What is leaching requirement (LR)?
Irrigation water always carries some dissolved salts, and as a crop uses water through evapotranspiration, those salts stay behind and concentrate in the soil. FAO Irrigation and Drainage Paper No. 29 (Ayers and Westcot, 1985) defines the leaching requirement (LR) as the fraction of applied irrigation water that must pass below the root zone to keep the soil's salinity from exceeding what the crop can tolerate.
This calculator applies FAO-29's steady-state formula to find LR from your irrigation water's salinity and the crop's salinity tolerance threshold, then shows the gross irrigation depth needed once that leaching fraction is added on top of a base net depth.
This calculator adjusts a net depth for salinity leaching alone; for a gross depth that also combines effective rainfall and irrigation efficiency, see the irrigation water requirement calculator. For the soil and water quality category more broadly, browse the full set of related tools.
How this leaching requirement calculator works
1. Leaching requirement
LR (FAO-29)
LR = ECw ÷ (5 × ECe − ECw)
ECw = salinity of the irrigation water (dS/m). ECe = the crop's soil salinity threshold (dS/m), the saturation-extract salinity at which yield begins to decline. Both are the same units as mmho/cm from older tables.
2. Adjusted gross depth
Gross depth
Gross depth = Net depth ÷ (1 − LR)
Since a fraction LR of the applied water passes below the root zone rather than being used by the crop, the applied depth must be increased so the remaining (1 − LR) fraction still meets the net depth.
When this calculator isn't the right tool
Use a different approach for:
- A gross depth that also includes effective rainfall and irrigation efficiency, not leaching alone. See the irrigation water requirement calculator.
- High-frequency drip irrigation, which FAO-29 treats with a different approach than the conventional-method formula used here.
Leaching requirement calculation example
Irrigation water at 1.1 dS/m, a crop with a 1.3 dS/m salinity threshold, and a net depth of 11.3 in:
- LR: 1.1 ÷ (5 × 1.3 − 1.1) = 1.1 ÷ (6.5 − 1.1) = 1.1 ÷ 5.4 ≈ 20.4%.
- Gross depth: 11.3 ÷ (1 − 0.204) = 11.3 ÷ 0.796 ≈ about 14.19 in.
The same idea in metric
Irrigation water at 0.8 dS/m, a crop with a 2.0 dS/m threshold, net depth 100 mm:
- LR: 0.8 ÷ (5 × 2.0 − 0.8) = 0.8 ÷ 9.2 ≈ 8.7%.
- Gross depth: 100 ÷ (1 − 0.087) ≈ about 109.5 mm.
Both examples use the exact FAO-29 formula the calculator runs; enter your own water and crop numbers above for your result.
Practical guidance
Get ECw from an actual water quality test of your irrigation source rather than an assumed figure, since salinity can vary significantly between wells, surface sources and blended supplies. For ECe, use the specific crop's published salinity threshold from FAO-29's or FAO-48's crop tolerance tables rather than a generic figure, since tolerance varies widely by crop, from sensitive fruit trees to more tolerant field and forage crops.
Because LR only describes the fraction of water that must pass below the root zone under steady-state conditions, real-world leaching also depends on distribution uniformity, soil structure and drainage. A zone with poor uniformity may under-leach some areas and over-water others even when the average applied depth matches the calculated gross depth.
Check local drainage and water rights rules. Leaching water must go somewhere; confirm that subsurface drainage is adequate and that any return flow complies with local water rights and drainage regulations.
What changes leaching requirement
| Factor | Effect | What to check |
|---|---|---|
| Irrigation water salinity (ECw) | Higher ECw raises LR and the gross depth needed. | Use a current water quality test, not an assumed value. |
| Crop salinity threshold (ECe) | A lower (more sensitive) threshold raises LR for the same water. | Use the specific crop's published FAO-29/FAO-48 threshold. |
| Net depth | Gross depth scales directly with net depth at a fixed LR. | Base net depth on the crop's actual water requirement. |
| Irrigation method | This formula applies to conventional surface/sprinkler methods; drip's different wetting pattern uses a separate FAO-29 approach. | Confirm which method this water will actually be applied with. |
Frequently asked questions
What is leaching requirement (LR)?
Leaching requirement is the fraction of applied irrigation water that must pass below the root zone to keep salts from building up in the soil past what a crop can tolerate. FAO Irrigation and Drainage Paper No. 29 (Ayers and Westcot, 1985) gives the formula LR = ECw ÷ (5 × ECe − ECw), where ECw is the salinity of the irrigation water and ECe is the soil salinity threshold the crop tolerates.
How do I find ECw and ECe for my situation?
ECw comes from a water quality test of your irrigation source, in deciSiemens per meter (dS/m). ECe is the crop's salinity tolerance threshold, the soil saturation extract salinity at which yield begins to decline; FAO Irrigation and Drainage Papers No. 29 and No. 48 publish threshold ECe values for many agricultural crops.
How does LR translate into extra irrigation depth?
Gross depth = Net depth ÷ (1 − LR). Since a fraction LR of the applied water passes below the root zone rather than being used by the crop, the applied depth must be increased so the remaining (1 − LR) fraction still meets the crop's net water need.
What if my calculated LR is very high or the tool won't compute a result?
A very high or undefined leaching requirement means the irrigation water is too saline relative to what the chosen crop's threshold allows; the formula becomes physically meaningless once LR reaches or exceeds 100%. In that situation, consider a more salt-tolerant crop, a lower-salinity water source, or blending water sources, rather than relying on leaching alone.
Is dS/m the same as mmho/cm?
Yes. FAO Irrigation and Drainage Paper No. 29's guideline tables note that deciSiemens per meter (dS/m) and millimhos per centimeter (mmho/cm) are numerically equivalent, so older tables in mmho/cm can be used directly as dS/m.
Does this apply to drip irrigation?
This calculator uses FAO-29's leaching requirement formula for conventional (surface or sprinkler) irrigation methods. High-frequency drip irrigation maintains a different, generally more favorable salt distribution in the root zone, and FAO-29 gives a separate approach for that case.
Why does a lower ECe threshold (more sensitive crop) need heavier leaching?
A lower ECe threshold means the crop tolerates less soil salinity before yield suffers, so the same irrigation water salinity (ECw) requires a larger fraction of applied water to pass through and carry salts below the root zone, which is exactly what a smaller denominator in the LR formula produces.
How does this relate to the irrigation water requirement calculator?
This calculator adjusts a net depth for salinity leaching alone. The irrigation water requirement calculator combines net depth, effective rainfall, irrigation efficiency and a leaching fraction together into one gross depth; use that one if you need those other factors included as well.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- The LR formula and constant were confirmed directly against FAO-56's own citation of the FAO-29 leaching formula.
- The primary worked example's leaching requirement (about 20%) matches a University of Arizona Cooperative Extension worked example using the same ECw and ECe values.
- Both worked examples were hand-calculated and traced against the code; a guard blocks the result with a clear message when the calculated LR would reach or exceed 100%, avoiding a negative or divide-by-zero gross depth.
Sources
- Ayers, R.S. & Westcot, D.W. Food and Agriculture Organization of the United Nations. FAO Irrigation and Drainage Paper No. 29, Water Quality for Agriculture (leaching requirement formula, crop salt tolerance tables).
- Food and Agriculture Organization of the United Nations. FAO Irrigation and Drainage Paper No. 56, Chapter 8 (confirms the FAO-29 leaching formula, LR = ECiw ÷ (5ECe − ECiw)).
- University of Arizona Cooperative Extension. Water Management in Desert Agriculture (worked leaching requirement example).