Optimisation des paramètres de la procédure géomatique d'extraction des caractéristiques hydrologiques d'un bassin versant à partir d'un modèle numérique d'altitude
Modelling of water terrestrial superficial runoff, under the gravity action, is very useful in environmental sciences. Using a raster geographic information system (GIS), it is actually possible to generate the gridded waterway network on a watershed. The most frequently used drainage algorithm identifies the flow direction (aspect) from each cell to one of the eight adjacent cells (8D) according to the steepest elevation gradient. This drainage pattern allows the generation of the hydrographic network and subwatersheds. The basic input data is a digital elevation model (DEM), extracted from elevation contours and rasterised according to a grid cell size. The vectorial terrain network constitutes the only reference data needed. To control the hydrographic tree density, a minimal drainage area generating perennial streams has to be adjusted; but the error associated with the cartographic results can not be explicitly calculated owing to the hierarchical structure of the drainage algorithm. Classical Euclidean distance operations available in GIS are not adapted to quantify the deviation between the reference and the synthetic networks. This particular application requires the use of a drainage distance based on the flow path. Such concept allows the establishment of a unique error estimation that quantifies the spatial agreement between the generated and the reference networks, reflecting the accuracy of the drainage scheme from which all other hydrologic features are extracted. This optimisation technique has been tested on the Boyer River watershed located 25 km south-east of Quebec city and covering an area of approximately 220 km<sup>2</sup>. The 8D drainage algorithm used, which has been adapted and completed, allows for the consideration of low relief characteristics where elevations range from 275 m upstream to 10 m at the outlet in the Saint-Lawrence River. The reference DEM has been generated by interpolation of the elevation isolines available on the 1:20 000 numerical topographic maps. The optimal grid cell size for the drainage algorithm has been found to be 125 m. Monte-Carlo simulations have been performed to assess the accuracy changes resulting from a DEM rotation of 22,5° and from the introduction of random errors in the initial DEM. The present work constitutes a targeted advance in the field of environmental geomatics. It replaces multiple geometric indexes, generally used in hydrology to compare two networks, by a unique georeferenced measure well adapted to the gravitational nature of the runoff. Moreover, it proposes an objective and universal criterion to reach optimality, substituting traditional rules of thumb. Finally, it opens perspectives for watershed model construction relating land use exports to surface water quality.