Water Cycle (Hydrological Cycle)
Water is essential for all forms of life — no organism can survive without it. Precipitation (rain, snow, dew, slush, etc.) is the primary source of water on Earth. The water received from the atmosphere returns to it as water vapour, through evaporation and evapotranspiration.
This continuous movement of water within the biosphere is known as the Water Cycle or Hydrological Cycle.
Significance of Water on Earth
- Earth is called the watery planet, with about two-thirds of its surface covered with water.
- However, only a very small fraction of this water is available for animals and plants.
- Water is unevenly distributed across the planet:
- Nearly 95% of Earth’s water is locked chemically in rocks and does not participate in the cycle.
- Of the remaining 5%, about:
- 3% is in the oceans (saline water).
- 1% exists as polar ice caps.
Only about 0.6% is available as fresh water in the form of ground water, soil water, and atmospheric vapour.
Driving Forces of the Water Cycle
The water cycle is powered by two main forces:
- Solar radiation — provides the energy for evaporation and transpiration
- Gravity — causes precipitation and runoff to return water to oceans and lower levels.
Key Processes of the Water Cycle
Evaporation
- Water from oceans, lakes, rivers, ponds, and streams evaporates into the atmosphere due to the heat of the Sun.
- Plants also contribute through transpiration, releasing significant amounts of water vapour (collectively called evapotranspiration).
Cloud Formation
- Water vapour rises, cools, and condenses in the atmosphere to form clouds.
- Clouds drift with winds and may travel to different regions.
Precipitation
- When clouds encounter cooler air (especially over mountains and forests), water condenses further and falls as rain, snow, or dew due to gravity.
Water Balance on Earth
o 84% of water evaporates from ocean surfaces. o Oceans regain 77% of water through precipitation. o The remaining 7% is supplied by runoff from land via rivers to oceans, balancing the ocean’s evaporation deficit.
o Evaporation accounts for 16%. o Precipitation over land is 23%. |
