Water Harvesting at Kheri: Desilt the Johad or Sink a Recharge Shaft?
The gram panchayat of Kheri, a 620-household village on the edge of Jaipur district, must choose between two proposals under a ₹25 lakh state groundwater recharge grant. The village sits on weathered granite; observation wells show a fall of about 6 m over the last decade, and the block has been categorised 'over-exploited' in the latest dynamic groundwater assessment. The catchment above the existing masonry tank (johad) is 60 ha, mean annual rainfall is 480 mm, and the runoff coefficient estimated from neighbouring gauges is 0.35. The tank's designed gross storage is 45,000 m³, but a recent survey found it 30 per cent silted and the outlet sill damaged. Proposal P is desilting at ₹18 lakh. Proposal Q is a recharge shaft with a settling chamber below the waste weir at ₹6 lakh, estimated to recharge about 15,000 m³ a year. A single 90 mm storm in 2023 filled the tank in five hours, and the weir ran for two days.
1. Using the catchment figures, quantify the tank's effective storage and the volume that spills in an average year. What does the gap tell you about where the water is actually being lost?
Read the worked answer
Catchment yield
- Area = 60 ha = 6,00,000 m²; rainfall = 0.48 m.
- Gross rainfall volume = 6,00,000 × 0.48 = 2,88,000 m³.
- At runoff coefficient 0.35: 1,00,800 m³ of runoff per year.
Storage actually available
- Design capacity 45,000 m³, silted 30% → 31,500 m³ effective.
Spill
- 1,00,800 − 31,500 ≈ 69,300 m³ leaves the tank every year - roughly 69% of the yield.
The gap is not a rainfall problem; it is a storage-and-infiltration problem. Two different losses are being conflated: (i) capacity lost to silt, which is recoverable by desilting, and (ii) water that arrives faster than any tank of this size can hold, which only a recharge structure can convert into groundwater. The 2023 storm confirms (ii) - the tank filled in five hours and the weir ran for two days, so a large single-event volume bypasses storage entirely. Any proposal that treats desilting as a complete solution is answering only the smaller half of the question. Note the caveat: the 1,00,800 m³ figure ignores evaporation and upstream abstraction, so treat it as an upper bound, not a precise entitlement.
2. Within the ₹25 lakh ceiling, what should the committee fund, and on what reasoning? State two conditions that must be verified before a recharge shaft is approved.
Read the worked answer
Recommendation: fund both - P plus Q (₹18 lakh + ₹6 lakh = ₹24 lakh), leaving ₹1 lakh for monitoring. The two measures attack different losses and are complementary, not rival.
Cost-effectiveness comparison
- Desilting: ₹18,00,000 for 13,500 m³ of restored storage ≈ ₹133 per m³ of capacity.
- Recharge shaft: ₹6,00,000 for about 15,000 m³ recharged per year ≈ ₹40 per m³ - cheaper per unit, but it does not create any storage for direct use.
Because spill is 69,300 m³ and the shaft handles only 15,000 m³, the shaft is a first stage, not a full fix; the committee should record a phase-two shaft as a future proposal once the first year's monitoring data exist.
Two pre-conditions
- Aquifer condition - the shaft must be sited where the weathered granite is unconfined and permeable; a clay-rich horizon would simply pond water and waterlog the downstream plots.
- Water quality and sanitation - no soak pit, latrine or cattle shed within the recharge zone, and a settling chamber plus filter media must precede the shaft, otherwise turbid runoff carries silt and pathogens straight into the aquifer. Add post-monsoon water-level readings in three wells as a condition of the second instalment.