The buffer coverage was intersected with the zipcode polygons to allow identification of buffer regions by zipcode.
These data, originally from 1992 National Agricultural Statistics Service data, were tabulated by zipcode. Hamlett et al.
The spatial distribution of agronomic soil P, based on the fraction of samples exceeding 200 ppm P within a zipcode, is mapped in Figure 4.
To estimate the proportion of agricultural lands in a zipcode that is within variable source areas, a detailed stream layer was buffered by 150 ft (45.7 m) on each side and then intersected with land cover data.
Because both P source and P transport capacity are required for P to enter surface waters, a composite map showing potential impacts by P-index regulations was developed by adding the percent of soil samples exceeding 200 ppm P to the percent of agricultural lands within the 150 feet (45.7 m) stream buffers for each zipcode, then multiplying the result by the fraction of zipcode area that has agricultural land (Figure 8).