Sprinkler System Runtime Scheduler
Calculate the optimal sprinkler runtime (minutes per session) based on your plant's water requirements, your sprinkler's precipitation rate, soil type, slope, and local evapotranspiration conditions.
Results will appear here.
Formulas Used
1. Crop Evapotranspiration (ETc):
ETc = ET₀ × Kc
2. Net Irrigation Requirement:
Net_IR = max(0, ETc − Effective Rainfall)
3. Gross Irrigation Requirement:
Gross_IR = Net_IR ÷ (Distribution Uniformity × Slope Factor × Soil Factor)
4. Water per Session:
Water/Session = Gross_IR ÷ Sessions per Week
5. Runtime per Session:
Runtime (min) = (Water/Session ÷ Precipitation Rate) × 60
6. Volume Estimate:
Gallons = Inches × 623 (per 1,000 sq ft; 1 in × 1,000 ft² = 623 gal)
Assumptions & References
- Reference ET₀ is based on the Penman-Monteith equation (FAO-56 standard). Obtain local ET₀ from your state's CIMIS, AZMET, or similar network.
- Crop coefficients (Kc): Turfgrass ≈ 0.8, Cool-season lawn ≈ 0.9, Shrubs ≈ 0.5, Trees ≈ 0.5–0.9 (FAO-56, Allen et al. 1998).
- Typical sprinkler precipitation rates: rotary heads 0.4–1.0 in/hr; fixed spray heads 1.0–2.0 in/hr; drip varies widely.
- Distribution Uniformity (DU) of 75–85% is typical for well-maintained systems (Irrigation Association standards).
- Slope and soil factors reduce effective application to prevent runoff and promote infiltration.
- Cycle & Soak is recommended when runtime exceeds 10 min on clay soils or slopes to prevent runoff.
- 1 inch of water over 1,000 sq ft ≈ 623 gallons (USGS conversion).
- Effective rainfall is typically 75–80% of actual rainfall for irrigation scheduling purposes.
- Source: FAO Irrigation and Drainage Paper No. 56; Irrigation Association Best Management Practices.