Modelling raindrop impact and splash erosion processes within a spatial cell: a stochastic approach

A new approach is proposed to simulate splash erosion on local soil surfaces. Without the effect of wind and other raindrops, the impact of free-falling raindrops was considered as an independent event from the stochastic viewpoint. The erosivity of a single raindrop depending on its kinetic energy was computed by an empirical relationship in which the kinetic energy was expressed as a power function of the equivalent diameter of the raindrop. An empirical linear function combining the kinetic energy and soil shear strength was used to estimate the impacted amount of soil particles by a single raindrop. Considering an ideal local soil surface with size of I m x I m, the expected number of received free-failing raindrops with different diameters per unit time was described by the combination of the raindrop size distribution function and the terminal velocity of raindrops. The total splash amount was seen as the sum of the impact amount by all raindrops in the rainfall event. The total splash amount per unit time was subdivided into three different components, including net splash amount, single impact amount and re-detachment amount. The re-detachment amount was obtained by a spatial geometric probability derived using the Poisson function in which overlapped impacted areas were considered. The net splash amount was defined as the mass of soil particles collected outside the splash dish. It was estimated by another spatial geometric probability in which the average splashed distance related to the median grain size of soil and effects of other impacted soil particles and other free-falling raindrops were considered. Splash experiments in artificial rainfall were carried out to validate the availability and accuracy of the model. Our simulated results suggested that the net splash amount and re-detachment amount were small parts of the total splash amount. Their proportions were 0.15% and 2.6%, respectively. The comparison of simulated data with measured data showed that this model could be applied to simulate the soil-splash process successfully and needed information of the rainfall intensity and original soil properties including initial bulk intensity, water content, median grain size and some empirical constants related to the soil surface shear strength, the raindrop size distribution function and the average splashed distance. Copyright (c) 2007 John Wiley & Sons, Ltd.

黄土结皮的发育机理与侵蚀效应研究

了解土壤结皮的发育特征及其对侵蚀的影响作用,是认识土壤侵蚀机理、建立精准侵蚀预报模型的重要理论支持之一。本文通过模拟降雨试验,探讨有无雨滴打击作用下黄土结皮的发育过程以及前期有无结皮时结皮侵蚀效应的动态特征。结果表明:(1)随降雨的进行,黄土结皮发生层容重、抗剪强度(含水率为20%)不断增加,30min内形成厚度约3～4mm稳定结皮层;(2)消除雨滴打击后,黄土结皮发育过程同有雨滴打击时类似,但程度减弱。雨滴打击同湿润作用对黄土结皮形成的贡献作用基本相当;(3)降雨初期有、无结皮处理抗剪强度差别大,随后急剧下降并趋同,因而黄土结皮影响抗蚀性的作用微弱,累积溅蚀量的差异主要由溅蚀起始时间的变化引起;(4)黄土结皮的存在明显的减少入渗、增加径流,但结皮效应随降雨进行逐渐消失。溅蚀效应的函数表达式为C=0.6670ln(t)-3.2459,结合溅蚀量的计算式,可较为准确地预测前期无结皮时某降雨历时的累积溅蚀量。