Cells with higher values will tend to be located in drainage channels rather than on hillsides or ridges. In the Flow Accumulation layer each cell contains a value representing the number of cells upstream from that cell. The numbers in the Flow Direction layer show flow direction from each cell to its steepest downslope neighbor.ģ.- Flow Accumulation: This tool calculates the flow into each cell by accumulating the cells that flow into each downslope cell. Areas of internal drainage can cause problems later in the watershed delineation process.Ģ.- Flow Direction: Direction of flow must be known for each cell, because it is direction of flow that determines the ultimate destination of water flowing across the surface.
It is important to build a depressionless DEM for all subsequent hydrological analyses. Set the input surface to the Digital Elevation Model (DEM) of the study area.
The study area is from a small torrential basin in the central Pyrenees (Aras ravine), well known for a flash flood event that wiped out a campsite on 7th August, 1996 claiming the lives of 87 people.įirst, in the ArcToolbox window expand Spatial Analyst Tools > Hydrology.ġ.- Fill sinks in the Digital Elevation Model: We start by removing small imperfections in the digital elevation model using the Fill tool. The required data for this tutorial is a 5×5 m cell size Digital Elevation Model (DEM) obtained from the National Aerial Orthoimagery Dataset ( PNOA).
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