This dataset is a digital hydrologic unit boundary layer that is at the Subwatershed (12-digit) level, extracted from the NRCS HUC Basins layer that also contains delineations at the Subbasin (8-digit) and Watershed (10-digit) levels.

Each basin is identified by a unique Hydrologic Unit Code (HUC), as well a name, at each level. HUCs are identifiers as assigned to basin polygons by the U.S. Geological Survey (USGS). Data in this layer cover all of Massachusetts and small parts of surrounding states. The names of the statewide datalayers are NRCSHUC_POLY (polygons), NRCSHUC_ARC (arcs), and NRCSHUC_ANNO (annotation). The HUC is a hierarchical, numeric code that uniquely identifies hydrologic units. Hydrologic units are subdivisions of watersheds nested from largest to smallest areas and are used to organize hydrologic data. HUCs are constructed as follows:

• the first two digits identify the region,

• the first four digits identify subregions,

• the first six digits identify accounting units,

• the first eight digits identify cataloging units,

• the first ten digits identify watershed units, and

• the full twelve digits identify subwatershed units.

The USGS has developed the first 8-digit HUC for the United States, while the U.S. Department of Agriculture, Natural Resource Conservation Service (USDA-NRCS) within each state is developing the full 12-digit HUC.