Effect of grass basal diameter on hydraulic properties and sediment yield processes in gully beds in the dry-hot valley region of Southwest China

Vegetation is an important factor impacting the hydrodynamic processes of gully beds and further affecting the headward erosion of gullies. Gully erosion is one of the major contributors to severe land degradation in the Yuanmou dry-hot valley region of Southwest China where soil erosion rates are estimated ranging from 8000 to 20,000t·km−2·a−1, with a mean gully distribution density ranging from 3 to 5km·km−2. However, few studies have been performed in this area which focused on the influence of the aboveground part of grass on soil erosion under natural conditions in gullies. To quantify the temporal variation of hydraulic properties (i.e., shear stress (τ), Darcy–Weisbach friction factor (resistance f) and Sediment Concentration (SC) and analyse the change trends of hydraulic properties and SC in gully bed along with the downslope direction under different grass basal diameters, a series of in situ scouring experiments were conducted in development areas of gully erosion in the Yuanmou dry-hot valley region. With the grass basal diameter increased from 0 (no grass) to 17 to 43 to 70 to 98mm in gully beds, the mean shear stress of concentrated flow increased slightly (4.03 to 4.49Pa) and then decreased obviously (4.49 to 3.45Pa). On the other hand, increasing trends were observed in temporal variations of shear stress for every grass basal diameter and the increase rate varied from 0.05 to 0.18. Whereas no notable regular changes in shear stress were detected in the downslope direction for grass basal diameters of 0 to 43mm, obviously increasing trends were observed for the grass basal diameters of 70mm and 98mm. The resistance f experienced a notable increase with increasing grass basal diameter in this study. A logarithmic growth of resistance f was observed in the gully bed as the experiment progressed (f=aln(t+b), P<0.01), and increasing trends were detected for the resistance f in the downslope direction of the gully bed for all grass basal diameters although regression equation could only be fitted for grass basal diameter of 98mm (f98=2.438ln(DOH98−2.643), P<0.01). However, the SC showed an exponential decline with the scouring time. And the SC showed an increasing trend along with the downslope direction of the gully bed in all experiments. In addition, a negative correlation could be detected between SC and resistance f in all five grass basal diameter experiments. In this study, only the disposal for grass basal diameter of 98mm was clearly more effective than other disposals in conserving gully bed, which might because grass basal diameter (≥98mm) that could cover a relatively large section of gully bed could exert apparent impact on reducing runoff shear stress, increasing resistance coefficient and then further decline the sediment yield. In contrast, when grass basal diameter was <98mm, the reduction effect for soil erosion was very limited due to overland concentrated flow could detour around the grass base and erode the gully bed.