EmF n a remF na

X2 =-[Xi +Y O„,o +Y Ord>0} ; X3 = Xi/n ; X4 = X2 n The m et n variables are m_[(l-T)2 -R2 } . n_-m+R+l-T ; n m+R+l-T

The arrest of solar radiation by the vegetation is expressed by the extinction coefficient as follows (Berbigier and Bonnefond, l995):

2coshssinasin[ arccosi j }-cosasinh; lLa,hs )=-

i n-2arccos

-tghs tga

;rsinh s

Where a is the mean inclination of leaves that characterise also the orientation of a plant in a row and a row in a stand. Many authors have expressed the distribution of orientation by analytical formulas (Grancher et al , 1993). We can quote here:

The planophile distribution for a mean orientation of about 27°

The erectophile distribution for a mean orientation of about 63° g(a) = 2/n( 1 - cos(2 a))

The plagiophile distribution for a mean orientation of about 45° g(a) = 2/n( 1 - cos(4 a))

The extremophile distribution for a mean orientation of about 45° g(a) = 2/n( 1 + cos(4 a))

The spherical distribution for a mean orientation of about 57° g(a) = sin(a)

The variable (hs) in the extinction coefficient is the altitude of the radiation source (angular elevation above the horizon) which gives an indication on the hours of the day. Its presence in the expressions of the radiation flux point out their utility for monitoring, at the scale of hour, day and season, the farming productivity inside the oasis, the photo-sanitary state of plants and when testing a new species for a genetic biodiversity conservation in situ. In fact, the elevation of the radiation source is given by the following expression (Grancher et al., 1993):

Sinh = sinLat sinD + cosLat cosD cost D is the sun declination determined from

D = 0.0066241+0.406149 sin (0.0172029(T-81.95)) + 0.006675 sin (0.0344057(T-42.85)) + 0.003009 sin (0.0516086(T-21.42)) + 0.000149 sin(0.0688115(T-17.57)) Lat is the latitude of the location calculated by: Cos( t1 ) = - tg(L )tg(D) T: day number of the year t : hour angle of the sun (angular distance from the South, 1 hours = 15°) ti is the half of the astronomic day duration

From the expressions of the radiative flux we remark easily that they are expressed as function of the physiologic properties of every sort of plant, the geometrical characteristics of vegetation, plant densities and time. In fact the optical properties (reflectivity, transmittivity, absorbtivity) for a leaf, a plant or all the stand of plants, due to the re-diffusion phenomenon of solar radiation, depend on the state of the leaves surface, the disposition between leaves, branch and trunk for the same plant, its geometrical form, its position in the field and the orientation of row. The distinction of a type of vegetation from the other in our calculus is net when I remind that molecules arrangement for a kind of plant differs from that of an other and the optical properties belongs to the level of molecular diffraction. So the analysis of the optical properties gives indication about the sort of plant. Also, the reflectance ratio is deemed the photochemical reflectance index, and it is linearly related to the degree of down-regulation. Applying the technique to monitoring whole canopies requires modelling of the down-regulation process and of the propagation of radiation in the canopy. The geometrical or architectural effect is lucid in the optical properties signification and is plain in the meaning of the leaves inclination that characterise the orientation of a leaf in a plant, a plant in a row and a row in a stand. The presence of plant density in those equation is obvious in the signification of the leaf area index. This is calculated from the cumulated surface of all the components of the trees in high canopies like the forest or the traditional oasis. The presence of time appear in the meaning of the radiation source elevation which gives the hours of the day. So we can follow the plant productivity, photo-sanitary and architecture effect at any time asked. Also we can by knowing the need of tolerant plant to the efficient radiation and to oasis environmental conditions cited in the above paragraphs we can propose the plant species to install, their density and their orientation (architecture).

The solar radiation intercepted by a vegetable layer, situated between the leaf area index levels f and f + df, is generally considered equivalent to that absorbed and is determined by the balance between the received and the lost radiation:

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