Environmental Science / Unit 6: Water Resources and Pollution
6.1 Water Distribution and the Water Cycle
Notes
Water covers about 71% of Earth, but most of it is not usable by people.
- About 97% of Earth's water is salt water. Only about 3% is fresh, and most of that is frozen in glaciers and ice caps (roughly two thirds of freshwater). Most of the rest is groundwater. Less than 1% of freshwater is in lakes, rivers, and the atmosphere.
- The water cycle is driven by solar energy: evaporation, transpiration, condensation, precipitation, runoff, and infiltration.
- Groundwater is stored in an aquifer (porous rock or sand holding water). The top of the saturated zone is the water table. Recharge is the refilling of an aquifer by infiltration.
- A watershed is all the land that drains into a particular stream, river, or lake.
- Example: if 3% of 1,000 L of Earth's water were fresh, only about 30 L would be freshwater, and about 20 L of that would be ice.
Practice
About what percent of Earth's water is salt water?
Oceans hold nearly all water; only about 3% is fresh.
Where is most of Earth's freshwater stored?
Roughly two thirds of freshwater is frozen.
What is a watershed?
Everything that falls in a watershed flows to the same outlet, which is why land use affects water quality.
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