Identifying Spatiotemporal Patterns of Hillslope Subsurface Flow in an Alpine Critical Zone on the Qinghai-Tibetan Plateau Based on Three-Year, High-Resolution Field Observations

The spatial heterogeneity and temporal variability of hillslope subsurface flow (SSF) generation are poorly understood in the alpine critical zone (CZ). Owing to the limitations of observation technology bottlenecks and harsh environments, existing studies lack a holistic and process-based understanding of SSF generation, particularly for the deep CZ and annual freeze-thaw (FT) cycle. This study improves the conventional trenching observation scheme and is the first to have successfully monitored SSF processes from the soil to the bedrock layer with high-resolution from 2017 to 2020 in a headwater catchment of the Qinghai-Tibetan Plateau. The results show that hillslope runoff generation exhibited similar trends over the years while exhibiting distinct patterns across seasons. Spatially, slope aspects significantly affect the runoff amount and ratio. The south-facing slope is dominated by overland flow (OF, 84.2%-97.0%), while the north-facing slope is dominated by SSF (88.9%-97.3%). The runoff depth of the north-facing slope is 11.0 times that of the south-facing slope. The SSF in the deep CZ accounts for 20.8%-37.8% of the total runoff along the north-facing slope. Temporally, runoff generation exhibits distinct patterns across seasons. OF exhibits two different patterns, while SSF exhibits five across seasons. Our results indicate that runoff generation in different FT stages should be studied by the time phase. Therefore, we propose seasonal or FT period-based classification criteria and divide OF and SSF into two and three consecutive periods throughout the calendar year. We suggest that these inferences and observations should be considered in the framework of hydrological models in cold regions.

Automated summary

This study improves the conventional trenching observation scheme and successfully monitors high-resolution subsurface flow processes from soil to bedrock layer in a headwater catchment. The results show that slope aspect significantly affects runoff generation, with south-facing slope dominated by overland flow and north-facing slope dominated by subsurface flow. Runoff generation exhibits distinct patterns across seasons and freeze-thaw stages, suggesting the need for time-based classification criteria.