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water Article Raindrop Energy Impact on the Distribution Characteristics of Splash Aggregates of Cultivated Dark Loessial Cores Yu Fu 1 , Guanglu Li 1,2, *, Dong Wang 2 , Tenghui Zheng 1 and Mingxi Yang 2 1 Institute of Soil and Water Conservation, Northwest A&F University, No. 26 Xinong Road, Yangling 712100, China 2 College of Resources and Environment, Northwest A&F University, No. 3 Taicheng Road, Yangling 712100, China * Correspondence: [email protected]; Tel.: + 86-029-87082376 Received: 15 June 2019; Accepted: 18 July 2019; Published: 21 July 2019 Abstract: To determine the e ff ect of di ff erent rainfall energy levels on the breakdown of soil aggregates, this study analyzed the soil splash erosion amounts and the distribution of particle sizes under six rainfall conditions (rainfall energy: 2.41 × 10 - 5 –22.4 × 10 - 5 J m - 2 s - 1 and 1.29 × 10 - 4 J m - 2 s - 1 ) at five splash distances (from 0–10 cm to 40–50 cm). Cores of the size 10 × 20 cm of undisturbed cultivated dark loessial soil were selected in tree replicates as the research subject. The results indicated that splashed aggregates were distributed mainly at splash distances of 0–20 cm, which accounted for 66%–90% of the total splash erosion amount. The splash erosion amount significantly decreased exponentially with increasing splash distance for the same rainfall energy ( p < 0.01). The splash erosion amount significantly increased in the power function relationship with increasing rainfall energy at the same splash distance ( p < 0.05). A model was obtained to predict the splash erosion amount for rainfall energy and splash distance. The fractal dimension ( D ) of the aggregates showed a downward opening parabolic relationship with raindrop energy. The maximal value of the rainfall energy was 1.286 × 10 - 4 J m - 2 s - 1 , which broke the aggregates to the largest degree. Enrichment ratio ( ER ) values for fragments > 2 mm were close to 0. A particle size of 0.25 mm was the critical particle level for splash erosion. Keywords: raindrop energy; soil aggregate; splash distance; fractal dimension; Loess Plateau 1. Introduction Soil erosion reduces land productivity and soil fertility, destroys farmland, exacerbates flood disasters, and results in soil environment deterioration, which a ff ects land exploitation and the protection and utilization of soil and water resources [ 1 , 2 ]. Water erosion is the main type of soil erosion in the Loess Plateau. The aggregates caused by splash erosion are dispersed and broken, which is the initial stage of water erosion [ 1 4 ]. Raindrops fall from the air and impact the surface soil particles at a certain speed under the action of gravity. Some soil particles are separated from the soil and become loose particles, which is conducive to the formation and flow of surface runo ff . At the same time, the process provides abundant loose particles for subsequent runo ff transport [ 5 , 6 ]. Additionally, soil particle transport leads to reduced or blocked pores on the soil surface and reduced soil permeability due to soil crust formation [ 7 9 ].

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