Evapotranspiration (ET) Calculator
Calculate how much water your landscape loses each day using the FAO-56 Penman-Monteith equation - the international standard for ET estimation.
Location
Weather Conditions
Crop Settings
Daily Water Loss
Evaporation vs Transpiration
What This Means
Your landscape is losing about 5.2 mm of water per day. To replace this loss, you would need to apply approximately 3,250 gallons per acre daily.
Irrigation Runtime
Convert today's ET into sprinkler run time, adjusted for your system's distribution uniformity.
Weekly & Monthly Projections
Projected based on current conditions. Actual values will vary with daily weather.
Weather data powered by Open-Meteo.
The FAO-56 Penman-Monteith Equation
This calculator uses the internationally standardized Penman-Monteith equation from FAO Irrigation and Drainage Paper 56. It's the most accurate method for estimating reference evapotranspiration:
Understanding the Variables
Δ (Delta): Slope of the saturation vapor pressure curve. Increases with temperature, making hot days have higher ET.
γ (Gamma): Psychrometric constant. Depends on atmospheric pressure (elevation). ~0.067 kPa/°C at sea level.
Rn: Net radiation at the crop surface. The primary energy source driving ET.
es - ea (VPD): Vapor pressure deficit. The "drying power" of the air. Low humidity = high VPD = more ET.
u₂: Wind speed at 2m height. Wind removes humid air from the canopy, increasing ET.
Reference: FAO Irrigation and Drainage Paper 56
Crop Coefficients (Kc)
The crop coefficient adjusts reference ET (ETo) to actual crop ET (ETc). Different plants have different water needs:
| Vegetation | Kc Range | Notes |
|---|---|---|
| Cool-Season Turf | 0.80 - 0.90 | Kentucky Bluegrass, Fescue, Ryegrass |
| Warm-Season Turf | 0.60 - 0.80 | Bermuda, Zoysia, St. Augustine |
| Trees (Mature) | 0.40 - 0.70 | Deep roots reduce irrigation needs |
| Native Plants | 0.20 - 0.40 | Adapted to local rainfall |
Frequently Asked Questions
What is a good ET rate?
Reference ET (ETo) typically ranges from 2-3 mm/day in cool, humid conditions to 7-10 mm/day in hot, arid conditions. For most temperate regions during summer, 4-6 mm/day is typical. Your actual crop ET (ETc) depends on the crop coefficient - turfgrass typically uses 80-90% of reference ET, while established trees and shrubs may use only 40-60%. An ET of 5 mm/day means your landscape loses about 3,250 gallons per acre per day.
How does humidity affect evapotranspiration?
Humidity directly affects the vapor pressure deficit (VPD) - the difference between how much moisture the air can hold and how much it actually holds. Low humidity creates a high VPD, which drives faster evapotranspiration because the atmosphere can absorb moisture more readily. At 20% relative humidity and 30°C, VPD is about 3.4 kPa, producing high ET. At 80% humidity and the same temperature, VPD drops to about 0.85 kPa, reducing ET by roughly 40-50%.
What is the Penman-Monteith equation?
The FAO-56 Penman-Monteith equation is the internationally standardized method for calculating reference evapotranspiration, published by the Food and Agriculture Organization of the United Nations in Irrigation and Drainage Paper 56 (Allen et al., 1998). It combines energy balance (radiation) and aerodynamic (wind and humidity) approaches into a single equation. It requires four weather inputs: air temperature, relative humidity, wind speed at 2m height, and solar radiation. This calculator implements the full equation as defined in FAO-56.
How do I convert ET to irrigation run time?
Divide the daily ETc (mm/day) by your sprinkler application rate (mm/hour), then divide by your system's distribution uniformity (typically 0.65-0.85). For example: if ETc = 5 mm/day, your rotors apply 12 mm/hour, and uniformity is 0.75, then run time = 5 / 12 / 0.75 = 0.56 hours (about 33 minutes). You can find your application rate by placing catch cups in your irrigation zones and measuring collected water over a timed run.