Preprocessing a DEM and calculate flow direction

Raw, unprocessed DEMs have artifacts such as depressions. Artifacts are a result of the DEM acquisition process and must be removed before a DEM can be used for hydrological analysis, like catchment and stream delineation or hydrological modelling. There are several algorithms for filling sinks.  Here, we’ll use the core QGIS functionality for filling sinks and calculating flow directions with the Wang & Liu algorithm.