watershed delineation
TRANSCRIPT
DEM-Based Stream and Watershed Delineation
Francisco Olivera, Ph.D., P.E.Texas A&M University
Department of Civil Engineering
Watershed Delineation
Watershed delineation is the process of identifying the drainage area of a point or set of points.
For many years, Civil Engineers have been using USGS paper maps for watershed delineation.
The maps are in Universal Transverse Mercator (UTM) coordinates and in 1:24,000 scale.
Watershed Delineation
Water flows in the direction of the terrain steepest downhill slope.
Streamlines are orthogonal to the elevation contour lines.
Watershed Delineation
Streamlines do not flow towards drainage divides, and do not intersect them.
Drainage divides are found along the highest points of the terrain.
DEMs
Digital Elevation Models (DEMs) are grids of elevation.
DEMs store the same type of information contour lines do, but with a different data structure.
Watershed delineation can be based on DEMs rather than contour lines.
30-Meter DEMs
For the United States, 30-meter DEMs have been developed by USGS.
30-meter DEMs are consistent with 1:24,000 paper maps.
30-Meter DEMs
720
700
680
740
680700720740
720 720
30m ~ 90m
30-Meter and 3" DEMs
Hydrologic Functions
Flow direction codes
1
248
16
32 64 128
The eight-direction pour point algorithm (D-8) assigns a flow direction code to each cell, based on the steepest downhill slope as defined by the DEM.
The flow direction code indicates the cell – out of the eight neighbor cells – towards which the water flows.
Flow direction cannot be defined for cells within a terrain depression. DEMs have to be filled before determining flow directions.
67 56 49
53 44 37
58 55 22
1
67 56 49
53 44 37
58 55 22
1
60.152
2244 =−00.7
1
3744 =−Slope:
Hydrologic Functions
Hydrologic Functions
71
56
445369
74
78 72 69
4768
58 55
21
31
67
58
49 46
37 38
64 22
61 16
DEM
22
2 2 2
2
4 4
4 4
1 1 2 4 8
128
128 1 2 4
128 1 41
128
Flow direction grid Flow network(not a grid)
Function: Flow directionArgument: DEM
Flow Direction
Water flows to one of its neighbor cells according to the direction of the steepest descent.
Flow direction takes one out eight possible values.
0 0 0 0 0
0 1 1 2 1
0
0
0 0 0
3 8 5 2
1 1 20 0
1 24
Hydrologic Functions
71
56
445369
74
78 72 69
4768
58 55
21
31
67
58
49 46
37 38
64 22
61 16
DEM Flow network(not a grid)
Function: Flow accumulationArgument: Flow direction
Flow accumulation grid
Flow Accumulation
Flow accumulation is a measure of the drainage area in units of grid cells.
The cell itself is not included.
0 0 0 0 0
0 1 1 2 1
0
0
0 0 0
3 8 5 2
1 1 20 0
1 24
Hydrologic Functions
71
56
445369
74
78 72 69
4768
58 55
21
31
67
58
49 46
37 38
64 22
61 16
DEM Flow accumulation(> 2 cells)
Process: Stream delineationArgument: Flow accumulation
Stream Network
All grid cells draining more than a user-defined threshold value (blue streams) are part of the stream network.
All grid cells located downstream of user-defined cells (red streams) are also part of the stream network.
Hydrologic Functions
71
56
445369
74
78 72 69
4768
58 55
21
31
67
58
49 46
37 38
64 22
61 16
DEM Delineated streams
Function: Stream linksArgument: Delineated streams
Streams links
Stream links are assigned an identification number at random.
Stream links
Stream links are the segments of a stream channel connecting two successive junctions, a junction and an outlet, or a headwater and a junction.
Hydrologic Functions
71
56
445369
74
78 72 69
4768
58 55
21
31
67
58
49 46
37 38
64 22
61 16
DEM Stream links
Process: OutletsArgument: Stream links
Outlets
Outlets are assigned the identification number of their link.
Watershed Outlets
The most downstream cells of the stream segments (yellow/brown cells) are watershed outlets.
User-defined points (red dots) are also watershed outlets.
Modified Stream Links
User-defined outlets modify the stream links by subdividing the segment in which they are located into two segments.
Hydrologic Functions
71
56
445369
74
78 72 69
4768
58 55
21
31
67
58
49 46
37 38
64 22
61 16
DEM Outlets
Function: WatershedArgument: Flow direction and Outlets
Watersheds
Watersheds are assigned the identification number of their outlet.
Watershed Delineation
The drainage area of each watershed outlet is delineated.
Raster to Vector ConversionThe grids of stream links and watersheds are vectorized to obtain stream line and watershed polygon datasets.
Both datasets include a field Grid-Code in their attribute tables, which stores the stream link and watershed identification number.
Since the identification number of the streams and their corresponding watershed is the same, the attribute tables can be related.
Dissolving Spurious Polygons
Cells connected to the main watershed polygon through a corner are defined as a separate polygon (spurious polygon).
These polygons are dissolved into the main polygon.
Merging Watersheds
Adjacent watershed polygons can be merged into a single polygon, if they share their outlet or one flows into the other.
Merging Watersheds
The attribute tables are updated so that the watershed code (WshCode) of the streams is modified and the area of the new polygon is calculated.