How to Calculate Evapotranspiration
What is Evapotranspiration?
Evapotranspiration is water loss from soil/water bodies and plants. It's critical for irrigation design, water balance, and climate analysis.
Step-by-Step Guide
- 1Reference ET (ET₀): well-watered grass, calculated from temperature, humidity, wind, radiation
- 2Actual ET: ET₀ × crop coefficient (varies 0.2-1.2 by crop)
- 3Typical: 100-200 mm/month depending on climate
Worked Examples
Input
ET₀ = 5 mm/day, alfalfa (K_c = 1.0)
Result
Daily ET = 5 mm, weekly = 35 mm (irrigation need)
During growing season
Common Mistakes to Avoid
- ✕Using ET₀ without crop coefficient (over-irrigating)
- ✕Neglecting seasonal variation (summer ET much higher)
Frequently Asked Questions
Why does crop type matter?
Deep-rooted crops (alfalfa) use more water; shallow (lettuce) less. Reflectance and canopy architecture differ.
How do you measure ET?
Lysimeters (soil columns), energy balance models, or empirical equations from climate data.
Ready to calculate? Try the free Evapotranspiration Calculator
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