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104 lines
6.4 KiB
HTML
104 lines
6.4 KiB
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'>D-Infinity Contributing Area</h1>
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<div class='author'>(c) 2010 by David G. Tarboton</div>
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<div class='description'>Calculates a grid of specific catchment area which
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is the contributing area per unit contour length using the multiple flow
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direction D-infinity approach. D-infinity flow direction is defined as
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steepest downward slope on planar triangular facets on a block centered
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grid. The contribution at each grid cell is taken as the grid cell length
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(or when the optional weight grid input is used, from the weight grid).
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The contributing area of each grid cell is then taken as its own contribution
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plus the contribution from upslope neighbors that have some fraction
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draining to it according to the D-infinity flow model. The flow from each
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cell either all drains to one neighbor, if the angle falls along a cardinal
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(0, π/2, π, 3π/2) or ordinal (π/4, 3π/4, 5π/4, 7π/4)
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direction, or is on an angle falling between the direct angle to two adjacent
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neighbors. In the latter case the flow is proportioned between these two
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neighbor cells according to how close the flow direction angle is to the
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direct angle to those cells. The contour length used here is the grid cell
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size. The resulting units of the specific catchment area are length units
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the same as those of the grid cell size.</div>
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<p align="center"><img src="img/tardemfig.gif"></img></p>
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<div class='description'>When the optional weight grid is not used, the
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result is reported in terms of specific catchment area, the upslope area
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per unit contour length, taken here as the number of cells times grid cell
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length (cell area divided by cell length). This assumes that grid cell
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length is the effective contour length, in the definition of specific
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catchment area and does not distinguish any difference in contour length
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dependent upon the flow direction. When the optional weight grid is used,
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the result is reported directly as a summation of weights, without any
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scaling.</div>
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<div class='description'>If the optional outlet point shapefile is used,
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only the outlet cells and the cells upslope (by the D-infinity flow model)
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of them are in the domain to be evaluated.</div>
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<div class='description'>By default, the tool checks for edge contamination.
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This is defined as the possibility that a contributing area value may be
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underestimated due to grid cells outside of the domain not being counted.
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This occurs when drainage is inwards from the boundaries or areas with
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"no data" values for elevation. The algorithm recognizes this
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and reports "no data" for the contributing area. It is common
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to see streaks of "no data" values extending inwards from boundaries
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along flow paths that enter the domain at a boundary. This is the desired
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effect and indicates that contributing area for these grid cells is unknown
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due to it being dependent on terrain outside of the domain of data available.
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Edge contamination checking may be turned off in cases where you know it
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is not an issue or want to ignore these problems, if for example, the DEM
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has been clipped along a watershed outline.</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>D-Infinity Flow Direction Grid <div class='type'>Raster Grid</div></dt>
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<dd>A grid of flow directions based on the D-infinity flow method using
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the steepest slope of a triangular facet. Flow direction is determined
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as the direction of the steepest downward slope on the 8 triangular
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facets of a 3 x 3 block centered grid. Flow direction is encoded as
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an angle in radians, counter-clockwise from east as a continuous
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(floating point) quantity between 0 and 2π. The resulting flow in
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a grid is then usually interpreted as being proportioned between the
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two neighboring cells that define the triangular facet with the steepest
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downward slope.</dd>
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<dt>Outlets Shapefile <div class='type'>Point Shapefile (optional)</div></dt>
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<dd>A point shapefile defining the outlets of interest. If this input
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file is used, only the cells upslope of these outlet cells are considered
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to be within the domain being evaluated.</dd>
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<dt>Weight Grid <div class='type'>Raster Grid (optional)</div></dt>
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<dd>A grid giving contribution to flow for each cell. These contributions
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(also sometimes referred to as weights or loadings) are used in the
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contributing area accumulation. If this input file is not used, the
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result is reported in terms of specific catchment area (the upslope
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area per unit contour length) taken as the number of cells times grid
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cell length (cell area divided by cell length).</dd>
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<dt>Check for edge contamination <div class='type'>Boolean</div></dt>
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<dd>A flag that indicates whether the tool should check for edge
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contamination. Edge contamination is defined as the possibility that
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a contributing area value may be underestimated due to the fact that
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grid cells outside of the domain have not been evaluated. This occurs
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when drainage is inwards from the boundaries or areas with "no data"
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values for elevation. The algorithm recognizes this and reports "no data"
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for the impated cells. It is common to see streaks of "no data"
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values extending inwards from boundaries along flow paths that enter
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the domain at a boundary. This is the desired effect and indicates that
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contributing area for these grid cells is unknown due to it being
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dependent on terrain outside of the domain of available data. Edge
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contamination checking may be turned off in cases where you know this
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is not an issue, or want to ignore these problems, if for example, the
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DEM has been clipped along a watershed outline.</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 Specific Catchment Area Grid <div class='type'>Raster Grid</div></dt>
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<dd>A grid of specific catchment area which is the contributing area
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per unit contour length using the multiple flow direction D-infinity
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approach. The contributing area of each grid cell is then taken as its
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own contribution plus the contribution from upslope neighbors that
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have some fraction draining to it according to the D-infinity flow model.</dd>
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</dl>
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</body></html>
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