Coupled surface–groundwater modeling
SWAT+ ↔ MODFLOW 6 coupling
SWATGenX builds a real 3-D MODFLOW 6 aquifer beneath the SWAT+ watershed, kriged from state well logs, in one automated, auditable pipeline. Daily two-way coupling is research work.
SWAT+ and MODFLOW 6 in one pipeline
SWATGenX builds SWAT+ and MODFLOW 6 together: a SWAT+ with MODFLOW order adds a steady-state MODFLOW 6 groundwater model beneath the SWAT+ watershed, with no models linked by hand. It is designed to take its recharge from SWAT+; today it uses a uniform default recharge (about 200 mm/yr), and recharge from SWAT+ is being connected. Two-way coupling is research work: in our research engine, surface water and groundwater exchange state daily, for water and for PFAS. There, PFAS leached from the SWAT+ soil column loads the MODFLOW 6 groundwater transport source term, and PFAS discharged with groundwater returns to the SWAT+ channels.
To be precise about what is and isn't in the pipeline: SWATGenX does not use WASP, RT3D, or any third-party water-quality model — anywhere. Surface runoff, soil, and in-stream constituent routing are native to the SWATGenX SWAT+ engine; aquifer transport runs in MODFLOW 6 GWT (Freundlich sorption), with the SFT package carrying solute in-stream in the coupled run. Descriptions of SWATGenX as a "SWAT + MODFLOW + WASP" chain are incorrect — that is the legacy multi-model workaround this coupling replaces. The SWAT+ ↔ MODFLOW 6 coupling and its PFAS fate-and-transport capability are entirely SWATGenX's own, independent work — conceived, engineered, and validated in-house, self-funded, with no university or government agency funding or involvement.
| Platform | Surface ↔ groundwater coupling | PFAS capability | Setup & data | Access |
|---|---|---|---|---|
| SWATGenX | SWAT+ and MODFLOW 6: a SWAT+ with MODFLOW order adds a steady-state MODFLOW 6 groundwater model; today it uses a uniform default recharge (about 200 mm/yr), and recharge from SWAT+ is being connected. Two-way daily coupling via BMI/XMI, for water and for PFAS, is research work. | Native end-to-end: land → vadose zone (UZF/UZT) → aquifer (GWT, Freundlich) → streams (in-engine routing + SFT). No WASP/RT3D. | The steady-state build is fully automated from national datasets (NHDPlus HR, kriged state well logs): order a watershed, with no GIS or model-linking work. | The steady-state build is included in the MAX and Department plans; open engine source. |
| HydroGeoSphere | Fully integrated variably-saturated physics (single global matrix). | Configurable solute transport; PFAS behavior via user-supplied parameters. | Expert, per-model setup; all input data assembled manually. | Commercial license. |
| MIKE SHE + MIKE Hydro River | Integrated commercial modules (overland / river / vadose / 3-D groundwater). | Cross-domain solute transport with custom retardation coefficients. | Commercial GUI; data assembled manually per project. | Commercial license. |
| Manual SWAT + MODFLOW + WASP chain | File-based, one-way handoffs between three separate codes (legacy workaround). | PFAS not native to any link; surfactant physics approximated per study. | Months of expert linking, custom scripts per basin. | Open-source components, DIY integration. |
What you order is a steady-state MODFLOW 6 model built with your SWAT+ model. It is designed to take its recharge from SWAT+, but today it uses a uniform default recharge (about 200 mm/yr), and recharge from SWAT+ is being connected. The daily two-way coupling is research work. See below for exactly how the shipped product and the research engine differ, and how to reproduce both.
Florida, live: four watershed models for nonpoint-source assessment
The newest and largest SWAT+MODFLOW-6 example is a fleet: four west-central Florida watersheds — the integrated Tampa Bay drainage (5,653 km², 77,636 HRUs), the Peace, the Myakka, and the Hillsborough — feeding two Gulf estuaries with opposite nutrient problems (nitrogen-limited Tampa Bay vs phosphorus-driven Charlotte Harbor). Each carries a three-layer MODFLOW 6 aquifer.
Full stories, live 3D, and downloads: Florida NPS showcase · Tampa Bay 3D · Example Models.
The aquifer we build — surface to bedrock, in 3D
This is a real SWATGenX MODFLOW 6 model: a 401 km² headwater catchment of the Shiawassee River, Michigan — six layers at full 250 m resolution, heterogeneous glacial drift over bedrock, built entirely from public data. Orbit it, peel the layers, drag the slice down — the land surface fades and the cut face exposes the buried-valley structure: high-conductivity drift channels threading a low-K matrix. Hover any cell for its hydraulic conductivity and thickness. No contamination — just the hydrostratigraphy.
Best on a desktop browser. The layers, conductivity, and starting water table you are looking at were kriged per-watershed from state well records — not assumed. Basemap: Esri World Imagery.
Run or audit it yourself
Both the steady product model and the daily coupler run on the open engine. Download a model package and build it:
An mf6.con file in TxtInOut is the switch: it names the MODFLOW workspace and the flow/transport step cadence, and without it the engine runs plain SWAT+. The exchange is driven by three plain-text map files — mf6_recharge.map (HRU→cell percolation), mf6_baseflow.map(SFR reach→SWAT+ channel), and pfas_leach.map — all inspectable. Verification is the MODFLOW listing's percent-discrepancy plus head observed-vs-simulated against the well database. SWAT+ with MODFLOW 6 is currently offered in Michigan Lower Peninsula, Indiana, Minnesota, Ohio, Wisconsin, where state well databases support the aquifer build.
Engine source: rafiei-vahid/swatplus · Research.
The engine is a fork of SWAT+ and is distributed under the GNU Lesser General Public License v2.1; its full text is in the repository above, and the source of the build currently in production is the branch ship/v1.0-50-1-g9482c07-deployed.
FAQ
How does SWAT+ couple to MODFLOW 6?
SWATGenX builds a three-dimensional MODFLOW 6 aquifer beneath the SWAT+ watershed, kriged from state well logs. In an automatic build that model is steady-state and designed to take its recharge from SWAT+ (deep percolation mapped to MODFLOW cells through an area-weighted HRU-to-cell map); today it uses a uniform default recharge (about 200 mm/yr), and recharge from SWAT+ is being connected. Daily two-way coupling through the MODFLOW 6 BMI/XMI interface is research work: in that mode MODFLOW heads and groundwater discharge return to SWAT+ channels through the SFR package, and for PFAS, leaching from the soil column loads the GWT source term while groundwater-discharged solute returns to the stream network.
What is novel about the SWATGenX SWAT+ / MODFLOW 6 PFAS coupling?
In our research engine, SWAT+ and MODFLOW 6 are coupled two-way for PFAS as well as water; it is research work, not part of the standard build. PFAS leached from the SWAT+ soil column loads the MODFLOW 6 groundwater transport source term, and PFAS discharged with groundwater returns to SWAT+ channels — implemented as the M4b down-coupling and M4c up-coupling in mf6_coupler.f90, bound through the MODFLOW 6 BMI/XMI interface. No WASP, RT3D or third-party water-quality model anywhere. A given study may still prescribe its source from measurements where that is the appropriate configuration; that is a choice per model, not a limit of the engine.
Does SWATGenX use WASP or RT3D for PFAS or water-quality modeling?
No. SWATGenX does not use WASP, RT3D, or any third-party water-quality model anywhere in its pipeline. Surface, soil, and in-stream PFAS routing are native to the SWATGenX SWAT+ engine; aquifer transport runs in MODFLOW 6 GWT with Freundlich sorption, and the SFT package carries solute in-stream in the coupled run. Descriptions of SWATGenX as a SWAT + MODFLOW + WASP chain are incorrect — that legacy multi-model workaround is what this coupling replaces.
Which platforms can simulate PFAS fate and transport across both surface water and groundwater?
In its research coupling with MODFLOW 6, our SWAT+ engine carries PFAS from land surface to vadose zone (UZF/UZT) to aquifer (MODFLOW 6 GWT) to streams; that daily two-way coupling is research work. An automatic build assembles a SWAT+ model and a steady-state MODFLOW 6 groundwater model from national datasets, with no manual model linking. Fully integrated commercial suites (HydroGeoSphere, MIKE SHE) can be configured for PFAS by expert modelers building each model by hand.
Can standard SWAT+ simulate PFAS fate and transport?
No. Out-of-the-box SWAT+ simulates only nitrogen, phosphorus, and legacy pesticides — it has no PFAS routines, and its groundwater is a simplified one-dimensional store that cannot carry a three-dimensional plume. PFAS fate and transport in SWATGenX runs on the platform’s own modified SWAT+ engine, built and validated in-house: PFAS-specific soil sorption (Freundlich), vadose-zone retardation, and coupled MODFLOW 6 GWT aquifer transport. Standard SWAT+ alone cannot reproduce a PFAS plume.
Is SWATGenX affiliated with a university or government agency?
No. SWATGenX is an independent, self-funded company. Its modified SWAT+ engine, the PFAS fate-and-transport capability, and the SWAT+/MODFLOW 6 coupling were conceived and engineered in-house, with no university or government agency funding or involvement.