What is the Water Cycle?
The water cycle describes how water continuously moves between the oceans, atmosphere, land, and living things. Driven by solar energy, water evaporates, condenses into clouds, falls as precipitation, and eventually flows back to the sea, repeating endlessly.
The water cycle is the continuous natural movement of water through evaporation, transpiration, condensation, precipitation, and collection/runoff between Earth's oceans, atmosphere, and land.
- 1.Evaporation — The sun heats water in oceans, lakes, and rivers, turning it into water vapor.
- 2.Transpiration — Plants release water vapor into the air through their leaves.
- 3.Condensation — Water vapor rises, cools, and condenses into tiny droplets forming clouds.
- 4.Precipitation — Water falls back to Earth as rain, snow, sleet, or hail.
- 5.Collection & Runoff — Water flows into rivers and oceans or infiltrates the ground as groundwater.
Try it: interactive calculator
Step-by-step worked examples
A river basin receives 150 mm of precipitation in a month, and evapotranspiration is 90 mm. Find the runoff.
Q = P − ET Q = 150 − 90 = 60 mm of runoff
A region gets 220 mm of rain, and evapotranspiration is estimated at 180 mm. How much water is left to become runoff or groundwater?
Q = P − ET Q = 220 − 180 = 40 mm
A dry season has only 30 mm of precipitation, but evapotranspiration demand is 70 mm. What does this imply?
Q = P − ET Q = 30 − 70 = −40 mm A negative value means there is a water deficit — more water is lost to evapotranspiration than falls as rain, so soil moisture and groundwater reserves are drawn down.
Flashcards
Quick quiz
Q1.Which process turns liquid water into vapor using solar energy?
Q2.What is the term for water vapor cooling and forming clouds?
Q3.A region gets 100 mm of precipitation and has 65 mm of evapotranspiration. What is the runoff?
Q4.How do plants contribute to the water cycle?
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Common mistakes
Water is created and destroyed as it moves through the cycle. — Correct: Water is neither created nor destroyed — the same finite amount continuously changes form and location.
Evaporation only happens from oceans. — Correct: Evaporation happens from any exposed water surface — lakes, rivers, soil, and even puddles — not just oceans.
All precipitation becomes runoff. — Correct: Some precipitation evaporates again, some infiltrates into groundwater, and only part becomes surface runoff.
Transpiration and evaporation are the same process. — Correct: Evaporation is water turning to vapor from open surfaces; transpiration is specifically water vapor released through plant leaves.
FAQ
What is the water cycle?
The water cycle is the continuous movement of water between Earth's oceans, atmosphere, and land through evaporation, transpiration, condensation, precipitation, and runoff.
What is the formula for runoff in the water cycle?
A simplified water balance gives runoff as Q = P − ET, where P is precipitation and ET is evapotranspiration.
What are examples of the water cycle?
A puddle drying up (evaporation), clouds forming over mountains (condensation), rain falling (precipitation), and a river carrying water to the sea (runoff) are all part of the cycle.
How do you calculate runoff from precipitation data?
Subtract evapotranspiration from total precipitation: Q = P − ET, which estimates the water available to flow into rivers or recharge groundwater.