Stream and Watershed Delineation from DEM’s David Maidment, Ph.D. and Francisco Olivera, Ph.D. Center for Research in Water Resources University of Texas at Austin Texas Department of Transportation (TxDOT) Water Resources Modeling Using GIS Austin, Texas

Overview zStudy area in West Austin with a USGS 30-meter DEM from a 1:24,000 scale map zEight direction pour point model (flow direction and flow accumulation grids) zStream network definition zWatershed delineation zDEM data sources

Study Region in West Austin Hog Pen Creek 4 km

Topographic Map 20 ft contour 100 ft contour Stream Center Line Austin West, 1:24,000 Scale

Direction of Steepest Descent

Watershed Delineation by Hand Watershed divide Drainage direction Outlet

200-Meter Mesh 1km Universal Transverse Mercator (UTM) Coordinates

100-Meter Mesh 100m 1km

30-Meter Mesh Standard for 1:24,000 Scale Maps

Austin West 30-Meter DEM Elevations in meters. ftp://ftp.tnris.state.tx.us/tnris/demA.html

USGS 30-Meter DEM

30 Meter DEM Cell Boundaries

DEM Lattice Points

DEM Cells Store Elevation at Lattice Point

DEM Elevations

DEM Elevations

Overview zStudy area in West Austin with a USGS 30-meter DEM from a 1:24,000 scale map zEight direction pour point model (flow direction and flow accumulation grids) zStream network definition zWatershed delineation zDEM data sources

Digital Elevation Model

cell size cell (cell value) Cell Definition

Eight-Direction Pour-Point Model

Slope: Direction of Steepest Descent

Flow Direction Grid

Austin West 30-Meter DEM Elevations in meters. ftp://ftp.tnris.state.tx.us/tnris/demA.html

Flow Direction Grid

Grid Network

Flow Accumulation Grid

Overview zStudy area in West Austin with a USGS 30-meter DEM from a 1:24,000 scale map zEight direction pour point model (flow direction and flow accumulation grids) zStream network definition zWatershed delineation zDEM data sources

Flow Accumulation Threshold Flow Accumulation > 5 cells

Stream Network Stream network for a 5-cell threshold drainage area.

Stream Network Stream network for a 13.5-acres threshold drainage area (200 cells).

+ =  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  Burned-in DEM streams = Mapped streams Burning-in the Streams

Filling in the Pits zDEM creation results in artificial pits in the landscape zA pit is a set of one or more cells which has no downstream cells around it zUnless these pits are filled they become sinks and isolate portions of the watershed zPit filling is first thing done with a DEM

Overview zStudy area in West Austin with a USGS 30-meter DEM from a 1:24,000 scale map zEight direction pour point model (flow direction and flow accumulation grids) zStream network definition zWatershed delineation zDEM data sources

Watershed Outlet

Watershed Draining to an Outlet

DEM-Based Delineation Found error hand delineation!!! Watershed and flow paths

Stream Segments (Links)

Stream Segments Grid 3 6

(>1000) Threshold at Stream Junction 1000 cell threshold exceeded at stream junction.

Same Cell Value Sub-Watersheds

Stream Vectorization Vector Streams Grid Streams

Stream and Sub-Watersheds

Overview zStudy area in West Austin with a USGS 30-meter DEM from a 1:24,000 scale map zEight direction pour point model (flow direction and flow accumulation grids) zStream network definition zWatershed delineation zDEM data sources

DEM Data Sources z30-m DEM’s from 1:24,000 scale maps z3" (100-m) DEM’s from 1:250,000 scale maps z15" (500-m) DEM for the US resampled from 3” DEM z30" (1-Km) DEM of the earth (GTOPO30)

30-m DEM’s zBest resolution standardized data source available for the US zCoverage of the country is incomplete zData by 7.5’ map sheets in UTM projection zLink for US: zLink for Texas: ftp://

3” DEM’s zDerived by US Defence Mapping Agency, available from USGS for the whole US. zData in geographic coordinates by 1:250,000 map sheet names (1°x 1°) cells in (1°x 2°) maps zNeeds to be projected to planar coordinates zLink:

7.5´ 1º 3" (100m) DEM 30m DEM Coverage of 30-m and 3" DEM’s

Cell Size 30m100m 30-m and 3" DEM’s

15" DEM zDerived by USGS from 3" DEMs by resampling each fifth point zBest resolution for seamless coverage of the US in a single grid zProjected cell size is 500m zLink:

30" DEM GTOPO30 zProduced by USGS from 3" grids and Digital Chart of the World topography zCoverage complete for the earth zProjected cell size is 1km approximately zData must be projected before use zLink:

30" DEM GTOPO30

Drainage in North America Source:

Summary Concepts (1) zTopographic maps are the traditional way of representing land surface terrain and streams. zWatersheds can be hand-delineated from these maps. zDEM’s of equivalent accuracy are now available for most map series in the US. zDEM cell elevation is at the cell center. zEight direction pour point model leads to flow direction and flow accumulation grids.

Summary Concepts (2) zStream network is defined as cells whose flow accumulation exceeds a threshold. zStreams can be divided into links and sub-watersheds defined for them. zWatershed outlet is the cell with highest flow accumulation. zResulting grids can be vectorized. zStandardized DEM data sources: 30m, 3”, 15”, 30”, which cover from local scale to the whole earth