GEF

Unit 2  |  Water Science and the Water Cycle 50 depends on the surface it falls on. Ground that is porous allows water to percolate through air spaces in the soil and rock. Once the water is underground, some of it becomes part of long-lived aquifers . Other water eventually enters wetlands, lakes, streams, and rivers at the surface. Precipitation such as rain and snow returns water to the land where it finds its way into overland flows or underground sources. Not all precipitation percolates into the ground. Land that is steep, frozen, or oversaturated from recent rainfall is not able to absorb much, if any, water. Impervious surfaces such as concrete, asphalt, and other human-constructed materials also do not allow water to percolate into the ground. In these conditions water flows over the land as surface runoff . In agri- cultural or urban areas, surface runoff frequently contains high levels of pollutants that can find their way into water bodies. Water that falls as snow or freezes into ice also has an impor- tant role to play in the earth system. Collectively known as the cryosphere , it includes seasonal snow cover, ice-covered lakes and sea ice, permafrost , and continental or mountain glaciers . The cryosphere plays a vital role in climate regulation by reflect- ing a much higher percentage of the sun’s energy back into PERCOLATE The movement of a liquid through a porous material. AQUIFERS Underground layers of porous rock that allow the movement of water between layers of non-porous rock. SURFACE RUNOFF The flow of water over land surfaces that are saturated or impermeable. CRYOSPHERE The portion of the Earth where water is maintained as snow or ice. PERMAFROST Permanently frozen soil that releases CO 2 and methane as it melts. GLACIER A large body of ice that per- sists for a long period of time.