Watershed Delineation Project

John Mackenzie
January 29, 1999

Data files, Arc export (e00) format, gzipped or WinZipped
Note: LAT-LON (decimal degrees) data will work with any ArcView view projection
New Castle County UTM-18N, NAD83 LAT-LON
Kent County (draft) UTM-18N, NAD83 LAT-LON
Sussex County (draft) UTM-18N, NAD83 LAT-LON



This page introduces a new project to develop and validate detailed watershed delineations for Delaware.  Until further notice, the data provided here are for review only.  I welcome any comments regarding their accuracy or usefulness: contact me at johnmack@udel.edu.  I hope that, once reviewed and corrected as necessary, these data will support more efficient targeting of water quality protection efforts.

Source data

These basin delineations are derived from two data sources:

  • 30-meter digital elevation models of Delaware (SPATLAB, 1995) 
    Digital elevation model (DEM) data are created for each 7.5-minute quad by rasterizing 1:24000-scale Digital Line Graph (DLG) hypsography (elevation contour) data at 30-meter resolution, and interpolating elevation values for all cells between contours.  The source hypsography data include 10-foot interval contours for northern New Castle County quads, and 5-foot interval contours for all other quads in Delaware.  These hypsography data were combined with estimated shorelines derived from Landsat TM band 4 (near infra-red) data before interpolation.  A 30-meter resolution composite DEM for each county was created by patching together DEM's for all quads covering any portion of the county.  The composited DEM was input to a GIS watershed delineation module as described below.
  • digital hydrography data in DLG format (Delaware Geological Survey and USGS, 1998).
    The DLG hydrography data are used to check the consistency of the preliminary basin delineations generated by the watershed analysis module, and to guide raster editing of basin boundaries as appropriate.  The spatial accuracy of these data is far superior to that of previously available hydrography data (USGS 1:100,000-scale DLG and TIGER).
Delineation procedures

The basin delineations were generated from the GRASS r.watershed module.  Complete on-line documentation is available from www.baylor.edu/~grass/.  Module execution times ranged from 8 to 18 hours per county on a 200-MHz Pentium II PC with 64MB RAM running Linux.  The module was run without a depression map, and set to delineate basins to an approximate minimum size of two square miles.

The r.watershed module outputs a basin map with even-number identifiers sequenced by principal outlet.  This map was masked to include only land cells, and then split into individual quads so tha boundary editing could be performed quad-by-quad..  The r.watershed module also generated a water accumulation mapping, where each cell value in the map records the number of up-gradient cells draining into it.  Cells with accumulation values of one are divides.  A map extracting just these divide cells was used to guide editing of basin boundaries for conformity with the DLG hydrography data.

Edit procedures

Some basins span both sides of stream reaches; where appropriate, these basins were separated and assigned even-odd sequential numbers.  Some small adjoining basins were combined (reclassed) into single basins.  Occasional boundary edits were necessary to correct boundaries crossing intermittent streams or stream headwaters, and make basins converge consistently with stream confluences.  In areas with significant ditching (particularly where man-made ditches connect natural streams in different basins) basin boundaries are not adjusted to conform with drainage alterations.   Basin delineations in coastal areas are necessarily imprecise due to limited terrain resolution and changeable stream channels.

Once the raster editing was completed for each quad, the edited quad basin maps were re-patched to obtain a full county basin map.  After checking for edit consistency at quad edges, the basin map was vectorized with only slight smoothing of boundary lines.  The vector maps are exported from GRASS to ArcINFO in Ungenerate format (polygon arcs and label points as separate files).  The ArcINFO modules GENERATE (to import arcs and label points), BUILD (to snap nodes and create topology), PROJECTDEFINE (to set UTM projection parameters), PROJECT (to create Lat-Lon version in decimal-degree units) BUILD (topology for Lat-Lon version) and EXPORT (no compression) were used to create the files provided here.

Data and narrative copyright John Mackenzie, 1999

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