Computer - assisted mesh generation based on Hydrological Response Units for distributed 1 hydrological modelling

21 Distributed hydrological models rely on a spatial discretization composed of homogeneous 22 units representing different areas within the catchment. Hydrological Response Units 23 (HRUs) typically form the basis of such a discretization. HRUs are generally obtained by 24 intersecting raster or vector layers of land uses, soil types, geology and sub-catchments. 25 Polylines maps representing ditches and river drainage networks can also be used. However 26 this overlapping may result in a mesh with numerical and topological problems not highly 27 representative of the terrain. Thus, a pre-processing is needed to improve the mesh in order 28 to avoid negative effects on the performance of the hydrological model. This paper 29 proposes computer-assisted mesh generation tools to obtain a more regular and physically 30 meaningful mesh of HRUs suitable for hydrologic modelling. We combined existing tools 31 with newly developed scripts implemented in GRASS GIS. The developed scripts address 32 the following problems: (1) high heterogeneity in Digital Elevation Model derived 33 properties within the HRUs, (2) correction of concave polygons or polygons with holes 34 inside, (3) segmentation of very large polygons, and (4) bad estimations of units’ perimeter 35 and distances among them. The improvement process was applied and tested using two 36 small catchments in France. The improvement of the spatial discretization was further 37 assessed by comparing the representation and arrangement of overland flow paths in the 38 original and improved meshes. Overall, a more realistic physical representation was 39 obtained with the improved meshes, which should enhance the computation of surface and 40 sub-surface flows in a hydrologic model. 41