Sizing a farm pond in a 45 ha micro-watershed
A village watershed committee has 45 ha of catchment above a natural depression where a farm pond is proposed. Mean annual rainfall is 720 mm, 80% between mid-June and September; the catchment has 12 ha of cultivated land, 25 ha of degraded pasture and 8 ha of scrub, and a runoff coefficient of 0.30 is assumed for the whole area. The pond is designed with a top water spread of 40 m × 25 m at full supply level, a maximum water depth of 3.0 m and 1:1 side slopes. The committee wants supplemental irrigation for 2 ha of wheat at 5 cm per irrigation, three times, and wants to keep 0.4 m of dead storage for silt. Evaporation and seepage together are estimated at 25% of live storage per year. The pond has no pucca spillway, and the excavated soil has been dumped as a bund on the downstream side. Silt inflow is expected from the untreated pasture.
1. Compute the annual runoff yield of the catchment and the gross and usable storage of the pond, then check whether the committee's irrigation plan is feasible. State every assumption.
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Catchment yield: 45 ha = 450,000 m2; 450,000 × 0.72 m × 0.30 = 97,200 m3/yr. Assumption: the 0.30 coefficient applies to the whole catchment and all rainfall occurs in one monsoon.
Pond geometry (1:1 side slopes, prismoidal formula):
- Top area at 3.0 m = 40 × 25 = 1,000 m2
- Bottom area = 34 × 19 = 646 m2
- Mid-depth area = 37 × 22 = 814 m2
- Gross storage = (3.0/6) × (1,000 + 4 × 814 + 646) = 0.5 × 4,902 ≈ 2,450 m3
Dead storage: area at 0.4 m = 34.8 × 19.8 ≈ 689 m2; volume ≈ 0.4 × (646 + 689)/2 ≈ 267 m3. Live storage ≈ 2,450 − 267 = 2,183 m3; after 25% evaporation and seepage, usable ≈ 1,637 m3.
Demand: 2 ha × 0.05 m × 3 = 20,000 × 0.15 = 3,000 m3.
Verdict: the plan is not feasible. Even ignoring all losses, gross storage (2,450 m3) is below the demand, and usable storage covers only about 55% of it - a shortfall of roughly 1,360 m3. Note also that yield (97,200 m3) is about 40 times the pond capacity, so the pond spills repeatedly: the constraint is storage, not rainfall. Finally, a catchment losing 5 t/ha/yr delivers 225 t of sediment; trapping half at bulk density 1.3 gives about 86 m3/yr, so 267 m3 of dead storage is filled in about three years, not the ten years normally assumed.
2. Recommend, with numbers, how to close the gap: reduce the command, deepen or widen the pond, or add a second structure. Which gives the most usable water per cubic metre excavated?
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Option 1 - reduce and re-schedule the command (no earthwork). One hectare at two irrigations of 5 cm = 1,000 m3, plus 25% losses = 1,250 m3, which fits within 1,637 m3 usable. This is implementable this season and should be the base plan: protective irrigation at crown-root initiation and flowering gives the best return per millimetre on wheat.
Option 2 - deepen from 3.0 m to 4.5 m. Bottom becomes 31 × 16 = 496 m2; mid-depth 35.5 × 20.5 = 728 m2; gross = (4.5/6) × (1,000 + 2,912 + 496) ≈ 3,306 m3. Dead storage about 350 m3, so usable ≈ (3,306 − 350) × 0.75 ≈ 2,217 m3. Additional excavation ≈ 855 m3 for about 580 m3 of extra usable water - roughly 1.5 m3 dug per m3 of usable water. Feasible only if the substratum is not rock and is reasonably impermeable.
Option 3 - widen to 60 m × 40 m at 3 m depth. Gross = (3/6) × (2,400 + 4 × 2,109 + 1,836) ≈ 6,336 m3; usable ≈ (6,336 − 400) × 0.75 ≈ 4,450 m3. Additional excavation ≈ 3,890 m3 for about 2,800 m3 of extra usable water - about 1.4 m3 per m3, marginally better than deepening but nearly four times the earthwork and land take.
Option 4 - lining. A 250-micron LDPE lining saves seepage but not evaporation, and evaporation alone (about 1.2 m/yr over roughly 900 m2, near 1,080 m3) is a large share of live storage; lining therefore gives a poor return unless seepage is shown to be high.
Recommendation: adopt Option 1 now, deepen rather than widen if funds permit, and treat the surplus spill as a recharge opportunity (a downstream recharge pit or dug well) rather than as waste. Deepening wins on cost per usable cubic metre and on land take; widening wins only if the subsoil is too rocky to deepen.
3. The pond has no spillway and the excavated soil forms the downstream bund. Describe the failure sequence you expect, give the design corrections, and identify one equity or legal issue the committee must settle.
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Expected failure sequence:
- A heavy storm fills the pond; with no spillway there is no freeboard, and water overtops the dumped bund.
- The bund is uncompacted fill on the downstream side, placed on a stripped or partly stripped surface, so it saturates quickly and loses strength; the outer face is at or steeper than 1:1 and slumps.
- Piping starts where water exits at the downstream toe; the pipe enlarges until the bund breaches, releasing the stored volume as a short, damaging wave onto downstream fields and possibly houses.
- Silt also reduces freeboard year by year, so the failure risk rises with age.
Corrections: design and construct a spillway sized for the peak inflow of the design storm (with a vegetated or masonry channel and a safe outlet), keep 0.3–0.5 m of freeboard, place the bund in compacted layers on a keyed foundation with a downstream batter of 1.5:1 to 2:1, add a toe drain, and provide a silt trap or desilting basin upstream with a scheduled desilting interval of about three years. Site the pond so that a breach cannot reach houses, and fence it for child and livestock safety.
Equity/legal issue: the water is harvested from a 45 ha catchment, so downstream users who rely on the same drainage line may claim customary use; the pond may sit on private land while the catchment is common land. The committee needs written consent from the panchayat and upstream/downstream landholders, a stated sharing rule for the command area, and clarity on who maintains and desilts the pond.