(GRASS Raster/Vector Program)
generates flowlines using a combined raster-vector
approach from input elevation elevin , aspect aspin and optionally barrier
barrierin raster maps.
Input elevation raster map with integer values should have values in centimeters preventing premature flowline stop in flat areas. Aspect used for input must follow the same rules as aspect computed in other GRASS programs (see r.slope.aspect, s.surf.tps). Barriers are defined by non-zero values in an input raster map barierin . Make sure that the resolution of your current region is set to the resolution of the elevation raster file.
There are three possible output maps which can be produced in any combination simultaneously: a vector file flout of flowlines, a raster map lgout of flowpath lengths, and a raster map dsout of flowline densities. Flowlines are constructed by straight line segments (vectors) with endpoints creating intersections of the flowline with edges of the imaginary mesh connecting centers of the grid cells. Flowlines are generated from each cell uphill by default; they can be generated downhill using the flag -d . A flowline stops if its next segment would reverse the direction of flow (from up to down or vice-versa), cross a barrier, or arrive at a cell with undefined elevation or aspect. Another option, skip =val , indicates that only the flowlines from every val th cell are to be included in flout . The default skip is 1. A high skip usually speeds up processing time and often improves the readability of a visualization of flout .
Flowpath length output flout gives the horizontal length of the flowline for given grid cell in units defined by your mapset. It is computed as the sum of the planar length of all straight flowline segments.
Flowline density output dsout represents the number of generated flowlines passing through the given cell. This program uses a modified multiple directions algorithm. Flowline density for each flowline passing between 2 grid cell centers is fractioned between these grid cells by distance. This algorithm smoothes flowline densities on convex (dispersing) slopes. Flowline lengths and densities can be computed for both uphill and downhill directionss.
r.flowmd [-d ] elevin=name aspin=name [barrierin=name] [skip=val ] [flout=name ] [lgout=name ] [dsout=name ]
Alternately, the user can simply type r.flowmd on the command line without program arguments. In this case, the user will be prompted for parameter values and flag settings using the standard GRASS parser interface.
Flag:
Parameters:
Flowlines generated uphill are used for computation of the flowpath length (slope lenghts) defining the distance between the grid cell and upland area. This can be used, for example, in water erosion modeling for the computation of LS factor in standard form of USLE. Flowlines can be also generated downhill from each grid point to simulate the topographic potential for quasi steady-state overland water flow. Downslope flowline density multiplied by grid cell area gives the upslope contributing area. This topographic parameter defines an area from which water inflows to the given cell. This area is an estimate of potential steady-state water flux and can be used in the modeling of water erosion and for computation of unit stream power based LS factor. The program has been designed for modeling erosion on hillslopes and has a very strict condition for ending the flowline. It is therefore not very suitable for the extraction of stream network or delineation of watersheds unless a DEM without pits or flat areas is available.