Watershed Delineation in ArcGIS

Watershed
Delineation
March 2019
Dr. Arthur Gill Green

What is This?
Watershed Delineation by Arthur Gill Green is licensed under a Creative Commons
Attribution-ShareAlike 4.0 International License.
Based on a work at http://greengeographer.com/.
• An open training module for learning
Geographic Information Science as applied
to watershed delineation.
• It uses data from the USGS (SRTM) and
software from ESRI (ArcMap 10.x).

Why Do This Training?
It’s free and you will learn how to:
• Get SRTM 1 Arc-Second (30 meter resolution) for
free to make Digital Elevation Models (DEM).
• Merge raster grids into mosaics.
• Derive streams, stream orders, basins, and
specific watersheds from the data.
• Convert raster grids into vector features.
• Calculate area and length.
• Create and analyze evidence for responding to
geographic questions.

Research Question
While working in Cameroon on a
transboundary international conservation
area near Tchabal Mbabo, we wondered…

Does the Faro River basin cross the international
border between Cameroon and Nigeria?
If it does, this is one reason to explore setting up a
transboundary international conservation area or
international watershed co-management plan.

Where are Tchabal Mbabo &
the Faro River?
• This photo is from the Tchabal Mbabo cliffs
looking down into the Faro River basin.
• This is a remote region, located in the west of
the Adamaoua Province of Cameroon.
Livelihoods are based around herding.
• The rapid drop of the cliffs provide many
microclimates and are home to rare
Afromontane and Sudano-Guinean flora and
fauna. For example, Prunus africana is one rare
species here.
Click below to see
the region on OSM.

What You Need
• Software: ArcMap 10.x
• A license to use Spatial Analyst.
• Access to the internet.
• 3-4 hours (depending on the size of the data
you download and your computer’s
processing abilities it could be even longer –
so choose a small area).

Outline
1. Get Data
2. Set Work Environment
3. Mosaic Rasters (put them together)
4. Find Sinks and Fill Sinks (create a depressionless DEM)
5. Flow Direction
6. Flow Accumulation
7. Basins and Watersheds
8. Stream Network
9. Converting to Polygons and Cleaning Up
10. Comparing Basin to Watersheds
11. Answering the Research Question

1. Get Data
This teaches you how to download SRTM data
from anywhere in the world.

Get Data
• We will use SRTM (Shuttle Radar
Topographic Mission) 1 Arc-Second Global
elevation data (~30 meter resolution) from
the USGS and NASA. Collected Feb. 2000.
• Get an account at:
http://earthexplorer.usgs.gov/
• I use data from the border of Cameroon and
Nigeria to look at where the Faro River is
located.
https://en.wikipedia.org/wiki/Faro_River
• You can select your own region. Make
sure to only take 1-2 SRTM image areas or
your computer may take a very long time to
process the data.
Source:
http://www2.jpl.nasa.gov/srtm/
mission.htm

• This is the http://earthexplorer.usgs.gov/ interface.
• Sign up for a free account.
• Once you have a free account, you can search for data
from any part of the world or manually outline your
area of interest (AOI) using your mouse to click on the
map.

• I manually outlined my area of interest using the above
points on the map.
• Once you have outlined your area of interest, you can
move on to searching the types of data available. We
are going to search for SRTM.

• After outlining your
area of interest (AOI),
move on to the Data
Sets tab.
• Type in “srtm” and you
should see SRTM 1 Arc-
Second Global.
• Select that, then click on
Results.

• You can show the SRTM image footprints by using the buttons on
the left.
• We see there are two SRTM images that fall within my AOI.
• I will have to download both of them and mosaic (merge) them into
a new raster.
• You can display metadata (SRTM files were collected in 2000 but
published in 2014). You can do individual or bulk downloads.

Product Specifications (some metadata)
Projection Geographic
Horizontal Datum WGS84
Vertical Datum EGM96 (Earth Gravitational Model
1996) ellipsoid
Vertical Units Meters
Spatial Resolution 1 arc-second for global coverage
(~30 meters)
3 arc-seconds for global coverage
(~90 meters)
Raster Size 1 degree tiles
C-band Wavelength 5.6 cm
Projection is geographic, so we will need to reproject to do
measurements (area, length) in meters.

Download Data
• Earth Explorer offers SRTM data as:
• Digital Terrain Elevation Data (DTED)
• Band interleaved by line (BIL) (a binary raster format)
• Georeferenced Tagged Image File Format (tif, tiff,
GeoTIFF)
• Any of these formats will work for this exercise.
• I downloaded the GeoTIFF.
• You should make a project directory (such as
“C:/WATERSHED/DATA”) and move/unzip the
files into that directory.
https://lta.cr.usgs.gov/SRTM1Arc

2. Set Work
Environment
This allows you you to know where your work
is being saved and to protect your data.

A Directory
Create a folder (directory) for your data call it
“watershed”. Put all your images and data
(and zip files) in the directory. You can create
this directory on your desktop, flash drive, or
elsewhere.
Once you have created your working
directory, you need to setup a geodatabase
(to control datasets) and your project
environmental variables so you know where
to find things.

Create a New Geodatabase
Open ArcMap.
Create a new file geodatabase in the same
folder as your data/images. Call the
geodatabase “watershed”.
You should create a ”file database” as it is a
more flexible way of storing data.

Create a New Geodatabase
You can create a
geodatabase by
following any of the
below:
• Using the
ArcToolbox
• Just creating a
geodatabase in the
ArcCatalog window

Map Document Settings
• Create a new mxd document. Save it as
“watershed.mxd” in the same folder as your data
and geodatabase.
• Enable Spatial Analyst tools Customize > Extensions >
Spatial Analyst.
• Search the “Hydrology” toolbox. You should be able
to see the tools in the image on the left.
• Set Map Document Properties in the following
steps.

Map Document Properties
Open up map
document properties
(in the File menu). Set
your default
geodatabase as the
new one you just
created.
Set relative pathnames
on.

Geoprocessing Settings
• Go to Geoprocessing > Environment and enter the following
settings using your own directory (folder) and geodatabase.
The below directories and geodatabase reflect only my files.
• Current determines where outputs are saved. Scratch
determines where temporary files are saved.

Load Data and Check
Properties
• Load your geodatabase and images into the mxd.
• Check your data properties by right-clicking on
the layer. This is needed for merging (mosaicing
the images).
• You should have 1 band and 16 Bit pixel depth.
None of the data values are negative so, we
should use a 16 Bit Unsigned when we mosaic
rasters in the next step. If you have negative
values, check other pixel types.

Spatial Analyst License
If you ever get the below error, you need to
activate your spatial analyst license.
Go to “Customize-> Extensions”

3. Mosaic Rasters
If you have downloaded multiple SRTM
images, you need to do this step to get them
all in one raster.

Mosaic Rasters
We need to combine satellite images to have one
raster.The function = Mosaic To New Raster.
You can access this via the Data Management Toolbox
(Raster-> Raster Dataset). Or you can search for it
using the ArcMap Search window.
The new raster name is “mosaicsrtm1.tif”.

Mosaic Output =
mosaicsrtm1.tif

Enhance Visualization in
Layer Properties -> Symbology:
Histogram Stretch & Color Scale

Nigeria/Cameroon border (using a basemap)

4. Find Sinks &
Fill Sinks
Creating a depressionless DEM

Find Sinks
• Sinks are a common problem in DEM. “A sink is a cell or set of
spatially connected cells whose flow direction cannot be
assigned one of the eight valid values in a flow direction
raster.”
• Sinks are often data aberrations and they will impact and
possibly ruin models of flow direction…. Yet, to find sinks we
need to first do a flow direction analysis which assigns cell
values that reflect flow direction.
Flow Direction Results, Source:
http://desktop.arcgis.com/en/arcmap/latest/tools/spatial-
analyst-toolbox/how-flow-direction-works.htm
Up to 4.7% of the cells in a 30 meter DEM
might be sinks.
Tarboton, D. G., R. L. Bras, and I. Rodriguez–
Iturbe. 1991. "On the Extraction of Channel
Networks from Digital Elevation
Data." Hydrological Processes 5: 81–100.
http://dx.doi.org/10.1002/hyp.3360050107

Flow Direction to Find Sinks
Sinks are assigned a value of the sum of their
possible directions.
The results do not give the actual flow
direction grid that we want, they give us a
raster with values 1-255. We can use this to
identify sinks.

I zoomed into a small AOI. These
dots are all sinks and peaks in the
data that we need to fix. Notice
how an apparent stream bed has
a number of low and high values.

Sinks
After running the Sink tool in the previous
slide, you need to run a Fill tool.

Fill
• Now we can fix our data.
• We need to run Fill on the original mosaic
raster. Then we will run the Flow Direction
again.
• Fill will create a depressionless DEM.

5. Flow Direction
We can run flow direction and be condifent in
our results.

Flow Direction
• Now we can do a flow
direction analysis on a
depressionless DEM. Notice
the difference in our result
values now versus using the
uncorrected data previously!
• We can include a drop raster
that models drops in
elevation too.

6. Flow Accumulation
We now need to determine how many cells
are flowing into each cell.

Flow Accumulation
• Now that we know flow direction we can do
a number of additional analyses.
• Flow Accumulation counts the number of
cells that flow into a particular cell.

Flow Accumulation
• Deriving streams from flow accumulation requires examining
your data and that you make a threshold decision.
• Here I have made two classes of the flow accumulation grid, as a
result any cells with more than 5000 cells flowing into them will
be recognized as part of the stream network.

AOI Flow Accumulation Classification

AOI Flow Accumulation Classification
over satellite image (visual check).

6. Basins and
Watersheds
We create basins (automatic) and watersheds
through manually identifying pour points.

Basins and Watersheds
• The function “Basin” will automatically
calculate the basins in your data set using
flow direction.
• There is a way for us to model specific
watersheds by establishing pour points and
looking at flow accumulation.
• Let’s look at how to do both of these and
then compare our results. Watersheds will
take longer, so let’s start by making basins.

Watersheds
• Pour points are the downstream outlets of
the watershed.
• You can upload a predetermined set of pour
points (based on known locations) in a vector
file or you can establish your own set of
points in the interface.

Watersheds
• In this process, you will first need to create a
geodatabase feature class for your points.
• You can do this in ArcMap (ArcCatalog
window) following ESRI instructions:
http://desktop.arcgis.com/en/arcmap/10.3/
manage-data/databases/create-a-feature-
class-in-a-database-in-arcgis.htm

Watersheds
• Call the new layer “pourpoints”.
• You’ll have to choose a coordinate system.
• We’ll use the same coordinate system as
our original satellite data, you can see
below: GCS_WGS_1984

Watersheds
• Click through accepting
defaults.
• When creating the new
feature class, create a
field called UNIQUEID
using Short Integer Data
Type.
• This will be used to
identify watersheds.

Pour Points
• Load the new layer
you created.
• Start editing the file.
• You may need to
activate the Editor
Bar and open the
Create Features
window to make the
points.

• Add points to
your new feature
class as close as
possible to the
stream network.
• Assign an
“uniqueid”
number to each
point.
• Save your edits
and stop editing
when done.

• Snap pour points to the highest
point of flow accumulation near
them.
• Snap to Pour Point will do this
and it will convert the points to
a new raster grid.
• Try several snap distances to
avoid having all points go the
same location (snap distance to
large) or miss the stream
network completely (snap
distance too small).
• We have a geographic
coordinate system so the
distance is measured in decimal
degrees (1 decimal degree is
~111 km at the equator but
changes further from the
equator).

• Snap pour point distance has big
impacts on watershed generation.
• Distance 5 (~555km) led all my
points to be collapsed to one
point.
• Distance 0 (no movement) led me
to have one good watershed and
one poor watershed.
• Distance 0.005 (roughly 555
meters) allowed me snap my
points to the highest flow cells and
keep three watersheds.

Original vector pour points (yellow) shown next to
the new raster pour points snapped to the highest
flow accumulation point within roughly 555 meters.

Generate Watershed
Using the three different snap pour
points raster to generate
watersheds, I found that the 555
meters snap measurement gave
the best results for this region.

Erroneous watershed generated
when Snap Distance = 0 km
Only one
watershed
generated
when Snap
Distance =
555km
Three
watersheds
generated when
snap distance =
555 meters

7. Stream Network
Using raster calculator and other functions to
create a vector stream network.

Creating a Stream Network
• In order to get our raster streams
into a vector format and to perform
some other analysis, we need to
make a raster that only shows our
streams.
• We will use Raster Calculator
(located in the toolbox Spatial
Analyst > Map Algebra).
• Use the formula on the following
slide to generate a new raster with
only the streams represented.

This conditional function creates a new raster “streams” wherein all cells
that had the flow accumulation value greater than 5000 will be given the
value “1” and all other cells no value.
Con(“FlowAcc” > 5000,1)

Creating a Stream Network
Linear raster
stream network.

• We can perform Stream Link (to assign
unique values to branches of the stream
network).
• Also, look at stream order using Shreve or
Strahler approaches.

• Shreve adds cumulatively
saying that 1+1=2, 2+3=5, and
2+2=4.
• Strahler says that when 1+1=2
and that 2+3 = 3 and that
2+2=3.
• This makes a big difference in
the number of stream orders
for a large region!
• I opted for Strahler.

We can now convert these ordered steams to a vector
(polyline) feature using Stream to Feature.
This gives us a feature
class with attributes for
the nodes and the order
(“grid code”) as well as
the length.
The length is in decimal
degrees, so we need to
fix this measurement by
projecting our data set
from a geographic
coordinate system to a
projected coordinate
system.

Projections and
Calculating Length
We need to project our data into a projected
coordinate system in order to accurately measure
length and areas.
There are two ways to do this calculation:
1.Project our data into a new feature class in a
geodatabase (automatically will calculate length and
area in meters).
2.Project our data into a shapefile, add data fields,
and calculate geometry for the new fields.
We’ll take this first approach using the geodatabase.

Projections
• First find a projected coordinate system that
is appropriate. UTM Zones will probably
work for you.
• For my data I used UTM Zone 33N (which
covers the majority of my region).

Find Your UTM Zone
http://www.dmap.co.uk/utmworld.htm

Projections
• EPSG (European Petroleum Survey
Group) assigns a unique number to all
projections.
• I add the EPSG number to my file
name to identify the projection. The
EPSG for UTM Zone 33N is 32633. You
can find your EPSG number using the
above link or by looking at any
projection via ArcCatalog or ArcMap.
• You can reproject directly to a feature
class (gdb) or a shapefile.

Project to a feature class
No geographic
transformation is
needed as we are
using the same
datum.

Calculating Length
(geodatabase)
• This is done for you in the geodatabase. See
Shape_Length (this is measured in meters
because we choose UTM Zone).

Calculating Length (shapefile)
If you took the shapefile approach, calculation will be different. Add a new
field, right click on the field and choose Calculate Geometry, Now you should
see the calculation in your attribute table.

You can now change the Symbology (try line
width and/or colors) on your new vector stream
network to visually represent the stream orders,
section length, or other attributes.

9. Convert to Polygons
and
Clean Up
Making the analysis accessible as vector files
so they can be used in a wide array of
platforms.

Basin to Polygon
• Open up the Basin raster attribute table and
sort by count.
• By selecting the row you will be able to
visually identify and select the basin near
your watershed.

Basin to Polygon
• The only function you need now is Raster to
Polygon.
• If you don’t select parts of the raster, then
you can get all the basin polygons using the
above function.

Basin to Polygon
Now you should have your new basin polygon
extracted as a vector file from all the basins originally
in the raster file.

Watersheds to Polygon
• Like Basins, we use Raster to Polygon.
• Yet, something funny happens… Open the
attribute table to see.

• We now have five watersheds because in
converting the raster to a polygon, some cells
were cut off and formed new features.
• Select one of the areas with small or no
Shape_Area (again this is in decimal degree
because we have not projected the raster data).
• Zoom to the selected area (this is an option in
the menu).

Here we see the original raster
watershed below the vector
watershed displayed on top.
We have two options, delete the
hanging area or merge the
features.
Given the small size, I simply
deleted the small polygon features
in the table (you may need to turn
on editing to delete).
I then projected the feature class
in the geodatabase. I now have my
three watersheds and the
measurement of area in meters.

10. Comparing Basin to
Watersheds

We now have a basin and
rivers.

We can compare our basin to
our watersheds and rivers.

We can compare our basin to
our watersheds and rivers.
• When we decided to locate pour points, we
ended up not capturing the entire basin and
even including part of another basin.
• This could impact field decisions.
• For example, if we were collecting flow
information with monitors located in the field
we might decide to change the location of our
pour points (monitors) to more accurately
represent the entire basin and maybe even
capture 1-2 more basins with the same amount
of monitors.

11. Answering the
Research Question

Does the basin or any small
watershed cross international
borders?

Does the basin or any small
watershed cross international
borders?
• Yes, the basin appears to cross the border, it is largely in
Cameroon with small parts of it in Nigeria.
• As well there were other basins that, appeared to cross
the border (largely in Nigeria with small parts in
Cameroon).
• We could continue on with this analysis identifying and
quantifying the overlap of basins throughout this region.
• Given our findings, we might suggest that the basin and
watersheds overlapping the border are reasons to
explore an international conservation area or an
international agreement on watershed management.

Looking Back
You now know how to:
• Get SRTM 1 Arc-Second (30 meter resolution) for free.
• Merge raster grids into mosaics.
• Derive streams, stream orders, basins, and specific
watersheds from the data.
• Convert raster grids into vector features.
• Calculate area and length.
• Create and analyze evidence for geographic questions.
More information about this region can be found from
organizations such as BirdLife International:
http://datazone.birdlife.org/site/factsheet/6112

End
Did you find an error?
Could something be more clearly explained?
Did you adopt and adapt these materials?
Let me know at:
arthur.green@mail.mcgill.ca
arthur.green@geog.ubc.ca
@greengeographer
Watershed Delineation by Arthur Gill Green is licensed under a Creative Commons
Attribution-ShareAlike 4.0 International License.
Based on work at http://greengeographer.com/.

Watershed Delineation in ArcGIS

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