Irrigation Zone Runtime Calculator
Calculate the required runtime for an irrigation zone based on your plant water needs, system precipitation rate, and distribution uniformity.
Typical range: 0.5–3.0 in/week depending on climate and season
Lawn grass ≈ 0.8, shrubs ≈ 0.5, trees ≈ 0.9, vegetables ≈ 1.0
Rotary heads ≈ 0.5–1.0 in/hr; spray heads ≈ 1.5–2.5 in/hr; drip ≈ 0.1–0.5 in/hr
Well-designed systems: 75–85%; older/poor systems: 50–65%
Number of days per week the zone will be irrigated
Subtract any effective rainfall received; enter 0 if none
Formulas Used
1. Crop Evapotranspiration (ETc):
ETc = ET₀ × Kc
2. Net Irrigation Requirement:
Net Irrigation = max(0, ETc − Effective Rainfall)
3. Gross Irrigation Requirement:
Gross Irrigation = Net Irrigation ÷ DU
4. Total Weekly Runtime:
Total Runtime (hr) = Gross Irrigation ÷ Precipitation Rate
5. Runtime per Session:
Session Runtime = Total Runtime ÷ Irrigation Days per Week
Assumptions & References
- ET₀ (Reference Evapotranspiration) is based on the Penman-Monteith equation (FAO-56 standard). Local ET₀ data is available from CIMIS, NOAA, or your local extension service.
- The Crop Coefficient (Kc) adjusts ET₀ for specific plant types and growth stages per FAO Irrigation and Drainage Paper No. 56.
- Distribution Uniformity (DU) accounts for uneven water application across the zone. ASABE standard S436 defines DU as the ratio of the average low-quarter application depth to the average application depth.
- Effective rainfall is the portion of precipitation that is actually available to plants (typically 75–100% of total rainfall for moderate events).
- Cycle-and-soak is recommended when session runtime exceeds 15 minutes to prevent surface runoff, especially on slopes or clay soils.
- Precipitation rate should be measured by catch-can test (ASABE S398) for best accuracy.
- This calculator assumes a single homogeneous zone. Mixed plant types or microclimates should be split into separate zones.