equilibrium.imposed.ET {bigleaf}R Documentation

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 = \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).

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)            
            

[Package bigleaf version 0.8.2 Index]