Surface Hydrologic Modeling for Watershed Delineation

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