This tool delineates the longest flowpaths for a group of subbasins or watersheds. Flowpaths are initiated along drainage divides and continue along the D8-defined flow direction until either the subbasin outlet or DEM edge is encountered. Each input subbasin/watershed will have an associated vector flowpath in the output image. longest_flowpath is similar to the `r.lfp` plugin tool for GRASS GIS. The length of the longest flowpath draining to an outlet is related to the time of concentration, which is a parameter used in certain hydrological models.

The user must input the filename of a digital elevation model (DEM), a basins raster, and the output vector. The DEM must be depressionless and should have been pre-processed using the breach_depressions_least_cost or fill_depressions tool. The basins raster must contain features that are delineated by categorical (integer valued) unique identifier values. All non-NoData, non-zero valued grid cells in the basins raster are interpreted as belonging to features. In practice, this tool is usual run using either a single watershed, a group of contiguous non-overlapping watersheds, or a series of nested subbasins. These are often derived using the watershed tool, based on a series of input outlets, or the subbasins tool, based on an input stream network. If subbasins are input to longest_flowpath, each traced flowpath will include only the non-overlapping portions within nested areas. Therefore, this can be a convenient method of delineating the longest flowpath to each bifurcation in a stream network.

The output vector file will contain fields in the attribute table that identify the associated basin unique identifier (BASIN), the elevation of the flowpath source point on the divide (UP_ELEV), the elevation of the outlet point (DN_ELEV), the length of the flowpath (LENGTH), and finally, the average slope (AVG_SLOPE) along the flowpath, measured as a percent grade.

## See Also

max_upslope_flowpath_length, breach_depressions_least_cost, fill_depressions, watershed, subbasins

## Function Signature

`def longest_flowpath(self, dem: Raster, basins: Raster) -> Vector: ...`

## Project Links

WbW Homepage User Manual Support WbW