Site-scale watershed modeling
HUC14 catchment models nested in the watershed
A contaminated catchment is a few cells wide in a basin model. Site-scale rebuilds that one HUC14 fine enough to hold the plume, the wellfield, and the vadose zone above them — while the watershed model above it supplies the boundary.
Most of what SWATGenX builds is basin-scale: a complete SWAT+ model for a HUC12 watershed, optionally coupled to a 3-D MODFLOW 6 aquifer. Site-scale points the same automated pipeline at one HUC14 catchment — the drainage area that actually holds the site, the plume, and the monitoring wells.
Where to cut a model, and how the model above it supplies the boundary, is a method question rather than a site one. It is worked through against already-built models — including where nesting is the wrong tool — on nested watershed modeling. This page is the case where the answer is a single catchment, and where the reason is almost always contaminant transport.
Why a nested site model
How it is ordered
You do not assemble anything by hand. In the Watershed Explorer you pick a point, generate the HUC12 watershed model, then drill in and select the HUC14 catchment to nest as the site model — the watershed run becomes the site model's boundary.
Open the Watershed Explorer to start, or read how models are built.
Worked examples
The exemplars below are contaminant-transport site models built on the nested HUC14 pattern. Both put land-surface loading, vadose-zone descent, and aquifer transport into one coupled MODFLOW 6 simulation at catchment scale.
The Wurtsmith build is documented end to end — nested workflow, vadose transport, and an interactive 3D plume — on the watershed-to-site PFAS page.
PFAS at site scale
The usual reason to build a nested site model is contaminant fate and transport, and PFAS is the flagship case. The HUC14 build carries a PFAS source term down through the vadose zone into a MODFLOW 6 / GWT aquifer with Freundlich sorption, while the HUC12 model above it supplies the regional flow field.
What separates this from a hand-built desktop model is where the starting numbers come from. The National PFAS Soil Inventory — 30,972 depth-resolved records — seeds the source term, and the groundwater lithology inventory supplies aquifer geometry. The model therefore has a defensible starting point before any site-specific boring or monitoring data is added, rather than after.
The mechanisms — sorption, air–water interfacial retardation in the vadose zone, plume growth, and groundwater discharge to streams, compared against HYDRUS, MODFLOW/MT3DMS+GMS and the dedicated PFAS tools — are on the PFAS fate-and-transport page, and the nested Wurtsmith build is documented method-by-method on the watershed-to-site PFAS page.
Start a site model
Model generation is free with an account; paid cloud calibration is there if and when you want it. Contaminant, vadose-zone, and site-investigation studies that need finer than a basin-scale model start here — and if your study area is a reach or a management zone rather than a single catchment, start on nested watershed modeling instead.
Page updated 2026-07-13. Platform-generated, expert-finished.
FAQ
What is site-scale (HUC14) watershed modeling on SWATGenX?
A single-catchment SWAT+ model — optionally coupled to MODFLOW 6 — built for one HUC14 catchment and nested inside a coarser HUC12 watershed model that supplies its boundary conditions. It exists for contaminant fate-and-transport, vadose-zone, and site-investigation studies that need catchment-scale context a basin-scale model alone cannot resolve.
Why build a HUC14 site model instead of using the HUC12 watershed model?
A HUC12 model resolves a basin at about 250 m — right for regional water balance and streamflow, but a single contaminated catchment, wellfield, or source area is only a few cells wide at that resolution. Nesting a HUC14 site model keeps the study footprint on the catchment that holds the plume, the monitoring wells, and the vadose zone, while the HUC12 model still supplies inflow and groundwater context. Choosing where to cut in the first place — gauge, HUC12 outlet, or single catchment — and knowing when nesting is the wrong tool are covered on the nested watershed modeling page.
How do I simulate PFAS fate and transport at my site?
Generate the HUC12 watershed model over your area in the Watershed Explorer, then drill in and rebuild the single HUC14 catchment around your site, coupled to MODFLOW 6 for three-dimensional groundwater transport. The watershed model supplies the site model’s inflow and groundwater boundary conditions, while the National PFAS Soil Inventory and the groundwater lithology inventory populate the source term and aquifer automatically; site-specific boring and monitoring data then refine it. The mechanisms and a worked Wurtsmith example are documented on the PFAS fate-and-transport and watershed-to-site PFAS pages.