Irrigation Zone Run Time Calculator
Calculate the required run time for each irrigation zone based on your plants' water needs, your system's precipitation rate, and local evapotranspiration conditions.
Typical range: 0.5–3.0 in/week. Check your local extension service or weather station.
Lawn ≈ 0.8, shrubs ≈ 0.5, vegetables ≈ 1.0, trees ≈ 0.5–0.9.
Rotary heads ≈ 0.4–1.0 in/hr; fixed spray heads ≈ 1.0–2.0 in/hr. Check head spec sheet.
Well-designed systems: 75–85 %. Older or mismatched systems: 50–70 %.
How many days per week this zone will run.
Enter measurable rainfall that actually soaks in (not runoff). Leave 0 if none.
Fill in the fields above and click Calculate.
Formulas Used
1. Crop Evapotranspiration (ETc)
ETc = ET₀ × Kc
ET₀ = reference evapotranspiration (in/week); Kc = plant/crop coefficient.
2. Net Irrigation Requirement (NetIR)
NetIR = max(ETc − Effective Rainfall, 0) [in/week]
3. Gross Irrigation Requirement (GrossIR)
GrossIR = NetIR ÷ (DU / 100) [in/week]
Accounts for non-uniform water distribution across the zone.
4. Total Weekly Run Time
Total Time = GrossIR ÷ PR [hours/week]
5. Run Time per Session
Time per Session = (Total Time × 60) ÷ Irrigation Days [minutes/session]
Assumptions & References
- ET₀ values are based on the Penman-Monteith method (FAO-56 standard). Local values are available from CIMIS, NOAA, or your county extension office.
- Crop coefficients (Kc) follow FAO Irrigation and Drainage Paper No. 56 (Allen et al., 1998).
- Precipitation Rate (PR) should be measured by catch-can test or taken from the manufacturer's spec sheet for matched head spacing.
- Distribution Uniformity (DU) is the low-quarter DU (DULQ) as defined by the Irrigation Association. A catch-can audit is the most accurate method.
- Effective rainfall is assumed to be 100 % of measured rainfall for simplicity; in practice, runoff and deep percolation reduce this value.
- This calculator assumes a single homogeneous zone. Mixed plant types or slopes may require zone-by-zone adjustments.
- For run times > 30–45 min, cycle-and-soak scheduling is recommended to prevent runoff, especially on slopes or clay soils.
- Reference: Irrigation Association, Landscape Irrigation Scheduling and Water Management (2005).