Estimating the Effect of Rain Splash on Soil Particle Transport by Using a Modified Model: Study on Short Hillslopes in Northern China

Splash erosion is an important soil erosion process in sloping lands. This study aims to improve the model of rain splash transport based on the results of previous studies and field experiments involving rainfall simulation. A field study was conducted to examine the effects of rainfall properties, herbaceous cover and surface flow on splash processes on hillslopes in northern China. On the basis of the experimental results, a comprehensive model of rain splash was established through the quantitative analysis of the interactive effects of rainfall kinetic energy, vegetation coverage and overland runoff depth on splash erosion rate and the probability density of splashed particles and maximum splash distance. The results showed that the estimated and observed values of splash transport exhibit high consistency and adaptability. However, several discrepancies were observed between the estimated and observed values for events with high vegetation coverage. These differences can be ascribed to the variation in overland runoff connectivity and the differences in soil surface cohesion at various wetness degrees. The proposed model provides insights into splash erosion characteristics and suggestions for erosion control practices on hillslopes.

Effects of vegetation lodging on overland runoff flow regime and resistance

Vegetation is a vital part of the natural environment. Variations in vegetation morphology produce changes in the mechanical and fluid characteristics of overland flow. Determining the effects of vegetation lodging on the overland runoff flow regime and resistance is a prerequisite for accurately simulating overland runoff and convergence, revealing the mechanism of overland flow propagation, and the design and management of vegetation protection, soil consolidation, and ecological slope engineering. To systematically study the effects of vegetation lodging on overland runoff, four planting vegetation lodging angles (α) and 10 test water depths were used to simulate experimental research with a 1.0% slope ratio. Experimental results show that the depth and state of vegetation inundation and the degree of lodging significantly influence the flow regime and resistance. Under the same water depth, higher values of α are associated with higher values of the flow velocity, Reynolds number, Froude number, and Darcy–Weisbach resistance coefficient (f), and lower values of the drag coefficient (CD). The overall result is enhanced turbulence in the flow field and weaker flow resistance. Numerical statistics and difference analysis indicate that, when the vegetation is non-submerged, a 10° increase in α produces a 9.30% decrease in f. In the submerged state, a 10° increase in α causes a 26.70% decrease in f. CD is greatly affected by the boundary water depth. Below some critical water depths, an increase of 10° in α reduces CD by 8.48%. Above the critical depth, a 10° increase in α decreases CD by 41.10%.

Commemorating the 50th anniversary of the Freeze and Harlan (1969) Blueprint for a physically-based, digitally-simulated hydrologic response model

<p>The year 2019 marked the 50th anniversary of a pioneering publication in hydrology. Allan Freeze and Richard Harlan published their <em>Blueprint for a physically-based, digitally-simulated hydrologic response model</em> (Freeze and Harlan, 1969) in <em>Journal of Hydrology</em>. Their vision was for a futuristic model that would integrate key processes and compartments in the hydrologic cycle: precipitation, evapotranspiration, overland runoff, infiltration and groundwater exchange (into and out of) surface water bodies, such as rivers and lakes. Today, the original Blueprint is a reality.</p><p>We recently published a paper in <em>Journal of Hydrology</em> to commemorate the 50 year anniversary of the original Blueprint paper (Simmons et al., 2019). In this talk, we present an overview of, and highlights from, this paper.</p><p>Through personal communications with Allan Freeze, we present the history and genesis of the Blueprint paper. We reflect on the uptake of the Blueprint into modern hydrology, the development of numerical models that enabled this, and the range of challenges being tackled by these models. Finally, we consider challenges and opportunities for the future of this area of modelling and hydrologic science.</p><p> </p><p><strong>Reference</strong></p><p>Simmons, C.T., Brunner, P., Therrien, R., and Sudicky, E.A., 2019. <em>Commemorating the 50th anniversary of the Freeze and Harlan (1969) Blueprint for a physically-based, digitally-simulated hydrologic response model</em>, Journal of Hydrology, https://doi.org/10.1016/j.jhydrol.2019.124309  </p>

Runoff on rooted trees

We introduce an idealised model for overland flow generated by rain falling on a hillslope. Our prime motivation is to show how the coalescence of runoff streams promotes the total generation of runoff. We show that, for our model, as the rate of rainfall increases in relation to the soil infiltration rate there is a distinct phase change. For low rainfall (the subcritical case) only the bottom of the hillslope contributes to the total overland runoff, while for high rainfall (the supercritical case) the whole slope contributes and the total runoff increases dramatically. We identify the critical point at which the phase change occurs, and show how it depends on the degree of coalescence. When there is no stream coalescence the critical point occurs when the rainfall rate equals the average infiltration rate, but when we allow coalescence the critical point occurs when the rainfall rate is less than the average infiltration rate, and increasing the amount of coalescence increases the total expected runoff.

The resistance effect of vegetation stem diameter on overland runoff under different slope gradients

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 three different slope gradients (s), three 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.