Watershed

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Watershed Delineation by ArcHydro

1. Introduction
2. OBJECTIVE
3. SRTM 90m Digital Elevation Model
4. Getting Started With Archydro
4.1 Data Setup
4.2 Watershed Delineation
5. Overlay delineated Drainage Line on Google Earth
6. Tutorial
1. Introduction
GIS is a powerful tool for developing solutions for water resources such as assessing water quality and managing water resources on a
local or regional scale. Hydrologists use GIS technology to integrate various data and applications into one, manageable system.
ArcGIS with Arc Hydro gives you the flexibility to combine watershed datasets from one map source with stream and river networks.
The suite of tools contained in Arc Hydro facilitate the creation, manipulation, and display of hydro features and objects within the
ArcGIS environment. Use ArcGIS Spatial Analyst for hydrologic analysis such as calculating flow across an elevation surface, which
provides the basis for creating stream networks and watersheds; calculating flow path length; and assigning stream orders.
2. OBJECTIVE
Watershed Delineation
Familiarize with the capabilities of ArcHydro.
3. SRTM 90m Digital Elevation Model

Shuttle Radar Topography Mission (SRTM) 90m resolution data for whole world is free available on
http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp . It is in WGS 1984 projection system. DEMs are digital records of terrain
elevations for ground positions at regularly spaced horizontal intervals. These grids are derived from standard topographic quadrangle
maps through the use of hypsographic data and /or photogrammetric methods.
4. Getting Started With Archydro

4.1 Data Setup
Step 1: Open ArcMap
http://nptel.ac.in/courses/105102015/gis%20done/case%20study/watershed%20delineation%20by%20archydro.html Page 1 of 19
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Step 2: Under the Customize menu of ArcMap, click the Extensions… button. You will see an extension entitled “Spatial Analyst”.
Turn on this extension.
Step 3: Go to Toolbar under the Customize menu of ArcMap, a list of tools will appear. Click the ArcHydro Tools.
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Step 4: The main interface of ArcHydro will open in ArcMap.
Step 5: Click on the icon to add the raster data. In the dialog box, navigate to the location of the data; select the raster file
Himachal.tif containing the DEM for Himachal Pradesh Area and click on the “Add” button.
Step 6: Click on the icon to add the vector data. In the dialog box, navigate to the location of the data; select the vector file
stream.shp containing the drainage for Himachal Pradesh Area and click on the “Add” button.
Step 7: Save the ArcMap Document as WatershedDelineation.mxd as desired location.
4.2 Watershed Delineation

Watershed delineation are all derived from the basic premise that water flows downhill, and in so doing it will follow the path with the
largest gradient (steepest slope). In a DEM grid structure, there exist at most 8 cells adjacent to each individual grid cell.(Cells on the
grid boundary are not bounded on all sides) Accordingly, water in one cell travels in 1 out of at most 8 different directions in order to
enter the next downstream cell. This concept is referred to as the 8-direction pour point model.
In this grid representation, water in a grid cell may flow only along one of the eight paths depicted by arrows. The number in each cell
represents the direction water travels to enter the nearest downstream cell, and the numbering scheme has been set by convention. The
numbers were determined from the series 2x x ={0,1,...7}.
Step 1: DEM Reconditioning

This function modifies a DEM by imposing linear features onto it (burning/fencing: The function needs as input a raw DEM and a
linear feature class (like the river network) that both have to be present in the map document.
Select Terrain Preprocessing | DEM Reconditioning.
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Select the appropriate Raw DEM (Himachal) and AGREE stream feature (stream).Change parameter according to following figure.
The output is a reconditioned Agree DEM (default name AgreeDEM).
Step 2: Fill Sinks
This function fills the sinks in a grid. If cells with higher elevation surround a cell, the water is trapped in that cell and cannot flow. The
Fill Sinks function modifies the elevation value to eliminate these problems.
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DEM alteration to allow delineation of inland catchments
Filling an artificial pit in the DEM
Select Terrain Preprocessing | Fill Sinks.
Confirm that the input for DEM is “AgreeDEM” (or your original DEM if Reconditioning was not implemented). The output is the
Hydro DEM layer, named by default “Fil”. This default name can be overwritten.
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Press OK. Upon successful completion of the process, the “Fil” layer is added to the map. This process takes a few minutes.
Step 3: Flow Direction

This function computes the flow direction for a given grid. The values in the cells of the flow direction grid indicate the direction of
the steepest descent from that cell.
Slope Calculations with the 8-direction Pour Point Model – A) Slope calculated for diagonal cells; B) Slope calculated for cells with common sides.
Flow Direction grid, contains cells with only the numerical values dictated by the 8-direction pour point model.
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Grid Operations – A) DEM Grid; B) Flow Direction Grid. Note: Area in red is from the previous figure
Physical Representation a Flow Direction Grid –A) with directional arrows; B) As a flow network
Select Terrain Preprocessing | Flow Direction.
Confirm that the input for Hydro DEM is “Fil”. The output is the Flow Direction Grid, named by default “Fdr”. This default name can
be overwritten.
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Press OK. Upon successful completion of the process, the flow direction grid “Fdr” is added to the map.
Step 4: Flow Accumulation
This function computes the flow accumulation grid that contains the accumulated number of cells upstream of a cell, for each cell in
the input grid. Flow accumulation grid is calculated from Flow direction grid.
Flow Accumulation – number of cells draining to a given cell (blue) along the flow network
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Select Terrain Preprocessing | Flow Accumulation.
Confirm that the input of the Flow Direction Grid is “Fdr”. The output is the Flow Accumulation Grid having a default name of “Fac”
that can be overwritten.
Press OK. Upon successful completion of the process, the flow accumulation grid “Fac” is added to the map.
Step 5: Stream Definition

With a flow accumulation grid, streams may be defined through the use of a threshold flow accumulation value. For example, if a value
of 5 were set as the threshold, than any cell with flow accumulation greater than 5 would be considered a stream.
This function computes a stream grid which contains a value of "1" for all the cells in the input flow accumulation grid that have a
value greater than the given threshold. All other cells in the Stream Grid contain no data.
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Stream Definition from the Flow Accumulation Grid and a threshold value – A) Grid cells with accumulation greater than or equal to 5 are considered stream cells
(red); B) Streams identified on the flow network (red); C) Stream Grid
Select Terrain Preprocessing | Stream Definition.
Confirm that the input for the Flow Accumulation Grid is “Fac”. The output is the Stream Grid. “Str” is its default name that can be
overwritten.
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A default value is displayed for the river threshold. This value represents 1% of the maximum flow accumulation: a simple rule of
thumb for stream determination threshold.
Put Number of cells “5000”. The threshold drainage area to generate a stream is then 5000 x 90 x 90 / 1000000 = 40.5 km2. A smaller
threshold will result in a denser stream network and usually in a greater number of delineated catchments.
Press OK. Upon successful completion of the process, the stream grid “Str” is added to the map.
Step 6: Stream Segmentation

The next step is to divide the stream network into distinct stream segments – this is useful if the purpose of the delineation is to
determine the individual Catchments. If only the overall watershed is desired, the delineation function could be used on the established
grids as long as the outlet cell is defined. For this discussion, Catchments are delineated
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Stream Links defined – A) Stream Grid representation, B) Stream Links (numbers) defined, link outlets (blue), watershed outlet (red)
This function creates a grid of stream segments that have a unique identification. Either a segment may be a head segment, or it may
be defined as a segment between two segment junctions. All the cells in a particular segment have the same grid code that is specific to
that segment.
Select Terrain Preprocessing | Stream Segmentation.
Confirm that “Fdr” and “Str” are the inputs for the Flow Direction Grid and the Stream Grid respectively. The output is the Link Grid,
with the default name “StrLnk” that can be overwritten by the user.
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Press OK. Upon successful completion of the process, the link grid “StrLnk” is added to the map.
Step 7: Catchment Grid Delineation

This function creates a grid in which each cell carries a value (grid code) indicating to which catchment the cell belongs. The value
corresponds to the value carried by the stream segment that drains that area, defined in the stream segment link grid
Select Terrain Preprocessing | Catchment Grid Delineation.
Confirm that the input to the Flow Direction Grid and Link Grid are “Fdr” and “StrLnk” respectively. The output is the Catchment
Grid layer. “Cat” is its default name that can be overwritten by the user.
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Press OK. Upon successful completion of the process, the Catchment grid “Cat” is added to the map. You can recolor the grid with
unique values to get a nice display.
Step 8: Catchment Polygon Processing

The three functions Catchment Polygon Processing, Drainage Line Processing and Adjoint Catchment Processing convert the raster
data developed so far to vector format. The rasters created up to now have all been stored in a folder named "Layers". The vector data
will be stored in a feature dataset also named "Layers" within the geodatabase associated with the map document.
Select Terrain Preprocessing | Catchment Polygon Processing.
This function converts a catchment grid into a catchment polygon feature.
Confirm that the input to the Catchment Grid is “Cat”. The output is the Catchment polygon feature class, having the default name
“Catchment” that can be overwritten.
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Press OK. Upon successful completion of the process, the polygon feature class “Catchment” is added to the map.
Open the attribute table of "Catchment". Notice that each catchment has a HydroID assigned that is the unique identifier of each
catchment within ArcHydro. Each catchment also has Shape Length and Area attributes. These quantities are automatically computed
when a feature class becomes part of a geodatabase.
Step 9: Drainage Line Processing

This function converts the input Stream Link grid into a Drainage Line feature class. Each line in the feature class carries the identifier
of the catchment in which it resides.
Select Terrain Preprocessing | Drainage Line Processing.
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Confirm that the input to Link Grid is “Lnk” and to Flow Direction Grid “Fdr”. The output Drainage Line has the default name
“DrainageLine” that can be overwritten.
Press OK. Upon successful completion of the process, the linear feature class “DrainageLine” is added to the map.
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5. Overlay delineated Drainage Line on Google Earth

Step 1: Click ArcToolbox and expand the Conversion Tools. Now select “To KML” and double click “Layer to KML”.
Step 2: Select the “DrainageLine” Shapefile in the first pull down menu.
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Step 3: In the Second pull down menu, Browse the Output location to Store KML file and give the name “Drainage”.
Step 4: In the Third pull down menu, give output scale 50,000.
Step 5: Click Ok. After completing the process, open “Drainage.kmz” in Google earth.
6. Tutorial
1. Cell size, Number of rows and columns of DEM, maximum and minimum elevation values in Himachal.tif.
2. How many DrainageLines and Catchments are there?
3. Perform same exercise with 500 thresholds and calculate number of catchment.
4. Make a screen capture of the attribute table of Fdr and give an interpretation for the values in the Value field using a sketch.
5. Generate contour from the DEM and create a layout view.
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4/11/17, 8(25 PM

Watershed Delineation by ArcHydro

3. SRTM 90m Digital Elevation Model

4. Getting Started With Archydro

Step 4: The main interface of ArcHydro will open in ArcMap.

Step 7: Save the ArcMap Document as WatershedDelineation.mxd as desired location.

4.2 Watershed Delineation

Step 1: DEM Reconditioning

Select Terrain Preprocessing | DEM Reconditioning.

The output is a reconditioned Agree DEM (default name AgreeDEM).

Step 2: Fill Sinks

DEM alteration to allow delineation of inland catchments

Filling an artificial pit in the DEM

Select Terrain Preprocessing | Fill Sinks.

Step 3: Flow Direction

Select Terrain Preprocessing | Flow Direction.

Step 4: Flow Accumulation

Select Terrain Preprocessing | Flow Accumulation.

Step 5: Stream Definition

Select Terrain Preprocessing | Stream Definition.

Step 6: Stream Segmentation

Select Terrain Preprocessing | Stream Segmentation.

Step 7: Catchment Grid Delineation

Select Terrain Preprocessing | Catchment Grid Delineation.

Step 8: Catchment Polygon Processing

Select Terrain Preprocessing | Catchment Polygon Processing.

This function converts a catchment grid into a catchment polygon feature.

Step 9: Drainage Line Processing

Select Terrain Preprocessing | Drainage Line Processing.

5. Overlay delineated Drainage Line on Google Earth

2. How many DrainageLines and Catchments are there?

5. Generate contour from the DEM and create a layout view.

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