## 1 Introduction  
Soil erosion by water is the most common soil degradation process globally, and in arable cropland it is consistently higher than soil formation [1]. Detailed knowledge of the processes that control erosion on arable croplands contributes to better application of soil management techniques that minimize and control soil erosion risk [2]. Splash erosion starts with the raindrop impact on the soil surface, which represents the first stage in soil erosion by water [3]. The detached soil particles transported by raindrop impact are deposited on the near-distance soil surface or are transported further by surface runoff if the infiltration capacity of the soil is reached [4]. The major splash erosion driver is the erosivity of rainfall, which can be expressed by parameters like rainfall intensity [5, 6, 7], kinetic energy (\(KE\)) [8, 9, 10], rainfall erosivity (\(EI_{30}\)) [11], or raindrop momentum [12]. Apart from the rainfall properties, the detachment of soil particles also depends on soil physico-chemical characteristics, such as infiltration capacity [13; 14], initial water content [15; 16], the ability to form stable aggregates and crusts [17; 18], organic matter content [19], texture, cohesion, porosity, capacity of ionic interchange, and clay content [20]. Splash erosion measurements on a small scale are usually done through splash cups or splash containers [21; 22; 23; 24; 25; 26]. Most of the splash erosion studies have been conducted in the laboratories using rainfall simulators, where by controlling the raindrop size and fall height, the \(KE\) of raindrops is adjusted [27; 28; 29; 30; 12]. However, rainfall simulators often do not reproduce the same rainfall drop size and velocity distribution characteristics as in nature [31]. As the velocity of raindrops is controlled by the height at which the nozzles are located, due to space or design limitations, sometimes raindrops cannot reach the terminal velocity of natural raindrops [32]. By applying different pressure at the nozzle, the raindrop velocity can be adjusted regarding to the raindrop size; however, large drops are unlikely to reach their terminal velocity, and consequently the \(KE\), of natural rainfall [33]. Nevertheless, laboratory experiments improve the consistency of the results by minimizing the effects of the various uncontrolled factors that are present in the field [34], and also allow experiments to be repeated. Splash erosion experiments under natural rainfall investigate the relationship between rainfall erosivity and splash detachment [3]. [PERSON] [23] observed the splash erosion under natural rainfall for 100 consecutive days, comparing four different soil textures. The \(KE\) of rainfall was calculated from the 10 min rainfall intensity values, using the formula from [PERSON] [35]. Splash erosion of the bare soil was significantly correlated with \(KE\). [PERSON] [36] collected data at 21 sites in Belgium using circular splash cups. He found that the product of rainfall \(KE\) and drop circumference are better at expressing the rainfall erosivity compared to \(KE\) and intensity, or when the 0.75 power of rainfall intensity is used. However, a detailed drop size distribution (DSD) was not available at the time, and the fall velocities of raindrops were based on data by [PERSON] [37]. Splash erosion under natural conditions is primarily affected by rainfall DSD. The ability of raindrop impact to cause splash erosion (rainfall erosivity) is mainly dependent on drop size and drop fall velocity [12]. Direct measurements of raindrop size and velocity provide precise information about the erosivity of rainstorms--namely, \(KE\). When the raindrop size and velocity is not directly measured, the rainfall \(KE\) is estimated from the experimentally based equations between rainfall intensity and \(KE\) from other studies. Theoretically obtained rainfall \(KE\) could underestimate or overestimate the real \(KE\)[38; 39; 40].