Every Drop Counts
Water Distribution and the Water Cycle
Learning Objectives
Describe how water is distributed on Earth
Explain the processes of the water cycle and trace water's path
Analyze why freshwater is a limited resource
Every Drop Counts
~5 minutesEvery Drop Counts
Earth is called the "Blue Planet" because water covers about 71% of our surface. From space, our world looks like a giant water world. But here's a troubling paradox: despite all that water, freshwater shortages threaten billions of people.
How can a planet covered in water have water shortages?
The answer lies in understanding WHERE Earth's water is, what STATE it's in, and how it MOVES through our planet's systems.
Essential Question: Why is freshwater considered a limited resource when so much of Earth is covered by water?
Where is Earth's Water?
~15 minutesWhere is Earth's Water?
Earth's Water Budget
Earth contains about 1.386 billion cubic kilometers of water. That sounds like a lot—but look at how it's distributed:
| Reservoir | Percentage | Notes |
|---|---|---|
| Oceans (saltwater) | 96.5% | Too salty to drink or use for agriculture |
| Ice caps, glaciers, permanent snow | 1.74% | Frozen; not readily accessible |
| Fresh groundwater | 1.05% | Underground; accessible with wells |
| Saline groundwater | 0.94% | Too salty for most uses |
| Freshwater lakes | 0.007% | Accessible surface water |
| Soil moisture | 0.001% | In soil; supports plant growth |
| Atmosphere | 0.001% | Water vapor, clouds |
| Rivers | 0.0002% | Tiny fraction but crucial for humans |
| Swamps/marshes | 0.0008% | Wetland ecosystems |
| Living organisms | 0.0001% | Water in plants and animals |
The Critical Math
Total freshwater: Only about 2.5% of Earth's water is fresh (not salty)
But of that freshwater:
• ~69% is locked in ice caps and glaciers
• ~30% is groundwater (much of it deep and hard to access)
• ~1% is surface freshwater (lakes, rivers, swamps)
Bottom line: Only about 0.5-1% of Earth's total water is readily accessible freshwater for human use!
- Ocean water: 96.5 liters (a full bathtub)
• Ice and glaciers: 1.7 liters (a small water bottle)
• Groundwater: 1.7 liters (another small bottle)
• Lakes, rivers, atmosphere, everything else: About 0.1 liters (2 tablespoons!)
All the rivers in the world—the Amazon, Mississippi, Nile, Ganges—together hold less than 2 tablespoons out of every 100 liters of Earth's water!
The Water Cycle
~15 minutesThe Water Cycle
Powered by the Sun
The water cycle (also called the hydrologic cycle) is the continuous movement of water through Earth's systems. The Sun's energy powers this cycle by: 1. Heating water to cause evaporation 2. Creating temperature differences that drive winds 3. Warming air so it can hold more water vapor
Major Processes
Evaporation
• Water changes from liquid to gas (water vapor)
• Requires energy (540 calories per gram of water)
• Occurs mainly from ocean surfaces (about 86% of all evaporation)
• Rate increases with temperature, wind, and surface area
Transpiration
• Water released by plants through their leaves
• Plants "breathe out" water as part of photosynthesis
• A large tree can release 400+ liters of water per day!
• "Evapotranspiration" = evaporation + transpiration combined
Condensation
• Water vapor changes back to liquid droplets
• Releases energy (540 calories per gram—same amount absorbed during evaporation)
• Forms clouds when water vapor condenses around tiny particles (dust, pollen)
• This released energy helps power weather systems
Precipitation
• Water falls from atmosphere as rain, snow, sleet, or hail
• Occurs when water droplets/ice crystals become too heavy to stay suspended
• Globally, precipitation equals evaporation (balanced system)
Infiltration
• Water soaks into the ground
• Becomes soil moisture or groundwater
• Rate depends on soil type, slope, vegetation, and saturation
Runoff
• Water flows over land surface to streams, rivers, and eventually oceans
• Carries sediments and dissolved materials
• Increases when ground is saturated or impermeable
The water cycle moves HUGE amounts of energy around the planet!
When water evaporates, it absorbs heat from the environment (cooling effect). When water condenses, it releases that heat (warming effect).
This latent heat transfer is what powers hurricanes, thunderstorms, and much of Earth's weather. A single hurricane can release energy equivalent to 10,000 nuclear bombs per day—all from water condensing!
Residence Time: How Long Water Stays
~10 minutesResidence Time: How Long Water Stays
What is Residence Time?
Residence time is the average amount of time a water molecule spends in a particular reservoir before moving to another part of the water cycle.
| Reservoir | Typical Residence Time | Implication |
|---|---|---|
| Atmosphere | 9 days | Weather changes rapidly |
| Rivers | 2-6 months | Quick renewal |
| Soil moisture | 1-2 months | Seasonal variation |
| Large lakes | 50-100 years | Slower renewal |
| Shallow groundwater | 100-200 years | Pollution persists for generations |
| Deep groundwater | 10,000+ years | Essentially non-renewable |
| Oceans | 3,000-4,000 years | Very slow cycling |
| Antarctic ice sheet | ~900,000 years | Climate archive |
Why Residence Time Matters
For Pollution:
• Atmosphere: Air pollution can be cleaned up relatively quickly
• Groundwater: Pollution persists for centuries—prevention is critical
• Oceans: Plastic and pollutants accumulate over millennia
For Resources:
• Short residence time = renewable (rain refills rivers)
• Long residence time = essentially non-renewable (deep aquifers)
When pollutants enter groundwater, they can persist for 100-10,000+ years.
Unlike air pollution (which clears in days) or river pollution (months), groundwater contamination affects many generations. An aquifer polluted today might not be clean until your great-great-great-grandchildren's era.
Prevention is far more important than cleanup for groundwater!
Summary
~5 minutesSummary
Key Concepts Review
Water Distribution:
• 96.5% of Earth's water is salty ocean water
• Only ~2.5% is freshwater (most frozen in ice caps)
• Less than 1% is accessible freshwater for human use
• Rivers hold only 0.0002% of Earth's water!
Water Cycle Processes:
• Evaporation: Liquid to gas (absorbs energy)
• Transpiration: Plants release water vapor
• Condensation: Gas to liquid (releases energy, forms clouds)
• Precipitation: Water falls as rain, snow, sleet, or hail
• Infiltration: Water soaks into ground
• Runoff: Water flows over surface to streams/oceans
Energy in the Water Cycle:
• Sun powers the entire cycle
• Latent heat transfer drives weather and storms
• Evaporation cools, condensation warms
Residence Time:
• Atmosphere: 9 days (rapid cycling)
• Groundwater: 100-10,000+ years (slow cycling)
• Long residence time makes pollution cleanup nearly impossible
Check Your Understanding
What percentage of Earth's total water is salty ocean water?
Where is MOST of Earth's freshwater located?
About what percentage of Earth's total water is readily accessible freshwater for human use?
What provides the energy that powers the water cycle?
When water vapor condenses to form clouds, it:
Why is groundwater pollution considered more serious than air pollution?
Which process releases water vapor from plants?
A large tree can release how much water per day through transpiration?
Match each water cycle process with its description:
Match each water reservoir with its typical residence time:
Match each reservoir with its approximate percentage of Earth's total water:
Which of the following are TRUE about the water cycle? (Select ALL that apply)
Select all that apply.
Put these water reservoirs in order from SHORTEST residence time to LONGEST: