Ryan Miller

4/8/2008

Geog 370

Lab #6

Question 1. Create a layout of the contour map, overlay points file, “points.shp” (complete with cartographic elements), print it out, and hand draw watershed delineations.Draw aline from basecamp tomountain pick (outlook), selecting the shortest, but the flattest route. Submit this copy to the TAs.
(-0.5) See the subwatershed..

Question 2. Why is it necessary to fill sinks in the DEM before delineating watersheds?

It is necessary to fill the sinks in the DEM before delineating the watershed because the data in the DEM is not uniformly accurate. The DEM may contain data errors within the grid and as a result, if you try to perform any analyses using an unfilled DEM, ArcMap will treat the gap in the data as a hole and will affect your results. Filling the sinks allows ArcHydro to analyze a more hydrologically correct map.

Question 3. How many directions are assigned when running the flow direction operation? How do you think this might influence the resulting delineation?

ArcHydro assigns 8 possible flow directions that water can flow, hence the name of the algorithm used, “The D8 Algorithm”. This discrete number of possible flow paths creates a simplified and idealized representation of how water really behaves. Consequently, the D8 Algorithm cannot account for the subtle variations in the infinite number of possible flow paths, so, ArcHydro for the sake of convenience simplifies all possible directions into a more useable format. Since ArcHydro only acknowledges 8 possible directions of water flow, there is the possibility that the resulting data may be skewed.

Question 4. On your flow accumulation layer, click on the in-stream point labeled "gauge". What is the value? How much area drains into that point?

I interpreted the ‘Count’ field representing the ‘value’ and the Pixel Value representing the amount of land area that drains into the ‘gauge’ point along the stream. (-0.5)

- drainage area: 6928*100=692,800m2

Question 5. Make two stream networks: one with threshold value 500 and name that “str_500”; and the other with threshold value 2000 and name that “str_2000”. What is the effect of changing the stream definition threshold? What does this suggest about the Horton stream order concept?

Changing the stream definition threshold changes the tolerance at which ArcHydro considers a location to be a stream. The lower the tolerance, as in the “str_500” layer, the less water is required to flow into a given point to be considered part of the stream network. While conversely, a higher tolerance, as in the “str_2000” layer, requires more water to flow into a point before it can be considered a section of the stream. Consequently, the lower tolerance value produces a more extensive network of streams and the higher tolerance value produces a more simplified version. As a result, Horton’s Law of stream orders provides an accurate representation of how water flow volumes in a river are compounded when two streams converge.

Question 6. What is the area of your defined subwaterhsed which has “gauge” as an outlet point in both “Catchment_500” and “Catchment_2000” watersheds? Are they same or different? How does this area compare to the value of accumulation discussed in question 4?

The area of sub-watershed with the “gauge” point as the outlet for Catchment_500 is: 2685, i.e. 268,500m2

The area of sub-watershed with the “gauge” point as the outlet for Catchment_2000 is: 6948 i.e. 694,800m2

The areas are nearly identical with some minor variation in pixel amounts between the Catchment_2000 and the result yielded from the identify tool.

Question 8. Create two layouts of the resulting watershed delineation with the DEM, stream and point data (complete with cartographic elements). Explain the difference between the two subwatershed maps.

The two areas are different because they represent different water threshold values. In the Sub-watershed_500 map, the sub-watershed is smaller because the amount of pixels (water) needed to flow into a given point in order to designate an area as a sub-watershed is much less than in the Sub-Watershed_2000 map. As well, the sub-watershed in Sub-Watershed_2000 is larger because more pixels (water) is needed to flow into a given location before it can be designated a sub-watershed.