Talk:Penman equation

is someone able to show a worked example for this, I don't know if I am using it correctly. Teeteetee 14:07, 5 May 2006 (UTC)

I should be able to, since my rain garden simulation model depends on it:)

I see nothing unverifiable about the original Penman equation, but I confess this is not the form of Penman's Equation most accurate for vegetation. The FAO56 standard covers that.

Priestly Taylor...

I may talk with my adviser, Bruce N. Wilson at the U of M, to muster a better explanation of this equation and the various forms for plant evapotranspiration (eg. Penman-Monteith). Time is always the issue. BrianAsh

Okay, here the equation is worked out, mostly with help from FAO-56: (http://www.fao.org/docrep/X0490E/x0490e06.htm)

Basically, the crux is to find the 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.

I was seeking the evaporation (sweat) rate for a human, so I discarded the radiation (and latent heat) terms. The initial values for this example are: temperature = 34, dew point = 10, elevation = 10 and wind speed = 10

423.70787202733 ETo reference evapotranspiration [W m-2]

1.1458764954493 rho a mean air density at constant pressure [kg m-3]

1012 cp specific heat of the air [J/(kg·K)]

4.1279690380751 es saturation vapour pressure [kPa],

1.1236872033495 ea actual vapour pressure [kPa],

27.733333333333 ra aerodynamic resistance [s m-1],

0.2298311011042 D slope vapour pressure curve [kPa °C-1],

0.06664547816281 g psychrometric constant [kPa °C-1].

1.1458764954493 rho a=n*28.9/1000

39.649705724888 n=PV/RT

8.3144 R

307 T air temperature at 2 m height [°C],

7.5 u2 wind speed at 2 m height [m s-1],

Baden

--187.132.3.243 (talk) 07:52, 20 March 2010 (UTC)