Evapotranspiration (ET) Calculator

This tool helps farmers, agronomists, and farm managers estimate crop evapotranspiration rates for irrigation planning. It uses standard meteorological and crop-specific parameters to calculate accurate ET values for field use. Adjust inputs to match your local conditions and crop type for reliable results.

Evapotranspiration (ET) Calculator

Calculate reference and crop-specific ET for irrigation planning

Meteorological Parameters

Crop Parameters

💡 Tip: Use local weather station data for most accurate results. Adjust crop stage as your crop matures.

How to Use This Tool

Follow these steps to calculate accurate evapotranspiration values for your farm:

  1. Gather local meteorological data: daily max/min temperatures, relative humidity, wind speed, solar radiation, and atmospheric pressure from your nearest weather station.
  2. Enter the values into the corresponding input fields, selecting the correct units for each measurement.
  3. Select your crop type and current growth stage from the dropdown menus.
  4. Click the Calculate ET button to generate results.
  5. Use the Reset button to clear all inputs and start a new calculation.
  6. Copy results to your clipboard for record-keeping or irrigation planning.

Formula and Logic

This calculator uses the FAO-56 Penman-Monteith equation, the global standard for reference evapotranspiration (ETo) calculations. The core formula for grass reference ETo is:

ETo = [0.408Δ(Rn - G) + γ(900/(T+273))u2(es - ea)] / [Δ + γ(1 + 0.34u2)]

Where:

  • Δ = Slope of saturation vapour pressure curve (kPa/°C)
  • Rn = Net radiation at crop surface (MJ/m²/day)
  • G = Soil heat flux density (assumed 0 for daily calculations)
  • γ = Psychrometric constant (kPa/°C)
  • T = Mean daily air temperature (°C)
  • u2 = Wind speed at 2m height (m/s)
  • es = Saturation vapour pressure (kPa)
  • ea = Actual vapour pressure (kPa)

Crop-specific ET (ETc) is calculated by multiplying ETo by the crop coefficient (Kc) for your selected crop and growth stage: ETc = Kc × ETo. Daily water need per hectare is derived by converting ETc from mm to m³ (1 mm = 10 m³/ha).

Practical Notes

Apply these real-world considerations to improve calculation accuracy for your agricultural operation:

  • Seasonal factors: ETo is typically 30-50% higher in summer than winter due to longer day length and higher temperatures. Adjust irrigation schedules seasonally.
  • Soil conditions: Sandy soils have higher infiltration rates but lower water holding capacity, requiring more frequent irrigation than clay soils for the same ETc.
  • Growth stage adjustments: Kc values increase as crops reach mid-season, when water demand peaks. Monitor crop development closely to update growth stage selections.
  • Microclimate effects: Sloped fields, windbreaks, and irrigation method (drip vs. flood) can alter actual ET rates by 10-20% from calculated values.
  • Yield variability: Consistent under-irrigation relative to ETc can reduce crop yields by 5-10% per 10% water deficit during mid-season.

Why This Tool Is Useful

Evapotranspiration is the largest component of farm water demand, making accurate ET calculations critical for:

  • Irrigation scheduling: Avoid over-irrigation (wasting water and nutrients) or under-irrigation (reducing yields).
  • Water budgeting: Plan seasonal water allocations for crops, especially in drought-prone regions.
  • Cost savings: Reduce energy and labor costs associated with unnecessary irrigation runs.
  • Agribusiness planning: Estimate water needs for crop insurance, loan applications, or expansion projects.
  • Student learning: Agricultural students can verify field data against standardized FAO-56 calculations.

Frequently Asked Questions

What is the difference between ETo and ETc?

Reference ET (ETo) measures water loss from a standardized grass surface, while crop ET (ETc) adjusts ETo for the specific water use characteristics of your crop and its growth stage. ETc is the value you should use for irrigation planning.

How often should I update my ET calculations?

Recalculate ET weekly during the growing season, or more frequently during periods of rapid temperature change or crop growth. Use rolling 3-5 day averages of weather data for more stable results.

Can I use this tool for orchard crops or permanent vegetation?

Yes, but you will need to use Kc values specific to orchard crops (e.g., 0.4-0.6 initial, 0.8-1.1 mid-season for citrus). The tool includes annual crop Kc values by default, but you can manually adjust the Kc result if using orchard-specific coefficients.

Additional Guidance

Maximize the value of your ET calculations with these tips:

  • Calibrate weather data: If your weather station is more than 5km from your field, apply a 5-10% correction to ETo values to account for microclimate differences.
  • Combine with soil moisture sensors: Use ET calculations as a baseline, and verify with in-field soil moisture probes to account for local soil conditions.
  • Account for irrigation efficiency: Drip irrigation systems typically have 85-95% efficiency, while flood irrigation may have 50-60% efficiency. Divide ETc by your system's efficiency to get total irrigation water needed.
  • Record historical data: Track ET values and crop yields over multiple seasons to refine irrigation schedules for your specific farm.