The Water Cycle

The water cycle (also called the hydrological cycle) is the never-ending journey of water between the Earth's surface and the sky. Water keeps moving — from oceans, lakes and rivers, up into the air, into clouds, down as rain, and back to the surface. The water you drink today has been on this journey countless times for billions of years.

The four main stages of the water cycle:

1. Evaporation — heat from the sun changes liquid water into water vapour (a gas). This happens most over oceans, lakes, rivers, and even from puddles in the road and damp clothes on a line. Plants also release water vapour through their leaves — this special form of evaporation is called transpiration.

2. Condensation — as water vapour rises into the cooler upper atmosphere, it cools down and changes back into tiny water droplets. These droplets gather around dust particles in the air to form clouds.

3. Precipitation — when the water droplets in clouds become big and heavy, they fall back to the ground. Precipitation can take several forms:

   • Rain — liquid water droplets (most common in Kenya).
   • Snow — frozen ice crystals (rare in Kenya; only seen on Mount Kenya).
   • Hail — large balls of ice (happens occasionally in the highlands).
   • Sleet — a mix of rain and snow.

4. Collection — water that falls onto the ground either:

   • Runs off into streams, rivers and eventually back to the ocean.
   • Soaks into the soil (infiltration), becoming groundwater that fills wells and underground aquifers.
   • Is absorbed by plants through their roots and later released back as transpiration.

Other important terms:

• Transpiration — water vapour released by plant leaves. Forests release huge amounts of water this way.
• Evapotranspiration — the combined effect of evaporation from soil + transpiration from plants.
• Infiltration — water soaking into the soil.
• Surface runoff — water flowing over the ground after rain.
• Groundwater — water stored beneath the surface in soil and rock.

Why the water cycle matters:

• It supplies fresh water to drink, to grow crops, to wash and to use in industry.
• It regulates climate — water vapour holds heat in the atmosphere; evaporation cools the Earth's surface.
• It shapes the land — rivers carve valleys; rain weathers rocks; floods deposit fertile soil on flood plains (like the Tana River delta).
• It forms ecosystems — wetlands, lakes, mangroves and rainforests all depend on consistent water cycle activity.

Human impact on the water cycle:

Human activities are changing the water cycle in three big ways:

• Deforestation reduces transpiration. With fewer trees, less water enters the atmosphere over land — leading to less rain. This is why areas around Mount Kenya, the Aberdares and the Mau Forest are protected as water catchment areas.
• Climate change is heating the atmosphere, causing more extreme rainfall, longer droughts and heavier floods. Kenya has seen this in recent years.
• Urbanisation covers soil with concrete, reducing infiltration and increasing runoff — making flooding worse in cities like Nairobi.

Common student mistakes to avoid:

• Calling water vapour "steam". Steam is hot water vapour you can see; water vapour from evaporation is invisible.
• Saying clouds are "made of smoke" or "made of gas". Clouds are made of tiny liquid water droplets (or ice crystals high up) — that is why they can fall as rain.
• Saying transpiration is the same as evaporation. Transpiration happens from PLANT LEAVES; evaporation happens from any wet surface.
• Confusing the order. The cycle goes evaporation → condensation → precipitation → collection → and BACK to evaporation. It is a cycle, not a one-way trip.

CBC Grade 4 introduces sources of water; Grade 5 covers the water cycle in detail (this lesson); Grade 6 extends to water conservation; Grade 7–9 Integrated Science deepens this into climate, weather, and the impact of pollution and climate change on the cycle — material that appears in KPSEA and KJSEA.