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

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: 1.Create a depressionless DEM 2.Calculate flow direction 3.Calculate flow accumulation 4.Create watershed Pour points 5.Delineate watersheds

12 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 elevation

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

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

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 1 3 5

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