The Borewell That Ran Dry
A high school in a drought-prone block of Balangir district has 620 students and one borewell. In 2019 the borewell yielded about 4,000 litres a day; in May this year it yielded 1,200 litres a day. The hand pump is the only drinking-water source for the school and for 40 nearby households. The headmaster has three proposals: (a) deepen the borewell at ₹1.5 lakh; (b) build rooftop rainwater harvesting on the 300 m² school roof at ₹2.2 lakh; (c) buy tanker water at ₹3,000 per 5,000-litre load, two loads a week through the eight-week summer. Local rainfall is about 700 mm a year, and about 80% of rain falling on a roof can be harvested. The school also has a small garden watered from the borewell. The village water committee wants a decision before the summer vacation.
1. Calculate the potential annual rainwater harvest from the school roof and compare it with the tanker volume and with the borewell's dry-season yield. Show the calculation and units.
Read the worked answer
Harvest = roof area × rainfall × runoff coefficient. = 300 m² × 0.700 m × 0.80 = 168 m³ = 1,68,000 litres per year.
Tanker: 2 loads/week × 8 weeks = 16 loads; 16 × 5,000 L = 80,000 L; cost = 16 × ₹3,000 = ₹48,000.
Borewell at May yield: 1,200 L/day × 56 days = 67,200 L (72,000 L for 60 days).
So the roof can theoretically capture about 2.1 times the tanker volume and about 2.5 times the borewell's yield over the eight-week summer. Even if only half the roof is connected, the harvest is still about 84,000 L, slightly more than the tanker supply. The calculation assumes all rain is captured; in practice some will overflow, so a recharge pit is needed.
2. Evaluate the three options on cost, sustainability and equity, and recommend one with conditions.
Read the worked answer
Deepening (₹1.5 lakh): one-time and relatively cheap per year, but it does not create water; it may lower the water table further, dry neighbouring wells, and fail if the aquifer is depleted. It treats the symptom.
Rooftop harvesting (₹2.2 lakh): higher capital cost but annual yield about 1.68 lakh L; over 10 years the cost is roughly ₹0.13 per litre, and it reduces runoff and can recharge groundwater through an overflow pit. It is sustainable if maintained.
Tanker (₹48,000 per summer): recurring, creates no asset, and is vulnerable to price and supply; it is the most expensive per litre over a few years.
Recommendation: rooftop harvesting plus a recharge pit and demand management (taps, leak repair, staggered use), with a maintenance fund and a written agreement with the 40 households. Deepen the borewell only if monitoring shows the water table has not fallen further. Equity matters because the school pump serves households; overflow recharge should be directed to a community well.
3. What three measurements would you record over the next monsoon to test whether the plan works?
Read the worked answer
- Rainfall and harvested volume: daily rain gauge readings, tank storage and overflow volume, so the actual harvest can be compared with the calculated 1.68 lakh L.
- Groundwater response: water level in a nearby observation well before and after the monsoon, and the borewell's yield in the following May. If the yield stays at or above 1,200 L/day and the observation-well level rises, recharge is working.
- Water quality and use: coliform and turbidity tests of stored water, plus a log of daily demand and tanker purchases. A fall in tanker use is the practical test of success. Record all three in a register kept by the water committee.