Flow Accumulation¶

Purpose¶

Flow accumulation computes upstream drainage area for each cell. Output value represents count of cells whose flow paths traverse the cell. Provides basis for stream network extraction.

Parameters¶

input_path¶

Path to flow direction raster. GDAL-readable format. Single band. UInt8 data type. Values 0-7 represent valid flow directions. Value 8 (undefined) not permitted (input must have all cells resolved). Value 9 (nodata) propagates to output.

output_path¶

Path for accumulation output. Written as GeoTIFF. Int64 data type. Inherits projection and geotransform from input. Nodata value set to -1. Values represent upstream cell count including the cell itself (minimum value 1 for cells with no upstream contributors).

chunk_size¶

Tile dimension in pixels. Default 2048. Set to 0 or 1 for in-memory processing.

progress_callback¶

Optional callback for monitoring long operations. See ProgressCallback API.

CLI Usage¶

overflow accumulation \
    --input fdr.tif \
    --output accum.tif

Python API Usage¶

import overflow

overflow.accumulation(
    input_path="fdr.tif",
    output_path="accum.tif"
)

The output raster values represent the total upstream cell count including self. The minimum value is 1 and values monotonically increase following flow paths downstream. Nodata cells have value -1 and are propagated down from the input.

Note

The flow direction raster MUST have all non-nodata cells contain valid defined flow directions (0-7) and must not contain cycles. These requirements are met automatically when using flow_direction() with resolve_flats=True on properly conditioned DEM.

Visualization¶

Input Flow Direction	Flow Accumulation Output

The input shows flow direction arrows overlaid on terrain with a valley down the center. Arrows converge toward the valley (↓) from both sides (↘ and ↙). The flow accumulation output shows how many upstream cells drain through each location. Higher values (darker blue) indicate greater drainage area, with the valley channel collecting flow from both sides. The pattern clearly identifies potential stream locations where accumulation values are highest.