bigleaf (version 0.7.0)

equilibrium.imposed.ET: 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)

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)

Details

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

$$ET = \Omega ET_eq + (1 - \Omega)ET_imp$$

where \(\Omega\) 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):

$$ET_eq = (\Delta * (Rn - G - S) * \lambda) / (\Delta + \gamma)$$

where \(\Delta\) is the slope of the saturation vapor pressure curve (kPa K-1), \(\lambda\) is the latent heat of vaporization (J kg-1), and \(\gamma\) 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):

$$ET_imp = (\rho * cp * VPD * Gs * \lambda) / \gamma$$

where \(\rho\) is the air density (kg m-3).

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

# NOT RUN {
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)            
            
# }