Transmission of Radiation by Sparse CanopieeSoil Reflectance Effects
257
If the leaves are not horizontal, Goudriaan (1988) suggests that the
beam reflection coefficient for a deep canopy can be approximated from
The reflection coefficient for diffuse radiation can be approximated by
substituting Kd for Khe (@) in Eq. (1 5.8).
If the canopy is not dense, then the effect of the soil may be
significant and the canopy reflection coefficient for beam irradiance
becomes (Monteith and Unsworth, 1990)
Neglecting second order terms like (p&, (@)12 and pi,,,, (@)ps results
in the following simplified equation:
Equation (15.10) is a good approximation to Eq. (15.9) in the PAR, but
in the NIR, relative discrepancies can approach five percent. The diffuse
forms of Eqs. (15.9) and (15.10) have Kbe(@) replaced by Kd and are
represented by pd,,, . ps is soil reflectance.
15.6 Transmission of Radiation by Sparse
Canopies-Soil Reflectance Effects
For a canopy with a high LAI, the transmission of beam radiation (including its scattered component) as a function of depth L in the canopy
is given by Eq. (15.6). If the canopy is not dense, and the LAI is low, then
radiation can be reflected from the soil and re-reflected from the leaves
to enhance the downwelling radiation stream. Monteith and Unsworth
(1990), give the following equation for determining the flux density of
radiation under the canopy:
If the second order terms are again neglected, then Eq. (15.11) simplifies
to Eq. (15.6), and this amounts to assuming that the ratio of upwelling to
downwelling radiation below L, for a deep canopy is equivalent to the soil
reflectance for a finite canopy. In the PAR wavelength band, Eq. (15.6)
may be a reasonable approximation to Eq. (15.1 I), depending on p,, but
in the NIR, relative descrepancies of ten percent or more can occur. The
beam radiation absorbed by the canopy can be approximated with
257
If the leaves are not horizontal, Goudriaan (1988) suggests that the
beam reflection coefficient for a deep canopy can be approximated from
The reflection coefficient for diffuse radiation can be approximated by
substituting Kd for Khe (@) in Eq. (1 5.8).
If the canopy is not dense, then the effect of the soil may be
significant and the canopy reflection coefficient for beam irradiance
becomes (Monteith and Unsworth, 1990)
Neglecting second order terms like (p&, (@)12 and pi,,,, (@)ps results
in the following simplified equation:
Equation (15.10) is a good approximation to Eq. (15.9) in the PAR, but
in the NIR, relative discrepancies can approach five percent. The diffuse
forms of Eqs. (15.9) and (15.10) have Kbe(@) replaced by Kd and are
represented by pd,,, . ps is soil reflectance.
15.6 Transmission of Radiation by Sparse
Canopies-Soil Reflectance Effects
For a canopy with a high LAI, the transmission of beam radiation (including its scattered component) as a function of depth L in the canopy
is given by Eq. (15.6). If the canopy is not dense, and the LAI is low, then
radiation can be reflected from the soil and re-reflected from the leaves
to enhance the downwelling radiation stream. Monteith and Unsworth
(1990), give the following equation for determining the flux density of
radiation under the canopy:
If the second order terms are again neglected, then Eq. (15.11) simplifies
to Eq. (15.6), and this amounts to assuming that the ratio of upwelling to
downwelling radiation below L, for a deep canopy is equivalent to the soil
reflectance for a finite canopy. In the PAR wavelength band, Eq. (15.6)
may be a reasonable approximation to Eq. (15.1 I), depending on p,, but
in the NIR, relative descrepancies of ten percent or more can occur. The
beam radiation absorbed by the canopy can be approximated with