Week 7: Freshwater Access

What is the hydrologic cycle?

The continuous movement of water through Earth’s systems — atmosphere, land, and oceans.

(think water cycle)

What drives the hydrologic cycle?

The Sun — it heats water, causes evaporation, forms clouds, and leads to precipitation.

What is a water reservoir?

A place where water is stored in the cycle (oceans, lakes, rivers, ice, underground).

What is a flux?

The movement of water among reservoirs (e.g., rainfall, river flow, evaporation).

Why is water considered a renewable resource?

Its total volume doesn’t change; it constantly recycles through the water cycle.

What happens in the oceanic subcycle?

Water evaporates from the ocean → forms clouds → rains back into the ocean.
Think: Ocean water takes a “round-trip vacation.”

What happens in the ocean-land exchange?

Ocean water evaporates → forms clouds → rains on land → rivers carry water back to the ocean.
Think: Water “visits” land, then “goes home.”

What happens in the terrestrial subcycle?

Water circulates between lakes, rivers, soil, plants, and the atmosphere.
Think: Land water has its own “travel network.”

What does water residence time mean? 

The average time water stays in one reservoir before moving to another.

About how much of the world is freshwater? How much of the world’s water is frozen and how much of the world’s water is accessible to water?

About 3% of the world’s water=freshwater: (Most water on Earth is saltwater in oceans (~97%), so only a tiny fraction is freshwater)

• About 2% of the world’s water= frozen (Stored in glaciers and ice caps. Hardly accessible because it’s frozen in remote areas like Antarctica and Greenland)

• About 1% of the world’s water= accessible to humans (Found in rivers, lakes, and shallow groundwater This is the water we can actually use for drinking, farming, and everyday needs

Mostly groundwater) 

What does “average river flow” mean?

The average yearly amount of water flowing in rivers — measures usable surface freshwater

What does “groundwater recharge” mean?

The rate at which groundwater refills through rain or snow soaking into the soil.

How much of Earth’s surface is arid (dry)?How much of the world’s population lives in arid regions? 

About one-third (⅓) of Earth’s land surface is arid (dry) . About one-fifth (⅕) is where the world’s population of the global population live in arid regions.

What are arid regions? And examples of arid regions?

Areas with very little rainfall (e.g., deserts and drylands). Sahara Desert, Middle East, parts of Australia, and the southwestern U.S.

Why do arid regions face water supply problems?

Low rainfall, limited freshwater, and high competition for limited water resources.

What is the single largest use of domestic water?

Flushing toilets (~25% of household use).

What is “hidden water consumption”?

Indirect water used to produce food and consumer goods (like growing crops or raising livestock).
🧠 Example: Eating a burger uses hundreds of gallons of water (to grow feed and raise cattle).

Human Water Use

• Minimum need: 50 L/day per person

• Flushing toilets = 25% of domestic use

• U.S. = highest water use per person

Environmental Impacts

• Groundwater depletion

• River & habitat loss

• Pollution & reduced biodiversity

• Water conflicts