GRASS GIS: r.watershed

NAME

r.watershed

DESCRIPTION

r.watershed generates a set of maps indicating: 1) the location of watershed basins, and 2) the LS and S factors of the Revised Universal Soil Loss Equation (RUSLE).

OPTIONS

-m: Without this flag set, the entire analysis is run in memory maintained by the operating system. This can be limiting, but is relatively fast. Setting the flag causes the program to manage memory on disk which allows larger maps to be processes but is considerably slower.
-4: 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.
elevation: Input map: Elevation on which entire analysis is based.
depression: Input map: Map layer of actual depressions or sinkholes in the landscape that are large enough to slow and store surface runoff from a storm event. Any non-zero values indicate depressions.
flow: Input map: amount of overland flow per cell. This map indicates the amount of overland flow units that each cell will contribute to the watershed basin model. Overland flow units represent the amount of overland flow each cell contributes to surface flow. If omitted, a value of one (1) is assumed. The algorithm is D8 flow accumulation.
disturbed.land: Raster map input layer or value containing the percent of disturbed land (i.e., croplands, and construction sites) where the raster or input value of 17 equals 17%. If no map or value is given, r.watershed assumes no disturbed land. This input is used for the RUSLE calculations.
blocking: Input map: terrain that will block overland surface flow. Terrain that will block overland surface flow and restart the slope length for the RUSLE. Any non-zero values indicate blocking terrain.
threshold: The minimum size of an exterior watershed basin in cells, if no flow map is input, or overland flow units when a flow map is given. Warning: low threshold values will dramactically increase run time and generate difficult too read basin and half.basin results. This parameter also controls the level of detail in the stream segments map.
max.slope.length: Input value indicating the maximum length of overland surface flow in meters. If overland flow travels greater than the maximum length, the program assumes the maximum length (it assumes that landscape characteristics not discernible in the digital elevation model exist that maximize the slope length). This input is used for the RUSLE calculations and is a sensitive parameter.
accumulation: Output map: The absolute value of each cell in this output map layer is the amount of overland flow that traverses the cell. This value will be the number of upland cells plus one if no overland flow map is given. If the overland flow map is given, the value will be in overland flow units. Negative numbers indicate that those cells possibly have surface runoff from outside of the current geographic region. Thus, any cells with negative values cannot have their surface runoff and sedimentation yields calculated accurately.
drainage: Output map: drainage direction. Provides the "aspect" for each cell. Multiplying positive values by 45 will give the direction in degrees that the surface runoff will travel from that cell. The value -1 indicates that the cell is a depression area (defined by the depression input map). Other negative values indicate that surface runoff is leaving the boundaries of the current geographic region. The absolute value of these negative cells indicates the direction of flow.
basin: Output map: 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. 0 values indicate that the cell is not part of a complete watershed basin in the current geographic region.
stream: Output map: stream segments. Values correspond to the watershed basin values.
half.basin: Output map: each half-basin is given a unique value. 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.
visual: Output map: useful for visual display of results. Surface runoff accumulation with the values modified to provide for easy display. All negative accumulation values are changed to zero. All positive values above the basin threshold are given the value of the threshold parameter.
length.slope: Output map: slope length and steepness (LS) factor. Contains the LS factor for the Revised Universal Soil Loss Equation. Equations taken from Revised Universal Soil Loss Equation for Western Rangelands (Weltz et al. 1987). Since the LS factor is a small number, it is multiplied by 100 for the GRASS output map.
slope.steepness: Output map: slope steepness (S) factor for RUSLE. Contains the revised S factor for the Universal Soil Loss Equation. Equations taken from article entitled Revised Slope Steepness Factor for the Universal Soil Loss Equation (McCool et al. 1987). Since the S factor is a small number (usually less than one), it is multiplied by 100 for the GRASS output map layer.

NOTES

r.watershed uses an algorithm designed to minimize the impact of DEM data errors. This algorithm works slower than r.terraflow but provides more accurate results in areas of low slope as well as DEMs constructed with techniques that mistake canopy tops as the ground elevation. Kinner et al. (2005), for example, used SRTM and IFSAR DEMs to compare r.watershed against r.terraflow results in Panama. r.terraflow was unable to replicate stream locations in the larger valleys while r.watershed performed much better. Thus, if forest canopy exists in valleys, SRTM, IFSAR, and similar data products will cause major errors in r.terraflow stream output. Under similar conditions, r.watershed will generate better stream and half.basin results. If watershed divides contain flat to low slope, r.watershed will generate better basin results than r.terraflow. (r.terraflow uses the same type of algorithm as ESRI's ArcGIS watershed software which fails under these conditions.) Also, if watershed divides contain forest canopy mixed with uncanopied areas using SRTM, IFSAR, and similar data products, r.watershed will generate better basin results than r.terraflow.

There are two versions of this program: ram and seg. Which is version is run depends on whether the -m flag is set.
The ram version uses virtual memory managed by the operating system to store all the data structures and is faster than the seg version; seg uses the GRASS segmentation library which manages data in disk files. Thus seg uses much less system memory (RAM) allowing other processes to operate on the same CPU, even when the current geographic region is huge.
Due to memory requirements of both programs, it is quite easy to run out of memory when working with huge map regions. If the ram version runs out of memory and the resolution size of the current geographic region cannot be increased, either more memory needs to be added to the computer, or the swap space size needs to be increased. If seg runs out of memory, additional disk space needs to be freed up for the program to run.

Both versions use the A^T least-cost search algorithm to determine the flow of water over the landscape (see SEE ALSO section). The algorithm produces results similar to those obtained when running r.cost and r.drain on every cell on the map.

In many situations, the elevation data will be too finely detailed for the amount of time or memory available. Running r.watershed may require use of a coarser resolution. To make the results more closely resemble the finer terrain data, create a map layer containing the lowest elevation values at the coarser resolution. This is done by: 1) Setting the current geographic region equal to the elevation map layer with g.region, and 2) Use the r.neighbors or r.resamp.stats command to find the lowest value for an area equal in size to the desired resolution. For example, if the resolution of the elevation data is 30 meters and the resolution of the geographic region for r.watershed will be 90 meters: use the minimum function for a 3 by 3 neighborhood. After changing to the resolution at which r.watershed will be run, r.watershed should be run using the values from the neighborhood output map layer that represents the minimum elevation within the region of the coarser cell.

The minimum size of drainage basins, defined by the threshold parameter, is only relevant for those watersheds with a single stream having at least the threshold of cells flowing into it. (These watersheds are called exterior basins.) Interior drainage basins contain stream segments below multiple tributaries. Interior drainage basins can be of any size because the length of an interior stream segment is determined by the distance between the tributaries flowing into it.

The r.watershed program does not require the user to have the current geographic region filled with elevation values. Areas without elevation data MUST be masked out, by creating a raster map (or raster reclassification) named MASK. Areas masked out will be treated as if they are off the edge of the region. MASKs will reduce the memory necessary to run the program. Masking out unimportant areas can significantly reduce processing time if the watersheds of interest occupy a small percentage of the overall area.

Zero data values will be treated as elevation data (not no_data).

EXAMPLE

Convert r.watershed streams map output to a vector layer.

  r.watershed elev=elevation.dem stream=rwater.stream
  r.null map=rwater.stream setnull=0
  r.to.vect -v in=rwater.stream out=rwater_stream

Set a nice color table for the accumulation map:

  MAP=rwater.accum
  r.watershed elev=elevation.dem accum=$MAP

  eval `r.univar -g "$MAP"`
  stddev_x_2=`echo $stddev | awk '{print $1 * 2}'`
  stddev_div_2=`echo $stddev | awk '{print $1 / 2}'`

  r.colors $MAP col=rules << EOF
    0% red
    -$stddev_x_2 red
    -$stddev yellow
    -$stddev_div_2 cyan
    -$mean_of_abs blue
    0 white
    $mean_of_abs blue
    $stddev_div_2 cyan
    $stddev yellow
    $stddev_x_2 red
    100% red
  EOF

AUTHOR

Charles Ehlschlaeger, U.S. Army Construction Engineering Research Laboratory

Last changed: $Date: 2006/11/20 05:26:57 $

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