Impact of Depth to Bedrock on Permafrost Hydrothermal Regime Simulation in CLM5.0: A Case Study of the Qinghai–Tibet Plateau

Abstract 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 processes and heat transfer. In this study, we evaluate the impact of DTB on permafrost hydrothermal regime in the 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 [Beiluhe (BLH), Tanggula (TGL), and Xidatan (XDT)] and the Qinghai–Tibet Plateau (QTP). Through four experiments with progressively increasing soil thickness, we find that permafrost simulations are significantly impacted by DTB. Shallow DTB_P results in excess water being redistributed and markedly overestimated active layer thickness (ALT) at three sites, while deeper DTB_SG improves simulations by enhancing the soil column’s water storage capacity and extending the depth of soil-parameterized heat transfer, though at the cost of reduced accuracy in upper-layer soil liquid water content simulations. At the regional scale, implementation of DTB_SG significantly reduces the 40-yr (1979–2018) spatially averaged ALT from 8.83 to 3.11 m across the QTP and from 9.04 to 2.55 m in the Three Rivers Source Region, producing more realistic spatial patterns and temporal variations. Our systematic analysis reveals that while soil liquid water simulations stabilize when DTB exceeds 4.0 m, thermal processes continue to benefit from greater DTB, establishing 4.0 m as a minimum threshold for reliable permafrost simulations in the QTP. These findings highlight the importance of appropriate DTB in permafrost regions.