Delineating the Stream Network and Watersheds of the Guadalupe Basin

Transcription

1 Delineating the Stream Network and Watersheds of the Guadalupe Basin Francisco Olivera Department of Civil Engineering Texas A&M University Srikanth Koka Department of Civil Engineering Texas A&M University Ashish Agrawal Department of Civil Engineering Texas A&M University March 2004 CONTENTS Brief Overview of Delineating Watersheds and Stream Networks Goals of the Exercise Computer and Data Requirements PROCEDURE 1. Add a DLL to ArcMap project 2. Get familiar with the DEM data 3. Set up the project 4. DEM Setup 5. Provide a Mask, Burn-in streams 6. Fill sinks, flow direction grid, flow accumulation 7. Construct the Basic Stream Network 8. Add a stream to the Stream Network 9. Rain Drop Tool 10. Add Outlets 11. Delineate streams and watersheds 12. Merge sub-basins Brief Overview of Delineating Watersheds and Stream Networks One of the most important applications of Digital Elevation Models (DEMs) is watershed delineation. ARC/INFO has built-in functions for watershed delineation from DEM's; however, the ArcGIS and its Spatial Analyst extension provide tools which are more convenient to use. These tools have been further developed by Dr. Francisco Olivera and Ashish Agrawal, into a watershed characterization system currently named PrePro2004,

2 where PrePro stands for PreProcessor of GIS data for hydrologic models. Goals of the Exercise In this exercise, you will learn how to use the PrePro2004 to process a digital elevation model and to delineate watersheds and stream networks from it. Computer and Data Requirements You need the following data for this exercise: This exercise was successfully done using ArcView 8.3, using Spatial Analyst Extension. The data needed for the exercise consists of the following datasets. BullCkNHD.shp - a shapefile of the NHD (National Hydrographic Dataset) stream network of the Bull Creek basin. bullckdem - an Arc/Info Grid of digital elevation for the Bull Creek Basin region. bullmask- - an Arc/Info Grid, which will be used as mask so that only that part of the DEM is processed that contains Bull Creek Basin. DLL A dynamic library link containing the tools for watershed delineation These files should be copied and unzipped from DATA.ZIP to a working directory on your computer. PROCEDURE 1. Add a DLL to ArcMap project Open ArcMap from the Windows start menu. Before proceeding further, save the document with what ever name you want to. Once you have done this, you can now add the required tools to the document. For this exercise, the tools are distributed as a DLL (Dynamic Library Link). The name of the DLL is WSDelineation.dll. To add the DLL, click Tools/Customize. In the Customize wizard that pops up, make sure that ToolBars tab is selected and click Add from file button. Now browse down to the working directory, select WSDelineation.dll and click Open and then OK. To add the toolbar corresponding to the watershed delineation, insert a check for Prepro2004 in the Toolbars listbox of Customize wizard. You can now observe that a toolbar is added to the document by name, Prepro Get familiar with the DEM data In the map document, click the Add Data button, and then pressing on Shift key, add bullckdem, bullmask and BullCkNHD.shp from your data directory to a data frame..click on don t build pyramids and then OK.

3 Let s see the properties of the DEM. Right click on the DEM and then click on Properties. In the Layer Properties wizard, choose the Source tab to view the grid properties. As it can be seen in the wizard, the DEM has Transverse_Mercator projection, cell size is 10 m, minimum elevation is m and maximum elevation is m. 3. Set up the project Setting up of project consists of defining the directory path for the utilities to store the results. To do this, click Prepro2004/Path Setup. In the form that pops up, select a drive using Select Drive combo box, then in this drive select a folder using Browse to the Folder listbox and finally inside the selected folder create a new folder by any name, by clicking on New Folder button. After entering the name for the folder, click OK. Finally, a message box pops up showing the path of the folder you have defined, click OK.

4 4. DEM Setup This step consists of specifying the DEM and verifying that the DEM has spatial reference defined to it. To do this, click Prepro2004/Watershed Delineator. A form named Watershed Delineation Tools pops up which contains a number of tabs, each containing different functions. Until the end of the exercise, this form will be used. To specify the DEM dataset, in the Watershed Delineation Tools form, click on the button containing folder icon, which is located under the tab named DEM. A form by name Select Option- Open DEM, pops up. In this form, select Select From View option and click OK. In the Select the layer form that pops up, select bullckdem and click OK. Up to this point you have specified the DEM required, now you will verify that the DEM is projected. To do this, in the Watershed Delineation Tools form, click

5 Projection button located under DEM tab. A wizard pops up, take a note of the information provided and click OK. Please make sure that Watershed Delineation Tools form is not closed until instructed. 5. Provide a Mask, Burn-in streams What you are now going to do is specify a mask concerning the study area. When a mask is used, only that part of the DEM will be processed, which falls within the mask. To do this, in the Watershed Delineation Tools form with DEM Setup active, under Mask/Burnin tab, insert a check mark for Mask. Now click on the folder icon for Mask. A form titled, Select Option-Mask pops up, in which select Select from View option and click OK. A new form titled Select a Layer pops up, in which select bullmask and click OK. What you are now going to do is raise the land surface cells that are off the streams by an arbitrary elevation amount so that the streams delineated from the DEM exactly match those in NHD. To do this, in the Watershed Delineation Tools form with DEM Setup active, under Mask/Burnin tab, insert a check mark for Burnin. Now click on the folder icon for Burnin. A form titled, Select Option-Burnin pops up, in which select Select from View option and click OK. A new form titled Select a Layer pops up, in which select BullCkNHD and click OK.

6 Normally, before you burn in the streams you have to ensure that the stream network is continuous and doesn't have gaps between each stream segment. If such gaps exist they have to be edited and closed before you burn in the streams. Likewise you have to edit out miscellaneous stream lines out in the landscape that are not connected to the stream network. In this case, NHD is a well constructed stream network and we can just go ahead and burn it in. 6. Fill sinks, flow direction grid, flow accumulation Up to this point of the exercise, your Watershed Delineator form should look something like that shown in the image below.

7 Most of the DEM data are accurate, however, aberrations do occur in the DEM which cause pits to form in the terrain. These pits need to be filled, otherwise they will cause the wrong flow direction. The Fill sinks function raises pit cell elevations so as to level the pits with the surrounding terrain. Only artificial sinks will be filled, since real sinks -- which we do not wish to remove from the DEM -- are treated differently. With the DEM grid filled, the flow direction grid can be calculated. Once flow direction grid is computed, the flow accumulation grid will be calculated. All these three functions (Fill sinks, flow direction and flow accumulation) will be applied at the same time. To do this, in the Watershed Delineation Tool form, click on FIL/FDIR/FACC button located under Mask/Burnin tab. It will take some time to execute these functions. Once it is done, a message pops up indicating the completion of the process, in which click OK to continue. Now minimize the watershed delineation tool form and try exploring the newly added layers to the Table of contents which are: TargetDEM, FACC, FDIR, FIL. To see only the Filled DEM, in the TOC, make all layers invisible except for FIL.

8 Similarly, to see only the Flow direction grid, in the TOC make all layers invisible except for FDIR..

9 Similarly, to see only the Flow accumulation grid, in the TOC make all layers invisible except for FACC. In the flow accumulation grid, keep in mind that the lighter the color of an individual grid cell, the more grid cells drain into that particular cell. Click on the Zoom In tool and zoom into a spot in the lower right corner where the two streams join in the grid network. Use the Identify tool to check individual cell values and understand how flow accumulation function counts the number of cells upstream of a particular cell. Follow a particular stream going downstream and see how the flow accumulation value increases as more drainage area is picked up. Focus on a junction and see how the flow accumulation downstream of the junction is the sum of the flow accumulations in the two upstream tributaries. See the example shown below.

10 7. Construct the Basic Stream Network Before you start to construct the stream network, you have to define the cell threshold or minimum stream drainage area. In the Watershed Delineation Tools form, click on Streams tab, enter a value of as the number of cells in the text box for Threshold and click Str button. A streams grid is formed, which has a value of 1 in each cell with a flow accumulation value larger than 15000, and NODATA on all other cells. After sometime a message box appears, saying that the function was successfully executed. Also four new layers would be added to the map (Monitoring Point, Reach, OUT and STR). STR is a raster containing streams, OUT is raster containing cells that are outlets of streams being flagged, Reach is line shapefile that correspond to the streams of STR and Montoring point is a shapefile containing outlet points corresponding to the outlet cells in OUT raster.

11 Now minimize the Watershed Delineation Tool form and make only STR active by making the other three invisible in the TOC. Also make TargetDEM and BullCkNHD layers visible. To see if the streams delineated are right or wrong we will compare them with BullCkNHD streams. To do this, drag the BullCkNHD layer below the STR layer. Now zoom into any stream covered by both the layers using Zoom In tool. You should notice that the stream grid STR overlays on the top of BullCkNHD with some of the streams of BullCkNHD not being covered. This is because the cell threshold is too large to identify some of the smaller streams represented by BullCkNHD.

12 8. Add streams to the Stream Network Although you do not want to delineate all of the extra streams described by the BullCkNHD, there could be one or few streams for which you do want to delineate the associated watersheds. For example a water right location or a stream gauging station might be located on that stream, and you might want to know the drainage area of that water right location or gauging station. If the stream grid (STR) does not define these streams, you can add the streams to the STR grid using the ADD NEW STREAMS button located on the Watershed Delineation Tool wizard. First of all zoom to the portion of the map where you want to add a stream. Next, click on the ADD NEW STREAMS button, then click on any point on the map, then right click on the map and click Stop Editing. In the form that pops up, click Yes, to save changes. To add the new stream, click on the PROCESS NEW STREAMS button located on the Watershed Delineation Tool wizard, which could be one of the streams in BullCkNHD that is not defined by the stream grid STR. You can now observe that the four layers: Monitoring Point, Reach, Out and STR are replaced by new ones with same names. The new STR layer will contain previous streams plus the extra stream that you wanted.

13 9. Rain Drop Tool This tool can be used to trace the path of a rain drop upon inducing one on the terrain. The associated tool creates a graphic to show the rain drop trace path. First of make all the layers in TOC invisible except the TargetDEM layer. Then make sure that the Watershed Delineation Tool is maximized. In this form under the Streams tab, click on Rain Drop (Graphic) button and then click on the DEM where ever you want to. After you have observed the graphic created, to clear the graphics, click Refresh Map button.

14 10. Add Outlets Usually, the outlet is defined as the last cell of each stream section; however, you can also manually define an outlet. For example, if you want to find out the drainage area of a specific water right location, you can define the water right location as an outlet. For this step, you will define one water right location on the stream you just added. First of all drag the BullCkNHD layer just below the Reach layer. Then Zoom into the stream you just added until you can see the individual cells. Make the Outlet/Inlet/Reservoir tab active in the Watershed Delineation Tool form. Then insert a check for Add Outlet/Inlet/Reservoir and let the Outlet option be selected. Click on the Add button in the Watershed Delineation Tools form, and then click on a cell and then, don t forget to right click on the map, then to click Stop Editing and finally on clicking Save on the prompt. You will see new points being added to the MonitoringPoint layer. In the MonitoringPoint attribute table, see the Type field. Scroll down in the table until you see a row that has UDOutlet for Type. 11. Delineate streams and watersheds Before delineating the watersheds, you will have to select the basin outlet point from MonitoringPoint shapefile. The outlet point is the one that is most downstream on the

15 stream network (see the figure below). There are two different stream networks here. So, we will discard the smaller one and take the bigger one into consideration. You will have to select the outlet of the bigger stream network. So, zoom in to the most downstream portion of the network using Zoom In tool (see the second image below). To select the outlet point, in the Watershed Delineation Tool wizard, insert a check for Include junctions for Delineation, click on Select button and finally draw a square around the outlet point to select it. Finally, click DELINEATE WATERSHED button to delineate watersheds and streams.

16 12. Merge sub-basins If you just want the drainage basins for the major streams, you can merge the subwatersheds. PrePro2004 allows you to merge two sub-basins at a time. Before you merge the sub-basins, check the number of rows of the Watershed layer. To do this, open its attribute table by right clicking on the layer and then clicking Open Attribute Table. Also open the Reach attribute table by right clicking on it and then clicking on Open Attribute Table and observe the number of streams. Right now we have one stream per sub-basin. It may so happen that some of the sub-basins may be too small, so what you do is merge them with their neighboring sub-basins. To merge two sub-basins, make sure that the Vectorization tab is selected in the Watershed Delineation Tools wizard. Insert a check in the box for MERGE SUBBASINS, minimize the Watershed Delineation Tools form, in the ArcMap document click on the Select Features tool and finally select the two sub-basins as shown in the figure below.

17 Once you have selected the features, maximize the Watershed Delineation Tools form, click on MERGE SUBBASINS. You will now see that the sub-basins you have selected are merged and the Watershed layer will now have one feature less than what was before. To see this, open its attribute table by right clicking on the layer and then clicking Open Attribute Table. Make a note of the number of features. Also open the Reach attribute table by right clicking on it and then clicking on Open Attribute Table and observe the number of streams. You can see that the number of sub-basins is one less than the number of the streams.

18