The Evaporation Potential Calculator, often referred to as the Potential Evapotranspiration (PET) Calculator, estimates how much water could be lost through evaporation and transpiration if water were not limited. It plays a vital role in hydrology, agriculture, irrigation planning, and climate studies. This tool helps users understand environmental water demand and supports informed decision-making for crop scheduling, drought monitoring, and water resource management.
Using the FAO Penman-Monteith equation, the calculator factors in temperature, wind speed, humidity, solar radiation, and other meteorological data to predict the evaporative capacity of the atmosphere under ideal conditions.
Formula of Evaporation Potential Calculator
PET = [0.408 × Δ × (Rn − G) + γ × (900 / (T + 273)) × u₂ × (es − ea)] / [Δ + γ × (1 + 0.34 × u₂)]
Where:
- PET = potential evapotranspiration (mm/day)
- Δ = slope of vapor pressure curve (kPa/°C)
- Rn = net radiation at crop surface (MJ/m²/day)
- G = soil heat flux density (MJ/m²/day)
- γ = psychrometric constant (kPa/°C)
- T = mean daily air temperature at 2 meters (°C)
- u₂ = wind speed at 2 meters (m/s)
- es = saturation vapor pressure (kPa)
- ea = actual vapor pressure (kPa)
This formula is widely accepted for accurate estimation of PET across varied climatic zones and is suitable for daily, weekly, or monthly timescales.
Helpful Reference Table
Here’s a simplified reference for PET values based on typical climate data, assuming average conditions:
| Avg Temp (°C) | Wind (m/s) | Radiation (MJ/m²/day) | PET Estimate (mm/day) |
|---|---|---|---|
| 15 | 2 | 10 | 2.5 |
| 20 | 2 | 15 | 3.7 |
| 25 | 2 | 20 | 5.2 |
| 30 | 2 | 25 | 6.8 |
| 35 | 2 | 30 | 8.1 |
These values serve as general estimates and may vary based on humidity, crop type, and altitude.
Example of Evaporation Potential Calculator
Let’s estimate PET using the following daily weather data:
- T = 25°C
- Rn = 20 MJ/m²/day
- G = 0 (assuming negligible soil heat flux)
- u₂ = 2 m/s
- es = 3.2 kPa
- ea = 2.0 kPa
- Δ = 0.18 kPa/°C
- γ = 0.066 kPa/°C
Step 1: Calculate numerator
= [0.408 × 0.18 × (20 − 0) + 0.066 × (900 / (25 + 273)) × 2 × (3.2 − 2.0)]
= [1.4688 + 0.066 × 3.055 × 2 × 1.2]
= [1.4688 + 0.483]
= 1.9518
Step 2: Calculate denominator
= 0.18 + 0.066 × (1 + 0.34 × 2)
= 0.18 + 0.066 × 1.68 = 0.180 + 0.1109 = 0.2909
Step 3: Final PET
= 1.9518 / 0.2909 ≈ 6.71 mm/day
Result: The potential evapotranspiration is approximately 6.71 mm/day under these conditions.
Most Common FAQs
Potential evapotranspiration (PET) is the maximum water loss possible under ideal conditions with unlimited water. Actual evapotranspiration (AET) is the real amount of water evaporated and transpired, which depends on water availability and soil moisture.
PET helps determine how much irrigation water is needed for crops. It allows farmers and agronomists to plan water supply schedules, prevent crop stress, and optimize yield by matching irrigation with atmospheric demand.
Yes. The Penman-Monteith method is robust and globally applicable, making it suitable for arid deserts, humid tropics, or temperate climates, provided that accurate input data is available.