G_bulk: Bulk aerodynamic conductance

In VEZY/DynACof: A Process-Based Model to Study Growth, Yield and Ecosystem Services of Coffee Agroforestry Systems

Description

Compute the aerodynamic conductance for sensible and latent heat above the canopy following Van de Griend and Van Boxel (1989).

Usage

G_bulk(
  Wind,
  ZHT,
  Z_top,
  Z0 = Z_top * 0.1,
  ZPD = Z_top * 0.75,
  alpha = 1.5,
  ZW = ZPD + alpha * (Z_top - ZPD),
  LAI,
  extwind = 0,
  vonkarman = Constants()$vonkarman
)

Arguments

Wind	Average daily wind speed above canopy (m s-1)
ZHT	Wind measurement height (m)
Z_top	Average canopy height of the taller crop (m)
Z0	Roughness length (m). Default: 0.1*Z_top
ZPD	Zero-plane displacement (m), Default: 0.75*Z_top
alpha	Constant for diffusivity at top canopy. Default: 1.5 following Van de Griend et al (1989).
ZW	Top height of the roughness sublayer (m). Default: ZPD+alpha*(Z_top-ZPD)
LAI	Leaf area index of the upper layer (m2 leaf m-2 soil)
extwind	Extinction coefficient. Default: 0, no extinction.
vonkarman	Von Karman constant, default to Constants()$vonkarman, 0.41.

Details

alpha can also be computed as:

alpha= (zw-d)/(Z_top-d)

The bulk aerodynamic conductance ga_bulk is computed as follow:

ga_bulk= 1/(r1+r2+r3)

where r1, r2 and r3 are the aerodynamic resistances of the inertial sublayer, the roughness sublayer and the top layer of the canopy respectively. Because wind speed measurements are more often made directly in the roughness sublayer, the resistance in the inertial sublayer r1 is set to 0 though. r2 and r3 are computed using the equation 43 of Van de Griend and Van Boxel (refer to the pdf of the web version of the help file for Latex rendering) :

r2= Integral{zh,zw}(1/K'')dz

with

K''= k x Ustar x (zw-d)

And:

r3= Integral{z2+z1)/2,zh}(1/K''')dz

with

K'''= Uz x (Kh/Uh)

Integration of r2 and r3 equations give:

r2= (log((ZPD-ZW)^2)-log((ZPD-Z2)^2))/(2*vonkarman*Ustar)

simplified in:

r2= (1/(vonkarman*Ustar))*log((ZPD-ZW)/(ZPD-Z2))

and finaly:

r3= (Uh/Kh)*log(Uh/U_interlayer)

Value

G_bulk

The bulk aerodynamic conductance (m s-1)

References

Van de Griend, A.A. and J.H. Van Boxel, Water and surface energy balance model with a multilayer canopy representation for remote sensing purposes. Water Resources Research, 1989. 25(5): p. 949-971.

See Also

G_interlay() and GetWind(), which is used internaly.

Examples

# The bulk aerodynamic conductance for a coffee plantation managed in agroforestry system:
G_bulk(Wind=3,ZHT=25,Z_top=24,LAI = 0.5,extwind = 0.58)

VEZY/DynACof documentation built on Feb. 3, 2021, 8:52 p.m.