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Surface Hydrologic Modeling for Watershed Delineation

This overview discusses the importance of watershed management, algorithms and techniques for automatically delineating watersheds, flow length calculations, and the conversion of raster data to vector format.

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Surface Hydrologic Modeling for Watershed Delineation

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  1. Surface Hydrologic Modeling(Watershed Delineation) ESRM 250/CFR 520Autumn 2009 Phil Hurvitz 1 of 40

  2. Overview • Watershed management • Definitions • Algorithms & Watershed delineation • Automatically delineating watersheds • Flow length • Raster to vector conversion 2 of 40

  3. Overview • Watershed management • Definitions • Algorithms & Watershed delineation • Automatically delineating watersheds • Flow length • Raster to vector conversion 3 of 40

  4. Watershed management • Important topic in modern landscape management • In the past, landscapes have been managed by ownership • Plant & animal species do not obey ownership boundaries • Need for physically or biologically based land divisions 4 of 40

  5. Watershed management 5 of 40

  6. Watershed management • Watersheds are physically & biologically meaningful • Various interest groups can agree on watershed boundaries • Watersheds are easily defined based on elevation models • Management activities applied on watershed-by- watershed basis 6 of 40

  7. Overview • Watershed management • Definitions • Algorithms & Watershed delineation • Automatically delineating watersheds • Flow length • Raster to vector conversion 7 of 40

  8. Definition of watershed • “The region draining into a river, river system, or body of water” American Heritage Dictionary • The upstream area of any given point on the landscape • Physically defined by drainage point and upstream area • Also known as basin, sub-basin, catchment, and contributing area 8 of 40

  9. Definition of watershed • A watershed can be defined at a broad regional scale, such as the Columbia River watershed or . . . 9 of 40

  10. Definition of watershed • At a small local scale, such as the Husky Stadium watershed 10 of 40

  11. Overview • Watershed management • Definitions • Algorithms & Watershed delineation • Automatically delineating watersheds • Flow length • Raster to vector conversion 11 of 40

  12. How it works: algorithm • Axiom: Water always flows downhill • For any point on a rasterrepresenting a landscape,a drop of water can be traced downhill • Direction of flow can bedetermined for every DEM cell • For any point on a raster representing a landscape,a flow pathway can be traced back uphill • Flow accumulation can be calculated for every DEM cell • Uphill back-tracing proceeds to a ridgeline or to the edge of the grid • Termination of uphill back-tracing defines the watershed boundary 12 of 40

  13. Watershed delineation: Steps • Watershed delineation steps are available in ArcToolbox: • Create a depressionless DEM • Calculate flow direction • Calculate flow accumulation • Create watershed Pour points • Delineate watersheds 13 of 40

  14. Watershed delineation: Steps 14 of 40

  15. elevation Watershed delineation: Creating a depressionless DEM • DEM must eventually drain off edge of grid • Areas of internal drainage will result in unprocessed areas • FILL routine fills in sinks or cuts off peaks creating a new grid with no drainage errors 15 of 40

  16. Watershed delineation: Flow direction • Every cell flows into another cell or off the grid edge • Flow direction is calculated as the direction of steepest downward descent • Flow direction is stored in numerically-coded schema • Flow direction values are not ratio or proportional • Flow direction is calculated for each cell, resulting in a new grid theme 16 of 40

  17. direction of flow is saved as a code number flow moves out of a cell in one of 8 directions Watershed delineation: Flow direction • Flow direction values are encoded in a raster 17 of 40

  18. north-flowing cells coded as 64 Watershed delineation: Flow direction • This is a bookkeeping scheme, not a ratio or proportion representing a measured phenomenon 18 of 40

  19. Watershed delineation: Flow direction • Individual cells/zones in the grid are coded for flow direction 19 of 40

  20. Watershed delineation: Flow direction • Individual cells/zones in the grid are coded for flow direction 20 of 40

  21. Watershed delineation: Flow direction • Individual cells/zones in the grid are coded for flow direction 20 of 40

  22. Watershed delineation: Flow accumulation • Each has just been coded for direction of flow • Cumulative flow is calculated from flow direction • Output grid is created where values are the number of tributary (upstream) cells • Lower accumulation values are ridge tops • Higher accumulation values are valleys & stream channels 21 of 40

  23. Watershed delineation: Flow accumulation • Higher-flow cells have a larger value 3 5 1 22 of 40

  24. Watershed delineation: Flow accumulation • Higher-flow cells have a greater value 23 of 40

  25. Watershed delineation: Flow accumulation • Legend can be altered to show only high-flow cells a single class legend to show only those cells above a threshold of flow accumulation 24 of 40

  26. Watershed delineation: Flow accumulation • Generated flow network should fit closely with reality only if: DEM matches ground condition Streams match ground condition note discrepancies 25 of 40

  27. Watershed delineation: Watershed “Pour points” • Watersheds are defined by outlets (pour points) • Pour points should be placed in high-flow pathways • Basins will be generated from pour point to ridgeline or to upstream sub-basin • Pour points should be numerically coded per sub-basin • Pour points should be converted to a grid layer 26 of 40

  28. Create as many pour points as necessary Zoom in to place pour point in center of high-flow cell Watershed delineation: Watershed Pour points 27 of 40

  29. Watershed delineation: Delineating watersheds • Preliminary steps are completed • Filled DEM • Flow direction • Flow accumulation • Pour points created & converted to grid • Run tool to create watersheds 28 of 40

  30. Watershed delineation: Delineating watersheds • Watersheds represent area upstream from pour points • Watersheds terminate at ridgelines, • uphill sub-basin boundary, or • edge of the raster 29 of 40

  31. Clearly visible with analytically hillshaded DEM Watershed boundaries stop at ridgelines Watershed delineation: Delineating watersheds 30 of 40

  32. Overview • Watershed management • Definitions • Algorithms & Watershed delineation • Automatically delineating watersheds • Flow length • Raster to vector conversion 31 of 40

  33. Watershed delineation: Automatic delineation • Basin tool • No user control • Pour points automatically selected by “intersection” of highest-flow pathways and grid edge 32 of 40

  34. Watershed delineation: Automatic delineation • Basin tool 33 of 40

  35. Overview • Watershed management • Definitions • Algorithms & Watershed delineation • Automatically delineating watersheds • Flow length • Raster to vector conversion 34 of 40

  36. Flow length • Flow distance for every cell to outlet 35 of 40

  37. Flow length • Flow distance for every cell to closest stream 36 of 40

  38. Overview • Watershed management • Definitions • Algorithms & Watershed delineation • Automatically delineating watersheds • Flow length • Raster to vector conversion 37 of 40

  39. Raster to vector conversion • Conversion from rasters to lines or polygons • Stream network as line shape • Stream links as points • Stream order (Strahler or Shreve) • Watershed grid theme a polygon theme 38 of 40

  40. Raster to vector conversion • Display watersheds with other data sets to verify modeling 39 of 40

  41. Raster to vector conversion • Watershed themes can be incorporated with other raster & vector analysis methods • Road & stream densities • Forest age analysis • Sedimentation effects • Habitat area in different basins • Animal movement analysis 40 of 40

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