Nicholas Holcomb

7/9

Lab 6 Surface Analysis > Contour would not highlight for me to click on it.

Question 2. Why is it necessary to fill sinks in the DEM before delineating watersheds? The sinks represent holes in the data, so if these holes are not filled out the map will think that all the data is draining into these holes. This could never occur in real life.

Question 3. How many directions are assigned when running the flow direction operation? How do you think this might influence the resulting delineation? 8 directions. This can alter the projection of water movement on the map. Streams are never straight, so if a stream flowed into one square and changed its path, the map would still compute it’s flow as the direction of the inital square it went in.

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? Fac : 4, point : gauge, 10mdem – 129.115, COUNT: 5,323.

- pixel value: 6928

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

Question5. 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? Blue 500 Pink 2000. Increasing the stream threshold will reduce the length of what ArcMap defines as a stream. Small streams with small drainage basins will not be counted if the threshold is high. The law of drainage areas states that the area of the basin is related to stream order in a geometric series. So the larger the basin, the larger the stream, but as streams become larger there are fewer streams in the basin.

- The lower threshold value produces a more extensive network of streams and the higher threshold value produces a more simplified version.

- Horton’s Law of stream orders provides an accurate representation of how water flow volumes in a river are converged when two streams converge.

- the drainage area increases as the stream order increases

Question7. 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 catchment for 500 will be a much smaller area because not as much wáter needs to flow into a point for it to be considered a stream.

- for cat_2000: 694,799 m2. It’s similar to the drainage area in Question 4, 692,800m2.

- cat_500 shows more fragmented subwatershed.

Missing maps -2