Chinese scientists have performed a detailed evaluation of the hillslopes in the Qilian Mountains, a crucial water source of the Qinghai-Xizang Plateau, offering insights for water resource management and ecological engineering planning.

This study offers new insights into soil hydrological processes at the hillslope scale in mountainous regions, especially on the Qinghai-Xizang Plateau, and its findings were published in the Journal of Hydrology, according to Lanzhou University (LZU).

Mountainous areas are important water sources. A thorough understanding of how hillslope soil moisture responds to rainfall and the mechanisms that control this response is essential for understanding how rainfall is partitioned into runoff, soil water storage, and groundwater recharge.

"We aim to uncover the mechanisms which govern hillslope soil moisture response to rainfall, especially in data-scarce alpine regions, through a comprehensive long-term multi-scale hydrological observation on the Qilian Mountains," said Zhang Baoqing, a professor at the College of Earth and Environmental Sciences of LZU.

The Qilian Mountains serve as a crucial ecological barrier in western China, spanning the border between northwestern Gansu and Qinghai provinces. The team set up a slope-scale soil moisture monitoring network with 15 sites on a grassland slope in the mountains.

This study relies on high-density in situ observations of a typical alpine grassland hillslope. Based on slope orientation and location, the 15 monitoring sites were categorized into semi-sunny slope, valley (middle slope), and sunny slope groups.

Scientists evaluated soil moisture response patterns using rainfall thresholds, soil moisture increments, and soil storage increments.

"We used the long-term field monitoring data to expose how soil moisture at different locations of the Qilian Mountains responds to rainfalls. More vividly, it is like a detailed scan of this vital water source of the Qinghai-Xizang Plateau," said Tian Jie, a professor at the College of Earth and Environmental Sciences of LZU.

The study found distinct small-scale spatiotemporal heterogeneity within the slope surfaces, with rainfall thresholds varying significantly at different points and a spatial pattern emerging that corresponds to slope direction.

After rainfall, the valley generally showed a greater increase in soil moisture, indicating that lateral unsaturated soil flow plays an important role in redistributing water along the slope and in storing water in the valley. In terms of temporal dynamics, rainfall is the dominant factor driving increases in soil moisture, according to the study.

Overall, the interaction of environmental factors, topography and vegetation contributes to the complexity and notable heterogeneity of soil hydrological processes on alpine slopes, the study noted.

The study also revealed that water tends to "move horizontally," with the middle and lower parts of the hillside acting as "water collection basins." At the same time, most rainwater remains on the surface.

"Thanks to this multi-scale observation and study, we uncovered the microscopic process of how rainwater becomes the precious water resource in the alpine mountains. It reminds us to take into consideration the spatial differences of rainfall in drafting water resources management and ecological protection projects in the mountains, especially in the plateau regions," Zhang said.