FAO 56 reference ET

The FAO 56 reference ET model is applied by default when selecting the FAO 56 climate model. The FAO 56 reference ET model uses the standard­ised Penman-Monteith equation for calculating reference evapotranspiration and it requires no additional input.

The reference ET is calculated as:

(A.3.1)   

Where:

ET₀: Reference evapotranspiration [mm/day],

R_n: Net radiation at the crop surface [MJ/m²/day],

G: Soil heat flux density [MJ/m²/day],

T: Air temperature at 2 m height [°C],

u₂: Wind speed at 2 m height [m/s],

e_s: Saturation vapour pressure [kPa],

e_a: Actual vapour pressure [kPa],

e_s-e_a: Saturation vapour pressure deficit [kPa],

Slope vapour pressure curve [kPa/°C],

Psychrometric constant [kPa/°C].

The FAO Penman-Monteith equation provides the evapotranspiration from a hypothetical grass reference surface and provides a standard to which evap­otranspiration in different periods of the year or in other regions can be com­pared and to which the evapotranspiration from other crops can be related.

A detailed description of the calculation procedure for the Penman-Monteith formulation is described in FAO 56, and contains the following calculation steps:

·         Derivation of all required climatic parameters from the daily maximum (T_max) and minimum (T_min) air temperature, altitude (z), mean wind speed (u₂) and geographical location.

·         Calculation of the vapour pressure deficit (e_s - e_a). The saturation vapour pressure (es) is derived from the mean temperature, which is assumed to be the average of T_max and T_min. The actual vapour pressure (e_a) is be derived from the minimum temperature, which is assumed to equal the dew-point temperature.

·         Determination of the net radiation (R_n) as the difference between the net short-wave radiation (R_ns) and the net long-wave radiation (R_nl). These variables are derived from geographical location, sunshine hours and vapour pressure. The effect of soil heat flux (G) is ignored for time steps smaller than 10 days as the magnitude of the flux in this case is relatively small and ET₀ is obtained by combining the results of the previous steps.