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Hydrological Modeling

Hydrological Modeling. Overview. Introduction Watershed delineation Automatic delineation Flow length. Introduction. Watershed management. Definition of watershed. “The region draining into a river, river system, or body of water” American Heritage Dictionary

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Hydrological Modeling

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  1. Hydrological Modeling

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

  3. Introduction

  4. Watershed management

  5. 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

  6. Watershed delineation

  7. 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

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

  9. 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

  10. 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

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

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

  13. 2. Flow direction Flow direction grid

  14. 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

  15. 3. Flow accumulation 3 5 1

  16. 3. Flow accumulation

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

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

  19. 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

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

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

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

  23. 5. Delineating watersheds

  24. Automaticdelineation

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

  26. Automatic delineation

  27. Flow length

  28. Flow length Flow distance for every cell to outlet

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

  30. Flow length Euclidean distance vs. flow distance

  31. Homework Read:Hydrological Modeling & Watershed Delineation, Map Layouts Study for the exam Presentations: 3/5 Luke, Jennifer, Josh 3/7 Chris, Prati, Carolyn

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