by Francisco Olivera
Graduate Research Assistant
University of Texas at Austin
Center for Research in Water Resources
Inland catchments constitute closed hydrologic systems that do not drain to the ocean as most watersheds do. In inland catchments water drains towards an inland pour point located within the basin, and not on the basin border. The analysis of inland catchments with Arc/Info-GRID is not straight forward, and requires a good understanding of the GRID conceptual hydrologic model. Hydrologic analysis with GRID is based on the digital elevation model (DEM) of the terrain, from which all the topographically-determined characteristics of the landscape are obtained.
When performing hydrologic analysis with GRID, the very first step consists in determining the flow direction with the FLOWDIRECTION function. The FLOWDIRECTION function has the DEM as argument, and determines the direction water takes when flowing due to gravity, i.e., direction of stepest descent. However, the flow-direction can not be determined for cells that are lower than their surrounding neighbor cells. In such a case, FLOWDIRECTION flags the lower cells with an error value, which affects all cells flowing towards them. The existance of lower cells is explained by DEM errors (which might be small for most practical purposes but critical for hydrologic modeling) or by inland catchments (in which the lowest cell constitutes a pour point). The purpose of the AML presented here is to identify which of the lower cells are DEM errors and which correspond to inland catchments.
Once the inland catchments have been identified, the terrain pits generated by DEM errors are corrected by filling the DEM with the FILL function, and those generated by inland catchments are flaged by assigning NODATA to their pour points, i.e. the lowest point of the pit.
The methodology used by the program to spot inland catchments consists in filling the DEM and comparing the area and depth of the filled zones with certain user-defined threshold values. All filled zones that satisfy the criteria are then treated as inland catchments and NODATA is assigned to their lowest cells in the original DEM. Once the inland catchment pour points have been assigned NODATA, the DEM can be filled to correct DEM errors.
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These materials may be used for study, research, and education, but please credit the authors and the Center for Research in Water Resources, The University of Texas at Austin. All commercial rights reserved. Copyright 1997 Center for Research in Water Resources.
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