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Trace the
Water Cycle

Follow water through every pathway in the hydrologic cycle: from evaporation to precipitation, from runoff to underground flow, and back again.

Earth & Space Science

What Is the Water Cycle?

The water cycle (also called the hydrologic cycle) describes how water moves continuously between Earth's surface, atmosphere, and underground. No water is created or destroyed: the same molecules have been cycling for billions of years.

Essential Vocabulary

Evaporation
Liquid water absorbs heat and becomes water vapor, rising into the atmosphere.
Condensation
Water vapor cools and changes back into tiny liquid droplets, forming clouds.
Precipitation
Water falls from clouds as rain, snow, sleet, or hail when droplets grow heavy enough.

More Key Terms

Runoff
Water flows over the land surface into streams, rivers, and eventually the ocean.
Infiltration
Water soaks into the soil and moves downward through rock layers.
Transpiration
Plants absorb groundwater through roots and release water vapor through their leaves.

The Complete Hydrologic Cycle

Ocean Cloud Land Groundwater Evaporation Precipitation Runoff Infiltration Transpiration Condensation forms clouds

Solar Energy Powers It All

The Sun provides the thermal energy that drives evaporation and transpiration. Without solar heating, liquid water would not gain enough kinetic energy to escape into the atmosphere as vapor.

Water Takes Multiple Pathways

Precipitation Runoff Infiltration Surface Storage Rivers/Ocean Groundwater Evaporation Back to clouds

Evaporation vs. Transpiration

Evaporation Transpiration
Source Open water surfaces (oceans, lakes, puddles) Plant leaves (stomata)
Mechanism Solar heat converts liquid to vapor Plants pull water from soil, release vapor
Scale ~90% of atmospheric moisture ~10% of atmospheric moisture
Requires Heat energy Living plants + heat energy

Earth's Water Reservoirs

97.2% Oceans 2.1% Ice Caps 0.6% Ground 0.1% Other

How to Trace a Water Molecule

1
Pick a starting location: Where is the water right now? (ocean, cloud, river, underground, glacier)
2
Identify the process: What changes its state or moves it? (evaporation, condensation, precipitation, runoff, infiltration, transpiration)
3
Determine the destination: Where does the water end up after that process?
4
Repeat: Continue tracing from the new location. The path eventually loops back, completing a cycle.

Example 1: Ocean to Rain

Ocean evaporate Water Vapor condense Cloud precipitate Rain on Land

Example 2: Rain to Groundwater and Back

Rain Soil Groundwater Spring/River Ocean Evaporation infiltrate seep down discharge flow reach

Example 3: The Plant Pathway

Soil Water Plant Roots Leaves Atmosphere Cloud absorb transport transpire condense precipitate

Residence Time: How Long Water Stays

Residence time is the average length of time a water molecule spends in a particular reservoir before moving to the next stage of the cycle.

Atmosphere
~9 days
Rivers
~2 weeks
Groundwater
100 to 10,000 years
Ocean
~3,000 years

Common Mistakes to Avoid

✕ One Fixed Path
Thinking water always follows the same route. In reality, water can take many different pathways each time it cycles.
✕ Water Is Created/Destroyed
Water changes state and location but is neither created nor destroyed. The total amount stays roughly constant.
✕ Forgetting Underground
Many students skip infiltration and groundwater flow. A huge amount of fresh water is stored underground, not just in rivers and lakes.

Process Summary Table

Process State Change Direction Energy
Evaporation Liquid → Gas Surface → Atmosphere Absorbs heat
Transpiration Liquid → Gas Plant → Atmosphere Absorbs heat
Condensation Gas → Liquid Atmosphere (forms clouds) Releases heat
Precipitation Liquid/Solid falls Atmosphere → Surface Gravity
Runoff Stays liquid Land → Rivers → Ocean Gravity
Infiltration Stays liquid Surface → Underground Gravity

Question 1 of 5

Which two forces primarily drive the water cycle?

Question 2 of 5

A water molecule evaporates from a lake, forms a cloud, falls as snow on a mountain, melts, and flows into a river. Which process is missing from this trace?

Question 3 of 5

Rain falls on a hillside with dense clay soil. What is the most likely immediate pathway for most of the water?

Question 4 of 5

How does transpiration differ from evaporation?

Question 5 of 5

A molecule of water has been trapped in a deep underground aquifer for 5,000 years. Which statement best explains this?