Application Efficiency: The Whole Arithmetic Behind Irrigation Saving

Application efficiency has one definition across the irrigation literature: the volume of water stored in the crop's root zone divided by the volume delivered to the field. The remainder is not lost in a physical sense - some percolates below the roots, some evaporates, some runs off - but it is lost in the sense that matters to the farmer. You paid to pump it and the crop did not use it.

The published range for flood irrigation under ordinary field conditions is 50% to 65%, improving modestly with laser levelling and shorter runs. Drip reaches roughly 90% when it is scheduled against crop demand, and falls below 75% when it is left on a fixed timer - because timed drip delivers water on time rather than on need.

From there the arithmetic is direct. If a crop genuinely needs 4,000 cubic metres per feddan per season inside the root zone, then at 55% efficiency you must pump 4,000 ÷ 0.55 ≈ 7,273 m³. At 90% you must pump 4,000 ÷ 0.90 ≈ 4,444 m³. The difference is 2,829 m³, or 39% of the original figure. That is where the 25-40% range we quote comes from: the lower end corresponds to well-managed flood on levelled land, the upper end to poorly managed flood.

The demand figure itself is not invented. It is computed the way FAO Irrigation and Drainage Paper 56 sets out: reference evapotranspiration for the locality multiplied by the crop coefficient for each growth stage, less effective rainfall. The coefficient for tomatoes at fruiting is not the coefficient at establishment, and anyone applying the same depth all season over-waters twice and under-waters once.

This is why installing a drip network on its own captures only part of the saving. The second part - the gap between 75% and 90% - comes from scheduling: when the valve opens and how long it stays open. That decision cannot be made without measuring soil moisture, because visual judgement of the surface errs consistently in the same direction: the surface looks dry while the root zone is still at field capacity.

The rule we press on every client: never accept a quoted saving percentage without the fraction that produces it. Ask for the estimated demand, the efficiency of the current system and the efficiency of the proposed one, then divide it yourself. If a supplier cannot give you all three numbers, the percentage they quoted is not a calculation. It is an advertisement.