# How Flow Accumulation works

The Flow Accumulation tool calculates accumulated flow as the accumulated weight of all cells flowing into each downslope cell in the output 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.

In the graphic below, the top left image shows the direction of travel from each cell and the top right the number of cells that flow into each cell.

Cells with a high flow accumulation are areas of concentrated flow and may be used to identify stream channels. This is discussed in Identifying stream networks. Cells with a flow accumulation of 0 are local topographic highs and may be used to identify ridges.

## Example

A sample usage of the Flow Accumulation tool with an input weight raster might be to determine how much rain has fallen within a given watershed. In such a case, the weight raster may be a continuous raster representing average rainfall during a given storm. The output from the tool would then represent the amount of rain that would flow through each cell, assuming that all rain became runoff and there was no interception, evapotranspiration, or loss to groundwater. This could also be viewed as the amount of rain that fell on the surface, upslope from each cell.

The results of Flow Accumulation can be used to create a stream network by applying a threshold value to select cells with a high accumulated flow.

For example, the procedure to create a raster where the value 1 represents the stream network on a background of NoData could use one of the following:

• Perform a conditional operation with the Con tool with the following settings:
• Input conditional raster : Flowacc

Expression : Value > 100

Input true raster or constant : 1

• Alternatively, run the Set Null tool with the following settings:
• Input conditional raster: : Flowacc

Expression: : Value <= 100

Input false raster or constant: : 1

In both examples, all cells that have more than 100 cells flowing into them are assigned 1; all other cells are assigned NoData. For future processing, it is important that the stream network, a set of raster linear features, be represented as values on a background of NoData.

This method of deriving accumulated flow from a DEM is presented in Jenson and Domingue (1988). An analytic method for determining an appropriate threshold value for stream network delineation is presented in Tarboton et al. (1991).

## References

Jenson, S. K., and J. O. Domingue. 1988. "Extracting Topographic Structure from Digital Elevation Data for Geographic Information System Analysis." Photogrammetric Engineering and Remote Sensing 54 (11): 1593–1600.

Tarboton, D. G., R. L. Bras, and I. Rodriguez–Iturbe. 1991. "On the Extraction of Channel Networks from Digital Elevation Data." Hydrological Processes 5: 81–100.