The processing time for these was measured in days or weeks, never hours. Output cells with a high flow accumulation are areas of concentrated flow and can be used to identify stream channels. Run the Flow Accumulation tool using the output raster from step 3 as the 'Input flow direction raster'. If you would like to allow or restrict tracing on edges with indeterminate or uninitialized flow, be sure to set the ITraceFlowSolver::TraceIndeterminateFlow property prior to calling this trace solver.. Run the Flow Direction tool using the output raster from step 2 as the 'Input surface raster' and checking 'Force all edge cells to flow outward'. Specify the location of the Output accumulation raster. The Flow Accumulation tool supports three flow modeling algorithms while computing accumulated flow. Output cells with a high flow accumulation are areas of concentrated flow and can be used to identify stream channels. Preferably the points we wish to use as input for the delimitation of a contributing area should be located along the stream definition (drainage network) determined by the flow accumulation raster output. Note: The Flow Direction tool creates a raster of flow direction from each cell to the steepest downslope neighbor. The current processing cell is not considered in this accumulation. The Flow Accumulation output raster will allow the delineation of the draining area to any specified point. Remarks. This is using ArcHydro. Note: For more information on this tool read Flow Direction (Spatial Analyst). The accumulated flow is based on the number of cells flowing into each cell in the output raster. Recommended workflow: The accumulated flow is based on the number of cells flowing into each cell in the output raster. ArcGIS searches for cells at the edge of the grid that have this amount of flow accumulation, and turns these cells into pour points. Use the output from Step 2 as the 'Input flow direction raster'. If no weight raster is provided, a weight of 1 is applied to each cell, and the value of cells in the output raster is the number of cells that flow into each cell. Compute watershed grids: fill, flow direction, and flow accumulation The Flow Accumulation tool calculates accumulated flow as the accumulated weight of all cells flowing into each downslope cell in the output raster. The FlowMethod parameter determines whether the trace solver traces upstream, downstream, or on all connected edges. Navigate to Spatial Analyst Tools > Hydrology > Flow Accumulation. The current processing cell is not considered in this accumulation. Output cells with a high flow accumulation are areas of concentrated flow and can be used to identify stream channels. The current processing cell is not considered in this accumulation. Run the Flow Accumulation tool. Note: Manual manipulation of the symbology on the flow accumulation raster shows the cells where water is prone to accumulate, which correlates with large cell values. Usage. Click OK. Here are watersheds generated by using Basin This method potentially creates a large number of watersheds, none of which match any boundaries that could be used for management planning. The result of Flow Accumulation is a raster of accumulated flow to each cell, as determined by accumulating the weight for all cells that flow into each downslope cell.. Using ArcGIS 9.3.1 I've had to create flow accumulation grids from flow direction grids ranging from 150ish to 230ish megs. The accumulated flow is based on the number of total or a fraction of cells flowing into each cell in the output raster. Cells of undefined flow direction will only receive flow; they will not contribute to any downstream flow. 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