HFAM Model Set Up Using DEM Data and GIS Analysis

Case Study: Baker River , Washington

Tracey Kenward


Introduction

The Hydrocomp Forecasting and Analysis Model was set up to model flows into Lake Shannon on the Baker River, WA. A Digital Elevation Model (DEM) for this region was obtained from the USGS National Elevation Database. Soils information was obtained from the Washington State Soil Geographic Database (STATSGO). ArcView GIS 3.2 was used to delineate reaches and land segments and to calculate watershed surface properties. Hydrologic and hydraulic properties required for HFAM were calculated from the GIS output.


Digital Elevation Data

Figure 1. Baker River DEM

DEMs are digital records of terrain elevations for ground positions at regularly spaced horizontal intervals. Coverages for the contiguous United States can be ordered from the National Elevation Database:

http://ned.usgs.gov/

Data for digital elevation, aerial photography, digital maps and Landsat images can be found at:

http://edcsns17.cr.usgs.gov/EarthExplorer/


STATSGO Soils Data

Figure 2. Washington State STATSGO

The State Soil Geographic Database (STATSGO) for Washington and all states is available at UNC libraries:

http://www.lib.unc.edu/reference/gis/datafinder/index.html?individual_dataset_details=1&data_set_id=120

This gives information on soil coverage for the entire state. For the Baker River , the STATSGO data was used to estimate percentage of rock outcrop for each land segment.


ArcView 3.2 Procedure

ArcView 3.2 and the Spatial Analyst were used for the GIS analysis. The hydro11.avx sample scripts that accompany Spatial Analyst were used to perform some of the spatial calculations. The ArcView on-line help provides additional information on the use of the hydrologic modeling extension and map calculator.

The hydrologic modeling menu (available by loading the hydro11.avx extension) was used to fill sinks in the DEM, to calculate spatial grids of flow direction and flow accumulation, and to create a stream network.

The watershed option was used to generate a watershed grid. The watershed grid was converted to a shapefile, which defines each area as a polygons. The watershed polygons were combined where necessary, and deleted when outside of the watershed, to provide a polygon for each reach. Reaches were numbered from 3000 to 3110.

Figure 3. HFAM Reaches

The watershed polygons were converted to a revised watershed grid. The map calculator (available by loading the spatial analyst extension) was used to generated a mask. The mask was calculated as the watershed grid divided by the watershed grid. This calculation results in a value of 0 outside the watershed and 1 within the watershed. This mask was used to exclude areas outside of the watershed from future calculations and graphics.

Figure 4. Watershed Mask

Slope, aspect and hill shade were calculated for the watershed area using the surface menu option (available by loading the spatial analyst extension). Hill shade requires entry of solar altitude and azimuth. The solar properties for a given location and date are available on-line through the data services provided by the U.S. Naval Observatory: http://aa.usno.navy.mil/AA/

Figure 5. Slope, Aspect and Hill shade

Elevation was reclassified into 200m bands using Analysis, Reclassify from the view window menu options (0-16). Aspect was reclassified as NE or SW (0 or 1).

Figure 6. Reclassified Elevation and Aspect

Land segments in HFAM are defined as hydrologically homogeneous units. For the 297 sq. mi. (769 sq. km.) Baker River , 423 land segments were defined by considering the elevation (200m bands, 0-16), aspect (NE/SW, 0/1) and location (reach number, 3000-3110). The map calculator was used to create a segment grid based on these three variables: segment value = reach# * 100 + elevation band * 10 + aspect category. The segment grid was converted to a shape file which creates polygons of similar segment values.

Figure 7. Segment Grid and Segment Polygons

The average elevation, slope, aspect and hill shade of each segment type was calculated by selecting Analysis, Summarize Zones. ArcView creates tables for each parameter that can be exported in Microsoft Excel format.

The average elevation and downstream channel elevation for each reach was calculated by selecting Analysis, Summarize Zones.

The view image was set to display both the reaches and the stream network.

Figure 8. Reaches and Stream Network

     Channel length in each reach was measured manually using the measure tool.

The upstream elevation of each reach was estimated by clicking the information on the upstream area.


Using the GIS Output

ArcView GIS was used to calculate the following parameters:

  • Reach area, downstream channel elevation, upstream channel elevation and channel length.
  • Segment area, elevation, slope, aspect and hillshade.
  • Segments in each reach

This data was imported into an Excel spreadsheet. The spreadsheet calculated the HFAM hydrologic and hydraulic parameters and generated the HFAM input files.

Relationships between the GIS output and the HFAM parameters were initially estimated from prior studies in the Baker and other watersheds and were included in Excel equations. These relationships were calibrated to the Baker watershed from simulated/recorded streamflows and simulated/recorded data at snow courses. A twenty-five year hydrometeorologic data base was used for calibration. Hourly hydrologic process calculations were used for 423 ‘land segments'.

A generic version of the Excel spreadsheet will be made available to other HFAM users to assist in generating HFAM input parameters from GIS output.

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