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The Stream Network Analysis toolset contains tools for creating and analyzing stream networks. The following table lists the tools in the toolset followed by a brief description of each.
| Tool | Description |
|---|---|
| Peuker Douglas | Creates an indicator grid (1,0) of valley form grid cells according to the Peuker and Douglas algorithm. With this tool, the DEM is first smoothed by a kernel with weights at the center, sides, and diagonals. The Peuker and Douglas (1975) method (also explained in Band, 1986), is then used to identify valley form grid cells. This technique flags the entire grid, then examines in a single pass each quadrant of 4 grid cells, and unflags the highest. The remaining flagged cells are deemed 'valley cells', and when viewed, resemble a channel network. This proto-channel network generally lacks connectivity and requires thinning, issues that were discussed in detail by Band (1986). |
| D8 Extreme Upslope Value | Evaluates the extreme (either maximum or minimum) upslope value from an input grid based on the D8 flow model. This is intended initially for use in stream raster generation to identify a threshold of the slope times area product that results in an optimum (according to drop analysis) stream network. If the optional outlet point feature is used, only the outlet cells and the cells upslope (by the D8 flow model) of them are in the domain to be evaluated. By default, the tool checks for edge contamination. Edge contamination is defined as the possibility that a result may be underestimated due to grid cells outside of the domain not being counted. This occurs when drainage is inwards from the boundaries or areas with no data values for elevation. The algorithm recognizes this and reports "no data" for the result for these grid cells. It is common to see streaks of "no data" values extending inwards from boundaries along flow paths that enter the domain at a boundary. This is the desired effect and indicates that the result for these grid cells is unknown due to it being dependent on terrain outside of the domain of data available. Edge contamination checking may be turned off in cases where you know this is not an issue or want to ignore these problems, if for example, the DEM has been clipped along a watershed outline. |
| Slope Area Combination | Creates a grid of slope-area values = (S^m)(A^n) based on slope and specific catchment area grid inputs, and parameters m and n. This tool is intended for use as part of the slope-area stream raster delineation method. |
| Leangth Area Stream Source | Creates an indicator grid (1,0) that evaluates A >= (M)(L^y) based on upslope path length, D8 contributing area grid inputs, and parameters M and y. This grid indicates likely stream source grid cells. This is an experimental method with theoretical basis in Hack's law which states that for streams L ~ A^0.6. However for hillslopes with parallel flow L ~ A. So a transition from hillslopes to streams may be represented by L ~ A^0.8 suggesting identifying grid cells as stream cells if A > M (L^(1/0.8)). |
| Stream Drop Analysis | Applies a series of thresholds (determined from the input parameters) to the input accumulated stream source grid (*ssa) grid and outputs the results in the *drp.txt file the stream drop statistics table. This function is designed to aid in the determination of a geomorphologically objective threshold to be used to delineate streams. Drop Analysis attempts to select the right threshold automatically by evaluating a stream network for a range of thresholds and examining the constant drop property of the resulting Strahler streams. Basically it asks the question: Is the mean stream drop for first order streams statistically different from the mean stream drop for higher order streams, using a T-Test. Stream drop is the difference in elevation from the beginning to the end of a stream defined as the sequence of links of the same stream order. If the T test shows a significant difference then the stream network does not obey this "law" so a larger threshold needs to be chosen. The smallest threshold for which the T test does not show a significant difference gives the highest resolution stream network that obeys the constant stream drop "law" from geomorphology, and is the threshold chosen for the "objective" or automatic mapping of streams from the DEM. This function can be used in the development of stream network rasters, where the exact watershed characteristic(s) that were accumulated in the accumulated stream source grid vary based on the method being used to determine the stream network raster. |
| Stream Definition by Threshold | Operates on any grid and outputs an indicator (1,0) grid identifing cells with input values >= the threshold value. The standard use is to use an accumulated source area grid to as the input grid to generate a stream raster grid as the output. If you use the optional input mask grid, it limits the domain being evaluated to cells with mask values >= 0 . When you use a D-infinity contributing area grid (*sca) as the mask grid, it functions as an edge contamination mask. The threshold logic is: src = ((ssa >= thresh) & (mask >=0)) ? 1:0 . |
| Stream Definition with Drop Anlysis | This tool combines the function of the "Stream Drop Analysis" tool and the "Stream Definition by Threshold" tool. It applies a series of thresholds (determined from the input parameters) to the input accumulated stream source grid (*ssa) grid and outputs the results in the stream drop statistics table (*drp.txt). Then it outputs a stream raster grid, which is an indicator (1,0) grid of stream cells. Stream cells are defined as those cells where the accumulated stream source value is >= the optimal threshold as determined from the stream drop statistics. There is an option to include a mask input to replicate the functionality for using the *sca file as an edge contamination mask. The threshold logic should be: src = ((ssa >= thresh) & (mask >=0)) ? 1:0 . |
| Peuker Douglas Stream Definition | This tool combines the functionality of the "Peuker Douglas," "D8 Contributing Area," "Stream Drop Analysis," and "Stream Definition by Threshold" tools in order to generate a stream indicator grid (1,0) where the streams are located using a method based on geomorphology. With this method, the DEM is first smoothed by a kernel with weights at the center, sides, and diagonals. The Peuker and Douglas (1975) method (also explained in Band, 1986), is then used to identify valley form grid cells. This technique flags the entire grid, then examines in a single pass each quadrant of 4 grid cells, and unflags the highest. The remaining flagged cells are deemed 'valley cells', and when viewed, resemble a channel network. This proto-channel network sometimes lacks connectivity, and/or requires thinning, issues that were discussed in detail by Band (1986). The thinning and connecting of these grid cells is achieved here by computing the D8 contributing area using only these valley cells. An accumulation threshold on the number of these cells is then used to map the channel network where this threshold is optionally set by the user, or determined via drop analysis. |
| Slope Area Stream Definition | This tool combines the function of the following tools: "Slope Area Combination," "D8 Extreme Upslope Value," "Stream Drop Analysis," and "Stream Definition by Threshold." It creates a grid where stream cells have a value of 1 and non-stream cells have a value of 0 using the area and slope threshold method to determine the location of the streams. With this method, streams are defined as beginning where an expression meets or exceeds a threshold (T). The expression is calculated by taking the slope (S) raised to an exponent (m) and then multiplied by the specific catchment area (a) raised to an exponent (n), i.e, (S^m)(a^n) >= T. Once a stream begins, it continues downslope, from there. This method was suggested by Montgomery and Dietrich (1992). (They used the slope exponent (m) of 2, area exponent (n) of 1, and a threshold (T) of 200 m^2 in their study). Please note that specific catchment area (a) is the contributing area per unit width (cell size) and has units of length. If specific catchment area is used for A, the units are contributing area per unit width (cell size), usually in m, so if the cell size is 15 m, a T equivalent to 200 m^2 is 200/15=13.3. |
| Move Outlets to Streams | Moves outlet points that are off a stream raster grid, downslope along the D8 flow direction until a stream raster grid is encountered. Input is a flow direction grid, a stream raster grid and an outlets feature. Output is a new outlets OGR file where each point has been moved to coincide with the stream raster grid, if possible. A field 'dist_moved' is added to the new outlets file to indicate the changes made to each point. Points that are already on the stream raster (src) grid are not moved and their 'dist_moved' field is assigned a value 0. Points that are initially not on the stream raster grid are moved by sliding them downslope along the D8 flow direction until one of the following occurs: a.) A stream raster grid cell is encountered before traversing the max_dist number of grid cells. In which case, the point is moved and the 'dist_moved' field is assigned a value indicating how many grid cells the point was moved. b.) More than the max_number of grid cells are traversed, or the traversal ends up going out of the domain (i.e., a no data D8 flow direction value is encountered). In which case, the point is not moved and the 'dist_moved' field is assigned a value of -1. |
| Stream Reach and Watershed | This function produces a vector network and OGR file from the stream raster grid. The flow direction grid is used to connect flow paths along the stream raster. The Strahler order of each stream segment is computed. The subwatershed draining to each stream segment (reach) is also delineated and labeled with the value identifier that corresponds to the WSNO (watershed number) attribute in the stream reach file. |
| Gage Watershed | Calculates Gage watersheds grid. Each grid cell is labeled with the identifier (from column id) of the gage to which it drains directly without passing through any other gages. |
| ConnectDown | For each zone in a raster entered (e.g. HUC converted to grid) it identifies the point with largest AreaD8. This is taken to be the outlet. A OGR file is created.Using flow directions each outlet is moved downflow a specified number of grid cells which is user controllable (Default is 1). The ID of the location the point has moved to is taken as iddown. Two OGR files are created one with the initial points and one with the moved points. Both contain id, iddown and AreaD8. |
| Watershed Grid to Shapefile | Translates the grid watershed representation into a polygon shapefile. Each shape in the Watershed Shapefile is assigned an identifier the same as the value identifier in the grid from which it was created, that in turn maps back to WSNO in the Stream Shape File. |