Kathleen Casanova
Lab 6
(4.5/9)
Question 1. Print out
(-1.0) hand drawn watershed delineation?
Question 2. Why is it necessary to fill sinks in the DEM before delineating watersheds?
Water cannot flow across depressions. Filling the sinks allows you to find the depressions that would obstruct the flow, and fill them so that the water is not lost in these depressions and can flow across these 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?
Only 8 directions are assigned for the flow direction. This simplifies the flow by only providing 8 classifications. This simplification is less detailed, and may present incorrect flow direction which could affect the delineation. The more detailed the flow direction, the more likely the delineation will be accurate.
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? (-1.0)
The FAC value of gauge is 21148, which is the amount of upstream cells contributing to that point. 21148 cells is the area that drains into this point – it is upstream so the value is high, if it were downstream it would be lower.
- pixel value: 6928
- 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?
My original threshold value was 450. Changing the definition of the threshold drastically changes the streams that are displayed on the map. The main streams are shown, but the branching off and the intricate network disappears with a higher threshold. If you choose a higher number than lots of water flow is not accounted for because they are not considered streams.
Horton’s law classifies streams almost inversely. He goes from the smallest value to largest. His definition of types depends on the converging of the larger type
(two type one converge to make type 2àtwo first order streams converge to make one second order stream). The threshold starts at what Horton classifies as type three with the largest value, and works towards lower values. Horton classifies streams by how many streams they drain to, instead of using
thresholds. His map would look very different from the one we’ve made with 2000 and 5000 thresholds.
- (-0.5) When applying the Horton stream order concept, the number of flow accumulation cells determines if the streams are low-order or high-order. If a low number of threshold is chosen, there will be more low-order streams flowing into a high-order stream. If a high number of flow accumulation cells is chosen, there will be less high-order streams, because there are less low-order streams flowing together.
Extra Credit: Trace the flow path between basecamp and the downstream, and basecamp and the ridge. Click this icon then click “outlook” point of the “points.shp” layer.
2625.112061 - 2568.595459 56.5166 to downstream
2625.112061 - 2731.680420 = -106.568 to ridge closest, but not directly located on.
- Pixel value: 2929.77(outlook)------2605.22(basecamp)------2237.23(gague), so 324.55 m and 367.99 m each for flow path.
- crow’s path: around 253m and 296m each.
- The water flow path is defined by flow direction.
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?
Monica told us this was extra credit. Thanks!
Question 7. 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? (-1.0)
The value for Catchment_2000 and Catchment_5000 watershed with gauge has a shape area of 700872.557144. They are the same Catchment_500 and Catchment 2000 have nineteen watersheds. The value of accumulation discussed above (21148) is higher than 2822 but lower than 86125.
- The cat_2000 area is 694,799 m2. It’s similar to the drainage area in Question 4, 692,800m2.
- The cat_500 shows more fragmented subwatershed and the area is 268,500m2
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. (-1.0)
There is no difference between the two watershed delineations. (No. They are different. See the cat_500 map) The maps differ in its display of streams. The first map has a stream threshold of 500 which means that the map displays more of the network of the streams. The second map shows less of the network but still has the major streams and knows it’s flow direction making it possible to have nineteen watersheds of the same size and shape for both maps.