Water is the most important component to our existence on Earth. Without water, life could not be sustained. All of the water on or near Earth makes up the hydrosphere. Let’s dive deeper into this high emphasis topic for the TASC Test Assessing Secondary Completion™ Science subtest.
Water in the hydrosphere includes:
- Moisture in the air
- Water below Earth’s surface
97.5% of Earth’s water is salt water and contained in the oceans. Only 2.5% of Earth’s water is fresh, but humans cannot drink all of it. According to NASA:
- 98.7% of fresh water is unusable because it’s frozen or underground.
- 1.3% of fresh water is surface water.
- However, less than 1% of all water on Earth is usable for humans.
The Water Cycle
Earth’s water is always moving and constantly changing states. The water cycle explains how water moves on, in, and above Earth.
The water cycle has no starting point since it is in constant rotation. Let’s start in the ocean, since that’s where most of Earth’s water is contained, and see where the water cycle takes us:
- The water cycle is driven by the sun.
- Some ocean water evaporates into the atmosphere as vapor from the sun’s heat.
- The vapor continues to rise from air currents.
- The higher the vapor travels, it condenses into clouds from a drop in temperature.
- Air currents move clouds around the globe.
- Cloud particles grow, collide, and fall out of the sky as precipitation.
- Precipitation falls to Earth due to gravity, landing in bodies of water or on land.
- Precipitation that falls on land flows as surface runoff, eventually making its way to a body of water (such as a river). This moves water back to the oceans.
- Runoff can also soak into the ground as infiltration, hydrating plants and trees.
- Excess water from plants and trees transpires back into the atmosphere.
- Since water is in constant circulation, the water cycle never ends.
Study this visual representation of the water cycle to understand how water flows through Earth’s hydrosphere for the TASC test.
Photo credit: NASA