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

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

Hydrologic Functions 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. 1 2 4 8 16 32 64 128 Flow direction codes

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

Hydrologic Functions Function: Flow direction Argument: DEM DEM 71 56 44 53 69 74 78 72 47 68 58 55 21 31 67 49 46 37 38 64 22 61 16 2 4 1 8 128 DEM Flow direction grid Flow network (not a grid)

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.

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

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

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 Function: Stream links Argument: Delineated streams 71 56 44 53 69 74 78 72 47 68 58 55 21 31 67 49 46 37 38 64 22 61 16 DEM 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 Process: Outlets Argument: Stream links DEM 71 56 44 53 69 74 78 72 47 68 58 55 21 31 67 49 46 37 38 64 22 61 16 DEM 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 Function: Watershed Argument: Flow direction and Outlets 71 56 44 53 69 74 78 72 47 68 58 55 21 31 67 49 46 37 38 64 22 61 16 DEM 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 Conversion The 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.