Penman-Monteith Equation Calculator

The Penman-Monteith Equation estimates potential evapotranspiration based on factors like temperature, humidity, wind speed, solar radiation, and surface characteristics. It’s a widely used tool in agriculture and hydrology to determine maximum water loss from vegetated surfaces, aiding in irrigation planning and water resource management.

Penman-Monteith Equation Calculator

Penman-Monteith Equation Calculator


ParameterSymbolValue (Example)UnitDescription
Air TemperatureT25°CDaily average air temperature
Relative HumidityRH50%Daily average relative humidity
Wind SpeedU2m/sDaily average wind speed
Solar RadiationRs20,000J/m²/dayDaily total solar radiation
Psychrometric Constantγ0.067kPa/°CPsychrometric constant (typically constant)
Slope of Saturation VaporΔ0.12kPa/°CSlope of the saturation vapor pressure curve
Atmospheric PressureP101.3kPaAverage atmospheric pressure
Latent Heat of Vaporizationλ2.45MJ/kgLatent heat of vaporization of water
Albedo (Surface Reflectivity)α0.20Albedo (reflectivity) of the surface
Canopy Resistancerc100s/mCanopy resistance (can be estimated)
Soil Heat FluxG0MJ/m²/dayDaily soil heat flux (often set to 0)
Latitudeφ35°Latitude of the location
Elevationz500metersElevation above sea level
Day of Yeardoy200Day of the year (Julian day)
Stefan-Boltzmann Constantσ0.0000000567MJ/(m²·K⁴)Stefan-Boltzmann constant


  1. What is the Penman-Monteith Equation?
    • The Penman-Monteith Equation is an empirical formula developed to estimate potential evapotranspiration (PET), which represents the maximum possible rate of water loss from a vegetated surface under specific meteorological conditions. It accounts for both the energy and aerodynamic components of the evapotranspiration process.
  2. What are the components of the Penman-Monteith Equation?
    • The Penman-Monteith Equation includes several meteorological parameters such as temperature, relative humidity, wind speed, and solar radiation, as well as physical characteristics of the surface (e.g., albedo, canopy resistance) to estimate potential evapotranspiration.
  3. Why is potential evapotranspiration important?
    • Potential evapotranspiration is crucial in agriculture, hydrology, and water resource management. It helps determine how much water a crop or landscape may require under specific environmental conditions, aiding in irrigation planning and water conservation.
  4. How is the Penman-Monteith Equation used in practice?
    • The Penman-Monteith Equation is used to estimate the potential evapotranspiration rate for a specific location and time period. This estimate can be used to inform irrigation scheduling, water resource planning, and understanding the water balance in various ecosystems.
  5. Can the Penman-Monteith Equation be used for actual evapotranspiration?
    • While the Penman-Monteith Equation is primarily used to estimate potential evapotranspiration, it can be modified to estimate actual evapotranspiration by incorporating a crop coefficient or a factor that accounts for the difference between potential and actual rates of water use.
  6. Are there simplified versions of the Penman-Monteith Equation?
    • Yes, simplified versions of the Penman-Monteith Equation exist for practical use. These simplified forms reduce the number of input variables and make the calculation more accessible for routine applications, such as irrigation scheduling. The most commonly used simplified version is the FAO-56 Penman-Monteith equation.
  7. What are the limitations of the Penman-Monteith Equation?
    • The Penman-Monteith Equation requires accurate meteorological data and knowledge of surface characteristics, which may not always be readily available. Additionally, it assumes steady-state conditions and does not account for the effects of intermittent rainfall or abrupt weather changes.
  8. Are there software tools or calculators available to use the Penman-Monteith Equation?
    • Yes, there are various software tools, spreadsheets, and online calculators available that use the Penman-Monteith Equation to estimate potential evapotranspiration. These tools simplify the calculation process and allow users to input the required meteorological data.
  9. How does climate change impact the use of the Penman-Monteith Equation?
    • Climate change can affect the accuracy of evapotranspiration estimates because it alters meteorological parameters like temperature and precipitation. Users of the Penman-Monteith Equation may need to consider climate change data and adjust their calculations accordingly for more reliable results.
  10. Is the Penman-Monteith Equation suitable for all regions and climates?
    • The Penman-Monteith Equation is widely used and adaptable to different regions and climates. However, it performs best in areas with well-defined seasons and consistent meteorological data. In regions with extreme conditions, modifications or alternative methods may be necessary.
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Remember that the Penman-Monteith Equation is a powerful tool for estimating potential evapotranspiration, but it should be used with consideration of local conditions and the availability of accurate meteorological data.

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