decoupling {bigleaf} | R Documentation |

## Canopy-Atmosphere Decoupling Coefficient

### Description

The canopy-atmosphere decoupling coefficient 'Omega'.

### Usage

```
decoupling(
data,
Tair = "Tair",
pressure = "pressure",
Ga = "Ga_h",
Gs = "Gs_ms",
approach = c("Jarvis&McNaughton_1986", "Martin_1989"),
LAI,
Esat.formula = c("Sonntag_1990", "Alduchov_1996", "Allen_1998"),
constants = bigleaf.constants()
)
```

### Arguments

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

`Tair` |
Air temperature (deg C) |

`pressure` |
Atmospheric pressure (kPa) |

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

`Gs` |
Surface conductance (m s-1) |

`approach` |
Approach used to calculate omega. Either |

`LAI` |
Leaf area index (m2 m-2), only used if |

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

`constants` |
Kelvin - conversion degree Celsius to Kelvin |

### Details

The decoupling coefficient Omega ranges from 0 to 1 and quantifies the
linkage of the conditions (foremost humidity and temperature) at the canopy surface
to the ambient air. Values close to 0 indicate well coupled conditions
characterized by high physiological (i.e. stomatal) control on transpiration
and similar conditions at the canopy surface compared to the atmosphere above
the canopy. Values close to 1 indicate the opposite, i.e. decoupled conditions and
a low stomatal control on transpiration (Jarvis & McNaughton 1986).

The `"Jarvis&McNaughton_1986"`

approach (default option) is the original
formulation for the decoupling coefficient, given by (for an amphistomatous
canopy):

`\Omega = \frac{\epsilon + 1}{\epsilon + 1 + \frac{Ga}{Gc}}`

where `\epsilon = \frac{s}{\gamma}`

is a dimensionless coefficient
with s being the slope of the saturation vapor pressure curve (Pa K-1), and `\gamma`

the
psychrometric constant (Pa K-1).

The approach `"Martin_1989"`

by Martin 1989 additionally takes radiative coupling
into account:

`\Omega = \frac{\epsilon + 1 + \frac{Gr}{Ga}}{\epsilon + (1 + \frac{Ga}{Gs}) (1 + \frac{Gr}{Ga})}`

### Value

`\Omega`

- the decoupling coefficient Omega (-)

### References

Jarvis P.G., McNaughton K.G., 1986: Stomatal control of transpiration: scaling up from leaf to region. Advances in Ecological Research 15, 1-49.

Martin P., 1989: The significance of radiative coupling between vegetation and the atmosphere. Agricultural and Forest Meteorology 49, 45-53.

### See Also

`aerodynamic.conductance`

, `surface.conductance`

,
`equilibrium.imposed.ET`

### Examples

```
# Omega calculated following Jarvis & McNaughton 1986
set.seed(3)
df <- data.frame(Tair=rnorm(20,25,1),pressure=100,Ga_h=rnorm(20,0.06,0.01),
Gs_ms=rnorm(20,0.005,0.001))
decoupling(df,approach="Jarvis&McNaughton_1986")
# Omega calculated following Martin 1989 (requires LAI)
decoupling(df,approach="Martin_1989",LAI=4)
```

*bigleaf*version 0.8.2 Index]