The Water Cycle
The Water Cycle
The water cycle (also called the hydrological cycle) describes the continuous movement and recycling of water through Earth's systems — oceans, atmosphere, land surface, and underground. No water is created or destroyed; it is perpetually recycled.
The total amount of water on Earth has remained approximately the same for billions of years. About 97.5% is saltwater in oceans; only 2.5% is freshwater, and most of that is locked in ice caps and glaciers. Less than 1% of Earth's water is accessible freshwater.
The water cycle is driven by two main forces:
- Solar energy — drives evaporation and transpiration
- Gravity — drives precipitation, surface run-off, and infiltration
Key Processes
Evaporation
Solar energy heats water at the surface of oceans, lakes, rivers, and soil, providing enough energy for molecules to escape from the liquid surface and enter the atmosphere as water vapour.
Factors that increase evaporation rate:
- Higher temperature
- Lower humidity (drier air)
- Greater wind speed (removes water vapour from above the surface)
- Larger surface area of water exposed
Approximately 86% of global evaporation comes from the oceans.
Transpiration
Plants absorb water from the soil through their roots and transport it upward through xylem vessels. Water exits through stomata in leaves as water vapour in a process called transpiration.
Transpiration is driven by the same factors as evaporation (temperature, humidity, wind, light). Plants open stomata in daylight to allow CO2 in for photosynthesis; water vapour simultaneously exits.
Evapotranspiration is the combined term for evaporation from surfaces AND transpiration from plants. Forests contribute enormously to evapotranspiration — tropical rainforests generate their own rainfall through this process.
Condensation
As water vapour rises in the atmosphere, the air cools (temperature drops approximately 6.5°C per 1000 m of altitude). Cooler air can hold less water vapour, so water vapour condenses onto tiny dust and salt particles called condensation nuclei, forming microscopic water droplets. Billions of these droplets together form clouds.
The altitude at which condensation begins is called the dew point — visible as the flat base of clouds.
Dew and frost form when the ground cools rapidly at night, causing water vapour in contact with the surface to condense directly onto grass, cars, and spider webs.
Precipitation
When water droplets in clouds collide and merge, they grow larger and heavier. When they become too heavy for air currents to support, they fall as precipitation:
- Rain — liquid water droplets (temperatures above 0°C)
- Snow — ice crystals that form when clouds are below 0°C; joins into snowflakes
- Sleet — partially melted snow or a mixture of rain and snow
- Hail — balls of ice formed in cumulonimbus clouds by strong updrafts
Surface Run-off
Precipitation falling on land may flow over the surface as run-off, collecting in streams and rivers that eventually drain into the ocean. The rate of run-off depends on:
- Soil saturation (already waterlogged = more run-off)
- Gradient (steeper slopes = faster run-off)
- Vegetation cover (plants intercept rainfall and slow run-off)
- Impermeable surfaces (concrete = all run-off, no infiltration)
Infiltration and Groundwater
Some precipitation soaks into the soil — this is infiltration. Water filtered through soil and rock layers accumulates in aquifers (underground stores of water in porous rock). Groundwater can:
- Slowly move through rock to emerge as springs
- Be extracted by wells and boreholes for drinking water
- Supply rivers during dry periods (baseflow)
Collection and Storage
Water collects in oceans, lakes, rivers, glaciers, snowpacks, and groundwater stores. The ocean is the largest reservoir, storing about 97% of all water. Water can remain in some stores for very different durations:
- Atmosphere: approximately 9 days
- River: weeks
- Groundwater: hundreds to thousands of years
- Glacier/ice sheet: thousands to millions of years
Human Impact on the Water Cycle
Human activities significantly alter the water cycle:
| Activity | Effect |
|---|---|
| Deforestation | Reduces transpiration; increases run-off and flood risk; reduces groundwater recharge |
| Urbanisation (building cities) | Impermeable surfaces increase run-off; reduce infiltration |
| Irrigation | Withdraws groundwater faster than it recharges |
| Climate change | Intensifies the cycle — faster evaporation, heavier precipitation events, melting ice caps |
| Dam building | Alters river flow; increases evaporation from reservoirs |
Why the Water Cycle Matters
- Distributes fresh water across the planet — essential for drinking, agriculture, and ecosystems
- Regulates climate — water vapour is the most important greenhouse gas; water cycle transfers heat between regions
- Shapes landforms — erosion, deposition, and sediment transport
- Supports all life — every organism needs water; the cycle ensures it is continuously renewed
- Freshwater is a limited resource — only about 1% of Earth's water is accessible; conservation is critical
Worked Example
Q: Trace a water molecule from the ocean to a cloud and back to land.
- Solar energy causes evaporation — a water molecule leaves the ocean surface and enters the atmosphere as water vapour.
- Rising air currents carry the vapour upward where it cools. At the dew point, vapour condenses around tiny dust particles, forming liquid droplets — this is condensation. Millions of droplets together form a cloud.
- As droplets grow and combine, they become heavy enough to fall as precipitation (rain, snow, or hail).
- On land, the water may flow along the surface as run-off into rivers, soak into the ground as infiltration (replenishing groundwater), or be absorbed by plant roots and released back to the atmosphere through transpiration.
- Eventually, the water makes its way back to the ocean — and the cycle begins again.
Key Terms
- Evaporation — change from liquid water to water vapour using solar energy
- Transpiration — loss of water vapour from plant leaves through stomata
- Evapotranspiration — combined evaporation and transpiration
- Condensation — water vapour cooling and forming liquid droplets; cloud formation
- Precipitation — water falling from clouds as rain, snow, sleet, or hail
- Run-off — water flowing over the land surface into rivers and streams
- Infiltration — water soaking into the soil, potentially becoming groundwater
- Aquifer — underground store of water in porous rock
Common Mistakes
- Confusing transpiration (water loss from plants) with respiration (energy release) — completely different processes
- Saying rain "creates" water — the water cycle recycles existing water; no new water is made
- Thinking evaporation only happens at boiling point — evaporation occurs at any temperature from any water surface
- Forgetting that groundwater is part of the water cycle — it is not separate
Tips and Tricks
- Water cycle order: Evaporation → Condensation → Precipitation → Run-off/Infiltration → back to ocean
- The Sun drives evaporation (energy); gravity drives everything falling back down
- Clouds form when rising air cools to the dew point — the flat base of a cloud marks this altitude
- Deforestation disrupts the cycle: fewer trees = less transpiration = less rainfall in some regions