The water cycle is the continuous movement of water between atmosphere, land and ocean. It is driven by solar energy and gravity. CBSE Class 9 and 11 cover it as part of biogeochemical cycles.
Core Concepts
Main processes
Evaporation from oceans, lakes and rivers. Transpiration from plants (together called evapotranspiration). Condensation into clouds. Precipitation as rain, snow or hail. Runoff over land. Infiltration into groundwater. Back to the ocean.
Let us trace a single water molecule through the complete cycle:
Solar energy heats the ocean surface. A water molecule gains enough kinetic energy to escape from the liquid surface into the atmosphere as vapour. About 86% of global evaporation comes from oceans.
Air currents carry the water vapour inland. The molecule may travel hundreds or thousands of kilometres from where it evaporated.
As the moist air rises (over mountains or by convection), it cools. Water vapour condenses on tiny dust or salt particles (condensation nuclei) to form cloud droplets. Millions of these droplets make a visible cloud.
Cloud droplets collide and merge until they are heavy enough to fall. The molecule returns to Earth as rain, snow, sleet, or hail, depending on temperature.
(a) Runoff: flows over the surface into streams and rivers, eventually back to the ocean. (b) Infiltration: seeps into the soil and becomes groundwater. (c) Absorption by plants: taken up by roots, used briefly in photosynthesis, then mostly transpired back to the atmosphere.
Oceans as the main reservoir
About 97% of Earth’s water is in oceans. Only 3% is freshwater, and most of that is locked in ice caps and glaciers. Available freshwater is less than 1% of total.
| Reservoir | Percentage | Accessibility |
|---|---|---|
| Oceans | 97.2% | Saline — not directly usable |
| Ice caps and glaciers | 2.15% | Frozen — largely inaccessible |
| Groundwater | 0.62% | Accessible but slow to recharge |
| Freshwater lakes | 0.009% | Readily accessible |
| Rivers | 0.0001% | Readily accessible |
| Atmosphere | 0.001% | In transit (as vapour/clouds) |
| Soil moisture | 0.005% | Available to plants |
The astonishing fact: all the rain, rivers, and lakes that sustain life on land come from less than 1% of Earth’s total water. This makes freshwater conservation not just important but essential.
Groundwater
Water that seeps into soil and pore spaces in rock. Aquifers store this water and supply wells and springs. Recharge takes years to centuries — groundwater can be mined faster than it is replenished.
Aquifer basics:
- Water table: The upper boundary of the saturated zone in soil/rock
- Aquifer: A rock or sediment layer that stores and transmits groundwater (sandstone, gravel)
- Aquitard: A layer that does not transmit water well (clay)
- Recharge zone: Where water enters the aquifer (usually at higher elevation)
- Discharge zone: Where water exits (springs, wells, rivers)
NEET and CBSE ask about groundwater depletion specifically in the Indian context. India is the world’s largest user of groundwater, extracting about 250 km3 per year. States like Punjab, Haryana, and Rajasthan are in critical condition.
Why Indian groundwater is at risk:
- 85% of India’s drinking water comes from groundwater
- Tube well irrigation has expanded massively since the Green Revolution
- Recharge depends on monsoon rainfall (limited to 4 months)
- Many aquifers are being extracted faster than recharge
- Water table dropping by 0.3-1 metre per year in many regions
Evapotranspiration
The combined loss of water from surfaces (evaporation) and plants (transpiration). A single large tree can transpire 400 litres per day. Forests significantly contribute to atmospheric moisture and local rainfall.
Transpiration’s role in the water cycle:
- About 10% of atmospheric moisture comes from plant transpiration
- Forests act as “biotic pumps” — they recycle moisture inland
- Amazon rainforest generates about 50% of its own rainfall through transpiration
- Deforesting the Amazon would reduce rainfall not just locally but across South America
Human impact
Deforestation reduces transpiration and increases runoff. Urbanisation reduces infiltration. Over-extraction of groundwater lowers water tables. Pollution contaminates rivers and aquifers. Climate change shifts rainfall patterns.
Detailed impacts:
| Human Activity | Effect on Water Cycle | Consequence |
|---|---|---|
| Deforestation | Less transpiration, more runoff | Floods, soil erosion, less local rainfall |
| Urbanisation | Concrete prevents infiltration | More surface runoff, less groundwater recharge |
| Dam construction | Alters river flow, creates reservoir | Downstream ecosystems disrupted, sedimentation |
| Groundwater pumping | Lowers water table | Wells go dry, land subsidence |
| Industrial pollution | Contaminates surface and groundwater | Unsafe drinking water, ecological damage |
| Climate change | Shifts rainfall patterns, increases evaporation | More extreme droughts and floods |
Water conservation
Rainwater harvesting, watershed management, drip irrigation, wastewater treatment and reuse. India has several traditional systems — stepwells in Gujarat, eris in Tamil Nadu.
Modern conservation strategies:
- Rainwater harvesting: Collecting roof runoff in tanks. Mandatory in some Indian cities. Can recharge groundwater or provide direct use.
- Drip irrigation: Delivers water directly to plant roots through tubes. Uses 30-50% less water than flood irrigation.
- Watershed management: Managing an entire drainage area as a unit — check dams, contour bunding, afforestation. Successful examples: Ralegan Siddhi (Maharashtra), Hiware Bazar.
- Wastewater recycling: Treating sewage for reuse in irrigation, industry, or groundwater recharge. Israel recycles 85% of its wastewater — India is below 10%.
| Method | Location | Scale | Impact |
|---|---|---|---|
| Rooftop harvesting | Urban (Chennai, Bengaluru) | Individual building | Reduces dependence on municipal supply |
| Johads (check dams) | Rajasthan (Alwar district) | Village | Recharged groundwater, revived rivers |
| Percolation tanks | Maharashtra | District | Raised water table 2-5 metres |
| Drip irrigation | Gujarat, Maharashtra | Farm | 30-50% water savings |
| Eris (tank systems) | Tamil Nadu | Region | Centuries-old, still functioning |
Worked Examples
Delayed monsoon means less evapotranspiration, less cloud cover, more direct sunlight and higher temperature. The water cycle is tightly coupled to temperature.
Trees transpire huge amounts of water that form clouds downwind. Cutting forests cuts the moisture supply and reduces local rainfall. Amazon deforestation is showing this effect in real time.
Water is neither created nor destroyed — it changes form (liquid, vapour, ice) and location (ocean, atmosphere, land, underground) but the total amount remains roughly constant. The same water molecules have been cycling for billions of years. The water you drink today may have been drunk by a dinosaur.
Chennai gets heavy rainfall during the northeast monsoon (Oct-Dec), but most water runs off into the sea because of poor drainage and concrete surfaces that prevent infiltration. During summer, the stored water runs out. The solution is not more rain but better water management — harvesting, storage, and recharge.
Common Mistakes
Saying the water cycle adds or removes water. It redistributes water; total amount is roughly constant.
Confusing evaporation and transpiration. Transpiration is specifically from plants.
Writing that groundwater is infinite. Many Indian aquifers are being mined faster than recharge.
Saying the water cycle works only during monsoon season. It operates year-round — evaporation, condensation, and precipitation happen continuously. Monsoon is just the most dramatic phase in India.
Assuming all precipitation becomes groundwater. Most precipitation runs off the surface into rivers. Only a fraction infiltrates into the ground. In urban areas with concrete surfaces, almost all precipitation becomes surface runoff.
Exam Weightage and Revision
The water cycle carries 2-3 marks in CBSE Class 9, usually as a diagram question. CBSE Class 11 tests it under biogeochemical cycles (3-5 marks). NEET occasionally asks about human impact on the water cycle in ecology questions.
| Question Type | CBSE Frequency | Marks |
|---|---|---|
| Labelled diagram of water cycle | Every year | 3 |
| Human impact on water cycle | Most years | 2-3 |
| Groundwater depletion | Most years | 2 |
| Water conservation methods | Most years | 2-3 |
| Difference between evaporation and transpiration | Occasional | 2 |
The guaranteed CBSE question: “Draw and label the water cycle showing evaporation, condensation, precipitation, runoff, and infiltration.” Practice this diagram with at least 5 labelled processes. Full marks require clear labels and arrows showing direction of water movement.
Practice Questions
Q1. What is the difference between evaporation and transpiration?
Evaporation is the physical process of water turning from liquid to vapour from any surface (ocean, lake, wet soil). It depends on temperature, humidity, and wind. Transpiration is the loss of water vapour specifically from plant surfaces (mainly through stomata in leaves). It is a biological process regulated by the plant (stomata opening/closing). Together, they are called evapotranspiration.
Q2. Why does urbanisation increase flood risk?
In natural landscapes, soil and vegetation absorb rainfall (infiltration) and release it slowly. In cities, concrete and asphalt cover the ground, preventing infiltration. Almost all rainfall becomes surface runoff, which flows rapidly into drains and rivers. The drainage system cannot handle the sudden volume, causing flooding. This is why cities like Mumbai and Chennai flood even with moderate rainfall.
Q3. A village in Rajasthan builds check dams (johads). How does this affect the local water cycle?
Check dams slow surface runoff and create temporary pools. This allows more water to infiltrate into the ground, recharging the aquifer. Over time, the water table rises, wells that had gone dry start yielding water again, and even seasonal streams may become perennial. The johad movement in Alwar district (led by Rajendra Singh) revived five rivers using this approach.
Q4. Why is the water cycle important for life on land?
The water cycle continuously redistributes freshwater from oceans (where 97% of water is) to land (where life needs it). Without the water cycle, there would be no rain, no rivers, no groundwater recharge, and no freshwater for drinking, agriculture, or ecosystems. All terrestrial life depends on this redistribution.
FAQs
How much water does a human need daily?
About 2-3 litres for drinking. But the total water footprint (including food production, industry, and sanitation) is about 3000-5000 litres per person per day. Most of this is “virtual water” — water used to grow food and manufacture products.
Is the total amount of water on Earth really constant?
Essentially yes. Water is neither created nor destroyed in the water cycle — it only changes phase and location. Tiny amounts are added from volcanic outgassing and tiny amounts are broken down by photolysis in the upper atmosphere, but these are negligible. The water cycling today is the same water that has been on Earth for billions of years.
What is a watershed?
A watershed (or catchment area) is the entire land area that drains into a particular river or lake. Managing a watershed as a single unit — planting trees on slopes, building check dams, preventing pollution at source — is more effective than managing individual water bodies in isolation.
Draw one diagram with all five main processes labelled. That is your one-page revision.
The water cycle is the engine behind rain, rivers and life on land. Every environmental question about water ultimately comes back to this cycle.