I am looking to calculate flow accumulation from a flow direction raster. In ArcGIS there is Flow Accumulation (http://desktop.arcgis.com/en/arcmap/10.3/tools/spatial-analyst-toolbox/flow-accumulation.htm), that seems to do the job. However, I am looking something in opensource software, but all that I have found require using a DEM.
Any function in GRASS or SAGA?
Best Answer
The following is a step-by-step process for how to take an ArcGIS D8 flow direction raster and use it to perform a D8 flow accumulation operation in Whitebox Geospatial Analysis Tools. I will preface this by saying that it would obviously be much easier if you had the original DEM from which the flow direction raster was derived. Also, I'm making the assumption that your flow direction raster was derived using the D8 flow algorithm and that it was derived using the ArcGIS flow pointer scheme.
The first step is getting your flow direction raster from ArcGIS into Whitebox GAT. There are several ways to do this, but I recommend using the Raster to Float tool in ArcGIS to convert your raster into a .flt file. This is one of the most efficient formats for getting data into Whitebox GAT. Next, open Whitebox GAT and add the layer to the map. A pop-up window will appear asking if you'd like to convert the raster to Whitebox format, and you simply need to click 'yes'.
The ArcGIS D8 flow direction raster uses the following scheme:
Which is different than Whitebox GAT's:
So you need to use the Reclass tool to convert the pointer values:
As for your question about the weight raster, you might want to take a look at the D8 Mass Flux tool, which allows for a weight (loading) file and two types of losses (efficiency, which is a proportional loss, and absorption, which is a loss amount).