Molly McCarter
29 March 2010
GEOG 370 601
Question1. Create a layout of the contour map, overlaying points file, “points.shp” (complete with cartographic elements, including authorship!), print it out, and hand draw watershed delineations.Draw aline from basecamp tomountain pick (outlook), selecting the shortest, but flattest route. Turn in this printed map to the TA at the beginning of lab next week.
Question2. Why is it necessary to fill sinks in the DEM to delineate watersheds?
Water cannot flow across grid cells that contain a sink (depression). Therefore I had to locate and identify these obstructions to flow so that my basin could be "hydrologically" correct (from the standpoint of "surface hydrology"). This “fill” accounts for these sinks/depressions and corrects/fills them. Water can’t flow across cells with depressions.
Makes places on the map surrounding the water bodies depicted on the map at a little lower elevation than before (more dark places on the map)
Question3. How many directions are assigned when running the flow directionoperation? How do you think this might influence the resulting delineation?
Eight flow directions. These eight flow directions are the same directions on a compass (north, south, east, west, northeast, northwest, southeast, and southwest). Restricting the flow to these eight directions makes it easier to tell which ways water in an area generally flow. Having a complete 360 degree flow direction would be difficult to read, and not give the map viewer a real sense of which way water is flowing (and from where) in an area. Having 8 directions of flow makes it easier to read which way water is flowing from different points on the map.
Question 4. On your flow accumulation map, click on the in-stream point labeled "gauge".What is the value? How much area drains into that point? (6928)
The pixel value is 6928
6928 *100 sq m = 692,800 sq m is the area that drains to that point
(each pixel represents 100 sq m)
(answer was confirmed with Emily Snow, 3/28)
Question5. What is the effect of changing the stream definition threshold? What does this suggest about the Horton stream order concept?
Changing the stream definition threshold changes how much area is required to generate sufficient runoff to incise the underlying bedrock and create and maintain a stream channel.
It literally changes the number of GRID cells that the various drainage networks are based on.
One can change it, therefore it is a user-defined threshold. This threshold is defined as a number of cells. This stream threshold definition can define the sub basin delineation during preprocessing. Changing the number of cells changes how far reaching the streams are.
The stream definition threshold defines the number of pixels which must flow to an area to make it significant enough to constitute a flow (the number of pixels that can flow into an area that can constitute a stream).
Horton’s stream order concept states that a stream segment with no tributaries is designated as a first-order stream. When two first-order segment join, they form a second-order stream; two second-order segments join to form a third-order segment, and so forth. In this step, the Blue lines that are shown (excluding the parts covered by the pink lines) are first order streams. The pink lines overlaid on the blue lines represent second and third order streams.
A smaller stream definition would make a lesser number of pixels flowing to an area constitute a flow. This smaller stream definition would make a more complex network (more branching) of water flows (streams), therefore making there be higher order of streams on the map. (Lower stream definition, more higher order streams)
Question6. Trace the flow path between base camp and the downstream, and basecamp and the ridge. Show the screenshot of the flowpath. Extra credit: How long is each flow path? What is the difference between the flow path and the crow's path for each? What defines the water flow path?
The flow path from the basecamp to the end of the Cockeysville (following the stream) is approximately 2388 meters. The crow’s path is much shorter, at approximately 2018 meters. The flow path from the ridge is approximately 3934 meters. The crow’s path is approximately 3251 meters. The flow path in both cases is longer because of the twists and turns that the water must make to follow the terrain of the earth. The water flow path is defined by this changing elevation and terrain. The crow’s path simply takes the shorted straight line route.
The flow from basecamp to the gauge is 388 meters. The crow’s path is shorter at 286 meters. The flow from the ridge to the gauge is 1864 meters. The crow’s path is shorter at 1508 meters.
Question7. What is the area of your defined catchment? How does this area compare to the value of accumulation discussed in question 4?
5 catchments flow to the gauge point. Below is the calculation for the sum of their areas:
268500 + 151200.000001 + 51500 + 59899.999999 + 163700.000001
= 694,800.000001
The area for the defined catchment (all catchments flowing to the gauge) is 694,800.000001. This area is very close to the area calculated in number 4 (692,800). Both questions were attempting to measure the amount of area that flows to the gauge, and therefore should have similar answers.
Question8. Create a layout of the resulting watershed delineation with the DEM, stream and point data (complete with cartographic elements).
Question9. How does this delineation compare to the hand-drawn contour map? How does the flow path in question 5 differ from the route drawn in question1.
This watershed delineation was created by processing actual terrain data (Digital Elevation Model, stream location, etc - actual data, more than just contour data) while the hand drawn delineation was based solely on the contour of the area.
The watersheds in this map were first delineated by creating a GRID that contained raster regions that represented catchments within the basin, then this data was made into Vector polygon data (lines made around the catchments).
The flow in question five is more accurate than (although looks very similar to) the flow hand drawn for number one because it is based on more than just contour data.
This map shows subwatersheds, the hand drawn map shows a more broad watershed.
The marking on this map are also more precise since they were done with a computer and not by hand.