Calculate the wind speed decrease in two steps:
Decrease the measured wind speed from measurement height until top of the canopy using the formulation of Van de Griend and Van Boxel (1989)
Decrease wind speed further with increasing canopy depth using an exponential extinction coefficient and a cumulated LAI above the target point.
GetWind( Wind, LAI_lay, LAI_abv, extwind = 0, Z_top, ZHT, Z0 = Z_top * 0.1, ZPD = Z_top * 0.75, alpha = 1.5, ZW = ZPD + alpha * (Z_top - ZPD), vonkarman = Constants()$vonkarman )
| Wind | Above canopy wind speed (m s-1) |
|---|---|
| LAI_lay | Leaf area index of the layer (m2 leaves m-2 soil) |
| LAI_abv | Cumulated leaf area index above the layer (m2 leaves m-2 soil) |
| extwind | Extinction coefficient. Default: |
| Z_top | Average canopy height of the taller crop (m) |
| ZHT | Wind measurement height (m) |
| Z0 | Roughness length (m). Default: |
| ZPD | Zero-plane displacement (m), Default: |
| alpha | Constant for diffusivity at top canopy. Default: |
| ZW | Top height of the roughness sublayer (m). Default: |
| vonkarman | Von Karman constant, default to |
The winspeed at the center of the layer (m s-1)
The function computes the average wind speed at the center of the canopy layer. It is considered
that the leaf distibution is homogeneous in the layer, so the LAI_lay parameter is used to
add half of the target layer to the cumulated LAI above:
$$WindLay=Wh*e^{^{\left(-extwind*\left(LAI_{abv}+\frac{LAI_{lay}}{2}\right)\right)}}$$
with Wh the wind speed at top of the canopy.
Note: the alpha parameter can also be computed as:
$$alpha=\frac{zw-d}{Z2-d}$$
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. Part of the code is taken from the MAESPA model.
# Windspeed in a coffee layer managed in agroforestry system GetWind(Wind=3,LAI_lay=4,LAI_abv=0.3,extwind= 0.58,Z_top = 24,ZHT = 25)#> [1] 0.7297131