Lab 3 - Watershed Analysis

This lab dealt with some more complicated concepts dealing with watershed analysis. We were introduced to the procedures necessary to delineate streams and watersheds from a digital elevation model (DEM), which was provided to us but originally obtained from the National Elevation Dataset (NED) through the Seamless Data Distribution System of the USGS. The first step to prepare the DEM was to fill the sinks. A sink is a depression with no visible outlet. Filling the sinks allows the water to flow properly throughout the area we are investigating. After filling the sinks, we created a flow direction raster, which utilizes the D8 algorithm to determine the direction of the flow. From this flow direction raster, we are able to create a flow accumulation raster, which is a calculation of the accumulated weight of all cells flowing into the downstream cell. Cells with higher values are cells where flow is concentrated and can be used to identify streams. From the flow accumulation raster, we created a raster of a stream network based on a threshold. The threshold value is the number of cells we set as necessary to denote a stream. Part of this lab was changing the threshold value and determining which we thought gave the best results when comparing our modeled stream network to the actual stream network. We converted our streams to features and from there, created a stream link and stream order raster. I found the stream order raster particularly interesting. Using the Strahler method, the stream order increases only if two streams of a lesser order meet. The Streve method stream order is determined by the sum of streams meeting irrespective of upstream stream order. In our lab, we used the Strahler method, but I feel that either gives one a good idea of the rate of flow or size of the stream, even though it doesn't seem to be quantifiable from this tool alone.

Next we worked with watersheds and pour points. The term "pour point" was a little confusing to me at first; I tended to keep thinking of the term as an inflow. What it basically is is where water "pours" out of the stream system, often to a larger body of water. To determine a pour point, we added a new feature class and digitized a pour point where the stream drained out to the ocean.Using the watershed tool, we created a flow direction raster using our pour point as an input, which results in a watershed containing the area that drains to that pour point. We also learned to snap a pour point to a stream if that pour point is not exactly on a stream cell.

For our final product, we were to select an existing watershed with only one pour point. We needed to show a map displaying a comparison of the modeled and existing streams, as well as a comparison between existing and modeled watersheds. I chose the Lumahai River watershed. I outlined the borders of the existing watersheds in red. I displayed the existing watershed as a darker blue and placed the modeled watershed on top to demonstrate any discrepancies between the two. I'm showing the modeled streams as orange so they stand out from the blue existing streams. The inset shows the larger extent of the island with the Lumahai River watershed displayed. I found it interesting that using hillshade as a separate raster looks much better than using the hillshade effect within the symbology window of the DEM. Some of the concepts of this week took me quite a bit of time to really understand and I feel a lot more is possible within watershed analysis, but I feel I learned some very useful techniques, especially when working with data at the National Weather Service and the hydrology department, as I should soon be.