Farm ponds and field bunds: catching the rain where it falls
On rainfed land the water arrives in a handful of storms. What matters is not how much falls but how much stays on the field after the runoff stops.
A district recording 700 mm of rain has enough water for two crops. Most of it leaves within hours of falling, taking topsoil with it. Water harvesting is not about the total; it is about the interval between the storm and the runoff leaving your boundary. A farm pond, fed by bunds, is how you hold some of it back.
Work out how much water crosses your field
Runoff is rainfall multiplied by a coefficient that depends on soil and slope. For a typical rainfed field on a 2–3% slope with medium soil, the coefficient is around 0.25 — a quarter of what falls, runs off.
On one hectare receiving 700 mm, that is 1,750 cubic metres leaving each year, most of it in three or four storms. Capturing even a third of that is enough for one supplemental irrigation on the whole field at the moment it matters most.
Bunds first, farm pond second
The order matters. A pond below an unbunded field silts up within three seasons, because the runoff arrives carrying the topsoil.
Contour bunds
On slopes up to 6%, bunds follow the contour so water spreads rather than concentrates. Mark the contour with an A-frame level or a line level — this is a half-day job and can be done with materials from the village.
Vertical interval between bunds is roughly 1 metre on gentle slopes, closing to 0.6 m as the slope steepens. Build the bund 0.5–0.75 m high with a base width three times the height, and compact it in layers rather than heaping loose soil.
Every bund needs a waste weir at the lowest point of its run. Without one, an unusual storm overtops the bund at its weakest point and cuts a gully that is worse than having no bund at all.
Field bunds on flatter land
On slopes under 2%, bunds along the field boundary hold water in place for infiltration. Plant the bund with a grass such as Vetiver — the root system is deep and fibrous, it binds the structure, and it needs no maintenance once established.
Sizing the farm pond
A farm pond of 20 × 20 × 3 m holds roughly 1,200 cubic metres, allowing for side slopes. That is about one supplemental irrigation of 100 mm over one hectare — which is the point of it. A farm pond is insurance against a three-week dry spell at flowering, not a source of assured irrigation.
Practical guidance:
- Site at the lowest point of the catchment, on the least productive land you have.
- Side slope of 1:1 in clay, 1:1.5 in lighter soil. Steeper sides collapse.
- Inlet through a silt trap — a small settling chamber upstream. Cleaning a silt trap is a morning; desilting a pond is a season.
- Spillway sized for the largest storm on record for the district, not the average.
- Fence it. This is not optional where there are children or cattle.
Seepage, and what to do about it
In sandy or gravelly soil an unlined pond can lose most of its water within weeks. Options in ascending cost: compacted clay blanket where clay is available locally; soil-cement lining; LDPE or HDPE film at 250–500 micron, which is the common subsidised option and lasts five to eight years if it is protected from sunlight and cattle.
An unlined pond in heavy black soil often needs no lining at all — check the soil first before paying for film.
Recharge instead of storage
Where the aquifer is the real store, a recharge structure beats a pond. A recharge shaft — a bored well backfilled with graded gravel and sand, fed through a silt trap — moves storm runoff into the aquifer rather than holding it on the surface where it evaporates.
This works only where the geology accepts it, and it must be filtered. Directing dirty runoff straight into an aquifer contaminates the groundwater that the whole village drinks. Get the site checked by the groundwater department before building one.
What it is worth
On rainfed land, one supplemental irrigation at flowering commonly lifts yield by 30–50%, because that is the stage where moisture stress translates directly into lost grain. The pond does not make the land irrigated. It removes the worst outcome, which on rainfed land is the outcome that matters.
Water that leaves your field in an hour took three months of soil with it.
Further reading: Pradhan Mantri Krishi Sinchayee Yojana funds the watershed works a farm pond normally sits inside.
