Lab 3- Watershed Analysis

In GIS Applications we took a look at watershed analysis. I thought I had an understanding of this topic prior to beginning this weeks lab but I quickly realized I had only touched the surface. Almost all of the data we used this week stemmed from a DEM file available form the National Elevation Dataset (NHD). I am little amazed with all of the analysis that can be done with a single DEM and I am sure this is going to happen every time we use another tool in ArcGIS during this class.

One of the most important things I learned was also one of the first steps in the lab - how to fill in sinks (pixels that are anomalies or extremely lower than the cells surrounding them) to hydrologically correct the elevation data prior to conducing analysis. This was a really simple process using the Fill Tool in the Spatial Analyst Toolbox but without it the water flow data could have turned out much different. Then we went through the process of understanding water flow and identifying stream paths. To to this we used the Flow Direction Tool, Flow Accumulation Tool, Con Tool, Stream Link Tool, Stream Order Tool, Snap Pour Point Tool, Watershed Tool, and Basin Tool.

All of these tools have water related names except the Con Tool and that is because the Con Tool is not a part of the Hydrography Toolset even though it was important to use for watershed analysis this week. We used the Con Tool to establish a flow accumulation threshold in order to identify stream paths. This was the part that confused me for a while but eventually it began to make sense. To me this means we told the computer that in order for a pixel/cell to be a part of a stream, it had to have a flow connection to at least x other cells (for the lab this was 200).

After using all of these tools I eventually had a map that showed a computer model of all the streams and stream orders on Kuauai Island as well as the watersheds based off of these modeled streams. The task after creating the models was to compare the modeled data with existing data that was create from aerial images. I thought the amount of streams, stream length, and stream order were important in the stream comparison. I thought comparing the total area size and the amount of high order streams in each watershed was also significant. My final graphic is below.

Lab 3 - Watershed Analysis

x