Seen from space, Earth is a cool blue oasis, bathed in water, decorated by clouds. For at least the last billion years, a staggering 326 million cubic miles of water has filled her oceans and rained gently from her skies. Every drop is ancient, nearly as old as the earth itself. The water running through your faucet today also fell as rain on ancient civilizations, was swallowed by a thirsty dinosaur, and rolled across an empty beach long before the dawn of life.
The complex, constant movement of water on Earth—from the oceans to the air, across the landscape, and through plants and animals—is called the water cycle. It is powered by solar energy, and aided by gravity. The water cycle includes all the processes that shift water—both those that physically move water, and those that convert water between solid, liquid, and gaseous states.
The cycle also contains many reservoirs, where water accumulates. Within these reservoirs, water is spread unevenly across the Earth’s surface. The great majority—more than 97%—lies in the oceans. Just over 2% is frozen in glaciers and ice caps. The rest—a meager half percent or so—is divided between the atmosphere, lakes and streams, and the ground.
Because most water is in the oceans, most water is salty, and stays salty for a long time. Within the vastness of the sea, water molecules may linger for millennia. Tides and waves move ocean water over short distances, while currents circulate the seas around the globe. Carrying warm water away from the equator, and cold water away from the poles, oceanic circulation moderates the planet’s temperature extremes.
Wherever the sun shines on the ocean, evaporation creates fresh water vapor out of salty seawater. Winds lift the moist air high into the atmosphere, and blow it about the globe. As air moves away from the warm ocean it starts to cool off. With sufficient cooling, water vapor changes into drops of liquid water— a process called condensation.
With continued condensation, tiny drops of water grow bigger and bigger and gather into clouds. The average airborne water molecule stays aloft for only 10 days or so, until it joins a water droplet heavy enough to fall from the sky. Most precipitation, be it rain, snow, sleet, or hail, drops right back into the ocean. There, the water circulates until evaporation claims it once again.
Some snow and rain also lands on the continents. In cold regions, at high latitudes or altitudes, where snow builds up in the winter and doesn’t melt in the summer, ice caps and snowfields grow and persist over thousands of years. The clean ice in glaciers and ice caps, most frozen well before the age of man, is both the purest and the largest storehouse of fresh water on Earth.
When ice and snow melt, or rain falls on land, water is pulled swiftly downhill by gravity. Some of it flows across the top of the ground, a process called runoff. Surface water gathers into rivers pauses for a time in lakes, and rushes down to the sea. As it flows across the ground, running water cuts into the earth, wearing down and reshaping the ground. Moving water is the most powerful geologic force sculpting the landscape.
Although the handiwork of runoff is visible everywhere on land, more precipitation actually infiltrates, or soaks into, the earth than runs off. More than 95% of the planet’s liquid fresh water is groundwater—water held within the ground. Some shallow groundwater doesn’t last long. Evaporation directly from the soil, and transpiration through plants, both transfer moisture back to the air. The process of transpiration is part of plant metabolism. Liquid water is absorbed by roots, and lifted to the leaves, where it is converted to vapor and lost to the sky.
Most groundwater is neither evaporated nor transpired. It slowly drains downward, slipping through tiny pores between soil grains, following cracks and caves in the bedrock, to seep into streams, lakes, and eventually, the sea. Along the way, the water interacts with the ground. Some pollutants, such as bacteria, are filtered out of the water, while some minerals, like sodium and arsenic, are picked up by the water. Earth’s oceans are salty because groundwater has been carrying dissolved mineral salts down to the sea for billions of years.
As the water cycle spins, the earth’s water moves from oceans to the atmosphere to the land, and back to the sea, over and over, and over again. However, the water cycle is not a closed system. Tectonic activity inside the planet pulls water out of the system when seawater is dragged down inside the planet at subduction zones, and also adds water into the system when steam erupts from volcanoes. Some moisture also leaves the outer edge of the water cycle, when vapor high in the atmosphere "leaks" into space. And, water can also enter the system from above, when icy comets collide with earth.
The water cycle is the most fundamental system operating on the surface of the earth. The varied processes of the water cycle control the global climate, shape the landscape, and allow life to exist.
The movement and distribution of water across the planet determines broad climate patterns. Evaporation powers vast weather systems like hurricanes and cyclones, while uneven precipitation nourishes rainforests or parches deserts. Other weather events, such as floods, droughts, and blizzards, are all aspects of the water cycle.
Runoff is unequalled in its ability to erode rock and soil, and reshape the face of the land. Mountains are worn low and canyons are carved deep, grain-by-grain from the relentless force of moving water.
For mankind, perhaps the most immediate impact of the water cycle is its influence on the quantity and quality of fresh water. Because of the unequal distribution of water around the cycle, fresh water is relatively scarce—only 0.65% of Earth's total water supply is neither salty nor frozen. And the quality of that vital water is at risk, because as it moves through the water cycle, it is exposed to a host of natural and manmade pollutants. As clouds drift above cities, and rivers run past factories and fields, their waters pick up industrial and agricultural wastes. As groundwater infiltrates through the soil, it gathers up both mineral compounds and buried toxic chemicals.
The global demand for clean, fresh water is increasing rapidly. At the same time, the supply is threatened by pollution and climate change. Scientists hope that by studying the complex workings of the water cycle, we will learn how to predict and preserve sufficient water resources to sustain the ever-growing world population.