Flow Accumulation

Draft: written, not yet checked by a person Operation · flow-accumulation

For each cell, how many cells (or how much area) drain through it, following the steepest downhill direction.

Pitfall. Engines differ on whether a cell counts itself, on units (cells or area), and on cells at the edge.

Pitfall. Run it on a filled DEM, or sinks stop the flow.

About this operation
Also called	contributing area, upslope area, D8 flow accumulation, catchment area
Group	`raster/hydrology`
Checks	`raster.nodata`
Pitfalls	2 listed above
Columbus tool	Not stated
Notes	Not stated
Status	active
Broader	Not stated
Narrower	Not stated
Related	`fill-sinks` `slope` `aspect`

Parameters

Not stated

Direction	Name	Type	Geometry	Note
Input	`raster`	raster	Not stated	Not stated
Output	`output`	raster	Not stated	Not stated

Engine	Tool	Parameters	Differences
`grass`	`r.watershed`	`raster` → `elevation` `output` → `accumulation`	Multiple flow direction by default; this mapping sets the s flag (single, D8). Also writes basins, streams and other maps when named. Without the a flag, cells that may receive flow from outside the region get negative counts (on a plane draining to one edge, every cell); this mapping sets it. Cells on the region's border drain straight out of it, so they do not feed their neighbours (on a 20 by 20 plane draining west, the top and bottom rows and the east column were all 1, interior rows counted 19 down to 1). Preset: `{"flag_s":true,"flag_a":true}`
`terra`	`terra::flowAccumulation`	`raster` Takes a flow direction raster from terrain(x, 'flowdir'), not the DEM.	None noted
`whiteboxtools`	`D8FlowAccumulation`	`raster` → `input` `output` → `output`	Left out, out_type gave specific contributing area (cell count times cell size) in the equivalence tests, although the tool declares cells as its default; this mapping sets cells. Counts include the cell itself. Takes a DEM by default, or a D8 pointer raster with pntr; out_type gives cells, catchment area or specific contributing area. Preset: `{"out_type":"cells"}`
`xarray-spatial`	`xrspatial.hydro.flow_accumulation_d8`	`raster` Takes a D8 flow direction raster (xrspatial.hydro.flow_direction_d8), not the DEM.	None noted