MODFLOW coupling

“SWAT+MODFLOW-6” refers to two related but different deliverables. Establish which one a given package contains before working with exchange files or BMI.

Two coupling modes (do not conflate)

• Automatic build: SWAT+ builds first; a sibling MODFLOW_250m steady-state grid is then built from annual-mean percolation (perc). This model runs on its own: heads and baseflow return to SWAT+ only in the engine's daily coupling, switched on by mf6.con (below). Automatic builds do not yet produce a transient MODFLOW model.
• Research path (research work, not a product capability): daily two-way BMI/XMI coupling inside the SWAT+ day loop. Activated only when TxtInOut contains mf6.con plus map files; the engine loads libmf6 at run time. Not included in standard packages.

Build pipeline (steady-state MODFLOW 6)

1. Order a SWAT+ build with a steady-state MODFLOW 6 model (or attach one to a finished SWAT+ workspace).
2. After the SWAT+ package is ready, a 250 m MODFLOW 6 grid is built from annual-mean SWAT+ percolation.
3. Percolation is mapped HRU→cell (area-weighted); the grid is assembled with standard MF6 packages (RCHA, SFR/DRN/GHB/WEL, …), solved steady-state, then optionally head-calibrated when observations exist.
4. Artifacts land as a sibling folder MODFLOW_{resolution}m under the same site directory — not inside Scenarios/.../TxtInOut.

{site}/
├── SWAT_MODEL_Web_Application/   # (or resolved model name) — SWAT+ package
│   └── Scenarios/Default/TxtInOut/
└── MODFLOW_250m/                 # sibling steady-state MF6 grid (not inside TxtInOut)
    ├── mfsim.nam
    ├── MODFLOW_250m.nam
    ├── *.dis *.npf *.ic *.ims *.oc *.sto *.tdis
    ├── *.rcha *.sfr *.drn *.ghb *.wel …
    ├── Grids_MODFLOW*
    ├── obs_vs_sim.csv / .png     # when head obs exist
    └── head_calibration_report.json

• SWAT+ and MODFLOW download as separate archives. The standard MODFLOW folder has no README and no mf6.con / exchange maps.
• Default grid spacing for chained builds is 250 m. You can also attach MODFLOW to an existing finished SWAT+ workspace from the product UI/API.
• Groundwater coverage for hosted coupled builds: Michigan Lower Peninsula, Indiana, Minnesota, Ohio, Wisconsin. The request point / HUC12 centroid must fall in coverage; sparse well density can refuse a build.

Units and the recharge link

• MF6 lengths in meters; time in days. Recharge is derived from SWAT+ percolation (often converted mm/yr → m/day). Well elevations commonly convert feet→meters (× 0.3048).
• Spatial link: an HRU↔cell area-weighted map places recharge on the grid. No daily exchange loop in the shipped archive.
• The stock SWAT+ gwflow module is a different mechanism and is not this external-MF6 path.

Running the MODFLOW package locally

1. Install USGS MODFLOW 6 (mf6). FloPy is optional for inspection/edits; the platform already ran MF6 at build time.
2. Unzip the MODFLOW_* package; keep the tree intact (mfsim.nam at the root of that folder).
3. From that directory, run mf6 (or point your runner at mfsim.nam). Re-read heads (.hds), budget (.cbc), and listing files.
4. SWAT+ still runs from Scenarios/Default/TxtInOut with a SWAT+ executable — independently of mf6 unless you assemble the research coupling below.

Daily two-way BMI/XMI coupling (research)

Daily two-way coupling is research work in the public SWAT+ fork. Its design, each SWAT+ day: percolation is pushed to MF6 recharge; SFR groundwater exchange is aggregated back to SWAT+ channels, meant to replace the native aquifer return. The current engine still adds that exchange on top of the native return (a double count being fixed), and no model we deliver runs this mode. An optional PFAS leach map feeds groundwater transport when present.

./mf6        ! 1: folder (inside TxtInOut) holding mfsim.nam
1            ! 2: groundwater FLOW step, in days (1 = daily)
30           ! 3: groundwater TRANSPORT step, in days (30 = monthly)
1.0          ! 4: (optional) recharge multiplier for calibration
             ! 5: (optional) full path to libmf6 if not on the default search path

• Place mf6.con in TxtInOut. Convention: MF6 workspace at TxtInOut/mf6/ containing mfsim.nam. Absent mf6.con ⇒ plain SWAT+ (no-op).
• Maps next to mf6.con: mf6_recharge.map (HRU→cell), mf6_baseflow.map (SFR reach→channel), optional pfas_leach.map.
• Needs a coupler-enabled swatplus binary (see Compile the SWAT+ fork) plus libmf6.so / libmf6.dll on the library path.
• Standard packages do not ship mf6.con or the exchange maps; assemble them only for the research daily-coupling path.

Failure modes

• Expected mf6.con missing — standard archives intentionally omit it; the BMI path requires adding con + maps + a coupler binary.
• MF6 fails to open packages — run from the MODFLOW_* directory; do not nest it inside TxtInOut unless assembling research daily coupling.

MODFLOW_250m package contents

File-by-file contents of the sibling MODFLOW_{resolution}m folder a coupled build produces. Assembled as a standard MODFLOW 6 simulation; the archive contains the entire folder.

The standard deliverable is a standalone steady-state MODFLOW 6 model built from SWAT+ annual-mean percolation. Lengths are meters, time is days. Run it locally with mf6 from the folder root (mfsim.nam). The archive (MODFLOW_Model_{site}_{res}m.zip) contains the whole tree, including intermediate rasters and grids.

MODFLOW 6 packages in the archive

Companion files (not MF6 packages)

mf6.con and exchange maps

The four TxtInOut files that activate daily two-way SWAT+/MODFLOW 6 BMI/XMI coupling: mf6.con and the mf6_recharge.map, mf6_baseflow.map, pfas_leach.map exchange maps.

Coupling activates when TxtInOut contains mf6.con and a coupler-enabled swatplus binary can dlopen libmf6.so. Absent mf6.con → plain SWAT+ (no-op). Comments after ! are stripped on every line of every coupling file.

mf6.con

./mf6        ! 1: folder (inside TxtInOut) holding mfsim.nam
1            ! 2: groundwater FLOW step, in days (1 = daily)
30           ! 3: groundwater TRANSPORT step, in days (30 = monthly)
1.0          ! 4: (optional) recharge multiplier for calibration
             ! 5: (optional) full path to libmf6 if not on the default search path

mf6_recharge.map — HRU → cell (down-coupling)

Optional; absent → MF6 keeps its own recharge. Header line, then one row per HRU–cell overlap link. Each day the coupler overwrites MF6 RECHARGE with Σ sepbtm(hru)/1000 × weight × rch_mult (SWAT+ mm/day → MF6 m/day).

mf6_baseflow.map — SFR reach → channel (up-coupling)

Optional; absent → up-coupling off. Header N_LINKS N_REACHES, then one row per link. The coupler reads SFR GWFLOW (m³/day; positive = stream→aquifer) and adds −GWFLOW as baseflow to the mapped SWAT+ channel. It is meant to replace the native aquifer return; the current engine still adds it on top (a double count being fixed).

pfas_leach.map — HRU → GWT source cell (PFAS down-coupling)

Optional; requires the PFAS module and a GWT model with an SRC package. Daily push: SRC load = Σ perc(kg/ha) × overlap_ha × 1e9 (µg/day); GWT concentrations are treated as ng/L on the way back.

How maps are produced, and coupled-run outputs

• Exchange maps are assembled offline from the SWAT+ HRU geometry and the MF6 grid (area-weighted HRU→cell for recharge / PFAS leach; SFR reach→channel for baseflow). They are not written by a standard website order.
• A research daily-coupling workspace also needs a daily-TDIS MF6 (and optional GWT+SRC) model next to mf6.con — typically under TxtInOut/mf6/.
• Coupled-run outputs under TxtInOut/mf6/: pfas.ucn (groundwater PFAS concentration per cell over time), pfas.lst (transport mass budget + percent discrepancy), MODFLOW_sfr.lst (flow budget incl. stream–aquifer exchange) — standard MODFLOW 6 formats.
• An automatic build's package holds none of these coupling files; it carries the steady-state sibling MODFLOW_250m archive.