Irrigation Application Efficiency

Application efficiency is the fraction of the water delivered to a field that ends up stored in the root zone where a crop can use it. Forty millimetres applied and 32 mm still in the profile afterwards is $32/40 = 80\%$. The missing fifth is not a rounding error; it went somewhere specific, and knowing where is what makes the number useful rather than merely discouraging.

There are three destinations. Deep percolation is water that drained below the roots, and it is the dominant loss on furrow systems and on any system that runs too long — the head of a furrow is wet for the entire set while the tail end is only wet at the finish, so the head is over-irrigated by construction. Runoff is water that never entered the soil, which happens whenever the application rate exceeds the infiltration rate, and it is a design fault more often than an operator one. Evaporation and wind drift take the rest, and they can be severe: an impact sprinkler throwing fine droplets on a hot, windy afternoon can lose 20 to 30% of its output before it lands.

Typical figures fall roughly where one would expect. Surface furrow irrigation runs 45 to 70%, unmanaged flood lower still. Set-move and centre-pivot sprinklers reach 65 to 85%, with low-pressure drop nozzles at the upper end because they release water close to the ground and cut drift almost entirely. Drip and micro-irrigation reach 85 to 95%, which is the strongest argument in their favour and the reason they dominate high-value horticulture despite the capital cost.

Two cautions keep this number honest. Efficiency cannot be raised above what uniformity permits — if the driest quarter of the field is getting 70% of the average depth, then irrigating that quarter adequately necessarily overwaters the rest, and the excess shows up here as deep percolation no scheduling can remove. And efficiency at the field scale is not efficiency at the basin scale: water that percolates below one farm's roots frequently returns to a river or an aquifer and is pumped again downstream, so a project that raises field efficiency does not always save any water at all. That distinction has embarrassed a great many irrigation modernisation schemes, and it is worth stating plainly whenever this number is used to justify one.