Characterization of Watersheds from DEMs using Spatial Analyst/ArcHydro Robert G. Burns, P.G. Engineering Geologist DWR – Division of Safety of Dams Watershed for Union Valley Reservoir (El Dorado County)

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 To learn more about DWR-DSOD: BEFORE AFTER St. Francis Dam

Hydrologic 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: Traditional – Topographic Map(s), Tape, Pencil, Eraser, Planimeter, Time (can take days…weeks), Coffee…

Delineating 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) – Minimum Polyline stream layer (may be necessary in flatter terrains) ArcHydroand ArcGIS 9.3 vs ArcGIS 10 SPATIAL ANALYST ESRI

Topographic MapDEM 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? 10m 30m Common DEM Resolutions

Sources of DEMs USGS Seamless Website Cal-Atlas - Geocommunity website – 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 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 DEM * Arc Hydro Tools 1a Fill Sinks 2.Flow Direction 3.Flow Accumulation 4.Stream Definition 5.Stream Segmentation 6.Catchment Grid Delineation 7.Catchment Polygon Processing 8.Drainage Line Processing 9.Adjoint Catchment Processing David R. Maidment

Delineating the Watershed Flow Path Tracing shows route water would travel off DEM Point Delineation produces a polygon representing the watershed.

Watershed as Analysis Mask Drainage Area: Area of irregular polygon evaluated upon delineation (Attributes table). VECTOR DATA PARAMETERSRASTER DATA PARAMETERS Drainage Area Centroid Lat/Lon of Centroid Mean Annual Precip (Rantz, 1969) New DEM representing watershed Mean Elevation (non-traditional) Longest Flow Path 10 and 85% Stream Reach Mean Annual Precip (PRISM) Elevation selection for snow melt. Vector to Raster NOAA’s HMR-58 Probable Max Precip Field Calcs/Geoprocessing ToolsSpatial Analyst / Raster Calculator Clarks’s Unit Hydrograph Parameters for HEC-1

Mean 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) 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 / 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

Epilogue 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 You