Implications of climate change on water quality and sanitation in climate hotspot locations: A case study in Indonesia

Water is inextricably linked to pillars of sustainable development by crosscutting the UN sustainable development goals (SDGs) through its close links with climate. Recent global water crisis has shown that climate change poses a higher danger than previously thought, affecting the water cycle and seriously impacting life and sustainable development (Zhang et al. 2023). We live in a time of climate abnormality. However, this is the new normal. Increased water contamination observed in recent years suggests the need for tools with unique ability to describe future trends and to forecast potential water quality variations amidst the persistent threat of climate change. Past studies indicated that high local temperatures and frequent/ intense precipitation due to climate change will increase the discharge of suspended solids and nutrient loading to water bodies (Rachmadi et al. 2020).

The debate around climate change is a multifaceted one. While climate change is real, current debates in the body of literature argue that the global COVID-19 pandemic has raised public awareness on the importance of universal and equitable access to water, sanitation, and hygiene as a critical issue for public health (Fu et al. 2021a). As a result of the new challenges, developing countries have become unlikely to implement integrated water management by 2030 due to water scarcity, water shortage, and water pollution induced by climate change over the past years because of human involvement (Fekete et al. 2022).

Equally attributable to those accepting climate change, the opponents of climate change counter argue that climate change is not a significant threat, but it is a natural phenomenon that humans cannot control. While water scarcity is a major threat to the current global environment and a critical issue for sustainable development, water consumption minimization, wastewater treatment, greenhouse gas (GHG) reduction, and water pollution issues remain complicated and strongly intertwined due to anthropogenic activities and conflict of interests among relevant stakeholders (Barbieri et al. 2023). Therefore, mitigation measures and actions require new methods to undertake feasible and applicable solutions (Zikra et al. 2015).

Recently, extreme weather events have increased in terms of frequency and severity and are increasingly driving displacement in different parts of the world. This highlights the need for appropriate knowledge, tools and institutional arrangements for future hazards assessment, adaptation planning, and effective systems development of climate risk management. Therefore, there is urgency for a holistic approach to integrate biophysical and social aspects by looking at environmental and human contexts for solving the complex issue of water supply in Indonesia because of its scarcity. It is vital to redouble strategic efforts in terms of climate protection to deal with looming consequences in countries' water resources due to climate change (Najafzadeh & Basirian 2023).

Additionally, the climate change impact on the planet is unprecedented. Long-term changes in the Earth's environment and climate system are significant. The danger posed by climate change is enormous. Climate change is happening globally, but the countries and people least responsible and least resourceful are affected disproportionately. As tangible evidence, extreme weather conditions have been increasingly taking place globally recently, and the impacts of climate change are clearly noticeable worldwide. In developing countries with insufficient resources, heat waves, droughts and water shortages lead to serious hardship. A future where the world does not cross the 1.5 °C threshold is still within reach, but it requires drastic change and substantial investment in mitigation through appropriate water management practice (Avtar et al. 2019a).

According to Gentilucci et al. (2018), water management is accountable for 90% of successful adaptation to climate change, while the International Water Association found that about 20% of CO₂ emissions depends on appropriate water management policies. Aside from being affected by climate change, food production contributes to CO₂ emissions. A third of global GHG emissions is from food and agriculture (Gentilucci et al. 2019). Except Brunei and Singapore situated in the region, agriculture is among the top GHG-generating sectors, which requires climate mitigation action. Rice production is the highest contributor to methane (CH₄) emissions globally. Southeast Asia (SEA) produces 30% of the global supply of CH₄ (Fu et al. 2018).

For this reason, there is a growing need to provide a comprehensive overview of the current and project impacts of climate change on water quality and sanitation in Indonesia by highlighting the challenges and opportunities of adapting water quality and sanitation to climate change in the country. It is necessary to provide an overview of the effects of climate change on water resources and propose innovative solutions for developing countries by using Indonesia's water challenge as a case study for other countries that currently encounter similar problems to strengthen their resilience.

A preliminary study has been undertaken by Widyarani et al. (2022), who investigated domestic wastewater in Indonesia with respect to generation, characteristics and treatment. In spite of its novelty, their work did not take into account the effect of climate change on water resources in Indonesia such as drought and water stress. This calls for transformational changes and points to wide-ranging solutions, from adopting water-efficient technologies to ‘off-setting’ CO₂ emissions with water supply restoration.

To strengthen its novelty, this work reviews the current situation of water resources based on a baseline assessment dataset of water resources in Indonesia. For this purpose, a climate change model based on the Representative Concentration Pathway (RCP) 8.5 is used to predict various scenarios in Jakarta (its capital) when sea level rises. This study also critically evaluates climate change-related disrupted long-standing assumptions about water management, while providing an overview of the effects of climate change on water resources. Drinking water safety and legislation status and relevant legislation and standards are analyzed as a response to a growing concern over chronic challenges that undermine water supply nationwide. Wastewater treatment and infrastructure (surface water treatment, information on water treatment facilities and its distribution networks) are also discussed. The implications and applications of this knowledge and pathways for future work are also presented.

SEA is among the regions hardest hit by climate change. The region is projected to encounter increasingly frequent and destructive storms, heatwaves, droughts, and other unpredictable weather events, which will impact food production. The Asian Development Bank (ADB) estimated that the region's GDP could decline by 11% by the end of 2100 due to a sharp decline in agriculture, fishing, and tourism. This would affect one in three people living in the SEA region, who are employed in the agricultural sector (or approximately 190 million farmers) (Khanzada et al. 2023). The volatility in food supply would also lead to higher food prices, greater food insecurity, and potentially, create social instability.

Due to their physiology, climate change will increase children's risks to water and heat stress, food insecurity, rising air pollution, increased risk of vector-borne diseases, acute respiratory infections, diarrheal diseases and malnutrition. Children born in East Asia and the Pacific are experiencing a six-fold increase in climate-related disasters, as compared to those born 50 years ago, while the frequency and intensity of disasters will continue to rise. Therefore, there is an urgent need to build the resilience of children, prepare and prioritize them to foster their climate adaptation in anticipation of frequent and severe climate hazard (Sallan et al. 2023).

With an area of 1.9 million km², Indonesia is the world's largest archipelagic state with the second longest coastline (108,000 km). The country has a high vulnerability due to climate change for the next 20 years due to its growing economic activities and population, abundant low-lying areas, and reliance on the agricultural sector (Handayani & Asyary 2019). In 2020, its population was estimated at 275 million (3.5% of the world's population), ranked as the fourth most populous country. Its gross domestic product (GDP) in 2020 was estimated at USD 1,186 billion (0.1% of the global economy) (Herlinda et al. 2020).

Indonesia's climate is hot and humid with rainfalls occurring in areas that are at or near sea level throughout the year. On average, the annual rainfall across the country is 2,800 mm, while its water supply rate is 130 L/person/day with 92% of water sources for coverage in rural and urban areas (Avtar et al. 2019b). With increasing variability and magnitude of rainfall patterns, 25% of the country's population is projected to experience water scarcity by 2030 (Avtar et al. 2020). According to Han et al. (2023), water scarcity is defined as the lack of access to adequate quantities of water for human and environmental uses.

As temperatures rise, drought lingers, and the resource grows scarce, officials tend to blame water scarcity on physical forces that lie beyond their control. Pollution, industrialization, climate change, drought, or a failed monsoon combine to apply pressure on a finite local resource. Command-and-control approaches to dynamic water and people not only harm ecosystems and economies, but they also punish the poor, reward waste, undermine long-term reforms, and delay technological adoption. Hence, decentralized water is an approach which in time may become both an institutional possibility and an ecological necessity (Kurniawan et al. 2022a).

Indonesia is the largest country in the Southeast Asian region with respect to area. With 280 million in 2023, recently the country has encountered serious problems of water shortages (Wardhani et al. 2018, 2022). Current challenges are not only related to water availability (only 60% of Jakarta's clean water is supplied), but are also associated with water quality as water sources do not meet legislative standards because of anthropogenic contamination (Kurniawan et al. 2023a). Furthermore, water demand is much higher than the actual availability as the government only contributes 80% of the raw water supply (Kurniawan et al. 2023b), increasing water demand across all sectors and stressing existing water resources.

Indonesia's rapid urbanization, industrialization, and population growth over the past years have brought adverse consequences on water resources availability and quality (Ali et al. 2018). Its freshwater demand continuously increases to satisfy the needs of increasing population and growing economy. The changing climate further stresses water resources, making water availability limited and critical for Indonesia and challenging reliable supply of clean water (Furlong & Kooy 2017). Despite collective efforts to deal with climate change implications on water resources, people remain struggling to access water quality to fulfill their daily needs with >100 million Indonesians lacking reliable access to clean water on a daily basis (Ariyanti et al. 2020), highlighting the need for infrastructure upgrading and securing supplies. Even where access to water exists, water utility services are characterized by poor management, inadequate financing, low investment, low quality of service, and poor access, posing serious risks to public health, food security, and other ecosystem services. Hence, assessing locations where water quality is inadequate and incorporating the need of water security in national development plans are necessary to avoid inequality that subsequently hampers its economic growth (Kurniawan et al. 2011).

Water supply deficit in Indonesia was 5.5 hm³/year (Ardhianie et al. 2022), which is expected to be fulfilled with private vendors. The term ‘water supply’ refers to water provision by public utilities, commercial organizations, community endeavors, or individuals via pumps and pipes. This includes piped networks, self-pumped groundwater, and other sources of water. Depending on the vendor and location, private vendors may secure additional water from groundwater, surface water, or rainwater harvesting. They may purchase water from the same source as the piped supply and sell it in areas without reticulation.

Currently, Indonesia's average rate of non-revenue water (NRW) stands at 32% (Molekoa et al. 2019), unlike the global average of 20% or the country capital's NRW of 46% or around 280 million m³ (Ardhianie et al. 2022). NRW means water lost before it reaches consumers through water leaks. This has been achieved by real-time data monitoring throughout water distribution systems, covering issues like water quality, pressure, and flow rates. Water supply and demand forecasts for 2030, however, suggest that this could not be closed by increasing water supply from other sources. This highlights the country's vulnerability to climate change impacts, which impoverished minority groups in marginalized areas (Bakker 2003).

Indonesian basins also show water issues with respect to quality and quantity because of excessive groundwater extraction, which is likely the reason for the sinking of certain areas in the country. The problems affecting 10 million people are related to increased population, inefficient water management, low awareness in water saving, expanding industrial and agricultural production, lack of water treatment and poor surface water quality (Bawafi et al. 2020).

In spite of efforts in making clean water accessible to millions of people, one-sixth of the country's population (about 45.8 million) remain lacking access to clean water, while over 30% (about 82.5 million) do not have access to improved sanitation, locking them in poverty for generations (Cao et al. 2021), and only 2% (about 5.5 million) have access to sewerage (Emam et al. 2016). The whole situation makes water challenges in Indonesia unprecedented (Austin et al. 2017), as water scarcity also has hit the agricultural sector, making farming difficult and threatening community access to food. Unless tackled immediately, water scarcity and related problems will become a major obstacle for promoting sustainable development.

Indonesia has over 5,700 rivers managed within 135 river basin territories (Ariyanti et al. 2020). Key basins include five transboundary basins shared with Papua New Guinea, Malaysia, and Timor Leste. There are 30 inter-provincial basins, and 37 national strategic basins controlled by the central government (Table 1). Presently, water resource potential is around 15,000 m³ per capita annually, while the total water availability is 690 × 10⁹ m³ annually, higher than the country's demand (175 × 10⁹ m³/year). Papua and Borneo islands, inhabited by 15% of the country's total population, possess about 70% of its overall water resources (Kopp et al. 2017), which is much higher than global water supply average (8,000 m³ per capita annually). In 2021, the total potential of water supply in Java decreased to 1,200 m³ per capita annually (Fulazzaky 2014) because the increasing population positioned Indonesia's water resources as 6% of the world's share and 21% of Asia-Pacific's fraction (Strauss et al. 2021). As a result, water has become scarce due to growing consumption, lack of reservoirs and storage, and persistent environmental issues. About 70% of rivers in Indonesia are heavily polluted by domestic waste, which has been linked to 1.8 million deaths annually (Kulp & Strauss 2019), leaving communities with no choice but to consume unsafe water. Poor-quality drinking water related to poor sanitation is the cause of infant mortality caused by diarrheal disease (Suryono 2015).

Indonesia has abundant surface and groundwater sources for water supply (<8,000 watersheds) (Aldrian & Susanto 2003), but only 10% of rainfall infiltrates the underlying groundwater with the rest as surface water (Avtar et al. 2021). The cumulative water average across the country was estimated 4 × 10⁶ m³/year (Rochyatun et al. 2006). Although the country has an average of 55 m³/cap water stored in reservoirs nationwide, 33% of its river basins have their maximum capacity for exploitation (Kurniawan et al. 2021b). The river basins experience a growing competition for water, which in turn has led not only to an increasing, inequitable, and inefficient allocation of water, but also a widespread water degradation (Ulfat et al. 2023).

Climate change also impacts on increasing pressure on water supply, leading to water scarcity. As a result, the freshwater boundary has been crossed already in different locations, entering the country into a danger zone, as it suffers from water scarcity, water pollution, and damages caused by extreme events, such as flooding and droughts (Lo et al. 2009). Droughts are hitting harder and more often – up 30% since the 2000s (Zablotska et al. 2008). Drought continues to tighten its frightening grip on both land and life. Droughts hit the poorest the hardest, of which 85% of people affected by drought live in low- or middle-income countries. The countries are obviously less equipped with resources and have low levels of capacities, technology, and financial resources to cope with drought. In Africa such as Ethiopia, Kenya and Somalia, over the past 50 years, drought-related losses exceeded USD 70 billion, putting over 20 million people at risk of food insecurity across the continent (Batool et al. 2024). This contributed to reduced agricultural productivity and high food prices, leaving around 23 million people facing severe hunger (Kurniawan et al. 2024). As food shortages frequently happened, it is time to acknowledge that droughts, hardship and displacement evolved from an environmental issue into an economic crisis. The world has to act urgently to prevent future droughts from destroying development gains.

The amount of freshwater being used by humans and ecosystems exceeds the amount that can be replenished naturally, leading to a decline in freshwater availability and quality. The term ‘freshwater boundary’ refers to the limit of the Earth's freshwater resources that can be sustainably used by humans and ecosystems. Water sector is on the frontline of climate change impacts from drought to floods. Extreme weather events were responsible for displacing millions of people with the Java Island being one of the most disaster-prone regions in the country (Kurniawan et al. 2011). The situation is expected to worsen before any reversals in the trend will be observed.

With increased ocean warming, tropical cyclones also intensify, and so do aridity and fire weather, and extreme sea level events, which currently happen once in a century, will occur annually in more than half of all water level monitoring stations by 2100. About 3.6 billion people are highly vulnerable to climate change, with economically and socially marginalized urban residents suffering disproportionately from the effects of climate change (Kurniawan et al. 2021b). Ocean warming and ocean acidification also have already damaged fisheries and shellfish aquaculture (Kurniawan et al. 2022d).

It has been estimated that only 35% of the water is economically feasible to be reused (Han et al. 2023). This includes agriculturally used water and piped network water. Hence, its actual potency is about 400 m³ per capita annually lower than the lowest demand set by the United Nations (1,000 m³ per capita annually). Therefore, Java is projected to suffer from a water crisis in the future (Ali et al. 2018) along with the whole of Indonesia as climate change alters rain patterns, generating flooding and droughts. To generate resilience, sound water management and access to reliable delivery service remain vital to inclusive economic growth and social well-being (Goh et al. 2023).

By 2050, over 90 million people are predicted to live in Indonesian cities, which is expected to result in a massive increase in the country's need for clean water and sanitation services (Kurniawan et al. 2005). Tackling water scarcity, food and nutrition security simultaneously require a systemic and transformational approach in the way water is used to support agriculture across scales and landscapes. Careful planning of urban development can bring access to clean water and sanitation. It is vital for Indonesia to maintain health and grow food, while protecting the environment at the same time (Rochyatun et al. 2006). Thinking ahead and acting in advance of water supply has far lower costs than reacting and responding to its impacts in the long-term.

It goes without saying that the government needs to promptly put together a unified water management body. Currently, flood control is managed by the Ministry of Environment, depending on the body of water and the function. This is not an effective structure to address a crisis such as rapid flooding. The government needs to proactively invest in water management. Although 98% of flooding occurs in the countryside (Fu et al. 2023), smaller counties have difficulty securing funds. The central government must step up and provide support. Third, there needs to be legal and financial support to establish a net-zero GHG emissions policy to protect water resources from contributing to CO₂ emission during its treatment. While the European Union (EU) is a leader in this sector, the Indonesian government focuses its budget and other resources on the development of new capital, while promoting gradual economic, environmental and social reforms (Lo et al. 2012).

According to the ADB, Indonesia uses 80% of its water resources for agricultural applications (Suryono 2015). Declining aquifer storage and surface water base storage, changes in rainfall patterns, reduced water quality, and/or increased public use demands caused by climate change have generated pressure on the water, energy, food, and ecosystems (WEFE) nexus (Ikhzan et al. 2021).

Industrialization is the main source for pollutants detected in water resources. The limited water resources are polluted with industrial waste discharged without proper treatment into water bodies. Industrial activities such as agriculture, pulp and paper, and mining lead to decreased water quality, which has slowed down infrastructure development in water resource management in Indonesia. For example, water sources located in mining areas are polluted with heavy metals like mercury (Hg), which has been detected at different sampling points at concentrations up to 2.78 μg/L surmising that 50 rivers in the country did not meet drinking water criteria based on the Government Regulation (Kurniawan 2012). The following sections describe the identified needs.

In Indonesia, the main challenges for drinking water supply are sustainable water availability and proper quality. For example, the key water source for Indonesia's capital is the Jatiluhur Dam, on the Citarum River, situated 70 km southeast of Jakarta (Hasbiah & Kurniasih 2019). Water supply in Jakarta is frequently at risk because of inadequate dam maintenance, but mainly because of the pollutants in the Citarum watershed. Such pollutants have become an emerging issue nationwide (Juwana et al. 2022). As a result, alternative water supply sources are required locally to minimize the pressure on conventional water sources to increase (Pawitan & Haryani 2011).

Alternative sources of water have been intensively explored to meet clean water demand in Indonesia (Maryono et al. 2021). For example, communities in Sumatra and Papua have implemented rainwater harvesting as an essential water, which is believed could close the gap of supply–demand and reduce pressure on conventional water supplies. However, considering the US Environmental Protection Agency (EPA)'s guidelines for per- and polyfluoroalkyl substances (PFAS) and other contaminant of emerging concern in drinking water, rainwater would be judged unsafe to drink (Sara et al. 2023) and only used for non-potable consumption, thus increasing communities' vulnerability to climate change.

Stormwater is another alternative source explored by a few studies in Indonesia (Syafriana & Arifin 2020). Once again, the availability of this valuable water source requires its proper conversion into a safe-to-use resource by removing pollutants entrained in stormwater. To achieve this goal, it is fundamental to understand its pollutants loads/concentrations processes, contaminants build-up on urban surfaces during dry weather periods and wash-off during wet weather (Wijesiri et al. 2020). The lack of information related with the current situation in Indonesia related with the pollution load in either surface and groundwater as well as alternative water sources such as rain- or stormwater is a knowledge gap that urges attention because it may mean a life change difference for sustainable development mainly for underserved community members, usually highly vulnerable to climate change-driven injustice (Liang et al. 2022a).

In addition to domestic wastewater, industrial wastewater significantly contributes to water pollution. There would be a 20% reduction in wastewater if conventional sprinkler systems were replaced by micro-irrigation, which delivers water directly to plant roots. In 2021, the country's Ministry of Environment projected 12,000 medium/large and 82,000 small/medium industries with process polluting surface water. The agro-industrial sector is another source of water pollution because of the presence of high loads of nutrients and organic matter released by agricultural run off or livestock husbandry effluents. This is attributed to undeveloped sewage treatment systems and/or untreated wastewater, impacting the quality of water building-up phosphate ⁠, nitrate ⁠, and ammonia (NH₃) concentrations in receiving water bodies (Kulp & Strauss 2019).

Indonesia's rivers rank among the worst in Asia because of domestic sewage and agricultural runoff. Using groundwater as an alternative for water supply is also challenging because of significant pollution of groundwater nationwide. Few cities in Indonesia possess minimum sanitation facilities, forcing community members to dispose of their waste directly into local water bodies, generating pollution in shallow wells commonly used for drinking water and leading to epidemics of gastrointestinal infections (Fu et al. 2022a).

Table 2 summarizes the major wastewater management issues identified for Indonesia and a variety of suggested actions to address the identified issues (Kurniawan et al. 2010a).

The existing sewage coverage in a particular area needs to be expanded by integrating combined sewage and interceptors for system separation (Kopp et al. 2017). Combined sewage collects both storm water runoff and domestic sewage in a single pipe. This is often used in older cities, where the infrastructure was built before the separation of stormwater and sewage was required. Interceptors for system separation are used to separate the stormwater from the sewage. This method is used to prevent the overflow of sewage into rivers and other water bodies during heavy rainfall. Expanding centralized sewage coverage using combined sewage and interceptors for system separation improves the overall sewage system and reduces the risk of sewage overflow (Premakumara et al. 2014).

Indonesia can be slow in reversing rising water scarcity back into abundance by adapting its own endemic governance systems for today's modern urban networks. The time has come to call for an immediate action and collaboration between countries, states and generations. Pursuing a water management strategy is a critical first step in the right direction to tackle those problems (Fu et al. 2022b).

According to the World Meteorological Organization, July 2023 was officially the hottest month on record that our planet has ever seen. Global warming, with global temperatures currently 1.1 °C above pre-industrial levels, has led to frequent hazardous weather events including droughts. The world was on course for a temperature rise of 2.5–3 °C above pre-industrial levels in the next decades – way above the 2°C (Zhu et al. 2023). Hence, it would have to minimize 60% of GHG by 2035, relative to a 2019 baseline, to keep temperature increases within the 1.5 °C target (Han et al. 2023). As GHG levels in the atmosphere are high, the world risks temperature increase of 2.5–3°C above pre-industrial level without urgent actions.

According to the United Nations, 55 million people are already affected by droughts (Kurniawan et al. 2022e). Temperatures are predicted to get hotter, with 700 million people around the world expected to be at risk of being displaced by 2030 as a result of the droughts (Kurniawan et al. 2021a). By 2050, it is projected that one in four people globally (1.8 billion inhabitants) are likely to experience recurring water shortages. Of those, almost 5% is exposed to extreme drought (Mustakim & Pratama 2020).

Southeast Asia is one of the most severely affected regions, with seven out of the 10 most water-stressed nations located here (Batool et al. 2023a). The human and economic costs of drought are likely higher than that of any other hazard (Zhu et al. 2022). Unlike other disasters that attract global attention, droughts happen silently, often going unnoticed and failing to provoke an immediate global response. This perpetuates a cycle of neglect, leaving affected populations to bear the burden in isolation (Albadarin et al. 2017).

It is also undeniable that climate change is directly linked to environmental pollution and aggravated pollution exposure and health impacts (Quevedo-Castro et al. 2022). The exacerbated water stress conditions faced by Indonesia may get worse because of the effects of climate change combined with already observed rapid urbanization, increasing water demand and affecting water quality (Setiawati et al. 2023).

For this purpose, water quality index (WQI) can be used to describe the overall water quality by combining physical, chemical, and biological factors into a single value that ranges from 0 to 100 (Adelagun et al. 2021). It is an effective way to describe the quality of water by reducing the bulk of information into a single value (Lukhabi et al. 2023). The WQI is designed based on parameters such as pH, dissolved oxygen (DO), biochemical oxygen demand (BOD), total coliforms, and fecal coliforms.

Water stress is projected to impact up to two-thirds of the Indonesian population by 2025, including 150 million people living in low-lying regions, including small islands and major coastal cities, who may need mass relocation and become climate refugees should global warming remain ignored (NOAA 2020). This massive population shift will likely generate additional stress in the already overwhelmed water resources and generate a risk for desperate sectors of the population using water sources without the proper safeguard to ensure water security (Kurniawan et al. 2022c).

It has been suggested that even slight temperature increases possess the capability of setting off a chain of negative effects in aquatic ecosystems such as lower DO, increased pollutant loads, widespread HABs, or increased saltwater intrusion (Wilopo et al. 2021). Nevertheless, there is a significant lack of knowledge on the accurate effect that these temperature changes may have on sensible water quality parameters, identified as a knowledge gap worth exploring to generate new decision-making tools that help to predict water quality variations as a function of temperature changes.

Such information reports water quality degradation that has been marginally implemented and evaluates water quality in tropical regions. This can be undertaken by taking into account climate change-related impacts, sediments and nutrients transport, hydrological cycle variation with temperature, demographic growth, and fecal contamination (Quevedo-Castro et al. 2022). A few studies report predictive models that combine water quality and global climate change models, of which most of them are developed and applied for water bodies located in colder environments (Kurniawan et al. 2023c).

There are two different concepts to study future climate change, namely RCPs and climate models. RCPs are scenarios that describe the possible trajectories of GHG concentrations in the atmosphere, based on different assumptions about socio-economic development, emissions, land use, and other factors (Strauss et al. 2021). They are expressed in terms of radiative forcing, a measure of the imbalance in the Earth's energy budget caused by GHG (Fu et al. 2017). There are several RCPs: RCP2.6, RCP4.5, RCP6, and RCP8.5, which correspond to different levels of radiative forcing by 2100.

Climate models are mathematical representations of the physico-chemical processes that govern the climate system. They use numerical methods to simulate the interactions between the atmosphere, ocean, land, ice, and biosphere, and how they respond to natural and human influences. Climate models can be used to project the future climate under different RCP pathways, by taking into account the feedback and uncertainty in the climate system. Climate models can also be used to explore the past and present climate, and to test hypotheses and theories about climate change (Manna & Biswas 2023).

Developing climate models such as RCP 8.5 is an interesting opportunity, considering the need for quantifying temperature variation impact on water quality for tropical regions. For this purpose, the RCP 8.5 model has been developed to predict various scenarios in Jakarta, its capital, when the sea level rises. Sea levels rose by 15–25 cm between 1900 and 2018. A scenario in which the sea level rises of 2 m could put 67% of Jakarta underwater, overwhelming the capital at an early stage of sea level rise. If the world warms by 2 °C, as compared to the pre-Industrial era, then the levels will rise again by 43 cm by 2100 (Ikhzan et al. 2021). If it warms by 3–4 °C, sea levels could rise by as much as 84 cm. If there is no mitigation efforts are made RCP scenario 8.5, in 2100 it is projected that the height of sea level rise will reach 93–243 mm. If efforts are made maximum to withstand rising air temperatures (RCP 2.6), sea level rise is projected to reach 26–98 cm.

Based on the predictions of global mean sea level (GMSL) and relative sea level (RSL) using the RCP 8.5 model (Kopp et al. 2017), this indicated the possibility of sea level rise to 2 m in 2100, as compared to normal sea level in 2019. On SLR-1 scenario (1 m sea level rise), paddy fields below 1 m asl in 2019 will be submerged, so will the rice fields at a height of 2 m asl on SLR-2 (sea level rise of 2 m). Over-pumping of groundwater in Jakarta could result in some areas dropping from 3 to 1 m asl, while the rice fields affected by SLR-1 and SLR-2 turned into seawater. It is assumed that it can no longer be used for growing rice. The study area is a coastal area in the form of land that is less than 10 km from the coastline.

The results obtained earlier by Ikhzan et al. (2021) were different from those reported by Aldrian et al. (2022), who identified three dominant rainfall regions within Indonesia and their relationship to sea surface temperature. The projection of the annual wet day rainfall total (using extreme index PRCPTOT) and Consecutive Dry Days (using extreme index CDD) based on the CORDEX-SEA projection data between 2021 and 2040, mid- (between 2041 and 2060) and long-term (between 2081 and 2100) period was assessed by Aldrian et al. (2022).

According to the scientists, PRCPTOT is an indicator of rainfall extreme that measures the annual/seasonal rainfall total. It is one of the four different indicators of rainfall extreme used in the study, the others being CDD, frequency of extremely heavy rainfall (R50mm), and annual/seasonal maximum of daily rainfall (RX1day). Additionally, the CORDEX-SEA refers to the SEA Regional Climate Downscaling/Coordinated Regional Climate Downscaling Experiment–SEA. This climate model provides high-resolution climate projections for the region. The model has been used to test the projected precipitation extremes for the end of the 21st century over the region (Kurniawan et al. 2005).

In the RCP4.5 scenario, the PRCPTOT index for the near-term projection was projected not to change with the average reduction value for all provinces approaching zero. For the mid- and long-term projection periods, the PRCPTOT index under RCP4.5 scenario was projected to decline at an average rate of decline across all provinces by 3.8 and 4.1%, respectively, against the baseline period. In mid- and long-term projection, the decreases occurred in 91 and 88% of provinces, respectively, sequentially under the RCP4.5 scenario (Figure 5).

With the RCP8.5 scenario, the projection results show that the changes were greater than the scenario RCP4.5. The PRCPTOT index for the near-, mid-, and long-term projection periods under RCP8.5 scenario decreased with an average rate of all provinces of 2.3, 2.9 and 7.2% over the baseline period, respectively. The downward trend of this PRCPTOT value under RCP8.5 scenario occurred in most of the existing provinces (>80%) both in short, medium and long-term projections.

Because of climate change-related depleting water resources and growing demands, there is a growing concern about providing economically challenged, underserved population segments with clean water. Water quality-related challenges and opportunities resulting from climate change scenarios are reportedly unevenly experienced with poor socio-economic urban residents being disproportionately affected (Hawken et al. 2021). Failing to address changing urban waterscape poses a risk to become an environmental injustice related to wealth redistribution and population well-being for inhabitants of various locations within the same community (city center versus suburbs), which imply the need for effective strategies and mechanisms for addressing water dilemmas and ensuring the accountability of actors involved (Stoler et al. 2022).

The new water managing/governing approaches need to be adapted to protect broader environmental community values and rights. However, very few studies are available devoted to generating the basic information required such as water supply availability, socio-economic needs of residents, and their demographic conditions. The lack of information is identified as a knowledge gap that requires attention to produce the data. This avoids low-income and vulnerable communities to suffer the consequences of today's water mismanagement (Mardiatno et al. 2023).

Stormwater is generally considered a problem in urban settlements because it causes flooding when improperly managed and it has been reported to affect aquatic ecosystems (Goonetilleke et al. 2017). However, this assumption causes decision makers and stakeholders to overlook the potential to transform stormwater into a safe-to-use resource that can provide relief for overexploited conventional water sources such as surface water (rivers, streams, lakes, reservoirs, and springs) and underground water (aquifers). The term ‘conventional water sources’ refers to the natural sources of water that are commonly used for human consumption (Gavirc et al. 2019).

It is undeniable that population increases add to the impacts of climate change by exacerbating water scarcity. But in most cases, authorities continuously fail to see urban stormwater as the last available water resource for cities (Kordana & Slys 2020). In Indonesia, stormwater has historically been treated as waste and discarded immediately because of storage space and fixed cost barriers (Goonetilleke et al. 2017). The authors found a significant lack of consistency and stability of legal regulations for implementing sustainable stormwater management in the country, like other megalopolises in developing countries, where policy openness, household affordability, storage space, and lobbying from industries with vested interests were identified as the most significant barriers for stormwater reuse. Although recent studies have identified great potential for stormwater reuse in Indonesia, very little information is available (Kurniawan et al. 2023d).

There are legislative and regulatory framework and specific bodies that have a governance responsibility for water resources in Indonesia. Water Resources Law No. 7/2004 is the primary legislation governing water resources in Indonesia. The law covers the conservation of water resources, enhancement and efficiency in the use of water resources, control over water destructibility, planning, construction, operation and maintenance, water resource information system, empowerment and supervision, financing, right, obligation and role of community, coordination, settlement of dispute, class action and organization, investigation, criminal provisions, transitional provision, and conclusion (Putri et al. 2023).

In addition to the Water Resources Law, there are other laws, regulations, and policies that govern water resources in Indonesia (Liang et al. 2022b). For example, the Government Regulation 37/2012 concerns watershed management, while Government Regulation 26/2008 concerns the national spatial plan. The governance responsibility for water resources in Indonesia is shared among several government bodies, such as the Ministry of Public Works and Housing, the Ministry of Environment and Forestry, the Ministry of Energy and Mineral Resources, and the National Development Planning Agency (Sallan et al. 2023).

Based on the available studies, the need for fit-for-purpose treatment of stormwater effluents before agricultural use is evident because the presence and concentration of heavy metals (As, Cd, Cr, Pb, and Zn) have been found to be significantly influenced by the catchment characteristics and weather conditions eliminating the possibility of any further reuse until proper treatment is carried out (Alrabie et al. 2021). For this reason, storage facilities required, including Aquifer Storage and Recovery (ASR), need to be considered if structures are suitable. The lack of information on the potential for stormwater catchment, treatment, and reuse in Indonesia is a key knowledge gap worth exploring because stormwater could replace freshwater sources for crop production, provided that there is a proper fit-for-purpose treatment to ensure food security. This allows conventional water sources to be devoted to other uses such as drinking water or specific manufacturing applications (Kurniawan et al. 2023e).

The most ambitious goal for water bodies is to reach a good state, which requires that planning and implementations be considered from an integrated point of view (Satalova & Kenderessy 2017). ‘A good state’ refers to the condition of a water body that is healthy and sustainable for both human and environmental uses (Batool et al. 2023b). A water body is considered to be in a good state if it meets the requirements of the EU's Water Framework Directive. The criteria for determining whether a water body is in a good state include the biological, chemical, and physical quality of the water, as well as the quantity and flow of the water. The biological quality of the water is assessed by measuring the presence and abundance of different species of plants and animals, while its chemical quality is assessed by measuring the concentration of different pollutants in the water (Homem & Santos 2011). The physical quality of the water is assessed by measuring the structure and function of the water body, including the flow of water and the physical characteristics of the surrounding landscape. In order to achieve a good state for water bodies, planning and implementation must be considered from an integrated point of view. This means that all aspects of water management, including water supply, wastewater treatment, stormwater management, and ecosystem protection, must be considered together in a coordinated and holistic manner (Astuti et al. 2022).

For this reason, active public participation is necessary for a proper management of water resources within a basin because all the current and future users can share responsibility in decision-making and technology implementations that enhance water security. Therefore, providing access to actors representing different interests in the process, sharing information to improve communication, encouraging power delegation to community stakeholders, and developing interventions are vital to ensure basin management.

Emerging contaminants (ECs) originate from diverse sources with typical concentrations ranging from ng/L to μg/L (Rodriguez-Narvaez et al. 2019). They include natural and synthetic chemical compounds that are commonly present in water, which have only recently been recognized as significant water pollutants. The ECs group includes pharmaceutical and personal care products (PPCPs), pesticides, and hormones that are included in the endocrine-disrupting compounds (EDCs) group. To the best of our knowledge, little is known about the presence of ECs in surface water, groundwater, and wastewater in Indonesia, but their presence has significant consequences for food production and public health by considering that the actual trends are moving toward decreases in precipitation, surface water and groundwater levels and an increase in population (Kurniawan et al. 2023f).

An interesting adaptation technology is promoting water treatment to prevent the release of ECs into water bodies and to restore water bodies that are already contaminated. Different technologies capable of removing contaminants have been widely reported, some of which have proved to be cost-effective for EC removal. Adsorption has been extensively studied for the removal of several pollutants (Ahmed et al. 2015).

Table 3 includes a recompilation of results from using carbon-based materials as adsorbents for ECs. Carbon-based materials are made from heating biomass at a high temperature, either in the absence or presence of oxygen. They include various materials, such as activated carbon, biochar, hydrochar, and carbon nanotubes (CNT). Recently, the use of activated carbon, biochar, and hydrochar has been investigated for the adsorption of ECs because of their high porosity and specific surface area (Rivera-Utrilla et al. 2013). Rodriguez-Narvaez et al. (2017) conducted in-depth analyses on the use of carbon-based materials to treat wastewater contaminated with ECs and highlighted their cost-effectiveness and potential for use in developing countries because the production of waste biomass materials can be reused. This allows them to remove target contaminants cost-effectively (Sniatala et al. 2022).

Harmful algal bloom (HAB) events are hard to predict and quickly attain high densities, which have devastating effects on biota, sometimes with long-term impacts on lake ecology. Although high nutrient loads such as N and P are closely linked to HABs, the response of algal communities to nutrient load, pH, temperature, DO, and other water quality parameters is unknown, like the mechanisms for HABs in freshwater (Royer 2020; Kurniawan et al. 2023g).

Additionally, surface water quality has been reportedly affected by climate change (Quevedo-Castro et al. 2022) when combined with the effect of anthropogenic activities. For example, the increased concentrations of nutrients (macro- and micronutrients) have been reported to generate algal blooms in tropical lakes of Indonesia (Piranti et al. 2021). Increased water temperature related with climate change is expected to produce low DO, pH conditions in freshwater and marine ecosystems, which consequently is projected to generate increased conditions for larger-scale HABs (Griffith & Gobler 2020). HABs have been frequently reported to happen in Indonesian marine ecosystems (Likumahua et al. 2022), but significantly less information is available on HABs occurring in freshwater ecosystems. Considering the high concentration of nutrients found in different reservoirs in Indonesia (Mardiatno et al. 2023) the HABs might have occurred or continue happening in the future, even if these events are not recorded with the corresponding threat to water users (Kurniawan et al. 2022b).

Algal blooms are best managed at a local level because of the varying circumstances of each event. Local councils and state water authorities should investigate suspected outbreaks and alert potential users to avoid using unsafe water for food production or other uses (Kurniawan et al. 2023h). The most effective and plausible method for stymieing algal blooms is to incorporate a comprehensive watershed management plan to minimize the nutrient load entering waterways through nonpoint sources. To implement this plan, information access and a better understanding of the conditions that trigger HABs are needed. The application of state-of-the-art technologies is the key to achieving these goals (Delinom et al. 2009).

To be sure, users must prevent algal blooms through comprehensive approaches that engage numerous actors in the development and integration of the best practices for their watersheds. Implementing such a broad system can be costly and require a highly coordinated approach among stakeholders. Nevertheless, costs can be reduced, and coordination can be achieved using technologies such as wireless communication, water-sensing arrays, or internet of things (IoT) (Kurniawan et al. 2023i).

For this reason, developing a cutting-edge application of state-of-the-art technologies to monitor and understand HABs will have long-term benefits for people in local communities. Deployment of such technologies in developing countries with significant vulnerability to climate change, such as Indonesia, will develop proper adaptation measures to ensure water security amid the diverse climate change scenarios.

Micro-organisms can infect plants and destroy crops, while increasing the risk of epidemic outbreaks that threaten human lives (Huesca-Espitia et al. 2017a). Among different micro-organisms, viruses are resistant to commonly used water disinfection processes (Garcia-Gil et al. 2020). This highlights the need for novel, cost-effective technique for their inactivation in water before it is used for food production or human consumption (Adelodun et al. 2020). While conventional secondary wastewater treatment processes can remove 2–3 log10-cycles of virus content through adsorption to the solid particles of activated sludge, membrane technologies such as ultrafiltration, nanofiltration, and reverse osmosis are efficient at removing viruses and/or solid-associated viruses (Zhang et al. 2024). However, they involve mechanical separation and not the inactivation of the pathogens (Bhowmick & Dhar 2019).

Other technologies such as advanced oxidation processes (AOPs) have been tested and identified as having great potential and providing a variety of options for pathogen inactivation (Sanchis et al. 2019). Using either solar disinfection, ozonation, wet oxidation, or even cold plasma to generate reactive oxygen species (ROS) has shown significant potential for inactivating bacteria and other pathogenic micro-organisms (Huesca-Espitia et al. 2017b). Other techniques have been reported including conventional chlorination, the use of alternative disinfectants, and the application of high-temperature CO₂ injection in water for pathogen inactivation (Garcia et al. 2019).

Particularly, AOPs are a promising option and have been tested for their capability to inactivate different pathogens in water (Aurioles-López et al. 2016). Fenton and Fenton-like processes are effective options for inactivating resistant micro-organisms (Bandala et al. 2011). Considering the threats posed by micro-organims as phytopathogens to decimate crops and put populations at the risk of waterborne disease outbreaks, developing cost-effective technologies to minimize the release of pathogens into the environment through wastewater effluents or their reuse in agriculture is a scientific task. There is a vast scientific literature and developed regulations available on this topic (Castillo-Ledezma et al. 2011, 2014).

The water stress problem can be caused by poor water infrastructure and management, deforestation, unsustainable farming practices, and pollution. In developing countries, the available infrastructure may not suffice to ensure water or food security, particularly in rural communities that lack access to resources to disinfect water using conventional water treatment processes (Kurniawan et al. 2013). This process is used to remove harmful pathogens from water to make it safe for consumption. The widely used technology is a combination of coagulation, sedimentation, filtration, and disinfection (González et al. 2008).

With respect to its limitation, this work was based on a single case study from Indonesia, which may not be representative of other climate hotspot locations or regions with different socio-economic and environmental conditions (Kurniawan et al. 2023b). Additionally, it did not provide a thorough quantitative assessment of the impacts of climate change on water quality and sanitation based on the WQI such as changes in water availability, pollution levels, health risks, or adaptation costs. Nor did this work consider the interactions and feedbacks between water quality and other sectors such as energy, agriculture, or urban development, which might affect the vulnerability and resilience of the systems (Kardono 2007).

Furthermore, this review relied on existing literature and data sources, which might have uncertainties, gaps, or biases in their methods, assumptions, or results. Therefore, this work did not explore the potential co-benefits or trade-offs of adaptation measures for water quality and sanitation, such as their effects on GHG emissions, biodiversity, or social equity (Hadipuro & Indriyanti 2009).

Resulting from this review, this work has identified knowledge gaps and needs for future studies such as:

• Assessing the impacts of climate change on water quality parameters based on the WQI, such as temperature, pH, nutrients, and pathogens, and their implications for public health and ecosystems.

• Developing and implementing innovative technologies for water quality monitoring, treatment, and reuse, such as sensors, decentralized systems, nature-based solutions, and circular economy (CE) approaches.

• Evaluating the effectiveness and co-benefits of adaptation measures for water quality and sanitation systems, such as infrastructure, rainwater harvesting, wastewater recycling, and ecological sanitation.

• Enhancing the governance and institutional capacity for water quality and sanitation management, such as strengthening the legal framework, improving the coordination and collaboration among stakeholders, and increasing the public awareness and participation (Mortazavian et al. 2019).

This implies the need to improve the governance and institutional capacity for managing water quality by enhancing the capacity of institutions responsible for water management, such as local governments, to implement legislation and policies that promote sustainable water use and sanitation practices.

As an implication of this work, scientific evidence obtained from this work will help policymakers to reduce urban water pollution through sound water management practices by bringing environmental impacts in the long-term for local communities in Indonesia. As understanding the Earth's changing climate and its consequences are scientific challenges of importance to society, this work eventually contributes to national adaptation plans, which can formulate appropriate actions to address local climate change impacts, which vary depending on the specific context and the extent of the pollution reduction measures implemented. It is vital that water professionals from various stakeholders can collaborate and share cutting-edge solutions to deal with future water risks. Through their dialogue, the world can uncover the best available water technology to explore solutions that may be ‘one-size-fits-all’ (Kurniawan et al. 2023j).

The recent situation of water management reported in this article has demonstrated that the country has been facing a water quality challenge due to serious and increased water pollution, population growth and sustainable progress in industrial and agricultural activities that consumes a lot of water sources (Carabineiro et al. 2011; Fu et al. 2021b). The increased demand for agricultural, industrial and urban water supplies has led to competition in the allocation of limited freshwater resources. The scarcity of freshwater might be alleviated by reusing water sources. This only happens if the risks of biological and chemical contamination are mitigated. The current legal regime must look to the future and address knowledge gaps in existing frameworks. Therefore, the present legal framework needs to be forward-looking and address the gaps in the frameworks.

This article also has identified knowledge gaps and research needs to improve the understanding and management of climate change impacts on water quality and sanitation in Indonesia. This includes assessing the impacts of climate change on water quality parameters, developing innovative technologies for water quality monitoring, treatment, and reuse, evaluating the effectiveness of adaptation measures for water quality and sanitation, and enhancing governance for water quality and sanitation management.

Among the key findings of this study, only 10% of rainfall infiltrates the groundwater, while 70% of its rivers are heavily polluted by domestic waste. In addition, water availability decreased to 1,200 m³/year in 2020, with only 35% of the resources being economically feasible for reuse. The water supply deficit in Indonesia was about 5.5 hm³/year with about 67% of the population's water demand satisfied in 2021. Although this might be fulfilled with private vendors, water supply/demand forecasts in 2030 suggest that the gap could not be closed by simply increasing water supply. This implies that climate change possesses serious threats on Indonesia's water resources in the future unless it is anticipated and tackled properly.

The article has also identified the key drivers and barriers for adaptation in the water sector, such as governance, institutional capacity, and water technology. It was found that the wastewater treatment system in Indonesia is insufficient to supply clean water to the local community. The water resources used to supply the needs of inhabitants have become unsustainably utilized. Hence, currently water supply is limited. While searching for alternative sources of groundwater is a necessary solution, Indonesia needs to change how economic development is pursued (Kurniawan et al. 2021c). The country needs to put forward and adopt strong policies and measures to provide ways out for the problems.

To underscore its scientific value, the findings reported in this review article are applicable to deal with water quality and sanitation challenges in Indonesia by having identified key drivers and barriers for adaptation in the water sector such as governance, institutional capacity, finance, and technology, to contribute to climate change adaptation with a water management transition. As implications of this work, the country needs to unlock institutional, technological, and financial lock-ins for meaningful progress toward diversifying its water supply source. Through adaptation and circularity, it can scale them up for a sound water resource management.

With the COP28 in Dubai (United Arab Emirate) fast approaching, sustainability experts are sounding the alarm for cooperation among countries to increase resilience, adaptation, and mitigation efforts. While water stress could be a future cause of conflict, the opposite is also true. Collaboration on water could serve as a bridge for peace because while conflicts are over boundaries, drought does not recognize borders. With the frequency and severity of drought events increasing, as reservoir levels dwindle and crop yields decline, as the world continues to lose biological diversity and famines spread, transformational change is needed.

The outcomes of this work are expected to be a major contribution in identifying and implementing cost-effective water management policies to minimize impacts and strengthen community resilience in developing countries vulnerable to climate change impacts (Pouretedal & Sadegh 2014). It is anticipated that this work forms a critical basis for the identification and assessment of alternatives of water supply solutions and their contribution to build resilience against extreme weather (WHO 2009; Ma et al. 2022). This work provides stakeholders with insights into the challenges faced by the country's water systems due to climate change and offers various suggestions to address the challenges. By making progress in understanding how extreme events, their consequences and required responses may look like under possible climate change, this work contributes to the development of new techniques to analyze national datasets to determine the socio-economic impacts of droughts in Indonesia and advance knowledge of the associated impact pathways (Iskandar et al. 2020; Kurniawan et al. 2023k).

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

The authors declare there is no conflict.