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

Detail

Why Earth is called the "Blue Planet"

• Water is rare in the solar system. No other known planet has large bodies of liquid water on its surface.
• Almost three-fourths (about 71%) of Earth's surface is covered by water. That is why Earth looks blue from space.

• Astronauts gave Earth the name "Blue Planet" after they saw it from space (NCERT Class 6).

• Total water on Earth: about 332.5 million cubic miles, which is roughly 1,386 million km³ [2].

The hydrological cycle (water cycle)

• Hydrological cycle: water keeps moving between the oceans, the atmosphere, the land surface, the ground below the surface, and living things (organisms).
• It moves in three forms: liquid (water), solid (ice and snow) and gas (water vapour).

• It has been running for billions of years. All life on land depends on it.

• The total amount of water stays the same. It only moves from one store to another.

• So the amount of renewable fresh water is fixed, but people's demand for it keeps growing.

• The cycle, step by step (NCERT Figure 12.1):

• The Sun heats the oceans → water evaporates and becomes vapour.
• Rising air cools → the vapour condenses into tiny droplets → clouds form.
• The droplets grow heavy → precipitation (rain, snow or hail).
• On land the water does one of three things:
  • it runs off into streams (surface runoff),
  • it soaks into the soil (infiltration) and refills groundwater,
  • or it is stored as ice and snow and later melts (snowmelt runoff).
• Rivers and groundwater (through discharge and springs) carry the water back to the oceans, and the cycle starts again.

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 first melting into liquid.

• It happens on dry, windy and sunny ice surfaces, such as high Himalayan glaciers and polar ice.

• Condensation: water vapour cools and turns back into liquid droplets. Clouds, fog and dew form this way.

• Precipitation: water falls from clouds to Earth as rain, snow, sleet or hail.
• Exam trap: "Hydration" is NOT a process of the hydrological cycle. Hydration is a kind of chemical weathering, in which minerals in rock take in water and swell.

How Earth's water is shared out

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] (about 69%)
— Groundwater	about 30% of fresh water [2]
— Surface water (lakes, rivers, soil moisture, air, living things)	only about 1.2% of fresh water [2]

• Key point: less than 1% of all water on Earth is fresh water that people can easily reach.
• Most fresh water is frozen. It lies mainly in the Antarctic and Greenland ice sheets and in mountain glaciers, such as those of the Himalaya.
• Groundwater is the largest store of fresh water we can actually use. For this reason it matters most to India.

Key process terms

Evapotranspiration

• Evapotranspiration: the total water lost to the air from two sources.
• Evaporation: water from soil and from water surfaces turns into vapour.

• Transpiration: plants give off water vapour through tiny pores in their leaves (stomata).

• Why it matters: it tells us how much water a crop needs. Irrigation is planned on this basis.

• It is highest in hot, dry, windy places, such as Rajasthan and the Punjab–Haryana plains in summer.

Infiltration

• Infiltration: water soaks down into the soil and recharges (refills) groundwater.
• What controls it:
• Soil type: water sinks in quickly through sandy soil and slowly through clayey soil.
• Slope: steep slopes let water run off before it can sink in.
• Vegetation: roots and leaf litter slow the water down and help it sink in.

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

• Paved ground cuts infiltration. Concrete and tar stop water from sinking in → more water runs off → urban floods get worse, and groundwater is not refilled.

Surface runoff

• Surface runoff: the part of rain or snowmelt that flows over the land into streams and rivers.
• Runoff is high when the ground is:
• steep (water flows away fast),
• bare (no plants to slow it),
• paved (it cannot soak in),

• saturated (already full of water, so it cannot take in more).

• Result: high runoff brings flash floods, soil erosion and silt in rivers and reservoirs.

Where the moisture in the air comes from

• Oceans supply most of the water vapour in the air. They provide roughly 86% of all evaporation on Earth. The rest comes from land, lakes and plants.
• For example, the moisture in India's south-west monsoon comes mainly from the Indian Ocean, the Arabian Sea and the Bay of Bengal.

Why a "water planet" faces a water crisis

• Fresh water is fixed, but demand keeps rising.

• Population, irrigation, industry and cities all use more water every year.

• Pollution makes the usable supply smaller. Sewage, industrial waste and farm chemicals spoil rivers and groundwater.

• India's water stress (official figures):
• India's average yearly water availability per person was 1,486 m³ in 2021. This figure comes from the Central Water Commission's study "Reassessment of Water Availability in India using Space Inputs, 2019" [4].

• A country is water-stressed if each person gets less than 1,700 m³ a year. It faces water scarcity below 1,000 m³ [4]. India is already water-stressed.

• Groundwater in India (Dynamic Ground Water Resource Assessment 2024, by CGWB and the States):

• Total yearly groundwater recharge: 446.90 BCM (billion cubic metres). Water that can be taken out each year: 406.19 BCM. Water actually taken out each year: 245.64 BCM [3].
• Stage of groundwater extraction: 60.47% for the whole country [3]. This is the share of extractable groundwater that is actually being used.
• Compared with 2017, recharge went up by 15 BCM and extraction went down by 3 BCM [3].
• Recharge from tanks, ponds and water conservation structures rose from 13.98 BCM (2017) to 25.34 BCM (2024) [3]. This shows that helping infiltration works.
• Assessment units in the "Safe" category rose from 62.6% (2017) to 73.4% (2024) [3].

Prelims Hooks

• Hydration is NOT part of the hydrological cycle. 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.
• Sublimation = ice changing straight into vapour, with no liquid stage.
• Earth's water: oceans about 96.5% (USGS; NCERT says 91%). Fresh water about 2.5%.
• Fresh water, from largest share to smallest: glaciers and ice caps (about 68–69%) > groundwater (about 30%) > surface water and other stores (about 1.2%).
• Water-stress thresholds: below 1,700 m³ per person per year = water-stressed. Below 1,000 m³ = water-scarce. India was at 1,486 m³ in 2021 (CWC).
• Stage of groundwater extraction in India (2024): 60.47%. Annual recharge was 446.90 BCM.
• Infiltration falls on steep, paved or saturated ground and during very heavy rain. Runoff rises under the same conditions.
• Evapotranspiration = evaporation + transpiration. It is used to work out how much water crops need.

Mains Points

• Paradox of the water planet (GS-I/III): about 71% of Earth's surface is water, but less than 1% of all water is fresh and easy to reach. Fixed supply, rising demand and pollution make water scarce. India, at 1,486 m³ per person in 2021, is already water-stressed (CWC, 2021).
• City floods are a breakdown of the water cycle: concrete and tar cut infiltration → surface runoff rises → drains overflow → floods. Groundwater is not refilled either, so the same cities face shortages in summer. Responses: rainwater harvesting, permeable pavements, and reviving lakes and tanks.
• Groundwater governance: groundwater is about 30% of the world's fresh water and India's main buffer for irrigation. The 2024 assessment shows extraction at 60.47%, more recharge from water conservation structures (13.98 → 25.34 BCM since 2017), and more "Safe" units (73.4%). This supports community recharge programmes, but over-pumped regions still need demand-side reform.
• Climate link: warmer air holds more moisture, so the cycle speeds up. This brings heavier rain bursts and longer dry spells, and glaciers, the largest store of fresh water, melt faster. Water security for Himalayan river basins is at risk.