Does LAK automatically suppress RCH and EVT when lake cells are wet?
F TW
Hi all,
We are modelling seasonal pools (which appear in the wet season and vanish in the dry) and want to use RCH, EVT, and LAK without predefining the shifting pool borders. To avoid double-counting, our intended setup is:
- RCH active everywhere (represents areal recharge).
- LAK rainfall set to 0.0 (so no double-count with RCH).
- EVT always active with vegetation ET = A.
- LAK evaporation adds the
“open-water minus vegetation” increment = B.
- When a cell is wet (LAK active): total ET = A + B = open-water ET.
- When dry (no LAK): ET = A only.
We were told that in MF6, when a LAK cell is wet, RCH and EVT are automatically suppressed for the underlying (vertical connected) aquifer cell. We couldn’t find this in the docs/examples, so we built a minimal iMOD Python test (quite a lot of work for a test):
- Case 1 (LAK wet, with LAK rainfall+evap): expect RCH and EVT suppressed.
- Case 2 (LAK dry): expect RCH/EVT active.
Result: In the cell beneath a wet LAK cell, RCH and EVT fluxes are the same as in a non-lake cell, NO suppression observed in the CBC. This suggests no automatic switching between LAK and RCH/EVT.
Questions:
1. Is there an official reference showing that LAK should suppress RCH and EVT in underlaying connected cell when a lake cell is wet?
2. If such switching exists, how is it enabled / specified?
Otherwise, our split-ET approach above seems appropriate.
Thanks in advance, Martin
Richard Winston
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F TW
F TW
Hi Richard, thanks for your suggestion. Since my 19 December message, I extended my MF6 test to make the switching behaviour unambiguous.
Setup (MF6 6.6.3, iMOD-python 1.0.0)
- Grid: 1 layer, 1×2 cells (each 100×100 m). Left cell = LAK; right cell = plain aquifer.
- LAK connection: vertical; negligible bed resistance (large conductance).
- Forcing (uniform):
- LAK: rainfall = 0.0000 m/d, evaporation = 0.0020 m/d;
- RCH: 0.0020 m/d;
- EVT: 0.0020 m/d.
- Initial stage: 0.1 m above lake bottom.
- 10 daily stress periods → lake evaporates to dry, then remains dry.
Results (per-cell flows for area of 10,000 m²: 0.002 m/d → 20 m³/d):
- SP 1–3 (lake wet):
- Lake cell: stage declines to bottom.
- Aquifer beneath lake: RCH = 0, EVT = 0 m³/d.
- Neighbour aquifer cell: RCH = 20, EVT = 20 m³/d.
- SP 4–10 (lake dry):
- Aquifer beneath former lake: RCH = 20, EVT = 20 m³/d.
- Neighbour aquifer cell: RCH = 20, EVT = 20 m³/d.
Conclusion
- When a lake cell is wet (active), MF6 suppresses RCH and EVT in the connected aquifer cell.
- When the lake cell is dry (inactive), no suppression
occurs.
This is exactly the switching behavior we need for seasonal surface water with unfixed extents.
I couldn’t find this spelled out in the public docs/examples. I’m planning to open a MODFLOW 6 GitHub issue to request that this interaction be documented (or pointed to if it already exists).
Martin