Flow accumulation

Functionality

The Flow accumulation operation performs a cumulative count of the number of pixels that naturally drain into outlets. The operation can be used to find the drainage pattern of a terrain.

As input the operation uses the output map of the Flow direction operation.
The output map contains cumulative hydrologic flow values that represent the number of input pixels which contribute any water to any outlets (or sinks if these have not been removed); the outlets of the largest streams, rivers etc. will have the largest values.

Example:

The Flow direction operation determines the natural drainage direction for every pixel in a Digital Elevation Model (DEM). Based on the output Flow direction map, the Flow accumulation operation counts the total number of pixels that will drain into outlets.

Calculating flow directions from a DEM (steepest slope)	Output flow direction map	Calculating flow accumulation	Output flow accumulation map

For more information, see Flow accumulation : algorithm.

Input map requirements:

The input map should be a raster map that is produced by the Flow direction operation, i.e. a raster map using system domain FlowDirection.

Domain and georeference of output map:

The output raster map will always use system domain Value. The operation sets the value range of the output map from 0 to 999999999, and sets the step size to 1.

The output raster map uses the same georeference as the input raster map.

Tips:

When displaying an output Flow accumulation map, you can use Logarithmic Stretching in the Display Options dialog box of the raster map.
The output Flow accumulation map is by default displayed using system representation Pseudo. If you wish, you can also create your own representation.
To show, from an output Flow accumulation map e.g. named outmap, only the pixels that receive water from for instance 25 or more input pixels, you can use MapCalc on the command line. Type for instance:

out25a = outmap >= 25		(out25a with default domain Bool)
out25b = iff(outmap >= 25, outmap, ?)		(out25b with default domain Value)