decoupling {bigleaf}

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 "Jarvis&McNaughton_1986" (default) or "Martin_1989".

LAI

Leaf area index (m2 m-2), only used if approach = "Martin_1989".

Esat.formula

Optional: formula to be used for the calculation of esat and the slope of esat. One of "Sonntag_1990" (Default), "Alduchov_1996", or "Allen_1998". See Esat.slope.

constants

Kelvin - conversion degree Celsius to Kelvin
cp - specific heat of air for constant pressure (J K-1 kg-1)
eps - ratio of the molecular weight of water vapor to dry air (-)
sigma - Stefan-Boltzmann constant (W m-2 K-4)
Pa2kPa - conversion pascal (Pa) to kilopascal (kPa)

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)