Maarg Manthan · Topic 5.4

Hydrological (Water) Cycle: Steps and Storage

Environment & Ecology › Nutrient Cycling (Biogeochemical Cycles) · Topic 5.4

The hydrological or water cycle is the continuous circulation of water between the oceans, atmosphere, land and living things, driven by solar energy and gravity. Water evaporates from oceans, lakes and soil and is released by plants as transpiration, together called evapotranspiration. The vapour rises, cools and condenses into clouds, then returns to the surface as precipitation (rain, snow, sleet or hail). On land it runs off into streams and rivers, infiltrates into soil and becomes groundwater, or is stored in ice, lakes and glaciers, and finally flows back to the sea. About 97 percent of Earth's water is salty ocean water, and only about 3 percent is fresh, most of it locked in ice caps and glaciers, so usable freshwater is a tiny fraction.

5 min read · UPSC Prelims · Environment & Ecology · Nutrient Cycling (Biogeochemical Cycles)

At a Glance

Driver Sun and gravity
Ocean share About 97 percent
Fresh water About 3 percent
Most fresh water In ice and glaciers
Air residence About nine days
Exam link UPSC Prelims, Environment & Ecology: Nutrient Cycling (Biogeochemical Cycles)

Where Does It Fit?

This topic follows the carbon cycle topics. It covers the water cycle, which carries nutrients and energy across the planet, and prepares for the nitrogen cycle. Previous topic: Human Impact on the Carbon Cycle and Effects of High Greenhouse Gas Concentration. Next topic: Nitrogen Cycle and Human Impact on It.

What Are the Steps of the Water Cycle?

Step	What happens
Evaporation	Liquid water becomes vapour from oceans, lakes, rivers and soil, driven by solar heat. Oceans give about 86 percent of global evaporation
Transpiration	Plants take up water through roots and release vapour through leaf pores called stomata. With evaporation it forms evapotranspiration
Sublimation	Ice and snow turn directly into vapour
Condensation	Rising vapour cools and turns into droplets on dust or salt particles, forming clouds
Precipitation	Rain, snow, sleet or hail fall from clouds. About 78 percent falls on oceans
Run-off and infiltration	Water flows over land into rivers and the sea, or soaks into soil and recharges groundwater
Groundwater flow	Slow movement of underground water that may emerge in springs or rivers (base flow)

Where Is Water Stored and How Do People Affect the Cycle?

Oceans: about 97 percent of water
Ice caps and glaciers: about 69 percent of fresh water
Groundwater: about 30 percent of fresh water
Surface water: lakes, rivers and wetlands, a very small share
Atmosphere: a small share, but vital for weather
Biosphere: water held in living things
Deforestation: reduces transpiration and rainfall recycling, and increases run-off
Urbanisation and dams: paved surfaces cut infiltration, groundwater is over-pumped and dams alter flow

What Else Should You Remember?

Residence time: about nine days in the air, weeks to months in rivers, years in lakes, decades to centuries in groundwater and thousands of years in ice and oceans
Water table: the upper level of saturated ground
Aquifer: a rock layer that stores and yields water
Recharge: refilling of aquifers through infiltration
Interception: rainfall held on leaves before reaching the ground
Throughfall and stemflow: water dripping from leaves and flowing down trunks
Overland flow: run-off across the surface
Watershed: area draining to a river or outlet
Humidity: vapour in the air
Latent heat: energy absorbed in evaporation and released in condensation, driving storms
Evapotranspiration: combined loss from soil, water and plants
Transpiration share: plants return a large share of land rainfall to the air, especially in the Amazon
Fresh water in India: India has about 4 percent of the world’s fresh water for about 17 percent of its population
Groundwater use: India is the largest user of groundwater; check the latest Central Ground Water Board assessment
Monsoon: provides most of India’s annual rain, in the June to September season
Climate change: warmer air holds more water, so rain gets more intense but less even
Glacier melt: Himalayan glaciers feed rivers, so melting changes flows
Cloud seeding: artificial rain by adding particles to clouds
Rainwater harvesting: capturing rain to recharge groundwater
Watershed management: soil and water conservation across catchments
Water footprint: water used to make goods and services
Virtual water: water embedded in traded goods
Link to previous: see Human Impact on the Carbon Cycle and Effects of High Greenhouse Gas Concentration
Link to water resources: see Water Resources and Water Pollution macro
Link to wetlands: see Coastal and Wetland Ecosystems macro
Caution: the water cycle is open to the atmosphere and has no true single start
Water use and cycle: irrigation moves groundwater into evaporation, adding moisture to the air
Hydrology instruments: rain gauges, river gauges and satellites measure the cycle
Exam hint: learn the percentages and that evaporation is mostly from oceans

Why Does It Matter for the Exam?

Steps: evaporation to run-off
Storage: ocean, ice, groundwater
Percent: 97 and 3
Current affairs link: Atal Bhujal Yojana and groundwater
Common errors: saying most fresh water is in rivers or lakes

As an analytical point, the water cycle ties climate, forests and human use together. Because usable fresh water is so scarce, protecting recharge areas and using water efficiently matter more than ever.

Why Is This Topic Important?

Steps Evaporate, condense, fall
Storage Oceans, ice
Impact Forests, dams

What Came Next?

The next topic covers the nitrogen cycle and human impact on it. Next topic: Nitrogen Cycle and Human Impact on It.

Key People and Terms

Evapotranspiration Evaporation plus transpiration
Infiltration Water entering soil
Aquifer Water-bearing rock
Water table Top of saturated zone
Run-off Surface flow
Watershed Drainage basin

Exam Corner

Points to Remember

Oceans hold 97 percent.
Fresh water is 3 percent.
Most fresh water is in ice.
Oceans give most evaporation.
Plants add transpiration.

Do Not Confuse With

Evaporation and transpiration From water surfaces versus from plants
Infiltration and run-off Soaking in versus flowing over
Water table and aquifer Level versus rock store

Memory Hook

Sun lifts, cloud cools, rain drops, rivers return; ocean owns it all, ice owns the fresh.

Mains Angle

Questions ask for steps and storage. These points help.

Steps Cycle
Storage Pools
Impact Human

A Question You May Face

An original practice question, not a past paper question.

“Describe the hydrological cycle and discuss how human activities and climate change are altering it.”

How to Answer

Steps Cycle.
Storage Pools.
Human Dams, forests.
Climate Intensity.

GS Relevance

Prelims: steps, storage, percentages, residence time. GS Paper 1: physical geography; GS Paper 3: water resources.

Frequently Asked Questions

What is the hydrological cycle?

The hydrological cycle, or water cycle, is the continuous movement of water between oceans, atmosphere, land and living things. Water evaporates, condenses into clouds, falls as precipitation, and returns to the sea by rivers and groundwater. Solar energy and gravity drive it, and it redistributes heat and fresh water around the globe.

What is transpiration?

Transpiration is the loss of water vapour from plants, mainly through tiny pores called stomata on leaves. Roots draw water from the soil, and most of it passes through the plant and evaporates. Together with evaporation from soil and water surfaces it is called evapotranspiration, and it contributes much of the moisture in forest regions.

Where is most of the world's fresh water stored?

About 97 percent of Earth's water is salty, and only about 3 percent is fresh. Of this fresh water, roughly two thirds is locked in ice caps and glaciers and nearly a third is groundwater. Rivers, lakes and the atmosphere hold only a tiny fraction, so easily usable water is very scarce.

What is the difference between infiltration and run-off?

Infiltration is the process by which rain soaks into the soil and moves down to recharge groundwater, while run-off is the water that flows over the surface to streams and rivers. Forests and porous soils increase infiltration, while paved surfaces and steep slopes increase run-off and floods.

How does deforestation affect the water cycle?

Deforestation reduces transpiration and interception, which can lower local rainfall, and it cuts infiltration so that run-off, soil erosion and floods increase while groundwater recharge and dry-season flow fall. Large-scale loss, as in the Amazon, may also change regional rainfall patterns.

PYQ Practice — Statement Analysis

1 Oceans hold about 97 percent of Earth's water.

True

Mostly saline.

2 Most fresh water is stored in rivers and lakes.

False

Mostly in ice caps and glaciers.

3 Transpiration returns water from plants to the air.