1 Geomorphic mapping in endorheic catchments in the Spanish Pyrenees: An integrated GIS analysis of karstic features

Digital elevation models (DEMs) can be a useful tool to assess morphologically and hydrologically active processes. This paper presents a method to obtain accurate DEMs, an analysis of DEM errors and the benefits of GIS techniques for mapping geomorphological features and active processes in the karstic endorheic catchment with three permanent lakes (Estaña lakes) in the External Ranges of the Spanish Pyrenees. Field work revealed the lack of accuracy in the original DEM. A method was then proposed to obtain a hydrologically correct DEM using a Total Topographic Station (TTS) and a Global Positioning System (GPS). On a regular grid, 246 topographic points were recorded with a GPS and 237 points were measured with a TTS on areas of gentle slope. The mean and standard deviation of the error of the corrected DEM were –1.5 and 3.2 m, respectively. By using a combined flow algorithm, a flow accumulation map was derived from the corrected DEM and the study area was divided into fifteen sub-catchments. The values of the indices of watershed circularity and elongation have a strong variation and the map of flow length also showed strong asymmetry. Maps of land use, topography and lithology were created in order to establish the basis for mapping main geomorphological features. The new DEM also provided a basis for calculating the topography factor (LS) in RUSLE, an empirical soil erosion model. A map showing the error of the original DEM indicates that the highest error values occur in the main karstic features of the geomorphological map. Hence, the proposed methodology including DEM correction is useful for an accurate assessment of topographical and geomorphological features in karstic environments.