The impact of micro-topography on the interplay of critical zone architecture and hydrological processes at the hillslope scale: Integrated geophysical and hydrological experiments on the Qinghai-Tibet Plateau

Abstract

As the Third Pole of the Earth, Qinghai-Tibet Plateau (QTP) has been significantly affected by climate change, which makes it an ideal and unique site for critical zone (CZ) research. However, we have limited knowledge of the subsurface CZ structure, hydrologic processes, and their interplays, due to extremely scarce observations in such a harsh environment. In this study, we performed a suite of hillslope-scale experiments that integrated geophysical, hydrological, pedological, and hydrometric observations to investigate the characteristics of the subsurface CZ structure, hydrologic processes, pedological attributes, and their interplays influenced by micro-topography. The results reveal that the subsurface CZ structure and pedological attributes exhibit systematic differences in the aspect gradient (north-facing and south-facing slope). The thickness of the soil, saprolite, weathered layer on the north-facing slope is 0.3 m, 0.2 m, 0.6 m, thicker than that of the south-facing slope. The soil porosity, saturated water content, capillary water content, field capacity, saturated hydraulic conductivity of the shallow soil layer (0–10 cm) in the north-facing slope is 77.2%, 140.1%, 128.2%, 125.8%, 0.67 mm·min−1, which is 8.7%, 26.3%, 24.0%, 22.6%, 0.52 mm·min−1 larger than those of the south-facing slope. In contrast, those properties of the deep soil layer (80-90 cm) in the north-facing slope is 44.3%, 34.3% 32.0%, 29.8%, 0.06 mm·min−1, which is 11.5%, 19.8%, 21.0%, 18.7%, 0.28 mm·min−1 smaller than those of the south-facing slope. The bulk density of the shallow soil layer in the north-facing slope is 0.54 g·cm−3, which is 0.06 g·cm−3 smaller than that of the south-facing slope. In contrast, the bulk density of the deep soil layer is 1.30 g·cm−3, which is 0.26 g·cm−3 larger than that of the south-facing slope. The subsurface CZ structure and micro-topography cause the hydrological processes on the south-facing slope is dominated by surface flow, while those on the north-facing slope is dominated by subsurface flow. The subsurface flow on the north-facing slope accounts for 94.5% of the total runoff, while the surface flow on the south-facing slope accounts for 97.9% of the total runoff. In turn, the dominant runoff regime leads to a decrease of thickness of the soil layer from the top to the bottom of the south-facing slope, with severe slope creep and landslides on the north-facing slope. Our results suggest that the subsurface CZ structure and dominating runoff regime are significantly affected by the micro-topography gradient, which should be parameterized for large-scale research.