## 4 Discussion
They suggested that rainfall effects on soil loss are based on one-hundredth of the product of rainfall energy for a given storm and the maximum 30 min intensity of the storm. Using this criteria, it is more important to simulate storms based on the EI value (product of total storm energy E and its maximum 30 min intensity I), rather than the intensity or amount of water. Additionally, the availability of large drops is essential when the height of fall is limited. The additional mass of large drops is needed to compensate for the failure of the falling drops to reach terminal velocity, considering that the greatest number of rainfall simulations were conducted with drop diameters larger than 2.0 mm [11]. On the other hand, for natural rainfall, the maximal median volume diameter (D\({}_{0}\)) usually does not exceed 2.5 mm [46] and it is inherent for relatively heavy rains, while for frequent relatively light rains, the D\({}_{0}\) is significantly smaller [47]. Regarding that, it is necessary to emphasize the ability of needles to produce drops smaller than 2 mm. The generation of drops by 32G, 33G, and 34G needles would require the establishment of a significantly higher hydraulic pressure in the hydraulic system of the rainfall simulator (Figure 2b). Such requirements could potentially lead to technical difficulties in developing the simulator due to the possibility to generating too much pressure and possible leakage [3]. On the other hand, with an enlargement of the needle diameter, the pressure would not increase significantly, but the dripping speed may become more sensitive to changes in hydraulic pressure (Figure 2b). Considering that pressure regulation is the primarily way to regulate the dripping speed [11], such an issue could impair the rainfall simulator use. These claims may explain the large number of simulators using 27G needles (0.40 mmOD Nom) [11; 30]. Besides being easily available on the market (due to their medical use), hypodermic needles generate drops whose diameters correspond to the size of drops that can be found in natural rainfall of almost any intensity [48]. Furthermore, needles require very low or no pressure to prevent dripping at the lowest dripping speed. Additionally, their sensitivity to a change in dripping speed with pressure allows reasonably good dripping speed regulation. The terminal velocity of drops increases with drop diameters up to 9 m\(\cdot\)s\({}^{-1}\) for drops larger than 4 mm (Figure 4b). The falling height necessary to achieve the terminal velocity of drops increases with the increment of the drop diameter, reaching almost 11.5 m to reach 99% of the terminal velocity for drop diameters between 3 and 4 mm, after which the needed height decreases slightly for larger drops (Figure 4a). Reaching 95 and 90% of the terminal velocity requires a significantly shorter falling height with a relatively low decrease in the achieved kinetic energy compared to the 99% height. Drops with a diameter less than 1.0 mm require around 2.0 m to reach 95% of the terminal velocity, while larger drops require significantly more than 2 m. For example, drops with a diameter of 2 mm require a falling height of more than 5.6 m to obtain 95% of their terminal velocity, which corresponds to 90% of their kinetic energy at terminal velocity (Figure 4c). Comparing the attained results with those from [31], there is an obvious similarity regarding the relation between the drop diameter and discharge, and the relations between water pressure in the system and dripping speed and discharge. However, there are slight deviations between those relations. Possible reasons for them vary, such as the wall thickness of larger drippers and a difference in the tip geometry. Moreover, a slight difference in the kinetic energy of falling drops is possible due to differences between models that are used for the calculation of the falling drop velocity.