Flow Direction

Flow direction is a raster-based hydrological analysis that determines the direction water would flow out of each cell in a DEM based on the principle that water follows the path of steepest descent. The most common method, the D8 (deterministic eight-node) algorithm, evaluates the eight neighboring cells and assigns the flow direction to the neighbor with the steepest downhill gradient, encoding the result as a direction value.

The D8 algorithm, the simplest and most widely used approach, assigns all flow from each cell to a single neighboring cell, producing discrete flow paths that converge into well-defined channels. The D-Infinity (D-inf) algorithm, developed by Tarboton, allows flow to be partitioned proportionally between two downhill neighbors based on the actual slope direction, producing more realistic divergent flow on hillslopes. Multiple flow direction (MFD) algorithms distribute flow to all downhill neighbors weighted by slope, better representing sheet flow on gentle terrain. Each method involves trade-offs between computational simplicity and hydrological realism.

Before computing flow direction, the DEM must be hydrologically conditioned by filling sinks (spurious depressions caused by data artifacts or natural features) that would trap flow and prevent it from reaching the drainage network. Breaching algorithms provide an alternative to filling by carving channels through barriers.

Flow direction is required as an input for computing flow accumulationFlow AccumulationFlow accumulation counts the number of upslope cells that drain through each cell in a raster, creating a surface tha..., delineating watersheds, tracing flow paths, and extracting stream networks. It underpins virtually every GISGISGeographic Information Systems (GIS) enable users to analyze and visualize spatial data to uncover patterns, relation...-based hydrological analysis, from simple watershed mapping to complex distributed rainfall-runoff modeling.