Leaching Requirement (Irrigation)

Well water in dry country arrives loaded with salt, and the crop drinks the water while leaving the salt behind. A season applying 600 mm at 1 dS/m — roughly 640 milligrams of salt per litre — drops close to four tonnes of salt per hectare, and it stays put until somebody pushes it below the roots. An irrigation designer who sizes the depth from evapotranspiration and application efficiency alone meets the problem as white crust on the surface and a weak field margin, at which point the fix costs drainage rather than water.

The fraction comes from the Rhoades equation: LR = ECw ÷ (5 × ECe − ECw), both conductivities in decisiemens per metre. ECw is the irrigation water, straight from the well or canal analysis — a river typically runs 0.2 to 0.5, a brackish crystalline-basement well 1 to 3. ECe is the salinity the chosen crop tolerates in the soil saturation extract: beans near 1.0, maize 1.7, cotton 7.7, barley above 8. The 5 in the denominator belongs to surface and sprinkler irrigation; for high-frequency drip, FAO swaps in twice the salinity that kills yield outright, which lands lower, so this screen errs on the safe side there.

The model runs in steady state: free drainage under the root zone, with no shallow water table and no compacted layer holding water up, and no meaningful rainfall, since rain leaches for free and cuts the calculated requirement. It covers total salinity only. Sodicity is a separate matter, read through SAR and ESP, and sodic water calls for gypsum rather than more water, since leaching without fixing sodium disperses clay and seals the profile. On units: µS/cm over a thousand gives dS/m, mmho/cm equals dS/m, and a 1:1 or 1:5 extract reading, being more dilute, sits lower and inflates the fraction if it lands in that field.