GEOG 370 – Spring 2008
Lab #6: Drainage network delineation
Due Dates:
Tuesday Labs: Tuesday, April 8 by 11AM
Wednesday Labs: Wednesday, April 9 by 2PM
Thursday Labs: Thursday, April 10 by 11AM
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.
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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 thirdorder 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).
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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
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Part 1 – Watershed delineation by hand
Before you begin, load the ArcHydro Toolbar (View > Toolbars > Arc Hydro Tools).
1. Download data under GPS lab
1) Download “10mdem” and “points.shp” from the data folder to your 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).
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5) Be sure to set the output shape file name “contour” and put it to your personal class folder.
6) 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.
Question 1. 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 a line from
basecamp to mountain pick (outlook), selecting the shortest, but the flattest route. Submit this
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copy to the TAs.
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 watershed. Ultimately you will delineate the smaller subwatershed 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")
Use the ArcHydro toolbar Terrain Preprocessing > fill sinks.
This creates a new GRID called "Fil" which will look almost identical to your 10mdem, except
minor depressions have been filled to enable water to flow across grid cells.
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All the grids and shape files created within ArcHydro are stored in a folder named "Layers" that
is automatically created in your working directory (your student folder in geog370 afs directory).
Question 2. Why is it necessary to fill sinks in the DEM before delineating 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.
Question 3. How many directions are assigned when running the flow direction operation? 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.
On the 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 headwater zone to the outlet of
the watershed.
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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 watershed. note how the values increase downstream.
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?
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, some questions that you are
answering are 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.
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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?
Use Flow path tracing icon (
) to 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.
(extra credit)
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?
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. Try with two stream
networks that you made previous step and name those “Lnk_500” and “Lnk_2000” each.
6. Now you can delineate your sub watersheds within the watershed basin
On the ArcHydro Toolbar, click on Terrain Preprocessing > Catchment Grid Delineation.
Preserve the default parameters defined by ArcHydro. This process creates a GRID named
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"Cat" containing raster regions that represent catchments within the basin. ArcHydro defines
one catchment for each stream segment in the basin. Try with two stream segmentations that
you made previous step and name those “Cat_500” and “Cat_2000” each.
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 shape file of the watershed named "Catchment". 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. Try with two stream segmentations that you
made previous step and name those “Catchment_500” and “Catchment_2000” each.
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?
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 shape file named “DrainageLine” containing the stream segments defined in
the Lnk GRID as poly lines. 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. Make two drainage lines: “DrainageLine _500” and “DrainageLine _2000”.
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.
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