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Flow tracing and watershed analysis

Geospatial Analysis and Modeling: Lecture notes Helena Mitasova, NCSU MEAS. Flow tracing and watershed analysis. Outline. cumulative terrain parameters based on flow tracing: flow path length, flow accumulation,

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Flow tracing and watershed analysis

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  1. Geospatial Analysis and Modeling: Lecture notes Helena Mitasova, NCSU MEAS Flow tracing and watershed analysis Geospatial Analysis and Modeling MEA592 – Helena Mitasova

  2. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Outline • cumulative terrain parameters based on flow tracing: flow path length, flow accumulation, • methods for computing flow direction (D*8, Dinf), flow tracing (SFD,MFD, uniform, weighted) • methods for flow tracing through depressions and flat areas • extracting watershed boundaries, computing watershed hierarchies

  3. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Cumulative flow parameters • Parameters and features: • flow path length, • flow accumulation and stream networks • watersheds and ridge lines Computed using flow lines

  4. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Cumulative flow parameters • Computed using flow lines • follow gradient direction (steepest slope) • perpendicular to contours

  5. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Computing flow direction • What is flow direction? • How to compute it? • D-8 and D-infinity

  6. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Computing flow direction: D8 • D8 algorithm: • uses 8 directions representing aspect discretized to 0, 45, 90, ... degrees • estimated from elevation differences between the given grid cell and its 8 neighboring cells

  7. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Computing flow direction: Dinf • D-infinity or vector-grid algorithm uses a floating point value of aspect estimated by approximation function (e.g. polynomial or spline)

  8. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow routing • Tracing flow in the gradient direction to compute • flow path length • flow accumulation

  9. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow path length • length of the flow line from the given cell to the outlet used to compute time to concentration (steady state flow) • hillslope length - from the given cell to a flat or depression, used for erosion modeling using Universal Soil Loss Equation (USLE) - measure of steady state flow depth for a uniform hillslope

  10. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow path length • how far is the given cell from the outlet • what is the length of a hillslope above a given cell

  11. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow accumulation • number of cells draining into a given cell = number of flow lines passing through each cell • size of the upslope contributing area (horizontal, in cell units) • measure of steady state flow depth

  12. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow accumulation • measure of steady state flow depth when raindrop from the farthest grid cell reaches the outlet

  13. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow routing: methods • single flow direction (SFD) moves entire unit of flow into a single downslope cell: flow dispersal on hillslopes with convex tangential curvature is not captured • multiple flow direction (MFD) partitions flow into two or more downslope directions • both can be implemented with D8 or Dinf

  14. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow accumulation: D8 • High resolution DEM: SFD D8

  15. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow accumulation: D8, Dinf • High resolution DEM: SFD D8 and Dinf

  16. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow accumulation: SFD, MFD • High resolution DEM: MFD D8

  17. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow routing: methods • uniform: one unit is routed from each cell • weighted: each cell is assigned weight proportional to amount of water it produces (rainfall excess available for runoff) , e.g. based on soil properties or land cover

  18. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow accumulation: weighted 30m DEM < flowacc. SFD D8 uniform

  19. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow accumulation: weighted • 30m DEM • < flowacc. • SFD D8 • uniform • Landuse>

  20. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow accumulation: weighted 30m DEM < flowacc. SFD D8 uniform Landuse> Weights> (rainfall excess) < weighted flowacc.

  21. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Stream extraction • Semi-automated stream mapping: extracting connected stream network from flow accumulation map using selected threshold • Stream raster map is derived using map algebra • Result is converted to vector representation • Stream origin is dynamic, often driven by groundwater: additional information is needed • curvature • groundwater, geology

  22. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow accumulation: stream extraction Flow accumulation from 30m NED, method: SFD D8 and a vectorized extracted stream network

  23. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow accumulation: stream extraction Flow accumulation from SRTM 90m and IFSARE 10m DEMs patched together and reinterpolated to 30m resolution method: SFD D8

  24. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Stream networks from SRTM Stream network and watershed boundaries from SRTM DEM Flow through large water bodies is indicated by straight stream segments: routing through flats method: SFD D8

  25. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow routing: flat areas • to create connected stream network flow needs to be routed through flats and depressions • integer DEMs, lakes or filled depressions create flat areas • flat areas: zero slope and undefined aspect • solutions: • iterative assignment of direction from the first draining cell • imposed gradient (small change in elevation)

  26. Geospatial Analysis and Modeling MEA592 – Helena Mitasova DEM depressions • depressions “trap” flow • sources of depressions: • real features • noise, measurement errors • artifacts from processing

  27. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Treating depressions • routing through false depressions: • filling fill-in works well for small depressions

  28. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Treating depressions • routing through false depressions: • filling, carving fill-in does not create flats carve-in

  29. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Treating depressions • routing through false depressions: • filling, carving, hybrid, fill-in hybrid carve-in

  30. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Treating depressions • routing through false depressions: • filling, carving, hybrid, least cost path fill-in least cost path hybrid carve-in

  31. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Treating depressions Flowaccumulation: MFD with depr. filling • Depressions • in lidar-based DEM

  32. Depressions in IFSARE DSM River profile from SRTM DEM

  33. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Filling for IFSARE DSM DSM: DEM+vegetation and structures, depressions due to gaps in vegetation 3D view of the original and filled DEM with streams derived by Rivertools

  34. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Resolving large depression in IFSARE DSM most common approach: fill-in least cost path Both are SFD, D8, 10m resolution DEM

  35. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Comparison of algorithms r.watershed: least cost path r.terraflow fill in rivertools fill in measured sites all are SFD D8

  36. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Treating depressions: carving • modifying DEM along with interpolation or flowrouting • carving based on stream data • hydrologically conditioned DEM: no depressions • NOT hydrologically correct - all potential wetlands are removed 50ft NCFMP DEM

  37. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Stream extraction: accuracy • Measured as horizontal accuracy of stream centerline or stream banks for large rivers • topographic maps - the lowest accuracy, old • extracted from lidar-based DEMs - better, but accuracy low in coastal plane, improvements expected with new lidar mapping • digitized from high res orthophotos and on ground surveys - most accurate except forested areas • structures require additional information (bridges are represented as dams in DEMs)

  38. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow lines up and down • dowslope flowlines - converge in valleys • upslope flowlines - converge on ridges

  39. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Flow lines upslope: dune ridges • upslope flowlines - converge on ridges change in ridge mean slope indicates whether dune is stabilizing angle between ridge and crest: transformation from crescentic to parabolic

  40. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Watersheds • watershed - important land management unit • water and mass from a watershed drains to a single point: outlet • other terms for watershed: (drainage) basin, catchment, contributing area • watersheds can be organized into hierarchies based on the size of contributing area USGS Hydrologic units: drainage areas of major rivers or multiple smaller rivers: see more at http://water.usgs.gov/GIS/huc.html

  41. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Watershed boundaries • GIS watershed analysis: • find watershed boundaries for a given outlet - contributing area from which water flows into a given grid cell or stream segment • partition area into watersheds with the given approximate size

  42. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Watershed boundaries • methods: same as for flow accumulation • starting from outlet, trace all cells going upslope using reversed flow direction and classify them with ID • snapping outlet to the stream - why it is needed • incomplete contributing area - given DEM does not cover entire drainage basin - flow accumulation and contributing area cannot be computed (handled e.g., as negative values of flow accumulation)

  43. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Watershed boundaries • exterior and interior watersheds in urban area highway

  44. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Watershed hierarchies • interior and exterior subwatersheds

  45. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Watershed for a given outlet • Contributing area: flowaccumulation*cell area • Watershed boundaries traced from a given outlet

  46. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Watershed and stream data • EDNA - hydrologicaly relevant parameters from DEMs http://edna.usgs.gov/ • HydroSHEDS worldwide watersheds and rivers based on 15sec DEM derived from 90m SRTM http://hydrosheds.cr.usgs.gov/ • national hydrologic data set - USGS, from topomaps

  47. Geospatial Analysis and Modeling MEA592 – Helena Mitasova Summary and references • Gruber and Peckham 2008, Chapter 7 in Hengl, T. and Reuter, H. I., 2008, Geomorphometry: Concepts, Software, Applications, Elsevier • Chang Ch.15, Neteler Ch. 5.4.4 • software: • JGRASS, LandSERF • TauDEM (ArcGIS and MapWindow extension), • TAS • TerraSTREAM (for massive data sets), • SAGA GIS, gv SIG

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