GIS in Watershed Analysis
RESM 493q
Wed Nov 4
EPA Basins graphic
Applied GIS for Nat Sci
Why Watershed Analysis with GIS?

Watershed analysis is a spatial issue

Used to analyze regional stressors
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Regional stressors
1.
Stream sedimentation
2.
Habitat loss
3.
Forest fragmentation
4.
Acid mine drainage
5.
Acid rain
6.
Flooding
7.
Invasive and non-native
species
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Impacts of human activities
Timber
Mining
Agriculture
Stream alterations
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Watershed based frameworks

Integrative approach

Legislative roots in the 1972 Clean Water Act

Goal to clean up and protect US water bodies from point and nonpoint sources
Designated uses – evaluated as part of the Clean Water Act
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Watershed cataloging units
USGS Regions (2 digit)
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Watershed cataloging units
USGS Sub-regions (4 digit)
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Watershed cataloging units
USGS Sub-basins (8 digit)
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Watersheds for WV
USGS 8-Digit sub-basins in WV
n=33
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Watersheds for WV
USGS 8-Digit sub-basins in WV
n=33
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Watersheds for WV
NRCS 10 and 12 watersheds in WV
(5th and 6th level)
n=342
n=745
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Watersheds allow us to examine issues

Streams not meeting designated uses
Cold water fishery
Warm water fishery
Safe for water contact
recreation
Public water use
Agricultural use
Industrial water use
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Protect water resources

High quality streams
DNR Stocked streams
Tier 2.5 - Reproducing
trout streams
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Issues

More local watersheds needed

One to one relationship between land cover runoff and receiving
stream segment

Track runoff from land to stream
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Example of scale differences
DNR Stocked streams
Tier 2.5 - Reproducing
trout streams
Impaired streams
Watershed boundaries
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Example study area
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Previous method

Delineate watershed boundaries using the topography to guide
us
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Manual method
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subwatersheds
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GIS and water resources

Geographic information systems (GIS) is a valuable tool in
water resources management
GIS in watershed management:

Elevation surface is key

Delineate watersheds

Track flow from a point

Find intermittent stream paths

Calculate drainage areas
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GIS use in the hydro cycle
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Surface hydrology

Determine where the water came from and where it is going

Describe the behavior of water as it moves over the surface of
the earth
• Analysis starts with the creation of a
hydrologically correct surface (no sinks or peaks
interrupting drainage direction)
• Include the entire drainage
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
Rainfall - runoff
relationships

Runoff - above curve

Losses - below curve
Runoff
Surface hydrology
Time
Runoff
 depression storage
 evapotranspiration
 infiltration
Rainfall
 interception
Time
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Runoff curve from gauge
Source: www.americanwhitewater.org
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GIS and surface hydrological modeling

Attempts to explain the hydrological cycle (best suited for
surface runoff)

Tools are available in GIS to measure, quantify and display
surface runoff

Watershed based (must consider the entire drainage to
effectively model interactions)

Begins with an elevation surface
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Elevation surface or grid

The starting point for all hydrological modeling in GIS

USGS Digital elevation model 30m or 10m elevation cells, or 3m for WV

Each cell or grid represents a value for the elevation
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Sinks

Depressions in the DEM where water gets trapped

A sink prohibits calculating future flow direction grid values.

A sink occurs when all neighboring cells are higher than the processing
cell.

Sometimes they are natural features!
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Flow direction

Created from an elevation surface

Direction values are assigned

Flow direction grids are used in many hydro GIS functions
32
64
16
8
128
1
4
2
Flow direction map
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Flow direction

Created from an elevation surface

Direction values are assigned

Flow direction grids are used in the other flow functions
64
16
8
128
1
4
2
Flow direction map
32
64
16
8
128
1
4
2
Flow direction map
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Flow accumulation

Created from a flow direction grid

Accumulates all uphill water into each cell

Cells with high flow accumulation area are areas of concentrated
flow

Output cells with a flow accumulation of 0 may be used to identify
ridges or watershed boundaries.
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Flow accumulation

The accumulated flow is based upon the number of cells flowing into each
cell in the output grid. The current processing cell is not considered in this
accumulation.
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Summary of spatial techniques and tools available within a GIS
Elevation grid
Fill sinks
Any sinks?
Yes
Flow direction
No
Flow accumulation
Allows for additional landscape based analysis
Stream delineations
Watersheds
Stream order
Riparian areas
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GIS data needed for hydro work
Elevation
Flow
direction
Flow
accumulation
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Applications
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Lab work of basic hydro functions

ArcGIS9 toolbar
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Delineate watersheds
Interactive
delineation
for a point
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Delineate watersheds
Hydro toolbar
in ArcGIS 9
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Track flow from a point
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Track flow from a point
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Find intermittent stream paths
Mapped
streams
from the
1:24,000
topomap
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Find intermittent stream paths
Mapped
streams
from the
1:24,000
topomap
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Find intermittent stream paths
Intermittent,
debris slides,
accumulators,
path of easiest
descent exist
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Find intermittent stream paths
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Calculate drainage area
Flow accumulation grid = tells us the
number of cells of a certain area that
flow to a point
Drainage
area?
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Calculate drainage area
Flow accumulation grid = tells us the
number of cells of a certain area that
flow to a point
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Calculate drainage area
So, if there are 280,721 cells that flow to that
location…
and each cell is 3m by 3m in size (9sq meters)
Then
The total drainage area is (280,721) * (9) =
2,526,489 sq meters
Or
2,526,489 sq meters * 0.00024718 = 624 acres
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Stream Flow Modeling

Why try to estimate it?

Important for water quality
modeling

Permitting discharge sites

Biological importance

Recreation

Flood forecasting

Influenced by human factors

More USGS gauges off-line
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Using drainage area to estimate stream flow

More of a “back of the envelope” approach

Uses ratio between drainage area and gauged flow to estimate flow

Assumes stream flow is the same per unit area as a nearby gauge

Best for sites near gauged locations (ratio .3 to 1.5) and on the
same watershed
Upstream Sites
Drainage area = 15
Downstream Site
Drainage area = 75
Gauge
Drainage
area = 50
Area good to go
for estimating
flow with the
ratio method!
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Steps for ratio method
1.
Find USGS gauge location downstream and in the same watershed
2.
Summarize the recorded flow from gauge (USGS web site)
3.
Find the FAC values at gauge and location of interest
4.
Apply ratio of flow to drainage area at gauge to all upstream
drainage areas in the FAC grid
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USGS gauge data (http://waterdata.usgs.gov/
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Example

Big Creek gauge at mouth of the watershed has a drainage area of
187,003 FAC units and an average May flow of 56 cfs
USGS gauge or drainage
area value
-----------------------------------
Drainage area value at a site
=
----------------------------------Flow at a site
Known or given flow
187,003
------------------56
=
Cross multiply…
56 * 95,000
=
56 * 95,000
------------------187,003
=
28.4
95,000
------------------?
187,003 * (?)
(?)
= Flow in cfs at site
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Assumptions / Limitations
• lack of interception, evapotranspiration, baseflow from
groundwater
• distribution of rainfall
• limited to natural systems (no dams or impoundments)
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Determine the land cover in the riparian area

Use the gaplulc grid to first select all forested, grassland or natural
cover
Values:
1,2,3,11,13,14,15up
are healthy riparian
cover classes
Reclass to show
problem areas
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Zonal statistics

If watersheds are unique you could
find the min, max, mean, elevation
for each stream
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Summary notes

Best to perform hydro analysis at the watershed level not county

Many of these global operations are very computer intensive, limit
the size of the study area as much as possible
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Using the Hydrological
Modeling Extension in
ArcMap
RESM 493q
Lab #1 Thurs, Fri Nov 5,6
Applied GIS for Nat Sci
Getting started….
NOTE: the newfdr28 grid has been derived from an elevation grid with
sinks already filled.

Select the watershed you are interested in using as the study area

Set it as the analysis extent and analysis mask in the spatial analyst
options

Create a clipped flow direction grid for your study area by using the
statewide flow direction grid (newfdr28) found in grids/other-grids.

Set the Spatial Analyst options for Analysis Mask and Extent (review lab
if you forget)
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Adding the Hydro tools extension to the toolbar
1.
Make sure the spatial analyst extension is turned on and its toolbar
is added to the display
2.
Add the Hydrology Modeling toolbar to the ArcMap window by
making sure it is checked
3.
You should have a
tool bar added to
your display which
looks like this:
NOTE: if working outside of 317 Percival, you
will need to download and install the Hydro
Modeling extension found on the class website
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Using the hydro tools extension

If you are starting with a DEM for your study area, you must first fill
any sinks

Compute flow direction

Compute flow accumulation
NOTE: If you clipped the fdr grid in the grids/other-grids
directory for your study area, then you
only have to calculate a flow accumulation grid
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Using the hydro tools extension

To use the hydrological modeling watershed and raindrop tools,
you must first enter the properties for the fdr and fac grids
After the properties setup you are good to use
the watershed and raindrop tool on the extension
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Steepest path determination (rain drop path)

After setup of the properties, the rain drop icon will be available
meaning you may click on any location in your display to track the
overland flow across the landscape
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Interactive watershed delineation (point-n-click)

After setup of the properties, the icon will be available meaning you
may click on any location in your display to delineate a watershed
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Synthetic streams (stream networks)

Use the “stream network as feature” to delineate stream paths
based on an area calculation you specify
# of cells * cell size2 = drainage area
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Watershed delineation by drainage area size

By drainage area size
NOTE: IF cell size of grid = 20m
Then, 500 cells would delineate 200,000 square meter or ~ 50 acre watersheds
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To delineate stream networks or watersheds by
size
A. If you desire 200 acre stream networks or watersheds….
Convert 200 acres to meters squared
200acres * 4,046.85 = 809,370 meters squared
B. Next, determine the cell size of your FAC grid, if 20 meters then the
cell size squared is 400
C.
Finally, divide your meters squared number by the cell size
squared to get the number of cells
809,370 / 400 = 2,023 cells
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