Enter weather, site, and crop inputs
Example data table
| Location | Day | Tmax/Tmin (C) | RH mean (%) | Wind (m/s) | Rs (MJ/m2/day) | Method | ET0 (mm/day) | ETc @ Kc 0.95 (mm/day) |
|---|---|---|---|---|---|---|---|---|
| Warm urban garden | 200 | 32 / 20 | 55 | 2.0 | 18.0 | FAO-56 PM | ~5.2 | ~4.9 |
| Coastal backyard | 140 | 27 / 18 | 70 | 3.5 | 16.0 | FAO-56 PM | ~4.2 | ~4.0 |
| Data-limited plot | 90 | 22 / 10 | — | — | — | Hargreaves | ~3.0 | ~2.9 |
Formula used
Reference evapotranspiration (ET0) is estimated as daily water loss from a well-watered reference surface. This tool offers two common approaches.
Uses radiation, air temperature, humidity, and wind:
ET0 = [0.408·Delta·(Rn − G) + gamma·(900/(T+273))·u2·(es−ea)] / [Delta + gamma·(1 + 0.34·u2)]
Uses temperature range with extraterrestrial radiation:
ET0 = 0.0023·(Tmean + 17.8)·sqrt(Tmax − Tmin)·Ra
Crop evapotranspiration (ETc) adjusts for plant type: ETc = ET0 · Kc.
Estimated water volume over a period uses: Liters ≈ (ETc_total_mm · Area_m2) / (Efficiency/100), because 1 mm over 1 m2 equals 1 liter.
How to use this calculator
- Pick the method that matches your available weather data.
- Enter latitude, altitude, and day of year for solar geometry.
- Add daily Tmax and Tmin. Include wind and humidity if available.
- Provide solar radiation or sunshine hours, or choose estimation.
- Set Kc, garden area, days, and efficiency to estimate liters.
- Press Calculate. Use the download buttons for CSV or PDF.
Daily ET0 patterns and key drivers
Reference evapotranspiration (ET0) commonly ranges from 2–4 mm/day in mild periods and 5–8 mm/day during hot, windy, sunny conditions. In the calculator, higher net radiation (Rn) and wind speed (u2) raise ET0, while higher humidity reduces the vapour-pressure deficit (es−ea) and lowers ET0. When humidity or radiation is missing, the temperature-based option can be used, but daily differences of 10–25% are possible.
Location and day-of-year effects
Latitude and day-of-year set extraterrestrial radiation (Ra) and daylight hours (N). For the same air temperature, a midsummer day can deliver 20–40% more Ra than early spring, increasing ET0 even before humidity and wind are considered. Altitude changes air pressure and the psychrometric constant (gamma), subtly shifting aerodynamic demand.
Radiation input choices and typical uncertainty
Measured solar radiation (Rs) provides the most stable daily estimate. When only sunshine hours are available, the Angstrom approach scales Rs by n/N, which works well for open sites. If data is limited, the temperature-range option uses kRs (about 0.16 inland or 0.19 coastal) and usually carries higher uncertainty during cloudy spells.
From ET0 to crop demand and irrigation volume
Plant demand is calculated as ETc = ET0 × Kc. Typical Kc values are about 0.30–0.60 for sparse cover and 0.85–1.05 for many vegetables at full canopy. For example, ET0 = 5.2 mm/day with Kc = 0.95 gives ETc ≈ 4.9 mm/day. Over 7 days, that is about 34 mm. Because 1 mm over 1 m² equals 1 litre, a 25 m² bed needs roughly 850 litres before efficiency adjustment.
Consistency checks that improve field decisions
Use realistic bounds: Tmax should exceed Tmin, humidity should stay within 1–100%, and Rs is often 10–25 MJ/m²/day on clear summer days. Irrigation efficiency typically falls between 60–90%. If ET0 looks unusually high, verify wind units (m/s vs km/h) and confirm that latitude sign is correct. Re-run the tool weekly as weather shifts. Mulch and partial shade can cut real water loss, so adjust irrigation accordingly outdoors.
FAQs
1) Which method should I choose?
Use the FAO-56 option when you have wind, humidity, and a radiation input. Choose Hargreaves when you only trust Tmax and Tmin. Compare both for a few days; large gaps suggest missing or inconsistent weather data.
2) What if I do not have solar radiation values?
Select Sunshine hours if you know daily bright-sun hours. Otherwise choose Estimate from temperatures. Enter kRs as 0.16 for inland or 0.19 for coastal locations, then review results against local watering experience.
3) How do I select a crop coefficient (Kc)?
Kc reflects canopy size and growth stage. Sparse seedlings often use 0.30–0.60, while many vegetables at full canopy use 0.85–1.05. If uncertain, start with 0.85 and adjust after observing leaf wilting or runoff.
4) Why is the liters value higher than the ETc total suggests?
Liters are adjusted by irrigation efficiency. If efficiency is 75%, the tool divides by 0.75 to account for losses from uneven coverage, wind drift, or runoff. Improve uniformity or use drip to reduce the required applied volume.
5) Can I use the calculator for containers or protected gardens?
Yes, as an estimate, but containers heat faster and sheltered areas reduce wind. Use a slightly lower Kc for shaded sites, and validate by measuring how quickly soil dries. Frequent, smaller irrigations often work better in pots.
6) What period should I enter in the Days field?
Use 1 day for daily scheduling, 3–7 days for weekly planning, or match your irrigation interval. Update inputs whenever a heatwave, rain, or seasonal shift changes typical temperatures, humidity, or sunshine.