DHSVM was configured to cover the Upper and Lower Chehalis basins (Hydrologic Unit Codes, or HUCs 17100103 and 17100104). These do not include areas draining to the estuary downstream of Aberdeen. The area extends into the Olympic National Forest to the North, the Willapa Hills to the South, and Onalaska to the East with streamflow output corresponding to locations of interest draining 55 sub-basin areas. In DHSVM, the watershed is subdivided into a uniform square grid of cells, or model elements, with spatial resolution generally ranging between 10 m and 150 m; for this study we use a resolution of 150 m. The spatial distribution of the soil and vegetation characteristics of the watershed are captured at the scale of the 150 m Digital Elevation Model (DEM) used as a primary input to DHSVM. As discussed below, some features of the stream network were difficult to resolve, even at this resolution, specifically due to (1) inconsistencies in the average downstream directions when elevations were averaged to a 150 m grid, and (2) the close vicinity of headwater gridcells at sub-watershed boundaries. Terrain analysis methods for digital streamflow network and watershed delineation (available in ArcGIS) were used to resolve the spatial distribution of elevation characteristics at the 150 m scale using flow pathways developed at the 30 m scale.
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