# Evaporation - Penman Equation (Shuttleworth modification)

## Description

The Penman equation describes evaporation (E) from an open water surface, and was developed by Howard Penman in 1948. Penman’s equation requires daily mean temperature, wind speed, air pressure, and solar radiation to predict E. Simpler Hydrometeorological equations continue to be used where obtaining such data is impractical, to give comparable results within specific contexts, e.g. humid vs arid climates.

Numerous variations of the Penman equation are used to estimate evaporation from water, and land. Specifically the Penman-Monteith equation refines weather based potential evapotranspiration (PET) estimates of vegetated land areas.

In 1993, W.Jim Shuttleworth modified and adapted the Penman equation to use SI, which made calculating evaporation simpler. The resultant equation is shown here.

Related formulas## Variables

E_{mass} | evaporation rate(mm/day) - Note: this formula implicitly includes the division of the numerator by the density of water (1000 kg/m^3) to obtain evaporation in units of mm/day (dimensionless) |

m | slope of the saturation vapor pressure curve(kPa/K) (dimensionless) |

R_{n} | net irradiance(MJ/m^2*day^1) (dimensionless) |

γ | psychrometric constant(kPa/K) (dimensionless) |

U_{2} | wind speed(m/s) (dimensionless) |

δ_{e} | vapor pressure deficit(kPa) (dimensionless) |

λ_{v} | latent heat of vaporization(MJ/kg) (dimensionless) |