Rugosite du paysage et evapotranspiration potentielle a l'echelle regionale

Abstract

Contrary to the microclimatic influences of a single windbreak, those of a network of parallel belts are still imperfectly known.A simple generalization of the individual case by taking into account a number of obstacles and the characteristics of the adjacent surfaces seems problematic; by considering a network of windbreaks as constituting the elements of a regional system, one may be able to deduce a roughness applicable to a regional scale.The roughness depends on height, porosity and relative spacing of the constitutive elements. Every natural surface and, particularly, organized networks of tree belts, may be characterized by two classical parameters: the roughness parameter z0 and the zero-plane displacement D. A theoretical study of the variation of z0 and D for varying relative spacings shows the existence of an optimal spacing (about 5 to 10 times the height of the windbreaks according to experiments) which will create a maximum relative roughness.The influence of surface roughness on the microclimate is studied theoretically, assuming sufficiently large homogeneous surfaces, and neglecting local advection. An increase in roughness tends to increase the turbulent transfer of momentum. As Reynold's analogy is no longer valid near a surface of high roughness, the creation of isolated cells leads to a reduction of heat and mass transfer through the surface. By using Chamberlain's empirical expression for this effect, the calculated wind function f (u) in Penman's equation may be written as follows: f(u) ∼ u∗n, with 0,55 ⩽ n ⩽ 1∗For near-smooth surfaces n tends to become 1. For rougher surfaces n is closer to 0,75. These results agree well with those of Brutsaert (1965).As networks of tree belts create new roughness elements in the landscape concerned, the evaporative power (Ea) and, thereby, the regional evapotranspiration (ETP) are expected to decrease. This would be in agreement with the experimental observations of several authors. For the purposes of rational land use, from the point of view of climatology, a maximum roughness in areas of strong winds could be achieved by planning an optimal spacing of the windbreaks, in accordance with the different species of windbreaks used. This is, however, not valid in transitional zones, where the increase of the drag effects due to the changes of topography may produce opposite effects.