Delineating the Watershed of your Stream Gage

Delineating the Watershed of your Stream Gage

1.      Create a folder mywatershedproject under D/drive and other folder under the created folder with the name processeddata

2.      Determine the out let point of your watershed (you can use google earth, GPS, or topomap) for this exercise use Google earth select a point  behind oda ya’a campus  as an outlet of Legedara river ( just After join with River Waleme) lat 6⁰ 26^’ 36^’’  and long 38⁰ 16^’ 48^’’ save the point as  Station_01064500

3.      Initialize the necessary extension

·         Spatial Analyst Extension (Tools – Extensions – checkmark Spatial Analyst)

4.      Add dem data  ASTGTM_dem1.img

5.      Make sure your DEM  has a projected coordinate system :

·         Before proceeding with watershed delineation or any other kind of hydrological analysis, your data sets need to be in a projected coordinate system in which the horizontal units of the xy coordinates are the same as the vertical elevation units of your DEM

6.      Mask the DEM with the extent of your interest 

·         You can use  create feature / from previous exercises/ for this exercise use drawing tool

You can convert graphics you draw on your map into shapefiles or geodatabase feature classes. The Convert Graphics To Features command, which is available from the Drawing menu on the Draw toolbar or by right-clicking a data frame in the table of contents, supports all the graphic types you can draw with the tools in the graphics palette on the Draw toolbar, including circles, curved lines, and freehand lines. You can also convert graphic text into annotation feature classes.

Steps:

a)      Make sure you are in data view.

b)      Draw a graphic (rectangular) which circumscribe your watershed

c)       In the table of contents, right-click the data frame containing the graphics you want to convert to features and click Convert Graphics To Features.

a.       You can also click the Drawing menu on the Draw toolbar and click Convert Graphics To Features.

d)     Click the Convert drop-down arrow and click the type of graphic to convert.

e)      By default, only the selected graphics will be converted. Uncheck Selected graphics only if you want to convert all graphics.

f)       Click the option for the output coordinate system you want to use.

g)      If you want the graphics to be deleted once you've converted them to features, check Automatically delete graphics after conversion.

h)      Click the Browse button and navigate to a location to save the exported data mywatershedproject /processeddata.

i)        Type the name for the output data source as (Graphic).

j)        Click the Save as type drop-down arrow and choose the output type. The output can be either a shapefile or a geodatabase feature class (annotation can only be stored in a geodatabase).

k)      Click Save.

l)        Click OK.

7.      Extract by mask

1.       From Arctoolbox Click the plus sign in front of the Spatial Analyst Tools

2.        Click the plus sign in front of Extraction

3.        Double click on extract by mask function

4.        Select raster input map (ASTGTM_dem1.img)

5.        Select raster or feature mask data (Graphic)

6.        Change the name of the folder and file name of your output data

7.        Click ok

8.      Now use the hydrology tools

   

Conceptual overview of watershed delineation:

Flow Direction: One of the keys to deriving hydrologic characteristics of a surface is the ability to determine the direction of flow from every cell in the raster.  There are eight valid output directions relating to the eight adjacent cells into which flow could travel. This approach is commonly referred to as an eight-direction (D8) flow model 

                                                                                                      

Calculating the direction of flow

The direction of flow is determined by the direction of steepest descent, or maximum drop, from each cell. This is calculated as follows:

 maximum_drop = change_in_z-value / distance * 100

When a direction of steepest descent is found, the output cell is coded with the value representing that direction.

If all neighbors are higher than the processing cell, it will be considered noise, be filled to the lowest value of its neighbors, and have a flow direction toward this cell.

However, if a one-cell sink is next to the physical edge of the raster or has at least one NoData cell as a neighbor, it is not filled due to insufficient neighbor information. To be considered a true one-cell sink, all neighbor information must be present.

If two cells flow to each other, they are sinks and have an undefined flow direction. 

Cells that are sinks can be identified using the Sink tool. To obtain an accurate representation of flow direction across a surface, the sinks should be filled before using a flow direction raster.

The output of the Flow Direction tool is an integer raster whose values range from 1 to 255. The values for each direction from the center are:  

• If a cell is lower than its eight neighbors, that cell is given the value of its lowest neighbor, and flow is defined toward this cell. If multiple neighbors have the lowest value, the cell is still given this value, but flow is defined with one of the two methods explained below. This is used to filter out one-cell sinks, which are considered noise.
• If a cell has the same change in z-value in multiple directions and that cell is part of a sink, the flow direction is referred to as undefined. In such cases, the value for that cell in the output flow direction raster will be the sum of those directions. For example, if the change in z-value is the same both to the right (flow direction = 1) and down (flow direction = 4), the flow direction for that cell is 1 + 4 = 5. Cells with undefined flow direction can be flagged as sinks using the Sink tool.
• If a cell has the same change in z-value in multiple directions and is not part of a sink, the flow direction is assigned with a lookup table defining the most likely direction. See Greenlee (1987).
• The output drop raster is calculated as the difference in z-value divided by the path length between the cell centers, expressed in percentages. For adjacent cells, this is analogous to the percent slope between cells. Across a flat area, the distance becomes the distance to the nearest cell of lower elevation. The result is a map of percent rise in the path of steepest descent from each cell.

When calculating the drop raster in flat areas, the distance to diagonally adjacent cells (1.414 * cell size) is approximated by 1.5 * cell size to increase the processing speed by using integer calculations.

• When using the NORMAL option, a cell at the edge of the surface raster will flow toward the inner cell with the steepest drop in z-value. If the drop is less than or equal to zero, the cell will flow out of the surface raster.

This tool takes a surface as input and outputs a raster showing the direction of flow out of each cell.
If the Output drop raster option is chosen, an output raster is created showing a ratio of the maximum change in elevation from each cell along the direction of flow to the path length between centers of cells and is expressed in percentages.
If the Force all edge cells to flow outward option is chosen, all cells at the edge of the surface raster will flow outward from the surface raster.

9.      Sink (Optional )

 Sink: This step uses your Flow Direction grid to identify sinks in your DEM. These are areas
surrounded by higher areas, so that there is no external drainage. Sometimes sinks are real, but
often DEMs have erroneous sinks. Regardless of whether they are real or not, for the watershed delineation process to work, we need a “depressionless DEM,” i.e., a DEM with no sinks. The
Output data set is Sink_FlowDirxx.

10.  Fill  ( as initial work or if there is sink)

 Fill: This step will fill the sinks of your DEM (DemClip). Note that doing this correctly requires
care and an iterative process which we don’t detail in this tutorial. Instead, we will do a basic Fill
command. The result of this step will be a depressionless DEM (Fill_Demclip1) which will in turn be
the basis of the rest of the process

11.   Flow Direction – for this initial process, set the Input Surface Raster to Fill_DemClip1. For
Output flow direction raster, navigate to your ProcessedData folder, and call the new
raster FlowDir_DemC1, then press OK, as show below:

12.   Flow Accumulation (may take a long time)

13.  Snap Pour Point

A)    Add a pour point  

       I.            can you do that from the previous exercises)

    II.            Before proceeding, you need to measure how far your stream gage is from an area of high accumulation on your Flow Accumulation grid. It is likely that your stream gage is on top of or near an area shaded white, indicating high flow accumulation.

B)    Activate the Snap Pour Point tool in ArcToolbox with specifications similar to
what you see below, using the distance you measured plus a little extra as your
Snap Distance. Make sure to call the output raster, SnapPour:

C)     Make sure the result is a single cell directly on top of a high accumulation area (colored white) and near your stream gage, as you see here:

14.   Watershed:

Your results should look something like this:

ü  If you think your result is correct, you can convert this raster data set to a GIS vector polygon (shapefile) so that you can have it permanently for mapping. To do this:
1. in ArcToolbox choose Conversion Tools – From Raster – Raster to Polygon

ü 

ü  Fill in the dialog box similar to what you see below (use your Watersh_Flow data set):

ü  For mapping purposes, you may make the new polygon hollow (no fill) with a thicker outline using the Symbology properties

15.  Flow lines for your stream network

This process requires your Flow Accumulation raster. The basic conceptual process is to reclassify all cells that meet a certain accumulated flow threshhold to be 1, and all other cells to be no data. To do this you can apply a conditional statement to your Flow Accumulation raster in which all cells with a value greater than a threshold number (e.g., 100) are reclassified to be 1, and all other cells
receive a “no data” value.

A.    Go to Spatial Analyst Tools – Map Algebra – Raster Calculator.

B.      Use the Raster Calculator to create a conditional statement
Con("FlowAcc_Flow1" > 1500, 1)
- Name the new data set that will be produced streamnet. Note: Pay attention to spaces when writing your expression.

C.     Zoom into it to see what it looks like – it is a detailed drainage network for the area covered by your Flow Accumulation grid.

o   Note that you can use a different threshold value. The value we used, 1500, is arbitrary. If you re-do this step with a different threshold value, you must change the output raster name – e.g., streamnet2, streamnet3, etc.

D.    To create a vector drainage line data set from this raster file, you can use the Stream to Feature tool under ArcToolbox – Spatial Analyst – Hydrology Tools. Fill in the dialog box similar to what you see below (note we specified 100 in our data name to remind ourselves we used 100 as the accumulation threshold):

Now

A.    East to west and north to south profile of the watershed?

B.     Calculate the area of the watershed

C.     The characteristics of the drainage net may be physically described by:
(i) the number of streams (ii) the length of streams
(iii) stream density (iv) drainage density