potential.ET {bigleaf}

Potential Evapotranspiration

Description

Potential evapotranspiration according to Priestley & Taylor 1972 or the Penman-Monteith equation with a prescribed surface conductance.

Usage

potential.ET(
  data,
  Tair = "Tair",
  pressure = "pressure",
  Rn = "Rn",
  G = NULL,
  S = NULL,
  VPD = "VPD",
  Ga = "Ga_h",
  approach = c("Priestley-Taylor", "Penman-Monteith"),
  alpha = 1.26,
  Gs_pot = 0.6,
  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 variables; optional

Tair

Air temperature (degC)

pressure

Atmospheric pressure (kPa)

Rn

Net radiation (W m-2)

G

Ground heat flux (W m-2); optional

S

Sum of all storage fluxes (W m-2); optional

VPD

Vapor pressure deficit (kPa); only used if approach = "Penman-Monteith".

Ga

Aerodynamic conductance to heat/water vapor (m s-1); only used if approach = "Penman-Monteith".

approach

Approach used. Either "Priestley-Taylor" (default), or "Penman-Monteith".

alpha

Priestley-Taylor coefficient; only used if approach = "Priestley-Taylor".

Gs_pot

Potential/maximum surface conductance (mol m-2 s-1); defaults to 0.6 mol m-2 s-1; only used if approach = "Penman-Monteith".

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)
Rd - gas constant of dry air (J kg-1 K-1) (only used if approach = "Penman-Monteith")
Rgas - universal gas constant (J mol-1 K-1) (only used if approach = "Penman-Monteith")
Kelvin - conversion degree Celsius to Kelvin (only used if approach = "Penman-Monteith")

Details

Potential evapotranspiration is calculated according to Priestley & Taylor, 1972 if approach = "Priestley-Taylor" (the default):

LE_pot,PT = (\alpha * \Delta * (Rn - G - S)) / (\Delta + \gamma)

\alpha is the Priestley-Taylor coefficient, \Delta is the slope of the saturation vapor pressure curve (kPa K-1), and \gamma is the psychrometric constant (kPa K-1). if approach = "Penman-Monteith", potential evapotranspiration is calculated according to the Penman-Monteith equation:

LE_pot,PM = (\Delta * (Rn - G - S) + \rho * cp * VPD * Ga) / (\Delta + \gamma * (1 + Ga/Gs_pot)

where \Delta is the slope of the saturation vapor pressure curve (kPa K-1), \rho is the air density (kg m-3), and \gamma is the psychrometric constant (kPa K-1). The value of Gs_pot is typically a maximum value of Gs observed at the site, e.g. the 90th percentile of Gs within the growing season.

Value

a data.frame with the following columns:

ET_pot

Potential evapotranspiration (kg m-2 s-1)

LE_pot

Potential latent heat flux (W m-2)

Note

If the first argument data is provided (either a matrix or a data.frame), the following variables can be provided as character (in which case they are interpreted as the column name of data) or as numeric vectors, in which case they are taken directly for the calculations. If data is not provided, all input variables have to be numeric vectors.

References

Priestley, C.H.B., Taylor, R.J., 1972: On the assessment of surface heat flux and evaporation using large-scale parameters. Monthly Weather Review 100, 81-92.

Allen, R.G., Pereira L.S., Raes D., Smith M., 1998: Crop evapotranspiration - Guidelines for computing crop water requirements - FAO Irrigation and drainage paper 56.

Novick, K.A., et al. 2016: The increasing importance of atmospheric demand for ecosystem water and carbon fluxes. Nature Climate Change 6, 1023 - 1027.

See Also

surface.conductance

Examples

# Calculate potential ET of a surface that receives a net radiation of 500 Wm-2
# using Priestley-Taylor:
potential.ET(Tair=30,pressure=100,Rn=500,alpha=1.26,approach="Priestley-Taylor")    

# Calculate potential ET for a surface with known Gs (0.5 mol m-2 s-1) and Ga (0.1 m s-1)
# using Penman-Monteith:
LE_pot_PM <- potential.ET(Gs_pot=0.5,Tair=20,pressure=100,VPD=2,Ga=0.1,Rn=400,
                          approach="Penman-Monteith")[,"LE_pot"]
LE_pot_PM

# now cross-check with the inverted equation
surface.conductance(Tair=20,pressure=100,VPD=2,Ga=0.1,Rn=400,LE=LE_pot_PM)