Flow Accumulation

Flow accumulation is a raster-based hydrological analysis that computes, for each cell in a DEM, the total number of upstream cells whose flow paths pass through that cell. It is derived directly from the flow directionFlow DirectionFlow direction analysis determines the downhill direction of water flow from each cell in a digital elevation model, ... grid and produces an output raster where cell values represent the contributing area (or number of contributing cells) upstream of each location.

Cells with low flow accumulation values represent ridge tops and hillslopes where little upstream area contributes flow. Cells with high values represent valley bottoms and stream channels where many upslope cells converge. By applying a threshold to the flow accumulation raster (e.g., selecting cells with accumulation greater than 1,000), analysts can extract synthetic stream networks that closely match real drainage patterns. The appropriate threshold depends on the DEM resolution and the desired stream network density.

Hydrologists use flow accumulation to extract stream networks, calculate upstream drainage areas for any point, and identify pour points for watershed delineation. Erosion modelers use contributing area as a proxy for runoff volume in soil erosion equations like USLE. Environmental scientists identify areas of concentrated flow where erosion risk or pollutant transport is highest. Civil engineers use flow accumulation to identify natural drainage paths that must be accommodated in site development plans.

Flow accumulation depends on the flow directionFlow DirectionFlow direction analysis determines the downhill direction of water flow from each cell in a digital elevation model, ... grid as its primary input. Together with watershed delineation, stream ordering, and snap pour point tools, it forms the core hydrology toolset in GISGISGeographic Information Systems (GIS) enable users to analyze and visualize spatial data to uncover patterns, relation.... Weighted flow accumulation allows analysts to route other quantities (such as rainfall or pollutant load) through the flow network rather than simply counting cells.