The Hydrologic Cycle - Introduction
Water on Earth is always moving and changing. Its moving and repeating changes make a cycle. As water goes through its cycle, it can be a solid (ice), a liquid (water), or a gas (water vapor).
The hydrologic cycle (also known as the water cycle) is all about storing water and moving water around Earth. The biggest storehouse of water is the ocean, but water is also in the sky. The sky, or the atmosphere, is the "superhighway" that moves water around the globe. About 97% of water on Earth is in the form of salt water. About 3% of water is fresh water, and only about 1% of that water is liquid. They water cycle helps move that fresh, liquid water around. It's really amazing that land animals (including humans) are even around with that little bit of water available to drink. |
There are three states of matter: solid, liquid, and gas.
You can see here that water exists as a solid (ice,) a liquid, or gas (water vapor). We say that water can exist in three different "states."
How does water change?
Evaporation
Transpiration
Water can also evaporate from plants. Evaporation from plants is called transpiration. The heat from the sun also causes this transpiration. When plants lose water to transpiration, it creates a vacuum inside the plant. This vacuum is what draws water up from the ground. This is how plants absorb water in the soil.
|
In the image above on the left, there is a structure called "stoma." Can you find it? This is a microscopic opening in the plant's outer layer, much like a pore in your own skin. This is where water vapor is released from in transpiration.
Condensation
As water vapor rises in the sky, the temperature decreases. If heat is taken away from water vapor, it condenses. "Condense" means to come together and become more dense. When water vapor condenses in the sky it turns into clouds. This process is called condensation. When you drink a cold beverage, you may see water droplets on the outside of your glass. This is also called condensation. Condensation is the process by which water vapor condenses into clouds or liquid. If enough heat is taken away, the liquid water condenses further and becomes ice. You may already know that when water turns to ice, this is called "freezing." But really, freezing happens when any liquid turns to solid. |
Precipitation
When the liquid in a cloud gets heavy enough, gravity will pull it down to the ground in the form of rain, hail, sleet, or snow. We call this precipitation. You may see the word "precipitation" instead of rain on the news forecast, and it is usually measured in inches.
Snow is a special kind of precipitation sometimes called crystallization. Crystallization is the process by which a substance becomes a solid, sort of like freezing. But in crystallization, crystals form. |
It's Okay to be Smart - The Science of Snowflakes
Runoff
Snow melt, rainfall, and other forms of water that move on land (this can include agricultural irrigation systems) eventually run into larger bodies of water, such as the ocean. This is called runoff.
Runoff pouring into the ocean can contain nutrients, as the water picks up nutrients and minerals as it flows over land. Since water is the universal solvent, it can dissolve these minerals and nutrients from rocks and soil and carry them out to sea. Because of this characteristic, water runoff can also contain hazardous chemicals. When humans pollute the land with chemical waste from factories, treatment facilities, and agricultural pesticides, water picks these hazardous materials up as it moves. This causes serious detrimental effects for marine life. You will learn more about this during our unit on Human Impact. |
Watersheds
The entire area of land, including the mountain, hillsides, and valleys that drains fresh water into a river or lake is called a watershed.
Watersheds are very important. They collect rainfall and melting snow. Watersheds drain into the larger bodies of water, such as rivers, streams, lakes, oceans, and aquifers. A healthy watershed allows us to have clean water to drink and pollution-free lakes, rivers, oceans, and aquifers. For more on the importance of watersheds and the role of human impact on them, click here. |
Los Angeles County is part of a watershed. The San Gabriel River flows from the Angeles National Forest, through the San Gabriel Mountains, through Los Angeles and Orange County, to the Pacific Ocean at Seal Beach. The San Gabriel River drains a watershed basin area of 713 square miles (1,850 km2).
The image below shows the major dams built along the San Gabriel River, which control the flow of water, control flooding, and create water reservoirs. |
Groundwater
The last part of the hydrologic cycle we will discuss is groundwater. As mentioned above, precipitation is absorbed into the ground where it can be stored for very long periods of time. The groundwater storage is known as an "aquifer." Eventually the water may find its way back out to the ocean.
In the picture on the right, you can see the relative storage times for the groundwater. For example, water may be in an aquifer for just days or years before a tree may soak it up. But water can store under ground for thousands of years if it is deep enough. Much of our drinking water comes from this groundwater. Agriculture in California also uses groundwater. California has been in an ongoing drought for the last few years. Our underground water is drying up, creating a crisis. Governor Jerry Brown, Jr. declared California in a State of Emergency in January 2014 because of the drought. |
Resources |
VocabularyHydrologic Cycle
Evaporation Transpiration Condensation Precipitation Crystallization Runoff Groundwater Watershed |