**1. Introduction**

The world's urban population has grown rapidly from 1.019 billion in 1960 to 4.117 billion in 2017 [1]. It is estimated that the population will reach 9.7 billion by 2050 [2]. The excessive population growth will cause urgent water problems like water shortage and water quality degradation in urban areas. In the 21st century, the global water sector faces quality and quantity challenges, which are highly related to climate change and population growth [3]. The 2018 Global Risk Report shows that most of the high risks (high-likelihood and high-impact) issues are water-related either directly or indirectly and are currently being exacerbated by climate change [4]. Water crises have become one of the five most significant risks in terms of their societal impacts. Additionally, the breaking of economic growth and unbalanced urbanization can also contribute to water shortage [5,6]. It is predicted that due to population and industrial growth the percentage of water scarcity will increase by 50% in developing countries and decrease by 18% in developed countries by 2025 [7]. One upcoming water scarcity event will occur in Cape Town, where it is supposed to be the first city to experience day zero but will not be the last if these threats still hold [8,9].

However, due to the growing complications of water-related issues such as water shortage, water deterioration, and aging infrastructure, traditional techniques, and management for drinking water supply have gradually shown their drawbacks and incapability to address these water issues [10]. Climate change and anthropic activities exacerbate the water issues by reducing water quantity and deteriorating water quality. Especially, the limited effort on ecological maintenance results in the pollution being increasingly found in the water distribution system for the public around the world. There is an urgent need for modernized water supply technologies to alleviate current water concerns by improving water supply efficiency and approaching sustainable water management globally [11]. Traditionally, engineers and researchers get used to re-sizing water supply systems. However, upgrading the existing water distribution network is time-consuming and costly. Instead, retrofitting the water system with smart components such as sensors, controllers, and a data center can achieve real-time monitoring, transmitting, and controlling in water systems for decision-makers, which is a more cost-effective and sustainable approach to address the water challenges [12].

To date, automated control technology (ACT) and information communication technology (ICT) are applied to tackle existing problems in water distribution networks, where both technologies play critical roles in large-scale ACT and ICT applications. A number of study cases around the world consider using smart water metering to monitor the water consumption and further track leakage and pipe burst issues in water distribution networks [7]. The real-time measurements can be utilized to improve the accuracy of hydraulic model calibration and forecasting. Real-time control is commonly applied in pumping, valve operation, and scheduling. The water supply efficiency significantly benefits from automatic control technology but the electricity energy efficiency needs optimization in practical applicability. If matched with appropriate and effective ICT or ACT solutions, in the form of a smart water system (SWS), city-wise water issues can be appropriately addressed and managed [13]. In SWS, progress can be made via smart metering (real-time monitoring that transmits data to the utility) and intelligent controlling (real-time feedback and action). For example, the Western Municipal Water District (WMWD) of California utilities have used the SCADA system to manage real-time alarms and automatically operate plants and networks [14]. The implementation of SCADA has been associated with 30% savings on energy use, a 20% decrease in water loss, and a 20% decline in disruption [15]. In Brisbane City, Australia, the web-based communication and information system tools are used by governments and municipalities to deliver relevant water information to the public, as well as to provide early warnings [16]. Another SWS case is in Singapore, where a real-time monitoring system called WaterWiSe was built, utilizing wireless sensor networks and data acquisition platforms to improve the operational efficiency of the water supply system [17]. Moreover, in San Francisco, the automated real-time water meters are installed among those communities for more than 98% of their 178,000 customers to transmit hourly water consumption data to the billing system via wireless sensing networks [18]. This access to frequently updated water consumption information allows engineers to detect water quality events and localize pipe leaks faster than traditional water systems that are still using existing manually-read meters [19]. Given these ACT and ICT applications in water sectors, smart water concepts therefore emerge and inspire SWS to be widely accepted by large amounts of stakeholders.

The terms "smart water grid," "smart water supply system," "smart water system" or "smart water network" have been widely spread. The concept of SWS in the urban water field is gaining great impetus among academia, government, and industry, drawing attention from international communities (SWAN, EWRI, HIC, and CCWI) to top-level organizations (IWA, AWWA, AWC-Asian Water Council). Other international collaboration projects (e.g., i-WIDGT from EU [20], CANARY from US [21], SEQ from the Australian water resources department [22], and Smart City reports [23]) are providing professional support to smart urban water infrastructure all over the world [24–26]. Although researches on SWS are speeding up to meet the demand of industry and government, the conceptual, technical and practical gaps between providers and clients are still not well bridged. The influences of SWS could be more significant and essential if priorities are precisely defined and

implemented into sensing technology domains in water contexts [27]. As a consequence of the lack of a systematic consensus from conceptual, technical, and practical perspectives, investigating the current architectures, initiatives, and applications of SWS around the world must be required to help generate a better understanding of the definitions, characteristics, and future trends of SWS. While review studies related to SWS and smart grid areas have already been undertaken [28–31], a review of SWS in conceptual and comprehensive prospects, with a further notification on the definition, architecture, and metrics, is missing. To address this research gap, this paper is going to analyze the existing SWS concepts, identify the possible metrics for SWS, and establish a more systematic SWS architecture, to enlighten future research on its implementations.

This review paper is structured as follows: firstly, the current definitions and historical development of SWS are presented and discussed in the introduction section, reviewing the evolution of SWS from the past to the date and analyzing the weakness of the current water system. Secondly, given the previous literature of SWS, a new architecture of SWS with five layers is put forward and demonstrated explicitly. Then, by reconciling definitions and architectures, two metrics of SWS are proposed to characterize its properties. Finally, recommendations on future research directions are given for smart water system development.
