Most water is salty
Oceans hold most of Earth’s water. Freshwater is a much smaller share, and much of it is stored as ice or groundwater. Lakes and rivers are important to people and ecosystems, but hold only a small part of the total.
Water on Earth
Why can a water-rich planet still face water shortages?
Class notes
Open PDF →Select a page to enlarge it. Open the PDF for the complete note set.
Availability is not the same as total supply
Water may be far from the people or ecosystems that need it, frozen, difficult to access, polluted or unavailable at the right time. A region’s supply also changes with weather, season, infrastructure and competing needs.
Fresh does not mean drinkable
Freshwater has relatively low salinity compared with seawater. Potable water is suitable for drinking. Clear, fresh water may still contain harmful organisms or chemicals, so appearance alone cannot prove safety.
Words worth knowing
- Salinity
- The amount of dissolved salts in water.
- Potable
- Suitable for drinking, based on the relevant water-quality requirements.
- Reservoir
- A place where water is stored; it can be natural or constructed.
- Groundwater
- Water in the saturated pores and fractures below the ground.
Zoom into Earth’s water
Change the whole you are measuring. Compare percentages of all water with percentages of freshwater.
Rounded USGS estimates: 97.5% saline, 2.5% fresh; freshwater is about 68.7% ice, 30.1% groundwater and 1.2% surface/other. These describe storage, not drinking-water safety.
A water-rich planet with water shortages
TED-Ed · Animation
Supply is more than a global total
Open on YouTube →Separate the total amount of freshwater from water that can reach people when and where it is needed. The film’s projections reflect its 2018 publication.
Make the denominator clear
A fictional planet has 100 L of water, including 4 L of freshwater and 1 L of accessible freshwater. The accessible amount is 1% of total water but 25% of its freshwater. These are teaching numbers, not estimates for Earth.
Key connections
- Total water
- Freshwater
- Location & quality
- Availability
Try it yourself
A graph says 70% of freshwater is frozen. Is that 70% of all water?
Give me a hint
Identify what the percentage is out of.
Compare with a worked response
No. The whole is freshwater, a subset of all water. Multiply by the freshwater share only if the total-water percentage is needed.
Make the connection
Why could two regions with the same annual rainfall have different water availability?
Compare your reasoning
Compare seasonal timing, population, storage, water quality, infrastructure and ecosystem needs. A yearly total hides when and where the water arrives.
Put the evidence to work
Read further: USGS · Where is Earth’s water? →