Using ArcGIS for Hydrology and Watershed Analysis:

Transcription

1 Using ArcGIS for Hydrology and Watershed Analysis: A guide for running hydrologic analysis using elevation and a suite of ArcGIS tools Anna Nakae Feb. 10, 2015

2 Introduction Hydrology and watershed analysis are important for many applications, including land management, salmon habitat assessment, and contaminant fate and transport. By using elevation, in the form of a USGS Digital Elevation Model (DEM) and the USGS s hydrological unit 10 dataset, a variety of watershed analysis can be performed, including watershed and catchment delineation, stream location and order, and flow direction. Though some of this analysis may seem unnecessary as the USGS has some of these datasets already compiled, like a stream network, this analysis has applications in very small catchments where the USGS may be lacking data. Data For this manual, the model was run for the South Fork of the Nooksack River, in Whatcom County of Western Washington (Figure 1). The datasets used were the 10 meter DEM mosaic of Western Washington, created by the USGS and compiled by Western Washington University in 2007 and the USGS HU10 watersheds from All of the maps created for this manual were made by Anna Nakae, made in February of 2015, and are used with permission. Figure 1. The study area used to run the hydrologic model was the South Fork of the Nooksack River. The basin is located in the northwest corner of Washington State, in Whatcom County. The South Fork is an applicable study area because it contains smaller watersheds and catchments within the main basin. Model The majority of tools used for this analysis are found in the Hydrology toolbox in the Spatial Analyst toolset. The model is set up to run as a tool and could run with little to no prior knowledge of the Hydrology toolbox. The steps and figures are included to help the reader understand the underlying 2

3 reasons for various tools and their associated outputs. However, explicit instructions for all tool inputs, parameters and outputs, are not included and if are necessary, the reader is asked to refer back to the model (Figure 2). Figure 2. The model for the analysis was created using ArcGIS The model can be used like a tool, with all the inputs set as parameters. Additionally, can be used for slightly different applications besides ones described in this manual as several tools within have parameters that can be modified. This model includes the tools for pre- processing and watershed and catchment delineation. The process to create the geometric network were completed in ArcMap and are described later in the manual. Pre- processing Like all GIS projects, some level of pre- processing is necessary. The DEM and stream network where clipped to the South Fork Nooksack basin and all were re- projected to NAD 1983 UTM Zone 10. The clipped DEM was then smoothed using the Fill tool in order to remove any sinks due to incorrect elevation data (Figure 2). This is an important step as any sinks or towers created by incorrect values could result in incorrect analysis later. Also included in the pre- processing is the creation of a Hillshade (Figure 2). Though this is not necessary for the analysis, it is a very useful dataset to have for map creation to share and communicate the results of the analysis. Part One Watershed and Catchment Delineation a. Flow Direction- The first step in watershed delineation is determining where the streams are. This is found through several step; the first of which is finding the direction of flow through the DEM. This is done using the Flow Direction tool (Figure 3). 3

4 Figure 3. The output for the flow direction tool is a very bright map with pixels assigned one of eight numbers. Each number corresponds to a direction and indicates which direction a drop of water would flow if were placed in the cell. Even in this preliminary step, ridgelines are beginning to become visible. b. Flow accumulation- After the direction of flow is known, it can be used to find the accumulation of water in each pixel with the Flow Accumulation tool (Figure 4). Water that aggregates in certain areas will create a stream, and it is from the flow accumulation dataset that the stream network is created. 4

5 Figure 4. The output from the flow accumulation tool is a raster were pixels are assigned vales of how many other cells flow into it. Darker colored cells indicate areas were more water will accumulate, which is indicative of where a stream or river would appear. The main channel is very apparent, while the lighter, smaller tributaries are less visible. From this raster, a threshold of how much accumulation is enough to count it as a stream will be applied and a stream network created. c. Stream Network- The flow accumulation dataset is used to create the stream network by determining a threshold value for what is considered a stream. In this case, a threshold of 500 pixels was chosen, that is, one pixel had to have at least 500 other pixels flow into it to be considered a stream. The threshold was applied using Raster Calculator. d. Stream Segments- The stream dataset is then divided into small segments based on flow direction. This is done using the Stream Link tool (Figure 5). The output is an important intermediate output necessary for other tools later. 5

6 Figure 5. The Stream Link tool output is a raster that contains each separate segment of stream or river. The stream dataset, created in Step C, has been divided into individual segments based on the direction of flow. Though this tool may seem unnecessary, it is needed in order to complete later analyses, like flow direction and stream order. e. Stream Order- The stream segments are used to calculate the hierarchy of the stream network (Figure 6). There are several methods that one can use to determine the order. Which method that one uses will be based on which definition of what each order represents the user prefers. 6

7 Figure 6. The output from the stream order tool is a raster where each stream segment is given a value 1-6. Order 1 streams are the initial headwaters. Where two order 1 streams meet, it becomes an order 2. Where two order 2 streams meet, the segment is assigned a 3. This continues up to 6, which is the main Nooksack channel. The ordering was estimated using the Strahler technique, which can be changed in the model if necessary. f. Flow Length- With the outputs created at this point in the model, the distance water would have to travel any cell to the most downstream or upstream cell can be calculated (Figure 7). Though the output raster gives the distance in number of cells, the distance in meters or miles can be found using Raster Calculator. 7

8 Figure 7. The output of the Flow Length tool is a raster where pixels are assigned a value based on how many pixels away the cell is from a point. In this case, it was the most downstream point, though this is a parameter that can be altered in the model. The flow length can be converted to meaningful units, not just pixels, by using raster calculator to multiply each cell by the cell size. In this case it would be 10 as each cell is 10 meters. One may notice in this raster there are small portions that are very lightly colored along the edges of the watershed. These sections actually do not flow into the main basin and will be clipped out for later steps. g. Delineating Watersheds- Creating watersheds requires several steps (Figure 8). i. Delineate the main basin using the flow direction raster. Though this is not necessary for the watershed delineation tool, it is a useful layer to have for watershed mapping. ii. Convert the raster stream network to a vector stream network. This step is again not necessary for the watershed delineation but allows for better cartography and is important later on iii. Create an Outlet dataset by creating a new feature class and then editing it to have features at intersections of order 5 and 6 streams and where the network exits the basin. Make sure the points snap to the stream. iv. Run the Watershed tool with the Outlet dataset as the Pour Points. 8

9 Figure 8. The watersheds were delineated using the Watershed tool, with the pour points being the outlet dataset that was created by the user. Pour points were created at intersections of order 5 and 6 streams and where the stream network exited the basin. This network was used as in addition to the flow dataset created earlier to create the watershed raster that delineates 6 different watersheds within the main basin. One will notice that the stream network looks much crisper in the map, which is due to the conversion from a raster to a polyline dataset. This not only makes for better looking maps, but will also be important for the flow direction analysis. h. Delineating Catchments- The final step in Part One is the delineation of the small catchments within the main basin. How a catchment is defined can vary. The user could create a new pour point dataset based on stream order criteria, which was done for the watersheds, or can be based on stream segments, which is done in this example (Figure 9). 9

10 Figure 9. Using Watershed tool again, very small catchments can be delineated. Instead of using a point dataset for the pour points, the stream link dataset was used. The result is a raster which delineates a catchment for each stream segment. This type of data could be useful for projects like tributary restoration. Part Two- Geometric Networks This part of the analysis is not included in the model, and is done within ArcMap and ArcCatalog. Geometric networks are a collection of features in a feature dataset that are spatially related. They create a power data structure that can be used for a variety of applications, including flow direction. All feature in a dataset must be vectors. Luckily, the stream network has already been converted to a polyline. The catchment, watershed and basin boundaries must also be converted to polygons before they can be added. The outlet for the basin also must be added to the dataset. In order to calculate the flow direction for the network, a sink or a point where all flow goes towards, must be identified. For this example, that is the outlet. In order to set the outlet as the sink, the role of source/sink must be assigned when it is added to the feature dataset. To turn this on, start an editor session with the network with in ArcMap and in the Outlet table, change the AncillaryRole attribute to Sink. To see the flow direction, add the Utility Network Analyst toolbar while in ArcMap and turn on Display Arrows and Set Flow Direction, both while still in an editor session. Arrows designating direction 10

11 should appear. Save edits and end editing session. The result is the direction of stream flow for the whole basin (Figure 10). Figure 10. The flow direction was calculated from the geometric network. The basin outlet, a subset of the outlet dataset created earlier in the model, was set as the sink. The sink is the point where all the streams ultimately flow towards. From this final step, one can see very clearly the main ridgelines that divide the basin. Conclusion The combination of the Hydrology toolbox and geometric networks creates a suite of powerful analysis that can be completed with a limited number of beginning datasets and relatively little pre- processing. The datasets and networks created with this model could be expanded to include additional analysis and can be altered to fit the needs of a slightly different type of watershed analysis. 11