| Area (m²) | Cutoff (mm) | Recent (mm) | Forecast (mm) | Conf. (%) | Sens. (%) | Moist. Now/Target (%) | Planned (min) | Rate (mm/hr) | Effective rain (mm) | Decision | Saved (L) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 100 | 5 | 3 | 6 | 70 | 100 | 35 / 45 | 30 | 10 | 7.20 | Cutoff | 500 |
| 50 | 8 | 1 | 5 | 50 | 90 | 30 / 40 | 25 | 12 | 3.15 | Reduce | ~90 |
| 120 | 4 | 0 | 4 | 30 | 110 | 60 / 55 | 20 | 8 | 1.32 | Cutoff | 320 |
- Weighted forecast rainfall: WeightedForecast = ForecastRain × (ForecastConfidence ÷ 100)
- Total rainfall: TotalRain = RecentRain + WeightedForecast
- Effective rainfall (sensor response): EffectiveRain = TotalRain × (Sensitivity ÷ 100)
- Planned irrigation depth: PlannedMM = ApplicationRate × (DurationMin ÷ 60)
- Planned water volume: PlannedLiters = PlannedMM × AreaM²
- Postpone estimate: PostponeDays = EffectiveRain ÷ DailyET
- Measure or log recent rainfall for your site.
- Enter forecast rainfall and confidence from your source.
- Set sensor sensitivity to match your device behavior.
- Enter current and target soil moisture for the root zone.
- Provide planned run time and application rate for the zone.
- Click calculate to get a cutoff or reduced schedule.
- Export CSV or PDF for records and scheduling notes.
Cutoff thresholds reduce waste and runoff
Rain sensors prevent unnecessary watering when natural rainfall already supplies the root zone. A cutoff threshold expresses the minimum effective rain depth that should pause scheduled irrigation. Setting it too low can create frequent stops from light showers, while setting it too high can delay watering after meaningful rain. Use a threshold that matches soil type, plant density, and typical irrigation depth. Review results weekly to align schedules with plant demand and soil infiltration.
Confidence-weighted forecasts improve automation
Forecast totals are uncertain, so weighting them by confidence avoids overreacting to low-probability storms. In this calculator, forecast rain is multiplied by the confidence percentage to create a conservative expected amount. Combine that expected amount with measured recent rainfall to estimate near-term wetting. This approach supports timers, controllers, and maintenance decisions during changing weather windows.
Sensitivity converts rainfall into sensor response
Different sensors and mounting locations respond differently to the same storm. Sensitivity adjusts the combined rainfall to reflect device behavior, canopy interception, and microclimate. Values below 100% suit sheltered sensors or fast-drying sites; values above 100% suit shaded sensors or slow-drying soils. Calibrate by comparing measured rain to real shutoff events across several storms.
Soil capacity links rain depth to moisture gain
Soil water capacity represents how many millimeters of water are needed to move from dry to fully replenished. The calculator converts effective rain into an estimated moisture increase, then compares current moisture to your target. This helps avoid watering when the profile is already adequate. Pair this with a moisture probe reading taken consistently at the same depth.
Document results to tune seasonal schedules
Exported CSV and PDF reports support troubleshooting and seasonal adjustments. Track effective rain, recommended runtime, and liters saved to spot patterns such as overwatering, poor uniformity, or ET changes. Update application rate after nozzle replacements and recheck thresholds at the start of hot or rainy seasons. Consistent logging delivers more reliable irrigation outcomes over time for each irrigation cycle.
1) What does “effective rain” mean here?
Effective rain is the combined recent rainfall and confidence-weighted forecast, adjusted by your sensitivity setting. It represents the rain amount your sensor and site are likely to “feel” for cutoff decisions.
2) How do I choose a cutoff threshold?
Start with one typical irrigation depth for that zone, then adjust. Sandy soils often need smaller thresholds, while clay soils can use higher thresholds. Validate using plant response and avoided runoff after storms.
3) Why include forecast confidence?
Confidence prevents overreacting to uncertain predictions. A low confidence forecast contributes less to effective rain, so irrigation is not paused unless rain is more likely. This supports stable schedules when weather models disagree.
4) My sensor stops watering too often. What should I change?
Lower sensitivity or increase the cutoff threshold. Also check placement: roof overhangs, shade, and wind exposure can change drying time. Use several storms to calibrate settings and avoid one-off adjustments.
5) How can I estimate application rate?
Run the zone for a known time and measure water depth with several catch-cans or straight-sided cups. Average the depths, convert to millimeters per hour, then update the calculator so liters and savings reflect reality.
6) What does “postpone days” represent?
It is an estimate of how many days the effective rain can cover, based on daily ET. It does not replace soil measurements, but it helps you decide how long to delay the next run during similar conditions.