Indicator-based rural water service sustainability assessment: a review

The aim of this paper is to present the state of the art on the sustainability indices of rural water services in order to identify gaps in knowledge. The methodological approach used was to conduct online searches using the databases of Google Scholar, Web of Science, Pub Med, the International Water Association, and ResearchGate. Several indices have been identified in the literature including the Canadian Water Sustainability Index, Index of Drinking Water Adequacy, the Sustainable Water Governance Index, equity index in water and sanitation, WASH performance Index, Sustainable Water Management Index, and Index of water service quality. However, these indices do not provide an integrative, contextualized and prospective analysis of the sustainability of water services. In addition, these indices select only a few evaluation criteria deemed relevant. In other words, these indices choose to make visible certain phenomena and therefore make others invisible, according to the logic of action of each and to certain social compromises. These difficulties therefore encourage the development of a new index to provide an integrative, contextualized and prospective analysis of the sustainability of rural water services.


INTRODUCTION
Access to safe drinking water is a fundamental and essential human right, essential to health (Bos et al. ).
People without access to water services have less opportunity to realize their potential (Watkins ). Access to an improved water source significantly reduces waterborne diseases (Armah ; Pullan et al. ). The formulation and implementation of international policies such as the Millennium Development Goals (MDGs) and the Sustainable Development Goals (SDGs) has made significant progress in access to safe drinking water over the past three decades. From 2000 to 2017, the proportion of the population with access to safely managed water service increased from 61% to 71%. At the same time, the proportion of the rural population with access to safely managed water services has increased from 39% to 53% (WHO & UNICEF ).
The presence of water supply systems in rural areas does not mean that people have access to reliable and sustainable water services (Majuru et al. ; Penn et al. ). Indeed, drinking water supply in rural areas faces problems and challenges related to inadequacies in maintenance operations and financial management generated by the service Previous literature reviews of the indexes for the analysis of water resource management have been conducted, including by OECD (), Juwana et al. () and Schweitzer et al. (). However, the number of indices developed each year is constantly increasing (Bandura ). This paper is therefore intended to update the review of literature on the indices to analyze water resources management with particular attention to clues to assess the sustainability of water services in rural areas in order to identify knowledge limits and lines of research.

WATER SERVICE DEFINITION AND PRINCIPLE
Water service refers to all the activities and means put in place to provide people with equitable access to quality water, in sufficient quantities and at an affordable price (pS-Eau ). In practice, there is a trend to confuse the service itself, generally defined in terms of the quantity of water, of a given quality, accessible to users, and the system (material) used to provide it. However, the difference between the system and the service is fundamental. The system is the means used to provide the service. For example, a drill equipped with a manual pump provides one type of service, while a home water distribution system provides another (Moriarty et al. ). which is a term used to describe and differentiate the quality of service received by users. According to these authors, four components or principles are used to characterize water services. These include the amount of water, water quality, accessibility and the reliability of the service.

SUSTAINABILITY OF WATER SERVICE
The concept of sustainability applied to the natural environment was introduced by the World Commission for Environment and Development (WCED ). The original definition of sustainability appears to be an intergenerational compromise to meet current needs without compromising the ability of future generations to meet their own. Thus, action is considered sustainable when it is economically profitable, has limited negative impacts on the natural environment, responds equitably to societal expectations and is enshrined in a context of good governance (Edjossan-Sossou ).
This definition has been widely used in many industries and has been adapted to an operational definition. In water services management, Pezon () proposed a normative and functional definition that is the most widely accepted.
According to Pezon, a water service is sustainable if it distributes water that meets standards and ensures the renewal of its capital on the basis of a rate acceptable to users.
However, this definition does not allow policies and managers to anticipate what is uncertain and what may call into question service sustainability. In response to these shortcomings, Brochet () proposed a definition that takes into account elements in terms of external sustainability. According to Brochet (), a drinking water service is sustainable if, while respecting the legal framework in force, it is able to ensure the renewal of its capital, without external financing and adapt to possible contextual variations, on the basis of a rate acceptable to subscribers.

Selection of components and indicators
Components and indicators are the main elements of an Index. Therefore, their selection is extremely important. . The relevance of the data is a qualitative assessment of the value provided by these data. Speed reflects the length of time between data availability and the event or phenomenon they describe, while data accessibility reflects the ease with which data can be located and accessed from original sources. The consistency of the data reflects the extent to which they are logically connected and mutually consistent.

Normalization of components and indicators
In this section, only the most widely used methods of standardizing the values of indicators will be presented.

Min-Max method
The Min-Max method is the most commonly used method in the development of indices. The Min-Max method allows an index to be centered between the extreme values of the sample. Algebraically, the Min-Max method translates into the following Equations, (1a) and (1b): where S i : value of indicator i; X i : current value of indicator i; X min : minimum value of indicator i; X max : maximum value of indicator i.

Ranking method
The ranking method is a simple method that allows the value of indicators to be obtained on the basis of their relative importance. This method is used to compare the values of a particular indicator for different countries or areas. Once the values of the indicators are obtained, they are ranked in increasing or decreasing order, thus allowing the ranking to be defined. Algebraically, this method translates into Equation (2).
where S i : value of indicator i; and X i : actual value of indicator i. The ranking method was used in the development of the Technology Achievement Index (Cherchye et al. ).

Percentage of annual differences over two consecutive years
This method calculates the value of an indicator based on the difference between the current value and the value of the previous year. The use of this method is therefore conditional on the availability of a series of time data on the identified indicator. The general equation of this method is presented below: where S i : value of indicator; X it : current value of indicator i on date t; and X itÀ1 : current value of indicator i to date t-1.

Category scale method
The category scale method assigns values to indicators based on previously defined criteria. It is used for qualitative indicators for which quantitative data are not available.
Algebraically the category scale method translates into Equation (4).
where S i : value of indicator i; X i : Current value of indicator i; Z j : the category for X i that meets the j test; and n : the number of categories.

Distance to a reference
Distance to a reference is a standardization method that associates scores with performance in a domain in reference to a threshold chosen more or less arbitrarily. This threshold may be the performance of the reference country in the initial year. Thus, the values of a country's indicator subindexes are assessed on the basis of their relative condition relative to the benchmark. Algebraically this method translates into Equation (5).
where S i : value of indicator i; X i : current value of indicator i; and X r : current value of reference r.

Weighting dimensions and indicators
Overall, there are two broad categories of methods for weighting components and indicators: statistical methods and participatory methods (Nardo et al. ).

Statistical methods
The main statistical methods used for weighting indicators  If the correlation existed, the second step is to identify common factors, representing the groups of indicators. In PCA, the factors are known as components. The third step is to determine the contribution of each indicator to its corresponding factor using the factor load analysis. Next, the final step is to calculate weights based on the factor load analysis and common factors.

UCM method
The

Aggregation
In index development, aggregation is done sequentially as illustrated in Figure 3 assuming the sustainability index has dimensions or components, indicators, and sub-indicators. In general, there are two main methods of aggregation in the development of indices: the arithmetic method and the geometric method.

Arithmetic method
The arithmetic method is the most commonly used method of aggregation. This method allows for a summary of weighted values of dimensions, indicators, and sub-indicators ( Juwana et al. ) as illustrated by Equation (6).

Geometric method
The geometric method establishes aggregation by multiplying the weighted values of components, indicators and subindicators (Swamee & Tyagi ) as presented in Equation (7).
where I: aggregate index; N: number of indicators to be aggre-

EXISTING WATER SERVICE SUSTAINABILITY INDEXES IN RURAL AREAS
The water sustainability indexes in the literature can be categorized as global water sustainability indexes and regional water sustainability indexes. Global water sustainability indexes have a general scope and can be applied to all contexts without much change. Regional water sustainability indexes are region-specific and cannot be applied to other contexts without modification.

Water poverty index
The Water Poverty Index (WPI) is a mathematical data-driven tool which measures water stress at household and community levels, and is designed to aid decision makers to determine priority needs for interventions in the water sector. The five key components of WPI are combined using the following equation: SI is calculated by users with the index defined as a geometric average of the criteria of M performance (C i m ) for the ith water user: The index was applied with successful results to the Rio

Equity index for water and sanitation
The Equity Index (EI) in water and sanitation was developed by Luh et al. () to measure the progressive realization of the human right to water and sanitation. The index is composed of one structural, one process and two outcome indicators. Thus, EI is the uniformly weighted average of three indexes: a Structural Index (SI), Process Index (PI) and Outcome Index (OI), as given by Equation (10).
EI was applied to 56 States in 2010 in order to measure state progress in realizing substantive equality for the right to water (Luh et al. ). This study demonstrated that EI score is not dependent on factors such as achieved level of coverage or gross national income. However, different weightings to SI, PI and OI components may be addressed by futures studies in order to take into account the relative importance of components, and sensitivity analysis is required to determine which sample weights would fit best from both human and legal standpoints. In addition, the four indicators used to construct the Equity Index were selected based on the availability of data, and as more data becomes available, additional carefully-selected indicators should be included in the index and will help to provide a more representative EI value.

WASH performance index
The They found that progress toward equity in sanitation is significantly associated with governance indicators and suggested that enabling an environment for WASH contributes to progress in sanitation equity. synthetic index representing quality of service was computed as follows:

Molinos
where QS j is the quality of service of jth RDWSS; n ¼ 1, 2, … N where (n) is the number of indicators comprising the quality service synthetic index; W n is the weight of indicator n; Hence, future development on this issue should focus on addressing uncertainty.

Regional water sustainability indexes
Canadian water sustainability index  (12)). that, by addressing five components, attempts to measure sustainability in the context of integrated urban water management. The index of a particular area/region is the sum of all weighted components (Equation (13)).
where X i refers to the standardized value of component i of the index for a particular community; w i is the weighting applied to component i. The tool has been applied to two southern African cities, Hermanus and Maputo. However, the validity of SIIUWM is questioned as a result of the biases introduced by weight allocation. The tool could be improved by future studies by developing weighting schemes through the adoption of a more robust methodology for selecting weights.

Arab water sustainability index
Ali () proposed the Arab Water Sustainability Index (AWSI), a conceptual framework incorporating a variety of physical, socio-economic and environmental water statuses in the Arab region. Four thematic components were proposed for the AWSI to reflect a useful and significant distribution: water congestion, dependence, scarcity, and environmental sustainability. The structure of the AWSI is developed by mathematical aggregation. The final index is obtained using Equation (14).
where w i the weight associated to indicator i; and X i the standardized value of indicator i. The AWSI has been applied in 22 Arab countries with significant results. To upgrade the structure of the index, it is recommended to combine categories based on infrastructure, environment protection, and culture.  that highlight the human dimension of water management in times of scarcity and unequal access to water and sanitation services.

Sustainable cities water index
The Sustainable Cities Water Index (SCWI), developed by Arcadis, explores three aspects that make up robust, efficient and healthy water environments to develop an indicative ranking of cities. The index is a tool to help inform future water improvement and long-term sustainability (Arcadis ). Arcadis () breaks down water sustainability into three fundamental elements ( Figure 5). The SCWI was applied to 50 global cities from 31 countries across all continents of the world by Arcadis (). They found that cities need to make greater investment to improve their resiliency to extreme weather events and unforeseen water shortages.

Sustainable water management index
The Sustainable Water Management Index (SWaM_Index) proposed by Maiolo & Pantusa () is a tool to measure the sustainability of water management and assesses the effects of policies taken to achieve sustainability. Its structure is derived from European legal frameworks for water resource management. The index consists of three pillars: the artificial system (AS), the natural system (NS) and socio-economic and institutional aspects (SEI). The first step in calculating the SWaM_Index is the preprocessing of the data. Once the data is pre-processed, the indicators are standardized using the standardization method against a baseline distance. Aggregation is done sequentially and the SWaM_Index is calculated using Equation (15).
where P p : is the aggregate value of the pillars. WASH Index has still to undergo further development in order to address these issues.
Other water sustainability frameworks These indices refer to the current or past situation of the aspect of the service they are measuring. Their ability to give The objective of the tool is to provide a better understanding of program design, priorities and decision-making within the context of the sector level.
Each country assessment will be unique and may require the investigation of complementary areas.
IRC & Aguaconsult () a valid picture of sustainability in a coherent time horizon is therefore limited. This difficulty therefore encourages futures studies to seek to incorporate a long-term approach that is not only retrospective but also forward-looking. In addition, these indices are based on a forecasting logic of the investments that water service managers at all levels will need to make and can be expected to ensure the sustainability of the service. However, the sustainability of water services cannot be reduced to predictable aspects alone (Brochet ). It must include elements of adaptation to external shocks, which some authors (Walker

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.