Watershed Modeling
Using ArcView
Timothy N. Loesch
2000 Minnesota GIS/LIS Consortium
Watershed Modeling Workshop
 Welcomes and Introductions
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Who the heck am I?
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Work in the DNR MIS Bureau - GIS Section
First involved in GIS in 1985
Interest in Education, Programming and Hydro
e-mail - tim.loesch@dnr.state.mn.us
Who the heck are you?
• Name, Rank & Serial number
• Background Bio Information...
• ArcView Experience?
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 Class Content
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Overview Hydrologic Modeling Process in GIS
Overview of Digital Elevation Models
Explanation of Processes and Procedures
Attributes Derived from DEMs
Implementation in ArcView
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 Class Timetable/Agenda
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9:00 - 9:15 - Welcome and Introduction
9:15 - 10:00 - Hydrologic Modeling in GIS
• Description of terms
• Discussion of Digital Elevation Models (DEM)
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10:00 - 10:15 - Preprocessing DEMs
• Filling Sinks
• Determining Sink Depth
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 Class Timetable/Agenda
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10:15 - 10:30 - Break!
10:30 - 11:30 - Surface Parameters
• Flow Direction
• Flow Accumulation
• Flow Length
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11:15 - 12:15 - Delineate Watersheds
• Delineation of Contributing Areas
• Delineation of Watersheds and Basins
• Summarizing Watershed Parameters
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 Class Timetable/Agenda
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12:15 - 12:45 - Lunch
12:45 - 2:30 - Map Overlay and Predictive
Equations
2:30 - 2:45 - Break
2:45 - 4:00 - Extending the Hydrologic Model in
3D
4:00 Wrap-up and questions….
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 Hydrologic Modeling in GIS
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The Shape and characteristics of the earth’s surface
is useful for many fields of study.
Understanding how changes in the composition of
an area will affect water flow is important!
• What happens when residential development occurs?
• How does this affect the watershed?
• How can these affects be mitigated?
– Best management Practices
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Topographic Maps
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Traditional watershed delineation has been done manually
using contours on a topographic map.
Outlet Point
A watershed boundary
can be sketched by
starting at the outlet
point and following the
height of land defining
the drainage divides
using the contours on a
map.
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 Drainage system - The area upon which
water falls and the network through which it
travels to an outlet.
 Drainage Basin - an area that drains water
and other substances to a common outlet as
concentrated flow (watersheds, basins,
catchments, contributing area)
 Subbasin - That upstream area flowing to an
outlet as overland flow
 Pour Point - A location at which the
contributing area can be determined.
 Drainage Divide - The boundary between
two basins. This is an area of divergent flow.
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 Stream Network A stream network is
composed of those areas where concentrated
flow is occurring.
 Stream Ordering
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Shreve - When two links intersect, their
magnitudes are added and assigned to the
downslope link.
Strahler - Stream order only increases when
streams of the same order intersect
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 Curvature - Curvature quantifies the shape of
the surface to understand erosion and drainage
process
• convex (bulging)
• concave (bowing)
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Profile Curvature - Curvature of a surface in
the direction of slope.
• Concave - slow flow and deposition
• Convex - Increasing slopes and erosion
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Planform Curvature - Curvature of a surface
perpendicular to direction of slope
• Convex - Divergent flow indicating ridges
• Concave - Convergent flow indicates valleys
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 Raster Data Structure
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Much of the data we will use in this class will be
“Raster” data.
Raster formatted data is much more suitable for
many types of landscape modeling, including
hydrologic analysis.
Inputs such as elevation can only be processed as a
raster data set
Raster is Faster, Vector is Corrector
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Raster
Vector
Real World
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 Most Raster data is created
by Rasterizing existing
Vector data
 Transparent grid is placed
on map.
 Each cell is assigned a
numeric value according to
the dominate attribute type
found in cell.
 Integer and Real values are
valid
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 Digital Elevation Models (DEM)
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To do all of this stuff you need data.
A DEM is a digital representation of the elevation
of a land surface.
X,Y and Z value
The USGS is the major producer of DEM’s in the
Nation
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• DEM’s consist of an array of data representing elevation
sampled at regularly spaced intervals
Y
ELEVATION
VALUES
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The elevation values of the DEM can be grouped into
intervals. Each interval is then represented by a different
gradient color.
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 Two Scales of DEMs Available for Minnesota
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1:24,000 Scale
• Level 1 - 30 meter spacing
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Errors up to 15 meters inherent in data
Developed using automated methods from air photos
Systematic errors evident as banding
Not appropriate for hydrologic modeling
• Level 2
– Matches map accuracy of 1:24,000 scale quads
– Developed by scanning published quads
– Appropriate for hydrologic modeling
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1 Degree (~250,000) scale - 93 meter spacing
– Appropriate for regional analysis
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Metro Area done by the Met Council
For more information look at:
http://www.lmic.state.mn.us/bmap90/dem/dem.htm
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ArcView Exercise #1 - Viewing DEMs
1) Start ArcView by clicking on the ArcView Icon on the desktop that says HYDROMOD.APR
2) This opens an ArcView Project that we will be working in for the remainder of this workshop. It has the
Spatial Analyst Extension Loaded and a couple of predefined Views and Themes.
3) Open the View called Digital Elevation Model.
4) This View contains a digital elevation model (DEM) for a small (7400 acres) watershed in Rice county
Minnesota. This View contains 4 themes:
Watershed Basin - Watershed delineated in 1995
Roads and Highways - From MNDOT
Streams and Ditches - Digitized from USGS Digital Raster Graphics (DRGS)
Original DEM - A hydrologically adjusted 1:24,000 scale Level 2 DEM from the USGS with a
cell resolution of 10 meters.
5) Since we want all of this to be bounded by the incredible graphics you can create using GIS let’s start by
creating a Hill-shaded view of the Original DEM Theme. To do this make the “Original DEM” theme active
and then accessing the Compute Hillshade option in the SURFACE menu. Accept the default values of
Azimuth and Altitude, then press OK to generate the grid. Once it’s done processing a new theme will be
added to the view called “Hillshade of Original DEM”. Turn it on and view it. It’s a pseudo-3D perspective
based on a light source in the upper left. Then Turn it off.
Now hide the legend of this theme by making it active and then using the THEME menu, Hide/Show Legend
option.
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ArcView Exercise #1 - Viewing DEMs
6) We can now use the hillshade theme to act as a brightness theme for our elevation.
To do this access the legend editor for the “Original DEM” theme. Then press the
Advanced button in the lower left hand corner of the Legend Editor dialog. Fill out
this dialog as it appears to the right. Then press the OK button and Apply the legend.
NICE HUH?!
7) Turn on the other themes in your view (Roads and Highways, Streams and Ditches, and Watersheds). This
shaded relief view is really a great way to view data. It adds a nice perspective to the landscape and when the
watersheds and streams are displayed you can do some valuable visual analysis and interpretation.
8) Now add a couple more themes. There are two images in the C:\HYDROMOD subdirectory that you should add
to this View. One is a Digital Orthophoto Quadrangle (DOQ.TIF) and one is a scanned 1:24,000 USGS quadrangle
map called a Digital Raster Graphic or DRG (DRG.TIF). To add these to your View document use the VIEW menu,
Add Theme option. Make sure you set the Data Source Type selection to IMAGE. Select the two images and then
press the OK button.
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ArcView Exercise #1 - Viewing DEMs
9)Edit the property sheets of these images by making them active and using the THEME menu, Properties option.
Set the name of the DOQ to “Digital Ortho Photograph” and the name of the DRG to “Digital Raster Graphic”.
10) Then re-order these themes so that the Digital Ortho Photo theme is on the bottom of the table of contents and
the Digital Raster Graphic theme is on the top.
11) Edit the legend of the Digital Raster Graphic Press the Colormap button as shown. Set the color of Class 1 to
transparent. Then turn on the image and do some exploring by zooming in and out and panning around to learn
more about our project area.
12) Now Save Your Project and minimize ArcView.
Transparent
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Colormap button
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Class 1
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Elevation data draped
on Shaded Relief
Notice that this watershed is
characterized by a flat upper
end with few tributaries and a
large valley, more relief,
increased drainage in the
lower stretches.
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 Preprocessing DEMs
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DEMs typically require some type of preprocessing
prior to hydrologic modeling to remove errors
inherent in the data. This type of processing can
greatly increase the accuracy of a DEM.
Methods of producing DEMs rarely include a
“Drainage Enforcement” Algorithm to remove
these errors.
Primary error found in DEMs are “Sinks”
• A sink is an erroneous depression created by the
DEM interpolation routine
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An example Sink
100
100
100
100
100
97
96
95
Stream
100-meter Elevation
contour
100
100
Sink Area
100
98
99
100
100
100
100
100
100
100
Cell Elevation
101
Cells containing the contour are assigned the value of the contour,
all other cells are interpolated. Sinks are always possible in areas
where contours converge near a stream.
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 Preprocessing DEM’s (continued)
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Drainage Enforcement
• Using existing hydrographic features to remove
these errors is essential to produce accurate
watershed basins.
• Sinks are usually small and cause drainage basins to
be incorrectly delineated
• If a stream is not present elevations in sink areas are
raised to elevation of saddle so that water can drain
• If a stream is present elevations of saddles are
lowered to that of the sink so that water can drain.
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 Preprocessing DEM’s (continued)
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ArcView can Fill sinks but not enforce other types
of hydrologic information
ANUDEM program from Australian National
University which is used to enforce known
drainage patterns
Arc/Info has a routine called TOPOGRID which is
an implementation of this software
• Must be careful not to remove real sinks
• Lake areas should be flat!
• Handles point or contour elevation data
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 The flat areas in this shaded
elevation model are wetlands
and lakes.
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 ANUDEM uses stream data to
enforce drainage.
 Stream data must have correct
directionality and connectivity.
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• This DEM is considered to be
“Hydrologically Adjusted”
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ArcView Exercise #2 - PreProcessing DEMs
1) ArcView should still be started and the project, HYDROMOD.APR should still be loaded. If not, then start
ArcView and Open the project which can be found in the C:\HYDROMOD folder.
2) Using the mouse, make the theme “Original DEM” active.
3) To remove sinks we will use the DNR HYDRO menu Fill Sinks option (do it now). This option will process the
DEM and remove all sinks that are found. In this implementation of the fill routine you do not have any options for
specifying size or area of sink to remove. It simply iterates through the data until all sink areas are removed.
4) You will be presented with a prompt “Create Sink Shapefile and Statistics?”. This will find the sinks that were
filled and create a shapefile of them with statistics that represent the size and depth of the filled areas. Answer YES to
the prompt. Again, ArcView will buzz a little and when done you will be asked to supply a name for this new file Take the default name SINKPOLY.SHP and make sure it’s in the C:\HYDROMOD folder.
5) When complete, you will have two new themes added to your view document. A Grid theme called “Filled Original
DEM” and a polygon theme called “Sinks with Statistics”. Turn off all your other themes and turn on the “Sinks with
Statistics” shapefile. See anything? There’s one small polygon in the center of the watershed. To view the statistics,
make the “Sinks with Statistics” active and then use the THEME menu, Table option to open the table. You will see
that the sink is 700 square meters in size and has a depth of 1 meter.
That’s pretty good but remember, this is a hydrologically adjusted DEM.
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ArcView Exercise #2 - PreProcessing DEMs
6) Now turn off all your themes and turn on the “Filled Original DEM” theme and make it active. You
probably don’t notice much here and it’s not the same color scheme as the original. Let’s create a new legend
using the DNR Hydro menu, Interval Legend option.
Use an interval of 1, a Base value of 290 and a Max Value of 375
and press the OK button.
Then select a color scheme of interest. One of my favorites is “Full Spectrum” but you can choose your own
or try a few if you’d like
8) Wow, edit the legend and change the NO Data value color to transparent. Then use the THEME menu
Hide/Show Legend option to hide that really long legend and make some room on our table of contents.
Then, move the two new themes down a couple of notches on the Table of Contents so that you can display
the vector themes (Roads and Watersheds) on top of them.
9) You now have a depressionless DEM, one that can be used for further Hydrologic Analysis. We want to
save this file so, make the “Filled Original DEM” theme active and then use the THEME menu Save Data
Set option to save this file to C:\HYDROMOD\DEMFILL.
10) Now save your project!
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 Generating Surface Parameters
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Flow across a surface will always be in the steepest
down-slope direction
Known as “Flow Direction” this is the basis of all
further watershed modeling processes.
Once the direction of flow is known it is possible to
determine which and how many cells flow into any
given cell!
This information is used to determine watershed
boundaries and stream networks.
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 Slope - Many of the parameters that
we will discuss and generate in this
workshop are based on slope.
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Usually calculated on a 3x3 window with
the center cell being the target cell.
Slope is calculated from the center cell to
each of the 8 neighbors
Greatest slope is assigned to the center
cell
Flow direction is that way…..
12 13 13
11 10 10
8 10 9
Target Cell
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 Generating Surface Parameters - Flow Direction
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In ArcView Spatial Analyst, the output of a Flow
Direction is a grid whose values can range from 1
to 255 based on the direction water would flow
from a particular cell. The cells are assigned valued
as shown below.
Target Cell
32 64 128
16
1
8 4 2
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 Generating Surface Parameters - Flow Direction
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If a cell is lower than its eight neighbors, that cell is
given the value of its lowest neighbor and flow is
defined towards this cell.
If a cell has the same slope in all directions the
flow direction is undefined (lakes)
If a cell has the same slope in multiple directions
and is not part of a sink the flow direction is
calculated by summing the multiple directions
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 Flow Direction
Original Surface
100
Flow Direction Surface
100
100
100
97
96
95
100
98
99
100
100
100
100
100
100
101
100
94
2
2
2
1
1
1
128
128
128
128
64
32
128
64
64
32
80
1
100
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 Flow Direction
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 Generating Surface Parameters - Flow
Accumulation
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If we know where the flow is going then we can
figure out what areas (cells) have more water
flowing through them than others.
By tracing backwards up the flow direction grid we
can figure the number of cells flowing into all cells
in a study area
Accumulated flow is calculated as the accumulated
number of all cells flowing into each downslope
cell.
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 Generating Surface Parameters - Flow
Accumulation
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For an accumulation surface the value of each cell
represents the total number of cells that flow into
an individual cell
Cells that have high accumulation are areas of
concentrated flow and may be used to identify
stream channels.
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Flow Direction Surface
2
Flow Accumulation Surface
2
2
1
1
1
128
128
128
128
64
32
128
64
100
32
80
1
0
128
0
0
0
3
8
15
0
2
2
0
0
0
0
0
0
0
0
18
0
64
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Flow Accumulation Surface
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 Generating Surface Parameters - Flow Length
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Another parameter that can be calculated is flow
length
Flow length is defined as the length of the longest
flow path within a drainage basin or watershed.
This measure is often used to calculate the time of
concentration of a basin.
ArcView calculates the flow length for each cell
within the basin. The cell values represent the
length of flow to an outlet from that point.
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Flow Direction Surface
2
Flow Length Surface
2
2
1
1
1
128
128
128
128
64
32
128
64
100
32
80
1
128
144.6
114.6
84.6
30
102.3
72.3
42.3
144.6
114.6
84.6
72.3
84.6
156.9
126.9
114.6
126.9
0
132.3
64
0
30
Assuming 30 meter sq. cells
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Flow Length Surface
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 The Whole Process - a Flowchart
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ArcView Exercise #3 - Generating Land Surface Parameters
1) ArcView should still be started and the project, HYDROMOD.APR should still be loaded. If not, then start ArcView
and Open the project which can be found in the C:\HYDROMOD folder. Make sure that the “Digital Elevation Model”
View is open and active.
2)The first step in this process is to create the Flow Direction theme. We use an elevation theme as our base so make the
“Filled Original DEM” theme active and then use the DNR HYDRO menu Flow Direction option to create this grid.
The computer will whir for awhile and then add a new theme called “Flow Direction”. Turn this theme on. Notice the
cell values in the legend. Remember that 1 represents east flow direction etc. You may also notice that this is really
showing us the aspect of the landscape. Edit the legend for this theme and then press the Load button to load the legend
file: C:\HYDROMOD\FLOWDIR.AVL. Use the defaults and apply the legend.
4) Now we’re ready to create the Flow accumulation grid. To do this, make the “Flow Direction” grid active and then
use the DNR HYDRO menu Flow Accumulation option to create this theme. You will then be requested to enter a
“Flow Accumulation Weight Grid”. We don’t have one at this time so press the CANCEL button. At this point the
computer will process for a bit and then add a new theme to your View called “Flow Accumulation” (surprise).
5) Turn on this grid and have a look. All you really see in this display is one area that look like they could be stream
channel. What you’re looking at here is an artifact of the legend. To create a better looking view of this surface we can
modify the legend to show more of the concentrated flow network. To do this create a legend that has the following
classes:
0 - 100
100 - 10000
10000 - 25000
25000 - 50000
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ArcView Exercise #3 - Generating Land Surface Parameters
Then colorize the classes and apply the legend. You should see a much better display of the drainage
channels in the landscape. Let’s assume that any cell with less than a 100 cell accumulation is most likely
overland flow so make this class transparent in your legend. Also make the NO DATA class symbol
transparent.
6) Now let’s calculate flow length for every cell in the watershed. To do this make sure that the “Flow
Direction” theme is active and then press the DRN HYDRO menu Flow Length option.
7) You will be asked if you want to calculate the flow length to an outlet. In other words, do you want
ArcView to find an outlet and calculate flow length from there. The answer in this case is yes. ArcView will
then create the flow length grid called “Downstream Flow Length” and add it to your view document.
8) Examine the grid. What does this tell you? Save your project……
9) Let’s look at how stream ordering can be created from this “Flow Accumulation” theme. To save some
time I’ve already created these data sets. You will find two Grid Data sets in the C:\HYDROMOD
subdirectory, one called STRAHLER and one called SHREVE. Add these two themes to your View
Document. Notice the difference between the two systems, most notably the difference in maximum stream
order. You could convert these to Vector representations of flow channels if desired.
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ArcView Exercise #3 - Generating Land Surface Parameters
10) Another way to visualize flow paths is to create a vector version of them. Do this using the DNR Hydro
menu Create Stream Network option. You will be required to enter a Flow Accumulation theme, a Flow
Direction Theme, and a flow accumulation threshold. For the Flow accumulation threshold use a value of 25.
11) Save the file using the name provided.
12) Once complete a new theme will be added to your view called Stream Network Shape - (25). Turn it on.
This shows you all flow paths that drain an area of 25 or more cells (250 square meters).
If you look close, you’ll see that these drainage paths follow the flow direction grid…..
13) If you’d like you can create another one of these and try a threshold of 10 and see what happens…
14) Save your project and minimize ArcView….
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 Generating Watersheds and Catchments
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Now you are ready to start delineating watershed
basins
Watersheds or Catchment areas are the basis for
many hydrologic analysis
Watersheds can be created for any spot in a dataset.
• Culverts, bridges, gauging stations etc.
• Points can be interactively selected by the user or
you can specify a minimum catchment size.
• Points become the lowest point on the boundary of
the watershed and are therefore referred to as pour
points
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 Generating Watersheds and Catchments
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Pour points can be input as an entire theme or they
can be created by selecting a point on the screen.
Using a Flow Accumulation Theme you can
specify a minimum sized watershed and let Spatial
Analyst automatically generate watersheds for an
entire area.
• Generally not useful for most purposes
• Arbitrarily assigns watershed outlets
• Has trouble assigning leftover areas….
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Pour Point
Contributing Area
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Minimum Catchment Area = 5000 cells
Minimum Catchment Area = 10000 cells
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ArcView Exercise #4 - Watershed Delineation
1) ArcView should still be started and the project, HYDROMOD.APR should still be loaded. If not, then start
ArcView and Open the project which can be found in the C:\HYDROMOD folder. Make sure that the “Digital
Elevation Model” View is active and open.
2) What we’ll do first is create some watersheds for the entire study area based on a couple of points that are stored in
a file. These points represent stream gauges on the watershed and can be found in the file
C:\HYDROMOD\GAUGES.SHP. Add this theme to your project and turn it on.
3) To create the watersheds, make the “GAUGES.SHP” theme active and then use the DNR HYDRO menu
Watershed from Points option. You will then be requested to supply the grid theme that represents the Flow
Directions. Select the “Flow Direction” theme from the list and press the OK button.
You will be prompted with a “Conversion Extent” window. Set the Output Grid Extent to “Same as Original DEM”
and set the “Output Grid Cell Size” to 10.
4) You will then be required to specify the Field in the GAUGES Shapefile who’s values will be used to assign ID’s to
the new watersheds. Use the ID Field in this case.
5) Give the output file a name. Take the default in this case. The computer will crank the numbers for a while and then
add a new theme to the view called “Calculated Watersheds”. You will see that ArcView has created 3 basins for this
area based on the points in the file. To get a better view of them edit the legend and use the Advanced button to
specify a brightness theme for this theme. Use the theme “Hillshade of Original DEM” as the brightness theme. Then
apply the legend and view the data. Wow! What a country! Now turn off that theme.
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ArcView Exercise #4 - Watershed Delineation
6) As discussed in the lecture, you can also interactively create watershed by selecting a point of interest. To do this
we need to use the Watershed Tool (the tool with a “W” in it). Select the tool and then press the mouse on the View
on any location on the map that you want to see a drainage area. You will be requested to select a Flow Direction and
Flow accumulation grid once you select an area and a watershed will be generated and a theme added to your view
document for that theme. Try it and turn on the theme.
7) Interpretations? What do you think? Did it produce the watershed you expected? This process is REALLY
sensitive to where you place the point. If it’s not on a flow path, then it won’t create a watershed you expect. To help
this process, Turn on the Flow Accumulation theme to show you where the flow paths are. You may need to zoom in
quite a ways in order to select a point that falls on a flow path.
8) Try it for some other locations in the study area. Try creating a catchment area above a point where a road crosses
a stream. In the words of the infamous Bart Simpson, “Cool Man”.
9) Try using the RainDrop Tool. This tool will follow the flow direction from the point chosen to the outlet. You can
see where the water will flow if it fell on that spot. Use the RainDrop Tool (the Tool with an “R” in it) to select a
point on the View, you will be requested to supply a Flow Direction Grid and the flowpath will be created.
10) Save your project and experiment a bit.
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 Summarizing Information for Watersheds.



Once you have created watersheds for an area many times
you want to summarize other land surface variables that
fall within the watersheds.
This type of Information derived from raw data increases
your Understanding a system and how changes to that
system (land use etc) affect the process.
Two types of reports can be created
• Statistical summaries of numeric data (elevation, precip)
• Cross-tabulations of Nominal Data (land use, soil types)

This can be done easily with ArcView.
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 Summarizing Information for Watersheds.
Statistical Summary
Cross-Tabulation
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 Summarizing Information for Watersheds.

This type of information can then be used in reports or fed
to other analytical programs for further analysis.
• Hydrologic models
– HEC, HydroCAD, SWRRB, others….


• Graphs, charts can be used to view data
When using existing programs create a summary of
required inputs and see if you can generate them from the
spatial layers available to you via GIS.
More and more is being done to interface GIS and
Hydrologic models.
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ArcView Exercise #5 - Summarizing Watershed
1) ArcView should still be started and the project, HYDROMOD.APR should still be loaded. If not, then start
ArcView and Open the project which can be found in the C:\HYDROMOD folder.
2) What we’ll do first is summarize some information by watershed. Let’s start with land use/cover. In the
C:\HYDROMOD subdirectory you will see a GRID theme called LULCXRA3 that contains this information.
Add this theme to the View document and then edit the legend and load the legend file
C:\HYDROMOD\LULCXRA3.AVL. Then apply the legend and view the data.
3) Since this is nominal data (the numbers represent land use classes) you can’t perform statistics, you can
only do a cross-tabulation. To do this use the ANALYSIS menu Tabulate Areas option. We’ll use the
LULCXRA3 Theme as our Row theme with the DESC field as the Row Field. We want to summarize by
watershed so choose the “Calculated Watersheds” theme you created during Exercise 4 or as shown below.
When you are done, press the OK button. At this time a table will be created that shows the results.
Remember that the units here are in square meters.
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ArcView Exercise #5 - Summarizing Watershed
5) Close this table and make sure that your View document is active
6) Now let’s create some statistics for the elevations within each of the watersheds. To do this make your
“Calculated Watersheds” theme active and then use the ANALYSIS menu Summarize Zones option. You will
be presented with two inputs you need to provide (see examples below)
A) Pick Theme containing variable to Summarize: “Filled Original DEM”
B) Select Statistic to Chart: Area
7) A new table and a new chart will be created. What do you think? What could you do with these? Of course
you would pretty them up and make them interpretable but for now, we’ll just move on...
8) That’s all folks! Save your project and quit ArcView.
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 Map Overlay is the one of the bread-and-butter
abilities of GIS
 Cell-by-cell overlay is the process of taking two or
more raster files and comparing the values of each
of the files, cell by cell.
 The result is a map that identifies map coincidence
with the values of the output file based on some
user-defined process.
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 Raster Files can also support “Mapwide Overlays”


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
Take a mathematical equation and “plunge” it
through a set of registered map layers.
Each map layer is considered a variable
Each location is considered a case
Each value considered a measurement
 Very important stuff for modelers
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 Using the Flow Direction and Flow Accumulation
processes it is easy to compute a mass balance for
each cell in terms of:

S = P - I - F - E where,
•
•
•
•

S = surplus water per cell
P = input precipitation
F = infiltration
E = Evaporation
Cumulative flow over the net is then obtained by
accumulating S over the Flow Direction grid.
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 Predictive equations are another way to use map
overlay.
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
The Water Available for Runoff equation is used by hydrologists
use to calculate the amount of water that will be available for
runoff for a specific rainfall event, based on the type of soil and
land cover present at a location.
Using this information you could predict the amount of water
available for runoff at every cell in a watershed
This could then be accumulated across a flow direction surface
Very important to a road engineer who has to design a road that
crosses that drainage. After all, if they put in a culvert that is too
small there could be problems.
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ArcView Exercise #6 - Calculating Water Available for Runoff
PROJECT OVERVIEW
Let’s assume that you are managing a set of watersheds in Rice county and you wanted to determine the total
amount of surface water runoff that could be potentially derived from these watersheds. One of the classic
methods of producing this type of information is to use the Natural Resource Conservation Service’s (NRCS)
“Water Available for Runoff” equation. This equation predicts the amount of water that will be available as
runoff based on the land cover and soil conditions, present at a certain spot, based on a given rainfall amount.
That is, if I get an inch of rain, how much can I expect to produce as runoff on a cornfield in clay soils?
The landcover/soil information is based on a value called the Curve Runoff Number or CN. The CN is
derived by examining the Hydrologic soil grouping of a soil and the landcover present. CN values range from
0 – 100 and are an indication of the ability of the soil/landcover mix to absorb moisture. Concrete has a value
close to 100. Sandy soils covered by forbes have values in the 30’s.
These values are derived from tables produced by the NRCS that compare landcover with hydrologic soil
groups. Hydrologic soil group values range from A – D. “A” soils are light, sandy porous, well drained soils
while “D” soils are heavy, clayey, compact, and poorly drained.
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ArcView Exercise #6 - Calculating Water Available for Runoff
To predict the total water available for runoff you need to use the following equation:
Q = (sqr(p – 0.2 * (1000/c – 10))) / (p + 0.8 * (1000/c – 10))
Where: Q = total water available for runoff
p = precipitation
c = curve number
Thus, at a minimum we need two map variables, Precipitation and Curve number. Once we have these two
grids the equation can be “Plunged” through these map layers to produce an output layer that shows the water
available for runoff at each location in the watershed.
The flowchart on the next page illustrates the flow of control for this analysis. Flowcharts are useful because
you can map out the process, the commands and files you will need, and those you will create, during the
analysis.
Sometimes analysis can get confusing because of the number of files created during the process. This file
proliferation can be hard to manage if you don’t have a plan. On-the-fly planning is not the best way.
As you go through the analysis, print the name of the files you create on the flowchart sheet. The
documentation is important.
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ArcView Exercise #6 - Calculating Water Available for Runoff
1) ArcView should still be started and the project, HYDROMOD.APR should still be loaded. If not, then start
ArcView and Open the project which can be found in the C:\HYDROMOD folder.
2)Close the “Digital Elevation Model” View and open the “Runoff View” and have a look. You will see three
of the four themes that we need to do this project. The “Precipitation” theme represents rainfall from a
particular storm, “Hydrologic Soil Group” represents the hydrologic soil group for the soils in the watershed
and “Land Use and Cover” is the land cover in the basins. Look at the property sheets of these themes to get
an idea of their sources and view the data to get an idea of their spatial distribution.
3) Look at the flow chart, the first thing we need to do is create the curve runoff number grid by combining
the Hydrologic Soil Group and Land Use and Cover themes. To do this use the ANALYSIS: Map Calculator
option. Enter the following map equation:
[Hydrologic Soil Group].Combine( { [Land Use and Cover] } )
This command creates a new grid by performing a spatial coincidence overlay analysis of two or more grids.
The values in the new grid represent zones of unique combinations of values in the input grids. A new theme,
called “Map Calculation 1” will be added to your View.
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ArcView Exercise #6 - Calculating Water Available for Runoff
4) Open the table for this new theme by making the theme active and using the THEME: Table option. This
table shows you that there are 28 unique combinations of Hydrologic soil groups and Landcover. Each of
these soil/landcover combinations is then assigned a value of Curve runoff based on a table you can get from
the NRCS.
Normally you would edit the table, add a new item called CN (for curve number) and then enter in the values
based on the soil and landcover combination.
To save time I have already created this file so you can make this new theme active and then use the EDIT:
Delete Theme command to remove the theme. Then add a GRID theme using the grid: C:\HYDROMOD\CN.
Then edit the legend of this theme and create Unique Legend using the VALUE field. This field stores the
values of curve number for each cell.
5) Now we’re ready to do the analysis. To do this we will use the ANALYSIS: Map Calculator option.
You need to enter the equation for water available for runoff as:
( [Precipitation] - 0.2.AsGrid * (1000.AsGrid / [CN] - 10.asgrid)).sqr / ([Precipitation] + 0.8.AsGrid *
(1000.AsGrid / [CN] - 10.AsGrid))
Be careful when you enter this equation. It has to be just right for it to work properly. Use the mouse to enter
as much of this information as you can. You will need to add some parenthesis by hand to make it work.
Once you have entered the equation press the Evaluate command to execute the analysis.
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ArcView Exercise #6 - Calculating Water Available for Runoff
6) The machine will chuck away for a while (if you entered the equation properly) and when complete will
add a new theme to the view. This theme, called “Map Calculation 1”, contains cell’s whose values represent
the total amount of water that will be available for runoff based on the input rainfall event.
Edit the properties of this theme and name it “Water Available for Runoff”. Edit the Legend and classify and
symbolize as you see fit.
This grid, in itself, is an important piece of information. It is obvious that in the upper reaches of the
watersheds there is a few areas that have a high potential for soil erosion because they have a high potential
water available for runoff value.
7) Let’s take it a step further. Remember the Flow Accumulation grid that we created in earlier exercises. In
those exercises we created the accumulation grid where the grid values represented the number of cells that
would flow through it. Well, the Flow Accumulation command can take a weight grid and accumulate the
values of the cells in the weight grid and assign that value to the cells in the output grid.
To create the Weighted Flow accumulation theme we need the Flow Direction theme that was created earlier
in this Workshop. Let’s get that Theme. Open the View “Digital Elevation Model” and make the “Flow
Direction” theme active and then use the EDIT menu Copy option to copy this theme to the ArcView
Clipboard.
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ArcView Exercise #6 - Calculating Water Available for Runoff
Now close the “Digital Elevation Model” View and make the “Runoff View” active and then paste the theme
in the clipboard to this View using the EDIT menu Paste option.
Now, make the Flow Direction theme active and then run the DNR HYDRO: Flow Accumulation command.
You will then be asked to provide an Accumulation weight grid. Use the “Water Available for Runoff Grid”.
The resulting theme is a grid whose cells are coded based on the accumulated flow as water available for
runoff. Of course, this is a highly idealized view of the world and does not factor in things like friction, flow
length, timing and ponding but it is a start.
8) Now close all of your open ArcView windows, save your project and minimize ArcView.
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 There are a variety of other types of hydrologic
variables that can be generated using DEMs.
 Flow Direction Grids are the basis for a wide
range of dynamic modeling tools
 These can be calculated using ArcView and
Spatial Analyst
 See van Deursen and Burrough (1998) for more
information.
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 Wetness Index

wetness index = ln(As / tanB),
where:
• As = Contributing Catchment
Area in meters squared
• B = Slope of cell measured in
degrees
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 The Stream Power Index

w = As / tanB
 This is directly proportional to the
stream power

P = pgq tanB where:
• p = density of water,
• g = acceleration due to gravity
• q = overland flow discharge per unit
width
 An indicator of erosive power of
overland flow
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 The Sediment Transport Index

t = [As / 22.13]^0.6 * [sinB / 0.0896]^1.3
 This index characterizs the process of
erosion and deposition
 Presents the effects of topography on soil
loss
 Can vary along the length of a stream
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 These tools are useful for quantifying the properties and
characteristics of the watershed you happen to be
investigating.
 May help find areas for wetland restoration or areas where
bank stabilization is needed
 Relate stream processes to fish distributions
 Plan fish structures and habitat improvements
 Measure the effects of management practices on the
watershed.
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ArcView Exercise #7 - Calculating More Land Surface
Variables
1) ArcView should still be started and the project, HYDROMOD.APR should still be loaded. If not, then start
ArcView and Open the project which can be found in the C:\HYDROMOD folder.
2) Make sure that the “Digital Elevation Model” View is open and active. We’ll experiment with some of the
new tools we’ve discussed in the lecture part of this workshop. Rather than cluttering up an already cluttered
view we’ll make a new view and copy the “Filled Original DEM” into that view. Do the following:
a) make the “Filled Original DEM” Theme active
b) Use the EDIT menu Copy option to copy the theme to the clipboard
c) Make the Project Window active and create a new View.
d) Use the EDIT menu Paste option to paste the theme to this new View
e) Edit the View’s Property sheet and name the View “Extended Variables”
3) Now make the Wetness Index Map by making the “Filled Original DEM” Theme active and then
accessing the HYDRO menu Wetness Index option. Your little silicon subordinate will dutifully start
processing and when done, add a new theme called “Wetness Index” to your View. Open the Theme’s
property sheet and view the comments, they tell you the equation used for this analysis….
Edit the legend to make it visible. You could generate some statistics from this baby…..
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ArcView Exercise #7 - Calculating More Land Surface
Variables
4) Now make the Stream Power Index Map. Do this by making the “Filled Original DEM” Theme active and
then accessing the HYDRO menu Stream Power Index option. This will take a bit as the machine needs to
calculate a few temporary variables. When done a new theme called “Stream Power Index” will be added to
your View. Again, view the property sheet and see what equation was used…..
Edit the legend to make it visible.
5) Now make the Sediment Transport Index Map. Do this by making the “Filled Original DEM Theme active
and then accessing the HYDRO menu Sediment Transport Index option. The machine will crunch away and
create this derived product. Once it’s done it will add a new theme called “Sediment Transport Index”. View
the Property sheet.
6) Now, close the open windows in ArcView, save your project and minimize ArcView
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 Where is this all going?


More and more effort is being put into integration
of Hydrologic Models and GIS
Several Web sites are available
•
•
•
•
•
http://www.ce.utexas.edu/prof/maidment/
http://www.ciesin.colostate.edu/USDA/hydrology/models_soft.html
http://www.ecgl.byu.edu/wms.htm
http://www.esri.com/news/arcuser/arcuser498/hydrology.html
http://www.esri.com/library/userconf/proc98/PROCEED/TO400/PAP
400/P400.HTM
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 Where is this all going?

Right now most efforts are being put into software
that bridges the gap between existing software and
GIS.
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 Several vendors and Universities are putting
together integration Software
 ESRI has a really good CD that comes with some
training materials, software and data
 Obtain this CD by contacting ESRI at your local
office.
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 To Conclude



This was a brief introduction to the Hydrologic
Tools available within ArcView
Realm of Possibilities was the intent of this
workshop
Much more can be done with this stuff, all it takes
is some imagination and ideas on how to apply this
information.
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