Irrigation Water Requirement Calculator

Calculating Irrigation Water Needs

Crops and landscapes need water to replace what's lost through evapotranspiration (ET) — the combined evaporation from soil and transpiration from plants. Irrigation must supply the deficit between ET and natural rainfall.

The Net Irrigation Requirement Formula

IR_net = ET_crop − P_eff

IR_gross = IR_net / Efficiency

Where:

• ET_crop = crop water requirement (mm/day or mm/season) = K_c × ET₀
• P_eff = effective rainfall (mm) — precipitation that doesn't run off or percolate past the root zone
• Efficiency = irrigation system efficiency (drip: 90–95%; sprinkler: 75–85%; flood: 50–60%)
• K_c = crop coefficient (0.3–1.2 depending on crop and growth stage)
• ET₀ = reference evapotranspiration (from weather data, typically 3–8 mm/day)

Practical Examples

Example: Wheat crop, K_c = 1.05, ET₀ = 6 mm/day, P_eff = 1 mm/day, drip irrigation (90% efficiency). ET_wheat = 1.05 × 6 = 6.3 mm/day. IR_net = 6.3 − 1 = 5.3 mm/day. IR_gross = 5.3 / 0.9 = 5.9 mm/day = 59 m³/ha/day. For 100 ha field: 5,900 m³/day = 68.3 L/s pump requirement.

Applications

Irrigation scheduling, reservoir sizing, pump and pipeline design, and water allocation planning for agriculture.