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Formula Used
The calculator uses an evapotranspiration approach. First, crop water depth is estimated: ETc = (ET0 × Kc) − EffectiveRain. Negative values are treated as zero.
Net daily volume converts depth over area into water volume. For imperial units: Gallons = Inches × Area(ft²) × 0.623. For metric units: Liters = mm × Area(m²).
Gross demand accounts for losses using efficiency: Gross = Net ÷ (Efficiency/100). Weekly and per-event values scale from the daily gross demand.
How to Use This Calculator
- Select units, then enter your watered garden area.
- Enter daily reference ET0 from a local weather source.
- Choose a plant preset or type your Kc value.
- Add effective rainfall if your soil stores some rain.
- Set irrigation efficiency based on your system quality.
- Pick watering days per week for per-event guidance.
- Optionally add emitters and runtime to validate output.
Example Data Table
| Scenario | Area | ET0 | Kc | Rain | Efficiency | Gross Daily Need |
|---|---|---|---|---|---|---|
| Vegetable bed | 200 ft² | 0.18 in/day | 0.70 | 0.02 in/day | 80% | 19.64 gal/day |
| Shrub border | 350 ft² | 0.16 in/day | 0.55 | 0.00 in/day | 75% | 25.57 gal/day |
| Drought tolerant mix | 500 ft² | 0.14 in/day | 0.40 | 0.01 in/day | 85% | 15.02 gal/day |
| Small lawn | 800 ft² | 0.20 in/day | 0.85 | 0.00 in/day | 65% | 130.57 gal/day |
| Raised beds (metric) | 25 m² | 4.0 mm/day | 0.70 | 0.5 mm/day | 85% | 102.94 L/day |
Examples are illustrative. Use local ET0 and site conditions for best planning.
Evapotranspiration-driven planning
Daily irrigation demand is best estimated from evapotranspiration (ET). ET0 represents atmospheric thirst under reference conditions, while Kc adjusts for plant type and canopy. The calculator computes ETc depth per day and converts that depth across your irrigated area into a daily volume. This produces a weather-responsive target instead of a fixed timer setting.
Selecting a realistic Kc
Kc varies with species, growth stage, and planting density. Vegetables and flowers often range 0.65–0.80, shrubs 0.45–0.65, and turf commonly 0.80–0.95. Use presets as a starting point, then refine using observed stress, leaf color, and local guidance. Overstating Kc inflates water demand and can cause runoff or disease.
Rainfall, soil storage, and efficiency
Not all rainfall is effective. Light rain may evaporate, while heavy rain can exceed infiltration. Enter only the portion that wets the root zone. Efficiency accounts for losses from wind drift, evaporation, leaks, and uneven coverage. Drip systems often reach 80–90%, while spray zones may be 55–75%. Lower efficiency increases the gross volume needed to deliver the same net root-zone water.
Reading the outputs for scheduling
Gross daily need is the practical amount your system must apply. Weekly and 30‑day totals help with budgeting and storage planning. If you water fewer days per week, the calculator divides the weekly demand by watering days to estimate per‑event volume. Use this per‑event target to set controller run times, and verify that your soil can absorb the application rate without puddling.
Validating with flow and exports
The optional flow check converts emitter flow and runtime into actual applied volume. If applied is higher than gross demand, reduce minutes or split cycles. If lower, increase runtime or add emitters. Exporting results to CSV supports recordkeeping across seasons, and PDF output is useful for sharing settings with clients or crew. Update values weekly to match season changes and plant growth.
FAQs
What is ET0, and where do I find it?
ET0 is reference evapotranspiration from weather networks or local extension services. Use daily values for your area. If only weekly ET is available, divide by seven for an approximate daily ET0.
How do I choose an irrigation efficiency value?
Use 85–90% for well-designed drip, 75–85% for mixed drip or micro-spray, and 55–75% for spray zones. Lower values fit windy sites, poor pressure regulation, or uneven coverage.
Why does effective rainfall reduce the daily need?
Rain that reaches the root zone offsets plant water demand. Only count rainfall that infiltrates and is stored in the soil. If your soil is compacted or runoff occurs, effective rainfall may be much lower.
What does “gross daily need” mean?
Gross daily need is the amount your system must apply to deliver the net requirement at the roots. It includes losses from evaporation, drift, and distribution nonuniformity based on your efficiency setting.
How can I use the flow check to set runtime?
Enter emitter flow, number of emitters, and runtime minutes. The tool compares applied volume to gross demand and estimates a recommended runtime. Split watering into multiple short cycles if runoff appears.
Should I recalculate often?
Yes. Recalculate after major temperature shifts, seasonal canopy changes, new mulch, pruning, nozzle repairs, or controller changes. Regular updates keep schedules aligned with real plant demand and reduce water waste.