Vegetation is an important part of the natural environment and has resistance effects on overland runoff, which can effectively reduce hydraulic erosion. The effect of vegetation stem diameter and slope gradient on flow resistance is thus worthy of further study. The influence of 3 different slope gradients (s), 3 vegetation stem diameters (d) and 12 levels of unit discharge (q) on the flow resistance of a slope was simulated to systematically study the effect of vegetation stem diameter and slope gradient on overland runoff. The diameter of the vegetation stem and the slope gradient were found to have a significant resistance effect on overland runoff. Under the same slope gradient, the Darcy–Weisbach resistance factor (f) increased with an increase in the vegetation stem diameter. Under experimental conditions, the rate of change of f was analysed by linear regression, and as d increased by 1 mm, f increased by an average of 49.9%. For a given vegetation stem diameter and vegetation distribution pattern, the greater the slope gradient, the smaller the value of f, and as S increased by 1.0%, f decreased by an average of 24.5%. These results are important to optimize the slope vegetation distribution in farmland conservation.

In this study, we investigated the origin of the overland flow roughness problem and divided the current overland flow roughness research into three types, as follows: the first type of research takes into account the effects of roughness on the volume and velocity of surface runoff, flood peaks, and the scouring capability of flows, but has not addressed the spatial variability of roughness in detail; the second type of research considers that surface roughness varies spatially with different land usage types, land-cover conditions, and different tillage forms, but lacks a quantitative study of the spatial variability; and the third type of research simply deals with the spatial variability of roughness in each grid cell or land type. We present three shortcomings of the current overland flow roughness research, including (1) the neglect of roughness in distributed hydrological models when simulating the overland flow direction and distribution, (2) the lack of consideration of spatial variability of roughness in hydrological models, and (3) the failure to distinguish the roughness formulas in different overland flow regimes. To solve these problems, distributed hydrological model research should focus on four aspects in regard to overland flow: velocity field observations, flow regime mechanisms, a basic roughness theory, and scale problems

Soil erosion of farmland is a major issue faced by the agricultural industry. Control of water-induced erosion can be improved through the simulation of different configurations of vegetation row direction to analyze the effects of farmland vegetation row directions RD on the hydraulic characteristics of flow. The farmland vegetation row directions were simulated and the anisotropic influence of the farmland vegetation row direction on flow hydraulic characteristics was experimentally analyzed. Three groups of farmland vegetation row directions with angles θ of 15°, 45°, and 90° with respect to the flow direction FD were configured. Experimental results show that different angles caused variation in hydraulic parameters velocity V, Reynolds number Re, Froude number Fr, and Darcy–Weisbach friction factor λ along the directions of flow. The rates of change of λ, Re, and Fr were analyzed statistically, where θ was increased every 10°; the average rate of decrease of λ was 5.2% and the average rates of increase of Re and Fr were 2.2% and 3.4%, respectively. These results indicate that the vegetation distribution pattern on farmland has an anisotropic influence on the hydraulic characteristics of flow. This investigation provides valuable references for the optimization of farmland vegetation distribution and soil erosion control.

To explore the characteristics of overland flow resistance under the condition of sparse vegetative stem coverage and improve the basic theoretical research of overland flow, the resistance characteristics of overland flow were systematically investigated under four slope gradients (S), seven flow discharges (Q), and six degrees of vegetation coverage (Cr). The results show that the Manning roughness coefficient (n) changes with the ratio of water depth to vegetation height (h/hv) while the Reynolds number (Re), Froude number (Fr), and slope (S) are closely related to vegetation coverage. Meanwhile, h/hv, Re, and Cr have strong positive correlations with n, while Fr and S have strong negative correlations with n. Through data regression analysis, a power function relationship between n and hydraulic parameters was observed and sensitivity analysis was performed. It was concluded that the relationship between n and h/hv, Re, Cr, Q, and S shows the same law; in particular, for sparse stem vegetation coverage, Cr is the dominant factor affecting overland flow resistance under zero slope condition, while Cr is no longer the first dominant factor affecting overland flow resistance under non-zero slope condition. In the relationship between n and Fr, Cr has the least effect on overland flow resistance. This indicates that when Manning roughness coefficient is correlated with different hydraulic parameters, the same vegetation coverage has different effects on overland flow resistance. Therefore, it is necessary to study overland flow resistance under the condition of sparse stalk vegetation coverage.