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)

• Standard deliverable: sequential one-way steady-state. SWAT+ builds first; a sibling MODFLOW_250m grid is then built from annual-mean percolation (perc). Heads and baseflow are not fed back into SWAT+ in this mode. Transient MODFLOW is out of scope for standard builds.
• Research path: live 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 (one-way steady state)

1. Order a coupled SWAT+/MODFLOW build (or attach MODFLOW 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, Wisconsin, and Ohio. The request point / HUC12 centroid must fall in coverage; sparse well density can refuse a build.

Units and one-way 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)

Live coupling is built into the public SWAT+ fork. Each SWAT+ day: percolation is pushed to MF6 recharge; SFR groundwater exchange is aggregated back to SWAT+ channels (replacing native aquifer return to avoid double counting). 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, replacing native aquifer return to avoid double counting.

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.
• Ordinary website packages ship none of these coupling files; only the one-way sibling MODFLOW_250m archive is standard.