Characterization of Watersheds Characterization of Watersheds from DEMs using Spatial from DEMs using Spatial

Analyst/ArcHydroAnalyst/ArcHydro

Robert G. Burns, P.G.Robert G. Burns, P.G.Engineering GeologistEngineering GeologistDWR – Division of Safety of DamsDWR – Division of Safety of Dams

Watershed for Union Valley Reservoir

(El Dorado County)

DWR – Division of Safety of Dams DWR – Division of Safety of Dams A regulatory agency serving California for 81 years.

Formed in 1929 after the 1928 failure of the 2-year-old St. Francis Dam in Southern California.

Mission: To protect people against loss of life and property from dam failure.

~ 1250 Jurisdictional

Dams

www.water.ca.gov/damsafety/To learn more about DWR-DSOD:

BEFORE

AFTER

St. Francis Dam

Hydrologic Review of Spillway CapacityHydrologic Review of Spillway Capacity

• Watershed (Drainage Basin)– What is the areal extent of watershed capable of

capturing water? – Need to define the watershed boundary upstream

of the drain point of interest.

MSU

Delineating a Watershed: TraditionalDelineating a Watershed: Traditional– Topographic Map(s), Tape, Pencil, Eraser,

Planimeter, Time (can take days…weeks), Coffee…

Delineating a Watershed : GISDelineating a Watershed : GIS

Software: ArcGIS, Spatial Analyst, ArcHydro Tools.

Learning Curve: With familiarity, time is significantly reduced from traditional method.

Data: Digital Elevation Model (DEM) – MinimumPolyline stream layer (may be necessary in flatter terrains)

ArcHydroArcHydroandand

ArcGIS 9.3 ArcGIS 9.3 vs vs

ArcGIS 10ArcGIS 10

SPATIALANALYST

ESRI

Topographic Map DEM

Thematic raster data (a grid) representing ground surface topography. Each equally-sized cell in the grid is populated with a single value representing the elevation of that cell.

What is a Digital Elevation Model?What is a Digital Elevation Model?

10m

30m

238 243 248 250

236

232

224

241 245 249

238 241 247

233 239 243

Common DEM Resolutions

Sources of DEMsSources of DEMs

• USGS Seamless Website - http://seamless.usgs.gov

• Cal-Atlas - http://www.atlas.ca.gov/

• Geocommunity website - http://spatialnews.geocomm.com/

– 30m and 10m of 7.5 Minute USGS Quadrangles

Need enough DEM data to contain the resultant watershed.May require merging multiple DEM tiles with Spatial Analyst.

Recommendations: Use 10m for small (acres, few sq mi) watersheds…30m for larger. (Vert Res: 1m)For flatter terrains, chances of successful delineation increase with 10m DEMs. (Vert Res 0.1m)

Three Overall ProcessesThree Overall Processes

• Preparing and Pre-Processing the DEM– DEM is data which does not know what you are

attempting to do with it.– The need here is to create a functional model

based upon the data of interest.• ArcHydro Tools

• Delineating the Watershed• Using the Watershed as an Analysis Mask to

Evaluate other Parameters of Interest

Preparing and Pre-Processing the DEMPreparing and Pre-Processing the DEM

Terrain PreprocessingData Management Terrain Preprocessing

1 DEM Manipulation Data Management DEM Manipulation2 Flow Direction Level DEM

Flow Direction with Sinks DEM ReconditioningAdjust Flow Direction in Lakes Assign Stream Slope

3 Flow Accumulation Burn Stream Slope4 Stream Definition Build Walls5 Stream Segmentation Sink Prescreening

Flow Direction with Streams Sink EvaluationDrainage Line from Stream Depression EvaluationStream Segmentation from Drainage Line Sink SelectionCombine Stream Link and Sink Link 1a Fill Sinks

6 Catchment Grid Delineation7 Catchment Polygon Processing8 Drainage Line Processing9 Adjoint Catchment Processing

Drainage Point ProcessingLongest Flow Path for CatchmentsLongest Flow Path for Adjoint CatchmentsAccumulate ShapesSlopeSlope Greater than 30Slope Greater than 30 and Facing NorthWeighted Flow Accumulation

>

*

Arc Hydro ToolsArc Hydro Tools

1a Fill Sinks2. Flow Direction3. Flow Accumulation4. Stream Definition5. Stream Segmentation6. Catchment Grid Delineation7. Catchment Polygon Processing8. Drainage Line Processing9. Adjoint Catchment Processing

David R. Maidment

Delineating the WatershedDelineating the Watershed

Flow Path Tracing shows route water would travel off DEM

Point Delineation produces a polygon representing the watershed.

Watershed as Analysis MaskWatershed as Analysis Mask

Drainage Area: Area of irregular polygon evaluated upon delineation (Attributes table).

VECTOR DATA PARAMETERS RASTER DATA PARAMETERS

Drainage AreaCentroidLat/Lon of CentroidMean Annual Precip (Rantz, 1969)

New DEM representing watershedMean Elevation (non-traditional)Longest Flow Path10 and 85% Stream ReachMean Annual Precip (PRISM)Elevation selection for snow melt.

Vector to RasterNOAA’s HMR-58 Probable Max Precip

Field Calcs/Geoprocessing Tools Spatial Analyst / Raster Calculator

Clarks’s Unit Hydrograph Parameters for HEC-1

Mean Annual Precip and SnowmeltMean Annual Precip and Snowmelt

Mean Annual Precipitation evaluated by clipping (geoprocessing) the Rantz, 1969 polygon and evaluating the weighted sum of contributing values.

Rule is applied within Spatial Analyst to identify cells with elevations between 6000 and 7000 feet.Cells satisfying argument are converted to vector polygons, and area evaluated.

Probable Max Precipitation(HMR-58)Probable Max Precipitation(HMR-58)

Mean Value of Clipped Surface PMP INDEX = 25.4 Inches

Interpolate Point (Krige) to SurfaceSelection of Points from HMR-58 Model

Here: A portion of NOAA’s HMR-58 point layer is used to approximate a raster-surface. The watershed polygon feature is then used to evaluate a PMP Index for the watershed.

Limitations / DifficultiesLimitations / Difficulties

• Method works well in hilly/mountainous terrain.• Method does not work effectively in flatter terrains (e.g.

San Joaquin/Sacramento Valley)– DEM Reconditioning (AGREE DEM)

• NHD Polyline– Low Res based on 1:100,000 scale DEMs.– Hi Res based on 1:24,000 scale DEMs

EpilogueEpilogue

• I prepared a procedure manual for DSOD.– Likely need to revise when Arc Hydro Tools for

version 10 becomes available.• At least an hour to demonstrate the steps in

manual…longer if instructing.• Class in DWR Training Center?

– Assessment of demand.

Thank YouThank You