Water Harvesting and Pond Sizing Calculator
The dry season is won or lost by storage. Size the pond from your own catchment and demand before you dig.
Rain that falls on your block can be kept for the weeks the rain forgets. This calculator pairs catchment yield (area × rainfall × runoff) with irrigation demand (daily use × season length), then sizes the pond needed to bridge the gap: usable storage, gross volume with dead storage, and surface area at your chosen depth. It follows on from the irrigation scheduler, which tells you the daily use this tool stores.
Pond and catchment calculator
Runoff yield vs irrigation demand → storage volume and pond footprint.
Graded block surface, roof or road that drains to the pond
Bare/tilled soil 0.2–0.4 · grassed 0.35 · roof or compacted 0.7–0.9
From the irrigation scheduler
How long the pond must carry you without rain — 10–21 typical
Banks, evaporation and sediment reduce the useful average
The formulas#
Annual runoff = catchment (ha) × rainfall (mm) × runoff coefficient × 10
← 1 mm over 1 ha = 10 m³
Annual withdrawal = daily use × irrigation season (days)
Supply ratio = annual runoff ÷ annual withdrawal
Usable storage = daily use × days of supply
Gross volume = usable storage ÷ (1 − dead storage) ← 15% assumed
Surface area = gross volume ÷ average depthWorked example#
4 ha catchment, 1,200 mm rain, 0.6 runoff, 70 m³/day for 180 days, 14-day store, 3 m deep:
Annual runoff = 4 × 1200 × 0.6 × 10 = 28,800 m³/yr
Annual withdrawal = 70 × 180 = 12,600 m³/yr
Supply ratio = 28,800 ÷ 12,600 = 2.29× ✓ catchment can refill the pond
Usable storage = 70 × 14 = 980 m³
Gross volume = 980 ÷ 0.85 = 1,152.9 m³
Surface area = 1,152.9 ÷ 3 = 384 m² (≈ 0.04 ha)Reading the results#
| Output | Why it matters |
|---|---|
| Annual runoff | What your catchment can actually deliver — the ceiling on any storage scheme |
| Supply ratio | Below ~1.2× the pond never refills in an average year; you are harvesting once, not yearly |
| Days of supply | The buffer between you and the next rain — match it to the local dry-spell statistics |
| Gross volume | The number to quote an earthmoving contractor |
| Surface area | Land take, and the evaporation losses that scale with it |
Next steps#
- Daily demand comes from the irrigation scheduler; system sizing from the irrigation calculator.
- Catchment and storage practice: the water harvesting guide.
- Rainfall risk and dry spells: drought and extreme weather.
- Water quality and salinity limits: salinity and water quality.