equilibrium.imposed.ET: Equilibrium and Imposed Evapotranspiration

In bigleaf: Physical and Physiological Ecosystem Properties from Eddy Covariance Data

Equilibrium and Imposed Evapotranspiration

Description

Evapotranspiration (ET) split up into imposed ET and equilibrium ET.

Usage

equilibrium.imposed.ET(
  data,
  Tair = "Tair",
  pressure = "pressure",
  VPD = "VPD",
  Gs = "Gs_ms",
  Rn = "Rn",
  G = NULL,
  S = NULL,
  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 input variables
Tair	Air temperature (deg C)
pressure	Atmospheric pressure (kPa)
VPD	Air vapor pressure deficit (kPa)
Gs	surface conductance to water vapor (m s-1)
Rn	Net radiation (W m-2)
G	Ground heat flux (W m-2); optional
S	Sum of all storage fluxes (W m-2); optional
missing.G.as.NA	if TRUE, missing G are treated as NAs, otherwise set to 0.
missing.S.as.NA	if TRUE, missing S are treated as NAs, otherwise set to 0.
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	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 (-) Pa2kPa - conversion pascal (Pa) to kilopascal (kPa)

Details

Total evapotranspiration can be written in the form (Jarvis & McNaughton 1986):

ET = Ω ET_eq + (1 - Ω)ET_imp

where Ω is the decoupling coefficient as calculated from decoupling. ET_eq is the equilibrium evapotranspiration rate, the ET rate that would occur under uncoupled conditions, where the heat budget is dominated by radiation (when Ga -> 0):

ET_eq = (Δ * (Rn - G - S) * λ) / (Δ + γ)

where Δ is the slope of the saturation vapor pressure curve (kPa K-1), λ is the latent heat of vaporization (J kg-1), and γ is the psychrometric constant (kPa K-1). ET_imp is the imposed evapotranspiration rate, the ET rate that would occur under fully coupled conditions (when Ga -> inf):

ET_imp = (ρ * cp * VPD * Gs * λ) / γ

where ρ is the air density (kg m-3).

Value

A data.frame with the following columns:

ET_eq	Equilibrium ET (kg m-2 s-1)
ET_imp	Imposed ET (kg m-2 s-1)
LE_eq	Equilibrium LE (W m-2)
LE_imp	Imposed LE (W m-2)

Note

Surface conductance (Gs) can be calculated with surface.conductance. Aerodynamic conductance (Ga) can be calculated using aerodynamic.conductance.

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.

Monteith, J.L., Unsworth, M.H., 2008: Principles of Environmental Physics. 3rd edition. Academic Press, London.

See Also

decoupling

Examples

df <- data.frame(Tair=20,pressure=100,VPD=seq(0.5,4,0.5),
                 Gs_ms=seq(0.01,0.002,length.out=8),Rn=seq(50,400,50))            
equilibrium.imposed.ET(df)            
            

bigleaf documentation built on Aug. 22, 2022, 9:09 a.m.