In the 21st century, due to population growth and a constant increase in population density in large cities, have generated a crisis of water scarcity in the main metropolises of the world. In Mexico 107 of the 653 aquifers were overexploited (CONAGUA 2018). The study case of this article is centered in the Metropolitan area of Mexico City (CDMX), where the aquifer is overexploited even though it supplies only 55% of water for consumption. Other sources for the supply are the Lerma system, which supplying 14%, and the Cutzamala system supplying 29%. In these systems, the water is imported into the basin and must be raised from its origin into its destination up to 1,000 m located 127 km away. This situation implies that 86% of operating costs are spent in electricity (Breña 2009). The remaining 2% of the water supply is therefore taken from local open to surface sources (springs and Magdalena River). Precipitation in CDMX concentrates 68% between June and September, generating some floods in this period.
This justifies the importance of taking advantage of rainwater for consumption in different activities. Rainwater harvesting (RWH) consists of collecting from selected surfaces, especially roofs and terraces, then storing, and finally employment of collected rainwater. Although this collected water, due to its quality, does not meet the parameters to be considered as drinking water, with the application of simple processes, generally only filters, can achieve the quality of drinking water. This collected water can also be used for non-potable uses, such as flushing toilets, washing clothes, cleaning, and for watering the gardens. RWH has proved to be an excellent tool seen from perspectives, such as: stakeholders, water supply operators, policy makers, and entrepreneurs (Concha et al. 2020). Because they generate savings in billing costs, access to this resource, reduction of supply interruptions, improve health, and even business opportunity by selling and maintaining RWH systems. Social benefits can also be considered, such as long-term availability of water, improvement of water quality, reduction of flooding and mitigation of environmental impacts.
Concha et al. (2020) carried out an evaluation of the potential RWH in CDMX, considering various types of users, local rainfall, and the costs of paying for water. This study focuses on financial benefits, considered as savings on the bill paid by property owners and entrepreneurs. RWHs are more beneficial for homeowners and entrepreneurs that have larger demands, large rooftop areas, and more users paying the high tariff (Concha et al. 2020), and not for small home users, who are often targeted to address a small escalation of RWH. The previous occurs as a direct consequence of the non-linear rate of water collection. This results in turns out that RWHs are economically attractive for domestic users, uncommon in CDMX, with catchment areas up to ranging from 150 to 300 m2 and with a high demand for potable water (> 1,500 L/day). Other economically attractive sites have large catchment areas, for instance, the case studied by Zavala et al. (2018), which corresponds to a logistics company in CDMX, where the sum of the different catchment areas accounts for accounts for more than 17,000 m2 and is sufficient to supply the water demand for the entire company. On the other hand, Ward et al (2010) found that in the UK, RWH in large commercial buildings may be more financially viable than smaller domestic systems. Other RWH study that have found feasible conditions to implement in groups of buildings is that of Cook et al. (2013) in which a group of 46 homes in a peri-urban area of Brisbane, Australia, was considered.
Choice for rainwater tank size for RWH does not obey any practical rule, because it depends on the criterion considered to achieve a certain objective, but mostly, on the relationship capture-consumption which in turn is defined based on the amount of rainfall of the place, and the number of user consumption habits. In most cases, the cost of a single rainwater tank (RWT) represents a significant portion of the initial investment. In the case studied by Zavala et al. (2018), the cost of the RWT represented more than half of the total cost of the system; however, despite this, it resulted in a feasible, reliable, and economically viable solution. Hanson and Vogel (2014) employed a method to estimate the volume of storage required by using regional regression equations considering climatic differences at 231 locations throughout the continental United States. These authors conducted continuous simulations of several days with information on the demand, the collection area, and values for local precipitation. This information allowed to determine the storage-reliability-yield (SRY) relationships and how large the RWT should be. Lawrence and Lopes (2016) determined the reliability of RWT in a performance analysis for three Texas cities under different rainfall conditions and multiple scenarios. The previous highlights demonstrate the importance of optimizing RWT sizing. A simple technique for the selection of RWT sizing is called the Rippl method and has been used by various authors (Matos et al. 2014; Santos and Taveira-Pinto 2013; Shadeed and Alawna 2021). In this method, the optimal storage size is determined by calculating the maximum cumulative positive difference in the dynamics of the RWT with regards to inlets and outlets. As a result of this methodology, the size obtained for the tank is large, and for the areas where the rainy season is dominant, it means that the collected water could be stored for weeks. This situation is an unwanted condition when considering water quality. Sultana (2022) studied the optimum tank size to maximize water savings considering potential of a RWH with a 10,000 m2 rooftop area in a college campus located in California, USA, where weather is semi-arid. The study consisted of a simple daily water balance model comparing the intended capability to capture 100% of the water, against the scenario of sizing the tank using reliability curves (efficiency, spill, and reliability), in which this last case leads to a significant reduction in the tank size of about 40%.
In this article, a daily estimation of the balance between inputs such as rainwater harvesting, storage (different RWTs dimension) and outputs such as water consumption, is performed. In addition, in this study a new approach is pursued in which the behavior of isolated buildings as well as groups of buildings, called clusters, are modeled, and then compared. All these connected buildings, can collect, store, and finally consume water as a single RWH system
Azcapotzalco campus of the Metropolitan Autonomous University (UAM Azc) is in the north of CDMX and has presented problems with the availability of drinking water between the months of March to May. The university community is made up of approximately 18,000 people, including academics personnel, students, and workers. This problem has encouraged adoption of water saving actions, and additionally, has enforced the purchase of water in trucks, a situation that implies increasing expenditures, as opposed to obtaining the water supply from the public network.
All activities conducted in UAM-Azc induce different amounts of water consumption which can be considered through different periods: quarterly, inter-quarterly and vacation. In the quarterly period, due to lectures there is a major attendance of students representing the largest group of the University. During the inter-quarterly period there are no lectures, only a small number of students attend to conduct other minor activities. During the vacation period only the presence of security personnel and reduced number of students and academics can be seen. The campus community, according to data from UAM (2019), accounts for 14,839 students (67.2%); 6,063 academics (27.5%) and 1,170 workers (5.3%).
Solorzano et al. (2019) proposed a methodology to model the amount of collected water, storage, and consumption of rainwater in a RWH located the W building of UAM Azc. This building has an atypical behavior that generates a surplus in the intake-consumption ratio; this is a consequence of the fact that it houses a reduced number of users in both lecture rooms and research laboratories, also owns a vast roof area (1,974 m2). The results of Solórzano et al. (2019) allows to suggest that the excessive volume of water in building W can be used in one or more neighboring buildings that have a deficit, because their catchment area is small or because the number of users is large.
This proposal aims to be a tool that solves or at least mitigates the local problem of water scarcity, but at the same time it is a methodology that can be applied in other places. The proposed system consists of collecting rainwater on the surface of 8 buildings in UAM Azc, to subsequently store it, and use it for non-drinking purposes.