Automatic Watershed Delineation using ArcSWAT/Arc GIS

Transcription

1 Automatic Watershed Delineation using ArcSWAT/Arc GIS By: - Endager G. and Yalelet.F

2 1. Watershed Delineation This tool allows the user to delineate sub watersheds based on an automatic procedure using Digital Elevation Model (DEM). User specified parameter provides limits that influence the size and number of sub watershed created. In addition, users have the option of importing pre-defined watershed boundary and an associated stream network. Purpose The watershed delineation carries out advanced GIS function to aid the user in segmenting watersheds in to several hydrologically connected sub-watersheds for use in watershed modeling with SWAT. Application The watershed delineation tool uses and expands ArcGIS and Spatial Analyst extension function to perform watershed delineation.the delineation process requires a Digital Elevation Model (DEM) in ESRI grid format. The user also has the option of importing and using a pre- defined digital stream network in ArcView shape file or geodatabase feature class (polyline) format. Once the delineation is finished, a detailed report (topographic Report) is added to the current project and several layers will be added to the current map, including: Basin, Watershed, Reach, Outlet and monitoring point. The topographic report the elevation distribution within the watershed ( or hydrologically not connected watersheds)and within each sub-watershed unit (sub basin)the layer added to the map contain the parameter of the watershed(s) characterization. Key Procedures Load the DEM (optional) Define the working area (Mask ) (optional) Load the stream network to be used for the delineation Preprocess the DEM Specify the minimum sub- watershed area (critical source area) Review and edit the stream network points Run the calculation of the sub basin parameter (optional) locate the reservoir Page 1

3 Create SWAT run with example dataset 1. Open ArcMap and choose A new empty map 2. Start ArcMap Customize -> Extensions -> make sure that Arc Swat project manager, swat watershed delineator and Spatial Analyst extension are checked. 3. From the Customize -> Toolbars menu, make sure that the ArcSWAT toolbar is checked. Explanation: Create an ArcMap project file using the ArcSWAT extension of ArcGIS 10.1 That will link GIS databases with the ArcSWAT hydrologic model. Create new SWAT project / Setup working directory / Setup Geodatabases 4. SWAT Project Setup New SWAT Project save current document? No Project Setup dialog box appears Project Directory E:\SWAT_Example SWAT Project Geodatabase automatically set to SWAT_Example.mdb and the raster geodatabase will be set as RasterStor.mdb. The SWAT Parameter Geodatabase will be set to the SWAT2009.mdb database located in your ArcSWAT installation folder (Figure-1) click OK. Explanation: The Project Directory is where all of the project files will be stored Page 2

4 Figure A new ArcSWAT project is created. Figure-2 Processing the Elevation Dataset 1. Choose Watershed Delineator Automatic Watershed Delineation Watershed Delineation box appears DEM Setup Load from Disk go to E:\Example1 Select DEM Add Explanation: The Digital Elevation Model (DEM) grid is added as a GIS layer to the ArcMap project file. Page 3

5 Figure DEM projection setup Click on the DEM Properties box appears Check the units for the Z Unit make sure that the Z Unit in the DEM Properties box is meters Ok Page 4

6 Note:- You can exit the Watershed Delineation and reopen it at anytime 3. Mask----- Manually delineate Optional Focusing on the watershed of interest using a pre-prepared mask Speeds up GIS processing Peg mask Click the folder icon next to the mask box select Load from Page 5

7 Disk Ok select amask Add Explanation: The mask covers the watershed that you plan to simulate with a little overlap around the watershed boundaries. The mask is pre-prepared and is added as a GIS layer to the ArcMap project file. Later examples will show how to manually create a mask. The mask is used to exclude areas outside of your selected watershed. Therefore only the data inside of the mask area are utilized in the SWAT simulation thereby speeding up computer processing time. Defining the stream network Use the threshold area method 3. Go to the Stream Definition box peg DEM-based go to the Flow direction and accumulation box and click on the icon End of DEM grid preprocessing Ok in the Area box change the value to 1000 Ha (In the example 1, set the threshold to 500. The smaller the number, the more detailed the stream network generated.) go to the Stream network box click on the icon End of stream preprocessing Ok Explanation: The watershed boundaries and the stream network in the watershed is calculated from the DEM using the flow direction and flow accumulation method. The locations of the subbasin outlets are also defined. Minimum, maximum and suggested subbasin areas (in hectares) can be defined. Defining outlet and inlet definition 4. Go to the outlet and inlet definition peg Subbasin outlet click the folder icon and select E:\Example1\outlet.dbf Add Ok Ok we will also add subbasin outlets manually. Click the ADD button of Edit manually Click the outlet points moving the mouse (see the figure below). It shows red dots. after adding outlet points, click the right Page 6

8 mouse button and stop editing yes 5. Go to the Whole watershed outlet(s) box and click on the icon use the mouse to draw a box around the watershed outlet (note: you may have to zoom-in if outlets are close-together to make sure the box is drawn around the correct outlet) 1 outlets selected Ok Explanation: Selecting the watershed outlet will define the boundary of the main watershed. Delineate the watershed 6. Go to the Delineate watershed box and click on the icon Watershed delineation is done Ok Explanation: After this step the delineated watershed with all subbasins will be added as a GIS layer to the ArcMap project file. Calculate the subbasin parameters 7. Go to the calculate subbasin parameters box and click on the icon (wait patiently!) Subbasin parameter calculation successfully done ok In this exercise, we will add a reservoir. Click add reservoir button. make sure the added reservoir point shown in both the map and the MonitoringPoint layer. click the right mouse button stop editing Page 7

9 yes back to the watershed delineation window Exit Explanation: This function calculates basic watershed and subbasin characteristics from the DEM. A Topographic Report can be opened by going to the Watershed Delineator tab Watershed Reports Topographic Report Ok. Each subbasin is given a unique ID number. Note: - watershed parameters like area, length of the main water course and slop can be found easily using the automatically delineated watershed. Page 8