1034 
2) Day 157 (June 6th): Phenological stage is “Begin ear growth”, or “Heading”. The AFRCWHEAT2 simulation 
gives 787 shoots/square meter: 250 Main Shoots with 11 leaves, 219 Tillers 1 with 8 leaves, 77 Tillers 2 with 8 
leaves, 108 Tillers 2 with 7 leaves and 133 Tillers 3 with 6 leaves. Various sizes of leaves are obtained for Tillers 
on same category depending on age. From leaf 6 on Main Shoot, leaf 5 on Tiller 1, and so on, the complete leaf is 
described by a sheat area and a lamina area. The model gives information on the senescent parts, which 
consequently have different optical properties. To avoid very expensive computational time, the area tested for 
the radiosity computation is limited to a 25 x 25 cm square. This leads to 417 leaves, but 679 leaf elements 
because sheats and laminas have different dimensions and orientations, and some leaves are turned down, as it 
can be seen on figure 4. Due to the high density, the x-y positions are randomly chosen. 
Figure 4: Observed various orientations of the highest leaves. The bottom leaves are senescent at this period and 
show a rather planophile distribution, (from Soltner, 1978). 
3 - RESULTS 
Figure 5 gives examples of radiance (red channel) reflected by a wheat field according to the radiosity equations 
applied to the canopy architecture described in the previous section. The figures 5a, 5b and 5c correspond to the 
following angular conditions, where 0s is the Sun zenith angle, 9v is the Satellite zenith angle, and y is the 
relative azimutal angle: 
a) 0s:O degre, 0v: 60 degres, y : 0 degre. 
b) 0s: 30 degres, 0v: 30 degres, y : 135 degres. 
c) 0s: 30 degres, 0v: 60 degres, y : 135 degres. 
We must point out that soil is clearly visible. In fact, it obviously has too much importance, regarding the chosen 
period, and this is due to the difficulty to correctly represent the architecture of leaves when they are numerous, 
long and with various curves. Our too simple description leads to elements which are crossing each others, which 
is completely irrealistic. However, the hot spot effect is perfectly retrieved as it can be observed on figure 6, 
where reflectances in the principal plane are plotted for several view angles. 
(a) 
(b) 
(0 
Figure 5: Rendering of wheat field canopy with the radiosity method, in visible wavelength (red). Illuminations 
and viewing conditions are explained in the text.