture.
