Evapotranspiration

Compute the daily evaporation or transpiration of the surface using the Penman-Monteith equation.

PENMON(
  Rn,
  Wind,
  Tair,
  ZHT,
  Z_top,
  Pressure,
  Gs,
  VPD,
  LAI,
  extwind = 0,
  wleaf = 0.068,
  Parameters = Constants()
)

Arguments

Rn	Net radiation (MJ m-2 d-1)
Wind	Wind speed (m s-1)
Tair	Air temperature (Celsius degree)
ZHT	Wind measurement height (m)
Z_top	Canopy top height (m)
Pressure	Atmospheric pressure (hPa)
Gs	Stomatal conductance (mol m-2 s-1)
VPD	Vapor pressure deficit (kPa)
LAI	Leaf area index of the upper layer (m2 leaf m-2 soil)
extwind	Extinction coefficient. Default: `0`, no extinction.
wleaf	Average leaf width (m)
Parameters	Constant parameters, default to `Constants()`, if different values are needed: Cp specific heat of air for constant pressure (J K-1 kg-1) Rgas universal gas constant (J mol-1 K-1) Kelvin conversion degree Celsius to Kelvin H2OMW conversion from kg to mol for H2O (kg mol-1) GBVGBH conversion from water conductance to heat conductance

Value

$ET$, the daily (evapo|transpi)ration (mm d-1)

Details

The daily evapotranspiration is computed using the Penman-Monteith equation, and a set of conductances as : $$ET=\frac{\Delta\cdot Rn\cdot10^6+\rho\cdot Cp\cdot\frac{VPD}{10\ }\cdot GH}{\ \Delta+\frac{\gamma}{\lambda\ }\cdot(1+\frac{GH}{GV})}\ $$ where $\Delta$ is the slope of the saturation vapor pressure curve (kPa K-1), $\rho$ is the air density (kg m-3), $GH$ the canopy boundary layer conductance (m s-1), $\gamma$ the psychrometric constant (kPa K-1) and $GV$ the boundary + stomatal conductance to water vapour (m s-1). To simulate evaporation, the input stomatal conductance $Gs$ can be set to nearly infinite (e.g. $Gs= 1\cdot e^9$).

Note

If wind=0, it is replaced by a low value of 0.01

References

Allen R.G., Pereira L.S., Raes D., Smith M., 1998: Crop evapotranspiration - Guidelines for computing crop water requirements - FAO Irrigation and drainage paper 56.

Examples