GIS for Watershed Modeling Exercise
Slope calculation using ArcGIS
Save the following to a text file ‘elev.txt’
ncols
5
nrows
4
xllcorner
0
yllcorner
0
cellsize
100
NODATA_value -9999
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Based on the elevation grid:
1. Manually assign eight direction pour point model grid
2. Draw flow direction grid
3. 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 into
ArcMap. 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 the Identify button on the grid created to verify that the numbers correspond to the values in the table above.
Open Tools  Extensions and verify that the Spatial Analyst function 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 layer slope of elev, then select Make 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 elevation
Open the Toolbox (not the spatial Analysis tool bar). Open ArcToolbox, select Spatial Analyst Tools  Hydrology  Flow
Direction, 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
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Flow accumulation is calculated from flow directions.
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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 FlowDir as the input flow direction raster, save flow accumulation raster as FlowAcc.
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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 processing
Now 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, Str as stream
grid to be generated
About threshold:
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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 definition
processing. Input the dialog as following. FlowDir as the flow direction grid and Str as 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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