Explain why accessible freshwater is a small fraction of Earth's total water, and describe causes and responses to freshwater scarcity, including in California.
Earth is often called the "water planet," but very little of that water is actually usable freshwater. About 97 percent of Earth's water is salty ocean water, leaving only about 3 percent as freshwater, and most of that 3 percent, nearly 70 percent of it, is locked up as ice in glaciers and ice caps. That leaves a surprisingly small slice of accessible freshwater in rivers, lakes, and underground aquifers to supply drinking water, farming, and every freshwater ecosystem on the planet.
California, despite its size and population, receives most of its precipitation unevenly, heavier in the north and in mountain snowpack, lighter in the south and the Central Valley, and stores much of its water supply in reservoirs and underground aquifers that can be pumped down faster than they naturally refill during drought years. Freshwater scarcity is managed through several strategies: conservation, using less through efficient fixtures and farming methods, recycling and reusing water, desalination, removing salt from ocean water at high energy cost, and careful allocation between agricultural, urban, and ecosystem needs.
Mastery looks like: They can state the approximate freshwater percentage and its distribution, and explain at least two real strategies for managing scarcity.
Common stumbles: Assuming "97 percent water" means water is not actually scarce; forgetting that most freshwater is frozen and not immediately accessible.