Emily Madara

711960088

(8.5/9, Nice statement!)

LAB 6

Part I:

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.

[Printed out]

(-0.5) See the subwatershed line.

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

Water cannot flow across grid cells that contain a sink or depression. It is necessary to locate and identify these obstructions to flow so that the DEM is hydrologically correct as far as the standpoint of "surface hydrology" goes. If we do not fill the sinks they will act as “bottomless pits” and it will seem as though water drains into them. The minor depressions have been filled to enable water to flow across grid cells.

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

Establishing the flow direction for each cell in the DEM allows us to determine the hydrologic flow along connected GRID cells so the resulting delineation shows which GRID cells are connected by the directional flow. There are eight directions assigned to the flow. This influences the delineation because water flows in all directions and not just eight so it is a simplification of all the directions that water flows. The resulting delineation will be simpler.

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?

The flow accumulation value at the in-stream point labeled “gauge” is 6928 which means 6928 grid cells drain into that point. Each grid cell represents one square meter which means there are 0.692800 square kilometers that drain into the stream point “gauge.”

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?

The lower the threshold the more stream channels will be defined because if there is a lower threshold, all the parts of the river/stream with a low flow accumulation will show up as streams, but with a higher threshold, only the segments assigned a higher flow accumulation value will be defined as a stream segment. With the layer str_500 it looks like there are more stream segments since the threshold is lower and there appears to be less when looking at the str_2000 layer. The streams that are defined at the lower threshold jus extend from the str_2000 layer. This would suggest that the Horton stream order concept is accurate because as the first-order stream segments join together they form second-order stream segments, which join to form third-order stream segments. As each order increases, the size of the stream decreases as these are small tributaries that connect to the main first-order stream. It is the small tributaries that are defined as streams when the threshold is lower and that do not appear on the map with a higher threshold. The length of streams in successive stream orders increases following a geometric relationship and the number of streams within each order decreases with order in a linear fashion. When there are more lower-order streams, the number of higher-order streams decreases.

Question 6. Create the layout of the resulting flow path with point data (complete with cartographic elements). How does this flow path differ from the route drawn in question 1? What is the difference between the flow path and the crow's path for each? What defines the water flow path?

[Extra credit!! ]

Question 7. What is the area of your defined subwatershed 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?
In Catchment_500 “gauge” is in Area 52. The shape length is 2660.0 meters and the shape area is 268500.0 square meters. In Catchment_2000 “gauge” is in Area 11. The shape length is 4520.0 meters and the shape area is 694800.0 square meters. These two areas are different because they were based off of different stream definitions which would affect how much water runoff there is going into these areas. Catchment_500 is smaller because there are more small tributaries in this area since the lower threshold defined more so the water flows in the direction of those small tributaries and runs off into other areas. Because there were so many streams defined, there were also more catchment polygons created so they end up being smaller. In Catchment_2000 there are less streams defined because the higher threshold did not define smaller, lower-order streams. The area is bigger because it encompasses the streams with higher flow accumulation and does not split off into smaller catchment areas. The value of accumulation for the “gauge” point is still the same because the flow accumulation does not change depending on the stream threshold defined because there is still going to be the same amount of water which flows in that direction since “gauge” is near a higher-order stream.

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.