Evapotranspiration Calculator
Estimate potential evapotranspiration (PET) for a month using average temperature, daylight hours, and month length. Useful for irrigation scheduling and hydrological water-balance studies.
About this calculator
Potential evapotranspiration (PET) quantifies how much water would evaporate from a vegetated surface if soil moisture were unlimited. This calculator uses a simplified Thornthwaite-style temperature method. The formula is: PET = 16 × (10 × avgTemp / 20) × (daylight / 12) × (daysInMonth / 30), where avgTemp is in °C. The first term (10 × avgTemp / 20) scales PET relative to a reference temperature of 20°C. The daylight ratio adjusts for seasonal day-length differences, and the monthly-length ratio normalizes for months that are longer or shorter than 30 days. Higher temperatures and longer days both drive greater evapotranspiration. Results are typically expressed in millimeters per month and feed directly into irrigation demand calculations and watershed models.
How to use
Assume avgTemp = 25°C, daylight = 14 hours, and daysInMonth = 31. Step 1 — temperature factor: 10 × 25 / 20 = 12.5. Step 2 — daylight factor: 14 / 12 ≈ 1.167. Step 3 — month-length factor: 31 / 30 ≈ 1.033. Step 4 — multiply: 16 × 12.5 × 1.167 × 1.033 ≈ 241 mm/month. This tells you that under these warm, long-day conditions roughly 241 mm of water would evapotranspire, guiding how much supplemental irrigation is needed.
Frequently asked questions
What is the difference between potential evapotranspiration and actual evapotranspiration?
Potential evapotranspiration (PET) assumes unlimited soil water availability, representing the maximum possible water loss from a vegetated surface driven purely by atmospheric demand. Actual evapotranspiration (AET) is constrained by how much water is actually in the soil; it equals PET only when soil moisture is plentiful. In dry conditions, AET falls well below PET, which is why irrigation is needed to bridge the gap. Comparing PET to rainfall gives a quick picture of regional water deficit or surplus.
How do daylight hours affect the evapotranspiration calculation?
Longer daylight hours mean more solar energy is available to drive the evaporation process, so the daylight ratio in the formula scales PET upward during summer. A location with 16 hours of daylight will compute a daylight factor of 16/12 ≈ 1.33, boosting PET by 33% compared to the 12-hour reference. This is why mid-summer irrigation demand is typically highest even at moderate temperatures. Accurate daylight data, which varies with latitude and season, is therefore essential for realistic monthly PET estimates.
Why is the temperature method for evapotranspiration useful for farmers and hydrologists?
The temperature method requires only three readily available inputs — average monthly temperature, daylight hours, and days in the month — making it practical when detailed radiation or humidity data are unavailable. Farmers use monthly PET estimates to schedule irrigation, ensuring crops receive enough water without wasteful over-application. Hydrologists incorporate PET into water-balance models to predict streamflow, groundwater recharge, and reservoir storage. Despite its simplicity, the method provides reasonable accuracy for planning purposes across a wide range of climates.