Subsurface Structure Regulates Water Storage in the Alpine Critical Zone on the Qinghai-Tibet Plateau

The subsurface critical zone (CZ) structure in alpine regions has a profound influence on water storage. The primary focus of this work is to reveal that the organic layer (A), leaching-deposit layer (B), saprolite layer (C) and freeze–thaw processes regulate changes in subsurface liquid water storage (CSWS). Here, we selected six representative ecosystems along an elevation gradient (4221~3205 m) in the Qinghai Lake Basin Critical Zone Observatory (QLBCZO) on the Qinghai-Tibet Plateau. We performed in situ field surveys, ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) measurements to investigate the subsurface CZ structure and liquid water storage (SWS). The results showed that the saprolite layer (thickness of 0.84~41.85 m) was the main subsurface liquid water storage reservoir, with a monthly average value of 595.49 mm. It occupied 82.12% of the total water storage of layers A, B and C. The average seasonal frozen thickness (SFT) ranged from 0.33 m to 1.60 m. SFT contributed most to CSWS, with 27.95% during the thawing period, and precipitation contributed most, with 19.87% during the freezing period. The SWS of the saprolite layers compared to that of layers A and B increased the most, by 39.41 mm and 45.88 mm in the thawing and nonfreezing periods, respectively, and that of layer B decreased maximally by 52.50 mm in the freezing period. This study contributes to advancing the mechanistic understanding of the interactions between the subsurface CZ structure and water storage processes and provides high-quality data with which coupled ecohydrological models can be developed.