Question2. Why Is It Necessary to Fill Sinks in the DEM Before Delineating Watersheds?

Sam Franklin

April 8, 2008

9/9

Question1. 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.

Question2. Why is it necessary to fill sinks in the DEM before delineating watersheds?

It fills in any holes that would be read as “bottomless pits” that might have been created in the initial data collection. The holes are filled in to match the elevation of surrounding terrain smoothing over any random errors that may exist.

Question3. How many directions are assigned when running the flow directionoperation? How do you think this might influence the resulting delineation?

There are eight directions assigned when running the flow direction operation. This would influence the data because it only takes into account 8 directions out of possibly an infinite amount. By oversimplifying the equation it will make the result different than what may actually happen or even different from an equation that assigns 20 directions.

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 stretched value is 255 while the pixel value or the amount of area that drains to this point is 6928.

- 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?

The 500 threshold creates more streams and makes existing streams longer than the 2000 threshold. According to Horton, drainage density is measured by stream length divided by basin area. Because we are changing the threshold we are changing the stream length and thus the drainage density by defining the threshold to create a larger basin area.

- 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?

For Catchment_2000 the area that drains into gauge is 649,800. And for Catchment_500 the area that drains into the point gauge is 268,500. They are different because Catchment_2000 defines a larger catchment area because the threshold is much larger. The segmentation in catchment_500 makes them smaller watershed drainage areas. This is different from the value of accumulation because before we found the area that drains to one point based on pixels, now we are measuring an area that creates a stream based on either 500 or 2000 threshold.

They are different because catchment_2000 has a larger area that drains into the point“gauge”. This is different from the value found in Question 4 because at this point we have created and defined larger polygon areas that flow to this point whereas before it was a smaller area.

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

- cat_500 shows more fragmented subwatershed.

Question8. 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 map above is the one with the 500 value. The map below has the 2000 value. You can easily notice that the 500 map has a larger stream system. This map has more catchment zones and they are much smaller than the 2000 map. This is because the watershed in the 500 defines a stream as where at least 500 grids flow to one point. The 2000 is a map that represents a water shed that requires at least 2000 grids to flow to one point. The catchments created are larger with the 2000 map, where there are more catchments which are smaller but overall include more of the map.