Quantify how Earth's water is distributed among oceans, ice, and groundwater, and distinguish physical from economic water scarcity.
About 97% of all water on Earth is saline ocean water, leaving only about 3% as freshwater. Of that small freshwater share, roughly 70% is locked up in ice caps and glaciers, about 29% is groundwater, and less than 1% is readily accessible surface freshwater -- the lakes and rivers people and ecosystems depend on most directly. The usable fraction of all Earth's water is therefore tiny once both percentages are combined.
Groundwater is stored in aquifers, underground layers of rock or sediment that hold water. Some aquifers recharge (refill) quickly from rainfall; others, like the Ogallala Aquifer beneath the central United States, formed over thousands of years and recharge extremely slowly, yet are being withdrawn for irrigation far faster than they refill -- a stock-and-flow problem straight out of Week 2. This points to an important distinction: physical scarcity means a region genuinely lacks enough water, while economic scarcity means water exists nearby but a community lacks the infrastructure or investment to access it safely.
Mastery looks like: They can correctly compute a nested percentage of Earth's total water and can distinguish physical from economic water scarcity using a specific example.
Common stumbles: Treating all freshwater as equally accessible; assuming groundwater is an unlimited, instantly renewable resource rather than a stock that can be depleted.