Runoff, Sediment Yield and Shear Strength Characteristics of Slopes With Different Degradation Types of Alpine Grasslands in the Yellow River Source Region

Abstract

The Yellow River source region, a critical component of the Qinghai-Tibet Plateau (QTP), plays an essential role in regional ecosystems and hydrological cycles. This study investigated the hydrological and mechanical effects of varying degrees of degradation of grassland slopes under rainfall conditions in the Yellow River source region, aiming to elucidate the impacts of grassland degradation on slope runoff generation, sediment yield, and soil stability. Field-simulated rainfall experiments and laboratory direct shear tests were conducted to examine the hydrological responses and mechanical properties of root-soil composites on lightly, moderately, and severely degraded grassland slopes (LDS, MDS, and SDS) under rainfall intensities of 40 mm·h⁻¹ and 80 mm·h⁻¹. The results demonstrated that with increasing degradation severity, the initial runoff time significantly decreased, while both runoff volume and sediment yield increased markedly. At a rainfall intensity of 40 mm·h⁻¹, the average runoff rates for LDS, MDS, and SDS were 0.281 L·min⁻¹, 0.344 L·min⁻¹, and 0.518 L·min⁻¹, respectively. These rates increased to 1.391 L·min⁻¹, 1.710 L·min⁻¹, and 2.154 L·min⁻¹, respectively, under the higher rainfall intensity of 80 mm·h⁻¹. The sediment yield was positively correlated with the degree of degradation; at 80 mm·h⁻¹, the peak sediment yield of severely degraded grassland reached 5.924 g·min⁻¹, which was 9.3 times greater than that of lightly degraded grassland, with a cumulative sediment yield of 292.694 g, which was 10.07 times greater. Furthermore, rainfall significantly increased the moisture content of the shallow soil layer, reducing the shear strength of the soil. Post-rainfall, the shear strength of the shallow soil layer (0–20 cm) decreased significantly, with cohesion reductions of 48.47% for lightly degraded grasslands and 35.42% for severely degraded grasslands, whereas the deep soil layer (40–60 cm) presented no significant change in cohesion, underscoring the critical role of vegetation roots in maintaining shallow soil stability. The synergistic effect of vegetation's three-dimensional structure (canopy-litter-root system) established a hierarchical rainwater regulatory mechanism in slope hydrology, providing vital theoretical and practical insights for soil and water conservation and ecological restoration in the Yellow River source region.