Sustainability characteristics of drinking water supply in the Netherlands

Developments such as climate change and growing demand for drinking water threaten the sustainability of drinking water supply worldwide. To deal with this threat, adaptation of drinking water supply systems is imperative, not only on a global and national scale, but particularly on a local scale. This investigation sought to establish characteristics that describe the sustainability of local drinking water supply. The hypothesis of this research was that sustainability characteristics depend on the context that is analysed and therefore a variety of cases must be analysed to reach a better understanding of the sustainability of drinking water supply in the Netherlands. Therefore, three divergent cases on drinking water supply in the Netherlands were analysed. One case related to a short-term development (2018 summer drought), and two concerned long-term phenomena (changes in water quality and growth in drinking water demand). We used an integrated systems approach, describing the local drinking water supply system in terms of hydrological, technical, and socio-economic characteristics that determine the sustainability of a local drinking water supply system. To gain a perspective on the case study findings broader than the Dutch context, the sustainability aspects identified were paired with global aspects concerning sustainable drinking water supply. This resulted in the following set of hydrological, technical and socio-economic sustainability characteristics: (1) water quality, water resource availability, and impact of drinking water abstraction; (2) reliability and resilience of the technical system, and energy use and environmental impact; (3) drinking water availability, water governance, and land and water use. Elaboration of these sustainability characteristics and criteria into a sustainability assessment can provide information on the challenges and trade-offs inherent in the sustainable development and management of a local drinking water supply system.


Introduction
Climate change, combined with a growing drinking water demand, threatens the sustainability of the drinking water supply worldwide.The goal set for drinking water supply in Sustainable Development Goal (SDG) 6.1 (UN, 2015) is "to achieve universal and equitable access to safe and affordable drinking water for all by 2030".Worldwide drinking water supply crises are visible, resulting from a combination of limited water resource availability, lacking or failing drinking water infrastructure, and/or increased drinking water demand due to short-term events or long-term developments (WHO, 2017).Still, nearly 10 percent of the world population is fully deprived of improved drinking water resources (Ekins et al., 2019), and additionally, existing drinking water supply systems are often under pressure.For instance, two recent examples of water crises were reported in Cape Town, South Africa, and São Paolo, Brazil (Sorensen, 2017;Cohen, 2016).To deal with such challenges and threats to safe and affordable drinking water, adaptation of the current drinking water supply system is imperative, not only on a global and national level but also on a local scale.
In the Netherlands, for instance, the national drinking water supply currently meets the indicator from SDG 6 (UN, 2018) on safely managed drinking water services and safely treated wastewater.At the same time, the more specific goals on (local) water quantity, quality, and ecology, as set by the European Water Framework Directive (WFD), are not met yet (European Environment Agency, 2018).Consequently, drinking water supply in the Netherlands does not meet all SDG 6 indicators, for instance when considering impact on water-related ecosystems (Van Engelenburg et al., 2018), of water pollution (Kools et al., 2019;Van den Brink and Wuijts, 2016), or of water shortage (Ministry of Infrastructure and Environment and Ministry of Economic Affairs and Climate Policy, 2019).Additionally, future developments, such as the uncertain drinking water demand growth rate (Van der Aa et al., 2015) and the changing climate variability (Teuling, 2018), may put the sustainability of the Dutch drinking water supply under pressure in the future.
The abstraction of groundwater or surface water from the hydrological system, and subsequent treatment to drinking water quality before being distributed to customers, requires local infrastructure (typically a drinking water production facility embedded in a distribution network of pipelines).Although the daily routine of drinking water supply has a highly technical character (Bauer and Herder, 2009), the sustainability in the long-term depends on the balance between technical, socio-economic, and environmental factors.This balance is especially complex for the local drinking water supply, which is intertwined with the local hydrological system and local stakeholders through its geographical location.
Because of the interconnections between physical, technical, and socio-economic factors as well as across space, organizational levels, and time, adaptation of the local drink-ing water supply to current and future sustainability challenges calls for an integrated planning approach (Liu et al., 2015).Integrated models have been developed to understand the complex interactions between the physical, technical, and socio-economic components in various water systems (Loucks et al., 2017).However, a systems analysis to assess local drinking water supply and to identify sustainability challenges on a local scale has not yet been developed.
This research aimed to propose a set of sustainability characteristics that describe the drinking water supply system on a local scale to support policy-and decision-making on sustainable drinking water supply.To reach this aim, cases on drinking water supply were analysed using a conceptual framework.The selected cases represented a short-term event and long-term developments that affect water quality and water resource availability, the technical drinking water supply infrastructure, and/or the drinking water demand.The system boundaries were set to drinking water supply on a local scale.While the drinking water supply on a local scale is also affected by outside influences from different organizational and spatial scales, the analysis accounted for these external influences too.The hypothesis of this research was that sustainability characteristics depend on the context that is analysed, and therefore, a variety of cases must be analysed to reach a better understanding of the sustainability of drinking water supply in the Netherlands.

Method
Sustainable water systems can be defined as water systems that are designed and managed to contribute to the current and future objectives of society, maintaining their ecological, environmental, and hydrological integrity (Loucks, 2000).This study focused on the sustainability of drinking water supply systems on a local scale -in short, local drinking water supply systems.The boundaries of these systems were set by the area in which drinking water abstraction is embedded.The system can be approached from different perspectives.The socio-ecological approach considers relations between the socio-economic and environmental system, whereas the socio-technical approach considers the socio-economic and technical system (Pant et al., 2015).In this study, we combined both approaches by describing the local drinking water supply system in terms of hydrological, technical, and socioeconomic characteristics that determine the sustainability of a local drinking water supply system.
Drinking water supply in the Netherlands is of a high standard compared to many other countries.The SDG 6 targets on safe and affordable drinking water and sanitation and wastewater treatment are basically met.But the Dutch government and drinking water suppliers are also challenged to meet the other goals set in SDG 6, such as the improvement of water quality, increase in water-use efficiency, and protection and restoration of water-related ecosystems.In ad-dition the standards on water quantity, quality, and ecology, as set by the European Water Framework Directive (WFD), have not been achieved yet (European Environment Agency, 2018).
The adopted research approach consisted of four steps.The first step was the selection and analysis of three drinking water practice cases in the Netherlands, aiming to identify the main Dutch sustainability aspects in these cases.Three Dutch cases were selected based on their impact on the sustainability of drinking water supply in the Netherlands, considering a short-term event with limited water resource availability and long-term, ongoing developments on water quality, and growing drinking water demand and water resource availability.The cases are illustrated with Vitens data (Van Engelenburg et al., unpublished, 2020).
In the second step, the cases were analysed using the DP-SIR framework (Driver, Pressure, State, Impact, Response; Eurostat, 1999; see Sect.2.1).The sustainability aspects of these cases were identified in the descriptive results of the DPSIR analysis.The results were combined with Dutch governmental reports on these events and developments (Ministry of Infrastructure and Environment and Ministry of Economic Affairs and Climate Policy, 2019;Vitens, 2016) and cross-checked with Vitens staff.The sustainability aspects were categorized into hydrological, technical, and socioeconomic aspects.This resulted in a set of relevant sustainability aspects, which is presented in Appendices A-C.The following step was used to broaden the perspective from the drinking water supply in the Netherlands to a more general perspective by cross-checking the set of sustainability aspects with the targets and indicators in Sustainable Development Goal 6 (hereafter referred to as SDG 6; see Appendix D; UN, 2015) and the WHO Guidelines for Drinkingwater Quality (WHO, 2017).The sustainability aspects, as identified in the analysis, were categorized into nine hydrological, technical, and socio-economic sustainability characteristics.In the final step of the study, each sustainability characteristic was elaborated further into five sustainability criteria that describe the local drinking water supply system.The results are described in Sect.3. A detailed description of the resulting sustainability criteria is presented in Appendix E.

Case analysis method
To reach the aim of this research to support policy development on sustainable drinking water supply, three practice cases were analysed to identify the main sustainability aspects in these cases using the DPSIR (Driver, Pressure, State, Impact, Response) systems approach (Eurostat, 1999).Drivers describe future developments, such as climate change and population growth.Pressures are developments (in emissions or environmental resources) as a result of the drivers.The state describes the system state that results from the pressures.In this research, the aim is to describe the sys-tem state of the drinking water supply system in terms of local hydrological, technical, and socio-economic sustainability characteristics (see Sect. 2.1).The changes in system state cause impacts on system functions, which will lead to societal responses.DPSIR was originally developed to describe causal relations between human actions and the environment.It has also frequently been used for relations and interactions between technical infrastructure and the socio-economic and physical domain (Pahl-Wostl, 2015;Hellegers and Leflaive, 2015;Binder et al., 2013).
The DPSIR approach was used for the analysis of the three selected drinking water supply cases to obtain an overview of the impact of drivers, pressures, and responses to the state of the drinking water supply system.Although the framework has been applied on different spatial scales, Carr et al. (2009) recommend using the framework in a place-specific manner to ensure that local stakeholder perspectives are assessed as well.With the research focus at the local drinking water supply system, these local perspectives were implicitly included.The drivers, pressures, and responses can be on local and higher organizational and/or spatial scales, thus ensuring that -where essential -relevant higher scales are accounted for too.
DPSIR has previously been used for complex water systems by various well-known researchers in this field, such as Claudia Pahl-Wostl.In Binder et al. (2013), a comparison was made between various frameworks, which concluded that DPSIR is a policy framework that does not explicitly include development of a model but aims at providing policyrelevant information on pressures and responses on different scales.In Carr et al. (2009), the use of DPSIR for sustainable development was evaluated.Although the authors were critical regarding the use of the DPSIR framework on national, regional, or global scales, they considered application on a local scale appropriate.They concluded that practitioners could use DPSIR for local-scale studies because it assesses the place-specific nuances of multiple concerned stakeholders more realistically.In Van Noordwijk et al. (2020), DPSIR was used to understand the joint multiscale phenomena in the forest-water-people nexus and, thus, diagnosed issues to be addressed in local decision-making.Therefore, DPSIR was considered an appropriate framework for meeting the aim of the research.
The impact of developments on different temporal scales to the drinking water supply system must be considered as well.The long-lived, interdependent drinking water supply infrastructure is resistant to change due to design decisions in the past which cause path dependencies and lock-ins (Melese et al., 2015).In addition, consumer behaviour, governance and engineering, and the interaction between these processes cause lock-in situations that limit the ability to change towards more sustainable water resources management (Pahl-Wostl, 2002).For this reason, the case analysis was performed considering both short-and long-term pressures, impacts and responses. https://doi.org/10.5194/dwes-14-1-2021 Drink.Water Eng.Sci., 14, 1-43, 2021

Case selection
In this research, three drinking water supply cases in the Netherlands were selected.The case studies were analysed to find sustainability aspects caused by the identified pressures and short-and/or long-term responses in each case because short-term shocks have different impacts and call for other responses than long-term stresses (Smith and Stirling, 2010).
The cases therefore focused on short-term events and longterm developments.All three cases also related to targets set in SDG 6 (UN, 2015).The DPSIR analysis of the case studies is presented in Appendices A-C.

Case 1: 2018 summer drought
Summer 2018 in the Netherlands was extremely warm and dry, causing water shortages in the water system and a long period of extreme daily drinking water demand, resulting in a record monthly water demand in July 2018 (Ministry of Infrastructure and Environment and Ministry of Economic Affairs and Climate Policy, 2019; see Illustration case 1).The driver in this case is the extreme weather condition, which caused several pressures, such as high temperatures, high evaporation, and a lack of precipitation.These pressures did not only cause drought damage to nature, agriculture, and gardens and parks as well as limited water availability in the surface water and groundwater systems, they also resulted in an extremely high drinking water demand.Data on drinking water supply volumes (Van Engelenburg et al., unpublished, 2020) showed that the extreme drinking water demand during summer 2018 put the drinking water supply system under high pressure, causing extreme daily and monthly drinking water supply volumes that exceeded all previously supplied volumes (see Fig. 1).The capacity of the system was fully exploited but faced limitations in abstraction, treatment and distribution capacity.

Illustration case 1: 2018 summer drought
Within the Vitens supply area, the average daily supply volume during the summer period June-August over the years 2012-2017 was approximately 965 000 m 3 d −1 .During the period 27 June-4 August 2018, the daily supply volume exceeded this average summer volume by approximately 28 %, with an average volume of nearly 1 240 000 m 3 d −1 (Fig. 1a).On 25 July 2019, the maximum daily water supply reached nearly 1 390 000 m 3 d −1 , which was 42 % above the baseline daily supply (Fig. 1a).The monthly drinking water supply volume in July 2018 of 38 million m 3 per month was an increase of 18 % compared to the previous maximum monthly supply volumes (Fig. 1b).Although the drinking water supply infrastructure was designed with an overcapacity to meet the regular demand peaks, the flexibility to more extreme peaks or to long periods of peak demand is limited.The high drinking water abstraction volumes added up to the water shortages in both the groundwater and the surface water system that was caused by the lack of precipitation and high evaporation during the summer (Ministry of Infrastructure and Environment and Ministry of Economic Affairs and Climate Policy, 2019).To ensure an acceptable surface water quality for the drinking water supply, measures were taken to reduce salinization (Ministry of Infrastructure and Environment and Ministry of Economic Affairs and Climate Policy, 2019).
To reduce the drinking water use, a call for drinking water saving was made, and locally, pressures in the drinking water distribution system were intentionally lowered to reduce the delivered drinking water volumes (Ministry of Infrastructure and Environment and Ministry of Economic Affairs and Climate Policy, 2019).The problems caused by the summer drought raised a discourse on (drinking) water use and saving, including discussions on controversial measures such as a progressive drinking water tariffs, with tariffs dependent on the consumed drinking water volume and differentiation between high-grade and low-grade use of (drinking) water (Ministry of Infrastructure and Environment and Ministry of Economic Affairs and Climate Policy, 2019).The results of this case analysis are presented in Appendix A.

Case 2: groundwater quality development
This case focused on the impact of the groundwater quality development in the Netherlands on the drinking water supply.Analysis of the state of the resources for drinking water supply in the Netherlands in 2014 pointed out that, although the drinking water quality met the Dutch legal standards, all water resources are under threat by known and new pollutants (Kools et al., 2019).In the Netherlands, 55 % of the drinking water supply is provided by groundwater resources (Baggelaar and Geudens, 2017).Long-term analysis of water quality records of Dutch drinking water supply fields shows that the vulnerability of groundwater resources to external influences, such as land use, strongly depends on hydrochemical characteristics (Mendizabal et al., 2012).Monitoring results show that, currently, groundwater quality is mainly under pressure due to nitrate, pesticides, historical contamination, and salinization (Kools et al., 2019).Nearly half of the groundwater abstractions for drinking water are affected by an insufficient groundwater quality, and it is expected that, in the future, the groundwater quality at more abstractions will exceed the groundwater standards set in the European Water Framework Directive (European Union, 2000).In addition, traces of pollutants such as recent industrial contaminants, medicine residues, and other emerging substances have been found, indicating that the groundwater quality will likely further deteriorate (Kools et al., 2019).
Groundwater protection regulations regarding land and water use by legal authorities will help to slow down groundwater deterioration (Van den Brink and Wuijts, 2016).However, strategies to restore groundwater quality will often not be effective in the short term because already existing con- taminations may remain present for a long period of time, depending on the local hydrological characteristics (Jørgensen and Stockmarr, 2009; see Illustration case 2).The impact of contamination cannot be undone unless soil processes help to (partially) break down contaminants.Thorough monitoring for pollution is therefore essential for following groundwater quality trends and for responding adequately to these trends (Janža, 2015).Due to the expected deterioration of the raw water quality1 , different and more complex treatment meth-ods are necessary to continuously meet the drinking water standards (Kools et al., 2019).In general, a more complex treatment method leads to higher energy use, use of additional excipients, water loss, and the production of waste materials, which will lead to a higher water tariff and to a higher environmental impact (Napoli and Garcia-Tellez, 2016).The results of the analysis are presented in Appendix B.  (Van Thiel, 2017).This development resulted in a decreasing total yearly drinking water demand volume in that same period, despite the population growth in the Netherlands (Baggelaar and Geudens, 2017).However, 2013 was a turning point at which the total yearly drinking water demand volume in the Netherlands started to grow again (Baggelaar and Geudens, 2017).The trend in the period 2013-2019 shows a strong increase in drinking water demand (see Illustration case 3).Delta scenarios have been developed for the Netherlands, projecting a drinking water demand development varying between a decrease of 10 % to an increase of 35 % in 2050 compared to 2015 (Wolters et al., 2018).
The drinking water demand growth rate for the period 2013-2019, as is seen within the Vitens supply area, compares to the growth rate in the maximum delta scenario of 35 % growth from 2015 to 2050 (See Illustration case 3).

Illustration case 3: drinking water demand growth
The increase in normalized drinking water supply volume as supplied by Vitens between 2015 and 2019 is 4.5 % (Fig. 3).Due to this recent demand growth, the reserve capacity within the existing drinking water supply infrastructure is already limited.The drinking water demand growth rate for the period 2015-2019 compares to the growth rate in the maximum Delta scenario of 35 % growth from 2015 to 2050 (Fig. 3).If this growth rate is not tempered through a significant reduction in the drinking water use, this would require a large extension of the drinking water supply infrastructure.
If this strong growth rate continues, it will put serious pressure on the drinking water supply.This will partially be due to limitations in the technical infrastructure but also partially due to limitations in the water resource availability caused by insufficient abstraction permits or a possibly negative impact on the hydrological system and stakeholders.Given the inflexibility of drinking water supply infrastructure to change, an integrated strategy is necessary to meet this uncertain development in the drinking water demand.To find sustainable solutions for the future, not only the technical infrastructure aspects must be solved.It also requires strategies on water saving, expansion of permits, development of new abstraction concepts using other water resources, as well as stakeholder processes in the design and use of the local drinking water supply system.This case is basically an extension of the first two cases in that the growing water demand amplifies the aspects caused by the drought in 2018 and the groundwater quality development.The results of the analysis of this case study are presented in Appendix C.

Sustainability characteristics of drinking water supply
In this section, the sustainability characteristics are presented, each elaborated further into five sustainability criteria.A detailed description of the resulting sustainability criteria can be found in Appendix E.

Hydrological sustainability characteristics
The following three hydrological sustainability characteristics are proposed that summarize the hydrological aspects affecting the drinking water supply as found in the case studies: water quality, water resource availability, and impact of drinking water abstraction (Table 1).
Water quality includes the monitoring and evaluation of current water quality and the trends and expected future development of the water quality and emerging contaminants, as described in the case of "groundwater quality development".In the WHO Guidelines for Drinking-water Quality (WHO, 2017), the importance of microbial aspects as a global water quality aspect with a health impact is additionally monitored, such as bacteriological contamination due to untreated wastewater or emergencies.The WHO Guidelines for Drinking-water Quality (WHO, 2017) also require monitoring of water quality aspects without a health impact, such as salinization, water hardness, and colour, which affect the acceptability of the drinking water (WHO and UNICEF, 2017).
Water resource availability for drinking water supply can be differentiated into the surface water and groundwater availability, as illustrated in case 1 -"2018 Summer drought".Other sustainability aspects are the vulnerability of the surface and/or groundwater system to the water quality being permanently affected by land use, as illustrated in the case of "groundwater quality development".The water resource availability can also be limited due to smallor large-scale emergencies caused by natural hazards, such as droughts, floods, earthquakes, or forest fires (WHO and UNICEF, 2017) that will put the sustainability of the local drinking water supply under pressure., 2003-2019(Van Engelenburg et al., unpublished, 2020), compared to the projected Delta scenarios on drinking water demand growth (Wolters et al., 2018), ranging between a decrease of 10 % and an increase of 35 % in 2050 compared to 2015.The normalized annual drinking water supply volume excludes the impact of extreme weather conditions on the actual supplied annual volumes of drinking water. https://doi.org/10.5194/dwes-14-1-2021 Drink.Water Eng.Sci., 14, 1-43, 2021 The impact of the drinking water abstraction on the hydrological system entails the impact on both the surface water system and the groundwater system and also the balance between the annual drinking water abstraction volume and the annual recharge of the (local) water system.Whether the impact of the abstraction is or can possibly be hydrologically compensated is another sustainability aspect.The spatial impact of the local drinking water abstraction facility may also be a sustainability aspect because a drinking water facility requires a certain water storage area or reservoir, which might have a significant spatial impact in the area and, thus, might affect local stakeholders.

Technical sustainability characteristics
The following three technical sustainability characteristics are proposed that summarize the technical aspects for the drinking water supply as found in the case studies: reliability and resilience of the technical infrastructure and energy use and environmental impact of the drinking water supply (Table 2).
The reliability of the supply system is defined in this research as "the (un)likeliness of the technical system to fail" (Hashimoto et al., 1982).The current technical state of the drinking water production facility and the distribution infrastructure and the complexity of the water treatment are important technical sustainability criteria for the local drinking water supply system.Other technical criteria that should be considered are the supply continuity of the facility, which stands for the capability to meet the set legal standards for drinking water supply under all circumstances and the operational reliability to solve technical failures without any disturbance of the drinking water supply.
In this research, the resilience of the drinking water supply system is defined as "the possibility to respond to shortand long-term changes in water demand or water quality" (Hashimoto et al., 1982).Climate change and other developments in water demand and quality call for the use of more resilient technologies and processes and may require upgrades of water treatment and storage capacity (WHO and UNICEF, 2017).The cases of "2018 summer drought" and "drinking water demand growth" emphasize the importance of the available abstraction permits and the treatment and distribution capacity compared to the annual and peak water demand, respectively, for the resilience of the local drinking water supply system.Furthermore, the flexibility of the treatment method determines whether a drinking water supply system can deal with variation in, or deterioration of, wa- ter quality and emerging contaminants, which are the sustainability aspects found in the case of "groundwater quality development".Energy use and environmental impact include the sustainability aspects from the cases of "groundwater quality development" and "drinking water demand growth".This entails the energy use of abstraction, treatment, and distribution and the environmental impact of additional excipients, wastewater, and other waste products of the treatment.Especially when the raw water quality deteriorates, the required water treatment methods become more complex.In general, this leads to large investments and an increased energy use and environmental impact, e.g. when advanced membrane filtration methods are required.Additional global sustainability aspects are the reliability of the energy supply and the renewability of the energy that is used (WHO, 2017).

Socio-economic sustainability characteristics
A total of three socio-economic sustainability characteristics are proposed that summarize the socio-economic aspects affecting the drinking water supply as found in the case studies, namely drinking water availability, water governance, and land and water use (Table 3).
The drinking water availability can be quantified by the percentage of households connected to the drinking water supply.A sustainable local drinking water supply provides sufficient drinking water of a quality that meets the national or international drinking water standards at a tariff that is affordable to all households (UN, 2015).In the Netherlands, by law the drinking water tariff must be built on a cost-recovery, transparent, and non-discriminatory basis (Dutch Government, 2009).Water-saving strategies will reduce the drinking water demand growth and, therefore, will contribute to the sustainability.Drinking water safety is a prerequisite for public health and sustainable drinking water supply.The WHO guidelines consider water safety plans essential for providing the basis for system protection and process control and for ensuring that water quality issues present a negligible risk to public health and that the drinking water is acceptable to consumers.Therefore, the WHO Guidelines for Drinking-water Quality (2017) monitor the availability of water safety plans, including emergency plans on how to act in case of drinking water supply disturbances, shortages, or drinking water quality emergencies (WHO and UNICEF, 2017).A water safety plan can be built on various safety protocols.
Water governance focuses on policies and legislation, enforcement, and compliance of regulations.Good governance also includes decision-making processes that consider differhttps://doi.org/10.5194/dwes-14-1-2021 Drink.Water Eng.Sci., 14, 1-43, 2021 ent stakeholder interests to ensure accountable, transparent, and participatory governance (UNESCAP, 2009).The availability of (inter-) national and local policies and legislation on drinking water supply as well as on water management, including regulations and permits, and the level of compliance of the drinking water supplier to these policies and legislation, is important for socio-economic sustainability.The sustainability of the local drinking water supply is also characterized by the stakeholders' interests related to the presence of a local drinking water abstraction and by how local authorities weigh these interests in their decision-making processes.A final aspect in water governance that reaches further than local stakeholder interests is the risk of small-or large-scale emergencies for the drinking water supply caused by human activities or conflicts (WHO and UNICEF, 2017).
The local land and water use, at surface and subsurface level, affects the water quality and quantity.It may have resulted in historical contaminant sources, causing point or non-point water pollution, but it may also lead to emerging contaminants that provide new risks to water quality.Addi-tionally, water use for other purposes may limit the availability of water resources for drinking water.Regulations to protect water quality or water quantity may cause limitations on local land and water use.Financial compensation for suffered economic damage due to the impact of the abstraction or the limitations caused by protection regulations can be an important aspect for the sustainability of the drinking water supply system.

Use of DPSIR systems approach
In this study, we used an integrated systems approach to analyse the local drinking water supply system, combining hydrological, technical, and socio-economic aspects of the system.The analysis of the three selected cases with DPSIR supported the identification of the aspects that shape the sustainability of the local drinking water supply system.The case analysis did indeed help to account for differences between short-term and long-term developments and for the impact of external influences that come from the national and international scale.
The applied DPSIR approach is a linear socio-ecological framework originally developed to identify the impact of human activities on the state of the environmental system (Binder et al., 2013).However, the local drinking water supply system is a complex rather than linear system because the impact of pressure on one system element could lead to pressure on another system element.This complicated the identification of pressures and impacts.For instance, high temperatures and lack of precipitation caused a higher drinking water demand and surface water quality deterioration.Both consequently presented pressures with an impact on the resilience and reliability of the technical drinking water supply infrastructure.Although this hampered the analysis, the use of DPSIR supported a systematic analysis of the local drinking water supply cases and helped to identify the sustainability aspects.Use of a different integrated systems approach would not have led to a significantly different outcome for the case analysis.A next step could potentially be to use the identified system characteristics for system dynamics analysis and modelling.However, this is beyond the scope of this current research.

General applicability of the sustainability characteristics
To increase the general applicability of the results from the analysis of the Dutch cases on drinking water supply, the identified sustainability aspects were related to worldwide acknowledged sustainability aspects by cross-checking with international policies on drinking water supply.This put the aspects in a broader perspective, which may contribute to the transferability of the proposed sustainability characteristics and criteria to other areas.Assessments to understand the sustainability challenges and the impact of future developments and adaptation options are seen as powerful tools for policy-making (Ness et al., 2007;Singh et al., 2012).The sustainability characteristics, as proposed in this research, may be used to develop a sustainability assessment for the local drinking water supply system that can help to identify sustainability challenges and trade-offs of adaptation strategies.Trade-off analysis supports decision-making processes and makes these processes more transparent to local stakeholders (Hellegers and Leflaive, 2015).Based on the local situation and data availability, adequate indicators and indices can be selected to quantify the sustainability characteristics in a certain area (Van Engelenburg et al., 2019).

Conclusions
The aim of this study was to identify a set of characteristics that describe the sustainability of a local drinking water supply system in the Netherlands to support policy-and decision-making on sustainable drinking water supply.The use of the DPSIR systems approach was an adequate method for the analysis of the cases.The results of the analysis of the three cases confirmed the hypothesis that sustainability is contextual, resulting in different sustainability aspects in the various cases.The combined results of the analysis of three different practice cases contributed to a better understanding of drinking water supply in the Netherlands.Crosschecking of the results of case analysis with international policies on drinking water supply provided a wider context than the Netherlands and has thus contributed to the general applicability of the identified sustainability characteristics.
Based on the presented analysis, the following set of hydrological, technical, and socio-economic sustainability characteristics is proposed, respectively: (1) water quality, water resource availability, and impact of drinking water abstraction; (2) reliability and resilience of the technical system and energy use and environmental impact; (3) drinking water availability, water governance, and land and water use.An elaboration of the sustainability characteristics into more detailed criteria may further increase the value of the results of this research in the process of the development of policies on sustainable drinking water supply in the Netherlands. https://doi.org/10.5194/dwes-14-1-2021 Drink.Water Eng.Sci., 14, 1-43, 2021 Appendix A: Results of analysis case 1: 2018 summer drought Changing the design standard of distribution pipelines to limit risk of temperature rise.
Drinking water quality, treatment method, and distribution infrastructure.
Increasing abstraction volume, resulting in increasing impact to land use.
Stakeholder complaints by agriculture and nature.
Increased societal pressure on the reduction of the impact of drinking water abstraction.
Drinking water demand, abstraction volume, impact of abstraction, land use, stakeholders, agriculture, nature, and drinking water suppliers.
Exceedance of abstraction permits and limiting the resilience of the technical infrastructure.
Enforcement procedures by legal authorities.
Extension of drinking water abstraction permits and watersaving strategies.
Drinking water demand, abstraction volume, abstraction capacity, abstraction permit, resilience of abstraction, legal authorities, water regulations, water legislation, and saving drinking water.
Shortage of drinking water during peak demand due to insufficient resilience of treatment infrastructure.
Reduced drinking water supply volume.
Adjustment of resilience and reliability of treatment infrastructure.
Treatment volume, treatment capacity, drinking water shortage, reliability of the treatment, resilience of the treatment, drinking water standards, drinking water demand, and drinking water suppliers.Lowering drinking water pressure to reduce drinking water volume.
Adjustment of resilience and reliability of distribution infrastructure.
Distribution capacity, resilience and reliability of distribution, drinking water suppliers, drinking water volume, and drinking water standards.
Major disturbances could cause a serious disruption of the supply.
Maximum personnel deployment by drinking water suppliers.
Investments to improve the resilience and reliability of technical infrastructure by drinking water suppliers.
Drinking water demand, reliability of technical infrastructure, and drinking water suppliers.
High energy use and environmental impact of extreme drinking water production.
-Incorporating impact to energy use and environmental impact in the design of measures to improve the resilience and reliability of technical infrastructure.
Drinking water demand, energy use, environmental impact, and drinking water suppliers.
https://doi.org/10.5194/dwes-14-1-2021Drink.Water Eng.Sci., 14, 1-43, 2021 The summer affected the drink-ing water use as follows: filling of swimming pools, watering gar-dens, and extra showering all led to a very high drinking water demand.Additionally, there also were requests from concerned cit-izens about adding drinking water to refill ponds that dried up due to the extreme drought.
Drinking water suppliers increased the abstraction volume to meet the increased drinking water demand.
The drought (re-)initiated a dis-course on water-saving strategies, including controversial measures such as progressive drinking water tariffs and differentiation between high-grade (household and sanita-tion and food production) and low-grade (pools, gardens, and process water) use.

Extreme weather event
High evap-oration and no precipi-tation.
Drought, falling water discharges and groundwater levels, and damage to groundwater-dependent ecosystems and agriculture.
Water use limitations, water au-thorities applied existing drought water policy, and risks in water quality.
Development of additional water shortage policy for water manage-ment and water governance.The drought caused falling water discharges and groundwater lev-els; thus, river discharges declined, springs and brooks dried up, and vegetation withered or even died due to low groundwater levels and high temperatures.Groundwater-dependent ecosystems such as wet-lands and agricultural produce suf-fered due to the drought.
Limitations in water use from wa-ter system.Water authorities ap-plied the special water policy that was developed for periods with low water availability.Drinking water supply has a high ranking because of its high societal rele-vance.
In some ecologically vulnerable ar-eas, there is a water policy to re-solve local surface water shortages by supplementing them from larger water bodies such as rivers.This affects the local surface water qual-ity and may also affect the ground-water quality.
Discourse and policy development for water management and wa-ter governance, aiming at a fur-ther prioritization and limitations of water use during water short-age and retention of surface wa-ter and groundwater during periods with sufficient water availability.Societal support for drinkingwater-saving strategies.
Customers, drinking water availability, drinking water suppliers, and water saving.
Because of the visible damage to vegetation due to the drought, customers started to worry about the drinking water availability.
Drinking water suppliers communicated that there still was sufficient drinking water, but people were asked to spread the drinking water use to reduce the peak demand.Later that summer, there was a call for customers to save water.
The drought raised awareness among customers that there are limits to the drinking water availability, thus creating (some) societal support for (drinking) water saving.

Extreme weather event
No precipitation.Declining surface water discharge and quality.
Drinking water supplies took measures to safeguard raw water quality.

Development of additional policies
on water quality protection.
Surface water discharge, surface water quality, drinking water suppliers, raw water quality, water management policies, and water use.
Due to the lack of rain, the share of industrial wastewater and treated sewage water in the surface water discharge increased, which caused the water quality in surface waters to deteriorate.
Drinking water suppliers that use surface water as a resource took measures to safeguard the raw water quality.
The surface water discharge and quality problems may induce the development of water management policies that aim to reduce the impact of treated sewage and industrial wastewater by a reduction in water use or improvement of treatment.

Extreme weather event
Declining surface water quality.Groundwater quality deterioration.
No response possible due to lack of water.

Development of additional policies
on water quality protection.
Groundwater quality, surface water quality, water shortage, surface water discharge, and water management policies.
The impact of an incidental warm and dry summer on the groundwater quality is limited, but when comparable droughts happen frequently, the groundwater quality may deteriorate due to the impact of a declining surface water quality.
In some surface water bodies, refreshment was required to guard the surface water quality, but due to the lack of precipitation, there was a water shortage, so insufficient water was available for this refreshment.The fact that surface water discharge and quality may affect groundwater quality supports the need for water management policies that aim to refresh water bodies and to reduce the impact of treated sewage and industrial wastewater.
Ensuring sufficient refreshment due to high demand.
Changing the design standard of distribution pipelines to limit risk of temperature rise.
Drinking water quality, treat-ment method, and distribution infrastructure.
The extreme temperatures led to an increased surface water temper-ature, and soil temperature, that may have affected the drinking wa-ter temperature in the distribution infrastructure.This introduces a drinking water quality risk.
When surface water is the main resource for drinking water, the water quality risk will be limited by a treatment method that en-sures the bacteriological quality of the drinking water.Sufficient re-freshment within storage and high stream velocities in pipelines re-duces the risk of temperature rise in the distribution infrastructure.
The risk of drinking water qual-ity aspects caused by increased drinking water temperature due to climate change may have conse-quences for the design of the dis-tribution infrastructure.

Extreme weather event
High drink-ing water demand.
Increasing abstraction volume re-sulting in increasing impact on land use.
Stakeholder complaints by agricul-ture and nature.
Increased societal pressure on the reduction of the impact of drinking water abstraction.
To meet the high drinking wa-ter demand, the abstraction volume rose to a high level.In some lo-cal areas, the impact of the abstrac-tion added up due to the extreme drought and high temperatures, af-fecting the land use.
Stakeholders in agriculture and na-ture complained about the impact of the extra abstraction on their land use.
The drought impact enlarged the societal pressure on drinking wa-ter suppliers to reduce the impact of local drinking water abstraction on the water system.Enforcement procedures by legal authorities.

Table A2.
Extension of drinking water abstraction permits and water-saving strategies.
Drinking water demand, abstraction volume, abstraction capacity, abstraction permit, resilience of abstraction, legal authorities, water regulations, water legislation, and saving drinking water.
To meet the high drinking water demand, the abstraction volume rose to a high level.The available abstraction capacity, combined with the high abstraction volumes, led to the exceedance of the abstraction permits.Some local drinking abstractions exceeded the monthly permitted volume, and some abstractions even exceeded the yearly permitted volume, failing drinking water regulations.This compromised the resilience of the abstractions.
Legal authorities (provinces and water boards) started enforcement procedures to meet the water regulations.The legal authority urged the drinking water supplier to stay within these limits.However, the drinking water legislation also had to be met to ensure a continuous supply of good quality drinking water at all times.
The exceedance of the abstraction permit limits set off enforcement actions by the government, resulting in an increased need for additional abstraction permits and drinking-water-saving strategies to reduce the drinking water demand.

Extreme weather event
High peak demand for drinking water.Shortage of drinking water during peak demand due to insufficient resilience of treatment infrastructure.
Reduced drinking water supply volume.
Adjustment of resilience and reliability of treatment infrastructure.
Treatment volume, treatment capacity, drinking water shortage, reliability of the treatment, resilience of the treatment, drinking water standards, drinking water demand, and drinking water suppliers.
To meet the high peak demand, the treatment volume rose to a high level.In some parts of the drinking water supply, there was insufficient treatment capacity, causing a temporary shortage in drinking water during peak demand, compromising the reliability of the treatment.These limitations showed that the treatment is not resilient for this extreme peak demand.
There is no response available when the treatment capacity is insufficient, except by reducing the drinking water supply volume.Exceeding the treatment capacity (by, for example, increasing the filter flow velocity or reducing the cleansing frequency of the filters) would introduce the risk of not meeting the drinking water standards.
The drought identified various locations in the technical infrastructure where the treatment capacity was not reliable at the peak drinking water demand, which led to drinking water suppliers solving these local treatment aspects.To adjust all aspects will take several years.
https://doi.org/10.5194/dwes-14-1-2021Drink.Water Eng.Sci., 14, 1-43, 2021 In some parts of the drinking water supply there was insuffi-cient distribution capacity due to hydraulic limitations, insufficient storage capacity, or age and quality of the pipelines.In some areas, this caused unintended low drinking water pressures.These limitations put the reliability of the distribu-tion under pressure and showed that the distribution capacity was not resilient for this extreme peak demand.
To reduce the drinking water vol-ume that was supplied, drinking water suppliers lowered the drink-ing water pressure intentionally in some areas.The impact of this pressure reduction is a decreased drinking water volume from taps.By reducing the drinking water pressure, the distributed drinking water volume was reduced; how-ever, this also led to a falling short of the mandatory drinking water standards in some areas.
The drought identified locations in the technical infrastructure where the distribution capacity was not reliable at peak demand, which led to drinking water suppliers solving these local distribution aspects.To adjust all aspects will take several years.

Extreme weather event
High peak demand for drinking water.
Major disturbances could cause a serious disruption of the supply.
Maximal personnel deployment by drinking water suppliers.
Investments to improve resilience and reliability of technical infras-tructure by drinking water suppli-ers.
Drinking water demand, re-liability of technical infras-tructure, and drinking water suppliers.
The high peak demand required a maximal exploitation of the techni-cal infrastructure.To ensure the re-liability of the drinking water sup-ply, many parts of the infrastruc-ture are designed to be redundant, which limits the impact of dis-turbances for customers.However, a major disturbance in the infras-tructure, such as the failure of a large transportation pipeline, could have led to a disruption in the sup-ply because the resilience was lim-ited due to limited reserve capac-ity and reduced maintenance dur-ing the extreme drinking water de-mand period.
To ensure the reliability of the drinking water supply, distur-bances are always solved with top priority.During the extreme peak period, drinking water sup-pliers had all personnel put on standby to immediately solve any disturbances.
The drought identified locations in the technical infrastructure that were not reliable at peak demand, which led to drinking water sup-pliers solving these local aspects and, where necessary, creating re-dundancy to decrease the risk of disturbances and, thus, improve the reliability.

Short-term response
Long-term response

Extreme weather event
High peak demand for drinking water.High energy use and environmental impact of extreme drinking water production.
-Incorporating impact on energy use and environmental impact in the design of measures to improve the resilience and reliability of technical infrastructure.
Drinking water demand, energy use, environmental impact, and drinking water suppliers.
The magnitude and duration of the peak demand forced a maximal exploitation of the technical infrastructure, causing a maximal energy use and environmental impact.
There was no short-term response available to reduce the energy use and environmental impact.
The drought identified locations in the technical infrastructure that were not reliable at peak demand, which lead to drinking water suppliers solving these local aspects.Energy use and environmental impact are important aspects that are considered in the design of the solutions for these aspects.
Groundwater quality, surface water quality, monitoring and evaluation, and water saving.
Soil energy systems may affect groundwater quality.
Monitoring and evaluation of water quality development and research.
Groundwater protection regulations.
Groundwater quality, groundwater pollution, research, monitoring and evaluation, regulations, and groundwater quality protection.
Local and upstream land and water use affects the surface water quality.
Monitoring and evaluation of water quality development.
Policy and measures to meet water legislation to protect and improve water quality and quantity.
Surface water quality, land and water use, contaminants, monitoring and evaluation, water legislation, and water quantity.
Diffuse and point sources of pollution affect surface water and groundwater quality.
Monitoring and evaluation of water quality development.
Measures to remove historical sources of pollution and to prevent new sources of pollution.
Groundwater quality, nutrients, organic micro-pollutants, other contaminants, surface water quality, monitoring and evaluation, water legislation, and water quality protection.
Emerging contaminants in surface and groundwater require new drinking water treatment methods.
Enforcement of groundwater protection regulations on pollution incidents and monitoring and evaluation.Surface water quality deteriorates due to limited surface water dis-charge.
Monitoring and evaluation of wa-ter quality development.
Water legislation on water qual-ity and quantity protection and drinking-water-saving strategies.
Surface water quality, surface water discharge, monitoring and evaluation, water legisla-tion, water quality and quantity, and saving drinking water.
In summer, the surface water qual-ity deteriorates due to limited sur-face water discharge combined with increasing contributions of in-dustrial and treated sewage water recharges compared to natural dis-charges due to the lack of summer precipitation.
Monitoring and evaluating water quality development is necessary to be able to respond timeously to a changing surface water quality.
Land and water use must meet wa-ter legislation as set by the Eu-ropean Water Framework Direc-tive and national water legisla-tion to protect and improve water quality and quantity.Further im-provement in sewage and wastew-ater treatment will reduce the im-pact on the surface water qual-ity.Drinking-water-saving strate-gies can also lead to reduction in treated sewage water recharges and industrial recharges.

Changing climate variability
Surface water quality de-terioration.
Groundwater quality deteriorates due to deteriorating surface water quality.
Monitoring and evaluation of wa-ter quality development.
Improvement of sewage and wastewater treatment and water-saving strategies.
Groundwater quality, surface water quality, monitoring and evaluation, and water saving.
Groundwater quality may be af-fected by the deteriorating surface water quality during summer peri-ods through natural or artificial in-filtration of surface water.
Monitoring and evaluating water quality development is necessary to be able to respond timeously to a changing surface water quality.
Further improvement in sewage and wastewater treatment will re-duce the impact on the surface water quality.(Drinking-) water-saving strategies can also lead to reduction in treated sewage water recharges and industrial recharges.There is a transition towards renewable energy resources, not only wind and solar energy but also towards using soil energy.Groundwater quality may be affected by the use of soil energy due to the risk of groundwater pollution by soil energy systems and the risk of leakage through aquitards that protect aquifers.
Research on, and monitoring and evaluating of, the impact of soil energy on the groundwater quality (including temperature impact) is necessary to avoid the introduction of new sources of pollution by soil energy systems.
Regulations on soil energy help to limit the risk for groundwater quality.A policy is developed to exclude vulnerable groundwater systems that are used for drinking water supply from soil energy use for groundwater quality protection.Diffuse and point sources of pol-lution affect surface water and groundwater quality.
Monitoring and evaluation of wa-ter quality development.
Measures to remove historical sources of pollution and to prevent new sources of pollution.
Groundwater quality, nutrients, organic micro-pollutants, other contaminants, surface water quality, monitoring and evalua-tion, water legislation, and wa-ter quality protection.
Groundwater quality is affected by diffuse and point sources of pol-lution such as nutrients, organic micro-pollutants, and other con-taminants caused by historic land and water use.Groundwater can be influenced by (historic and current) surface water quality through nat-ural or artificial infiltration of sur-face water.
The impact of historical contam-inations will proceed further into the groundwater system and can-not be undoneunless soil pro-cesses help to break down con-taminants.Monitoring and evaluat-ing are necessary to be able to re-spond timeously to a changing wa-ter quality.
Historical contaminations from past land use will affect the groundwater quality for a long period of time due to the low stream velocity of groundwater.Some historical point pollution may be removed through soil and groundwater remediation, but dif-fuse pollution cannot be removed.However, according to the water legislation in the European Water Framework Directive, additional measures must be taken to reach the set goals on water quality protection in 2027.Emerging contaminants, such as new industrial pollutants, medicine residues, and microplastics, may pose new threats to the groundwater and surface water quality and, consequently, the raw water quality, especially when they cannot be removed using the currently available treatment methods.The changes limit the resilience and reliability of the drinking water treatment.
Groundwater protection regulations on land and water use aim to reduce the risk of pollution to avoid groundwater quality deterioration.This includes regulations for small incidents with point pollution such as those caused by a car accident, for example, which are to be reported and solved immediately by removing the source of pollution.Continuous enforcement of these regulations is essential.Monitoring and evaluating are necessary to be able to respond timeously to a changing water quality.
According to the water legislation in the European Water Framework Directive, known sources of pollution must be reduced and new sources of pollution must be prevented.This may include prohibition by law or measures for reducing the use of specific chemical products.To deal with emerging contaminants, it is essential to limit or remove the contaminant source.If all these measures fail, the contaminants must be removed by the drinking water treatment.
Other or new drinking water treatment methods may be required.New treatment methods may cause an increase in energy use and environmental impact (excipients, wastewater, and waste materials).
This may lead to a higher drinking water tariff.
Enforcement of groundwater pro-tection regulations on land use change and monitoring and evalu-ating.
Combination of extensive land use functions with drinking water ab-straction.
Land use change, groundwa-ter quality, sources of pollution, groundwater protection regula-tions, water use, enforcement of regulations, monitoring and evaluating, drinking water ab-straction, extensive land use, nature, agriculture, and water system.
Land use change may cause groundwater quality deterioration due to the risk of the diffusion of point sources of pollution.The impact may be limited if land use changes towards less polluting land use functions.
Groundwater protection regula-tions on land and water use aim to reduce the risk of pollution to avoid groundwater quality deteri-oration.This includes regulations on land use change developments.Continuous enforcement of these regulations is essential.Monitor-ing and evaluating is necessary to be able to respond timeously to a changing water quality.
Combining extensive land use functions, such as nature and sus-tainable agriculture, with drinking water abstraction in local areas to reduce the groundwater qual-ity deterioration rate, depending on the land use and hydrological and chemical characteristics of the wa-ter system.Monitoring of drinking water quality; in case of emergencies, measures are taken to safeguard the drinking water quality.

Adjustment of treatment methods
to be able to continue to meet the drinking water standards.
Raw water quality, drinking water standards, water quality, vulnerability of the water system for contamination, treatment methods, reliability and resilience of treatment, drinking water quality, emergencies, energy use, environmental impact, and drinking water tariffs.
The raw water quality of the abstracted groundwater or surface water determines the treatment that is necessary to meet the legal drinking water standards.When water quality deteriorates in general, due to the vulnerability of the water system for contamination, different and more complex treatment methods become necessary to ensure the reliability of the treatment in order to meet the drinking water standards.The resilience of the treatment method or capacity may be insufficient to respond to variability in raw water quality.The drinking water quality is constantly monitored and checked with drinking water standards.In the case of drinking water quality emergencies, local measures are taken, such as temporary boiling instructions to customers or temporary additional treatment to safeguard the drinking water quality.A deteriorating raw water quality may require the adjustment of treatment methods to meet the drinking water standards and to ensure the resilience and reliability of the treatment.In general, a more complex treatment method leads to a higher energy use and a higher environmental impact due to additional use of excipients, water loss, and waste materials, which will lead to a higher drinking water tariff.If the raw water quality is under extreme pressure, adjustment of treatment methods may not be possible.This can ultimately lead to the decision to close the local drinking water abstraction and force the drinking water supplier to find and develop a replacing abstraction location.
Monitoring and evaluating water quality development.
Increase in resilience and reliabil-ity of drinking water treatment.
Surface water quality, ground-water quality, resilience and re-liability of the treatment, moni-toring and evaluating, raw wa-ter quality, energy use, envi-ronmental impact, and drinking water tariffs.
Especially surface water quality can show strong water quality vari-ations.They can enforce a tem-porary interruption of the surface water intake.Groundwater quality is more stable and, therefore, less vulnerable to incidental changes.However, incidents can cause a permanent change in the ground-water quality.It depends on the re-silience and reliability of the treat-ment whether sudden variations in raw water quality can be handled well.
Monitoring and evaluating is nec-essary to be able to respond timeously to changing water qual-ity.
To handle a varying or deterio-rating raw water quality, the re-silience and reliability of the drink-ing water treatment must be ex-tended.This may require innova-tions in treatment, which can lead to large investments, and higher energy use and an increase in the environmental impact of the treat-ment.This may lead to a higher drinking water tariff.

Appendix C: Results of analysis case 3: drinking water demand growth
Table C1.Summary of the impact, short-and long-term response, and sustainability aspects in case 3 -drinking water demand growth (for complete results of the case study, see Table C2).

Impact
Short-term response Long-term response Sustainability aspects A limited water resource availability will affect the drinking water availability.
Water resource availability, drinking water availability, resilience of drinking water supply, drinking water demand, and water legislation.
A water quality deterioration affects the resilience and reliability of the drinking water treatment.
Water quality, drinking water treatment, reliability of treatment, and drinking water standards.
A growing drinking water demand will put the reliability and resilience of the technical infrastructure under pressure.
Drinking water suppliers must adapt the technical infrastructure to the growing water demand.Water-saving strategies may reduce the growth rate, which will limit the required extension of the technical infrastructure.
Drinking water demand, reliability of technical infrastructure, drinking water suppliers, drinking water availability, treatment, energy use, environmental impact, and drinking water tariff.
A declining drinking water demand may also put the resilience of the technical infrastructure under pressure.
Research on the potential risks of a decline in drinking water demand.
Adaptation strategies that increase the resilience of the infrastructure to growth and a decline in the drinking water demand.
Drinking water demand, reliability, and resilience of technical infrastructure.
Water resource availability, drinking water availability, resilience of drinking water supply, drinking water demand, and water legislation.
A limited water resource availabil-ity will affect the drinking water availability.The abstraction per-mits may be insufficient to meet the drinking demand, and possi-bilities to extend the permits will be minimal.This will put the re-silience of drinking water supply to respond to changes in drinking water demand under pressure.This may cause frequent exceedance of permit conditions or failure to ad-here to the drinking water legisla-tion.
Water quality, drinking water treatment, reliability of treatment, and drinking water standards.
If the water quality deteriorates, this will affect the raw water quality of the water abstracted for drinking water production.The available drinking water treatment facilities may not be resilient to these changes.This affects the reliability of the water treatment, potentially causing an exceedance in drinking water standards.
Drinking water suppliers must adapt the technical infrastructure to the growing water demand.Water-saving strategies may re-duce the growth rate, which will limit the required extension of the technical infrastructure.
The overall capacity of the tech-nical infrastructure determines whether the supply is resilient in response to a higher drinking water demand.The drought in 2018 dis-played the technical limitations in parts of the drinking water supply system, putting the reliability of the technical infrastructure under pressure.
Depending on the effectiveness of the water-saving strategies that are developed, the technical lim-itations must be solved to meet the growing drinking water de-mand.Drinking water suppliers must solve the local aspects to ensure drinking water availabil-ity.Because these adjustments take time, drinking water suppli-ers must start solving the aspects now.This requires substantial in-vestment and also leads to an in-creased energy use and environ-mental impact, which may result in an increased drinking water tariff.Research on potential risks of a decline in drinking water demand.
Adaptation strategies that increase the resilience of the infrastructure to growth and a decline in the drinking water demand.
Drinking water demand, reliability, and resilience of technical infrastructure.
If, at some point, the socioeconomic developments reverse the drinking water demand growth, the reliability and resilience of the technical infrastructure will be put under pressure.Especially when the focus is on dealing with a growing water demand, there is the risk of an over-dimensioning of the technical infrastructure.This will put the drinking water quality under pressure in the case of a decreasing drinking water demand.
While working on solutions for the growing drinking water demand, it is important to consider the potential risks of a decreasing demand.
The chosen adaptation strategies for a growing drinking water demand must also be resilient and reliable under a decreasing drinking water demand. https://doi.org/10.5194/dwes-

Hydrological compensation
The extent to which the impact of abstraction is compensated hydrologically.
Small impact or impact is hydrologically compensated with a technical measure.
There are possibilities for hydrological compensation of the impact on the abstraction, but they are not operational yet.
There is a significant impact on the abstraction, but there are no possibilities for hydrological compensation.https://doi.org/10.5194/dwes-14-1-2021

Figure 3 .
Figure 3. Development of the normalized annual drinking water volume supplied by Vitens (drinking water supplier), the Netherlands, 2003- 2019 (Van Engelenburg et al., unpublished, 2020), compared to the projected Delta scenarios on drinking water demand growth(Wolters et al., 2018), ranging between a decrease of 10 % and an increase of 35 % in 2050 compared to 2015.The normalized annual drinking water supply volume excludes the impact of extreme weather conditions on the actual supplied annual volumes of drinking water.
use, water authorities, water policy, water management, water governance, and water availability.

Table 1 .
Summary of proposed hydrological sustainability characteristics, hydrological aspects from case studies (see Appendices A-C), relevant SDG * indicators and WHO Guidelines for Drinking-water Quality (WHO, 2017) aspects, and hydrological sustainability criteria.
* SDG -Sustainable Development Goal; see Appendix V for a summary of SDG 6 targets and indicators related to sustainability characteristics (UN, 2015).

Table 2 .
Summary of proposed technical sustainability characteristics, technical aspects from case studies (see Appendices A-C), relevant SDG * indicators and WHO Guidelines for Drinking-water Quality (WHO, 2017) aspects, and technical sustainability criteria.
* SDG -Sustainable Development Goal; see Appendix V for a summary of SDG 6 targets and indicators related to sustainability characteristics (UN, 2015).

Table 3 .
Summary of proposed socio-economic sustainability characteristics, socio-economic aspects from case studies (see Appendices A-C), relevant SDG * indicators and WHO Guidelines for Drinking-water Quality (WHO, 2017) aspects, and socio-economic sustainability criteria.
* SDG -Sustainable Development Goal; see Appendix V for a summary of SDG 6 targets and indicators related to sustainability characteristics (UN, 2015).

Table A1 .
Summary of the impact, short-and long-term response, and sustainability aspects in case 1 -2018 summer drought.In the subsequent TableA2, the full results of the case study are presented.

Table A2 .
Results analysis of case 1 -"2018 summer drought".For each pressure, the response to and impacts on the state of the local drinking water supply system are described.The cells in italics refer to TableA1.

Table B2 .
Results analysis of case 2 -"groundwater quality development".The cells in italics refer to TableB1.

Table C2 .
Results of the analysis of case 3, drinking water demand growth, were additional to the analysis of the first two cases.The cells in italics refer to TableC1.

Summary of Sustainable Development Goal 6 targets and indicators related to sustainability characteristicsTable D1 .
Summary of the Sustainable Development Goal 6 targets and indicators related to sustainability characteristics.