Partitioning of long wave radiation between plants

Net long wave radiation of the canopy is normally considered implicitly through the partitioning of net radiation between plants and soil following equations (4.67) - (4.71). It is also possible to explicitly calculate the long wave radiation balance of the plants taking the plant temperature into account (see Switch LongRadCanopy). This is important when the downward long wave radiation to the surface below the canopy is of special interest, for instance for snow melt in dense forest stands. In this case, short wave and long wave balances are calculated separately, short wave following equations (4.67)  - (4.71)  and long wave as described below. Plants are assumed to absorb long wave radiation from above and below following Beer’s law, eq.(4.1), and to emit radiation as a function of the plant temperature upwards and downwards.

For a single plant, the long wave radiation balance is then:

                      (4.72)

where R_lnet,j is the long wave net radiation for a plant, R_ol,j is the long wave radiation emitted by a plant, and R_ol,ground the long wave radiation emitted by the ground (snow and/or soil) surface below the canopy. A_l,j is the plant leaf area index and -k_rn is the extinction coefficient. The long wave radiation emitted by a plant, R_ol,j, is calculated as:

                                                (4.73)

where T_j is the plant surface temperature.

The long wave radiation emitted from the ground, R_ol,ground, is calculated as:

                                       (4.74)

where T_ground is the ground temperature.

For a canopy of two or more plants the distribution is made following the notation used in equations (4.67) - (4.71)  . Each plant absorbs and emits long wave radiation in relation to its contribution to the total leaf area index within a height segment ∆H_i in the horizontal zone ∆x_k according to:

(4.75)

 

 

where R_il is the downward long wave radiation from the segment above, and R_ol is the upward long wave radiation from the segment below. Calculations are made in two steps. First, the downward components are accumulated from the top of the canopy to the ground surface:

        (4.76)

 

starting with the downward long wave radiation from the atmosphere for i=1. Second, the upward components are added starting with the upward long wave radiation from the surface for the lowest canopy layer.