Water Resources

Why water is scarce, multi-purpose river projects and their problems, and traditional and rooftop rainwater harvesting.

Key ideas

Water can be scarce even where it is plentiful, because of over-use, pollution, unequal access and growing towns and industry.
Only a small part of the Earth’s water is fresh, and much of that is frozen or underground.
Multi-purpose projects serve irrigation, electricity, flood control and water supply together.
Large dams can disrupt rivers and sediment, submerge land and forests, displace people and cause water-sharing disputes.
Traditional harvesting includes guls and kuls in the Western Himalayas, inundation channels in Bengal, khadins and johads in Rajasthan, and tankas under houses.
Rooftop harvesting stores rain in tanks or lets it soak in to recharge groundwater; Meghalaya uses bamboo drip irrigation.
The Jal Jeevan Mission aims at piped drinking water for every rural home, and Atal Jal manages groundwater with village help.

Remember

Projects and rivers	Bhakra-Nangal: Sutlej–Beas; Hirakud: Mahanadi; Sardar Sarovar: Narmada
Rooftop water	1 mm of rain on 1 m² = 1 litre; litres = area × rainfall (mm) × share collected
Tankas	underground rain tanks of Bikaner, Phalodi and Barmer
Old water works	Bhopal lake: 11th century; Hauz Khas tank, Delhi: time of Allauddin Khilji

Watch out

Forgetting that only part of the rain is collected: multiply by the share given, such as 80%.
Mixing up projects and rivers: Hirakud is on the Mahanadi, Sardar Sarovar on the Narmada.
Thinking scarcity only means too little rain: over-use and pollution cause it too.

Worked example

A school roof of 130 square metres gets 700 mm of rain a year, and 80% can be collected (1 mm on 1 m² = 1 litre). (a) How many litres? (b) If the school uses 800 litres a day, how many full days will it last? (c) Give two benefits of rooftop harvesting.

Working: 130 × 700 × 80%; ÷ 800

Answer: (a) 72,800 litres (b) 91 days (c) It saves water, recharges groundwater and reduces flooding.