Oh Forrest
, pan evapotranspiration figures are so passe for the turf manager. Below is more specific ET/o method for the modern GCA and his faithful sidekick turf professional...
Derivation of the FAO Penman-Monteith equation for the hypothetical grass reference crop:
With standardized height for wind speed, temperature and humidity measurements at 2 m (zm = zh = 2 m) and the crop height h = 0.12 m, the aerodynamic and surface resistances become (Boxes 4 & 5):
ra = 208/u2 s m-1, (with u2 wind speed at 2 m height)
rs = 70 s m-1
(1 + rs/ra) = (1 + 0.34 u2)
Rn and G is energy available per unit area and expressed in MJ m-2 day-1. To convert the energy units for radiation to equivalent water depths (mm) the latent heat of vaporization, l is used as a conversion factor (Chapter 1). The conversion from energy values to equivalent depths of water or vice versa is given by (Eq. 20):
By substituting cp with a rearrangement of Eq. 8:
and considering the ideal gas law for r a:
where TKv the virtual temperature, may be substituted by:
TKv = 1.01(T+273)
results in:
[MJ m-2 °C-1 day-1]
where
cp specific heat at constant pressure [MJ kg-1 °C-1],
r a mean air density at constant pressure [kg m-3],
ra aerodynamic resistance [s m-1],
g psychrometric constant [kPa °C-1],
e ratio molecular weight of water vapour/dry air = 0.622,
l latent heat of vaporization [MJ kg-1],
u2 wind speed at 2 m [m s-1],
R specific gas constant = 0.287 kJ kg-1 K-1,
T air temperature [°C],
P atmospheric pressure [kPa],
[MJ m-2 °C-1 day-1]
or, when divided by l (l = 2.45),
[mm °C-1 day-1]
All of which goes to show that we are certainly in over our heads... :-/