Water cycle - SS2 Biology Lesson Note

The water cycle, also known as the hydrological cycle, is the continuous movement of water on, above, and below the Earth's surface. It describes how water circulates through various stages and reservoirs, driven by solar energy and gravity. The water cycle consists of the following main processes:

 

1.       Evaporation: Solar energy heats bodies of water such as oceans, lakes, and rivers, causing water to change from a liquid to a gaseous state, forming water vapor. This process also occurs from moist soil and transpiration from plants.

2.       Condensation: As the water vapor rises into the atmosphere, it cools down and condenses into tiny water droplets or ice crystals, forming clouds. This occurs when the air temperature decreases or when the water vapor comes into contact with particles, such as dust or pollution, called condensation nuclei.

3.       Precipitation: When the water droplets in the clouds combine and grow larger, they eventually become too heavy to remain suspended in the air. As a result, precipitation occurs in the form of rain, snow, sleet, or hail, and falls to the Earth's surface.

4.       Runoff: Precipitation that falls on the land's surface flows downhill as runoff. It collects in streams, rivers, and other water bodies, eventually making its way back to the oceans. Runoff also replenishes groundwater, which is stored in underground aquifers.

5.       Infiltration: Some precipitation seeps into the ground, a process known as infiltration. This water can be absorbed by soil and plants or percolate through porous rocks to become groundwater.

6.       Transpiration: Plants absorb water through their roots and release it into the atmosphere through small openings in their leaves called stomata. This process is known as transpiration and is an essential component of the water cycle.

7.       Sublimation and Melting: In addition to evaporation, water can also change directly from a solid (ice or snow) to a gaseous state through sublimation, or from a solid to a liquid through melting.

The water cycle is a crucial natural process that ensures the availability of freshwater for various purposes, including drinking, irrigation, and sustaining ecosystems. It plays a vital role in regulating Earth's climate by transporting heat energy from the tropics to the polar regions. Human activities, such as deforestation, urbanization, and pollution, can impact the water cycle and have consequences on water availability and quality.