- hydrology
- modeling
The Soil and Water Assessment Tool is a watershed model that routes rainfall through a basin's soils, land cover, and channels to estimate runoff, sediment, and nutrient loads over time. It works at the HUC scale, dividing a basin into subwatersheds so land-use change in one area can be traced to loading downstream.
SWAT is the kind of model that stands behind a nutrient total maximum daily load: it apportions a reach's total phosphorus and nitrogen load among its sources, which is what a load budget and its allocations are built on. The platform treats a SWAT output as a modeled [inference] — useful for attributing and comparing loads, but distinct from a measured discharge monitoring report value, and only as good as the land-cover and soil inputs it was run on.