Week 3 - AppsGIS - Watershed Analysis

Figure 1. Final map comparing modeled and given streams and a watershed on the island of Kuauai.

This week we worked on watershed analysis of the Hawaiian island of Kuauai, comparing modeled results with actual streams and watersheds. First, we performed watershed delineation using streams as pour points. In order to do so, the digital elevation model (DEM) was filled-in using the Fill tool to remove any sinks. Most of the sinks removed were in the low elevation areas to the west side of the island. Now that the model is hydrologically correct, the Flow Direction tool was used to establish how the streams will flow. Following this, we used the Flow Accumulation, with the flow direction raster as an input, which resulted in a stream network:

Figure 2. Modeled stream network.

We then set a condition that all of the streams are defined by having at least the flow of  200 cells accumulated downstream. The resulting raster was turned into a feature class via the Stream to Feature tool. We additionally created a stream order raster that used the Strahler method to order the streams created via the Conditional tool.

The next part of our analysis was delineating a watershed using stream segments (Created with the Stream Link tool) as the pour points. Using the Basin tool, we then used the edges of the DEM to delineate drainage basins:

Figure 3. Delineated basins using the edges of the DEM.

Alternately, we used the river output as the pour point to delineate watersheds. This required us to use Editor to mark the pour point at the mouth of the river of the largest watershed (dark green in the image above), known as the Waimea watershed. This pour point was at the edge of the DEM, which is why it matched the basin result above. We also used a pour point in the middle of the DEM, which was a gauging station used by USGS. This station was not on a modeled stream, so the Snap Pour Point tool was used to correct for this. The Watershed tool was again used to create a watershed raster for the specified gauging station:

Figure 4. Watershed raster based off of USGS gauging station.

Finally, we compared our modeled results from above with streams delineated based off of aerial photos, and previously mapped watersheds. For the streams, it was quite apparent that modeled streams are quite different than given streams at extreme elevations, however, they "match" quite nicely at mid-elevation.

Figure 5. Modeled streams (light blue) compared to given
streams (dark blue) at low elevations.

Figure 6. Modeled streams (light blue) compared to given streams (dark blue) at high elevations.

Figure 7. Modeled streams (light blue) compared to given 
streams (dark blue) at mid-elevation.

For the watershed analysis, I chose the Wainiha watershed to model, with the pour point located at the output of the river. Looking at the modeled and given watersheds, they lined up quite nicely. 

Figure 8. Modeled watershed (light purple) compared to given 
watershed (red outline). There was little excess in the modeled
output, but the northernmost point was "missing".

The analyses we used this week were very interesting, and I can see how they will be highly beneficial later down the line. I liked the fact that we performed the analysis using different tools and methods so that we can see the options available to us.