Water Harvesting — Catchments, Ponds and Storage
Rain arrives when you don't need it and disappears when you do. Storage is how a farm decouples the two.
Every square metre of your block is a potential catchment. Water harvesting is the business of catching that runoff when it falls, storing it without losing it to seepage or evaporation, and releasing it into irrigation during the dry weeks. The drought reference covers the demand side; this guide is the supply side.
Catchments: where the water comes from#
| Catchment type | Runoff coefficient | Notes |
|---|---|---|
| Bare / tilled soil | 0.20–0.40 | Erodes while it runs — hold it with grassedways |
| Grassed orchard floor | 0.30–0.45 | The realistic figure for a mulched banana block |
| Gravel or compacted yard | 0.50–0.70 | Useful around packhouses and roads |
| Metal roof | 0.80–0.90 | The best yield per hectare you will find |
Rule of thumb: 1 mm of runoff over 1 ha = 10 m³. A 4 ha grassed block at 1,200 mm annual rain with 0.4 runoff yields roughly 19,200 m³ a year — often more than a smallholding's entire dry-season irrigation need. Run the numbers in the pond calculator.
Move water to the pond with graded floors, grassedways or earthen channels — not bare rills, which end as gullies. A sediment trap before the pond is not optional: silt is the single biggest cause of storage loss.
Siting and building the pond#
- Low point in the catchment so gravity fills it; never pump where gravity works.
- Natural bowl or valley key — less excavation per cubic metre stored.
- Clay foundation if unlined; a dispersive or sandy site needs a liner regardless.
- Setback from trees — roots tunnel through embankments. Bananas are shallow-rooted but still keep the planting line off the wall.
- Animal and child safety: fencing, gentle bank slopes (≤3:1 on the outside), and no deep, bare vertical banks.
Freeboard, spillway, dead storage#
Three water levels do the work:
- Dead storage — the bottom ~15%, where sediment settles; the intake sits above it.
- Operating storage — what the pump draws from, sized by the pond calculator for your days of supply.
- Freeboard — empty space above operating level for storms and surge.
Lining options#
| Liner | Cost | Suitability |
|---|---|---|
| Compacted clay (60–90 cm) | Low–medium | The standard answer where suitable clay exists |
| HDPE / geomembrane | Medium–high | Any soil; expect 15–25 years |
| Bentonite additive | Medium | Treats silty soils in place |
| None (good natural clay) | Free | Test with a hole test before trusting it |
The hole test: dig a spade-deep hole, fill it, and time it. If it drains in under 4 hours, you need a liner.
Losing water: evaporation and seepage#
Seepage is a lining question, not a depth question. In dispersive soils it continues for years unless the pond is sealed.
Operating through the dry season#
- Pre-fill — top up fully before the last reliable rains; never enter the dry season half-full.
- Meter the take — withdrawal against daily demand tells you honestly how long the store lasts.
- Prioritise — stored water goes to flowering and bunch-filling blocks first; established vegetative mats tolerate dry better.
- Watch quality — pond water concentrates salts as it evaporates; monitor EC and blend with fresh inflow when you can.
- Desludge every 3–5 years — the sediment trap catches much of it; dredging the rest restores capacity cheaply compared to new excavation.
Next steps#
- Size it: pond calculator.
- Know your daily number: irrigation scheduler.
- When the storage runs out anyway: drought and extreme weather.
- Quality of what you stored: salinity and water quality.
- Context for the whole scheme: water management.