- hydrology
- stormwater
A hydrologic soil group sorts a soil into one of four classes — A, B, C, or D — by its infiltration rate when thoroughly wet. Group A soils (sand, gravel) soak up rain and shed little runoff; group D soils (clay, shallow-to-bedrock) infiltrate poorly and run off fast. The classification comes from the NRCS soil survey.
HSG is the soil half of the runoff calculation: paired with land cover it fixes the curve number that sets how much of a storm becomes runoff. It matters twice for a development record. It decides how sensitive a site is to the impervious cover a data-center campus adds, and — because the group can shift between survey editions — choosing the wrong HSG (say C instead of B) silently changes the modeled runoff, a drift the platform pins by sourcing the group explicitly rather than defaulting it.