New Castle County 4.3MB	Kent County 3.3MB	Sussex County 5.3MB

These permeability maps were derived from binary 1-meter resolution impervious cover rasters obtained from Mike Mahaffie at the Delaware Office of State Planning Coordination. The source files are available from the Office of State Planning Coordination webiste. The source rasters have three data values: 1=background (outside the county); 2=pervious surface; 3=impervious surface. At 1-meter cell resolution, they are well suited for localized stormwater modeling, but may be cumbersome for large-area analyses: the mosaicked Sussex County file has about 56,000 rows, 59,000 columns, and 3.34 billion cells.

I converted the source layers into 1M-resolution binary rasters (1=impervious; 0=not). I then set the output raster resolution to 10 meters and used Arc's Focal Statistics tool to sum the square meters of impervious surface in each 10x10 cell. The source data for Sussex County are distributed as two separate rasters covering east and west halves of the county. I processed these separately and mosaicked the final permeability maps to create a single county-wide map.

Incorporating surface permeability in watershed analyses

Surface permeability is a primary determinant of surface runoff volume. When development creates impervious cover and reduces landscape permeability, surface runoff, erosion and flood risk increase, groundwater recharge is reduced, and stream flows exhibit more variability over time.

Arc's FlowAccumulation utility uses a flow direction raster to model cumulative down-gradient flow volumes. The default output is a synoptic accumulation map that does not distinguish surface runoff from subsurface recharge of streams. But FlowAccumulation also supports inclusion of a weight raster that could index variable landscape permeability. These weights would index the amounts of received water that each cell either absorbs as groundwater recharge or passes down the steepest-descent path as immediate surface runoff.