Author
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MARTZ, LAWRENCE - UNIV SASKATCHEWAN, CANADA |
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Garbrecht, Jurgen |
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Submitted to: Computers and Geosciences
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 12/1/1999 Publication Date: N/A Citation: N/A Interpretive Summary: Drainage properties of a landscape can be determined by computer if the landscape topography is provided in a computer readable digital format. However, difficulties in the computer identification of drainage properties can arise in the presence of depressions in the digital topography. Depressions are low areas surrounded by higher terrain. Drainage is well defined towards and into a depression, but drainage out of a depression cannot be defined. Such a drainage sink with no apparent outflow leads to difficulties for computer evaluations. Since most digital topographies have depressions, it is important to overcome this problem, so that digital topographies can be usefully applied in runoff, erosion, nutrient transport and environmental investigations. A computer procedure is proposed to overcome this problem. The procedure consists of identifying and breaching any downstream flow blockage that may cause the depression, and, if breaching is not possible, by raising the digital topography within the depression until the depression no longer exists. The procedure was found to be robust and successful in simple, as well as in complex terrain. The procedure has made it possible to conduct a comprehensive computer analysis of the landscape topography and to identify drainage properties for use in runoff, erosion, nutrient transport and hydrologic investigations. Technical Abstract: This paper presents a new algorithm that is applied in conjunction with conventional depression filling to provide a more realistic treatment of depressions that are likely due to overestimation errors. The algorithm lowers the elevation of selected cells on the edge of the closed depression to simulate breaching of depression outlets. Application of this breaching galgorithm prior to depression filling can substantially reduce the number and size of depressions that need to be filled, especially in low relief terrain. Removing or reducing the size of a depression by breaching implicitly assumes that the depression is due to a spurious flow blockage caused by elevation overestimation. Removing a depression by filling, on the other hand, implicitly assumes that the depression is a direct artifact of elevation underestimation. Although the breaching algorithm cannot distinguish between overestimation and underestimation errors in a Digital Elevation Model, a constraining parameter for breaching length can be used to restrict breaching to closed depressions caused by narrow blockages along well defined drainage courses. Applying the constrained breaching algorithm prior to a conventional depression-filling algorithm allows both positive and negative elevation adjustments to be used to remove depressions. The breaching algorithm was incorporated into the DEM pre processing operations of the Topographic Parameterization software system. The effect of the algorithm is illustrated by the application of TOPAZ to a DEM of a low-relief landscape. |
