Hydrosphere Facts: Understanding the Water Cycle
The hydrosphere is one of the four spheres of the Earth system; the other three are the geosphere or lithosphere (solid rocky part of Earth), atmosphere (gases surrounding Earth), and biosphere (living things).
The hydrosphere consists of all of the water on, in and surrounding Earth. The water in the hydrosphere is distributed among the ocean, glaciers and ice caps, groundwater, surface water and water in the atmosphere in the form of water vapor and clouds. The water is present in all three phases: solid, liquid and gas. Life on Earth depends on the water of the hydrosphere.
Earth is known as the water planet for good reason. More than 71% of Earth’s surface is covered with water. The total amount of water on Earth is approximately 333 million cubic miles (1,386 million cubic kilometers). Only a small fraction of the total amount of water on Earth is available for drinking, washing and irrigating crops because most of it is either salty or frozen.
The global ocean, along with seas and bays, accounts for 97% of all the water on Earth, about 321 million cubic miles. Ocean water is salty and is not suitable for drinking or other everyday uses. Only 3% of the water on Earth is fresh (not salty). Over two-thirds of the freshwater is in the form of ice, in glaciers, polar ice caps or permafrost. Another 30% of the freshwater is groundwater. This leaves less than 1% of the freshwater, or 0.007% of the total water on Earth, on the surface in lakes and rivers that are easily accessible.
The Hydrologic Cycle
The hydrologic cycle, also known as the water cycle, describes how all the water in the hydrosphere continually moves between oceans, lakes, rivers, land and atmosphere. During the course of the water cycle, water changes state from liquid to gas and back to liquid. The energy that drives the hydrologic cycle comes from the sun. The steps in the water cycle are evaporation, condensation, precipitation and runoff. Transpiration is an additional element in the water cycle.
- Evaporation is the process by which water on the surface changes from a liquid to a gas state, water vapor. Energy is required for evaporation to take place.
- Condensation occurs when water vapor in the atmosphere cools and collects into water droplets, forming clouds. The water loses energy when it condenses.
- Precipitation consists of rain, hail, sleet or snow falling from clouds back to Earth’s surface.
- Runoff is water from precipitation that flows over the surface into rivers or streams, eventually returning to the ocean.
- Transpiration happens when water vapor returns to the air from the leaves of plants.
Water that does not run off soaks into the ground and fills the spaces between the soil and rock, necoming groundwater. This water may sometimes be accessed by wells.
Image credit: Terrie Schultz