Evaluation of the effects of grass and shrub cover on overland flow resistance and its attributes under simulated rainfall

Vegetation plays a crucial role in controlling soil erosion and regulating overland flow connectivity. However, the effects of vegetation coverage on the overland flow resistance mechanisms remain unclear. This study aimed to examine the impact of grass and shrub cover on the resistance to overland flow concerning a random patch distribution pattern. This was achieved through a series of simulated rainfall experiments. Thirty slope surfaces with different vegetation coverages, consisting of complete combinations of five grass and six shrub coverages, were tested under five rainfall intensities (60-120 mm h(-1)) and three slope gradients (8.75%-26.79%). The results indicated that the Darcy-Weisbach friction factor (f) values on unvegetated slopes ranged from 0.455 to 1.385, whereas those on vegetated slopes ranged from 0.500 to 5.624. Vegetation coverage increased flow resistance on hillslopes, and the relationship between f and the Reynolds number (Re) was influenced by vegetation cover and the slope gradient; thus, it showed a positive correlation when vegetation form resistance was dominant. In contrast, a negative correlation was observed. According to the t-test results, the linear superposition method had limitations in calculating overland flow resistance and was only applicable when the grass coverage was 10%. Moreover, as the coverage increased, significant differences (p < 0.05) emerged between the total resistance and the linear summation of the individual resistance components. Additional vegetation resistance plays a significant role in impeding the water flow, contributing up to 50% of the total resistance. Finally, an equation for calculating additional vegetation resistance was developed using dimensional analysis and multiple nonlinear regression. The accuracy of the equation was validated by comparing it with experimental results (NSE = 0.918, RMSE = 0.144). These findings emphasize the effectiveness of constructing grass-shrub communities and increasing vegetation coverage as measures for soil and water conservation.