## Surface conductance

Bigleaf.decouplingFunction
TODO; implement decoupling.

This stub is there to satisfy links im Help-pages.

Bigleaf.surface_conductanceFunction
surface_conductance(::Val{:PenmanMonteith}, Tair,pressure,VPD,LE,Rn,Ga_h;
G = 0.0, S = 0.0, Esat_formula=Val(:Sonntag_1990), constants)

surface_conductance!(df, method::Val{:PenmanMonteith}; G = 0.0, S = 0.0, kwargs...)

Calculate surface conductance to water vapor from the inverted Penman-Monteith equation or from a simple flux-gradient approach.

Arguments

• Tair : Air temperature (deg C)
• pressure : Atmospheric pressure (kPa)
• Rn : Net radiation (W m-2)
• df : DataFrame with above variables
• constants=bigleaf_constants(): Dictionary with physical constants

• LE : Latent heat flux (W m-2)
• VPD : Vapor pressure deficit (kPa)
• Ga_h : Aerodynamic conductance towater vapor (m s-1), assumed equal to that of heat
• G=0.0 : Ground heat flux (W m-2), defaults to zero
• S=0.0 : Sum of all storage fluxes (W m-2), defaults to zero
• Esat_formula=Val(:Sonntag_1990): formula used in Esat_from_Tair

Details

For Val(:PenmanMonteith), surface conductance (Gs) in m s-1 is calculated from the inverted Penman-Monteith equation:

$G_s = ( LE \. G_a \, \gamma ) / ( \Delta \, A + \rho \, c_p \, G_a \, VPD - LE \, (\Delta + \gamma ))$

Where $\gamma$ is the psychrometric constant (kPa K-1), $\Delta$ is the slope of the saturation vapor pressure curve (kPa K^-1), and $\rho$ is air density (kg m-3). Available energy (A) is defined as $A = R_n - G - S$.

Ground heat flux and total storage flux can be provided as scalars or vectors of the lenght of the DataFrame in the DataFrame variant. While the bigleaf R package by default converts any missings in G and S to 0, in Bigleaf.jl the caller must take care, e.g. by using G = coalesce(myGvector, 0.0).

For Val(:FluxGradient), Gs (in mol m-2 s-1) is calculated from VPD and ET only:

$Gs = ET/p \, VPD$

where ET is in mol m-2 s-1 and p is pressure. Note that this formulation assumes fully coupled conditions (i.e. Ga = inf). This formulation is equivalent to the inverted form of Eq.6 in McNaughton & Black 1973:

$Gs = LE \, \gamma / (\rho \, c_p \, VPD)$

which gives Gs in m s-1. Note that Gs > Gc (canopy conductance) under conditions when a significant fraction of ET comes from interception or soil evaporation.

If pressure is not available, it can be approximated by elevation using the function pressure_from_elevation

Value

NamedTuple with entries:

• Gs_ms: Surface conductance in m s-1
• Gs_mol: Surface conductance in mol m-2 s-1

References

• Monteith, J., 1965: Evaporation and environment. In Fogg, G. E. (Ed.), The state and movement of water in living organisms (pp.205-234). 19th Symp. Soc. Exp. Biol., Cambridge University Press, Cambridge
• McNaughton, KG., Black, TA., 1973: A study of evapotranspiration from a Douglas Fir forest using the energy balance approach. Water Resources Research 9, 1579-1590.

Examples

Tair,pressure,VPD,LE,Rn,Ga_h,G = (14.8, 97.7, 1.08, 183.0, 778.0, 0.116, 15.6)
Gs = surface_conductance(Val(:PenmanMonteith), Tair,pressure,VPD,LE,Rn,Ga_h;G)
isapprox(Gs.Gs_mol, 0.28, atol=0.1)