watershed modelling using gis

7/27/2019 Watershed Modelling using GIS

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GIS for Watershed Modeling Exercise

Slope calculation using ArcGIS

Save the following to a text file elev.txt

ncols 5nrows 4

xllcorner 0

yllcorner 0cellsize 100

NODATA_value -9999

57 55 47 48 48

5367

56 49 5253 52 45 42 4351 58 40 41 40

Based on the elevation grid:

1. Manually assign eight direction pour point model grid

2. Draw flow direction grid3. Hand calculate flow accumulation grid

4. Hand draw drainage line

Open ArcMap and ArcToolbox. Use the tool Conversion Tools To Raster ASCII to Raster to import this grid file intoArcMap. Specify a name and location for the Output raster (such as elev) and specify the Output data type as FLOAT.

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Use theIdentify button on the grid created to verify that the numbers correspond to the values in the table above.

Open Tools Extensions and verify that theSpatial Analystfunction is available and checked.

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Select Spatial Analyst Surface Analysis Slope on the Spatial Analyst toolbar.

At the dialog that appears select your output raster (elev )as the input surface and select the output in Degree or Percent. You will get

something like the following:

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You can make the slop raster permanent by right click the layerslope of elev, then selectMake Permanet:

Calculating Aspect:

Select Spatial Analyst Surface Analysis Aspect on the Spatial Analyst toolbar. At the dialog that appears select elev as the input surface.

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Calculate Flow Direction

-- Assign a flow direction to each grid cell based on the neighboring cell with the lowest elevationOpen the Toolbox (not the spatial Analysis tool bar). Open ArcToolbox, select Spatial Analyst Tools Hydrology FlowDirection, double click on Flow Direction

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Select elev as the input raster and specify names for output rasters (e.g. FlowDir).

Here is the Flow Direction grid:

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Compare the result with your hand drawing on the flow direction grid.

Calculate Flow Accumulation

Flow accumulation is calculated from flow directions.

It calculates the number of grid cells drain into an individual grid cell.

With ArcToolbox open, select Spatial Analyst Tools Hydrology Flow Accumulation, double click on Flow Accumulation

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Use FlowDiras the input flow direction raster, save flow accumulation raster as FlowAcc.

You will get flow accumulation grid. Notice, Cells with higher values are the ones that have low values on the elevation grid.

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Compare the result with your manually calculated flow accumulation grid.

Drainage line processingNow we will use ArcHydro to generate drainage line

1. Stream Definition

This step generates stream grid from stream accumulation grid.

From ArcHydro tool bar, select Tarrain Preprocessing Stream Definition. Use FlowAcc as flow accumulation grid, Stras stream

grid to be generated

About threshold:

Flow accumulation threshold value defines the minimum or number of cells drainage area upstream necessary to define a

stream.Cell size = 100 x 100 m

Each cell represents 10,000 m2 in area.If delineate the streams for a 10,000 cell flow accumulation threshold, the minimum drainage area upstream will be:

10,000 x 10,000 = 100,000,000 m2 or 100 km2

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Use the default value (0.15) for this example.

Stream Segmentation

Stream segmentation generate a grid of stream segments from flow direction grid and stream grid created from stream definitionprocessing. Input the dialog as following. FlowDiras the flow direction grid and Stras the Stream Grid. Output link grid will be

Lnk.

You will get something similar to the following:

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Drainage line processing

This process converts the stream link grid into a drainage line feature class that we are familiar with (we normally represent rivers,

streams as line features).

Select Terrain Preprocessing Drainage Line Processing. Input the dialog as following:

Does the drainage line feature look the same as your hand calculated one?

Catchment Grid & Catchment Polygon

Catchment area is the region of land whose water drains into a specified drainage line.

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Select Terrain Preprocessing Catchment Grid Delineation

Use FlowDir and Lnk grids as input.

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The catchment grid will be automatically added to the table of content.

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From catchment grid (Cat), we can derive catchment polygon. From Terrain Preprocessing menu, select Catchment Polygon

Processing:

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Drainage Points

Next, we need to generate drainage point feature class from Catchment and flow accumulation.

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Hydro Network Generation

We have already created drainage features drainage lines, drainage points and catchment polygons. Select Network Tools Hydro Network Generation, keep the default input as following:

This process generates two feature classes hydro junction feature class and hydro edge feature class. It also establish connectivity

between the drainage and network feature classes.

Set Flow Direction

This function set flow direction for the hydro network based on FlowDir grid.

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To view the flow direction, go to view Tool bars Utility Network Analyst. Make sure Utility Network Analyst is checked.

On the Network toolbar, select Flow Display Arrows. You should see:

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Use the Add Junction Flag tool on the Utility Network Analyst to place a flag on the basins outlet junction. This will place a

green square (representing a flag) on the junction.

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watershed modelling using gis

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