The pan evaporation method
FAO Irrigation and Drainage Paper No. 56 describes pans as a way to measure the integrated effect of radiation, wind, temperature and humidity on evaporation from an open water surface. Because a pan responds somewhat differently than a cropped surface does to the same weather, it isn't used directly; instead, an empirically derived pan coefficient (Kp) relates the two: ETo = Kp × Epan. FAO-56 notes that using pans to predict ETo for periods of 10 days or longer may be warranted, rather than relying on single-day readings.
The right Kp depends on the type of pan, how and where it's sited, and the general wind and humidity conditions, all of which this calculator asks for before looking up the coefficient in FAO-56's own published tables.
If you already have ETo from a weather station or another method and just need to take it further, see the irrigation water requirement calculator or the irrigation scheduling calculator.
How this pan evaporation calculator works
1. Average daily pan evaporation
Epan
Epan = Total evaporation over the period ÷ Number of days
FAO-56's own worked example averages a week of daily readings this same way before applying Kp.
2. Pan siting: Case A and Case B
FAO-56 defines Case A as a pan sited on a short green (grass) cover and surrounded by fallow soil, and Case B as a pan sited on fallow soil and surrounded by a green crop. At a large fetch, the air over the pan is more in equilibrium with the surrounding cover, carrying more water vapor and less heat in Case A than in Case B, which is why the Kp tables differ between them.
3. Pan coefficient (Kp)
FAO-56's Table 5 (Class A pan) and Table 6 (Colorado sunken pan) give Kp for combinations of wind speed class, mean relative humidity class, and fetch. This calculator sorts your entered wind speed and humidity into FAO-56's stated classes, then looks up Kp for the pan type, siting, fetch and those classes.
Wind speed classes (FAO-56)
Light: <2 m/s · Moderate: 2-5 m/s · Strong: 5-8 m/s · Very strong: >8 m/s
Relative humidity classes (FAO-56)
Low: <40% · Medium: 40-70% · High: >70%
4. Reference evapotranspiration
ETo
ETo = Kp × Epan
In US units, the calculator converts your wind speed from mph to m/s to match FAO-56's tables, and your pan reading is used in inches; the underlying Kp lookup and multiplication are identical either way.
When this calculator isn't the right tool
Use a different approach for:
- ETo from full weather-station data (temperature, radiation, humidity, wind). See the evapotranspiration calculator, which uses the FAO Penman-Monteith method FAO-56 treats as its standard.
- Carrying ETo through to a net or gross irrigation depth. See the irrigation water requirement calculator.
Pan evaporation calculation example
This reproduces FAO-56's own published example. A Class A pan is installed in a green area (Case A) surrounded by irrigated field crops, with a fetch of 1,000 m. Daily readings for a week are 8.2, 7.5, 7.6, 6.8, 7.6, 8.9 and 8.5 mm/day; mean wind speed is 1.9 m/s (light) and mean relative humidity is 73% (high).
- Average Epan: (8.2+7.5+7.6+6.8+7.6+8.9+8.5) ÷ 7 = 55.1 ÷ 7 ≈ 7.9 mm/day.
- Kp: Case A, light wind, fetch 1,000 m, high RH → Kp = 0.85.
- ETo: 0.85 × 7.9 ≈ about 6.7 mm/day.
The same idea in US units
A Class A pan sited on dry fallow soil (Case B), surrounded by a green crop with a 100 m fetch, wind speed 8 mph (about 3.6 m/s, moderate), and 55% relative humidity (medium), with 2.0 in of evaporation measured over 5 days:
- Average Epan: 2.0 ÷ 5 = 0.4 in/day.
- Kp: Case B, moderate wind, fetch 100 m, medium RH → Kp = 0.60.
- ETo: 0.60 × 0.4 = about 0.24 in/day.
Both examples use the exact FAO-56 table values and formula the calculator runs; enter your own pan readings above for your result.
Practical guidance
FAO-56 describes the Class A pan as circular, about 120.7 cm in diameter and 25 cm deep, made of galvanized iron or Monel metal, mounted on an open wooden platform 15 cm above ground level. It's filled to 5 cm below the rim and shouldn't be allowed to drop more than 7.5 cm below the rim before refilling. FAO-56 recommends siting the pan on a grassed area of about 20 by 20 m, open on all sides for free air circulation, preferably at the center or leeward side of a large cropped field, with daily readings taken in the early morning at the same time precipitation is measured.
FAO-56 also notes some practical adjustments: in extensive bare-soil areas with no agricultural development, listed Kp values for arid, windy conditions may need reducing by up to 20%; where pans sit in a small enclosure surrounded by tall crops, the listed coefficients may need increasing by up to 30% in dry, windy climates. Painting the pan a color other than the standard aluminium, letting water levels drop too far below the rim, or fitting screens over the pan can all introduce additional error.
Treat this as a planning estimate. FAO-56 recommends calibrating pan-based ETo against the Penman-Monteith method locally where possible, since site-specific factors aren't fully captured by the tables.
What changes Kp
| Factor | Effect | What to check |
|---|---|---|
| Pan siting (Case A vs. B) | The same fetch and weather give a different Kp depending on which surface the pan sits on. | Confirm whether the pan sits on green cover or fallow soil. |
| Fetch | A larger upwind buffer of the identified surface brings the air closer to equilibrium with it. | Estimate the real upwind distance in the prevailing wind direction. |
| Wind speed | Higher wind generally lowers Kp in Case A and raises Kp's spread in Case B. | Use the mean wind speed for the same period as the pan reading. |
| Relative humidity | Higher humidity generally raises Kp. | Use mean RH for the same period as the pan reading. |
| Pan condition | Non-standard paint, low water levels, or screens over the pan can bias readings. | Maintain the pan per FAO-56's siting and upkeep guidance. |
Frequently asked questions
What is the pan evaporation method for estimating ETo?
FAO Irrigation and Drainage Paper No. 56 relates pan evaporation to reference evapotranspiration through an empirically derived pan coefficient: ETo = Kp × Epan, where Epan is the measured evaporation depth and Kp adjusts for the pan type, its surroundings, wind speed and humidity. FAO-56 notes that using pans to predict ETo for periods of 10 days or longer may be warranted.
What is a pan coefficient (Kp) and why does it depend on wind, humidity and fetch?
A pan coefficient adjusts the measured evaporation from a water-filled pan to better represent evapotranspiration from a reference grass surface, since the two respond somewhat differently to the same weather. FAO-56 gives Kp values in tables that vary by wind speed class, mean relative humidity, and fetch, the upwind distance of the surrounding ground cover, because all three change how closely the air over the pan matches conditions over a cropped surface.
What's the difference between Case A and Case B pan siting?
FAO-56 defines Case A as a pan sited on a short green (grass) cover and surrounded by fallow soil, and Case B as a pan sited on fallow soil and surrounded by a green crop. The pan coefficients differ between the two because, at a large fetch, the air is more in equilibrium with the surrounding surface, carrying more moisture and less heat in Case A than in Case B.
What is "fetch" and how do I estimate it?
Fetch is the upwind distance of the identified ground cover, either the green crop for Case A or the dry fallow surface for Case B, before reaching the pan. FAO-56's tables use four fetch distances: 1, 10, 100 and 1,000 meters; choose the one closest to your actual upwind distance in the prevailing wind direction.
Class A pan vs. Colorado sunken pan: which should I use?
Use whichever pan you actually have installed and readings from; FAO-56 provides separate Kp tables for each. The Class A pan sits above ground and is the more common of the two; the Colorado sunken pan is set into the ground with its rim near soil level, which FAO-56 notes can give a better direct estimate of ETo but is harder to maintain and check for leaks.
How should I install and maintain a Class A pan?
FAO-56 describes the Class A pan as circular, about 120.7 cm in diameter and 25 cm deep, mounted on an open wooden platform 15 cm above ground, filled to 5 cm below the rim and not allowed to drop more than 7.5 cm below the rim. It recommends siting the pan on a grassed area of about 20 by 20 m, open on all sides, preferably at the center or leeward side of a large cropped field, with readings taken daily at the same time precipitation is measured.
How accurate is the pan evaporation method?
FAO-56 recommends it mainly for periods of 10 days or longer rather than single days, since day-to-day differences between pan and reference-surface behavior average out over longer periods. It also notes that local calibration against the Penman-Monteith method is recommended where possible, since environmental factors not fully captured in the tables can still affect results.
Can I use this for periods longer than a day?
Yes. Enter the total pan evaporation over the period and the number of days, and the calculator averages it to a daily rate before applying Kp, the same approach FAO-56 uses in its own worked example of averaging a week of daily pan readings.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Every Kp value in the calculator's lookup tables was copied and cross-checked directly against FAO-56's published Table 5 and Table 6.
- The worked US example was hand-calculated and traced against the code; the metric example reproduces FAO-56's own published worked example exactly.
- Wind and humidity class boundaries match FAO-56's stated ranges precisely.
Sources
- Food and Agriculture Organization of the United Nations. Allen, R.G., Pereira, L.S., Raes, D. & Smith, M. FAO Irrigation and Drainage Paper No. 56, Chapter 4: Determination of ETo — Pan evaporation method, Table 5 (Class A pan), Table 6 (Colorado sunken pan), and Example 21.
- USDA Natural Resources Conservation Service. National Engineering Handbook, Part 652, Irrigation Guide (general reference-ET and irrigation-scheduling terminology).