###### Abstract  
The aim of this study was to determine the suitability of a rainwater harvesting system to cover the water demand for indoor hydroponic lettuce cultivation located in Wroclaw (Poland). The analysis was performed on the basis of the recorded rainfall in Wroclaw in 2000-2019. The analyzed cultivation is located in a hall with an area of 300 m\({}^{2}\), where the lettuce is grown vertically by the hydroponic method. The calculations of the rainwater harvesting (RWH) system were carried out considering the selection of the tank capacity for the collected water. The operation of the water storage is simulated using a yield after spillage (YAS) algorithm. It was evident that the proposed system might be an auxiliary system that relieves the water supply network or supports other water recovery systems (e.g., the water vapor condensation in a cross-flow heat exchanger operating as an element of the air conditioning system, proposed in Part 1 of this study). The harvesting system for the selected vertical farming indoor hall covers an average of 35.9% of water needs and allows a saving of 146,510 L of water annually for the cultivation. An average water demand coverage increases up to 90.4%, which allows a saving of 340,300 L per year when the RWH system is combined with water recovery from exhaust air from the hall. a 2020  
the possibility of using a larger crop density, simply makes this method more efficient and effective than with soil [2]. As is evident, the soilless technology itself requires proper optimization to be more sustainable. Other considerations include water supply system, ventilation, HVAC (Heating, Ventilation, Air Conditioning), indoor air circulation, nutrient sanitation, and lighting. In each of these fields, some novel design and technological approaches can be taken. Various aspects of vertical farming were extensively reviewed in [PERSON] et al. [3]. In this article, the main focus is on a sustainable water source for indoor vertical farming. The first part of the study (a long-term analysis of the possibility of water recovery for hydroponic lettuce irrigation in an indoor vertical farm. Part 1: Water recovery from exhaust air [4]) analyzed a simple system of water recovery from exhaust air using a cross-flow exchanger. This method of water recovery only partially provides a water supply for irrigation throughout the year and therefore still depends on an external water supply for irrigation. This is disadvantageous because, in Poland, the water resources are relatively low when compared to the European average. According to [PERSON] et al. [5], the Polish average water resources per capita equals 1600 m\({}^{3}\) while the European average (4800 m\({}^{3}\)) is three times higher. Climate change and global warming affect the fresh water availability [6], hence the Polish water resources are constantly decreasing. With population growth and water usage, there is a risk that the country will have to face water shortages in the future [5]. Poland is located in a temperate climate in which rains are observed throughout the year. The highest average annual rainfall is over 1000 mm in the mountains, while in the lowlands it is between 450-650 mm (563 mm in Wroclaw). In those regions, water harvesting may be proposed as an alternative solution for water recovery for various purposes in households, and in industrial and residential buildings. In addition, water harvesting systems have the advantages of saving energy for pumping raw water [7; 8], relieving drainage, and reducing the risk of local flooding [5; 9; 10]. In fact, such a solution is used in many places, including greenhouses and vertical farms. For instance, in the _Trento AgroFarm_ project rainwater is collected and recovered to the fertigation system, and for non-potable usage (e.g., for the air treatment unit) [11]. Other examples of rainwater reuse for urban agriculture purposes are reported in [12].