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118 lines
7.4 KiB
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118 lines
7.4 KiB
HTML
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<head><link rel="stylesheet" type="text/css" href="help.css"/></head>
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<h1 class='module'>Stream Drop Analysis</h1>
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<div class='author'>(c) 2010 by David G. Tarboton</div>
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<div class='description'>Applies a series of thresholds (determined from
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the input parameters) to the input accumulated stream source grid (*ssa)
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grid and outputs the results in the *drp.txt file the stream drop statistics
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table. This function is designed to aid in the determination of a geomorphologically
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objective threshold to be used to delineate streams. Drop Analysis attempts
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to select the right threshold automatically by evaluating a stream network
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for a range of thresholds and examining the constant drop property of the
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resulting Strahler streams. Basically it asks the question: Is the mean
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stream drop for first order streams statistically different from the mean
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stream drop for higher order streams, using a T-test. Stream drop is the
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difference in elevation from the beginning to the end of a stream defined
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as the sequence of links of the same stream order. If the T-test shows a
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significant difference then the stream network does not obey this "law"
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so a larger threshold needs to be chosen. The smallest threshold for which
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the T-test does not show a significant difference gives the highest resolution
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stream network that obeys the constant stream drop "law" from
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geomorphology, and is the threshold chosen for the "objective"
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or automatic mapping of streams from the DEM. This function can be used
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in the development of stream network rasters, where the exact watershed
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characteristic(s) that were accumulated in the accumulated stream source
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grid vary based on the method being used to determine the stream network
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raster.</div>
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<p align="center"><img src="img/streamdrops.jpg"></img></p>
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<div class='description'>The constant stream drop "law" was
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identified by Broscoe (1959). For the science behind using this to determine
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a stream delineation threshold, see Tarboton et al. (1991, 1992), Tarboton
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and Ames (2001).</div>
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<ol>
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<li>Broscoe, A. J., (1959), "Quantitative analysis of longitudinal
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stream profiles of small watersheds", Office of Naval Research, Project
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NR 389-042, Technical Report No. 18, Department of Geology, Columbia
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University, New York.</li>
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<li>Tarboton, D. G., R. L. Bras and I. Rodriguez-Iturbe, (1991), "On the
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Extraction of Channel Networks from Digital Elevation Data", Hydrologic
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Processes, 5(1): 81-100.</li>
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<li>Tarboton, D. G., R. L. Bras and I. Rodriguez-Iturbe, (1992), "A Physical
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Basis for Drainage Density", Geomorphology, 5(1/2): 59-76.</li>
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<li>Tarboton, D. G. and D. P. Ames, (2001), "Advances in the mapping
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of flow networks from digital elevation data", World Water and Environmental
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Resources Congress, Orlando, Florida, May 20-24, ASCE, <a href="http://www.engineering.usu.edu/dtarb/asce2001.pdf">http://www.engineering.usu.edu/dtarb/asce2001.pdf</a>.</li>
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</ol>
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<h2>Parameters</h2>
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<dl class='parameters'>
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<dt>Number of Processes <div class='type'>Integer</div></dt>
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<dd>The number of stripes that the domain will be divided into and the
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number of MPI parallel processes that will be spawned to evaluate each
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of the stripes.</dd>
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<dt>Pit Filled Elevation Grid <div class='type'>Raster Grid</div></dt>
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<dd>A grid of elevation values. This is usually the output of the "Pit
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Remove" tool, in which case it is elevations with pits removed.</dd>
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<dt>D8 Flow Direction Grid <div class='type'>Raster Grid</div></dt>
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<dd>A grid of D8 flow directions which are defined, for each cell, as
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the direction of the one of its eight adjacent or diagonal neighbors
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with the steepest downward slope. This grid can be obtained as the
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output of the "D8 Flow Directions" tool.</dd>
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<dt>D8 Contributing Area Grid <div class='type'>Raster Grid</div></dt>
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<dd>A grid of contributing area values for each cell that were calculated
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using the D8 algorithm. The contributing area for a cell is the sum
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of its own contribution plus the contribution from all upslope neighbors
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that drain to it, measured as a number of cells or the sum of weight
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loadings. This grid can be obtained as the output of the "D8
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Contributing Area" tool. This grid is used in the evaluation of
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drainage density reported in the stream drop table.</dd>
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<dt>Accumulated Stream Source Grid <div class='type'>Raster Grid</div></dt>
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<dd>This grid must be monotonically increasing along the downslope D8
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flow directions. It it compared to a series of thresholds to determine
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the beginning of the streams. It is often generated by accumulating some
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characteristic or combination of characteristics of the watershed with
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the "D8 Contributing Area" tool, or using the maximum option
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of the "D8 Flow Path Extreme" tool. The exact method varies
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depending on the algorithm being used.</dd>
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<dt>Outlets Shapefile <div class='type'>Point Shapefile</div></dt>
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<dd>A point shapefile defining the outlets upstream of which drop
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analysis is performed.</dd>
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<dt>Minimum Threshold <div class='type'>Double</div></dt>
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<dd>This parameter is the lowest end of the range searched for possible
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threshold values using drop analysis. This technique looks for the smallest
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threshold in the range where the absolute value of the t-statistic is
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less than 2. For the science behind the drop analysis see Tarboton et
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al. (1991, 1992), Tarboton and Ames (2001). Default value <strong>5</strong>.</dd>
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<dt>Maximum Threshold <div class='type'>Double</div></dt>
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<dd>This parameter is the highest end of the range searched for possible
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threshold values using drop analysis. This technique looks for the smallest
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threshold in the range where the absolute value of the t-statistic is
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less than 2. For the science behind the drop analysis see Tarboton et
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al. (1991, 1992), Tarboton and Ames (2001). Default value <strong>500</strong>.</dd>
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<dt>Number of Threshold Values <div class='type'>Double</div></dt>
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<dd>The parameter is the number of steps to divide the search range
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into when looking for possible threshold values using drop analysis.
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This technique looks for the smallest threshold in the range where
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the absolute value of the t-statistic is less than 2. For the science
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behind the drop analysis see Tarboton et al. (1991, 1992), Tarboton
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and Ames (2001). Default value <strong>10</strong>.</dd>
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<dt>Spacing for Threshold Values <div class='type'>Choice</div></dt>
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<dd>This parameter indicates whether logarithmic or linear spacing
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should be used when looking for possible threshold values using drop
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ananlysis. Default is <strong>Logarithmic</strong>.</dd>
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</dl>
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<h2>Outputs</h2>
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<dl class='parameters'>
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<dt>D-Infinity Drop to Stream Grid <div class='type'>Text file (CSV)</div></dt>
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<dd>This is a comma delimited text file with the following header line:<br>
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<pre>Threshold,DrainDen,NoFirstOrd,NoHighOrd,MeanDFirstOrd,MeanDHighOrd,StdDevFirstOrd,StdDevHighOrd,T</pre>
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The file then contains one line of data for each threshold value examined,
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and then a summary line that indicates the optimum threshold value.
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This technique looks for the smallest threshold in the range where the
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absolute value of the t-statistic is less than 2. For the science behind
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the drop analysis, see Tarboton et al. (1991, 1992), Tarboton and Ames (2001).</dd>
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</dl>
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</body></html>
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