Hydrological cycle

Also called: Water cycle · Topic: Oceans: Hydrological Cycle, Relief of the Ocean Floor, Temperature and Salinity · NCERT: Class 6, Ch 2 "Oceans and Continents"; Class 7, Ch 3 "Climates of India"; Class 10, Ch 3 "Water Resources"; Class 11, Ch 10 "Water in the Atmosphere"; Class 11, Ch 12 "Water (Oceans)"

Meaning

The hydrological cycle (water cycle) is the endless movement of water between the oceans, the atmosphere, the land surface, the ground below the surface and living things (organisms). Water moves as a liquid (water), a solid (ice and snow) and a gas (water vapour). The total amount of water on Earth stays the same. It only moves from one store to another.

It matters because all life on land depends on it, and it has been running for billions of years. The amount of renewable fresh water it supplies is fixed, but people's demand for it keeps rising. This gap is the root of the water crisis.

Explanation

How the cycle works, step by step (NCERT Figure 12.1)

• The Sun heats the oceans, and water evaporates.

• Evaporation means liquid water turns into water vapour.

• Rising air cools, and the vapour condenses.

• Condensation means vapour cools and turns back into tiny liquid droplets.

• These droplets form clouds. Fog and dew also form this way.

• The droplets grow heavy and fall as precipitation.

• Precipitation is water falling from clouds as rain, snow, sleet or hail.

• On land, the water does one of three things:

• it flows over the land into streams (surface runoff),
• it soaks into the soil (infiltration) and refills groundwater,

• or it is stored as ice and snow, which melt later (snowmelt runoff).

• Rivers and groundwater carry the water back to the sea.

• Groundwater comes out through groundwater discharge and springs.

• Then the cycle starts again.

• Oceans supply most of the water vapour in the air. They give roughly 86% of all evaporation on Earth. The rest comes from land, lakes and plants.

Stores and their processes (NCERT Table 12.1)

Store (component)	Processes
Oceans	Evaporation, evapotranspiration, sublimation
Atmosphere	Condensation, precipitation
Ice and snow	Snowmelt runoff
Surface runoff	Streamflow, freshwater storage, infiltration
Groundwater	Groundwater discharge, springs

• Sublimation: ice turns straight into water vapour without melting into liquid first.

• It happens on dry, windy and sunny ice surfaces.

• Evapotranspiration: the total water lost to the air from two sources.

• Evaporation from soil and water surfaces.
• Transpiration: plants give off water vapour through tiny pores in their leaves (stomata).
• It tells us how much water a crop needs, so irrigation is planned using it.

How Earth's water is shared between stores

• About 71% of Earth's surface is water. Astronauts called Earth the "Blue Planet" after seeing it from space (NCERT Class 6).
• Total water on Earth is about 332.5 million cubic miles, or roughly 1,386 million km³ [2].

Store	Share
Oceans (salt water)	about 96.5% of all water [2] (NCERT: about 91%)
Fresh water (total)	about 2.5% of all water [2]
— Glaciers and ice caps	over 68% of fresh water [2]
— Groundwater	about 30% of fresh water [2]
— Surface water (lakes, rivers, soil moisture, air, living things)	about 1.2% of fresh water [2]

• Less than 1% of all water on Earth is fresh water that people can easily reach.
• Groundwater is the largest store of fresh water we can actually use.

What controls infiltration and runoff

• Infiltration and runoff work like a seesaw. When one goes down, the other goes up.
• Infiltration is controlled by:
• Soil type: water sinks quickly through sandy soil and slowly through clayey soil.
• Slope: on steep slopes, water runs off before it can sink in.
• Vegetation: roots and leaf litter slow the water and help it sink in.

• Rain intensity: very heavy rain falls faster than the soil can take it in.

• Runoff is high when the ground is:

• steep (water flows away fast),
• bare (no plants to slow it),
• paved (concrete and tar stop it soaking in),

• saturated (already full of water).

• High runoff causes damage.

• It brings flash floods and soil erosion.
• It fills rivers and reservoirs with silt.

Where it occurs

• Oceans: the main source of evaporation. The moisture in India's south-west monsoon comes mainly from the Indian Ocean, the Arabian Sea and the Bay of Bengal.
• Frozen stores of fresh water: mainly the Antarctic and Greenland ice sheets, and mountain glaciers such as those of the Himalaya.
• Sublimation: happens on high Himalayan glaciers and polar ice, where the surface is dry, windy and sunny.
• High evapotranspiration: hot, dry, windy places such as Rajasthan and the Punjab–Haryana plains in summer.
• Indian cities: paved ground cuts infiltration, so runoff rises and causes urban floods. The same cities also face water shortages because their groundwater is not refilled.
• Groundwater in India (Dynamic Ground Water Resource Assessment 2024, by CGWB and the States):
• Total yearly recharge was 446.90 BCM (billion cubic metres) [3].
• The amount that can be taken out each year is 406.19 BCM, and the amount actually taken out is 245.64 BCM [3].
• Recharge from tanks, ponds and water conservation structures rose from 13.98 BCM (2017) to 25.34 BCM (2024) [3].

Don't confuse with

• Hydration: this is chemical weathering, in which minerals in rock take in water and swell. It is NOT a process of the hydrological cycle.
• Sublimation vs evaporation: in sublimation, ice turns straight into vapour with no liquid stage. In evaporation, liquid water turns into vapour.
• Evapotranspiration vs evaporation: evapotranspiration = evaporation plus transpiration from plant leaves. Evaporation alone leaves out the plants.
• Water-stressed vs water-scarce: a country is water-stressed below 1,700 m³ per person per year and water-scarce below 1,000 m³ [4]. India, at 1,486 m³ in 2021, is water-stressed but not yet water-scarce [4].

Prelims Hooks

• Which of these is NOT part of the hydrological cycle? The answer is Hydration. Evaporation, sublimation, condensation, precipitation, infiltration and springs are all part of it.
• Store and process pairs (NCERT Table 12.1): Oceans – evaporation, evapotranspiration, sublimation. Atmosphere – condensation, precipitation. Ice and snow – snowmelt runoff. Groundwater – discharge, springs.
• Earth's water: oceans hold about 96.5% [2] (NCERT says about 91%). Fresh water is about 2.5% [2].
• Fresh water stores, from largest to smallest: glaciers and ice caps (over 68%) > groundwater (about 30%) > surface water and other stores (about 1.2%) [2].
• India's stage of groundwater extraction (2024) was 60.47% [3]. This is the share of extractable groundwater actually being used. Units in the "Safe" category rose from 62.6% (2017) to 73.4% (2024) [3].
• Infiltration falls and runoff rises on steep, bare, paved or saturated ground, and during very heavy rain.

Mains Points

• Why a water planet faces a water crisis (GS-I/III):
• About 71% of Earth's surface is water, but less than 1% of all water is fresh and easy to reach.
• Supply is fixed, while demand from population, irrigation, industry and cities keeps rising. Pollution from sewage, industrial waste and farm chemicals makes usable supply even smaller.

• India's water availability was 1,486 m³ per person in 2021, below the 1,700 m³ stress line [4].

• Urban floods show the cycle breaking down (GS-III, disasters):

• Concrete and tar cut infiltration, so surface runoff rises, drains overflow and cities flood.
• Groundwater is not refilled, so the same cities run short of water in summer.

• Responses include rainwater harvesting, permeable pavements, and reviving lakes and tanks.

• Groundwater governance and climate change (GS-III):

• Compared with 2017, recharge went up by 15 BCM and extraction went down by 3 BCM [3]. Recharge from conservation structures nearly doubled. This shows that community recharge works. Over-pumped regions still need to cut demand.
• Warmer air holds more moisture, so the cycle speeds up. This brings heavier bursts of rain, longer dry spells and faster melting of glaciers, which are the largest store of fresh water. Water security in Himalayan river basins is at risk.

Related concepts

• Distribution of the Earth's water
• Evapotranspiration
• Infiltration
• Surface runoff

Read more

• The blue planet: Earth's water and the hydrological cycle