Hydrologic Analysis

Francisco Olivera, Ph.D., P.E.Srikanth Koka

Department of Civil EngineeringTexas A&M University

Watershed Delineation

Watershed delineation is the process of identifying the drainage area of a point.

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 (blue arrows) are orthogonal to the contour lines.

Drainage divides (red line) are found along the highest points of the terrain.

Streamlines do not flow towards drainage divides, and do not intersect them.

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

30-Meter DEMs

700

680

30m ~ 90m

30-Meter and 3" DEMs

Hydrologic Functions

Flow direction codes

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

4 4

4 4

4 8

22

2 2 2

2

1 1 2

128

128 1 2 4

128 1 41

128

71 58

49 46

37 38

Hydrologic Functions

56

445369

74

78 72 69

4768

58 55

21

31

67

64 22

61 16

DEM Flow direction grid Flow network(not a grid)

Function: Flow directionArgument: DEM

DEM

Flow Direction

0 0

2 1

5 2

71 58

49 46

37 38

Hydrologic Functions

Function: Flow accumulationArgument: Flow direction

56

445369

74

78 72 69

4768

58 55

21

31

67

64 22

61 16

DEM Flow network(not a grid)

0 0 0

0 1 1

0

0

0 0 0

3 8

1 1 20 0

1 24

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.

71 58

49 46

37 38

56

445369

74

78 72 69

4768

58 55

21

31

67

64 22

61 16

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

DEM Flow accumulation(> 2 cells)

Process: Stream delineationArgument: Flow accumulation

Stream Network

All grid cells draining more than a user-defined threshold value are part of the stream network.

Red streams correspond to a lower threshold than the blue streams.

71 58

49 46

37 38

56

445369

74

78 72 69

4768

58 55

21

31

67

64 22

61 16

DEM Delineated streams Streams links

Hydrologic Functions

Function: Stream linksArgument: Delineated streams

Stream links are assigned an identification number randomly.

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.

71 58

49 46

37 38

56

445369

74

78 72 69

4768

58 55

21

31

67

64 22

61 16

DEM Streams links Outlets

Hydrologic Functions

Process: OutletsArgument: Stream links

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.

71 58

49 46

37 38

56

445369

74

78 72 69

4768

58 55

21

31

67

64 22

61 16

DEM Outlets Watersheds

Hydrologic Functions

Function: WatershedArgument: Flow direction and Outlets

Watersheds are assigned the identification number of their outlet.

Watershed Delineation

The drainage area of each watershed outlet is delineated.

Raster to Vector Conversion

The grids of stream links and watersheds are vectorized to obtain stream line and watershed polygon feature classes.

Both feature classes 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.

Merging Watersheds

Adjacent watershed polygons can be merged into a single polygon, if they share their outlet or one flows into the other.