Sensitivity of Permafrost Hydrothermal Regime to Depth-to-Bedrock in Land Surface Modeling: A Case Study of the Qinghai-Tibet Plateau

Depth-to-bedrock (DTB) determines soil thickness in land surface models and controls the active depth of hydrological and biogeochemical processes, particularly important in permafrost regions where soil thickness affects both hydrologic and heat transfer. In this study, we evaluate the sensitivity of permafrost hydrothermal regime to DTB parameterization in Community Land Model version 5.0 (CLM5.0) by comparing two datasets: DTB_P (default DTB in CLM5.0, developed by Pelletier) and DTB_SG (Shangguan-derived DTB) over three sites (BLH, TGL, and XDT) and the Qinghai-Tibet Plateau (QTP). Through four experiments with increasing soil thickness, we find significant sensitivity of permafrost simulations to DTB, shallow DTB_P results in excess water being redistributed and overestimated active layer thickness (ALT) at three sites (9.1 m and 10.5 m at BLH and TGL, exceeding 42 m at XDT), while deeper DTB_SG improves simulations by enhancing soil column's water storage capacity and extending the depth of soil-parameterized heat transfer, where both thermal conductivity and heat capacity vary with soil properties rather than using constant bedrock values. At the regional scale, implementation of DTB_SG significantly reduces mean ALT from 8.83 m to 3.11 m across the QTP and from 9.04 m to 2.55 m in the Three Rivers Source Region, producing more realistic spatial patterns and temporal variations. We conclude that while soil liquid water simulations stabilize beyond 4.0 m, greater soil thickness continues to benefit thermal processes, establishing a minimum DTB threshold of 4.0 m for reliable permafrost simulations in the QTP.