Water Poverty Index over the Past Two Decades: A Comprehensive Review and Future Prospects—The Middle East as a Case Study

Abstract

This paper summarises the evolution of the Water Poverty Index (WPI) application at different scales since its emergence. The review captures the main milestones and remarkable developments around the world. It sets the foundation for identifying the most appropriate version of the WPI, building on learning from previous versions. In addition, the paper sheds light on the linkages between the WPI and sustainable development goals and applications to fragile contexts. Therefore, it provides a synthesis of knowledge researchers and practitioners’ need in sustainable water resources management that helps boost human development in unstable/fragile arid and semi-arid contexts. The methodology included (i) WPI literature shortlisting and reviewing, (ii) review literature links WPI with sustainable human development and fragility, and (iii) data analysis, identification of gaps and future trends. Intensive research was found to address the limitations of the WPI. However, further research is needed to shortlist the multiple versions of the WPI and match them to their respective scale, purpose and context (including fragile contexts). In addition, a time-based WPI was rarely touched to forecast the impact of decisions on community welfare.

1. Introduction

Providing a reliable, sustainable and safe water supply for people worldwide has become a top priority in the international agenda [1]. However, water managers typically deal with competing priorities but with limited resources. More difficulty in making decisions were found in contexts where water was originally scarce. Therefore, the need has always been to have a comprehensive multidisciplinary tool to help managers and policy-makers take more informed decisions [2]. This need triggered the emergence of the WPI, which provides a standard framework of measurement to effectively communicate complex information to various politicians and policy-makers [3] and move towards a clearer understanding of waterscape complexities [4].

The Middle East is a region that spans southwestern Asia and north-eastern Africa. It consists of 17 countries. It is one of the most water-scarce regions of the world. In the past few years, some countries in this region have experienced a water and sanitation crisis, and there is anticipation that the water situation will continue to deteriorate in these countries Allan, 2001, as cited in [5,6]. Water use for agricultural purposes accounts for more than 90% of the water consumption in several countries [6]. Yet, the rapidly increasing non-agricultural water needs in the area cannot generally be met by further exploitation of water resources. Therefore, the best options to meet water demands is through either the development of expensive desalination facilities or the reallocation of water resources from agriculture [7].

A lack of appropriate actions in Middle Eastern countries could bring significant social and political challenges and risk exacerbating existing inequalities and regional tensions [6,7]. Despite this urgent need, limited research was found in the region exploring tools such as the globally tested WPI to support decision-makers to make more informed decisions. A literature review also revealed that testing the WPI for unstable countries (later defined as fragile countries) was rarely conducted. This could be attributed to the difficulties in data collection and analysis in these regions due to instability, lack of infrastructure, and security issues [8].

The WPI was first developed for local application participation of water managers, hydrologists, economists and other stakeholders to improve the effectiveness and equity of water allocations [9]. As part of that study, to demonstrate the global applicability of the method, the approach was applied to international data and reported for 147 countries worldwide [10].

The WPI emerged as a natural maturing of efforts to develop indices to measure water scarcity in the 1980s. It was found to be remarkably effective for providing a multidisciplinary and transparent tool to measure the water situation; it was reported to be useful for monitoring and benchmarking performance, evaluating policies, and making informed decisions [1,9,10,11,12]. It is easy to measure [2,9,13,14,15,16,17,18,19]. It is considered a robust tool to assist decision-makers in prospect development plans in the water sector [20]. The WPI is inclusive, and it concerns policy-makers, stakeholders, academics, donors, and resource managers [21].

The attraction of a dimensionless, holistic and easy-to-understand index such as the WPI is too important to be replaced by a simple, one-dimension and restrictive indicator [6]. Therefore, the WPI has attracted the attention of thousands of scholars and researchers worldwide as a holistic tool to assess the availability of water resources and access to them at different scales: international [6,10,21], national [6,11,15,18,21,22,23], district /basin [14,18,20,24,25], sub-basin [16,26,27,28] community [2,4,18,29,30] and livelihood group [31] and household level [3].

This comprehensive review of the vast array of research conducted since 2001 is intended to set the ground for more studies to fine-tune this tool by capturing the main milestones of this tool development and the pros and cons of each version of the WPI. Therefore, the main objective of this review is to inspire future research in the field of the WPI by providing a clear summary of the primary studies conducted in the past two decades and highlighting strengths, limitations and gaps where further research is needed, with special focus on the Middle Eastern context. The Middle East was identified intentionally as the case study in this review because of the diversity of contexts for countries composing this region from a fragility perspective and the values of components composing the WPI among the different countries. This overall diversity in the region optimises what has been learned from the WPI development journey and maximises the opportunities for future trends and research.

2. Materials and Methods

The research structure was conducted in three stages as follows:

• Shortlisting of literature included in this review covering the WPI and the case study

The first phase included retrieving the long list of topics related to the subjects and shortlisting them based on the below-mentioned criteria. Various science mapping applications/websites were used, such as Connected-papers [32] and Research Rabbit [33]. Both accessed by 3 August 2024 and before. The search at this stage was exclusively for the WPI, with special attention paid to Middle East water scarcity and management. All the reviewed articles at this step were for Scopus journals. This is attributed to the fact that (a) Scopus is a multidisciplinary database, (b) it has more research in the field of earth sciences than other databases, and (c) it has a broad coverage of journals favouring the natural sciences and natural resources management, especially water [34]. The Google Scholar search engine found about 4000 documents titled “Water Poverty Index,” and 94.3% of these articles had dates after 2006. A total of 714 articles were found, including the words “Middle East”. The main inclusion criteria used to select the database of studies reviewed are as follows:

• The articles reviewed are in English, the most frequent language.
• The articles reviewed intentionally included (a) articles covering theoretical foundations of the WPI; and/or (b) practical studies at global, regional, national, basin and local levels. This was done intentionally to cover the various scales on which the WPI was assessed and/or (c) cover the different WPI versions during this period.
• Articles that cover water scarcity in the Middle East
• The years included were from 2001 to 2023.

II.

The literature review links WPI with sustainable human development and fragility.

Given that one of the main objectives of this review is to set the ground for testing the WPI in fragile contexts to identify the correlation between the WPI, fragility and human development as well as sustainable development goals, literature about the topics below was added to the shortlist:

• SDGs and their relation to the WPI
• Human development and the relation to the WPI
• Fragile contexts and recommended tools to measure state fragility.
• General literature available focusing on

III.

Data analysis and identification of future trends and knowledge gaps

• The shortlisted articles were reviewed chronologically, from the oldest to the newest. The literature findings were categorised according to the topic and focus. A comparison between different studies was made, and the literature review was included. At this stage, alignments and contradictions in the literature were captured and reported. The highlights of each stage of the WPI development journey were captured and reported.
• MS Office software 2016 package was used to document the main findings of the reviewed literature, as well as the alignment, inconsistencies, and contradictions between various studies.
• The authors used the above analysis to identify knowledge gaps and future trends, as well as recommendation for further research.

3. Water Challenges Globally and in the Middle East at a Glance

Around 2.5% of the water on Earth would be abundant if it were evenly distributed [15]. In 1995, the global total water withdrawal was about 3790 km³/year, and consumption was at 61% of withdrawal; a forecast for 2025 showed that water demand will be approximately 5240 km³/year by 2025, which is only 12.3% of the total renewable global water resources.

Despite that, there is no global shortage in overall quantity. Only 0.266% of fresh water is readily available in rivers and lakes [35,36]. Scarcity is found at national and sub-national levels. In some areas, water shortage appears seasonally or spatially within the same region or even sub-region [35,37].

According to [38], the Middle East is a region characterized by numerous tension hotspots. Egypt, which relies entirely on the Nile for its water resources, must share these with 10 other Nile basin countries, particularly Ethiopia, where Lake Victoria—the primary source—is located, and Sudan, through which the river flows before reaching Egypt. Similarly, Iraq and Syria depend on Turkey for their freshwater resources, as the Tigris and Euphrates rivers originate there. Turkey can control the downstream flow of the Euphrates due to the numerous dams it has constructed on the river’s upper course. These dams are often used as leverage by Turkey against its neighbouring countries. According to experts, conflicts over these resources may escalate in the future due to rapid population growth and increasing demands.

Ref. [6] reported that in the Middle East and North Africa (MENA), annual available water is 1383 m³ per person per year compared to 8462 m³ globally. This amount is even less in the Middle East. In 2021, the value of total renewable water resources per capita was found to be less than 163 m³ in Palestine, Jordan, Bahrain, Yemen, Saudi Arabia, Qatar, and UAE, and it was only 4.7 m³ in Kuwait [39]. This puts those countries in absolute water scarcity [7]. Negative coping mechanisms were found in some of these countries, leading to environmental degradation. The latter reported that the financial cost of environmental degradation of water exacerbates the dilemma even more. For example, in the Gaza Strip in Palestine, the over-extraction of water resulted in most of its water being unpotable due to seawater intrusion [40]. In Iran, diseases and deaths resulting from poor sanitation and the subsequent collection and treatment of wastewater were an enormous cost to the nation. This resulted in a loss of 25% of Iranian GDP in 2002 [6]. In alignment with this, ref. [15] underscored that every cubic meter of discharged contaminated wastewater into water bodies and streams renders 8 to 10 cubic meters of pure water unsuitable.

4. Historical Perspectives and Theoretical Foundations

4.1. Evolution of Water Scarcity Indices and Empirical Examples from the Middle East

Much effort has gone into developing indicators of water poverty [35]. Some individuals may face being ”water poor” because they do not have enough water for their basic needs [10]. This can happen if water is not easily accessible, and they might need to walk a long distance to get it. Even if water is nearby, it might be limited for different reasons. Another reason people can be water-poor is if they are financially struggling (income-poor). In this case, even if water is available, they might be unable to afford it.

Historically, using indices and benchmarks to measure access to water is essential because they provide a standardised framework to evaluate progress towards achieving universal access to water. Using benchmarks can help policy-makers and international organisations assess progress towards achieving the UN’s Sustainable Development Goal 6, which aims to ensure all access to water and sanitation by 2030. Additionally, benchmarks help with local, national and regional comparisons, identifying areas of inadequate access and prioritising resources accordingly. Various indicators, indices, and benchmarks were used prior to the introduction of the WPI in 2002.

Table 1. The main water scarcity indicators, respective benchmarks and critical reviews.

Water Poverty Measurement Tool	Main Benchmarks
Indicators/indices that measure human and environmental water requirement
The minimum requirement [41,42]	50 Liters/capita.day (excluding food production)
Water Stress Index (WSI) [43]	[43] categorisation >1700 m3/year No stress 1000–1700 Stress (moderate water shortage) 500–1000 Scarcity (high chronic water shortage) <500 Absolute water scarcity
[41]	1000 m3 per capita per year as a standard that separates the two water conditions: no water stress and water stress
[44]	Advancing a methodology for describing water scarcity as a function of a country’s water balance against its projected needs,
[45] *	Ref. [46] have developed a Water Stress Indicator (WSI) that takes into account environmental water requirements, considered an essential parameter of available water resources                     WSI = ${\displaystyle \frac{Withdrawals}{MAR-EWR}}$ MAR is the mean annual runoff used as a proxy for total water availability, EWR is the estimated environmental water requirement WSI > 1 No stress 0.6 < WSI < 1 Stress (moderate water shortage) 0.3 < WSI < 0.6 Scarcity (high chronic water shortage) WSI < 0.3 Absolute water scarcity
FAO water stress level SDG 6.4.2	$WSL={\displaystyle \frac{\mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r} \mathrm{W}\mathrm{i}\mathrm{t}\mathrm{h}\mathrm{d}\mathrm{r}\mathrm{a}\mathrm{w}\mathrm{a}\mathrm{l}}{Renewablefreshwaterresources-environmentalflowrequirements}}$ Water Withdrawal: The total volume of water removed from rivers, lakes, and aquifers for agriculture, industry, and domestic purposes Renewable Freshwater Resources: The total volume of surface and groundwater resources generated through the hydrological cycle Water Stress Level: The ratio of total freshwater withdrawal to total renewable freshwater resources, expressed as a percentage. Low water stress: Less than 25% Moderate water stress: 25–50% High water stress: 50–75% Very high water stress: Above 75%
Water Resources Vulnerability Indices
Criticality Ratio (CR.)	Criticality Ratio (CR) was defined as the percentage of total annual withdrawals to available freshwater resources (Alcamo et al., 2000) 0–10%: no stress 10–20%: low stress 20–40%: mid stress 40–80%: high stress 80–100%: very high stress
Multidimensional approach
Social water scarcity index (SWSI) [45]	Social water scarcity/stress index (SWSI)                     SWSI = ${\displaystyle \frac{WCI}{\raisebox{1ex}{$HDI$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}$ WCI is the water crowding index (Falkenmark Index) <5: relative sufficiency 5–10: stress 10–20: scarcity >20: beyond the barrier

* Source: reproduced from [6] and [47] with updates from [39].

depicts the leading water scarcity indicators with their respective benchmarks.

Therefore, water scarcity indices are classified under two groups: the conventional indices. These indices were constructed to estimate future water vulnerability [47]. On the other hand, the multidimensional indices aim to develop a better understanding of the relationship between the physical extent of water availability, its ease of abstraction and use, and the level of household and community welfare. This will allow water policy-makers to make more rational and equitable decisions about water allocation [36].

The first group received several key critiques. Firstly, the indices here are mainly used to assess the water situation at global and national levels but discard the specificities of the local level. Brazil is in the “relative sufficiency” top class (>1700 m3/year/person), the dire situation of the Nordeste region being outweighed and masked by that of better-endowed regions Similar observations apply to other countries, such as Chile, Senegal, India, Mali or Iran [40,48]. Moreover, the indicator does not consider infrastructure that affects water availability for users, and the basic thresholds fail to capture significant variations in demand between countries due to factors like lifestyle and climate [40].

Second, up until the introduction of the WPI, water resources were the sole criterion for evaluating water problems. Ref. [48] argued that the relation between water withdrawals and the actual use (or depletion) of water is quite complex. In some basins, half of the diverted water returns to the river and flows to the sea, while in others, only about 5% of the total runoff makes it to the downstream end of the basin. Third, there is a growing awareness that more sophisticated indices are needed to understand better and solve increasing water problems [47,49]. Ref. [35] added one more critique to the first group, which looks at the existing situation. Nevertheless, it fails to differentiate between situational failures in human adaptive capacity and structural impediments. This is a severe failure in the first approach because the adaptive capacity can largely move a country from being water-stressed to being water-sufficient [42].

Furthermore, the conventional approaches to water scarcity also assume that population dynamics uniformly occur throughout all regions within a country [22,47,50]. These approaches also assume that resources within a watershed are uniformly accessible. In addition, conventional measures fall short of addressing the temporal dimension of water availability. Adding to these challenges, traditional metrics neglect to incorporate considerations related to water quality and its temporal fluctuations [15,42]. For example, ref. [48] gave a relevant example that some water resources come in the form of floods. In such a case, adding this water to the total renewable water resource will not help us much in understanding the situation. The latter reported that several indicators from this generation discarded the environmental water needs and disregards the question of virtual water, or whether shortfalls in water should be made up. According to [42], the indicators from this generation including the criticality ratio do not factor in consumptively used (or evapotranspired) water and how much is available for recycling through return flows.

Limited literature was found covering Middle Eastern countries concerning the indices mentioned above. Ref. [6] assessed these countries’ CR, SWSI and WCI. Unsurprisingly, 13 out of 17 countries were found to be at scarcity to absolute scarcity level (see

Table 2. Classification of Middle Eastern countries according to CR, SWSI, WCI.

Criticality Ratio	Countries’ Names	Social Water Scarcity Index	Countries’ Names	Falkenmark Index (WCI)	Countries’ Names
Very high stress	Jordan Syria Iraq Saudi Arabia Bahrain Qatar U.A.E Oman Yemen Israel Egypt	Beyond the barrier	Jordan Saudi Arabia Kuwait Qatar Bahrain UAE Yemen	Absolute scarcity	Palestine Jordan Saudi Arabia Kuwait UAE Bahrain Qatar Yemen Israel
High stress	Palestine Iran Cyprus	Scarcity	Palestine/ Israel	Scarcity	Egypt Syria Oman Cyprus
Mid stress	Lebanon	Stress	Yemen	Stress
Low stress	Turkey	Relative Sufficiency	Egypt Turkey Syria Lebanon Iraq Iran	No stress	Turkey Iraq Iran
No stress	NA
No data	Kuwait	No data		No data

Reproduced from [6].

below).

Ref. [6] reported that despite water scarcity, Saudi Arabia, Qatar, the United Arab Emirates and Kuwait are no longer classified as water-stressed. Because of the higher level of social adaptive capacity represented by a higher HDI, they are put into a relatively sufficient group. On the other hand, a country like Iraq, which has a low social-adaptive capacity, cannot handle water scarcity issues and is thus categorised as water-poor [27].

4.2. Key Theoretical Frameworks Influencing WPI Development

The water poverty index (WPI) is a holistic tool that deals with a wider range of issues linking the availability of water resources to the needs of both humans and the environment. This index is part of a group of water resources sustainability indices, such as the Canadian Water Sustainability Index (CWSI), Watershed Sustainability Index, and West Java Water Sustainability Index (JWSI), as cited by [10]. The CWSI is a composite water index developed by the PRI that measures community well-being concerning freshwater. The index incorporates a range of water-related data into a standardised evaluation framework. It comprises 15 indicators organised into five policy-relevant components: water quantity and quality, ecosystem vitality, infrastructure, socio-economic factors, and community capacity [51]. Likewise, the watershed sustainability index (WSI) was developed to integrate the hydrologic, environmental, life, and policy issues and the existing pressures and policy responses into one quantitative, dynamic, and aggregated indicator. The WSI uses a pressure–state–response function [52]. As for the WJSI, it is a water sustainability index that was developed to help improve the management of water resources in West Java. The WJWSI was specifically developed with the involvement of local water stakeholders and was based on the natural and socio-economic characteristics of West Java. The structure of the WJWSI is based on three components: conservation, water use, and policy and governance [53]

All these indices share a common goal: to help water resource managers, policy-makers, and stakeholders achieve water sustainability and convey this sustainability’s progress to the broader community. The WPI, in particular, has been widely utilised to steer water resources management towards a sustainable future [27].

The WPI was developed based on the concrete foundation of a theoretical framework. According to [36], the WPI is rooted in the works of Townsend (1979) and Sen (1981, 1985, 1995), expanded afterwards by Desai (1995). Desai’s work posits that poverty is a state of being that results from a lack of essential capabilities; drawing from Pigou’s (1920) basic needs approach, Sen has demonstrated that poverty stems from the absence of one or more fundamental conditions or skills necessary for a dignified existence. Ref. [9] observed a significant correlation between water poverty and poverty in general. From a livelihood perspective, ref. [37] argued a strong correlation between the WPI and the five basic capabilities identified previously by Desai (1995):

• Capability to stay alive/enjoy a prolonged life
• Capability to ensure biological reproduction
• Capability for healthy living
• Capability for social interaction
• Capability to have knowledge and freedom of expression and thought.

The Sustainable Livelihoods Framework assesses development impacts regarding various attributes, referred to as livelihood assets or capitals, and identified as natural, physical, financial, social, and human assets (Carney, 1998; Scoones, 1998, as cited in [30]). A strong correlation is found between the WPI and livelihood assets.

Table 3. Comparing WPI components and sustainable livelihood capitals.

WPI Component	Livelihood Asset	Subcomponents or Variables Used
Resources	Natural capital, as well as physical and financial capital, representing infrastructure	The measurement of ”Resources” in the Water Poverty Index (WPI) refers to the availability of groundwater and surface water resources. The most commonly used indicators are as follows: Ref. [10]: Internal freshwater flows; external in-flows; population. Ref. [18]: Assessment of surface water and groundwater availability using hydrological and hydrogeological techniques; quantitative and qualitative evaluation of the variability or reliability of resources; quantitative and qualitative assessment of water quality. Ref. [16]: Internal renewable freshwater resources; external freshwater resources and population. Ref. [15]: Ratio of total water withdrawals to available fresh water resources, Ratio of treated residual Ref. [23] Per capita annual water resources, dependency ratio and national rainfall index: Ref. [31]: Perceived changes in surface water and groundwater levels were measured to measure the quality, occurrence of illness from using surface water and groundwater, odour issues and groundwater quality parameters; and rainfall variability
Access	Social capital; financial capital	Accessibility of water resources to the general population, including the availability of freshwater in a community and the variability of water resources. The most commonly used indicators are as follows: Ref. [10]: Percentage of population with access to clean water; percentage of population with access to sanitation; percentage of population with access to irrigation adjusted by per capita water resources. Ref. [18]: Access to clean water as a percentage of households with piped water supply; reports of conflict over water use; access to sanitation as a percentage of the population; percentage of water carried by women; time spent in water collection, including waiting; access to irrigation coverage adjusted by climate and cultural characteristics. Ref. [16]: Percentage of population with safe access to clean water; percentage of population with access to sanitation and irrigation index Ref. [15]: Percentage of population with access to piped water and percentage of population with access to sanitation. Ref. [23]: Per capita annual water resources, dependency ratio and national rainfall index Ref. [31]: Access to safe drinking water inside the industry, daily water collection time including travel and waiting time, collection of water even when sick, security issues during the collection of water, access to improved wash room facilities inside the industry and access to improved sanitation and medication. For male industrial workers
Capacity	Human and social capital, including institutional issues, and financial capital for investment	Factors that influence the economic and social capacity of the community. Although it seems similar to the Human Development Index (HDI), the capacity component focuses more on indicators demonstrating the community’s water management and institutional capacities [10,54] and Liu et al. (2019) as cited by [12]. Below are some of the commonly used indicators: Ref. [10]: PPP (purchasing power parity) per capita income; under-five mortality rates; education enrollment rates; Gini coefficients of income distribution. Ref. [18]: Wealth equivalent to ownership of durable items; Mortality rate for children under five years; Educational level; Membership in water users’ associations; Percentage of households reporting illness due to water supply; Percentage of households receiving a pension, remittances or wages. Ref. [16]: PPP per capita income; under-five mortality rates and education enrollment rates Ref. [15]: Per capita incomeو under-one mortality rate; literacy rate Ref. [23]: GDP per capita (current USD), under-five mortality rates, percentage of the total population, undernourished, literacy rate, life expectancy of male, life expectancy of female, employment rate Ref. [31]: Affordability, financial help, access to institutional loans, duration of residence, political or NGO linkage, training in water, sanitation and hygiene issues, education ratio and roles in operation and maintenance.
Use	Physical capital; financial capital	The ”Use” component evaluates the amount of water used in different sectors (e.g., domestic, agricultural, and industrial use) and determines water consumption efficiency. Some of the used indicators are as follows: Ref. [10]: Domestic water use in liters per day; share of water use by industry and agriculture adjusted by the sector’s share of GDP. Ref. [18]: Domestic water consumption rate; agricultural water use, expressed as the proportion of irrigated land to total cultivated land; livestock water use based on livestock holdings and standard water needs; industrial water use (purposes other than domestic and agricultural). Ref. [16]: Domestic water use in liters per day and share of water use by industry adjusted by the sector’s share of GDP. Ref. [15]: Domestic water use in liters per day, share of water use by industry adjusted by sector’s share of GDP, share of water use by agriculture adjusted by sector’s share of GDP Ref. [23]: Per capita per day domestic water use, share of water use by agriculture adjusted by the sector’s share of GDP, share of water use by industry adjusted by the sector’s share of GDP Ref. [31]: Daily water requirement inside and outside the industry for domestic use, occurrence of violence and conflicts regarding water use
Environment	Natural capital	It measures environmental indicators related to water supply and management, indicating the pressure of human activities from the agricultural, industrial, and domestic sectors on the environment (Liu et al., (2019) as cited by [12]). Below are some of the commonly used indicators: Ref. [10]: Water quality; water stress (pollution); environmental regulation and management; informational capacity; biodiversity based on threatened species. Ref. [18]: People’s use of natural resources; Reports of crop loss during past five years; Percentage of households reporting erosion on their land. Ref. [16]: Water quality, water stress (pollution), environmental regulation and innovation, informational capacity and biodiversity based on threatened species Ref. [15]: Soil degradation/ erosion, water pollution, urban municipal waste collected as a percentage of urban municipal waste generated Ref. [23]: Water effects on the ecosystem Ref. [31]: Consumable fish species in surface water, reduction in fish species, damage and loss due to flood or drought, crop loss, drainage problems and reduction in vegetation cover.

depicts the correlation between the WPI and livelihood assets.

4.3. Emergence of the Multidimensional Approach to Water Poverty

Ref. [55] described the WPI as the ratio of available renewable water to the amount required to cover food production and the household uses of one person in one year under the prevailing climate conditions. According to [9,36], the WPI was developed through pilot projects carried out in the year 2000, intending to empower decision-makers to act impartially by allowing them to justify their choices based on a rational and transparent framework concerning water provision. This aims to have a more informed assessment of water stress at the household and community levels and to prioritise interventions in the water sector [6].

The WPI has continued, since its development in 2001, to the present, to prove to be an effective tool. For example, refs. [12,20] concluded that the WPI was identified as the most suitable method because of its nature as a multidisciplinary tool that incorporates spatial, environmental, and socio-economic factors related to water scarcity, allowing policy-makers to monitor resource availability and track the status of socio-economic factors that determine access and use.

4.4. The Development of WPI

The WPI formula developed over time, and various versions of the WPI have emerged since 2001 [6,29]. Figure 1 below is a chronological review of the WPI development:

4.4.1. Conventional WPI

In 2001, Caroline Sullivan linked physical estimates of water availability with socio-economic variables that reflect poverty [36]. The value of the WPI is estimated using the following formula:

$$\mathrm{WPI}={\displaystyle \frac{1}{3}}(w_{a}A+w_{s}S+w_t(100-T\left)\right)$$

where:

A: adjusted water availability (AWA) assessment as %.

S: the population with safe water and sanitation (%).

T: the index (e.g., between 0 and 100) represents the time and effort taken to collect water for the household.

w_a, w_s and w_t are weights given to each component (so that w_a + w_s + w_t =1).

Several studies like [16,25,37,54] have used the arithmetic mean to calculate the final value of each component and min–max normalisation to calculate the sub-components using the following formula:

$${Xi}^{*}={\displaystyle \frac{xi-xmin}{xmax-xmin}} \times 100\%$$

4.4.2. The Simple Time Analysis Approach

This version is to use a time analysis approach. Time is used as a numéraire to assess water poverty.

WPI = T/1000 m³

This is the time required per person to collect a quantity of 1000 m³ (as per Falkenmark categorisation). In cases where the water is provided by infrastructure, the value of the WPI would be equivalent to the wage-earning labour time required by residents to enable them to pay the appropriate fee for that level of water provision [36]. In rural areas, where infrastructure was less relevant, figure T would be based on the actual measurement of time required by persons in that household or community to collect the standard measurement unit (e.g., 1000 m³).

4.4.3. Holistic WPI

The holistic WPI formula, a weighted average of the five components, is as follows:

$$WPI={\displaystyle \frac{wrR+waA+wcC+wuU+weE}{wr+wa+wc+wu+we}}$$

Each component is standardised between 0 and 100; therefore, the WPI value is between 0 and 100; 100 represents the best situation. Where:

R: Resources—Physical availability of both surface- and groundwater.

A: Access—Access to water for human use.

C: Capacity—Effectiveness of people’s ability to manage water.

U: Use—Different uses of water, including domestic, agricultural and industrial.

E: Environment- Evaluation of the environmental integrity of water and the ecosystem.

4.4.4. Improved WPI Methods

WPI was tested by [1] in its additive (WPI _AD) and geometric (WPI _GE) formulas:

$$\mathrm{WPI}{\mathrm{AD}}_{}={\sum }_{i=R,A,C,U,E}wiXi$$

$$\mathrm{WPI}{}_{\mathrm{GE}}={\prod }_{i=R,A,C,U,E}Xiwi$$

The authors concluded that the weighted geometric function was found to be the most appropriate aggregation method for estimating the WPI because this does not allow compensability among the different components involved in the index formula [1]. A similar result was confirmed later by [25].

Ref. [6] suggested that efforts should continue to improve the tool, refining the mathematical structure and building more comprehensive data sets. An improved WPI (iWPI) was introduced to measure water poverty. Principal component analysis (PCA) was used to compute the final iWPI that was measured in Tunisia [21] and in the 50 African countries [23].

Regardless of the WPI calculation method or version, one fixed recommendation stands. Participatory data collection ensures community empowerment, deepened awareness of exact water issues and more relevant strategies to address those issues. In addition, a participatory approach was also recommended in determining the weights of components [2,6,10,15,56,57,58,59]. The participatory approach was also recommended for weighing WPI components, given that weights are inherently socio-political constructs.

4.5. Versions of the WPI Emerged

In the efforts to advance the WPI, several versions have been introduced since 2005. Below are the most important versions:

4.5.1. Water Wealth Index

Ref. [18] reviewed the structure of the WPI and attempted to generate an integrated indicator that focused more on the issues of food security and agricultural water use. The proposed Water Wealth Index (WWI) departs from the original configuration of the WPI in several respects. The resource component has been extracted and handled independently in this updated framework as a natural baseline endowment. Additionally, new components (Food, Health) have been introduced, replacing Access and Use, which are now encapsulated by the term productivity. The main components of the WWI are Food security (F), Health (H), Productivity (P), Environment (E), Institutional Capacity, Infrastructure (I) and Natural Baseline Endowment (NBE) (Quantity of water in the natural water cycle, and its quality and reliability). The WWI is calculated using the following formula:

$$WWI=\left(NBE,I\right)\mathrm{X}{\displaystyle \frac{w_{f}F+w_{h}H+w_{p}P{w}_{e}E}{w_f+w_h+w_p+w_e}}$$

The sum of weightings (w_f, w_h, w_p, w_e) should always be equal to 1.

4.5.2. Agricultural WPI

The Agricultural Water Poverty Index (AWPI) is used to evaluate water status and poverty in agriculture [56]. The AWPI was first introduced by [47]. The authors reviewed the role of sustainable water management in achieving agricultural sustainability. They proposed the AWPI to provide a holistic picture of vital issues for sustainable water management. The authors distilled key components of the AWPI and discussed its applications. The AWPI can be used to assess the agricultural water poverty among farmers and regions and to provide guidelines for sustainable water management in areas where agriculture is the dominant sector.

The AWPI was further developed by [48], who studied it using Q-methodology to understand stakeholders’ perceptions. Afterwards, ref. [56] made a comparison application of the analytic network process (ANP) and analytic hierarchy process (AHP) in the analysis of the agricultural water poverty index. Results revealed that the ANP model is more capable of analysing the AWPI than the AHP. Ref. [19] recently studied the AWPI in the semi-arid province of Hamedan in Iran. The value of the composite AWPI was developed using PCA, which is similar to the aforementioned iWPI.

4.5.3. Inclusive WPI

Ref. [57] introduced the concept of the inclusive WPI, that was upgraded to fit with the developing countries context. He based the IWPI on improved understanding for localised poverty; this is of greater importance in arid and semi-arid regions and, more specifically, in areas affected by climate change where water quantities are decreasing, thereby affecting the livelihood of communities who can barely access sufficient amounts water [60,61,62]. Thus, regarding the access of the poor and vulnerable to water and sanitation services, some indicators related to social and cultural aspects should be considered in designing a more adapted water poverty index because it will be more inclusive and pro-poor. The author adapted the WPI to be more inclusive by becoming inspired by the PADev approach. Therefore, the author recommended adding a new component to calculate the WPI: ”Cohesion” (Co). The concept of cohesion involves the ”force” that allows a community to remain united regardless of the divergent views and differences in socio-economic and professional categories. The developed form of the index is as follows:

IWPI = R^wr × A^wa × C^wc × U^wu × E^we × Co^wco,

The weight of each component is between 0 and 1, and the sum of those weights equals one. The weights are estimated in a participatory way.

4.5.4. Household Water Security Index

The Household Water Security Index (HWSI) was developed by [58]. It is based on the same components as the WPI. Nevertheless, they added a sixth component (institutions), which covers various aspects of institutional capacity, such as the degree of inclusiveness and integrated water resources management. The HWSI can be used as an effective and alternative measure of water security, and it is a helpful tool for water resources monitoring and policy. Boosting the HWSI is possible by implementing the following strategies:

(a)

Enhance the capacity of a household (human, cash, kind);

(b)

Improve current institutional arrangements (e.g., joining Water Users Associations (WUAs)), engage the local community, and ensure transparency.

The HWSI provides an innovative approach to the WPI, but it has several limitations. For example, the current HWSI calculation is based on cross-sectional data. In addition, normally households in a given area are not necessarily homogeneous due to differential resource access. Furthermore, the construction of the indexes has political relevance; The HWSI did not consider this. Thus, further study is recommended to assess the political implications of the HWSI.

4.5.5. Domestic Water Poverty Index

Ref. [60] used a PCA-based mean-weighted approach to introduce the Domestic Water Poverty Index (DWPI). The DWPI is constructed based on the fact that the scarcity of natural resources, including water, is not solely determined by physical availability but is also influenced by the level of human development and technological advancement in a society, as well as socio-economic and physical factors. The authors tested the correlation between the DWPI and HDI and found a clear positive correlation between water availability and human development. In addition, the test provided a comprehensive understanding of the factors contributing to community water poverty, enabling policy-makers to formulate context-specific plans. The DWPI was tested only at the regional level but not at other scales.

By reviewing all the above WPI versions, it can be concluded that the AWPI could be the WPI version that was mainly assessed. However, the opportunity for further research is possible in two directions: (a) through further assessing the versions of the WPI that were not sufficiently tested (e.g., the IWPI and DWPI); and (b) merging between more than one version such as using the inclusive approach to enhance the AWP/DWPI. In addition, further testing the water wealth index can be promising especially for countries where agriculture and food security are relevant.

5. Global Applications, Regional Perspectives, and Case Studies

Following to the emergence of the WPI in 2001, [9] undertook a pilot study that applied the WPI at the community scale. They presented the results with pentagram diagrams (where each point represents the score for one of the five components), allowing users to quickly classify the situation of water poverty in any location without losing its underlying complexities to make it more appealing, especially to policy-makers. Ref. [9] reported that the WPI produces sensible results, although they cautioned that the WPI is not definitive and is inaccurate. This is attributed to internal correlations between WPI components (especially access and capacity). Countries with different water access contexts were found to have similar WPI values. Therefore, at this level, the authors called for considering the components’ information rather than the single value of the WPI.

Ref. [15] reported a slightly different conclusion, being one of the few researchers who tested the WPI at national, regional and micro scales in Mexico in the same research. He used participatory rapid appraisal (PRA) tools and household surveys; he reported that the use of the WPI value without referring to its components was, in some cases, not reflecting the realities (e.g., related to access and use aspects). He concluded that only limited hypotheses can be made across scales as the current model used to calculate a water poverty index is relative, and results are only meaningful within the context from which they originate.

However, efforts have been exerted since that time to develop the WPI to become a more meaningful tool and method for sustainable water management. The analysis of the WPI ranged from small-scale community to national and global levels [57]. Nevertheless, studies in the following years confirmed similar results, including that the national WPI value masked the local level values at smaller scales [13,15,22,57]. This was unsurprising, given that [18] confirmed that the WPI was primarily designed for use at the community level, though the authors cite its ability to be applied to different scales depending on need.

Ref. [61] tried afterwards to address another weakness in the WPI (causality among different indicators). They conducted their research at a basin scale in 2008. The authors applied it at a community scale in 10 communities in Bolivia. They proposed an enhanced WPI (eWPI) that emphasises the importance of causality and thus incorporates cause–effect relationships whilst including sustainability issues. They defined indicators of pressure, state and societal responses for each variable used to calculate a WPI, hence arriving at what they describe as a causality-issue matrix. This causality response model was introduced in 1993 by the OECD (OECD, 1993 as cited in [61]).

Ref. [16], who assessed the WPI at the regional level (as mentioned above), reported two main critiques: First, they note that the final value of the WPI heavily depends on the number of countries or communities it is calculated for. This is because of the normalisation technique inherent within the WPI that assigns a score of 0 to the lowest-ranking country or community and 100 to the highest-ranking community or country. This means that having arid countries on the list will make the WPI value of a specific country higher than having humid countries on the list. Second, the authors found a high correlation among WPI components: access and capacity. The first weakness could be unavoidable, but it is suggested to consider homogeneous sampling of tested localities/ countries or be aware of this weakness. The second weakness was overcome through the aforementioned geometric formula.

The latter argued the appropriateness of calculating water poverty at the household scale instead of the community scale. Although logistically, assessing each household is much more challenging because similar data collection requirements exist at both scales. However, it was clear that the more micro level the calculation is made at, the more representative the WPI value is. This is evidenced by the relatively even strength of correlations between indicators and overall water poverty scores and the lack of significant correlation amongst indicators.

Ref. [17] compared villages within a specific geographical context. The study highlights how water resources are at risk due to the unregulated disposal of untreated industrial wastewater, household sewage, and the impact of climate change, thereby posing a significant threat to water availability at the household level in developing rural economies.

6. Water Poverty Index Measurement in Fragile Middle Eastern Countries

6.1. Water-Based-Fragility in the Middle East

The “state of fragility” refers to a condition or situation in which a country faces significant vulnerabilities, instability, and risks that threaten its social, economic, political, and environmental systems. Fragility often manifests itself in various forms, including but not limited to political instability, social unrest, economic vulnerability, insecurity and conflict, humanitarian crises, and environmental challenges [8]. These fragility aspects are available at different levels in several countries in the Middle East. In this context, water can ignite two types of conflicts: Firstly, internal tensions between cities and the countryside, agriculture and industry, internal agriculture and tourism stakeholders, as well as usage conflicts between users living in the same territory suffering from a water shortage. Secondly, tensions concern water quality and/or quantity between countries sharing the same sources. A total of 220 “upstream vs. downstream” basins (example: the case of the Tigris and the Euphrates) in the world are shared by at least two countries. Others are of the “right bank vs. left bank” type, in the case where the river defines the border [6]. This conflict can be latent in some cases, but it can be active in other cases.

The latter claimed that common management of water resources could be a factor in peace and cooperation even in the midst of war. The good example of this cooperation is that of India and Pakistan which, during the war between them in the 1960s, never stopped financing the development work they were carrying out together on the river Indu; there are 20 from here. The two states, thanks to international meetings on water, wish. Reviewing the WPI in the below graphs, it is observed that more fragile countries have a lower WPI value despite the available resources. Iraq, Syria and Palestine are clear examples of this observation.

6.2. WPI in the Middle East

Various studies consistently confirmed that most countries in the Middle East fall under the high and severe water poverty categories, with Jordan and Palestine ranked as the water-poor countries in the Middle East. Ref. [6] assessed the values of the WPI for Middle East Countries and reported the results graphically, as shown in Figure 2 below. By reviewing the maps below, it can be concluded that resources are not the primary factors dominating the WPI value. For example, despite the resource component for Iraq being in the highest category, because of the low values of Use, Capacity, Access and Environment components, the resultant WPI value is in one of the weakest categories.

According to [38], rapid population growth will largely cause very serious problems with the supply of drinking water. To escape from this critical situation, huge investments will be required to improve the existing infrastructure and create new equipment, namely production plants, distribution networks, sanitation stations, but also to modernize the irrigation systems. However, it can be argued that a more comprehensive momentum is needed to realise a sustainable change in water in the Middle East. It can also be argued that the fragility of several countries in the Middle East makes the support of decision-makers in comprehensively managing water resources in such a challenging context inevitable. Therefore, the WPI (or a tailored version of the WPI) could be one of the ideal multidimensional approaches to achieving this sustainable change.

Nevertheless, it can be easily found that despite the high water scarcity issues in the Middle East, the WPI value has not been measured for most countries since 2012. In other words, most of the WPI versions (e.g., Inclusive WPI, DWPI, AWPI, etc.), were not piloted in any/most Middle Eastern countries despite the evident need. Therefore, harnessing this tool with all its versions developed after 2012 can form a precious opportunity to help water managers in these countries make more informed and impartial decisions.

7. Critique, Emerging Trends, and Bridging Theory to Practice

7.1. Critical Analysis of WPI Weaknesses, Limitations, and Potential Biases

Various drawbacks compromise the WPI index’s significance, robustness, and utility as a meaningful policy tool. Common critiques include, firstly, the ad hoc selection of indicators used to calculate the index [35,61]. Secondly, the weighing and aggregation techniques influence the coherence and interpretability of final values. Thirdly, masking local-level variabilities through a larger-scale WPI was also a common area of critique [3,14,18,57]. Fourthly, the correlated pieces of information can lead to possible compensability among different WPI components [6,14,15,16,21,27,35,57]. In addition, the WPI as a blind value was found inadequate for assessing the complexity of water issues unless subcomponents were considered in the analysis [15,16,18,54].

Table 4. WPI’s main limitations and the respective mitigation measures.

#	Limitation	Mitigation
1	Refs. [35,62] criticised the ad hoc approach in the selection of indicators that compose the initial water poverty index of [37]. Data was found depending on data accessibility and the socio-economic structure of each country [1] and Kallio et al. (2017); Maheswari et al. (2017) as cited by [12]	According to [30], the better way to use this index is with national and official data in the sector. Ref. [15] recommended the usage of pre-determined variables to improve the WPI process.
2	National-level WPI could mask local-level variabilities [3,14,18,57]. Ref. [36] argued the four scaling issues related to WPI when integrating social and physical sciences: (a) how scale, extent, and resolution affect the identification of patterns; (b) how different levels on a scale explain different social phenomena; (c) how theoretical propositions about phenomena on one spatial, temporal, or quantitative level of a scale may be generalised to another level (up and down scaling), and (d) how processes may be optimised at particular points or regions on a scale (Gibson et al., 2000, as cited in [36]).	Ref. [15] reported that, however, it was clear that the more micro level the calculation is made at, the more representative the WPI value is. Ref. [23] stated that the international level of water poverty assessment may partially or entirely mask the local water poverty situation. Thus, conducting a thorough and reliable water poverty evaluation at different scales becomes crucial for effective management interventions. The author emphasised the necessity for targeted policy interventions and planning tailored to specific locations and varying levels to enhance the water poverty situation on the continent. Ref. [18] recommended the district level as the most cost-effective level, given that this level is typically surveyed and reported, and data is usually available at this level. A similar recommendation was relayed by [28].
3	Water issues are too complicated [16]. In addition, some indicators are correlated with the gross domestic product or HDI [15,57,64].	Cho et al. (2010) as cited by [6] used Principal Components Analysis (PCA) to reduce the number of weighted indicators. They arrive at a modified WPI (mWPI) that comprises indicators of Access, Capacity and Environment. They further reduced their model to include equally weighted indicators of Capacity and Environment justified by statistical tests that suggest these two indicators are most strongly correlated to the primary principal components of the WPI. Refs. [5,15] reported a similar conclusion, dropping the Resource component from calculating WPI. Multiplicative, geometric, and nonlinear functions have been suggested to address the limitations of the additive form [6,21,59,64,65].
4	Aggregating techniques can lead to inaccurate values of WPI due to poor weighting and possible compensability among the WPI components (Nardo et al., 2005, as cited in [14,15,49,57]).	Ref. [9] emphasised that the purpose of the WPI is political rather than statistical. In addition, refs. [3,15] recommended (a) determining weights in a consultative and transparent way with the local stakeholders (mainly experts); (b) Statistical methods to identify weights should be only used to help in decision making. Alternative weighting schemes proposed by [6,59,65] aimed to establish more appropriate and objective weights for different components. Ref. [14] recommended giving less attention to weights and focusing on the components’ values to inform decision-makers regarding water resource management issues. Ref. [31] recommended the application of different combinations of aggregation methods and weights to find the best-suited one for this scale.
5	Refs. [16,24] found the data collection process slow and painstaking.	Ref. [37] recommended seeking school students’ support to optimise awareness raising. Community-level data were collected using primary sources, while national-level data were collected from many secondary sources, such as different regional and national government departments. According to [30], available data should be utilised whenever possible rather than imposing data requirements without considering their availability. Ref. [18] recommended that regardless of the scale, secondary data should be used to optimise the efficiency and applicability of the WPI calculation. On the other hand, [24] concluded that agencies should also dedicate more resources to producing more data to have a more accurate score.

below depicts the main limitations with respective mitigation measures.

7.2. Emerging Trends and Lessons Learned from Real-World Implementations

According to recent studies, the main features of future trends for WPI are being shaped in two main ways:

(a)

Improving WPI as a composite index while maintaining the same original components: In this regard, [24] concluded that future trends focus on having more time-based data to have a more accurate calculation for the WPI. In addition, the authors recommended developing an application programming interface for automatic WPI updates. Moreover, ref. [28] recommended the inclusion of more indicators and the usage of unequal weights based on expert consultations for improved reliability. They recommended using smaller spatial scales (municipality and ward level) for clearer priority areas. The latter also recommended frequent updates for monitoring temporal changes in water poverty and evaluating existing policies. Furthermore, [3] recommended combining scholars’ and practitioners’ advice with statistical and geospatial analyses to have a comprehensive understanding. Refs. [29,56], who studied the enhanced WPI, recommended using structured approaches such as the AHP and ANP approaches to determine the weighting issue to achieve this target.

(b)

Add/remove some of the WPI components: As a trend, especially in the past decade, several studies have been found suggesting the introduction/elimination of a specific component. Ref. [4] calculated WPI after adding quality, quantity, and secondary source components while dropping environment and use components. The inclusive WPI also included cohesion as a main component while keeping the original five components. Ref. [3] used the same methodology but diversified the weighting method. The HWSI added to the five components the “institutions” [59]. However, it can be concluded that the literature trends towards adding more components to cover important aspects that traditional calculations did not include, such as the inclusiveness of people experiencing poverty and people with a disability. This upgrade improves the WPI by capturing new aspects of attention and making the WPI more responsive to emerging local and global interests.

7.3. Challenges in Transitioning from Theory to Practice and Strategies Employed

By calculating the WPI and its components, areas of improvement are identified. However, moving from theory to practice has several implications and requires political will to realise progress. Below are two examples:

• Despite the apparent global increase in water scarcity, ref. [35] argued that the water-scarcity issue can be solved if the water cost is increased. They emphasised that the paramount water policy concern in this context revolves around formulating a suitable pricing mechanism for water or creating a water market where prices are determined. As water prices experience an upward trajectory, there is a consequential exploration of new, albeit more expensive, water sources. This exploration increases supply volumes. Consequently, both marginal and average costs associated with the water supply are elevated. The newly tapped water sources may include deeper aquifers, surface water located at greater distances, reclaimed wastewater, brackish water subjected to desalination, and, at the furthest extreme, desalinated seawater. This could be true, but it is far from practical and inclusive to people experiencing poverty [35,57].
• Refs. [57,58] highlighted the necessity of considering a participatory approach in water resources management for effective and durable solutions. They emphasised that excluding communities from participating in their development affects the level of effectiveness of the water regulations, the level of communities’ ownership of these regulations and the increased level of sustainability. However, the inclusive and participatory approach requires certain governmental capacities and regulations to enforce this approach. Yet, such an enabling environment could be missing, especially in developing / fragile contexts.

8. Integration with Sustainable Development Goals (SDGs)

8.1. Alignment between the WPI and Relevant SDGs

Sustainable water access is a prerequisite to any development effort (Barker et al., 2000; Hansen and Bhatia, 2004; Wang et al., 2005, as cited in [27]). Therefore, water management should be the cornerstone of any sustainability program. Lack of sufficient water in some parts of the world has already impaired sustainability, decreased opportunities and is declining rural society ([2,47,64]; Hansen (2015) as cited by [27]);Therefore, this was acknowledged by prioritising access to clean water and sanitation as the sixth Sustainable Development Goal (SDG) adopted by the United Nations [65].

Given that agriculture is a primary source of provision and distribution of rural income and that access to water is a prerequisite for a feasible agribusiness, then improving the WPI has a proportional correlation to poverty alleviation and overall human development [60]. Therefore, it is impossible to escape extreme poverty without adequate access to water [2,18] for current generations while at the same time securing water availability for the needs of future generations [36]. This directly contributes to the very first SDG [66]. Therefore, proper water management and development (using tools such as the WPI) contributes directly to progress towards SDGs.

8.2. Contribution to Sustainable Water Management and Development

Ref. [29] reported that the WPI is a strategic approach and a quantitative tool that can be relied on for decision-making and understanding the complexity of water-related issues, especially the linkage between under-development and water scarcity. The WPI was also identified as a possible tool for monitoring progress towards increasing the number of people with access to water and indicating if the progress contributes to the wider water-related context, namely reduced hunger, improved food security and better health [9,45]. Ref. [27] reported that water poverty has a natural link to human development. This emerged first through using the Human Poverty Index (HPI), a derivative of the HDI, back in 2004. According to this index, a decent standard of living is assessed by the weighted average of two simple indicators: the proportion of the population without regular access to safe water and the proportion of moderately and severely underweight children below the age of five.

Much research also linked water and income [29,31,48]. Recently, ref. [29] assessed the relation between water poverty and income poverty. They reported that the relation between the two indices is evident but complicated. Rijsberman (2003), as cited in [27] reported that water supply is both a cause and a consequence of poverty, and access to consistent clean water sources is crucial to poverty alleviation.

By reviewing sustainable development goals, it can be found that there is a high level of alignment between several SDGs and the WPI. The most direct relation is SDG#6 (Clean water and sanitation). Despite all the alignment between SDG #6 and the WPI, minimal literature has been found that assesses the correlation between them. No literature was found nominating the WPI to be added as one of the proxy indicators to measure progress towards achieving SDGs in general and SDG#6 specifically. The alignment between SDG#6 and the WPI provides a promising opportunity to use the WPI and its components to benchmark progress towards achieving the SDGs’ targets. Yet, a significant data challenge is expected in linking them.

9. Conclusions

Historically, indices and benchmarks were used to measure current and forecast access to water and progress towards achieving the benchmark. The old generation of indicators was used from the 1980s to 2000. However, they were criticised for several limitations, including the following:

• The smaller geographical area was masked by the larger one (e.g., local scale by national scale), knowing that they were used to assess the water situation at global and national levels.
• Water resources were the sole criterion for assessing water problems. Therefore, other factors, like access, types of usage and environmental needs, that affect access to water were not considered.
• These indicators also did not consider the adaptive capacity of a country.

The multidimensional indices enabled a better understanding of the relationship between the physical extent of water availability, accessibility, and household and community welfare. The WPI has been the dominant multidimensional index over the past two decades, enabling water management to steer towards sustainability. The WPI has several advantages. Its multidisciplinary nature incorporates the spatial, environmental, and socio-economic aspects of water. However, several drawbacks of the WPI were reported in the literature, such as masking of the local scale by the national scale, the ad hoc determination of components’ weights, ad hoc selection of indicators, significant correlation among some indicators allowing for compensability, and other disturbances due to normalisation processes.

The AWPI and improved and inclusive WPI versions could be particularly assessed and matured. The weighted geometric function revealed itself to be the most appropriate aggregation method because it eliminates compensability among the different components. PCA usage afterwards in calculating the weights of indicators helped in a more accurate/consistent weighing of indicators. However, the journey to mature the tool is still ongoing. Improvement of existing WPI versions includes, firstly adding indicators such as gender and disability, cohesion and institutional capacity; secondly, systemising indicators’ weighing using recognised methods, such as the PCA; thirdly, mainstreaming of the participatory approaches for scholars and practitioners; fourthly, inclusiveness of the poor, marginalised and PwD, as well as considering gender equity; fifthly, systemising the selection of indicators to avoid the ad hoc approach and improve consistency; sixthly, considering smaller spatial scales for clearer local prioritisations.

The potential for the WPI’s further development is promising and great opportunities for further research are found to set the ground for future directions. First of all, measuring the WPI using time-based data to avoid a cross-sectional measurement to enable better monitoring of progress over time and future forecast. In addition, testing the correlation between the WPI and fragility using time-based data will help us to identify predicting the WPI variation according to the state of fragility. Given the diversified spectrum of contexts in the Middle East (in terms of state fragility, available resources, adaptive capacity, etc.), this will not only cover the knowledge gap in the WPI for the Middle Eastern countries, but will also prove/refute the hypothesis of a significant inverse relation between the WPI and fragility with showing the level of sensitivity according to the level of fragility in each country. It was found that countries in a fragile state could require a tailored WPI version. Testing this in the Middle East where fragility levels vary from one county to another, and more than 50% of countries are classified under the “absolute water scarcity” category, makes it an ideal region for this research.

Given the large number of WPI versions, it is necessary to mature some of the versions that were not tested enough at all scales and match each WPI version to its respective application and suitable context and scale. For example, the testing of the HWPI and DWPI at a local scale in an inclusive way (following the inclusive WPI) is expected to add a significant value to both versions. Likewise, testing an inclusive WPI at the local scale of fragile and/or vulnerable contexts, particularly in arid and semi-arid regions and, more specifically, in areas affected by climate change, could deliver promising results to such contexts. Moreover, given the multidimensional nature of the WPI, and its overlap with some of the SDGs (e.g., SDG #6) and the HDI, it would be promising to utilize the tool to track the progress in relevant SDGs and the HDI over time. Lastly, revitalising the WWI especially in fragile countries should be assessed, given the connection between fragility and WWI components.