potential_evapotranspiration_30min#

hydrometlib.evapotranspiration.potential_evapotranspiration_30min(rn, g, ta, rh, ws, pa, wind_height=None)[source]#

Calculate potential evapotranspiration (pet) [mm 30min-1]

PET is the maximum amount of water that could be evapotranspirated in a given climate, given a theoretical continuous expanse of vegetation covering the whole ground and a continuous supply of water.

\[PET = \frac{0.408 \, \Delta \, (R_n - G) + \gamma \, \dfrac{19}{T_a + 273} \, u_2 \, (e_s - e_a)} {\Delta + \gamma \, (1 + 0.34 \, u_2)}\]

where \(R_n\) and \(G\) are converted from W m-2 to MJ m-2 (30 min)-1 by multiplying by 0.0018, \(e_s - e_a\) is the vapour pressure deficit, and \(u_2\) is the wind speed at 2 m: corrected from wind_height when that is given, otherwise ws is taken to already be at 2 m.

The numerator constant 19 is the FAO-56 daily value (900) scaled approximately to a 30-minute step (900 / 48).

Note from FAO-56 on shorter timescales: “With the advent of electronic,automated weather stations, weather data are increasingly reported for hourly or shorter periods … When applying the FAO Penman-Monteith equation on an hourly or shorter timescale, the equation and some of the procedures for calculating meteorological data should be adjusted for the smaller time step”.

References

Parameters:

  • rn (Expr) – Net radiation [W m-2]

  • g (Expr) – Soil heat flux density [W m-2]

  • ta (Expr) – Air temperature [degC]

  • rh (Expr) – Relative humidity [%]

  • ws (Expr) – Wind speed measured at wind_height [m s-1]

  • pa (Expr) – Atmospheric pressure [hPa]

  • wind_height (Optional[Expr]) – Site attribute - optional height above ground of the wind sensor [m]. Give it as the single value it usually is, or as a column when the height varies row by row (for example if the sensor is moved). Omit it (or pass None) when the sensor is at the standard 2 m height, in which case no height correction is applied to ws.

Return type:

Expr

Returns:

Expression or Series computing PET [mm 30min-1]