1. Hydrologic Analysis Francisco Olivera, Ph.D., P.E. Srikanth Koka
Department of Civil Engineering
Texas A&M University

2. 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.

3. 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.

4. 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.

5. 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.

6. 30-Meter DEMs
720
700
680
740

7. 30-Meter DEMs
700
680

8. 30-Meter and 3" DEMs
30m
~ 90m

9. 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.
Flow direction codes

10. Hydrologic Functions 1 67 56 49 53 44 37 58 55 22 1 67 56 49 53 44 37 58 55 22
Slope:

11. Hydrologic Functions Function: Flow direction Argument: DEM DEM78 72 69 71 58 2 2 2 4 4 74 67 56 49 46 2 2 2 4 4 69 53 44 37 38 1 1 2 4 8 64 58 55 22 31
128
128 1 2 4 68 61 47 21 16
128
128 1 1 4
DEM
Flow direction grid
Flow network
(not a grid)

12. DEM

13. Flow Direction

14. Hydrologic Functions Function: Flow accumulation
Argument: Flow direction 78 72 69 71 58 74 67 56 49 46 1 1 2 1 69 53 44 37 38 3 8 5 2 64 58 55 22 31 1 1 20 68 61 47 21 16 1 24
Flow network
(not a grid)
Flow accumulation grid
DEM

15. Flow Accumulation
Flow accumulation is a measure of the drainage area in units of grid cells.
The cell itself is not included.

16. Hydrologic Functions Process: Stream delineation
Argument: Flow accumulation 78 72 69 71 58 1 2 3 8 5 20 24 74 67 56 49 46 69 53 44 37 38 64 58 55 22 31 68 61 47 21 16
DEM
Flow accumulation
(> 2 cells)

17. 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.

18. Hydrologic Functions Function: Stream links
Argument: Delineated streams 78 72 69 71 58 74 67 56 49 46 69 53 44 37 38 64 58 55 22 31 68 61 47 21 16
DEM
Delineated streams
Streams links
Stream links are assigned an identification number randomly.

19. 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.

20. Hydrologic Functions Process: Outlets Argument: Stream links DEM78 72 69 71 58 74 67 56 49 46 69 53 44 37 38 64 58 55 22 31 68 61 47 21 16
DEM
Streams links
Outlets
Outlets are assigned the identification number of their link.

21. 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.

22. Modified Stream Links
User-defined outlets modify the stream links by subdividing the segment in which they are located into two segments.

23. Hydrologic Functions Function: Watershed
Argument: Flow direction and Outlets 78 72 69 71 58 74 67 56 49 46 69 53 44 37 38 64 58 55 22 31 68 61 47 21 16
DEM
Outlets
Watersheds
Watersheds are assigned the identification number of their outlet.

24. Watershed Delineation
The drainage area of each watershed outlet is delineated.

25. 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.

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