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    The Water Cycle

    Water can be in the atmosphere, on the land, in the ocean, and underground. It moves from place to place through the water cycle.

    The Water Cycle

    All of the water on Earth makes up the hydrosphere. And that water doesn't stay still. It is always on the move. Rain falling today may have been water in a distant ocean days before. And the water you see in a river or stream may have been snow on a high mountaintop. Water is in the atmosphere, on the land, in the ocean, and underground. It moves from place to place through the water cycle.

    Where's the water?

    There are about 1.4 billion km3 of water (336 million mi3 of water) on Earth. That includes liquid water in the ocean, lakes, and rivers. It includes frozen water in snow, ice, and glaciers, and water that’s underground in soils and rocks. It includes the water that’s in the atmosphere as clouds and vapor.

    If you could put all that water together – like a gigantic water drop – it would be 1,500 kilometers (930 miles) across.

    Of all the water in the hydrosphere, the vast majority, about 97% of it, fills the ocean.  About 2% of the water on Earth is frozen in ice sheets near the poles and in glaciers. Sometimes the ice on Earth is included in the hydrosphere and sometimes it's seperated into a special part of the Earth system called the cryosphere. Most of the ice is in Antarctica, a smaller amount in Greenland in the Arctic, and a tiny fraction in mountain glaciers around the world. Most of the remaining 1% of Earth’s water is underground, in shallow aquifers, as soil moisture, or deep underground in rock layers. Only a small fraction of the water on Earth (0.03%) is in lakes, wetlands, and rivers.

    The main processes in the water cycle

    Credit: NASA

    Water's on the move.

    As it moves through the water cycle, water often changes from a liquid, to a solid (ice), to a gas (water vapor). Water in oceans and lakes is typically liquid; but it is solid ice in glaciers, and often invisible water vapor in the atmosphere. Clouds are tiny droplets of liquid water or small ice crystals.

    Water at the surface of the ocean, rivers, and lakes can become water vapor and move into the atmosphere with a little added energy from the Sun through a process called evaporation. Snow and ice can also turn into water vapor, which is a process known as sublimation. And water vapor gets into the atmosphere from plants, too, which is called transpiration.

    Because air is cooler at higher altitude in the troposphere, water vapor cools as it rises high in the atmosphere and transforms into water droplets by a process called condensation. The water droplets that form make up clouds. Water vapor can also condense into droplets near the ground, forming fog when the ground is cold. If the temperature is cold enough, ice crystals form instead of liquid water droplets.

    If the droplets or ice crystals within clouds grow in size, they eventually become too heavy to stay in the air, falling to the ground as rain, snow, and other types of precipitation.

    What happens to the rain and snow that fall?

    Around the world, each year, about 505,000 km3 (121,000 mi3) of water falls as rain, snow, and other types of precipitation.

    86% of those raindrops and snowflakes come from the ocean where 434,000 km3(104,000 mi3) of water evaporates into the atmosphere each year. Water eventually returns to the ocean as precipitation that falls directly into the sea and as precipitation that falls on land and flows to the ocean through rivers.

    Less water evaporates over the land than falls onto land as precipitation. Evaporation of water from the land happens directly from lakes, puddles, and other surface water. Also, water also makes its way into the atmosphere via a process called transpiration in which plants release water into the air from their leaves that was pulled up from the soil through roots. Collectively, the water evaporated from the land and from plants is called evapotranspiration.

    Some of the snow and ice that falls as precipitation stays on the land as a part of icy mountaintop glaciers or the ice sheets that cover places like Greenland and Antarctica. Some of the precipitation seeps into the ground and joins the groundwater that is often tapped by wells to provide water to farms, towns, and cities.

    A rainy day in the countryside near Xi’an, China

    Credit: L.S. Gardiner

    How long does water stay in a place before it moves?

    The length of time that particular water molecules stay in a part of the water cycle is quite variable, but water does stay in certain places longer than others.

    A drop of water may spend over 3,000 years in the ocean before evaporating into the air, while a drop of water spends an average of just nine days in the atmosphere before falling back to Earth.

    Water spends thousands to hundreds of thousands of years in the large ice sheets that cover Antarctica and Greenland. The oldest ice in Antarctica has been there for 2.7 million years. However, snow that falls in the winter may only stick around for a few days in mid-latitudes locations, where temperatures often rise above freezing causing the snow to melt, or up to six months closer to the Arctic, where temperatures stay below freezing all winter.

    Source : scied.ucar.edu

    The Atmosphere and the Water Cycle

    The atmosphere is the superhighway in the sky that moves water everywhere over the Earth. Water at the Earth's surface evaporates into water vapor which rises up into the sky to become part of a cloud which will float off with the winds, eventually releasing water back to Earth as precipitation. Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.

    Source : www.usgs.gov

    The Water Cycle

    Landscape sculptor. Climate driver. Life supporter. Water is the most important molecule on our planet.

    This page contains archived content and is no longer being updated. At the time of publication, it represented the best available science.

    A Multi-Phased Journey

    The water, or hydrologic, cycle describes the pilgrimage of water as water molecules make their way from the Earth’s surface to the atmosphere and back again, in some cases to below the surface. This gigantic system, powered by energy from the Sun, is a continuous exchange of moisture between the oceans, the atmosphere, and the land.

    Earth’s water continuously moves through the atmosphere, into and out of the oceans, over the land surface, and underground. (Image courtesy NOAA National Weather Service Jetstream.)

    Studies have revealed that evaporation—the process by which water changes from a liquid to a gas—from oceans, seas, and other bodies of water (lakes, rivers, streams) provides nearly 90% of the moisture in our atmosphere. Most of the remaining 10% found in the atmosphere is released by plants through transpiration. Plants take in water through their roots, then release it through small pores on the underside of their leaves. In addition, a very small portion of water vapor enters the atmosphere through sublimation, the process by which water changes directly from a solid (ice or snow) to a gas. The gradual shrinking of snow banks in cases when the temperature remains below freezing results from sublimation.

    Together, evaporation, transpiration, and sublimation, plus volcanic emissions, account for almost all the water vapor in the atmosphere that isn’t inserted through human activities. While evaporation from the oceans is the primary vehicle for driving the surface-to-atmosphere portion of the hydrologic cycle, transpiration is also significant. For example, a cornfield 1 acre in size can transpire as much as 4,000 gallons of water every day.

    After the water enters the lower atmosphere, rising air currents carry it upward, often high into the atmosphere, where the air is cooler. In the cool air, water vapor is more likely to condense from a gas to a liquid to form cloud droplets. Cloud droplets can grow and produce precipitation (including rain, snow, sleet, freezing rain, and hail), which is the primary mechanism for transporting water from the atmosphere back to the Earth’s surface.

    When precipitation falls over the land surface, it follows various routes in its subsequent paths. Some of it evaporates, returning to the atmosphere; some seeps into the ground as soil moisture or groundwater; and some runs off into rivers and streams. Almost all of the water eventually flows into the oceans or other bodies of water, where the cycle continues. At different stages of the cycle, some of the water is intercepted by humans or other life forms for drinking, washing, irrigating, and a large variety of other uses.

    Groundwater is found in two broadly defined layers of the soil, the “zone of aeration,” where gaps in the soil are filled with both air and water, and, further down, the “zone of saturation,” where the gaps are completely filled with water. The boundary between these two zones is known as the water table, which rises or falls as the amount of groundwater changes.

    The amount of water in the atmosphere at any moment in time is only 12,900 cubic kilometers, a minute fraction of Earth’s total water supply: if it were to completely rain out, atmospheric moisture would cover the Earth’s surface to a depth of only 2.5 centimeters. However, far more water—in fact, some 495,000 cubic kilometers of it—are cycled through the atmosphere every year. It is as if the entire amount of water in the air were removed and replenished nearly 40 times a year.

    This map shows the distribution of water vapor throughout the depth of the atmosphere during August 2010. Even the wettest regions would form a layer of water only 60 millimeters deep if it were condensed at the surface. (NASA image by Robert Simmon, using AIRS & AMSU data.)

    Water continually evaporates, condenses, and precipitates, and on a global basis, evaporation approximately equals precipitation. Because of this equality, the total amount of water vapor in the atmosphere remains approximately the same over time. However, over the continents, precipitation routinely exceeds evaporation, and conversely, over the oceans, evaporation exceeds precipitation.

    In the case of the oceans, the continual excess of evaporation versus precipitation would eventually leave the oceans empty if they were not being replenished by additional means. Not only are they being replenished, largely through runoff from the land areas, but over the past 100 years, they have been over-replenished: sea level around the globe has risen approximately 17 centimeters over the course of the twentieth century.

    Sea level has been rising over the past century, partly due to thermal expansion of the ocean as it warms, and partly due to the melting of glaciers and ice caps. (Graph ©2010 Australian Commonwealth Scientific and Research Organization.)

    Source : earthobservatory.nasa.gov

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