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62 lines
3.5 KiB
HTML
62 lines
3.5 KiB
HTML
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<html>
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<head><link rel="stylesheet" type="text/css" href="help.css"/></head>
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<body>
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<h1 class='module'>Length Area Stream Source</h1>
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<div class='author'>(c) 2010 by David G. Tarboton</div>
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<div class='description'>Creates an indicator grid (1, 0) that evaluates
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<tt>A >= (M)(L<sup>y</sup>)</tt> based on upslope path length, D8 contributing area
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grid inputs, and parameters <tt>M</tt> and <tt>y</tt>. This grid indicates
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likely stream source grid cells. This is an experimental method with
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theoretical basis in Hack's law which states that for streams <tt>L ~ A<sup>0.6</sup></tt>.
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However for hillslopes with parallel flow <tt>L ~ A</tt>. So a transition
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from hillslopes to streams may be represented by <tt>L ~ A<sup>0.8</sup></tt>
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suggesting identifying grid cells as stream cells if
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<tt>A > M (L<sup>(1/0.8)</sup>)</tt>.</div>
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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>Length Grid <div class='type'>Raster Grid</div></dt>
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<dd>A grid of the maximum upslope length for each cell. This is calculated
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as the length of the flow path from the furthest cell that drains to each
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cell. Length is measured between cell centers taking into account cell
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size and whether the direction is adjacent or diagonal. It is this
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length (<tt>L</tt>) that is used in the formula, A >(M)(L<sup>y</sup>),
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to determine which cells are considered stream cells. This grid can
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be obtained as an output from the "Grid Network" tool.</dd>
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<dt>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 of
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its own contribution plus the contribution from all upslope neighbors
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that drain to it, measured as a number of cells. This grid is typically
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obtained as the output of the "D8 Contributing Area" tool.
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In this tool, it is the contributing area (<tt>A</tt>) that is compared
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in the formula <tt>A > (M)(L<sup>y</sup>)</tt> to determine the transition
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to a stream.</dd>
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<dt>Threshold <div class='type'>Double</div></dt>
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<dd>The multiplier threshold (<tt>M</tt>) parameter which is used in
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the formula: <tt>A > (M)(L<sup>y</sup>)</tt>, to identify the beginning
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of streams. Default value <strong>0.03</strong>.</dd>
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<dt>Exponent <div class='type'>Double</div></dt>
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<dd>The exponent (<tt>y</tt>) parameter which is used in the formula:
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A > (M)(L<sup>y</sup>), to identify the beginning of streams. In
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branching systems, Hack's law suggests that <tt>L = 1/M A<sup>(1/y)</sup></tt>
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with <tt>1/y = 0.6</tt> (or 0.56) (<tt>y</tt> about 1.7). In parallel
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flow systems <tt>L</tt> is proportional to <tt>A</tt> (<tt>y</tt> about 1).
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This method tries to identify the transition between these two paradigms
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by using an exponent <tt>y</tt> somewhere in between (<tt>y</tt> about 1.3).
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Default value <strong>1.3</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>Slope Area Grid <div class='type'>Raster Grid</div></dt>
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<dd>A grid of slope-area values = <tt>(S<sup>m</sup>)(A<sup>n</sup>)</tt>
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calculated from the slope grid, specific catchment area grid, <tt>m</tt>
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slope exponent parameter, and <tt>n</tt> area exponent parameter.</dd>
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</dl>
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</body></html>
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