ArcMap

Flow Accumulation

Available with Spatial Analyst license.

Summary

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

Learn more about how Flow Accumulation works

Illustration

Flow Accumulation illustration
Flow_Acc = FlowAccumulation(Flow_Dir)

Usage

  • 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.

  • 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.

  • The accumulated flow is based on the number of cells flowing into each 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.

  • If the input flow direction raster is not created with the Flow Direction tool, there is a chance that the defined flow could loop. If the flow direction does loop, Flow Accumulation will go into an infinite loop and never finish.

  • The Flow Accumulation tool does not honour the Compression environment setting. The output raster will always be uncompressed.

  • See Analysis environments and Spatial Analyst for additional details on the geoprocessing environments that apply to this tool.

Syntax

FlowAccumulation (in_flow_direction_raster, {in_weight_raster}, {data_type})
ParameterExplanationData Type
in_flow_direction_raster

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

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

Raster Layer
in_weight_raster
(Optional)

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. For each cell in the output raster, the result will be the number of cells that flow into it.

Raster Layer
data_type
(Optional)

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.
String

Return Value

NameExplanationData Type
out_accumulation_raster

The output raster that shows the accumulated flow to each cell.

Raster

Code sample

FlowAccumulation example 1 (Python window)

This example creates a raster of accumulated flow into each cell of an input flow direction Grid raster.

import arcpy
from arcpy import env
from arcpy.sa import *
env.workspace = "C:/sapyexamples/data"
outFlowAccumulation = FlowAccumulation("flowdir")
outFlowAccumulation.save("C:/sapyexamples/output/outflowacc01")
FlowAccumulation example 2 (stand-alone script)

This example creates a raster of accumulated flow into each cell of an input flow direction IMG raster.

# Name: FlowAccumulation_Ex_02.py
# Description: Creates a raster of accumulated flow to each cell.
# Requirements: Spatial Analyst Extension

# Import system modules
import arcpy
from arcpy import env
from arcpy.sa import *

# Set environment settings
env.workspace = "C:/sapyexamples/data"

# Set local variables
inFlowDirRaster = "flowdir"
inWeightRaster = ""
dataType = "INTEGER"

# Check out the ArcGIS Spatial Analyst extension license
arcpy.CheckOutExtension("Spatial")

# Execute FlowDirection
outFlowAccumulation = FlowAccumulation(inFlowDirRaster, inWeightRaster, dataType)

# Save the output 
outFlowAccumulation.save("C:/sapyexamples/output/outflowacc02.img")

Environments

Licensing information

  • ArcGIS Desktop Basic: Requires Spatial Analyst
  • ArcGIS Desktop Standard: Requires Spatial Analyst
  • ArcGIS Desktop Advanced: Requires Spatial Analyst

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