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Vegetation plays a significant role in preventing desertification, conservation of soil and water. However, nowadays, there is still uncertainty regarding the mechanisms of flow resistance caused by dissimilar vegetation covers. To address this gap, a series of these was conducted in this study to investigate the influence of synthetic grass and stems on the variation of resistance on overland flows. Thirty vegetation configurations were selected (combining five synthetic grass coverage options and six synthetic stem coverage ones), and tested against five unit discharges (flow range = 0.28–2.22 L m−1 s−1) and four slope gradients (3.49%–20.78%). The results obtained showed that the unit discharge is the key driving factor for the transition from laminar to transitional flow. Furthermore, the relationship between the resistance coefficient (f) and the Reynolds number (Re) was not monotonically increasing or decreasing, but behaviours observed were specifically linked to each vegetation coverage. However, a critical coverage threshold was identified, and it corresponded to 2.72% when the slope gradient tested was 3.49%. This threshold decreased with the increase of the slope gradient. In addition, when the vegetation coverage was less than the critical threshold, the f was negatively correlated with Re, otherwise if the vegetation coverage was higher than the critical threshold identified the f–Re relation was positively correlated. The total flow resistance under synthetic grass and stem cover was unequal to the linear superposition of grain resistance and form resistance caused by synthetic stem and grass, which meant that the linear superposition approach is not applicable to overland flows. Finally, a model was developed to predict the flow resistance by applying the π‐theorem and the multiple nonlinear regression analysis and it has been validated against the experimental results confirming its accuracy and high performance (adj.R2 = 0.99, NSE = 0.94).

Grass distribution pattern (GDP) plays a crucial role in soil erosion, which is driven by overland flow on hillslopes caused by rainfall. To quantify the effects of GDPs on the hydrodynamic characteristics of overland flow, indoor scouring experiments were conducted in a hydraulic flume across eight flow discharges (3–45 L min−1), six slope gradients (2–12°), and five GDPs—a vertical strip pattern parallel to the slope direction (VP), chequerboard pattern (CP), small patches distributed in an ‘X’‐shape (XP), a banded pattern perpendicular to slope direction (BP), and a random pattern (RP). The results showed that GDP had a significant effect on the hydrodynamic parameters of overland flow when compared with bare slope (BS). For the five GDPs, the relative water depth, relative flow velocity, relative Froude number, and relative resistance coefficient were positively affected by flow discharge (p < 0.001), and negatively affected by slope gradient (p < 0.001); whereas the effects on the relative Reynolds number were inverted. Compared with the other patterns, the effects of BP on retarding flow was the best, as it maintained particularly remarkable characteristics affecting all five relative hydrodynamic parameters. Through parameterising GDP as a calculation factor, the prediction models of relative flow velocity (adj.R2 = 0.788**, Nash–Sutcliffe efficiency [NSE] = 0.692), and relative Froude number (adj.R2 = 0.756*, NSE = 0.674) were established via nonlinear regression analysis. The effects of GDP on overland flow revealed here can help inform optimal GDP designs for water and soil conservation on sloped environments.