Modelling of evaporation processes over tilted slopes by means of 3D GRASS raster

The purpose of the present work is to create a tool able to simplify the analysis of atmospheric phenomena in lower atmosphere; with specific application to up-slope winds on sloping terrain and the evaporation of water vapor from the vegetation. Models of atmospheric processes occurring close to the ground and GIS display a point of contact since most of the input variables of lower atmosphere phenomena are distributed on the land surface and the latter display have a strong interaction with the boundary condition; the land cover and variables like local slope, irradiance and exposition have in fact a large influence on the evolution of the wind and the evaporation processes. On the other hand, the intrinsic 3D character of such phenomena has so far prevented the development of atmospheric models within a GIS environment, since no tools for the management and elaboration of 3D data were available. The recent introduction of the 3D raster management modules in GRASS allowed implementation of a model for the evaluation of wind profiles and humidity distribution along a sloping terrain. This model exploits the power of the 3D GRASS map algebra (r3.mapcalc) to implement a simplified model generalizing Prandtl (1942) model for slope winds to take into account humidity and evaporation processes on the soil, in order to obtain wind velocity and virtual potential temperature profiles. Both this variables depends on direction and modulus of the slope normal therefore a method to evaluate the normal vector to the surface and the distance of a given point from the surface has been developed. Two applications to both an ideal valley and a real one, with validation of the model results, are presented.