GIS in Hydrology

1. GIS4048/5100 Applications in GIS GIS in Hydrology • Watershed management • Definitions • Algorithms • Watershed delineation • Automatically delineating watersheds • Flow length • Raster to vector conversion

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

3. 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. 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

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

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

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

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

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

10. How it works: algorithm • Axiom: 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 ArcToolbox: • Create a depressionless DEM • Calculate flow direction • Calculate flow accumulation • Create watershed Pour points • Delineate watersheds

12. 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

13. 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

14. direction of flow is saved as a code number • flow moves out of a cell in one of 8 directions Watershed delineation: Flow direction

15. north-flowing cells • coded as 64 Watershed delineation: Flow direction • This is a bookkeeping scheme, not a ratio or proportion

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

17. 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

18. Watershed delineation: Flow accumulation • Higher-flow cells have a larger value 3 5 1

19. 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

20. 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

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

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

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

24. Flow length • Flow distance for every cell to outlet

25. Flow length • Flow distance for every cell to closest stream

26. 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 as a polygon theme

27. Applications of watershed analysis • 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