Moisture-threshold and structure controls on soil thermal conductivity on the northern Qinghai–Tibet Plateau

• A moisture threshold governs when unfrozen STC exceeds frozen STC. • Crossover S r *​ predicted from measured properties matches field diagnostics. • Calibrated K e (S r ) schemes outperform the original Johansen scheme. • Bulk density, porosity, and quartz fraction shift S r * across sites. Soil thermal conductivity (STC) governs near-surface heat exchange and constrains simulations of active-layer evolution and permafrost change. Using a 10-year record from four Qinghai–Tibet Plateau sites (0–10 cm), laboratory Kersten number ( K e )–saturation ( S r ) calibrations, and a structure-aware Johansen implementation, we identify a moisture-threshold reversal: under low antecedent moisture the frozen state conducts less heat than the unfrozen state, while at higher moisture the conventional ordering returns. The crossover saturation S r * is traceable in calibrated K e – S r relations and observable from pre-freeze moisture, linking field diagnosis to model parameters. A compact, deployable correction follows: taper the frozen branch for S r < S r * , compute endmembers from measured bulk density, porosity, and quartz fraction (BD–n–q), and select the unfrozen K e ( S r ) form by soil class and dryness tendency. The scheme reduces unfrozen-season errors across the core sites and generalizes at an independent hold-out station (TGL) without site-specific tuning. The approach is transparent—inputs are observable and decisions are tied to S r * —and is most impactful in dry, coarse, and sparsely monitored regions.