Advanced GIS workshop for UCCE Geospatial Imaging & Informatics Facility HYDROLOGICAL ANALYSIS WITH ARCGIS In this exercise, you will perform several procedures common to spatial hydrological analysis, including stream and watershed delineation. You will use topographic (elevation) data to calculate flow direction and accumulation, as well as produce streams and watershed boundaries. All data can be found on your computer’s hard drive in C:\Workspace\CE\August22\Hydrology. You will be using this folder throughout this exercise, as it has proper read/write permissions. DATA: • dem2m : This Digital Elevation Model (DEM) product was produced by USGS from aerial flyovers performed by the National Geospatial Intelligence Agency (NGA) in 2004. While the raw data had a pixel size of 2 meters and an extent of much of the Bay Area, the dataset you’ll use in this lab was resampled to 8 meters and clipped to a small area in Alameda County to speed up data processing for the purpose of this lab. The original data had a horizontal accuracy of 0.3 m and a vertical accuracy of 0.7 m. More information is available at: http://casil-mirror1.ceres.ca.gov/casil/remote_sensing/urban_imagery/0.3meter_urban_area/ . • streamgages.shp : Point shapefile containing streamgage locations, downloaded from USGS National Water Information System (NWIS) at http://waterdata.usgs.gov/nwis . • naipclip.img: Clip of the National Agricultural Imagery Program (NAIP) imagery of Alameda County. Freely available for download at http://new.casil.ucdavis.edu/casil/remote_sensing/naip_2005/ . All data is in Universal Transverse Mercator (UTM) Zone 10 projection and NAD 83 datum. *** START THE EXERCISE ON THE NEXT PAGE! *** These are important points that you should read before you go any further! These are informative tips that may help you in the future!
Transcript
Advanced GIS workshop for UCCE Geospatial Imaging & Informatics Facility
In this exercise, you will perform several procedures common to spatial hydrological analysis, including stream and watershed delineation. You will use topographic (elevation) data to calculate flow direction and accumulation, as well as produce streams and watershed boundaries. All data can be found on your computer’s hard drive in C:\Workspace\CE\August22\Hydrology. You will be using this folder throughout this exercise, as it has proper read/write permissions. DATA: • dem2m: This Digital Elevation Model (DEM) product was produced by USGS from aerial flyovers performed by the
National Geospatial Intelligence Agency (NGA) in 2004. While the raw data had a pixel size of 2 meters and an extent of much of the Bay Area, the dataset you’ll use in this lab was resampled to 8 meters and clipped to a small area in Alameda County to speed up data processing for the purpose of this lab. The original data had a horizontal accuracy of 0.3 m and a vertical accuracy of 0.7 m. More information is available at: http://casil-mirror1.ceres.ca.gov/casil/remote_sensing/urban_imagery/0.3meter_urban_area/.
• streamgages.shp: Point shapefile containing streamgage locations, downloaded from USGS National Water Information System (NWIS) at http://waterdata.usgs.gov/nwis.
• naipclip.img: Clip of the National Agricultural Imagery Program (NAIP) imagery of Alameda County. Freely available for download at http://new.casil.ucdavis.edu/casil/remote_sensing/naip_2005/.
All data is in Universal Transverse Mercator (UTM) Zone 10 projection and NAD 83 datum.
*** START THE EXERCISE ON THE NEXT PAGE! ***
These are important points that you should read before you go any further!
These are informative tips that may help you in the future!
Advanced GIS workshop for UCCE Geospatial Imaging & Informatics Facility
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1. Open a new ArcMap document, and make sure that ArcToolbox is visible in ArcMap. If it is not, click the ArcToolbox button to make it
visible: All Hydrology tools are located under ArcToolbox|Spatial Analyst Tools|Hydrology. 2. Add the following data to ArcMap:
• streamgages.shp • dem2m
3. Calculate flow direction using the Flow Direction tool.
a. Set the Input surface raster to dem2m. b. Set the Output flow direction raster to flowdir stored in C:\Workspace\CE\Augsut14\Hydrology\.
c. Click OK, and view your results.
Advanced GIS workshop for UCCE Geospatial Imaging & Informatics Facility
Profile view of a sink
Profile view of a filled sink
A perfect flow direction should only contain eight values: 1, 2, 4, 8, 16, 32, 64, and 128, representing the eight adjacent cells into which water would flow. For example, for a cell with a value of 16, the water is flowing to the left (i.e. west). a. Open the attribute table for the flow
direction raster (right-click on flowdir and go to “Open Attribute Table”).
b. Right-click on the COUNT field, and choose to “Sort Descending” (see above screenshot).
You will see that while the eight values are the most plentiful in the grid, the grid also contains more than just the eight values there are other values that represent “sinks,” or areas where water flows into and not out (areas where all surrounding pixels are higher elevation). Sinks are depressions in the DEM, and they can be real depressions in the landscape (reservoirs, areas protected by levees, depressions in the ground like groundwater recharge areas, vernal pools, etc.), or they can be errors in the data. Therefore, it is important that we identify our sinks, and fill the ones that are data errors.
4. Identify sinks in your DEM using the Sinks tool.
a. Set the Input flow direction raster to flowdir. b. Set the Output raster to Sinks stored in C:\Workspace\CE\August22\Hydrology\.
c. View your results. All areas shown are sinks. Some are real-world sinks (e.g. reservoirs) and some are errors.
32 64 128
16 1
8 4 2
Advanced GIS workshop for UCCE Geospatial Imaging & Informatics Facility
Each sink has a unique ID. Turn off all layers in your layer list, except for the sinks. This way, you can view what your sinks really are. Looking close at the sinks in our DEM, it is clear from the sinks layer that the sinks in our DEM are most likely data errors or minor landscape depressions due to the fact that our study site is densely urbanized in the southwestern half of the image. Therefore, you can decide that the sinks should be “filled” or smoothed over. When a sink is filled, it is filled to its pour point, which is the minimum elevation along its watershed boundary. For the purposes of this exercise, we will fill all sinks in order to create smooth stream networks. 5. Fill all sinks in your DEM using the Fill tool.
a. Set the Input surface raster to dem2m. b. Set the Output surface raster to be dem2mfill. c. Leave the Z limit blank. (This is where you
would set a fill limit if desired.) d. Click OK.
Your newly filled DEM is added to your map. For the remainder of this lab, the example shown here will be using the data with ALL sinks filled. 6. Now that you have created your filled (or partially filled) DEM, you need to calculate a new flow direction with your filled DEM.
a. Go to Flow Direction. b. Set the Input surface raster to be dem2mfill. c. Set the Output flow direction raster to be fillflowdir. d. Click OK.
Notice how NOW there are only the 8 values in the fillflowdir dataset: 1, 2, 4, 8, 16, 32, 64, and 128.
Advanced GIS workshop for UCCE Geospatial Imaging & Informatics Facility
7. Calculate flow accumulation using the Flow Accumulation tool.
Your flow accumulation layer is added to the map, and represents the amount of water that would flow into each cell, assuming that all water became runoff and there was no interception, evapotranspiration, or loss to groundwater. This could also be viewed as the amount of rain that fell on the surface, upslope from each cell. Basically, the flow accumulation tool counts the number of cells that are flowing into it:
• Cells with a high flow accumulation are areas of concentrated flow and may be used to identify stream channels.
• Cells with a flow accumulation of zero are local topographic highs and may be used to identify ridges.
You might choose to use a weight raster when calculating the Flow Accumulation dialog if you wanted to take into account how much rain has fallen within a given watershed. In such a case, the weight raster may be a continuous raster representing average
rainfall during a given storm. Another reason to use a weight raster might be to account for different types of ground cover (smooth vs. rough). The resulting flow accumulation raster dataset should look like it does here. White areas are cells with very high flow accumulation (areas of concentrated flow), and can be used to identify stream channels.
Advanced GIS workshop for UCCE Geospatial Imaging & Informatics Facility
8. Create a stream network raster using your flow accumulation grid:
a. Turn on the Spatial Analyst toolbar by going to the View menu > Toolbars > Spatial Analyst. In addition, go to the Tools menu > Extensions, and make sure that Spatial Analyst is checked on.
b. In the Spatial Analyst toolbar drop-down menu, choose the Raster Calculator.
c. In order to create a stream network, you will need to specify a threshold for how many adjacent stream pixels make up a stream. Here we will specify a threshold value of 2000 pixels of accumulation (any pixel with more than 2000 pixels flowing into it will be part of the stream network).
d. Type in the blank area (as seen below):
streamnetwork = con([fillflowacc] > 2000, 1)
e. Click Evaluate.
The stream network raster is added to your map:
Advanced GIS workshop for UCCE Geospatial Imaging & Informatics Facility
9. Create a stream link raster using the Stream Link tool.
a. Set the Input stream raster to be streamnetwork.
b. Set the Input flow direction raster to be fillflowdir.
c. Set the Output raster to be streamlink. d. Click OK.
The stream link raster is added to your map. 10. Create a stream order raster using the Stream Order tool.
a. Set the Input stream raster to be streamnetwork.
b. Set the Input flow direction raster to be fillflowdir.
c. Set the Output raster to be streamorder. d. Set the Method of stream ordering to be either
STRAHLER or SHREVE. e. Click OK.
The stream order raster is added to your map. The stream ordering you choose is dependent on what kind of method you want to use. In the Strahler method, stream order increases when streams of the same order intersect. The intersection of two links of different orders, however, will not result in an increase in order. The Strahler method is the most common stream ordering method. The Shreve method accounts for all links in the network. The orders are additive, so the numbers from the Shreve method are sometimes referred to as magnitudes instead of orders. The magnitude of a link in the Shreve method is the number of upstream links.
Stream link raster result Stream order raster result
Advanced GIS workshop for UCCE Geospatial Imaging & Informatics Facility
11. Create a stream shapefile using the Stream to Feature tool.
a. Set the Input stream raster to be streamnetwork. b. Set the Input flow direction raster to be fillflowdir. c. Set the Output polyline features to be stream2000simp.shp (“simp” stands for simplify) d. Check Simplify polylines e. Click OK.
You can choose to simplify the polylines or not. Basically, simplifying the polylines smoothes the lines a bit. See the magnified comparison of simplified and non-simplified stream features below:
The stream shapefiles is added to your map: 12. Create a basins raster using the Basin tool.
a. Set the Input flow direction raster to be fillflowdir. b. Set the Output raster to be basin. c. Click OK.
The basins raster is added to your map, which is a raster delineating all drainage basins within the analysis window.
Simplified
Not Simplified
Advanced GIS workshop for UCCE Geospatial Imaging & Informatics Facility
Using the Watershed tool directly on the stream gages as pour point data produces tiny watersheds for the stream gages, since due to projection and registration issues, the stream gage point data do not fall exactly on areas of high flow accumulation (stream channels). Therefore, we must use the Snap Pour Points tool to first snap our pour points (streamgages.shp) to nearby areas of high flow accumulation. 13. Snap the pour points (streamgages.shp) to areas of high flow accumulation using the Snap Pour Points tool.
a. Set the Input raster or feature pour point data to be streamgages. b. Set the Pour point field to be SITENO. c. Set the Input accumulation raster to be fillflowacc. d. Set the Output raster to be snappourpoint. e. Set the Snap distance to be 175, since that is the approximate maximum distance the a stream gage from a major
stream (as delineated by our streamnetwork2000.shp dataset). f. IMPORTANT! Click on Environments, and under
General Settings, set the Extent to Union of Inputs. This is absolutely necessary for all the snapped pour points to fall exactly on the streams (otherwise, those points won’t search outside the input extent, which oftentimes doesn’t include areas where the streams actually are!)
g. Click OK. For example, in the magnified screenshot below, the stream gage is shown in yellow, while the new snapped pour point is show in red, placed in an area of high flow accumulation (on the stream channel).
C:\Workspace\CE\August14\Hydrol
Advanced GIS workshop for UCCE Geospatial Imaging & Informatics Facility
14. Create a watershed raster using the Watershed tool. Watersheds can be delineated two ways with the Hydrology Tools: • Raster: This is either a result from the Snap Pour Point
tool, or another raster that you have created, with NoData cell representing areas for pour points.
• Shapefile: This is a shapefile or other feature class that represent pour points; however, problems in watershed delineation (tiny watersheds) result if the points are not in areas of high flow accumulation (in or near stream channels).
a. Set the Input flow direction raster to be fillflowdir. b. Set the Input raster or feature pour point data to be snappourpoint. c. Set the Pour point field to be Value. d. Set the Outpur raster to be watershed. e. Click OK. The watershed raster is added to your map:
Advanced GIS workshop for UCCE Geospatial Imaging & Informatics Facility
15. Calculate flow length upstream and downstream using the Flow Length tool. Using the Flow Length tool, the length of the flow path, either upslope or downslope, from each cell within a given watershed can be determined. This is useful for calculating the travel time of water through a watershed. UPSTREAM calculates the longest upslope distance along the flow patch, from each cell to the top of the drainage divide, or ridge.
a. Set the Input flow direction raster to be fillflowdir. b. Set the Output raster to be upstream. c. Set the Direction of measurement to be UPSTREAM. d. Click OK.
The upstream flow direction is added to your map. DOWNSTREAM calculates the downslope distance along the flow path, from each cell to a sink or outlet on the edge of the raster.
e. Set the Input flow direction raster to be fillflowdir. f. Set the Output raster to be downstream. g. Set the Direction of measurement to be
DOWNSTREAM. h. Click OK.
The downstream flow direction is added to your map.
UPSTREAM flow direction DOWNSTREAM flow direction 16. Save your ArcMap project, and close ArcMap.
