## 5 Conclusions  
The research successfully established a relation between the drop sizes generated by hypodermic needles and the dripping speed at a corresponding water pressure. This research facilitates the future use of hypoderm needles as drippers and the construction of rainfall simulators. The rainfall simulator for this research was constructed to be simple and easily adaptable for various purposes in soil research. The rainfall simulator is described in detail so that its replication would be easy. By using a wide range of needle sizes, it is possible to simulate most natural rainfall drop sizes. Changes in needle diameters and dripping speeds, i.e., water pressures, affect the diameters of the generated drops. Generally, a reduction in the needle diameter leads to decreasing drop diameters. The largest measured diameter is 3.69 mm and is generated using a 16G needle at a dripping speed of 360 drops\(\cdot\)min\({}^{-1}\), while the lowest diameter of 1.42 mm is generated using a 32G needle at a dripping speed of 10 drops\(\cdot\)min\({}^{-1}\). The availability of large drops is essential when the height of fall is limited, while, on the other hand, for the natural rainfall with a maximal median volume diameter (D\({}_{0}\)) usually under 2.5 mm, it is necessary to emphasize the ability of needles to produce drops smaller than 2 mm. This paper provides data on the required falling height of drops and corresponding terminal velocity of fall and, more importantly, the achieved velocity. These can be used to estimate the kinetic energy of the drops, which is very important for the construction of rainfall simulators for soil erosion and conservation research. The obtained results show similarity with those from [31] regarding the relation between the drop diameter and discharge, and the relations between the water pressure in a system and dripping speed and discharge. However, although this study provides relevant information on drop sizes at different dripping speeds and water pressures using distinct standard hypodermic needles, drop size calibration should not be neglected during simulation experiments. The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app14166969/s1](https://www.mdpi.com/article/10.3390/app14166969/s1)]([https://www.mdpi.com/article/10.3390/app14166969/s1](https://www.mdpi.com/article/10.3390/app14166969/s1)): Figure S1: (a) Back side of the structural support (PL), (b) water tank (WT) with a water pump (WP), transparent hoses (H1, H2, and H5), plastic hose coupling (HK1), (c) front side of the structural support (PL) with elements of the DRS\({}_{u1}\), including transparent hoses (H3-H11), plastic hose coupling (HK2), syringes (S1-S5), flow regulators (FR1 and FR2), water filter (WF), three-pronged taps (T1-T5), pressure gauge (PG), air release chamber (ARC), water tank with dripper (WTD), dripper (D), and water drop measuring equipment such as the NeuLog Drop Counter Logger Sensor NUL-223 (DCLS), mobile stand (MS), NeuLog module-USB module USB-200 (NLU), and container for receiving drops during measurement in the form of a plastic cup with a volume of 250 mL (RC). Notes: taps on the Figure S1c are not adjusted into any specific position already described. Figure S2: (a) Nomograph for the calculation of dripping rainfall simulator's interconnected variables such as the drop diameter, dripping speed, dripper discharge, dripper density, rainfall intensity, and falling height and kinetic energy of the drops. (b) Nomograph for the calculation of the dripping rainfall simulator's interconnected variables such as the dripping intensity, dripper density, rainfall energy intensity, and kinetic energy of single and multiple drops [51]. Figure S3: Number and spacing of drippers in square and triangular patterns for a square-shaped water reservoir with drippers. Conceptualization, V.R., N.Z., N.A., C.S.S.F. and M.S. ; Methodology, V.R., N.Z., J.H.v.B., T.I., N.A. and C.S.S.F. ; Formal analysis, V.R., N.Z., J.H.v.B., T.I., N.A., C.S.S.F. and M.S.