###### Abstract  
Dripping rainfall simulators (DRS) for soil research generate water drops with different types of drippers, but metal tubes are most commonly used, often in the form of hypodermic needles. However, scientific papers using dripping rainfall simulators are often incomplete in terms of data on hypodermic needle characteristics, as well as data on drops produced by hypodermic needles under different water pressures. This study determines which drop sizes and dripping speeds are generated by various hypodermic needles at different water pressures. For the purpose of this study, a dripping rainfall simulator was designed and constructed for laboratory use. Water drops were generated with 11 different needles, ranging in size from 16 G to 32 G (tube gauge number), at different water pressures. Measured water drop sizes ranged from 1.42 to 3.69 mm at a dripping speed between 10 and 360 drops per minute and water head from 14 to over 1970 mm. Measured drop sizes, supplemented with data from previous studies, provided information on the relation between drop sizes and the size of the hypodermic needles. [PERSON]'s numerical model provided estimations of the fall velocity for different drop diameters and their kinetic energy for falling heights up to 11.5 m. The results of this research can be used to design dripping rainfall simulators for soil research. soil research; rainfall simulator with drippers; water drop diameter; dripping speed; water drop kinetic energy +
Footnote †: journal: Applied Sciences  
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Footnote 0: _Applied Sciences_  
drippers, such as plastic and glass tubes, drippers in the form of holes, irrigation drippers, and hanging yarn [10; 11]. Within the group of DRS, two groups of simulators can be distinguished, one group that has more than one dripper (DRS\({}_{>1}\)) [12; 13; 14; 15] and a specific group of simulators that has only one dripper (DRS\({}_{=1}\)) [16; 17; 18]. In the hydraulic system of DRS, the water is under relatively low pressure compared to the hydraulic system of the spraying rainfall simulators [19]. Since the liquid flow is relatively low, the drop on a dripper initially grows relatively slowly, establishing a quasi-static equilibrium between the force of gravity and the surface tension. However, at the moment when the volume of the drop reaches a critical value, the balance of forces is disturbed and the evolution of the drop is accelerated, leading to its elongation and eventual breaking off [20; 21; 22] with the establishment of a relatively low initial falling speed [23]. Metal capillary tubes are one of the most commonly used drippers, especially in the form of hypodermic needles [11]. Hypodermic needles are primarily used as medical tools and are usually relatively thin, hollow, stainless-steel tubes with a sharp tip on one end and a colored plastic adapter on the other end that connects the needle to a syringe. The size of the needles, i.e., the dimensions of their inner and outer diameters, is standardized according to the main system for measuring the diameter of hypodermic needles called Birmingham measurements [24]. Each needle is marked with a G mark and a number from 7 to 33, which corresponds to the size of the needle. Additionally, the needle adapters are made in different colors according to international standards ISO 6009:2016 [25] and ISO 9626:2014(E) [26] to facilitate the identification of the needle size [27]. Up to now, many soil studies with rainfall simulations have used hypodermic needles as drippers [6; 7; 28]. Hypodermic needles used in the function of rainfall simulator drippers are very suitable for soil research [1; 2; 29]. They generate water drops that match the size of natural raindrops [30], have standardized dimensions, can generate a wide range of drop sizes, are easy to use, are easily accessible on the market, have a relatively low price, have high durability, do not rust, and can be easily replaced. Ref.