potential.ET {bigleaf} | R Documentation |

## Potential Evapotranspiration

### Description

Potential evapotranspiration according to Priestley & Taylor 1972 or the Penman-Monteith equation with a prescribed surface conductance.

### Usage

```
potential.ET(
data,
Tair = "Tair",
pressure = "pressure",
Rn = "Rn",
G = NULL,
S = NULL,
VPD = "VPD",
Ga = "Ga_h",
approach = c("Priestley-Taylor", "Penman-Monteith"),
alpha = 1.26,
Gs_pot = 0.6,
missing.G.as.NA = FALSE,
missing.S.as.NA = FALSE,
Esat.formula = c("Sonntag_1990", "Alduchov_1996", "Allen_1998"),
constants = bigleaf.constants()
)
```

### Arguments

`data` |
Data.frame or matrix containing all required variables; optional |

`Tair` |
Air temperature (degC) |

`pressure` |
Atmospheric pressure (kPa) |

`Rn` |
Net radiation (W m-2) |

`G` |
Ground heat flux (W m-2); optional |

`S` |
Sum of all storage fluxes (W m-2); optional |

`VPD` |
Vapor pressure deficit (kPa); only used if |

`Ga` |
Aerodynamic conductance to heat/water vapor (m s-1); only used if |

`approach` |
Approach used. Either |

`alpha` |
Priestley-Taylor coefficient; only used if |

`Gs_pot` |
Potential/maximum surface conductance (mol m-2 s-1); defaults to 0.6 mol m-2 s-1;
only used if |

`missing.G.as.NA` |
if |

`missing.S.as.NA` |
if |

`Esat.formula` |
Optional: formula to be used for the calculation of esat and the slope of esat.
One of |

`constants` |
cp - specific heat of air for constant pressure (J K-1 kg-1) |

### Details

Potential evapotranspiration is calculated according to Priestley & Taylor, 1972
if `approach = "Priestley-Taylor"`

(the default):

`LE_pot,PT = (\alpha * \Delta * (Rn - G - S)) / (\Delta + \gamma)`

`\alpha`

is the Priestley-Taylor coefficient, `\Delta`

is the slope
of the saturation vapor pressure curve (kPa K-1), and `\gamma`

is the
psychrometric constant (kPa K-1).
if `approach = "Penman-Monteith"`

, potential evapotranspiration is calculated according
to the Penman-Monteith equation:

`LE_pot,PM = (\Delta * (Rn - G - S) + \rho * cp * VPD * Ga) / (\Delta + \gamma * (1 + Ga/Gs_pot)`

where `\Delta`

is the slope of the saturation vapor pressure curve (kPa K-1),
`\rho`

is the air density (kg m-3), and `\gamma`

is the psychrometric constant (kPa K-1).
The value of `Gs_pot`

is typically a maximum value of Gs observed at the site, e.g. the 90th
percentile of Gs within the growing season.

### Value

a data.frame with the following columns:

`ET_pot` |
Potential evapotranspiration (kg m-2 s-1) |

`LE_pot` |
Potential latent heat flux (W m-2) |

### Note

If the first argument `data`

is provided (either a matrix or a data.frame),
the following variables can be provided as character (in which case they are interpreted as
the column name of `data`

) or as numeric vectors, in which case they are taken
directly for the calculations. If `data`

is not provided, all input variables have to be
numeric vectors.

### References

Priestley, C.H.B., Taylor, R.J., 1972: On the assessment of surface heat flux and evaporation using large-scale parameters. Monthly Weather Review 100, 81-92.

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.

Novick, K.A., et al. 2016: The increasing importance of atmospheric demand for ecosystem water and carbon fluxes. Nature Climate Change 6, 1023 - 1027.

### See Also

### Examples

```
# Calculate potential ET of a surface that receives a net radiation of 500 Wm-2
# using Priestley-Taylor:
potential.ET(Tair=30,pressure=100,Rn=500,alpha=1.26,approach="Priestley-Taylor")
# Calculate potential ET for a surface with known Gs (0.5 mol m-2 s-1) and Ga (0.1 m s-1)
# using Penman-Monteith:
LE_pot_PM <- potential.ET(Gs_pot=0.5,Tair=20,pressure=100,VPD=2,Ga=0.1,Rn=400,
approach="Penman-Monteith")[,"LE_pot"]
LE_pot_PM
# now cross-check with the inverted equation
surface.conductance(Tair=20,pressure=100,VPD=2,Ga=0.1,Rn=400,LE=LE_pot_PM)
```

*bigleaf*version 0.8.2 Index]