What Our Engine Couples

This is what our engine couples, how it couples it, and what it does not couple.

An order delivers a SWAT+ model. In states with well-log coverage, a SWAT+ with MODFLOW order adds a steady-state MODFLOW 6 model, with layers, conductivity and the water table kriged from state well logs. The two are delivered side by side, not coupled.

In the engine, SWAT+ and MODFLOW 6 run in one process. Every day, soil percolation goes down as recharge to the MODFLOW cells, weighted by area overlap, and groundwater discharge comes back up to the SWAT+ channels. A fix to that two-way water balance is in progress.

Lakes and wetlands are routed in SWAT+ only. They are not coupled to the aquifer.

In every soil layer, PFAS sorbs to the soil and to the air–water interface, its dissolved concentration is capped at solubility, and it leaves by runoff, lateral flow, percolation and eroded sediment.

In streams and reservoirs it moves in dissolved and sorbed phases, with settling, resuspension and burial. It does not decay.

Leached PFAS loads the aquifer directly. There, MODFLOW 6 groundwater transport moves it, and every day, groundwater discharge carries it back to each gaining reach.

Separately, and offline, we model the vadose zone with MODFLOW 6 unsaturated-zone transport, which we modified so sorption depends on water content: one column per active cell with more than thirty centimetres to the water table, handing water and mass to the aquifer.

That vadose model takes SWAT+ percolation as an input file. It is not in the daily loop.

Soil and streams carry any number of compounds at once. The groundwater link carries one per run.

Every run writes the soil-phase PFAS mass balance. On the Rogue, it closes to better than one part in a million. Flow steps daily; groundwater transport, every thirty days by default.

Water quality runs in SWAT+ and MODFLOW 6 only: no WASP, no RT3D. The full table is at swatgenx.com.