Forest C near bare-soil in rev 62 — per-plant vs basin residue decomp, and cfac=1 hardwire

[Grace]

Hi all,

SWAT+ rev 2026.62, Editor v4.0.1, ~62% temperate deciduous forest watershed. Forest sediment ~5 t/ha/yr (10–100x realistic); cause is the daily USLE C-factor near bare-soil for forest. Traced in source — want to confirm I'm reading it right and how to fix it properly.

1) ero_cfactor.f90 hardwires bsn_cc%cfac = 1, so the APEX method (c = rsd_covfact * bio_covfact) always runs; the classic C_USLE,mn branch (theory manual eq 4:1.1.10) never executes, so plant usle_c_min and the codes.bsn c_fact flag are both inert. Intended?

2) Forest residue is starved at default: with basin rsd_decomp = 0.05, forest residue equilibrates ~400 kg/ha, forcing forest C ~0.5+. I only get realistic residue (~1,300–3,600 kg/ha) by dropping basin rsd_decomp to 0.02 — but that's basin-wide and slows decomposition + N/P mineralization on all cropland, disturbing a locked flow/TSS/TN/TP calibration.

3) No working per-land-use lever that I can find: plnt_decomp (per plant, my forest = 0.05) is set, yet the BASIN rsd_decomp is what actually moved forest residue — which governs? rsd_decomp.f90 reads pldb(idp)%rsdco_pl, but the basin value responded. Also, per-plant rsd_covfac/rsd_pctcov don't feed the C-factor (ero_cfactor uses basin rsd_covco), and bio_covfact stays ~0.6–0.9 for forest.

Questions:
- Is cfac=1 intended / usle_c_min deprecated?
- Per-plant plnt_decomp vs basin rsd_decomp — which controls forest residue decay, and should the plant value override the basin one?
- Any per-land-use path (calibration.cal?) to raise forest residue/cover without a basin-wide change?
- Is the weak bio_covfact for tall canopy expected?

Can share plants.plt / parameters.bsn / codes.bsn / ero_d output on request. Thanks!