The Challenge

Sub-watershed prioritization is only defensible when the hydrologic units being compared actually represent the full watershed. A visually plausible drainage network is not enough: stream thresholds, pour-point placement, raster resolution, and polygon conversion can all influence which cells are assigned to each basin.

The first stream-link catchment workflow produced 51 local drainage units totaling 87.672 km², leaving 2.988 km² of the authoritative watershed boundary outside those catchments. Rather than rank an incomplete set of units, the workflow was restructured around four major tributary basins and a separate Mainstem Local drainage area so the analytical zones represented the complete watershed.

The analytical goal was therefore twofold: derive comparable morphometric characteristics for the major tributaries, and prove through raster, vector, area, and overlap checks that the final zones were complete enough to support a relative prioritization.

"A ranking is only as defensible as the hydrologic units underneath it. Before prioritizing basins, the analysis first had to prove that the zones covered the watershed without meaningful overlap."

Process

  1. 1
    Prepare the watershed and elevation model Clipped USGS SRTM 1 Arc-Second Global elevation data to the Upper South Fork New River watershed and projected the analysis to WGS 84 / UTM Zone 17N (EPSG:32617) so distance, area, and terrain measurements could be calculated in metric units.
  2. 2
    Build the D8 hydrologic model Filled DEM depressions, generated D8 Flow Direction and Flow Accumulation rasters, and applied a 1,000-cell accumulation threshold — approximately 0.9 km² of contributing area at 30 m resolution — to define the modeled drainage network.
  3. 3
    Extract and structure the stream network Created Stream Link and Strahler Stream Order products, converted stream links to vector features, and used the ordered network to distinguish the main channel from major tributary connections.
  4. 4
    Delineate complete analytical basins Identified the major tributary mouths and delineated Basins 11, 14, 16, and 17. Cells inside the watershed but outside those four tributary basins were retained as Basin 9999, the Mainstem Local reference area, producing a complete set of analysis zones.
  5. 5
    Calculate morphometric indicators Combined elevation and slope zonal statistics with basin geometry and stream-network measurements to calculate drainage density, stream frequency, relief ratio, mean slope, bifurcation ratio, and circularity ratio for the major tributary basins.
  6. 6
    Rank, validate, and map the results Calculated the composite prioritization, checked basin union area against the watershed boundary, confirmed zero meaningful overlap, and traced the narrow Mainstem Local corridor back to value 9999 in the source raster before producing the final portfolio map.

What the Analysis Revealed

90.633 km² authoritative watershed area used for final coverage QA/QC
4 major tributary basins ranked in the final morphometric comparison
0.029% difference between the final zone union and watershed boundary area
  • Basin 14 — Very High: composite factor 2.167 across 16.227 km², the highest relative priority among the four tributary basins.
  • Basin 11 — High: composite factor 2.333 across 10.655 km².
  • Basin 16 — Moderate: composite factor 2.500 across 30.363 km², the largest ranked tributary basin by area.
  • Basin 17 — Low: composite factor 3.000 across 19.126 km².
  • Mainstem Local: 14.288 km² retained for spatial completeness but excluded from tributary ranking because it is not directly comparable to the four major tributary basins.
  • QA/QC: the final basin union measured 90.660 km² against the 90.633 km² watershed boundary, a 0.027 km² difference, with 0.000 km² of detected basin overlap.
Interpretation note: this is a relative morphometric prioritization, not a direct measurement of observed flood, erosion, or sediment-loss hazard. Results depend on the 30 m DEM, D8 flow model, 1,000-cell stream threshold, basin delineation logic, and selected morphometric indicators. The Mainstem Local unit is shown for completeness and is intentionally not ranked against the tributary basins.
Principle 03
Notice what's broken before anyone else does

The first 51 stream-link catchments covered only 87.672 km² of a 90.633 km² watershed. Instead of treating that intermediate output as final, the workflow isolated the missing area, rebuilt the analysis around complete tributary and mainstem zones, checked union area and overlap, and verified the unusual Mainstem Local corridor against the source raster before ranking. The QA/QC changed the analytical structure before it became a cartographic problem.