The Water Cycle » Transpiration, Evaporation and Condensation

What you'll learn this session

Study time: 30 minutes

The key processes in the water cycle: transpiration, evaporation and condensation
Factors affecting transpiration rates in plants
How evaporation occurs and what affects its rate
The process of condensation and its importance
How these processes interact in the global water cycle
Real-world applications and environmental impacts

The Water Cycle: Nature's Recycling System

The water cycle (or hydrological cycle) is the continuous movement of water on, above and below Earth's surface. It's one of nature's most important processes, ensuring that water - essential for all life - is constantly recycled and redistributed around our planet.

Key Definitions:

Water Cycle: The continuous movement of water between the Earth's surface, atmosphere and underground.
Transpiration: The process by which water vapour escapes from plants, primarily through leaf pores called stomata.
Evaporation: The process where liquid water changes into water vapour due to heat energy.
Condensation: The process where water vapour changes back into liquid water when cooled.

🌱 Transpiration: Plants' Hidden Water Movement

Transpiration is like plant sweating! Plants draw water from the soil through their roots, transport it up through their stems and release it as water vapour through tiny pores (stomata) in their leaves. This process is crucial for plants as it:

Helps transport minerals from roots to leaves
Cools the plant on hot days
Creates a 'pull' that draws more water up from the roots
Returns enormous amounts of water to the atmosphere - a large oak tree can transpire up to 150,000 litres per year!

💧 Factors Affecting Transpiration

Several factors influence how quickly plants transpire:

Temperature: Higher temperatures increase transpiration rates
Humidity: Lower humidity speeds up transpiration
Wind: Breezier conditions remove water vapour faster, increasing transpiration
Light intensity: Brighter light causes stomata to open wider
Leaf surface area: Larger or more numerous leaves = more transpiration

Evaporation: Water's Escape to the Atmosphere

Evaporation occurs when liquid water absorbs enough energy to change into water vapour. This happens constantly from oceans, lakes, rivers, puddles and even damp soil. About 90% of atmospheric moisture comes from evaporation from bodies of water, with the remaining 10% from transpiration.

The Science Behind Evaporation

Water molecules are always moving. When they gain enough energy (usually from heat), the fastest-moving molecules at the surface break free from the attraction of other water molecules and escape into the air as water vapour. This is why:

🌡 Temperature

Higher temperatures give water molecules more energy, speeding up evaporation. This is why puddles disappear faster on hot days!

🌬 Wind

Wind sweeps away water vapour that has already evaporated, allowing more water to evaporate. This is why hanging wet clothes outside on a windy day dries them faster.

🌞 Surface Area

A larger surface area exposes more water to the air, increasing evaporation. This is why spreading out a wet towel helps it dry faster than leaving it bunched up.

Condensation: Water Vapour Returns to Liquid

Condensation is the opposite of evaporation. It occurs when water vapour in the air cools down and changes back into liquid water. You've seen condensation when water droplets form on the outside of a cold drink on a warm day, or when your bathroom mirror fogs up after a hot shower.

☁ Cloud Formation

Condensation is responsible for cloud formation. As warm, moist air rises in the atmosphere, it cools. When it cools to the dew point (the temperature at which air becomes saturated with water vapour), condensation occurs. Tiny water droplets form around microscopic particles like dust, salt, or smoke in the air. These droplets cluster together to form clouds.

Different cloud types form at different heights and temperatures:

Cumulus: Fluffy, cotton-like clouds that form at lower altitudes
Stratus: Flat, sheet-like clouds that can cover the entire sky
Cirrus: High, thin, wispy clouds made of ice crystals

🌧 Precipitation

When water droplets in clouds grow large and heavy enough, they fall as precipitation:

Rain: Liquid water droplets
Snow: Ice crystals formed when water vapour condenses at temperatures below freezing
Sleet: Rain that freezes as it falls
Hail: Formed when strong updrafts in thunderstorms carry raindrops upward into extremely cold areas, where they freeze into ice pellets

The Complete Water Cycle: Putting It All Together

Transpiration, evaporation and condensation are all vital parts of the water cycle. Here's how they work together:

Water evaporates from oceans, lakes and rivers due to solar energy
Plants transpire water vapour into the atmosphere
Water vapour rises and condenses to form clouds
Water returns to Earth as precipitation (rain, snow, etc.)
Some water infiltrates into the ground, some runs off into rivers and eventually back to the oceans
The cycle begins again!

Case Study Focus: Amazon Rainforest and Transpiration

The Amazon Rainforest demonstrates the incredible power of transpiration. This vast forest:

Contains about 390 billion individual trees
Transpires about 20 billion tonnes of water into the atmosphere each day
Creates its own weather patterns through this "flying river" of moisture
Generates about 50-80% of its own rainfall through this cycle

Deforestation disrupts this cycle. When trees are removed, less water is transpired, potentially leading to drier conditions and affecting rainfall patterns across South America. Scientists have found that deforestation in the Amazon could reduce rainfall by up to 20% in some regions, showing how human activities can disrupt the water cycle.

Environmental Management Implications

Understanding the water cycle is crucial for environmental management because:

Water resource management: Knowing how water moves helps us plan water supplies for growing populations
Climate change impacts: Rising temperatures affect evaporation and transpiration rates, changing precipitation patterns
Flood prevention: Understanding the cycle helps predict and manage flood risks
Ecosystem protection: Many ecosystems depend on specific water cycle patterns

Practical Application: Measuring Transpiration

You can observe transpiration yourself with a simple experiment:

Take a small leafy plant and cover its pot with a plastic bag, sealing it around the stem
Place a clear plastic bag over some leaves and tie it closed around the stem
Place the plant in a sunny spot
After a few hours, you'll see water droplets inside the bag - this is transpired water!
Try comparing plants in different conditions (sunny vs. shady, windy vs. still) to see how these factors affect transpiration rates

Summary: The Water Cycle's Key Processes

The water cycle connects all water on Earth in a continuous system of movement and transformation:

Transpiration moves water from soil through plants to the atmosphere, cooling plants and helping transport nutrients
Evaporation transforms surface water into atmospheric water vapour, driven by heat energy
Condensation changes water vapour back to liquid water, forming clouds and eventually precipitation

These processes are essential for life on Earth, creating weather patterns, supporting ecosystems and providing the freshwater all living things need. Human activities that affect any part of this cycle can have far-reaching consequences for our environment and water resources.

🧠 Test Your Knowledge!