- hydrology
- watershed
A hydrologic unit code is the USGS spatial key for a hydrologic drainage area. The codes nest by scale: each added pair of digits subdivides the area, so a HUC-8 identifies a subbasin (the working unit for a total maximum daily load) and a HUC-12 a much smaller local subwatershed.
The HUC is the join key for hydrology in this record. It is how a facility's outfall is tied to the impaired water it discharges to, how NWIS gauges and nutrient loads are rolled up to a consistent drainage area, and how a watershed model like SWAT is bounded. Getting the HUC right is what keeps a downstream analysis anchored to the actual receiving system rather than a nearby but hydrologically separate one.