How much water can we use?
About three-fourths of Earth is covered with water, but about 96.5% is salty ocean water. Only about 2.5% is fresh, and most of that is ice and deep groundwater. So very little water is easy to use. It is renewed by the water cycle, but only if we do not waste or pollute it.
Water scarcity
Water scarcity means there is not enough good water for everyone's needs. Causes:
- More people need more water for homes and food.
- Farming uses the most water; wells pump out too much groundwater.
- Industries and cities use lots of water and electricity (from hydel plants).
- Pollution: even where there is water, it may be unsafe because of sewage, farm chemicals and factory waste.
Multipurpose river projects
A multipurpose project is a dam that is used for many things at once: irrigation, electricity, water for homes and industry, flood control, fish farming, boating and tourism. Jawaharlal Nehru called dams the "temples of modern India". Examples: Bhakra Nangal (Sutlej), Hirakud (Mahanadi), Sardar Sarovar (Narmada), Nagarjuna Sagar (Krishna).
Old India also had water works: a tank at Sringaverapura near Prayagraj, Bhopal lake, and Hauz Khas in Delhi.
Problems of big dams
- Rivers stop flowing naturally; silt collects in the reservoir instead of making the land fertile.
- Fish and river animals cannot move freely.
- Large areas of forest and villages are flooded; many people are displaced.
- Irrigation changed crops to water-heavy ones, causing salty soil.
- Sometimes dams even cause floods when too much water is let out.
This led to movements like the Narmada Bachao Andolan and the Tehri Dam Andolan.
Rainwater harvesting
- Guls and kuls: channels in the western Himalayas to take water to fields.
- Rooftop harvesting: in Rajasthan, rain collected in underground tankas for drinking.
- Khadins and johads: rain collected on farmland in Rajasthan to keep the soil moist.
- Inundation channels in West Bengal floodplains.
- Bamboo drip irrigation in Meghalaya: bamboo pipes carry spring water down the hill, drop by drop.
- Gendathur village (Karnataka) and Tamil Nadu (compulsory rooftop harvesting) are modern examples.
Try it
Use the last 3D step. A roof of 50 m² with 600 mm rain gives 50 × 600 = 30,000 litres in a year. If a family uses 100 litres of drinking water a day, how many days is that? (300 days.) Try your own school roof size.
Key formulas and definitions
- Water collected (litres) = roof area (m²) × rainfall (mm)
- Usable fresh water is only about 1% of all water
- Multipurpose project = irrigation + power + flood control + water supply + fish + tourism
Worked examples
1. A roof is 80 m² and the yearly rain is 500 mm. How much water can be harvested?
80 × 500 = 40,000 litres.
2. A school roof of 200 m² gets 1,200 mm rain. If 80% is collected, how many litres?
200 × 1,200 = 2,40,000 L; 80% = 1,92,000 litres.
3. A city near a river still faces water shortage. Why?
The river water may be polluted by sewage and factories, so it is not safe to use. Scarcity is about good-quality water too.
Common mistakes
- Thinking water scarcity happens only in deserts. Cities with rivers also face it due to pollution and overuse.
- Saying dams have no bad effects. They displace people, trap silt and harm fish.
- Mixing tanka (Rajasthan roof tank) and kul (Himalayan channel).
- Forgetting units: m² × mm gives litres, not cubic metres.