Water Cycle Explorer
Follow one water droplet through the ocean, the air, ice, rivers, soil, plants and groundwater. Choose its path or let chance decide, watch the years add up, and change the climate to see how Earth’s water responds.
Where next?
The droplet is in the ocean. Water typically stays there about 3,200 years.
Choose a way out for the droplet, or press Let chance decide. The time adds up as it travels.
The same code makes the same chance moves, so a class can follow one journey.
Change the conditions
Today’s climate. The chances are rounded classroom values that follow real patterns.
Journey log
Moves: 0 · Total time: 0 years
| Move | Place it left | How it moved | Typical time there | Total so far |
|---|---|---|---|---|
| No moves yet. The droplet is waiting in the ocean. | ||||
Each stop adds the typical (average) time water stays there. A real droplet could stay much shorter or longer.
Where is Earth’s water?
| Reservoir | Volume (km³) | Share of all water | Share of fresh water | Typical time there |
|---|---|---|---|---|
| Oceans, seas and bays | 1,338,000,000 | 96.5% | 0% (salty) | about 3,200 years |
| Ice caps, glaciers and permanent snow | 24,064,000 | 1.7% | 68.7% | mountain glaciers 20 to 100 years; Antarctica about 20,000 years |
| Groundwater (fresh and salty) | 23,400,000 | 1.7% | 30.1% | shallow 100 to 200 years; deep about 10,000 years |
| Ground ice and permafrost | 300,000 | 0.022% | 0.86% | Not part of the journey |
| Lakes (fresh and salty) | 176,400 | 0.013% | 0.26% | 50 to 100 years |
| Soil moisture | 16,500 | 0.0012% | 0.047% | 1 to 2 months |
| Atmosphere (water vapor and clouds) | 12,900 | 0.00093% | 0.037% | about 9 days |
| Swamp water | 11,470 | 0.00083% | 0.033% | Not part of the journey |
| Rivers | 2,120 | 0.00015% | 0.0061% | 2 to 6 months |
| Living things | 1,120 | 0.000081% | 0.0032% | about 1 week |
| Total | 1,385,984,510 | 100% | 35,029,110 km³ fresh |
Volumes: USGS Water Science School, “Where is Earth’s water?”, after Shiklomanov (1993). Typical times: Pidwirny, Fundamentals of Physical Geography (2006); living things: OpenStax Biology 2e. Typical times are averages and approximate.
How the water cycle works
Earth’s water is always on the move. It does not get used up or made new; the same water travels again and again between reservoirs, the places where water is stored: the ocean, the air, ice, lakes and rivers, the soil, the ground beneath it, and living things.
Two things keep it moving. The sun’s energy lifts water up. Sunlight warms the ocean and the land, so water evaporates, ice melts or turns straight to vapor, and plants pull water up from their roots and give it off from their leaves. Gravity brings water down. When rising vapor cools it condenses into clouds, and gravity pulls the drops down as rain and snow, then pulls the water downhill in rivers and down into the ground. In the explorer, arrows powered by the sun are orange and arrows pulled by gravity are blue.
- Evaporation (sun’s energy). Liquid water warms up and turns into water vapor, an invisible gas.
- Transpiration (sun’s energy). Plants give off water vapor through tiny holes in their leaves.
- Sublimation (sun’s energy). Ice and snow turn straight into water vapor without melting first.
- Condensation (cooling). Water vapor cools and turns into the tiny droplets and ice crystals that make clouds.
- Precipitation (gravity). Cloud droplets join until they are heavy enough to fall as rain or snow.
- Snow accumulation (gravity). Snow falls and piles up. Over years, buried snow is squeezed and freezes into glacier ice.
- Melting (sun’s energy). Ice warms up and turns into liquid water.
- Runoff (gravity). Water flows downhill over the ground and in streams and rivers.
- Infiltration (gravity). Water soaks down into the ground.
- Groundwater flow (gravity). Water creeps slowly through tiny spaces in soil and rock, downhill underground.
- Uptake by roots (sun’s energy). Roots draw water from the soil. The pull comes from water evaporating out of the leaves, so the sun powers it.
Where is Earth’s water?
Almost all of it, about 96.5%, is salty ocean water. Only about 2.5% is fresh, and most of that is out of easy reach: about 68.7% is frozen in ice caps and glaciers and 30.1% is underground. All the world’s rivers hold only about 2,120 km³, about 0.00015% of Earth’s water. The atmosphere holds about 6 times as much as the rivers, yet still only about 0.00093%.
Numbers that small don’t show up on an ordinary chart, so the explorer’s Where is Earth’s water? view uses zoom bars: each bar zooms in on the slivers of the one above, and says how many times bigger it is drawn. Try the 1-liter bottle units: if all of Earth’s water filled a 1-liter bottle, the rivers would be about 0.0015 mL, far less than a single drop.
How long does water stay in each place?
The average time water spends in a reservoir is its residence time. It depends on how big the reservoir is compared with how fast water flows in and out. The air holds little water and swaps it quickly, so a molecule stays only days. The ocean is enormous, so even though a lot evaporates every year, an average molecule waits thousands of years for its turn.
| Place | Typical time | Ways out (today’s climate) |
|---|---|---|
| Ice sheet | about 20,000 yearssource: about 20,000 years (Antarctica) | melting to the ocean (90%); sublimation to the atmosphere (10%) |
| Deep groundwater | about 10,000 years | groundwater flow to the ocean (70%); groundwater flow to a river (30%) |
| Ocean | about 3,200 years | evaporation to the atmosphere (100%) |
| Groundwater | about 150 yearssource: 100 to 200 years (shallow) | groundwater flow to a river (60%); groundwater flow to the ocean (15%); groundwater flow to deep groundwater (15%); groundwater flow to a lake (10%) |
| Lake | about 75 yearssource: 50 to 100 years | runoff to a river (45%); evaporation to the atmosphere (40%); infiltration to the groundwater (15%) |
| Mountain glacier | about 60 yearssource: 20 to 100 years | melting to a river (80%); sublimation to the atmosphere (20%) |
| River | about 4 monthssource: 2 to 6 months | runoff to the ocean (70%); runoff to a lake (15%); evaporation to the atmosphere (10%); infiltration to the groundwater (5%) |
| Soil | about 1.5 monthssource: 1 to 2 months | uptake by roots to living things (35%); evaporation to the atmosphere (25%); infiltration to the groundwater (25%); runoff to a river (15%) |
| Atmosphere | about 9 days | precipitation to the ocean (77%); precipitation to the soil (19%); precipitation to a lake (2%); snow accumulation to a mountain glacier (1%); snow accumulation to an ice sheet (1%) |
| Living things | about 1 week | transpiration to the atmosphere (100%) |
Typical times from Pidwirny, Fundamentals of Physical Geography (2006), and, for living things, OpenStax Biology 2e. Where the source gives a range, the explorer uses the middle of it. The chances are rounded classroom values; the 77% chance of rain or snow landing on the ocean matches the global water budget of Trenberth and others (2007).
Worked example: one round trip
A droplet evaporates from the ocean, falls as rain on land, soaks down into the groundwater, seeps into a river and flows back to the sea. How long does the trip take, and how likely is it?
| Move | Leaves | How | Time there | Total so far |
|---|---|---|---|---|
| 1 | Ocean | Evaporation (100%) | about 3,200 years | 3,200 years |
| 2 | Atmosphere | Condensation, then precipitation (19%) | about 9 days | 3,200 years |
| 3 | Soil | Infiltration (25%) | about 1.5 months | 3,200 years |
| 4 | Groundwater | Groundwater flow (60%) | about 150 years | 3,350 years |
| 5 | River | Runoff (70%) | about 4 months | 3,350 years |
How long? Add up the time at each place it leaves. The ocean (3,200 years) and the groundwater (150 years) take almost all of it; the air, the soil and the river together add only about 5.8 months. The whole trip takes about 3,350 years.
chance of the whole route = 100% × 19% × 25% × 60% × 70% ≈ 2%
How likely? Multiply the chance of each move: about 2 droplets in 100 that leave the ocean would follow exactly this route. Most go straight back to the sea as rain. Only one move on this route, evaporation, needed the sun’s energy; gravity did the rest. But without that first lift, the droplet could never have left the ocean.
Common misconceptions
- “Clouds are made of water vapor.” Water vapor is an invisible gas. Clouds are made of tiny liquid droplets or ice crystals that formed when vapor cooled and condensed.
- “The water cycle is one circle: ocean, cloud, rain, river, ocean.” Water takes many paths. Most of it simply goes from the ocean to the air and back to the ocean; some detours through ice, soil, plants or rock.
- “Water goes around the cycle quickly.” Some does, in days. But water can stay in the ocean for thousands of years, and in ice sheets or deep groundwater for tens of thousands.
- “Water only evaporates when it’s hot.” Evaporation happens at any temperature, from puddles, lakes, wet soil and leaves. Even ice and snow can turn straight into vapor (sublimation).
- “Groundwater is an underground lake or river.” Most groundwater fills the tiny spaces between grains of sand and gravel and the cracks in rock, and creeps along very slowly.
- “We are using up Earth’s water.” The total amount stays about the same. But fresh water we can use is scarce, and people can pump groundwater out much faster than it refills, or pollute it.
What this model simplifies
The diagram is a sketch, not a map: it is not to scale, and it puts a mountain glacier and a polar ice sheet side by side. Each place has one typical time, the middle of a published range, and every droplet is given exactly that average; real water molecules can leave a reservoir within minutes or stay for millions of years. Only the split of rain and snow between the ocean and the land is based on measurements; the other chances are rounded classroom values that follow the real pattern, and real chances change with place, season and weather. The climate buttons only show the direction of each change (for example, more melting when warmer) with made-up amounts, and they do not change the times. The model leaves out people (pumping groundwater, dams, farms and drinking water), animals, sea ice, permafrost and swamps in the journey, the difference between fresh and salty groundwater, wind carrying vapor from place to place, and the small amounts of water that enter and leave Earth’s interior at volcanoes and subduction zones. Switching off the sun or gravity is a thought experiment: it shows what each one does, not something that could happen. The reservoir volumes are one well-known estimate (Shiklomanov, 1993, published by the USGS); other estimates differ a little.
Water cycle journey activity
A one-page printable worksheet for grades 5–8, about 15 minutes: students follow one droplet with a shared chance code, record each place, process and typical time, then answer six questions about what drives each move, how long water stays, and where Earth’s water is.
Share the explorer with your class in one click. Students need no accounts or logins.
Share to Google ClassroomEvery path in this model
| From | Process | To | Chance today | Driven by |
|---|---|---|---|---|
| Ocean | Evaporation: evaporates from the sea surface | Atmosphere | 100% | The sun’s energy |
| Atmosphere | Condensation, then precipitation: falls as rain on the ocean | Ocean | 77% | Gravity |
| Atmosphere | Condensation, then precipitation: falls as rain on land and soaks into the soil | Soil | 19% | Gravity |
| Atmosphere | Condensation, then precipitation: falls as rain on a lake | Lake | 2% | Gravity |
| Atmosphere | Condensation, then snow accumulation: falls as snow on a mountain glacier | Mountain glacier | 1% | Gravity |
| Atmosphere | Condensation, then snow accumulation: falls as snow on a polar ice sheet | Ice sheet | 1% | Gravity |
| Mountain glacier | Melting: melts into a mountain stream | River | 80% | The sun’s energy |
| Mountain glacier | Sublimation: turns straight into vapor in the dry mountain air | Atmosphere | 20% | The sun’s energy |
| Ice sheet | Melting: flows to the coast, breaks off as an iceberg and melts | Ocean | 90% | The sun’s energy |
| Ice sheet | Sublimation: turns straight into vapor in the cold, dry air | Atmosphere | 10% | The sun’s energy |
| Soil | Uptake by roots: is drawn up by plant roots | Living things | 35% | The sun’s energy |
| Soil | Evaporation: evaporates from the ground | Atmosphere | 25% | The sun’s energy |
| Soil | Infiltration: soaks deeper, down to the groundwater | Groundwater | 25% | Gravity |
| Soil | Runoff: runs downhill over the ground into a river | River | 15% | Gravity |
| Living things | Transpiration: escapes from a leaf as vapor | Atmosphere | 100% | The sun’s energy |
| River | Runoff: flows downstream to the ocean | Ocean | 70% | Gravity |
| River | Runoff: flows into a lake | Lake | 15% | Gravity |
| River | Evaporation: evaporates from the river | Atmosphere | 10% | The sun’s energy |
| River | Infiltration: seeps through the riverbed into the ground | Groundwater | 5% | Gravity |
| Lake | Runoff: flows out of the lake into a river | River | 45% | Gravity |
| Lake | Evaporation: evaporates from the lake | Atmosphere | 40% | The sun’s energy |
| Lake | Infiltration: seeps through the lake bed into the ground | Groundwater | 15% | Gravity |
| Groundwater | Groundwater flow: seeps into a riverbed or spring | River | 60% | Gravity |
| Groundwater | Groundwater flow: seeps out under the sea at the coast | Ocean | 15% | Gravity |
| Groundwater | Groundwater flow: sinks slowly into deeper rock | Deep groundwater | 15% | Gravity |
| Groundwater | Groundwater flow: seeps into a lake from below | Lake | 10% | Gravity |
| Deep groundwater | Groundwater flow: creeps all the way to the sea floor | Ocean | 70% | Gravity |
| Deep groundwater | Groundwater flow: rises in a spring that feeds a river | River | 30% | Gravity |
Water cycle questions
What is the water cycle?
The water cycle is the nonstop movement of water between the ocean, the air, the land, ice, living things and the ground. Water changes between liquid, gas (water vapor) and solid (ice) as it moves, but the total amount of water on Earth stays about the same.
What drives the water cycle?
Two things. Energy from the sun warms water so it evaporates, melts ice and powers plants as they transpire: that lifts water up into the air. Gravity pulls it back down as rain and snow, and makes it flow downhill in rivers and soak down into the ground. In the explorer you can switch either one off as a thought experiment and watch the cycle stop.
How long does water stay in the ocean, or in the air?
On average a water molecule stays in the ocean about 3,200 years but in the atmosphere only about 9 days. Ice sheets can hold water for about 20,000 years and deep groundwater for about 10,000 years. These are averages: a real droplet can leave much sooner or stay much longer.
How much of Earth’s water is fresh water?
Only about 2.5%. About 96.5% of Earth’s water is salty ocean water. Most fresh water is frozen in ice caps and glaciers (about 68.7% of it) or underground (about 30.1%), so the lakes and rivers we use hold only a tiny part.
What is the difference between evaporation, transpiration and sublimation?
All three turn water into water vapor using energy, mostly from the sun. Evaporation is liquid water turning into vapor, from the ocean, lakes, rivers or wet soil. Transpiration is water vapor escaping from plant leaves. Sublimation is ice or snow turning straight into vapor without melting first.
Can I use the Water Cycle Explorer in class?
Yes. It is free with no sign-up and runs on school Chromebooks and tablets. Give the class a chance code so everyone follows the same droplet, or let each student pick their own and compare journeys. The printable 15-minute activity has an answer key on the For teachers page, and Challenge mode has 8 quick questions.