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1. http://radar.oreilly.com/archives/2008/05/where-20-video-googleesri-keyn.htmlhttp://radar.oreilly.com/archives/2008/05/where-20-video-googleesri-keyn.html

2. Hydrological Modeling

3. Overview • Introduction • Watershed delineation • Automaticdelineation • Flow length

4. Introduction

5. Watershed management

6. 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

9. Watershed delineation

10. How it works • Water always flows downhill • For any point on a grid representing a landscape,a drop of water can be traced downhill •  direction of flow is known for every DEM cell • For any point on a grid representing a landscape,a flow pathway can be traced back uphill •  flow accumulation is known for every DEM cell • Uphill back-tracing proceeds to a ridgeline or to the edge of the grid • Termination of uphill back-tracing defines watershed boundary

11. Watershed delineation • Steps (with ArcToolbox): • Create a depressionless DEM • Calculate flow direction • Calculate flow accumulation • Create watershed Pour points • Delineate watersheds

12. elevation 1. 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

13. 2. 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 calculated for each cell, resulting in a new grid theme

14. flow moves out of a cell in one of 8 directions 2. Flow direction direction of flow is saved as a code number

15. 2. Flow direction north-flowing cells coded as 64

16. 2. Flow direction Flow direction grid

17. 3. Flow accumulation • Each cell has been coded for direction of flow • Cumulative flow is calculated from flow direction • Output grid is created where values are the number of upstream cells • Lower accumulation values are ridge tops • Higher accumulation values are valleys & stream channels

18. 3. Flow accumulation 3 5 1

19. 3. Flow accumulation

20. 3. Flow accumulation single class legend shows high flow cells

21. 3. Flow accumulation Fit depends on accuracy of the DEM and stream layers

22. 4. 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 must be converted to a grid layer

23. Zoom in to place pour point in center of high-flow cell 4. Watershed Pour points • Create as many pour points as necessary

24. 5. Delineating watersheds • Preliminary steps are completed • Filled DEM • Flow direction • Flow accumulation • Pour points created & converted to grid • Run tool to create watersheds

25. 5. Delineating watersheds Watersheds represent area upstream from Pour points and terminate at ridgelines, uphill sub-basin boundary, or edge of the grid

26. 5. Delineating watersheds

27. Automaticdelineation

28. Automatic delineation Pour points automatically selected by “intersection” of highest-flow pathways and grid edge

29. Automatic delineation

30. Flow length

31. Flow length Flow distance for every cell to outlet

32. Flow length Flow distance for every cell to closest stream

33. Flow length Euclidean distance vs. flow distance

34. Homework Read:Hydrological Modeling & Watershed Delineation Finish: Assignment 7 Final: Tuesday June 8th , 4:30-6:30 MGH 030 CFR 520 posters will be due Thursday, June 10, 5:00pm