1 GIS in Hydrology Watershed management Definitions Algorithms Watershed delineation Automatically delineating watersheds Flow length Raster to vector

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GIS in Hydrology

• Watershed management• Definitions• Algorithms• Watershed delineation• Automatically delineating watersheds • Flow length• Raster to vector conversion

GIS4048/5100 Applications in GIS

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Overview

• Watershed management

• Definitions

• Algorithms & Watershed delineation

• Automatically delineating watersheds

• Flow length

• Raster to vector conversion

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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

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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

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Overview


• Definitions



• Flow length


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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

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• A watershed can be defined at a broad regional scale, such as the Columbia River watershed

or . . .

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• At a small local scale, such as the Husky Stadium watershed

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Overview


• Definitions



• Flow length


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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

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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

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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

elev

atio

n

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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

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• flow moves out of a cell in one of 8 directions

• direction of flow is saved as a code number

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• This is a bookkeeping scheme, not a ratio or proportion

• north-flowing cells • coded as 64

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• Individual cells/zones in the grid are coded for flow direction

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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

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Watershed delineation: Flow accumulation

• Higher-flow cells have a larger value

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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

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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

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Overview


• Definitions



• Flow length


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Watershed delineation: Automatic delineation

• Basin tool

• No user control

• Pour points automatically selected by “intersection” of highest-flow pathways and grid edge

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Overview


• Definitions



• Flow length


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Flow length• Flow distance for every cell to outlet

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Flow length• Flow distance for every cell to closest

stream

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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

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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

Documents

1 GIS in Hydrology Watershed management Definitions Algorithms Watershed delineation Automatically delineating watersheds Flow length Raster to vector