Water beneath the ground

People draw water from wells and handpumps. That is water that has seeped into the ground. Activity 7.5 shows how.

Cut three 1 L plastic bottles in half, make a small hole in each cap, and keep the tops upside down like funnels. Fill one with clay, one with sand and one with gravel, with a beaker under each. Pour 200 mL of water into each and collect what comes through in 10 minutes.

Water seeps fastest through gravel, slower through sand, and slowest through clay. The gaps between gravel pieces are wide; the gaps between clay particles are tiny. Water passes more easily where the gaps are wider.

Surface water seeping down through soil and rocks drop by drop is called infiltration. It happens more easily where the spaces between particles are wide, open and connected.

The water that seeps down is stored in the tiny spaces of sediments and in openings in rocks underground, as groundwater. The underground layers that hold water in their spaces (and let it flow slowly) are called aquifers. We reach this water by digging wells or drilling bore wells, from a few metres to hundreds of metres deep.

Groundwater is not unlimited. A growing population pulls out more and more of it. Fewer plants and more concrete in cities mean less rain can soak in. So groundwater is running low. Rainwater harvesting and recharge pits help put water back.

In Ladakh, people have a clever answer to spring water shortage. In winter, water from mountain streams is piped down and sprayed into the freezing air. It freezes as it falls, building up layer by layer into a tall cone called an ice stupa. In spring it melts slowly and gives water for farming through the summer.

The book also introduces Varahamihira, a sixth-century astronomer and mathematician of Ujjaini (today's Ujjain). His book Brihatsamhita gave ways to predict seasonal rain from cloud shapes, wind patterns and the positions of the stars and the moon.