Pan Evaporation ET & What Your Crop Drinks
Schedules irrigation
Enter pan evaporation, the pan coefficient and crop coefficient to get crop water use (ETc), the gross irrigation depth and the daily water volume for your field.
Pan & crop
Next: replace about 6.3 mm/day; on a 7-day basis multiply by the days between irrigations to get the depth to apply.
Pan method per FAO-56; Kpan varies with siting, humidity and wind — calibrate locally.
Pan evaporation ET — key facts
- ETc
- Epan × Kpan × Kc
- Kpan
- ≈ 0.65–0.85
- Kc mid-season
- ≈ 1.0–1.15
- Gross depth
- ETc ÷ efficiency
- 1 mm over 1 ha
- = 10 m³ (10,000 L)
- Drip / sprinkler
- ~90% / ~75% efficient
- Method
- FAO-56 pan
- Privacy
- Runs in your browser; nothing uploaded
A pan and a ruler can schedule your water
The same sun, heat and wind that dry out an open pan of water also pull water from your crop. That simple link is the basis of the pan evaporation method: read how far the pan dropped, adjust it with a pan coefficient and a crop coefficient, and you have the crop's daily water use — no weather station required. It's the most accessible way for a farmer to schedule irrigation on real evaporative demand instead of guesswork.
This tool computes ETc in mm/day, the gross depth after irrigation losses, weekly water use, and the daily volume in m³ for your field. Use it to decide how much and how often to irrigate as the weather and the crop stage change. Pair it with the Reference ET₀, Irrigation Scheduling and Drip Irrigation tools to run a precise, water-efficient system.
Schedule on real demand
Irrigate to what the weather actually evaporates.
No weather station
Just a pan and a ruler give crop water use.
Account for losses
Gross depth includes your system's efficiency.
Size each irrigation
Daily and weekly volume in m³ for the field.
Frequently Asked Questions
What is the pan evaporation method?+
It estimates crop water use from a simple evaporation pan. Water evaporates from the open pan at a rate driven by the same weather that drives crop water loss, so pan evaporation (Epan) — adjusted by a pan coefficient and a crop coefficient — gives the crop's water requirement. It's one of the easiest field methods for scheduling irrigation.
How is crop water use (ETc) calculated?+
ETc = Epan × Kpan × Kc. Epan is the measured daily pan evaporation in mm, Kpan is the pan coefficient (about 0.65–0.85 depending on siting, humidity and wind), and Kc is the crop coefficient for the current growth stage. For example 8 mm × 0.75 × 1.05 ≈ 6.3 mm/day.
What is the pan coefficient (Kpan)?+
Kpan converts open-pan evaporation to reference crop evapotranspiration. It's higher (around 0.85) for a pan in a humid, sheltered, grassed setting and lower (around 0.65) for a pan in dry, windy, bare surroundings. This tool offers typical presets; for best accuracy use a locally calibrated value.
What is the crop coefficient (Kc)?+
Kc scales reference ET to your specific crop and stage. It's low at the initial/seedling stage (≈0.4), rises through development, peaks at mid-season (often 1.0–1.15 for full canopy), and falls at maturity. Using the right stage Kc is the biggest factor in getting ETc right through the season.
What is the gross irrigation depth?+
ETc is the net water the crop uses; not all applied water reaches the root zone, so the gross depth = ETc ÷ system efficiency. Drip is about 90% efficient, sprinkler ~75% and surface ~55%. The tool applies your efficiency so you know the actual depth to pump.
How do I turn mm/day into litres for my field?+
One millimetre of water over one square metre is one litre, so 1 mm over a hectare (10,000 m²) is 10,000 litres = 10 m³. Enter your area and the tool gives the daily and weekly volume to apply, using the gross depth that accounts for irrigation losses.
How often should I irrigate using this?+
Multiply the daily ETc (or gross depth) by the days between irrigations to get the depth to apply each time, and check it against your soil's available water and the crop's allowable depletion. Light soils need shorter intervals; heavy soils hold more and can go longer between irrigations.
Where should the evaporation pan be placed?+
A standard Class A pan should sit on a low wooden platform over a grassed area, away from buildings and tall crops that block wind or cast shade, and be kept filled to the mark and clean. Poor siting changes the readings and the appropriate Kpan, so keep it consistent.
Is the pan method as good as Penman-Monteith?+
The FAO Penman-Monteith equation is the reference standard, but it needs several weather variables. The pan method needs only a pan and a ruler, and gives good practical estimates for scheduling when the pan is well sited and the coefficients are reasonable. Use whichever data you have — see the Reference ET₀ tool for the Hargreaves approach.
Are the coefficients here reliable?+
The Kpan and Kc presets follow FAO-56 typical ranges suitable for planning. Local climate, variety, canopy and pan siting shift the real values, so calibrate against your own conditions where you can, and adjust Kc as the crop moves through its stages.