ESRM 250 CFR 520 Introduction to GIS Surface

ESRM 250 & CFR 520: Introduction to GIS Surface Hydrologic Modeling (Watershed Delineation) ESRM 250/CFR 520 Autumn 2009 Phil Hurvitz © Phil Hurvitz, 1999 -2009 1 of 40

ESRM 250 & CFR 520: Introduction to GIS Overview Watershed management Definitions Algorithms & Watershed delineation Automatically delineating watersheds Flow length Raster to vector conversion © Phil Hurvitz, 1999 -2009 2 of 40

ESRM 250 & CFR 520: Introduction to GIS Overview Watershed management Definitions Algorithms & Watershed delineation Automatically delineating watersheds Flow length Raster to vector conversion © Phil Hurvitz, 1999 -2009 3 of 40

ESRM 250 & CFR 520: Introduction to GIS 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 © Phil Hurvitz, 1999 -2009 4 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed management © Phil Hurvitz, 1999 -2009 5 of 40

ESRM 250 & CFR 520: Introduction to GIS 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-bywatershed basis © Phil Hurvitz, 1999 -2009 6 of 40

ESRM 250 & CFR 520: Introduction to GIS Overview Watershed management Definitions Algorithms & Watershed delineation Automatically delineating watersheds Flow length Raster to vector conversion © Phil Hurvitz, 1999 -2009 7 of 40

ESRM 250 & CFR 520: Introduction to GIS Definition of watershed “The region draining into a river, river system, or body of water” American Heritage Dictionary þ The upstream area of any given point on the landscape þ Physically defined by drainage point and upstream area þ Also known as basin, sub-basin, catchment, and contributing area © Phil Hurvitz, 1999 -2009 8 of 40

ESRM 250 & CFR 520: Introduction to GIS Definition of watershed A watershed can be defined at a broad regional scale, such as the Columbia River watershed or. . . © Phil Hurvitz, 1999 -2009 9 of 40

ESRM 250 & CFR 520: Introduction to GIS Definition of watershed At a small local scale, such as the Husky Stadium watershed © Phil Hurvitz, 1999 -2009 10 of 40

ESRM 250 & CFR 520: Introduction to GIS Overview Watershed management Definitions Algorithms & Watershed delineation Automatically delineating watersheds Flow length Raster to vector conversion © Phil Hurvitz, 1999 -2009 11 of 40

ESRM 250 & CFR 520: Introduction to GIS How it works: algorithm Axiom: Water always flows downhill þ For any point on a raster representing a landscape, a drop of water can be traced downhill þ þ For any point on a raster representing a landscape, a flow pathway can be traced back uphill þ þ þ Direction of flow can be determined for every DEM cell Flow accumulation can be calculated for every DEM cell Uphill back-tracing proceeds to a ridgeline or to the edge of the grid Termination of uphill back-tracing defines the watershed boundary © Phil Hurvitz, 1999 -2009 12 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Steps Watershed delineation steps are available in Arc. Toolbox: 1. 2. 3. 4. 5. Create a depressionless DEM Calculate flow direction Calculate flow accumulation Create watershed Pour points Delineate watersheds © Phil Hurvitz, 1999 -2009 13 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Steps © Phil Hurvitz, 1999 -2009 14 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Creating a depressionless DEM must eventually drain off edge of grid Areas of internal drainage will result in unprocessed areas þ FILL routine fills in sinks or cuts off peaks creating a new grid with no drainage errors elevation þ © Phil Hurvitz, 1999 -2009 15 of 40

ESRM 250 & CFR 520: Introduction to GIS 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 þ Flow direction is stored in numerically-coded schema þ Flow direction values are not ratio or proportional þ Flow direction is calculated for each cell, resulting in a new grid theme þ © Phil Hurvitz, 1999 -2009 16 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Flow direction values are encoded in a raster direction of flow is saved as a code number flow moves out of a cell in one of 8 directions © Phil Hurvitz, 1999 -2009 17 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Flow direction This is a bookkeeping scheme, not a ratio or proportion representing a measured phenomenon north-flowing cells © Phil Hurvitz, 1999 -2009 coded as 64 18 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Flow direction Individual cells/zones in the grid are coded for flow direction © Phil Hurvitz, 1999 -2009 19 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Flow direction Individual cells/zones in the grid are coded for flow direction © Phil Hurvitz, 1999 -2009 20 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Flow direction Individual cells/zones in the grid are coded for flow direction © Phil Hurvitz, 1999 -2009 20 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Flow accumulation Each has just been coded for direction of flow Cumulative flow is calculated from flow direction þ Output grid is created where values are the number of tributary (upstream) cells þ Lower accumulation values are ridge tops þ Higher accumulation values are valleys & stream channels þ © Phil Hurvitz, 1999 -2009 21 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Flow accumulation Higher-flow cells have a larger value 3 5 1 © Phil Hurvitz, 1999 -2009 22 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Flow accumulation Higher-flow cells have a greater value © Phil Hurvitz, 1999 -2009 23 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Flow accumulation Legend can be altered to show only high-flow cells a single class legend to show only those cells above a threshold of flow accumulation © Phil Hurvitz, 1999 -2009 24 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Flow accumulation Generated flow network should fit closely with reality only if: DEM matches ground condition Streams match ground condition note discrepancies © Phil Hurvitz, 1999 -2009 25 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Watershed “Pour points” Watersheds are defined by outlets (pour points) Pour points should be placed in high-flow pathways þ Basins will be generated from pour point to ridgeline or to upstream sub-basin þ Pour points should be numerically coded per subbasin þ Pour points should be converted to a grid layer þ © Phil Hurvitz, 1999 -2009 26 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Watershed Pour points Create as many pour points as necessary Zoom in to place pour point in center of high-flow cell © Phil Hurvitz, 1999 -2009 27 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Delineating watersheds Preliminary steps are completed Filled DEM þ Flow direction þ Flow accumulation þ Pour points created & converted to grid þ Run tool to create watersheds © Phil Hurvitz, 1999 -2009 28 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Delineating watersheds Watersheds represent area upstream from pour points Watersheds terminate at ridgelines, uphill sub-basin boundary, or edge of the raster © Phil Hurvitz, 1999 -2009 29 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Delineating watersheds Clearly visible with analytically hillshaded DEM Watershed boundaries stop at ridgelines © Phil Hurvitz, 1999 -2009 30 of 40

ESRM 250 & CFR 520: Introduction to GIS Overview Watershed management Definitions Algorithms & Watershed delineation Automatically delineating watersheds Flow length Raster to vector conversion © Phil Hurvitz, 1999 -2009 31 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Automatic delineation Basin tool No user control þ Pour points automatically selected by “intersection” of highest-flow pathways and grid edge þ © Phil Hurvitz, 1999 -2009 32 of 40

ESRM 250 & CFR 520: Introduction to GIS Watershed delineation: Automatic delineation Basin tool © Phil Hurvitz, 1999 -2009 33 of 40

ESRM 250 & CFR 520: Introduction to GIS Overview Watershed management Definitions Algorithms & Watershed delineation Automatically delineating watersheds Flow length Raster to vector conversion © Phil Hurvitz, 1999 -2009 34 of 40

ESRM 250 & CFR 520: Introduction to GIS Flow length Flow distance for every cell to outlet © Phil Hurvitz, 1999 -2009 35 of 40

ESRM 250 & CFR 520: Introduction to GIS Flow length Flow distance for every cell to closest stream © Phil Hurvitz, 1999 -2009 36 of 40

ESRM 250 & CFR 520: Introduction to GIS Overview Watershed management Definitions Algorithms & Watershed delineation Automatically delineating watersheds Flow length Raster to vector conversion © Phil Hurvitz, 1999 -2009 37 of 40

ESRM 250 & CFR 520: Introduction to GIS Raster to vector conversion Conversion from rasters to lines or polygons Stream network as line shape þ Stream links as points þ Stream order (Strahler or Shreve) þ Watershed grid theme a polygon theme þ © Phil Hurvitz, 1999 -2009 38 of 40

ESRM 250 & CFR 520: Introduction to GIS Raster to vector conversion Display watersheds with other data sets to verify modeling © Phil Hurvitz, 1999 -2009 39 of 40

ESRM 250 & CFR 520: Introduction to GIS Raster to vector conversion Watershed themes can be incorporated with other raster & vector analysis methods Road & stream densities þ Forest age analysis þ Sedimentation effects þ Habitat area in different basins þ Animal movement analysis þ © Phil Hurvitz, 1999 -2009 40 of 40