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• Manning's n increased exponentially with increasing rainfall intensity. • Manning's correction Δ n r increased linearly with increasing rainfall intensity. • Two new equations for predicting Manning's n and Δ n r were established. • The effects of rainfall on the Manning coefficient of sheet flow cannot be ignored. Many hydrological and soil erosion models employ Manning's coefficient (n) to calculate the velocity of sheet flow (SF). However, Manning's coefficient of SF cannot be estimated accurately due to the strong influences of rainfall, slope gradient, ground surface roughness, sediment size, etc. The effects of rainfall, especially rainstorms, on Manning's n are usually ignored. In this study, flume tests were conducted with rainfall intensities ranging from 40 to 120 mm h−1, unit width inflow discharges ranging from 0.02 to 0.06 m2 min−1, bed surface roughness values ranging from 0.009 to 0.25 mm, and slope gradients ranging from 3° to 7°. Manning's n and the correction (Δ n r) induced by raindrops were calculated. The results indicated that Manning's n without rainfall increased with increasing surface roughness and decreasing unit width inflow discharge. In rainstorms, Manning's n increased with increasing rainfall intensity and surface roughness and decreasing unit width inflow discharge. Δ n r increased with increasing rainfall intensity and surface roughness and decreased slightly with increasing unit width inflow discharge. Manning's n significantly decreased with increasing Froude number (Fr) according to a power function, while Δ n r significantly decreased logarithmically with increasing Fr. The Δ n r values of this study were greater than 0 when Fr was less than approximately 2.15. For slope gradients of 3°, 5° and 7°, the rainfall increased n by 1.47–49.33 %, 0.44–21.81 % and 0.43–23.94 % for the cases of Fr < 2.15, respectively. This result indicates that the effects of rainfall on Manning's n of SF cannot be ignored for laminar flows and low Reynolds numbers (<900). Additionally, two empirical equations for predicting Manning's n and Δ n r under rainfall conditions were devised. The correction Δ n r due to raindrop impact was affected by the inflow or base flow regimes. This study experimentally provides basic insights into the effects of rainfall on Manning's coefficient. [ABSTRACT FROM AUTHOR]

[Display omitted] • Under low-cover vegetation, biocrust was dominant in slowing the flow velocity. • Vegetation and biocrust interacted with each other to create additional resistance. • The biocrust had a larger flow resistance than both grass and shrub. • Form resistance of vegetation and biocrust played a vital role in total resistance. Biological crust (BSC) is a common kind of ground cover that affects overland flow together with vegetation. It is important to understand the hydrodynamic mechanism of slope covered by both vegetation and BSC to clarify the water erosion dynamics. However, the coupling effects of vegetation and BSC on overland flow characteristics are still unclear. In this study, simulated rainfall was employed to explore the effects of different combinations of grasses, shrubs and BSC on the hydrodynamic characteristics of flow. The results showed that grasses, shrubs, BSC and vegetation combinations significantly reduced the flow velocity. Compared with the control check (CK), the flow velocities of the more BSC (MBSC), grass cover (GC) and shrub cover (SC) treatments were reduced by 50 %, 32 % and 14 %, respectively. The detention coefficient of different treatments increased with increasing BSC coverage. This result indicated that BSC played a dominant role in slowing the flow velocity under low-cover vegetation. The Froude number (Fr) of CK, GC, SC, and SC+GC were > 1 during rainfall, indicating supercritical flow. The average Fr of the other treatments was < 1, but Fr > 1 was found in the less BSC (LBSC) and SC+LBSC treatments. The total flow resistance under different vegetation combinations did not conform to the simple linear stacking relationship. The interaction effect between different covers generated additional resistance, which varied among different ground cover combinations. The relationship between resistance coefficient (f) and Reynolds number (Re) gradually changed from a negative correlation for bare slopes to a positive correlation when vegetation or BSC existed, indicating that form resistance played a dominant role. It was concluded that BSC had a dominant effect on the hydraulic characteristics of overland flow in all ground cover treatments. This research provides a theoretical basis for the mechanism of erosion dynamics on slopes with combined vegetation and BSC. [ABSTRACT FROM AUTHOR]