bigleaf (version 0.7.0)

potential.ET: 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")

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)

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.

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

# NOT RUN {
# 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)
# }