GIS II: Lab 3, Watershed Analysis

Goals and Background 

The goal of this lab was to use spatial analyst tools to understand watershed analysis and create a map of watersheds in the Adirondack region of New York State. Watershed analysis is a very important concept to understand and be able to use as it is applied in water management, water quality, and conservation to name a few. 

Methods

To begin, data from Cornell University and New York State GIS Clearinghouse was downloaded from the internet. This include a hydrology shapefile an the Adirondak State park polygon. ArcToolbox was heavily utilized in this lab. A 20 km buffer was added to the state park polygon as seen in Figure 1. 

Figure 1. A map showing a 20 km buffer around the state park polygon. The blue lines are the hydrology shapefile from Cornell. These are 2D and have no topographic relation.

An issue with this lab was that not all data was in the same projection. To correct this issue the project tool under data management was used and the projection of the state park shapefile wsa imported. The projection of this shapefile was NAD 1983 Zone 18 N. 

Because a watershed is a function of the topography there needs to be a spatial reference in place. The 30-arc second DEM of North America was imported from ESRI/ArcMap Online. This DEM was also in a different projection and using the transform tool was corrected to be in the same projection. We are only interested in the area of the park and using the clip tool, and new raster dataset was created and the original DEM removed.

The next step is to understand flow direction. Using the flow direction tool under the hydrology tab, nodes are created. Unfortunately, the output contains sinks, or areas that impede water flow. If this map was to be used for watershed analysis the results would be skewed and incorrect so sinks must be filled. The fill tool is used and new nodes are created for flow direction. The result is very trippy and can be seen in Figure 2. 

 

Figure 2. The new nodes are created for flow direction after sinks are filled.  Faint outlines of rivers and streams can be observed. 

Flow accumulation is the understanding of where water accumulates and creates channels. To understand watersheds, flow accumulation must be considered.  Using the flow accumulation tool, a new raster is created.

A source raster also must be created in order to delineate watersheds. This requires a threshold expression. Smaller thresholds yield more watersheds and contain much more detail of in-flowing streams. Larger thresholds yield less watersheds and are more reliant on the larger stream channels. The results figure has a threshold of 50,000.

Figure 3. Watersheds at a higher threshold.

Using the watershed tool,  watersheds for this area are now delineated. At 50k there are 108 different watersheds. The watersheds raster is then clipped to the park boundary buffer. This can be seen in the results section.

Results

Figure 4. The completed map for watershed analysis. The vectorstreams shapefile shows those streams to which contribute for a watershed. These are different from the initial hydrology shapefile as these streams contain a topography factor, are filled, and show accumulation. 

Sources

All materials from this lab are open for public consumption and are property of the University of Wisconsin-Eau Claire, ESRI, Cornerll University, and the New York State GIS Clearinghouse.