Maarg Manthan · Topic 6.1

The Hydrological Cycle: Components and Processes

Geography › Oceanography · Topic 6.1

The hydrological cycle is the continuous movement of water between the oceans, atmosphere and land. Water evaporates from the oceans, lakes and soil, and plants release vapour by transpiration. The vapour rises, cools and condenses into clouds, and falls as rain, snow or hail. On land it flows over the surface as runoff, soaks into the ground as infiltration, and is stored in rivers, lakes, glaciers and aquifers, before returning to the sea. The cycle is driven by solar energy and gravity, and has no beginning or end.

5 min read · UPSC Prelims · Geography · Oceanography

At a Glance

Driven by Solar energy and gravity
Main stores Oceans hold about 97 per cent of water
Fresh water About 2.5 to 3 per cent, mostly ice
Main source of vapour Oceans, about 86 per cent of evaporation
Return flow Runoff and groundwater to the sea
Exam link UPSC Prelims, Geography: Oceanography

Where Does It Fit?

This is the first topic of the macro on oceanography. It begins with the cycle that joins the oceans to the air and land, before the topics on the ocean floor, the properties of sea water and its movements. Previous topic: World Climate Types (Köppen Classification): Equatorial, Monsoon, Savanna, Desert, Mediterranean, Temperate and Polar Climates. Next topic: Relief of the Ocean Floor: Continental Shelf, Continental Slope, Deep Sea Plain, Oceanic Deep and Trenches.

What Are the Components and Processes of the Cycle?

In short: water evaporates, rises, condenses, falls and flows back, and the same water is used again and again.

Process	Meaning
Evaporation	Change of liquid water into vapour, mostly from the sea surface, driven by heat and wind
Transpiration	Release of water vapour from the leaves of plants; with evaporation from land, called evapotranspiration
Sublimation	Change of ice or snow directly into vapour
Condensation	Change of vapour into droplets, forming clouds, fog and dew; releases latent heat
Precipitation	Fall of water to the surface as rain, snow, sleet or hail, as in Types of Precipitation: Convectional, Orographic, Cyclonic, Frontal, Monsoonal
Interception	Water caught by leaves and buildings before reaching the ground
Infiltration and percolation	Entry of water into the soil, and its downward movement to the water table
Runoff	Flow of water over land into streams, rivers, lakes and seas
Groundwater flow	Slow movement of water underground, which feeds springs, wells and rivers

The cycle has a short loop between sea and air, in which most evaporation from the ocean falls back as rain on the ocean, and a long loop in which winds carry vapour to the land. About a tenth of the vapour from the oceans is carried over land, and returns through rivers and groundwater.

Where Is the Earth’s Water Stored?

Store	Share of the Earth’s water (approx.)	Typical time water stays
Oceans	About 97 per cent	Thousands of years
Ice caps, glaciers and permanent snow	About 1.7 to 2 per cent	Thousands to hundreds of thousands of years in ice sheets
Groundwater	About 0.6 to 0.7 per cent	Days in shallow soils to thousands of years in deep aquifers
Lakes, soil moisture, swamps and rivers	Less than 0.02 per cent	Days in rivers to years in large lakes
Atmosphere	About 0.001 per cent	About 9 days

Fresh water is only about 2.5 to 3 per cent of all water, and about two-thirds of that is frozen. The water in rivers and lakes that people use directly is a tiny fraction. The figures are rounded and vary a little between sources.

Why Is the Cycle Important, and How Do People Change It?

Heat transfer: evaporation takes up heat in the tropics and condensation releases it elsewhere, which moves energy through the atmosphere, as in Heat Balance of the Atmosphere: Conduction, Convection, Advection, Terrestrial Radiation, Albedo.
Fresh water: the cycle distils sea water and delivers it to land, so it supports farming, cities and ecosystems.
Landforms and soils: running water, groundwater and ice shape the land, as in Running Water Erosional Landforms: V-Shaped Valleys, Gorges, Canyons, Potholes, Incised Meanders, River Terraces.
Human changes: dams and canals store and divert runoff, irrigation increases evaporation, cities seal the ground and raise runoff, deforestation reduces transpiration, and pumping of groundwater faster than it is refilled drops the water table.
Climate change: a warmer atmosphere holds about 7 per cent more water vapour for each degree of warming, which intensifies the cycle, so heavy rain and floods become more frequent in some places, and droughts in others.

As an analytical point, the water of the Earth is almost fixed in amount, but its distribution in space and time is not, and water management is about handling this unevenness. This is the subject of Water Resources: Utilization, Water Contamination, Conservation Methods.

Why Is This Topic Important?

Basics Foundation of water resources
Numbers Distribution of water is asked
Climate Link with global warming

What Came Next?

The next topic describes the relief of the ocean floor: the continental shelf, slope, deep sea plain and trenches. Next topic: Relief of the Ocean Floor: Continental Shelf, Continental Slope, Deep Sea Plain, Oceanic Deep and Trenches.

Key People and Terms

Evapotranspiration Evaporation plus transpiration
Infiltration Entry of water into soil
Percolation Downward movement to the water table
Runoff Flow of water over land
Aquifer Rock that holds and yields groundwater
Water table Top of the saturated zone

Exam Corner

Points to Remember

Oceans hold about 97 per cent of the Earth’s water.
Most fresh water is locked in ice.
The cycle is driven by the Sun and gravity.
Water stays in the atmosphere only about 9 days.
Transpiration is vapour loss from plants.

Do Not Confuse With

Evaporation and transpiration From water surfaces versus from plants
Infiltration and percolation Entering the soil versus moving down to the water table
Runoff and groundwater flow Surface versus underground movement

Memory Hook

Sun lifts, cloud cools, rain falls, rivers run, and sea begins again.

Mains Angle

Questions link the cycle to water scarcity and climate change. These points add depth.

Balance Fixed water, uneven distribution
Human impact Dams, groundwater, land use
Climate Intensifying cycle

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

Cycle Processes and stores.
Loops Short and long.
Human changes Dams, groundwater.
Climate Intensified cycle.

GS Relevance

Prelims: components and processes of the water cycle, distribution of water, residence times. GS Paper 1: oceanography and water resources.

Frequently Asked Questions

What is the hydrological cycle?

The hydrological cycle is the continuous movement of water between the oceans, atmosphere and land. Water evaporates, condenses into clouds, falls as precipitation, and returns to the sea by runoff and groundwater flow. The Sun supplies the energy and gravity moves the water downhill.

What are the main processes of the water cycle?

The main processes are evaporation from water surfaces, transpiration from plants, condensation into clouds, precipitation as rain or snow, infiltration into the soil, percolation to the water table, and runoff into streams and rivers. Together they move water between the ocean, air and land.

How much of the Earth's water is fresh water?

Only about 2.5 to 3 per cent of the Earth's water is fresh, and about two-thirds of that is frozen in ice caps and glaciers. Groundwater is most of the remainder, while rivers, lakes and soil moisture make up a very small share of the total.

What is transpiration?

Transpiration is the loss of water vapour from the leaves of plants through tiny pores called stomata. Together with evaporation from soil and water surfaces, it is called evapotranspiration. Forests return a large amount of moisture to the air this way, which is why deforestation can reduce local rainfall.

How does climate change affect the water cycle?

A warmer atmosphere can hold about 7 per cent more water vapour for each degree of warming, so evaporation and the intensity of rainfall increase. This tends to bring heavier downpours and floods in some regions and longer, hotter droughts in others, while glaciers melt faster.

PYQ Practice — Statement Analysis

1 The hydrological cycle is driven by solar energy and gravity.

True

The Sun lifts water and gravity returns it.

2 Most of the Earth's water is fresh water.

False

About 97 per cent is salt water.

3 Transpiration is the release of water vapour by plants.