Water cycle and groundwater

The Sun sets water moving

Wet clothes dry faster on a sunny day, because the Sun’s heat speeds up evaporation. The same heat evaporates water from oceans, rivers and lakes all the time. Water exists in three states in nature: liquid in oceans and rivers, solid as snow and ice in the mountains and polar regions, and gas as water vapour in the air. In summer, some snow and ice melts and flows down as rivers; fresh snow in winter makes up for it.

The water cycle

• Evaporation: water in oceans, rivers and lakes is heated by the Sun and rises as water vapour.
• Transpiration: trees and plants also give off water vapour.
• Condensation: rising vapour cools and turns into tiny drops that form clouds.
• Precipitation: clouds bring the water down as rain, snow or hail.
• Back again: rainwater flows into ponds, rivers and oceans, or seeps into the ground.

This continuous movement is the water cycle. It carries water from the oceans to the land and back, refilling rivers, lakes and the ground, while the total amount of water on the Earth stays the same. Long ago, in the 6th century CE, the astronomer and mathematician Varahamihira of Ujjaini described ways to predict seasonal rainfall in his book Brihatsamhita, using clouds, winds, the stars and the moon.

How water seeps underground

Activity 7.5 compares three cut, upside-down plastic bottles filled with clay, sand and gravel. The same amount of water is poured into each, and the water that drips out in 10 minutes is collected.

Bottle	How fast water seeps
Gravel	fast
Sand	slower
Clay	slowest

Gravel pieces leave wide, open, connected spaces, so water passes easily; clay particles leave only tiny gaps. This seeping of surface water through soil and rocks is infiltration.

Groundwater and aquifers

The water that seeps down is stored in the spaces between sediments and in the cracks and openings of rocks. This is groundwater. The underground layers that hold it are aquifers. Wells, hand pumps and bore wells reach down into aquifers, which can lie a few metres or hundreds of metres below the ground.

Saving groundwater

Groundwater is not unlimited. A growing population uses more and more of it, fewer trees and more concrete surfaces in towns let less rainwater soak in, and so groundwater levels are falling. Rainwater harvesting collects rainwater instead of letting it run off, and recharge pits let it soak back into the ground.

In Ladakh, streams often dry up in spring, when the Sun is not yet strong enough to melt the mountain snow. In winter, people pipe stream water down and spray it into the freezing air, where it freezes layer by layer into a tall cone called an ice stupa. It melts slowly in spring and gives water for farming through the summer.

Why it matters

The water cycle is the reason rain keeps falling and wells do not run dry for good. Knowing how water moves, and how slowly the ground refills, helps us use water carefully, whether by harvesting rain at school or by not wasting it at home.

Read how heat moves through solids, liquids and gases in conduction, convection and radiation. Answers to the textbook questions are in the NCERT solutions for Chapter 7; see all of Chapter 7 notes.