Calculator Inputs
Formula Used
- Crop water use: ETc = ET0 × Kc
- Net irrigation depth/day: max(ETc − EffectiveRain, 0)
- Net depth for period: (Net/day × Days) − SoilWaterCredit, minimum zero
- Net volume: NetDepth(mm) ÷ 1000 × Area(m²)
- Makeup water (gross): Net ÷ Efficiency × (1+Losses) × (1+Leach)
- Runtime: GrossLiters ÷ Flow(L/min)
How to Use This Calculator
- Enter your irrigated area and select the correct unit.
- Set ET0 from local weather data, then choose Kc.
- Add effective rainfall if rain contributes to the root zone.
- Use soil water credit to reduce irrigation after soaking.
- Set efficiency and any extra losses from your setup.
- Optional: add leaching fraction for salinity management.
- Press Calculate to view makeup water and runtime estimates.
- Download CSV or PDF for records and irrigation planning.
Example Data Table
| Scenario | Area | ET0 | Kc | Rain | Days | Eff. | Loss | Leach | Makeup Water (approx.) |
|---|---|---|---|---|---|---|---|---|---|
| Summer vegetables | 120 m² | 6 mm/day | 0.95 | 0 mm/day | 1 | 85% | 5% | 0% | ~804 liters/day |
| Flower beds after light rain | 500 ft² | 0.20 in/day | 0.75 | 0.05 in/day | 3 | 80% | 0% | 0% | ~226 gallons/period |
| Salt-sensitive plants | 0.10 acre | 5 mm/day | 1.05 | 0 mm/day | 7 | 90% | 0% | 10% | ~19.8 m³/period |
Weather-driven irrigation demand
Makeup water starts with reference evapotranspiration (ET0), which reflects drying power from sun, wind, temperature, and humidity. The calculator applies a crop coefficient (Kc) to translate ET0 into crop water use (ETc) for your garden. This keeps scheduling tied to real weather instead of fixed minutes.
Turning depth into usable volume
Irrigation depth is converted into volume using area and unit conversions. A net depth of 10 mm over 100 m² equals 1.0 m³ or 1,000 liters. This is the foundation for comparing needs against tank capacity, hose output, or municipal billing units. For sandy soils, shorter and more frequent cycles often reduce deep percolation.
Rain, soil credit, and effective storage
Not all rainfall counts. “Effective rainfall” is the portion that reaches the root zone, while “soil water credit” accounts for water already stored after a soak. By subtracting these from ETc, the calculator estimates the true replacement requirement over your chosen planning period. Mulch, compost, and shading can increase effective storage by slowing evaporation at the surface.
Efficiency, losses, and leaching control
Gross makeup water is higher than net need because every system loses water. Drip lines often achieve 85–95% application efficiency, sprinklers can be 60–80% depending on wind and overlap. Extra losses can represent runoff or leaks, and a leaching fraction adds intentional surplus to manage salts. Track uniformity by checking wetting patterns at several spots in each zone.
Example data and operational planning
Example: Area 120 m², ET0 6 mm/day, Kc 0.95, rainfall 0, days 1, efficiency 85%, losses 5%, leach 0%. Net depth = 5.70 mm; net volume ≈ 684 L; makeup water ≈ 804 L/day. If flow is 20 L/min, runtime is about 40 minutes. Use these outputs to split irrigation into cycles, reduce runoff, and document water use across seasons. Re-check ET0 and Kc weekly during hot spells or rapid growth.
FAQs
1) What is “makeup water” in gardening?
It is the irrigation amount needed to replace plant water loss after subtracting effective rainfall and any usable soil moisture already stored during the selected period.
2) Where do I get ET0 values?
Use local weather services, agricultural stations, or smart irrigation controllers that publish daily ET0. If unavailable, use regional averages, then refine with observed plant response.
3) How do I choose Kc for my plants?
Kc depends on plant type and growth stage. Start with published ranges for your crop, then adjust slightly based on canopy density, mulching, and local microclimate.
4) What efficiency should I enter?
Use 85–95% for well-designed drip, 70–85% for micro-sprays, and 60–80% for sprinklers. If you see runoff or uneven coverage, choose the lower end.
5) What does “effective rainfall” mean?
It is rain that actually infiltrates into the root zone. Light showers may evaporate quickly, while heavy rain can cause runoff. Enter a conservative value if uncertain.
6) When should I add a leaching fraction?
Add it when using salty water, in arid climates, or where salts accumulate. Typical values are 5–15%, applied occasionally rather than every irrigation if drainage is limited.
7) How accurate is the runtime estimate?
It assumes your flow rate is steady and the system delivers water uniformly. Measure flow at the source, verify emitter performance, and recalibrate if pressure changes or zones differ.