Introduction

Water is one of the most important substances on Earth. Every living organism depends on it for survival. Plants need water for photosynthesis and growth, while animals rely on it for drinking, transporting nutrients, regulating body temperature, and carrying out essential chemical reactions. Although Earth has a vast amount of water, it is constantly moving between the atmosphere, land, rivers, lakes, oceans, and living organisms.

This continuous movement of water is called the water cycle (also known as the hydrological cycle). The water cycle is powered mainly by the Sun, which provides the energy needed for evaporation, and gravity, which returns water to Earth's surface as precipitation. Because water is continually recycled, the same water molecules have been moving around our planet for billions of years.

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What Is the Water Cycle?

The water cycle is the continuous movement of water between the Earth's surface, the atmosphere, and living organisms.

Water changes between three states during the cycle:

• Solid (ice and snow)
• Liquid (water)
• Gas (water vapour)

The main stages of the water cycle are:

• Evaporation
• Transpiration
• Condensation
• Precipitation
• Collection
• Infiltration
• Runoff

Together, these processes recycle Earth's water.

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Figure 1. The water cycle continuously moves water between the atmosphere, land, oceans, and living organisms.

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Stage 1: Evaporation

Evaporation is the process by which liquid water changes into water vapour.

The Sun heats water in:

• Oceans
• Lakes
• Rivers
• Puddles
• Soil

As water warms, some molecules gain enough energy to escape into the atmosphere as an invisible gas.

Evaporation is greatest on:

• Hot days
• Windy days
• Dry days
• Large bodies of water

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Transpiration

Plants also contribute water to the atmosphere.

Transpiration is the release of water vapour from plant leaves through tiny openings called stomata.

Together, evaporation and transpiration are sometimes called evapotranspiration.

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Figure 2. Water enters the atmosphere through evaporation from water surfaces and transpiration from plants.

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Stage 2: Condensation

As warm, moist air rises, it cools.

Cooling causes water vapour to change back into tiny liquid droplets.

This process is called condensation.

Condensation forms:

• Clouds
• Fog
• Mist
• Dew

Millions of tiny droplets combine to form visible clouds.

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Stage 3: Precipitation

When water droplets in clouds become too heavy to remain suspended, they fall back to Earth as precipitation.

Types of precipitation include:

• Rain
• Snow
• Sleet
• Hail

The type of precipitation depends mainly on the air temperature.

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Figure 3. Condensation forms clouds, and precipitation returns water to Earth's surface.

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Stage 4: Collection

After precipitation falls, water collects in:

• Oceans
• Lakes
• Rivers
• Wetlands
• Reservoirs

Some water remains on the surface, while some begins moving through the soil.

Eventually, much of it returns to the oceans, where the cycle begins again.

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Stage 5: Infiltration and Groundwater

Some water soaks into the ground.

This process is called infiltration.

Underground, water fills tiny spaces between soil particles and rocks, forming groundwater.

Groundwater:

• Supplies wells.
• Feeds springs.
• Keeps rivers flowing during dry periods.
• Provides water for many ecosystems.

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Stage 6: Runoff

Water that does not soak into the ground flows across the land.

This is called runoff.

Runoff carries water into:

• Streams
• Rivers
• Lakes
• Oceans

Runoff can also transport:

• Soil
• Nutrients
• Pollution

This is why protecting watersheds is important for maintaining clean water.

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Figure 4. Water either infiltrates into the ground or flows across the land as runoff.

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How Water Moves Through Ecosystems

Water moves continuously between living organisms and the environment.

For example:

• Plants absorb water through their roots.
• Animals drink water.
• Animals release water through breathing, sweating, and waste.
• Plants release water through transpiration.
• Decomposers return water to the soil as they break down organic matter.

This constant movement supports every ecosystem on Earth.

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Why Is the Water Cycle Important?

The water cycle is essential because it:

• Recycles Earth's freshwater.
• Supplies drinking water.
• Supports plant growth.
• Provides habitats for aquatic organisms.
• Transports nutrients.
• Regulates climate.
• Helps maintain healthy ecosystems.

Without the continuous movement of water, life on Earth could not exist.

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Factors That Affect Water Movement

Several environmental factors influence how quickly water moves through the water cycle.

Temperature

Higher temperatures increase evaporation.

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Wind

Wind removes moist air above water surfaces, increasing evaporation.

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Humidity

High humidity slows evaporation because the air already contains large amounts of water vapour.

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Vegetation

Forests increase transpiration and help water infiltrate into the soil.

Removing vegetation often increases runoff and soil erosion.

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Land Surface

Impermeable surfaces such as roads and parking lots prevent infiltration.

This increases runoff and the risk of flooding.

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Figure 5. Forests encourage infiltration, while urban areas increase runoff because less water can soak into the ground.

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Worked Example

Question

Place the following stages of the water cycle in the correct order.

• Condensation
• Collection
• Evaporation
• Precipitation

Solution

Correct order:

1. Evaporation
2. Condensation
3. Precipitation
4. Collection

Water then continues through infiltration, runoff, and eventually evaporates again.

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Real-World Connection

Cities often experience increased flooding because roads, pavements, and buildings prevent rainwater from soaking into the ground. Engineers reduce this problem by installing permeable pavements, rain gardens, green roofs, and stormwater ponds that slow runoff and allow more water to infiltrate into the soil. These solutions help protect ecosystems and improve water quality.

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Did You Know?

At any given moment, about 97% of Earth's water is found in the oceans. Of the remaining 3% that is freshwater, most is locked away in glaciers and ice caps. Less than 1% of Earth's freshwater is easily available in lakes, rivers, and shallow groundwater for humans and most terrestrial ecosystems to use.

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Key Terms

Collection – The gathering of water in oceans, lakes, rivers, and other bodies of water.

Condensation – The process by which water vapour cools and changes into liquid water.

Evaporation – The process by which liquid water changes into water vapour.

Groundwater – Water stored beneath Earth's surface in soil and rock.

Infiltration – The process by which water soaks into the ground.

Precipitation – Water falling from clouds as rain, snow, sleet, or hail.

Runoff – Water flowing across the land into rivers, lakes, or oceans.

Transpiration – The release of water vapour from plant leaves.

Water cycle (Hydrological cycle) – The continuous movement of water between the atmosphere, land, oceans, and living organisms.

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Key Takeaways

• The water cycle continuously recycles water through Earth's atmosphere, land, oceans, and living organisms.
• The main stages are evaporation, transpiration, condensation, precipitation, collection, infiltration, and runoff.
• The Sun provides the energy that drives evaporation, while gravity returns water to Earth as precipitation.
• Water moves through ecosystems by passing between the atmosphere, soil, plants, animals, and bodies of water.
• Temperature, wind, humidity, vegetation, and land use all influence how quickly water moves through the water cycle.