# Flow Accumulation function

## Overview

Creates a raster layer of accumulated flow into each cell. A weight factor can optionally be applied.

The result is a raster layer of accumulated flow to each cell, as determined by accumulating the weight for all cells that flow into each downslope cell. 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.

For more details, see the How flow accumulation works topic.

This is a global raster function.

## Notes

The Flow Accumulation function supports three flow modeling methods for computing accumulated flow, including D8, Multiple Flow Direction (MFD) and D-Infinity (DINF).

The flow direction raster can be created using the Flow Direction function.

• If the input flow direction raster is not created with the Flow Direction function, there is a chance that the defined flow could fail by going into an endless loop.
• Input flow direction can be created using the D8, MFD or DINF methods. The type of input flow direction raster between these three methods influences how the Flow Accumulation function partitions and accumulates flow for each cell.

For an input D8 flow direction raster, cells of undefined flow direction will only receive flow; they will not contribute to any downstream flow. A cell is considered to have an undefined flow direction if its value in the flow direction raster is anything other than 1, 2, 4, 8, 16, 32, 64, or 128.

For an input D-Infinity flow direction raster, a cell is considered to have undefined flow direction if its value in the flow direction raster is -1.

The accumulated flow is based on the number of cells, or fractions of cells, flowing into each receiving cell in the output raster. 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.

Output cells with a flow accumulation of zero are local topographic highs and can be used to identify ridges.

## Parameters

Parameter nameDescription

Flow Direction Raster

The input raster that shows the direction of flow out of each cell.

The flow direction raster can be created by running the Flow Direction function.

Weight Raster

An optional input raster for applying a weight to each cell.

If no weight raster is specified, a default weight of 1 will be applied to each cell.

Output Data Type

The output accumulation raster can be integer or floating point type.

• Float—The output raster will be floating point type. This is the default.
• Integer—The output raster will be integer type.

Flow Direction Type

Defines the type of the input flow direction raster.

• D8—The input flow direction raster is of type D8. This is the default.
• MFD—The input flow direction raster is of type Multi Flow Direction (MFD).
• DINF—The input flow direction raster is of type D-Infinity (DINF).