Transcript Raster and Vector Data - University of Nevada, Las Vegas

CEE 795
Water Resources Modeling and GIS
Lecture 5: DEM Processing and Watershed Delineation
(some material from Dr. David Maidment, University of Texas
and Dr. David Tarboton, Utah State University)
February 13, 2006
Learning Objectives:
• Perform raster based network delineation from digital elevation
models
• Perform raster based watershed delineation from digital
elevation models
Handouts:
Assignments:
Exercise #4
Duality between Terrain and
Drainage Network
• Flowing water erodes
landscape and carries
away sediment sculpting
the topography
• Topography defines
drainage direction on the
landscape and resultant
runoff and streamflow
accumulation processes
Study Area in West Austin with a USGS 30m DEM
from a 1:24,000 scale map
Watershed Delineation by Hand Digitizing
Watershed
divide
Drainage
direction
ArcHydro Page 57
Outlet
DEM Elevations
720
720
Contours
740
720
700
680
740 720 700
680
Hydrologic Slope
- Direction of Steepest Descent
30
30
80
74
63
80
74
63
69
67
56
69
67
56
60
52
48
60
52
48
67  48
 0.45
Slope:
30 2
ArcHydro Page 70
67  52
 0.50
30
Eight Direction Pour Point Model
32
64
16
8
128
1
4
2
ESRI Direction encoding
ArcHydro Page 69
32 64 128
Flow Direction Grid
16
8
ArcHydro Page 71
1
4
2
2
2
4
4
8
1
2
4
8
4
128 1
2
4
8
2
1
4
4
4
1
1
1
2
16
Flow Direction Grid
32 64 128
16
8
1
4
2
Grid Network
ArcHydro Page 71
Flow Accumulation Grid.
Area draining in to a grid cell
0
0
3
0
0
0
0
0
0
0
0
0
0
0
0
0
2
0
0
3
2
2
0
0
1
0
0
11
0
1
0
0
0
1
15
0
0
2
5
24
1
0
2
11
1
2
15
5
24
1
Link to Grid calculator
ArcHydro Page 72
Contributing Area Grid
1
1
1
1
1
1
1
4
3
3
1
1
1
1
12
1
2
1
1
2
16
1
1
1
3
6
25
2
1
1
1
1
4
1
3
12
2
1
3
1
1
3
1
1
2
1
16
6
25
2
TauDEM convention. The area draining
each grid cell including the grid cell itself.
Flow Accumulation > 5 Cell Threshold
0
0
0
0
0
0
3
2
2
0
0
0
11
0
1
0
0
1
15
0
0
2
5
24
1
Stream Network for 5 cell
Threshold Drainage Area
0
0
3
0
0
0
0
0
2
0
0
2
0
0
0
11
1
0
1
0
15
2
5
1
24
Streams with 200 cell Threshold
(>18 hectares or 13.5 acres drainage area)
Watershed Outlet
Watershed Draining to This Outlet
Watershed and Drainage Paths
Delineated from 30m DEM
Automated method is more consistent than hand delineation
The Pit Removal Problem
• DEM creation results in artificial pits in the
landscape
• A pit is a set of one or more cells which has
no downstream cells around it
• Unless these pits are removed they become
sinks and isolate portions of the watershed
• Pit removal is first thing done with a DEM
Pit Filling
Increase elevation to the pour
point elevation until the pit
drains to a neighbor
“Burning In” the Streams
 Take a mapped stream network and a DEM
 Make a grid of the streams
 Raise the off-stream DEM cells by an arbitrary elevation
increment
 Produces "burned in" DEM streams = mapped streams
+
=
AGREE Elevation Grid Modification
Methodology
PLAN
GRID
CELL SIZE
A
A
SECTION A-A
GRID CELL SIZE
ELEVATION
RESOLUTION
MODIFIED ELEVATION
ORIGINAL ELEVATION
KNOWN STREAM LOCATION
AND STREAM DELINEATED
FROM MODIFIED ELEVATION
STREAM DELINEATED
FROM ORIGINAL ELEVATION
Stream Segments
0
0
3
0
0
0
0
0
2
0
0
2
0
0
0
11
1
0
1
0
15
2
5
1
24
Stream Links in a Cell Network
1
1
2
1
2
3
5
3
3 5
4 4 4
4 4
6 6
6
ArcHydro Page 74
5
5
Stream links grid for the San Marcos subbasin
201
172
202
203
206
204
ArcHydro Page 74
209
Each link has a unique
identifying number
Vectorized Streams Linked Using
Grid Code to Cell Equivalents
Vector
Streams
Grid
Streams
ArcHydro Page 75
DrainageLines are drawn through the centers of cells on
the stream links. DrainagePoints are located at the
centers of the outlet cells of the catchments
ArcHydro Page 75
Catchments for Stream Links
Same Cell Value
Raster Zones and Vector Polygons
One to one connection
DEM GridCode
Catchment GridID
4
3
5
Raster Zones
Vector Polygons
Catchments
• For every stream
segment, there is a
corresponding
catchment
• Catchments are a
tessellation of the
landscape through a
set of physical rules
Catchments, DrainageLines and DrainagePoints of the
San Marcos basin
ArcHydro Page 75
Adjoint catchment: the remaining upstream area draining
to a catchment outlet.
ArcHydro Page 77
Catchment, Watershed, Subwatershed.
Subwatersheds
Catchments
Watershed
Watershed outlet points may lie within the interior of a
catchment, e.g. at a USGS stream-gaging site.
ArcHydro Page 76
Summary of Key Processing Steps
•
•
•
•
•
•
•
•
[DEM Reconditioning]
Pit Removal (Fill Sinks)
Flow Direction
Flow Accumulation
Stream Definition
Stream Segmentation
Catchment Grid Delineation
Raster to Vector Conversion (Catchment Polygon,
Drainage Line, Catchment Outlet Points)
Delineation of Channel Networks and Subwatersheds
500 cell
theshold
1000 cell
theshold
100 grid cell constant support area threshold stream delineation
1
0
1 Kilometers
Constant support
area threshold
100 grid cell
9 x 10E4 m^2
200 grid cell constant support area based stream delineation
1
0
1 Kilometers
constant support
area threshold
200 grid cell
18 x 10E4 m^2
How to decide on support
area threshold ?
AREA 2
3
AREA 1
12
Why is it important?
Examples of differently textured topography
Badlands in Death Valley.
from Easterbrook, 1993, p 140.
Coos Bay, Oregon Coast Range.
from W. E. Dietrich
Logged Pacific Redwood Forest near Humboldt, California
Topographic Texture and Drainage Density
Driftwood, PA
0
Driftwood, PA
1 Kilometers
Same scale, 20 m
contour interval
0
Sunland, CA
Sunland, CA
1 Kilometers
“landscape dissection into distinct valleys is limited
by a threshold of channelization that sets a finite
scale to the landscape.” (Montgomery and Dietrich, 1992,
Science, vol. 255 p. 826.)
Suggestion: One contributing area threshold does
not fit all watersheds.
Lets look at some geomorphology.
• Drainage Density
• Horton’s Laws
• Slope – Area scaling
• Stream Drops
Suggestion: Map channel networks from the DEM at
the finest resolution consistent with observed channel
network geomorphology ‘laws’.
• Look for statistically significant break
in constant stream drop property
• Break in slope versus contributing area
relationship
• Physical basis in the form instability
theory of Smith and Bretherton (1972),
see Tarboton et al. 1992
Summary Concepts
• The eight direction pour point model approximates
the surface flow using eight discrete grid
directions
• The elevation surface represented by a grid digital
elevation model is used to derive surfaces
representing other hydrologic variables of interest
such as
–
–
–
–
Slope
Flow direction
Drainage area
Catchments, watersheds and channel networks
Are there any questions ?
AREA 2
3
AREA 1
12