Editing Potential evapotranspiration (section)

===  FAO 56 Penman–Monteith equation (1998) ===
The '''[[Penman–Monteith equation]]''' refines weather based [[evapotranspiration]] (ET) estimates of vegetated land areas. This equation was then derived by FAO for retrieving the potential evapotranspiration ET<sub>0</sub>. <ref>{{cite book |last=Allen |first=R.G. |author2=Pereira, L.S. |author3=Raes, D. |author4= Smith, M.  |title=Crop Evapotranspiration—Guidelines for Computing Crop Water Requirements |url=http://www.fao.org/docrep/X0490E/x0490e00.HTM |accessdate=2007-10-08 |series=FAO Irrigation and drainage paper 56 |year=1998 |publisher=Food and Agriculture Organization of the United Nations |location=Rome, Italy |isbn=92-5-104219-5 }}</ref> It is widely regarded as one of the most accurate models, in terms of estimates.

:<math> ET_0 = \frac{0.408 \Delta (R_n-G) + \frac{900}{T} \gamma u_2 \delta e }{\Delta + \gamma (1 + 0.34 u_2)}
</math>

:''ET''<sub>0</sub> = Potential evapotranspiration, Water volume evapotranspired (mm&nbsp;day<sup>−1</sup>)
:&Delta; = Rate of change of saturation specific humidity with air temperature. (Pa&nbsp;K<sup>−1</sup>)
:''R''<sub>n</sub> = Net [[irradiance]] (MJ m<sup>−2</sup> day<sup>−1</sup>), the external source of energy flux
:''G'' = Ground heat flux (MJ m<sup>−2</sup> day<sup>−1</sup>), usually equivalent to zero on a day
:''T'' = Air temperature at 2m height (K)
:''u<sub>2</sub>'' = Wind speed at 2m height (m s<sup>−1</sup>)
:δ''e'' = [[Vapor pressure]] deficit (kPa)
:''γ'' = [[Psychrometric constant]] (''&gamma;'' ≈ 66 Pa&nbsp;K<sup>−1</sup>)

N.B.: The coefficients 0.408 and 900 are not unitless but account for the conversion from energy values to equivalent water depths: radiation [mm day<sup>−1</sup>] = 0.408 radiation [MJ m<sup>−2</sup> day<sup>−1</sup>].