The equation is widely used, and was derived by the United Nations Food and Agriculture Organization for modeling reference evapotranspiration ET0.
[1] Evapotranspiration contributions are significant in a watershed's water balance, yet are often not emphasized in results because the precision of this component is often weak relative to more directly measured phenomena, e.g., rain and stream flow.
In addition to weather uncertainties, the Penman-Monteith equation is sensitive to vegetation-specific parameters, e.g., stomatal resistance or conductance.
[2] Various forms of crop coefficients (Kc) account for differences between specific vegetation modeled and a reference evapotranspiration (RET or ET0) standard.
where rc refers to the resistance to flux from a vegetation canopy to the extent of some defined boundary layer.
The atmospheric conductance ga accounts for aerodynamic effects like the zero plane displacement height and the roughness length of the surface.
[5] While the Penman-Monteith method is widely considered accurate for practical purposes and is recommended by the Food and Agriculture Organization of the United Nations,[1] errors when compared to direct measurement or other techniques can range from -9 to 40%.
[6] To avoid the inherent complexity of determining stomatal and atmospheric conductance, the Food and Agriculture Organization proposed in 1998 [1] a simplified equation for the reference evapotranspiration ET0.
This is done by removing the aerodynamic terms from the Penman-Monteith equation and adding an empirically derived constant factor,
[9] The assumption that an air mass moving over a vegetated surface with abundant water saturates has been questioned later.