Here Is My Histogram for DEM in the Upper Watershed

Jen Winston

Exercise 2

Basic Terrain Analysis:

Here is my histogram for DEM in the upper watershed:

And here is my histogram for slope in the upper watershed:

And here is my histogram for aspect in the upper watershed:

Here is my histogram for DEM in the middle watershed:

And here is my histogram for slope in the middle watershed:

And here is my histogram for aspect in the middle watershed:

Q1. Differentiate between terrain characteristics in each of the subwatershed areas based on the DEM and aspect distributions. How are DEM and aspect distributed in each of the two subwatersheds?

As can be seen in the DEM histograms, the upper watershed has much higher elevations than the middle watershed. The upper watershed, for example, does not have any pixels with elevations less than 101 and has the greatest number of pixels at a value of 229. The middle watershed, on the other hand, has 474 pixels with an elevation of 0. The remainder of its pixels have average elevation between 50 and 100.

As can be seen in the slope histograms, the upper watershed is fairly flat, with the vast majority of pixels having a slope of less than 7. The middle watershed has a little bit steeper slopes, with most pixels falling between 2.5 and 13.5.

As can be seen in the aspect histograms, most of the steepest slopes in the upper watershed are facing in the northeast direction. Most of the steepest slopes in the middle watershed are facing in the southeast direction.

Q2. Compare the stream network and flow direction maps between the original and filled DEM. What is the major difference between them?

As can be seen in the maps below, the major difference in the stream networks is that the original DEM leaves stream reaches that are unconnected to anything because they fall in to a pit. The filled DEM connects these stream reaches to rivers downstream, allowing them to drain out of the watershed in the model.

Q3. Comment on the spatial distribution of depression cells. Where can you find major pits? How does filling pits affect the flow paths of this area? Why?

The pits filled in the middle watershed fall along the path of the river. Filling the pits along in this part of the watershed connects the flow paths. The unfilled DEM leaves orphan stream reaches that do not drain into anything. The flow accumulation model moves water into downhill pixels. Once water reaches a pit, in cannot get out because there are no downhill pixels, so the water gets trapped in a pit and the stream reach disappears. By filling those pits, water again has downhill pixels, so it reconnects orphan stream reaches to other reaches downstream.

There are a few pits in upper watershed on and around campus: one east of Boundary Street, between Rosemary and Franklin; one west of Boundary Street, between Hooper and Senlac; one north of Darys Street, between Battle Lane and Hillsborough Street; and a couple on the quad south of the planetarium. Similar to the middle watershed, filling the pits in this area connects the orphan streams to other reaches downstream.

Q4. How many hectares is the 1000 and the 500 grid cell threshold that defines the stream heads?

As can be seen in the map below, the 500 pixel stream is far more detailed than the 1,000 pixel stream, and extends much further upstream. Starting from the northwest corner of the map, the top pixel of each stream is, and the difference between the two is:

As can be seen in the map below, the 500 pixel streams also identify a number of branches that are not identified on the 1,000 pixels stream.

Q5. What range of drainage area (in hectares) appears to define a stream head in this area?