Influencing mechanism of microtopography on erosion hydrodynamics on bare slopes of pika mounds in alpine meadows in the Yellow River source area, Western China

• Plateau pika’s activities cause severe soil erosion in the Yellow River source area. • Rainfall intensity is an essential factor affecting microtopography and erosion hydrodynamics. • Surface cutting degree is the best indicator for characterizing erosion hydrodynamics. • Microtopographic changes can directly or indirectly affect erosion hydrodynamics. The mound-building behavior of plateau pika is one of the important factors leading to meadow degradation and soil erosion in the Yellow River source area of Western China, but little is known about the influencing mechanism of microtopography on erosion hydrodynamic characteristics of bare slopes of pika mounds. In this study, we analyzed microtopography and erosion hydrodynamic characteristics on bare slopes of pika mounds under subrainfall conditions through simulation of rainfall events at six intensities (all lasting 60 min), and revealed the influencing mechanism of slope microtopographic changes on erosion hydrodynamics by using redundancy analysis (RDA) and structural equation modeling (SEM). The results showed that the maximum average erosion depth was 4.159 cm, and the maximum erosion areas accounted for 93.40 % of the total area of runoff plots; intense erosion took place mainly in the upper half of the pika mounds, while deposition was mainly limited to the lower half of the pika mounds. Surface flow velocity, flow regime parameters, and erosion power parameters all showed an increasing trend with rainfall intensity (P < 0.05), while the resistance parameters showed a decreasing trend with rainfall intensity (P < 0.05); all slope flows were laminar ( Re < 500), but transitioned from slow flows to rapid flows at a higher rainfall intensity. The RDA results showed that surface cutting degree was the key factor influencing surface runoff volume and sediment yield and hydrodynamic parameters, and that the Reynolds number could be the best hydrodynamic parameter to characterize runoff volume and sediment yield. SEM analysis showed that microtopography influenced water erosion either directly (with a path coefficient of 0.856) or indirectly through hydrodynamic characteristics (path coefficient = 0.742). This research showed that microtopography of the bare slopes had a significant effect on water erosion, and was linked to hydrodynamic characteristics to build up a highly complex dynamic exchange process.