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How to create a watershed in GRASS GIS?
Note: DEM Sink-filling not needed, since GRASS GIS uses the A* least cost routing, see r.watershed manual: r.watershed elev= dem drain= fdir basin= catch stream= str thresh= So far, pretty straightforward. There’s abundant information on r.watershed.
How are the basins of a watershed divided?
Watershed basins are divided into left and right sides. The right-hand side cell of the watershed basin (looking upstream) are given even values corresponding to the values in basin. The left-hand side cells of the watershed basin are given odd values which are one less than the value of the watershed basin.
How is the slope of a watershed determined?
The left-hand side cells of the watershed basin are given odd values which are one less than the value of the watershed basin. The output length_slope raster map stores slope length and steepness (LS) factor for the Revised Universal Soil Loss Equation (RUSLE).
What do the labels mean on the output basin map?
The output basin map contains unique label for each watershed basin. Each basin will be given a unique positive even integer. Areas along edges may not be large enough to create an exterior watershed basin. NULL values indicate that the cell is not part of a complete watershed basin in the current geographic region.
How to calculate watersheds and basins in QGIS?
I am using height data from CIGAR (cell 39_03 to be exact). So what is the output I am trying to get? I would love to see a basin for each valley in the Alps. Compared to ArcGIS, where I remember a ton of steps to calculate watersheds/basins for a raster, the workflow in QGIS is straightforward thanks to the GRASS processing tools.
How to create a watershed in r.watershed?
Begin by rerunning the r.watershed command as in the first section, but adding a flow accumulation raster as the output: #set the region to the dem raster, and run the r.watershed module. g.region -p rast= dem # Choose threshold in map cells (try 10000 as a start).
What kind of search algorithm does are watershed use?
Since the S factor is a small number (usually less than one), the GRASS output map is of type DCELL. r.watershed uses an A T least-cost search algorithm (see REFERENCES section) designed to minimize the impact of DEM data errors.
What’s the by-4 flag for a drainage basin?
By -4 flag the user allow only horizontal and vertical flow of water. Stream and slope lengths are approximately the same as outputs from default surface flow (allows horizontal, vertical, and diagonal flow of water). This flag will also make the drainage basins look more homogeneous.
How to calculate the drainage area of grass?
Some other software, such as the ArcGIS extension “ArcHydro” can calculate the total upstream drainage area flowing through each stream reach. The same result is achieved in GRASS with a few hydrology modules and a database join. Begin by rerunning the r.watershed command as in the first section, but adding a flow accumulation raster as the output:
What does the output drainage raster map show?
Output drainage raster map contains drainage direction. Provides the “aspect” for each cell measured CCW from East. Multiplying positive values by 45 will give the direction in degrees that the surface runoff will travel from that cell. The value 0 (zero) indicates that the cell is a depression area (defined by the depression input map).