Hydrologic Cycle
The hydrologic cycle, sometimes called the water cycle, is the movement of water between the air, land and bodies of water. The hydrologic cycle includes numerous physical, chemical and biological processes. Almost 98% of the water on Earth is in liquid form. The rest is either bound up in ice, in the soil, in living organisms, or in the atmosphere. The hydrologic cycle is balanced on Earth because any water moving out of an environment is balanced by water moving in.
The hydrologic cycle also shows how water moves from one state to another. Evaporation is the changing of water from a liquid state to a gaseous state. Because of the heat of the sun, water evaporates from oceans, lakes, ponds, rivers, streams and glaciers and enters the atmosphere as gaseous water vapor. A small amount of water evaporates from the soil surface. In addition, water enters the atmosphere through transpiration from plants and respiration by animals and plants. Transpiration (sometimes called evapotranspiration) is evaporation of water from the leaves of plants. This is the primary way that plants move water and dissolved nutrients. Respiration is the process of releasing energy from food molecules. Water is produced during respiration and some of this water is released into the atmosphere as a gas.
Once in the atmosphere, wind can carry the water vapor around. Under cool atmospheric conditions, the gaseous water vapor molecules can condense into a liquid or solid form. This is how clouds are formed. If the liquid or solid drops get large and heavy enough, they fall to the ground as precipitation. Rain, snow, sleet, and hail are all forms of precipitation.
Most of this precipitation falls into the oceans, which cover about 71% of the Earth. However, there are several fates of the water that falls on the land. Much of it seeps into the ground until it reaches a zone where all of the cracks and spaces between soil particles are completely saturated with water. The top of this saturation zone is the water table. The groundwater in this saturated zone seeps slowly through the ground until it is discharged directly from the ground as a spring or into existing lakes, rivers, estuaries or oceans. In some locations, humans drill wells into the ground below the water table and use this groundwater as a source of fresh water for drinking and other purposes. Plants, whose roots reach into the soil, take up some of the water that percolates into the ground for their biological processes.
The remainder of the water that falls on the land is acted on by gravity, forming runoff. Small trickles of runoff eventually join together to form creeks and rivers, which eventually open into ponds, lakes, estuaries and oceans. Water evaporates once again during its passage to these bodies of water, and continues to evaporate from them, continuing through the hydrologic cycle. Because water is always being moved through the hydrologic cycle, it is constantly being renewed. The amount of time that a water molecule stays in a particular part of the hydrologic cycle is referred to as its residence time.
Humans have a great influence on the hydrologic cycle. By damming rivers we prevent runoff and change evaporation rates. Changes in land use such as cutting down a forest or building impervious structures such as roads and parking lots reduce the amount of transpiration and increase the amount of runoff. In addition, chemical pollutants can enter runoff, surface water and groundwater. This can have major impacts on water quality and organisms that live in and use this water, including human life. Overuse of water can reduce supplies of surface water and groundwater and change groundwater flow. Atmospheric pollution resulting from global warming may increase evaporation rates, which will result in further, perhaps devastating changes in global climate.
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