CEE 795 Water Resources Modeling and GIS

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CEE 795Water 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
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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

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Watershed Delineation by Hand Digitizing
Study Area in West Austin with a USGS 30m DEM
from a 124,000 scale map
Watershed divide
Drainage direction
Outlet
ArcHydro Page 57
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DEM Elevations
720
720
Contours
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720
700
680
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720
740
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Hydrologic Slope - Direction of Steepest Descent
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Slope
ArcHydro Page 70
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Eight Direction Pour Point Model
ESRI Direction encoding
ArcHydro Page 69
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Flow Direction Grid
ArcHydro Page 71
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Flow Direction Grid
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Grid Network
ArcHydro Page 71
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Flow Accumulation Grid. Area draining in to a
grid cell
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Link to Grid calculator
ArcHydro Page 72
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Contributing Area Grid
TauDEM convention. The area draining each grid
cell including the grid cell itself.
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Flow Accumulation gt 5 Cell Threshold
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Stream Network for 5 cell Threshold Drainage Area
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Streams with 200 cell Threshold(gt18 hectares or
13.5 acres drainage area)
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Watershed Outlet
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Watershed Draining to This Outlet
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Watershed and Drainage Paths Delineated from 30m
DEM
Automated method is more consistent than hand
delineation
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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

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Pit Filling

• Increase elevation to the pour point elevation
  until the pit drains to a neighbor

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(No Transcript)
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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


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AGREE Elevation Grid Modification Methodology
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Stream Segments
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Stream Links in a Cell Network
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ArcHydro Page 74
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Stream links grid for the San Marcos subbasin
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Each link has a unique identifying number
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ArcHydro Page 74
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Vectorized Streams Linked Using Grid Code to Cell
Equivalents
Vector Streams
Grid Streams
ArcHydro Page 75
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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
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Catchments for Stream Links
Same Cell Value
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Raster Zones and Vector Polygons
One to one connection
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Catchments

• For every stream segment, there is a
  corresponding catchment
• Catchments are a tessellation of the landscape
  through a set of physical rules

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Catchments, DrainageLines and DrainagePoints of
the San Marcos basin
ArcHydro Page 75
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Adjoint catchment the remaining upstream area
draining to a catchment outlet.
ArcHydro Page 77
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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
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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)

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Delineation of Channel Networks and Subwatersheds
500 cell theshold
1000 cell theshold
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100 grid cell constant support area threshold
stream delineation
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200 grid cell constant support area based stream
delineation
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How to decide on support area threshold ?
Why is it important?
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Examples of differently textured topography
Badlands in Death Valley.from Easterbrook, 1993,
p 140.
Coos Bay, Oregon Coast Range. from W. E. Dietrich
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Logged Pacific Redwood Forest near Humboldt,
California
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Topographic Texture and Drainage Density
Same scale, 20 m contour interval
Driftwood, PA
Sunland, CA
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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
• Hortons Laws
• Slope Area scaling
• Stream Drops

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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

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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

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Are there any questions ?