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.
Specifically the Penman–Monteith equation refines weather based potential evapotranspiration (PET) estimates of vegetated land areas.
[citation needed] The original equation was developed by Howard Penman at the Rothamsted Experimental Station, Harpenden, UK.
This equation assumes a daily time step so that net heat exchange with the ground is insignificant, and a unit area surrounded by similar open water or vegetation so that net heat & vapor exchange with the surrounding area cancels out.
Some times people replace Rn with and A for total net available energy when a situation warrants account of additional heat fluxes.
In 1993, W.Jim Shuttleworth modified and adapted the Penman equation to use SI, which made calculating evaporation simpler.