I want to create a flood map using Quantum GIS. First of all I create catchment area (flow accumulation). Then, I delineate a Channel Network by applying a threshold to flow accumulation. So I want to know, How can I determine threshold value and what is the mathemathical concept?

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    The threshold value in watershed delineation depends of your needs, size of the region and the resolution of the DEM that your analysis is based on. The value represents the number of raster cells that would make up a minimum catchment area. So if, for example, you want to delineate catchments of 10 sq km or more, then you need to calculate how many raster cells would total to 10 sq km. – Micha Feb 24 '15 at 7:00

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