The water cycle describes the sum of processes operative in the hydrosphere, a dynamic mass of liquid continuously on the move between the different reservoirs on land and in the oceans and atmosphere. The hydrosphere includes all the water in oceans, lakes, streams, glaciers, atmosphere, and groundwa-ter, although most water is in the oceans. The hydro-logic, or water, cycle encompasses all of the changes, both long- and short-term, in the Earth's hydrosphere. It is powered by heat from the sun, which causes water to change its state through evaporation and transpiration.
The water cycle can be thought of as beginning in the ocean, where energy from the sun causes surface waters to evaporate, changing from the liquid to the gaseous states. Evaporation takes heat from the ocean and transfers it into the atmosphere. An estimated 102 cubic miles (425 cubic km3) of water evaporate from the ocean each year, leaving the salts behind in the ocean. The water vapor then condenses into water droplets in clouds and eventually falls back to the Earth as precipitation. Ninety-two cubic miles (385 cubic km3) falls directly back into the ocean, but about 26 cubic miles (111 km3) of precipitation falls as rain or snow on the continents, transforming salty water of the oceans into freshwater on the land. Nearly three-fourths of this water (17 cubic miles, or 71 cubic km/yr) evaporates back to the atmosphere or is aided by the transpiration from plants, returning the water to the atmosphere. The other estimated 10 cubic miles (40 km3) per year of water runs across the surface, some merging to form streams and rivers that eventually flow back into the ocean, and other parts of this 10 cubic miles per year seeping into the ground to recharge the groundwater system. Humans are now intercepting approximately half of the fresh surface water for drinking, agriculture, and other uses, making a significant impact on the hydrological cycle. Water that seeps into the groundwater system is said to infiltrate, whereas the water that flows across the surface is called runoff.
Understanding the water cycle reveals that freshwater is a renewable resource, replenished and cleaned every year, and available for reuse. It must be used wisely, however, since the quantities are limited, and small amounts of contamination can make entire parts of the system unusable. Freshwater is also supplied unevenly in space and time, with some areas receiving little, and other receiving freshwater in abundance. The water may come in floods or may be withheld, causing drought and suffering. Controlling the flow and usage of freshwater across large regions is one of the major challenges facing humans as the population of the planet grows exponentially and the water supply remains the same.
The hydrologic cycle. Water evaporates from the oceans and forms clouds that cause precipitation to fall over the land and oceans. The rain and snow that falls on land can run off in rivers to the oceans, seep into the groundwater system, or be used by plants that then transpire the moisture back to the atmosphere. Water is continuously moving between the different parts of the hydrosphere.
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