Irrigation Requirement Calculator

Irrigation Requirement Formula

Worked Example

A loam (FC 28 %, PWP 14 %, ρb 1.4) with a 0.6 m root zone, 50 % depletion and ETc 5.5 mm/day:

1. Available water = (FC − PWP) × ρb × D = 0.14 × 1.4 × 600 mm = 117.6 mm
2. Net = 0.5 × 117.6 = 58.8 mm; interval = net ÷ ETc = 10.69090909 days

Gross irrigation depth (net ÷ efficiency): 84 mm · Crop water use ETc = Kc ET₀: 5.5 mm/day · Irrigation interval: 10.69090909 days · Gross volume per hectare: 840 m³/ha

GATE tip

If moisture is given by volume, skip the bulk density: d = (θ_FC − θ_PWP) × D.

More Irrigation and Drainage Engineering Calculators

• Sprinkler Irrigation Calculator

  I = 3600 q / (S_l S_m)

  CU = 100(1 − Σ|x − x̄| / (n x̄)); q in L/s, spacings in m

  Example: A 0.5 L/s sprinkler on a 12 × 18 m grid: Application rate = 8.33333 mm/h.

  Open the sprinkler irrigation calculator →

• Evapotranspiration Calculator

  ET₀ = p (0.46 T + 8.13)

  Hargreaves: ET₀ = 0.0023 Ra (T_mean + 17.8)√(Tmax − Tmin); ETc = Kc ET₀

  Example: A month with a mean temperature of 25 °C and p = 0.27: Reference evapotranspiration ET₀ = 5.3001 mm/day.

  Open the evapotranspiration calculator →

• Drain Spacing Calculator

  L² = (8K₂ d h + 4K₁ h²) / q

  d equivalent depth, h water-table height midway, q drainage coefficient

  Example: K = 0.5 m/day above and below drains, h = 0.6 m, d = 2 m and q = 5 mm/day: Drain spacing L = 33.2265 m.

  Open the drain spacing calculator →

Frequently asked questions

Sources

• The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures .

Last reviewed: 2026-10-15. Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.