Strong regulation of permafrost coverage on runoff recession in the high-altitude permafrost basin

Permafrost plays a crucial role in influencing regional water resources by impeding surface water infiltration and regulating surface runoff discharge. Great efforts have been made to investigate the hydrological effects of permafrost degradation, but the underlying mechanisms behind the impacts of permafrost change on runoff production remain unclear, especially in the high-altitudinal permafrost basins with high spatial heterogeneity and limited soil observations. Therefore, this study combines long-term discharge records, process-based model simulations and remote sensing measurements to investigate the characteristics of runoff recession processes, which directly reflect the variation of soil storages affected by permafrost changes. With 60-year daily discharge records from eight high-altitude permafrost basins and subbasins of the Tibetan Plateau, we first analyzed the long-term temporal evolution of runoff recession rates. Then taking the source region of the Yangtze River (SRYR) in the central Tibetan Plateau as an example, we further investigated the specific soil freeze/thaw (F/T) factors that impact the runoff recession rates, by modifying a process-based permafrost hydrology model and simulating the soil F/T dynamics and related hydrological responses.

The preliminary results show that permafrost coverage strongly impacts the storage-discharge relationships indicated by runoff recession rates. In basins with high permafrost coverage (>80%), the long-term runoff recession rates exhibit a significant decreasing trend across all recession events, and a discontinuous runoff recession process is generally observed during the autumn and early winter recession periods. With a reduction in the permafrost coverage, we did not observe a significant trend in the long-term recession rate except for the recession events during early freezing periods (around autumn). In basins with much lower permafrost coverage (<50%), no distinct long-term trend in the seasonal runoff recession rates is observed. The process-based model simulation results in the SRYR (~80% of permafrost coverage) further reveal strong regulation of permafrost on the runoff production. A slower autumn recession rate is often related to a delayed soil freeze onset, especially in the deep soils of the active layer, which facilitates a larger soil water reservoir. In addition, the observed discontinuous recession during fall and early winter runoff recession period may result from a delayed soil freeze onset and longer unfrozen state (e.g., a longer duration of zero-curtain), influencing the connectivity of groundwater flow channels. In the next phase, we plan to include more remote sensing observations, such as InSAR deformation, to further investigate how active-layer soil water dynamics and its F/T state affect regional runoff production and water balance. This study shall enhance our understanding of the fundamental influence of permafrost changes on river runoff and support predictions of permafrost hydrological responses to future climate changes.