Seventy-one percent of the Earth’s surface is covered in water that is constantly in motion. The total amount of water in the world never really changes, it just takes on new forms and moves to new places, from sky to sea to ground. This process is called the water cycle, and it is the reason we have clouds, rainfall, rivers, groundwater, and an ocean teeming with life.
One of the things that makes water such a unique molecule is that all of its stages (ice, liquid, and gas) can exist naturally on Earth. As one water molecule travels through the water cycle, it may pass through all three of these stages. During the process of evaporation, the water that gets collected in oceans, lakes, and rivers heats up at the surface and turns into a gas, sending gaseous water particles into the atmosphere. Humidity increases as more water vapor accumulates in the air.
As water vapor rises into the atmosphere, it begins to cool down, condensing back into a liquid phase. Water molecules are polar, so they are attracted to each other. When water droplets clump together in the sky, they form clouds.
When enough water vapor condenses in the atmosphere to make water droplets heavy enough to fall back down to Earth, precipitation occurs. Precipitation can come in many forms: rain, snow, hail. It can be caused by both rises and falls in temperature. When the temperature decreases, more water vapor condenses into a liquid. If it gets cold enough, the water freezes as it falls from the sky, creating snow or hail. Conversely, when the temperature increases, evaporation also increases. This brings more water into the atmosphere, causing rain. That is why warm, tropical places get so much rainfall. Precipitation of all kinds helps replenish freshwater reserves on Earth, like lakes, rivers, and groundwater.
As precipitation occurs, some of the water gets soaked up by the land, while some flows over the land’s surface and feeds into rivers and streams. This is called runoff, or surface runoff. The physical geology and topography of the land plays a big role in determining where the water will run. For instance, water is more likely to run quickly over something impervious, like a road, versus porous soil. It also likes to flow downhill and into previously-made canyons and gullies. Many years of runoff in the same place will cause erosion, carving out the land into channels where the water likes to run until the landscape is forever changed. That is how the Colorado River created the Grand Canyon.
The water that falls to Earth and does not flow over the land gets absorbed by the ground. An aquifer is an underground layer of permeable rock or sediment where groundwater gets stored. The water table is the level underground where this water sits. Eventually, groundwater can reenter the water cycle by seeping into other bodies of water or by getting pumped out of the Earth by humans.
It is possible for the same molecule of water to remain in the water cycle for over 100 million years before it gets broken apart by photosynthesis or other natural processes. That means that the same droplet of water that passed through a dinosaur’s body could be passing through your own body right now. Because of the interconnectedness of the water cycle, every dam we build, every well we dig, every piece of trash we drop on the ground has the power to affect water sources hundreds of miles away. And eventually, almost all water ends up in the ocean. That is why it is so important to be aware of how our actions can affect the environment around us, because everything we do has the potential to impact the resources that we all need to survive.
Check Out this Cool Interactive Watercycle: http://water.usgs.gov/edu/watercycle-kids-adv.html