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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

36

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

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Unique Basins