**2. Literature Review of SWS**

SWS is a multidisciplinary term. A ScienceDirect search for "Smart" or "Intelligent" in the title, abstract, and keyword gave a total of 31,527 article results. However, most of them belong to smart transportation, smartphone, and smart grid fields. If "water" is included in the search, the number goes down to 9847. Further searching "Smart Water System" made the results decline to 9517. By adding "framework" to the "Smart water system" only 4026 articles remained. Given that searching results, we conducted the literature review by considering relevant references from the selected papers. The step-by-step literature searching rules were summarized below: (1) "Smart" or "Intelligent" in title, abstract, and keyword with 31,527 article results; (2) "Smart Water" in title, abstract, and keyword with 9517 article results; (3) "Framework" and "Smart Water" in title, abstract, keyword, and body with 4026 article results; (4) "Structure", "Layer", "Framework" and "Smart Water" by manual filtering with 32 article results. These 32 final pieces of literature, including four article forms such as forums, papers, reports, and presentations gain high popularity in multiple sources like Google Scholar, SCOPUS, and ScienceDirect. All the papers come from the recent 10–15 years; they provide a wide presentation of the smart water system for the readers, which include the typical arguments of the framework of SWS. With insight from these literatures in Table 1, there are 17 papers, 10 technical reports from different well-known organizations including the International Telecommunication Union [32], U.S Environment Protection Agency [33], UK Department for International Development [34], UN Global Opportunity Committee [35–38], and Colorado State University [39]. Four key presentations and one International forum [40,41] are also taken into consideration in this review. As the number of studies that we reviewed is limited, this paper does not cover all aspects of SWS.


**1.** Classification of the literature considering the framework in SWS.

**Table** 


**Table 1.** *Cont*.


**Table 1.** *Cont*.

A graphical statistical data overview of the 32 selected literature is presented in Figures 1–4. In Figure 1, these pieces of literature are classified into four types including the forum, presentation, report and paper based on their literature formats. In Figure 2, they are reclassified into three types—academia, industrial, and governmental based on their published organizations. As Figure 1 shows, paper and report take up the highest percentage while key presentations and forums occupy only a small portion, which shows that current SWS studies are mainly documented by papers and reports for efficient sharing. The 22 publications out of a total of 32 from academia in Figure 2 imply that most SWS researchers are from universities and academic research institutions. There is a lack of industrial and governmental inputs for such interdisciplinary work in the SWS field. In Figures 3 and 4, it is clear how the percentages of each type of publication weight in each classification group. For example, in Figure 3, among those SWS papers, the highest percentage of publications are from academia. In contrast to the data presented in Figure 3, Figure 4 shows the different picture that there are equal numbers of all four literature formats in industrial publications.

**Figure 1.** Literature overview: the number of publications for smart water systems (SWS's) definition.

**Figure 2.** Literature overview: the number of the organization for SWS's definition.

0 5 10 15 20 25

**Figure 3.** Literature overview: the number of literature types for different organizations.

**Figure 4.** Literature overview: number of studies for literature type.

The following is a list of works of literature related to the framework of the smart water system in Table 1. We categorized these papers according to the smart water definition, structures (Instrument layer, Property layer, Function layer, Benefit layer, Application layer), and metrics, which correspond with the components of the proposed SWS framework below. The definition provides theoretical support for SWS structures, and the metrics can be used to evaluate the SWS performance. These five basic layers inside structures consist of a comprehensive SWS architecture. The relationship among definitions, layers, and metrics make SWS work, which is considered as the reason for the literature classification in this study. In Table 1, "Instrument" represents the instrument layer of SWS including the physical and ACT and ICT components. "Property" means the property layer of SWS containing the components like systems attributes. "Function" represents the function layer of SWS such as data fusion in the data center. "Benefit" represents the benefit layer including features like water quality security and energy saving. "Application" represents the application layer such as commercial and educational applications. "Metrics" relate to the methods applied to evaluate the smart water system. "Future research" stands for research direction recommendations regarding the smart water system. Each reference might include but not all components (definition, layers, and metrics) of the smart water system. A black solid circle was used to mark the elements that references have covered. It can be found that most of the pieces of literature have definitions of SWS. However, none of them have covered all key layers. Since SWS is built for different purposes, the structure of SWS may vary from case to case. Furthermore, among all those pieces of literature that we reviewed, only 7 of them discussed metrics for SWS. How to assess the performance of SWS was not fully explored based on currently available literature. It was evident that there is still great potential to improve the consensus and understanding of SWS. Table 1 also shows that over 90% of the relevant literature has recommended future research directions for SWS. This paper will summarize these suggestions later in Section 5.
