Surface water and groundwater
India gets about 4,000 km³ of rain and snow each year, but only part of it can be used. Surface water is in rivers, lakes, ponds and tanks. About 1,869 km³ flows in rivers, but only about 690 km³ can be used because of steep land, floods and uneven flow. The Ganga, Brahmaputra and Barak carry most of it.
Groundwater is rain that soaks into the soil and fills gaps in rocks. About 432 km³ can be recharged each year. It is heavily used in the north-west and in parts of the south, where water tables are falling fast. The Ganga–Brahmaputra plains still have large reserves.
Lagoons and backwaters
A lagoon is a body of shallow salty water cut off from the sea by a sand bar. The long coastline of India has many lagoons and lakes, especially in Kerala, Odisha and West Bengal. The water is brackish (a bit salty), so it is not good for drinking, but it is used for fishing, growing some paddy varieties and coconuts, and boat tourism in the Kerala backwaters.
Water demand and use
India is mainly a farming country, so most water goes to irrigation: about 89% of surface water and 92% of groundwater. Industry uses about 2% of surface water and 5% of groundwater; homes use about 9% and 3%. Demand from industry and cities is rising fast.
Why irrigation is needed
- Rain comes only in a few months.
- Some areas are dry.
- High-yield seeds need regular water.
- Irrigation allows more than one crop a year.
Over-pumping in Punjab, Haryana, western Uttar Pradesh and Rajasthan has made water tables drop, and in parts of Rajasthan and Maharashtra fluoride has increased; in West Bengal and Bihar arsenic has been found in groundwater.
Water quality and its deterioration
Water quality means how pure water is, without harmful dirt, germs or chemicals. Water becomes polluted when towns pour sewage, factories pour chemicals and farm fertilisers wash into rivers. Such water carries disease-causing germs and poisons fish. Rivers like the Ganga and Yamuna are badly polluted in stretches near big cities. Laws such as the Water (Prevention and Control of Pollution) Act 1974 and the Environment Protection Act 1986 exist, but they need strict use and public awareness.
Water conservation and management
Fresh water is limited, so we must conserve it. Ways: use efficient methods like drip and sprinkler irrigation, stop leaks, reduce use in homes and factories, and recycle and reuse treated waste water for flushing, gardens, cooling in factories and fire-fighting. Water bodies should be protected from pollution. In villages, people can rebuild old tanks and ponds.
Rainwater harvesting
Rainwater harvesting means catching rain where it falls and storing it or sending it into the ground. Roofs can be linked to tanks; pits, trenches and old wells can recharge groundwater. It is cheap, raises the water table, lowers fluoride and salt levels, reduces flooding in streets and gives water in dry months. Traditional systems include tanks in the south, johads and kunds in Rajasthan. Many cities now require rainwater harvesting in new buildings. Quick estimate: litres per year ≈ roof area (m²) × rain (mm) × 0.8.
Watershed management
A watershed is the land area from which rain drains into one stream or river. Watershed management saves and wisely uses surface and ground water in that area: building check dams and percolation tanks, planting trees, making contour bunds and stopping runoff. It also improves soil, farms, forests and people's income together.
Examples: Haryali, a central scheme that lets village panchayats manage watersheds; Neeru-Meeru (Water and You) in Andhra Pradesh; and Arvary Pani Sansad in Alwar, Rajasthan, where villagers built johads and made rules for sharing water. Community participation is the key to success.
Key formulas and definitions
- Usable water ≈ 690 km³ (surface) + 432 km³ (ground) ≈ 1,122 km³ per year
- Irrigation share: about 89% of surface water, 92% of groundwater
- Harvested rain (litres) ≈ roof area (m²) × rainfall (mm) × runoff factor (≈0.8)
- 1 mm of rain on 1 m² = 1 litre
- Lagoon: shallow salty water cut off from the sea by a sand bar
- Watershed: land draining into one stream
Worked examples
1. Find the total usable water if surface = 690 km³ and ground = 432 km³.
690 + 432 = 1,122 km³ per year.
2. A school roof is 100 m². The town gets 600 mm of rain. How much water can be harvested (factor 0.8)?
100 × 600 × 0.8 = 48,000 litres a year.
3. A family uses 400 litres a day. For how many days will 48,000 litres last?
48,000 ÷ 400 = 120 days.
4. Why is the water in Kerala's backwaters not used for drinking?
It is brackish (mixed with sea water), so it is too salty to drink. It is used for fishing, paddy, coconuts and boat tourism.
5. Why do water tables fall fastest in Punjab and Haryana?
Farmers grow water-hungry rice and wheat and pump groundwater with tube wells much faster than rain can refill it.
6. Explain how Arvary Pani Sansad saved water in Alwar.
Villagers built small earthen dams (johads) and formed a water parliament that set rules for sharing and using river water. The river started flowing again and wells filled up.
Common mistakes
- Saying all river water can be used. Only about 690 of 1,869 km³ is usable because of land shape and uneven flow.
- Thinking factories use the most water. Farming uses by far the most.
- Calling a lagoon a freshwater lake. Lagoon water is brackish.
- Thinking watershed management is only about building dams. It includes trees, soil, farms and people's participation.