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Application of GIS in Application of GIS in Watershed AnalysisWatershed Analysis
Dr. A.K.M. Saiful IslamInstitute of Water and Flood Management (IWFM)Bangladesh University of Engineering and Technology (BUET)
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Watershed analysis using ArcGIS
This training will be on:
1. Digital Elevation Model (DEM)2. Flow direction3. Flow accumulation4. Identify and removing sink5. Stream Network6. Stream Links7. Watershed analysis
Input Data
DEM data of Bangladesh (Resolution 900m)
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How to turn on Hydrologic Modeling
1. First turn on Spatial Analyst
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Hydrology Tools
Activate ArcToolbox from window menu
Expand “Hydrology” from “Spatial Analyst” Tools
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1.Digital Elevation Model
• Add digital elevation model of Bangladesh
• Click on + icon and select bgd900dem
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DEM of Bangladesh (Resolution 900m)
This DEM will be used To calculate Flow direction
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2. Flow direction
Flow directions simplified: “Dump a bucket of water on your surface and see which of way it flows!”
Shows the direction water will flow out of each cell of a filled elevation raster.
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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Flow Direction for cell1014 1011 1004
1019 1015 1007
1025 1021 1012
+1 +4 +11
-4 +8
-10 -6 +3
+0.71 +4 +7.78
-4 +8
-7.07 -6 +2.12
1
1 1.414
Cell Elevation Elevation difference Distance weight
Distance weight Gradient Flow direction
32 64 128
16 1
8 4 2
Flow direction Number
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Flow direction using ArcToolbox
• Click flow direction and select bgd900dem as input surface raster
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Flow direction raster
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3. Flow accumulation
Flow accumulation simplified: “Accumulate flow for each cell considering ALL uphill cells!”
Used to estimate rainfall volume within a watershed and delineates drainage.
A flow accumulation raster tabulates for each cell the number of cells that will flow to it.
A flow accumulation raster records how many upstream cells will contribute drainage to each cell ( the cell itself is not counted).
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Flow Accumulation in cell
1014 1011 1004 996
1019 1015 1007 999
1025 1021 1012 1003
1033 1029 1020 1003
Cell Elevation Flow direction
0 0 1 2
0 0 2 5
0 0 1 1
0 0 0 0
Flow accumulation
From To0 11 22 33 44 5
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Flow accumulation using Arc Toolbox
• Click flow accumulation and select flowdir_bgd91 as input surface raster
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Flow accumulation raster
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4. Identify and removing sink
• Sink is surrounded by higher cell. It can artificially terminate streams by trapping flow.
• It is caused by possible error in the elevation data and need to be removed before attempting hydrology analysis.
Sink
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Filling Sinks
• Filling sinks is an iterative process.
• Sink cell values are changed to equal neighboring elevation cell values. Lowest value is used.
• Use Merge to put new sink values in the elevation theme. Test again for more sinks.
Fill to lowest Neighboring cell
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Identify sink using Arc Toolbox
• Click on Sink from hydrology
• Select flowdir_bgd91 as input flow direction raster
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Sink raster
• A total of 4621 sinks found
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Fill Sink using Arc Toolbox
• Click on Fill from hydrology
• Select bgd900dem as input flow direction raster
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Filled sink DEM
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Use filled sink DEM
Now create Flow direction raster aging using this filled sink DEM.
Create Flow Accumulation raster using this new Flow direction raster.
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Flow Direction raster using filled sink DEM
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Flow Accumulation rasterusing filled sink DEM
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5. Stream network
• You can use the flow accumulation grid to identify drainage courses by extracting those cells with the highest accumulated flow values ( cell contains the most water are stream channel).
• You can create a drainage network of any detail by choosing the appropriate volume values (300 is used for this data).
• Choice of threshold value can be arbitrary. It should be corresponding to high resolution topographic maps of filed maps.
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Stream Network using ArcToolbox
• Select Flow accumulation as input conditional raster
• Set 1 as constant value
• Expression: Value > 300
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Stream Network raster
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Stream network on Flow accumulation raster
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6. Stream Links
• After a stream network is derived from a flow accumulation raster, each section of the stream raster line is assigned a unique value and is associated with a flow direction.
37
35
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Arbitrarily assign unique values to each link of the stream networks.
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Stream link using Arc Toolbox
• Select Stream network as input stream raster
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Stream Link
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Stream Order
• Assign a numeric order to stream link. Use either Shreve (true) or Strahler (false) ordering methods.
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Stream order using Arc Toolbox
• Select streaml_con_1 as input stream raster.
• Stream ordering method is STRAHLER
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Stream Order raster
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Steam as Shape object
• The stream link raster file can be converted into line feature class.
• Streams can be represented as shape file.
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Stream to Feature using Arc Toolbox
• Select Streaml_con_1 as input stream raster
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Stream as Shape file
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7. Watershed Analysis
• Watersheds delineate a drainage area.• Drainage basin, water sheds, basin, catchments area,
and contributing area are synonymous terms referring to an area of drainage that contributes water to a common outlets (pour points).
PourPoint
Watersheds
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Water shed using Arc Toolbox
• Select stream link as input raster
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Watershed
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Change symbology of watershed
• Double click and change as Unique values
Recommended