Lab 3 - Watershed Analysis

This week in GIS Applications we covered the topic of watershed analysis. The analysis itself requires a lot of steps, although thankfully they are fairly intuitive and standardized.

Our final map output for this week shows a comparison of a modeled watershed and the actual limits of a defined watershed - for my lab example I had focused on an area of the Anahola Stream Watershed, on Kuauai Island, Hawaii.

View of a modeled watershed versus reality.

To create my model I selected a pour point location along a stream on the edge of my DEM (shown above as the 'landscape' underneath the main map elements). I then ran the watershed analysis tool (which can be found in ArcToolbox under Spatial Analysis > Hydrology). My input layer was a previously created raster file which showed the flow direction for all stream segments as well as my selected pour point. The result was a raster file showing the watershed extent in light green above - although for display and analysis purposes I had converted the raster to a polygon file.

What is striking is how much smaller my modeled watershed is in comparison to the extent of the actual Anahola Stream Watershed. Note that only the connected stream segments are shown within my modeled watershed extent. I believe that my watershed extent is directly due to my choice of pour point location... in fact the watershed area would be even smaller had I selected a pour point further upstream.