Talk:Penman–Monteith equation

Unit problems

I did my best to disambiguate the terms "conductivity" and "conductance". I replaced the link to conductivity with a link to hydraulic conductivity, and a link to conductance with a link to fluid conductance. The context was this line:

g_a = Conductivity of air, atmospheric conductance (m s^-1)

There is a problem here: are we treating "conductivity of air" and "atmospheric conductance" as synonyms? If so, the way these terms are defined in the linked articles requires incompatible physical units. I don't know enough about this topic to fix the problem. Hopefully someone who knows better can help. That is why I tagged with a "clarify" template. CosineKitty (talk) 19:38, 7 February 2010 (UTC)

I believe you are misguided in understanding the conductance concept. It is the inverse of resistance, and the concept is discussed in detail in FAO-56: (http://www.fao.org/docrep/X0490E/x0490e06.htm). Basically, it models how well water is available to be vapourised.

For atmospheric conductance (m s-1). FAO-56 P-M uses aerodynamic resistance [s m-1], (208/u2) or conductance's inverse, so the formula is otherwise the same if you remove the stomatal resistance terms from FAO-56, which is a Penman-Monteith equation.

Baden --187.132.3.243 (talk) 05:41, 21 March 2010 (UTC)

Thank you for the work on this article. I believe this article needs some clarification/modification with regards to the formulas for the equation. First ETo is defined as the Reference evapotranspiration in the text but as the water volume transpired in the equation. I believe the former is the common designation. The two formulas - for mass water evapotranspiration rate, E, and volumetric water transpiration rate, ETo are equivalent with λv moved to the other side of the equation. This makes the two evapotranspiration terms equal, which is not reasonable. The article would be helped greatly by expansion to describe how \Delta, c_p , and \delta_q , are related to wind speed, solar radiation, and relative humidity, as stated in the text. As it stands the input parameters do not appear in the equations as set out.

Haresfur (talk) 00:06, 28 July 2012 (UTC)

unit problems 2

You talk about energy and say it is in Watt per square meter. But Watt is not energy, it is power. Energy would be Wh/m² or Joule/m². --2001:9E8:A53F:3300:37B6:381B:8794:903B (talk) 11:20, 29 June 2022 (UTC)