Nursery Water Budget Under a Three-Hour Borewell Window
You manage a 1.8-hectare seedling nursery attached to a social forestry range. The nursery holds 4,800 polybag seedlings. In summer each seedling needs 1.5 litres of water a day. The overhead tank holds 12,000 litres, but it is open and loses about 8 per cent of its stored water each day to evaporation before use. The drip system delivers 2,400 litres per hour. Power is available only from 06:00 to 09:00 and from 18:00 to 20:00. A borewell can supply 1,200 litres per hour for up to five hours, but the same pump supplies the neighbouring village, which has priority for two hours each morning. A tanker delivers 6,000 litres for ₹900. Tomorrow the tank is empty, and the borewell can run for only three hours because of a motor repair. The Range Officer wants a written water plan by this evening and asks whether one tanker is enough.
1. Calculate the daily water requirement after evaporation loss. How many tanker loads are needed if the borewell runs for only three hours?
Read the worked answer
Seedling demand = 4,800 × 1.5 = 7,200 litres/day.
Because the tank loses 8 per cent before use, the stored water must be more than the demand. Required storage ≈ 7,200 ÷ 0.92 = 7,826 litres (about 7,830 L).
Borewell supply in 3 hours = 3 × 1,200 = 3,600 litres.
Remaining need = 7,826 − 3,600 = 4,226 litres.
One tanker gives 6,000 litres, so one tanker is enough, with about 1,774 litres left over. Total supply would be 9,600 litres, which is below the 12,000-litre tank capacity. Two tankers are not needed unless the borewell fails completely or the tanker arrives late.
2. Set out a practical irrigation and filling schedule for tomorrow using the available power hours. What is the main operational risk?
Read the worked answer
Drip runtime needed = 7,826 ÷ 2,400 = 3.26 hours, i.e. about 3 hours 16 minutes.
A workable schedule:
- Before 06:00: tanker discharges 6,000 litres into the tank.
- 06:00–09:00: borewell adds 3,600 litres; drip runs for 3 hours and uses 7,200 litres. Net tank position remains workable if filling and irrigation are managed together.
- 18:00–18:16: drip runs for the remaining 16 minutes and uses about 640 litres.
A safer version is to run the drip for 3 hours in the morning and 20 minutes in the evening, leaving a small buffer. The main risk is the tanker arriving after 09:00: the morning block would then have only 3,600 litres from the borewell and could not deliver 7,200 litres. The plan must make the tanker arrival a precondition, with a named person checking the tank level at 05:30.
3. The Range Officer asks for two low-cost measures that could cut the nursery demand by 10 per cent. Quantify the saving and explain the limitation.
Read the worked answer
A 10 per cent cut means saving 0.10 × 7,826 ≈ 783 litres/day, bringing the requirement to about 7,043 litres/day before loss.
Two low-cost measures:
- Mulch each polybag with paddy straw or leaf litter. This reduces surface evaporation from the bag by roughly 15–20 per cent and also suppresses weeds.
- Use 50 per cent shade net over the beds during peak summer and group bags more closely, while keeping paths clear. This lowers direct sun and wind drying.
A third no-cost step is to repair leaking drip joints and avoid watering paths. The limitation is that mulch and shade reduce evaporation, not the plant's transpiration need. They cannot replace the basic 1.5 litres per seedling if the plants are already stressed. The saving must be verified by checking bag weight or soil moisture, not assumed from the design.