GEOG 370 – Spring 2010
Lab #5:Drainage network delineation
Due Date:
Week of March 29at the beginning of your lab
(1 week)
Fundamentally the earth's surface is organized into watersheds. DEMs (digital elevation models) are frequently used by geographers in environmental analysis. Watershed managers and planners are interested in different types of environmental conditions within distinct segments of watersheds (e.g., hillslopes, terraces, floodplains, etc.), while hydrologists and geomorphologists are interested in the spatial variability in processes throughout a watershed. Obtaining this information requires delineation of the drainage system, which includes the stream channel network and smaller catchments within the basin. In addition, every watershed can be characterized by geometric properties related to its linear, areal and relief properties. These properties are related to the position of a stream within the watershed, and can be used to compare watersheds. In this lab you will utilize raster GIS procedures to work with USGS DEMs. You will use ArcHydro, state-of-the-art GIS technology for watershed sciences, to delineate the drainage basins and drainage network of Cockeysville, located in the Baltimore County, Maryland.
BACKGROUND
A drainage basin is an area defined by a topographic boundary that diverts all runoff to a single outlet. The topographic boundary that separates runoff between two basins is the drainage divide. The delineation of drainage basins can be done manually using topographic information. On the other hand, the widespread availability of elevation data in digital format has bolstered the development of automated tools that can be used to delineate drainage basins and their associated stream network. Most of the hydrologic modeling tools work with elevation data in raster format (digital elevation models or DEMs). A DEM is a grid of cells in some coordinate system having land surface elevation as the value stored in each cell. In ArcGis, hydrologic flow is modeled using an eight direction pour point model. This model considers that runoff from a given cell in a DEM will flow towards one of its eight neighbors, and that this neighbor is the one that represent the greatest slope between adjacent cells. These techniques require the calculation of the direction of the flow and the number of cells draining to every cell in the DEM. Using these features, the watershed and sub-watershed boundaries, and the associated stream network can be modeled in a relatively straightforward way.
Once the stream channels within a certain watershed have been identified, the quantification of some intrinsic characteristics related to the morphometry of these elements can be used to identify certain general properties. First, an important quantifiable characteristic of stream networks is related to the hierarchical arrangement of stream channels. Different methods can be used to classify streams according to their position in the network, but the most commonly used is the method proposed by the famous hydrologist Robert Horton. According to this system, 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. When a lower order segment joins a higher order segment, there is no change in river order (Figrue 1).
Figure 1. The Horton’s law of the stream order
After the stream segments have been ordered following a given scheme, some interesting relationships have been observed. For example, the bifurcation ratio (also know as the law of stream numbers) defined as the ratio between the number of streams of a given order to the number in the next order, has been found to vary around the value from 3 to 5 in basins where geology is reasonably homogeneous. The law of stream lengths (Figure 2) suggests that the length of streams in successive stream orders increases following a geometric relationship. Similarly, the number of streams within each order decreases with order in a linear fashion. In addition, The law of drainage areas (Figure 3) states that the area of the basin is related to stream order in a geometric series. Finally, one important areal measurement is drainage density, which is simply defined as: Dd = Sum of stream lengths / Basin Area. Drainage density is an important index to geomorphologists and hydrologists because it provide a quantitative index of how dissected a drainage basin is. The controls on drainage density are climate (which supplies precipitation for runoff and stream incision) and geology (resistance to erosion and as a control on runoff through infiltration).
Figure 2. The law of stream lengths
Figure 3. The law of drainage areas
Before continuing with this lab, be sure that you understand the concepts summarized above and explained in more detail in the following additional sources:
- PhysicalGeography.net. Chapter 10: Introduction to the Lithosphere: (aa) The Drainage Basin Concept, and (ab) Stream Morphometry
Part 1 – Watershed delineation by hand
Before you begin, load the ArcHydro Toolbar (View > Toolbars > Arc Hydro Tools).
1. Copy the data
1) Copy “10mdem” and “points.shp” from the data folder to your folder and open it in ArcMap.
2) Check out the “points.shp” table by right clicking and Open attribute table. You can find three data points: gauge, outlook, and basecamp.
2. Create contours
1) On the Spatial Analyst dropdown, choose Surface Analysis > Contours
2) Put “10mdem” in the Input surface box.
3) Choose the contour interval (CI).
The CI is simply the vertical difference in elevation between successive contour lines. Put 10 into this blank.
4) The default at zero should work fine, or, select the elevation at which you would like to begin contouring (base contour).
5) Be sure to set the output shape file name “contour” and put it to your personal class folder.
5) Edit the Layer Properties to change the color and line thickness of the contour lines.
Right click in the “contour”, go to “properties”, and click “Symbology” tab. You can find some options for editing visualization. For your information, an editing example is attached below. You can choose whatever colors, classification method, class numbers, and so on to make your map more recognizable.
Question1. A. 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.B. Draw aline from basecamp tomountain peak (outlook), selecting the shortest, but flattest route. Turn in this printed map to the TA at the beginning of lab next week.
Part 2 – Watershed delineation with ArcHydro Tool
In this portion of the lab you will work with the DEM to perform some basic hydrologic analysis of the Honey Creek watershed. Ultimately you will delineate the smaller subbasins within the watershed, and also the drainage network.
Now you will perform several automated types of hydrologic analysis based on your “10mdem” GRID.
1. Fill the sinks in your basin
Water cannot flow across grid cells that contain a sink (depression). Therefore you have to locate and identify these obstructions to flow so that your basin is "hydrologically" correct (from the standpoint of "surface hydrology")
With the honey_basin active, Use the ArcHydro toolbar Terrain Preprocessing > fill sinks.
This creates a new GRID called "Fil" which will look almost identical to your honey_basin, except minor depressions have been filled to enable water to flow across grid cells.
All the grids and shapefiles created within ArcHydro are stored in a folder named "Layers" that is automatically created in your working directory.
Question2. Why is it necessary to fill sinks in the DEM to delineate watersheds?
2. Create a flow direction GRID for your DEM (Fdr)
This process establishes the flow direction for each cell in your DEM so that the procedures that follow will be able to determine the hydrologic flow along connected GRID cells.
On the ArcHydro toolbar, click on Terrain Preprocessing > Flow Direction. Note the new grid named fdr, the colors represent flow directions for water. You will use this GRID for further analysis.
Question3. How many directions are assigned when running the flow directionoperation? How do you think this might influence the resulting delineation?
3. Create a flow accumulation GRID (Fac)
This will create a new GRID that shows number of cells upstream any given cell in your DEM. The colors represent the cumulative increase in grid cells with downstream distance, i.e. the cumulative increase in cells that contribute water to the main basin.
Onthe ArcHydro toolbar, click on Terrain Preprocessing Flow Accumulation. Not a really exciting looking grid, but if you change the color scheme by Layer properties like below, you will note that the values of flow accumulation increase from the headwaters zone to the outlet of the watershed.
If you use the "identify" tool and click on a single cell, the values represent the upstream cells contributing flow to this point in the basin. note how the values increase downstream.
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)
4. Delineate the drainage network GRID (Str)
This will create a GRID representing stream channels (Str)
On the ArcHydro Toolbar, click on Terrain Preprocessing > Stream Definition. You will need to set the number of flow accumulation cells that define if a stream channel is created.
Note: This is a very important decision to make. Fundamentally, the question you are answering is how much area is required to generate sufficient runoff (and energy!) to incise the underlying bedrock and create and maintain a stream channel. Keep this in mind and be sure to note how many GRID cells that your various drainage networks are based on.Experiment with different #s and note the change in stream network delineation and drainage density.
Here is an example showing the stream network delineated from default value, 456 (blue) and 2000 (pink) grid cells.
Question5. What is the effect of changing the stream definition threshold? What does this suggest about the Horton stream order concept?
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?
5. Run the stream segmentation
On the ArcHydro Toolbar, click on Terrain Preprocessing > Stream Segmentation. Preserve the default parameters defined by ArcHydro. This process creates a GRID named "Lnk" which has a unique value for all the pixels belonging to a single stream segment.
6. Now you can delineate your sub watersheds within the watershed basin
OntheArcHydroToolbar, click on Terrain Preprocessing > Catchment Grid Delineation. Preserve the default parameters defined by ArcHydro. This process creates a GRID named "Cat" containing raster regions that represent catchments within the basin. ArcHydro defines one catchment for each stream segment in the basin.
7. Create a vector layer of the sub watersheds you just created
On the ArcHydro Toolbar, click on Terrain Preprocessing > Catchment Polygon Processing. This creates a shapefile of the watershed named "Cathcment".Take a look at the statistics in the data table. Pay special attention to the field Shape_Area. This field contains the area of each catchment that ArcHydro calculates automatically.
Question7. What is the area of your defined catchment? How does this area compare to the value of accumulation discussed in question 4?
8. Create a vector version of the stream segments you have modeled.
On the ArcHydro Toolbar, click on Terrain Preprocessing > Drainage Line Processing. This process creates a shapefile named “DrainageLine” containing the stream segments defined in the Lnk GRID as polylines. Again, note the stats in the table (Specially the Shape_Length field). Also, note that each stream segment has a field called GridID, which is unique for each stream segment.
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.
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