Sizing a drip system before you dig the trench

A drip irrigation system that waters the first three rows well and the last three poorly is not a maintenance problem. It was designed wrong, and no amount of flushing will fix it. The design is four numbers, and any farmer can check them before signing for an installation.

1. Peak crop water requirement

Size for the worst week of the season, not the average. The requirement is reference evapotranspiration multiplied by the crop coefficient at its peak stage.

For most of peninsular India, peak reference evapotranspiration runs 6–8 mm per day in April and May. A mid-season vegetable crop has a coefficient near 1.05. So peak demand is around 7.5 mm per day, which over an acre is roughly 30,000 litres a day.

If the pump and mainline cannot deliver that inside the hours you can actually run the system, the design has already failed.

2. Emitter discharge and spacing

Emitter choice follows soil texture, because it governs how water spreads underground:

On sand, a high-discharge emitter drives water down past the root zone before it spreads sideways. On clay, a low-discharge emitter on close spacing puts more pipe in the field than the crop needs.

3. Drip irrigation pressure at the far end

This is where installations fail. A non-pressure-compensating emitter rated 4 lph at 1.0 bar delivers considerably less at 0.6 bar. Friction loss along a lateral is real, and slope adds or subtracts on top of it.

The standard is that discharge between the first and last emitter on a lateral should not vary by more than 10%. In practice that means:

• Laterals under 60 m on 12 mm pipe, under 100 m on 16 mm.
• Pressure-compensating emitters wherever ground slope exceeds about 2%.
• A pressure gauge at both the head and the far end of a representative lateral. Two gauges cost almost nothing and are the only way to know.

Ask the supplier for the lateral length calculation. If they cannot produce one, they did not do it.

4. Filtration matched to the water source

Emitters block, and what blocks them depends on the source. Borewell water carries sand and, if it is hard, precipitates calcium. Canal and pond water carries organic matter and algae.

• Borewell — hydrocyclone followed by a screen filter, 120 mesh.
• Open source — sand media filter followed by a screen filter.
• Both — a screen filter immediately before the fertigation injection point.

Clean the filter when the pressure difference across it reaches 0.5 bar. Waiting until flow visibly drops means the emitters have already taken silt.

Fertigation, once the hydraulics are right

A venturi is cheap and adequate for a single block; it costs 10–20% of line pressure, so the design must allow for that. A dosing pump gives an accurate rate and does not steal pressure, and pays back on a larger holding.

Run the sequence in three parts: twenty minutes of clear water to fill the lines and wet the soil, the fertiliser injection, then at least fifteen minutes of clear water to flush. Skipping the final flush is how algae and salt build up inside the laterals.

Water-soluble grades only. Ordinary DAP will precipitate inside the line and the blockage will be permanent. Keep calcium and phosphate in separate injections — together they form an insoluble solid in the pipe.

Drip irrigation maintenance that keeps the design honest

Open the lateral ends and flush at high velocity once a fortnight. Acid-treat once a season to clear carbonate scale, chlorinate to clear organic slime, and log the pressure readings each month. A rising head pressure with falling far-end pressure means blockage is developing, and it says so weeks before a plant shows stress.

Further reading: Pradhan Mantri Krishi Sinchayee Yojana sets the subsidy norms and design standards most drip irrigation installers quote against.