Water Cycle

The Amazing Water Cycle

Did you know that the water in your glass could have been in a puddle that a dinosaur like T. rex stepped in 65 million years ago? How is that? It’s because of the water cycle, the process that describes water’s constant movement from the Earth to the sky. Another name for the process is the hydrologic cycle. “Hydro” means water.

The amount of water on Earth remains about the same, and it’s been here since the planet formed billions of years ago. Water constantly changes form. Sometimes it’s the liquid we drink and swim in. Sometimes it’s frozen solid like snow and ice. Other times it’s a gas called water vapor. You’ve seen water vapor in the steam from a teakettle. Water’s ever-changing states allow it to move from the ground to the clouds and back to Earth as rain. The Sun drives the process, recycling the same water over and over. That’s how T. rex’s puddle could end up in your glass.

How'd It All Get Here?

The elements that combine to make water—hydrogen and oxygen—were found in the dust, gases, and other “stuff” that formed Earth. The molten rock, called magma, that made up early Earth contained water. Some of the water was able to escape from the magma and reach the atmosphere. As the Earth gradually cooled off, the water was able to settle on the surface and billions and billions of liters later the oceans were formed.

As the Cycle Turns

There are four stages in the water cycle. The cycle is never-ending, so there’s really no beginning or end, and any stage could be stage 1.

Stage 1: Evaporation

Most of Earth’s water is in the ocean. When the Sun warms the water, some of it turns into the gas water vapor. This change is called evaporation, and it’s how most water vapor gets into the air. Another way is from plants. Plants draw water from the ground through their roots. Some of the water is lost through the leaves in a process called transpiration. That water evaporates too. A third way for water vapor to get into the air takes place in very cold climates. Snow and ice can turn directly into water vapor in a process called sublimation. The Sun’s heat fuels this process too. Where does all that water vapor go? It rises high into the atmosphere on air currents.

Stage 2: Condensation

We are surrounded by water vapor and even breathe it in. The amount of water vapor in the air is called humidity. We can’t see it because it’s an invisible gas, but you can feel humidity on warm, sticky days. As water vapor rises higher into the air, it cools off. When it gets cool enough, the vapor changes back into liquid water in a process called condensation. When you see a cloud, you are looking at a puffy collection of condensed water. If it’s cold enough close to the ground, fog forms. Fog is a ground-level cloud.

Stage 3: Precipitation

Moved around by the air, clouds gather more water. Very tiny water drops join other drops, eventually growing heavy enough to fall from the cloud. This is rain. Rain is a form of precipitation. Other forms are snow, sleet, and hail. The rain brings the water back to Earth where it may end up on the ground or in an ocean or other body of water.

Stage 4: Collection

In this stage the water is collected or stored back on Earth. A lot of it ends up back in the ocean. Some falls on land in the form of rain or snow. If you’ve ever watched water run down a storm drain, you’ve seen runoff. That’s precipitation that flows over land. Rain on land may run into streams and rivers and find its way back to the ocean. Some will soak deep into the ground and gather in pockets of groundwater called aquifers. People who get their water from wells are tapping into an aquifer. A fancy word for large collections of water such as oceans or aquifers is reservoir. Earth’s largest reservoirs are the oceans, which hold 97 percent of all water.

Colder Cousins

Another 2 percent of Earth’s water is ice. Precipitation that falls in a very cold climate may remain frozen. Snowcaps on the tops of mountain peaks are one example. Glaciers are huge sheets of ice that can be many thousands of kilometers in area and hundreds of meters thick. Most of Earth’s glaciers are found at the North and South Poles. These vast areas of ice hold as much as 70 percent of the freshwater on the planet. It may seem as if the water in a glacier would be suspended from the water cycle, but that’s not true. As you already know, some of the ice turns into water vapor through sublimation. And even though snowcaps and glaciers can last thousands of years, they also melt. The freshwater from the melting is an important source of water for many people and animals living nearby.

The Cycle of Life

We depend on the water cycle to bring freshwater to the planet in rain and other precipitation. Without it, all living things would die. While most of Earth’s water is in the salty oceans, land-living plants and animals rely on the 1 percent of freshwater that isn’t frozen and is found in precipitation, streams, lakes, and aquifers.

Climate Control

The water cycle also drives Earth’s climate. The oceans act as planetary air conditioners that cool Earth. Without them, the land would soak up all the Sun’s heat and be too hot. So why don’t the oceans boil? It’s because of evaporation. The Sun warms the oceans just as it does the land, but that heated-up surface water gets carried away in water vapor. A basic law of science is that warm air rises. The air, warmed by the Sun and the heated-up water vapor, moves higher into the atmosphere. The moving air drives weather patterns worldwide, shifting the clouds and bringing wind, rain, drought, and sunny days alike.

Carving Out a Landscape

The movement of water on Earth’s surface shapes it. Because of gravity, water flows downhill. Rain falling on mountains or hills will race down the slopes, collecting as it goes to form streams. When snow melts on a mountain peak, the same thing happens. All that runoff collects into larger streams and rivers that keep flowing toward the planet’s “low spots”—the oceans. The flowing water cuts into the soil and rock to create a streambed. A dramatic example of how water shapes the land is the Grand Canyon, which was cut out of rock by the Colorado River over millions of years.

Too Little or Too Much of a Good Thing

The amount of water on Earth remains the same and is always in balance. After a heavy rainfall, for example, lots of water will evaporate, keeping the cycle steady. But people can affect the water cycle, creating water shortages or flooding.

The more water people demand for their homes, businesses, and farms, the more quickly freshwater gets used up. Since the water cycle replaces the water at its own steady pace, it’s possible for people to use up available freshwater before it can be replaced. In addition, people pollute water when they pour out wastes, pesticides, or fertilizers. These pollutants may go straight into bodies of water, sink into groundwater, or get washed into streams by rain. The ever-growing human population means there will be more pressure on freshwater resources.

People also change the landscape, and that affects water levels, too. The growth of cities and of paved and built-on land means that water runs off much more quickly than it does over bare ground. People have cleared forests, including vast tropical rainforests, and filled in wetlands —swamps, bogs, and marshes—that hold large amounts of water. Fewer wetlands and natural areas and more paved ground mean a greater chance of flooding during heavy rains. Aquifers may also not be filled up as quickly because the rainwater runs off without the plants and bare ground to “catch” it so it can soak in.

Water Cyclist

Scientists who study the water cycle include engineers, geologists, climatologists, and chemists. Making sure there is freshwater available to everyone who needs it is the most obvious reason to study the water cycle. Since it is the engine behind our weather, understanding the cycle helps prepare us for floods, droughts, or extreme storms—not to mention how to dress each day!

On a larger scale, climatologists want to learn how the water cycle affects Earth’s climate. One way is to monitor the amount of surface ice. The less ice, the more bare ground is exposed to the Sun. Bare ground soaks up sunrays, helping to warm the planet, while ice reflects the rays away from Earth. Melting polar ice may also affect sea levels and change the salt content of seawater. We don’t really know yet how these things might change our global climate, but global warming and its affects on the water cycle are a big concern to many scientists.

As part of an international mission to study Earth’s water cycle, NASA launched the satellite Aqua in 2002. Aqua monitors evaporation, clouds, precipitation, soil moisture, vegetation, and snow and ice levels. Aqua’s data are helping scientists learn how the water cycle affects global climate.