Jakarta, “the sinking city”, is the current capital city of Indonesia. Located on the Java Sea, this coastal city is home to nearly 30 million people within the greater-Jakarta area. Jakarta has grappled with water management issues for decades, leading to several current day water-related crises. Access to a reliable, potable water supply is extremely limited as there is a significant disparity between those with piped water access and those without. Citizens without piped water access have consequently relied heavily on groundwater and have dug thousands of unregulated wells as a result. This has led to a second water crisis – the chronic overextraction of Jakarta’s underground aquifers. Land subsidence is of the utmost concern as this sinking city is placed at high flood risk from the surrounding ocean. Approximately 40% of Jakarta now lies below sea level as a result and predictive models suggest that the entire city will be underwater by 2050 (Gilmartin, 2019). Compounding these problems, the climate crisis has led to significant sea level rise as glaciers and ice caps continue to melt (Intergovernmental Panel on Climate Change, 2019; Lindsey, 2022). As the city of Jakarta continues to sink and sea levels rise, millions of citizens within Jakarta are at extremely high risk of flooding, particularly during monsoon season (Figure 1). Thousands of residents have already been forced to abandon their homes in search of improved conditions and higher ground (Garschagen et al., 2018).

As a means of addressing these challenges, the current Indonesian government has made a proposal to move the capital city entirely. As a concept, the idea itself is not novel; several Indonesian government authorities have made similar proposals since the 1950s. The plans, however, have never actually been followed through – until now. The new capital, which will be known as Nusantara, is planned to be built in the East Kalimantan region of the island of Borneo (Figure 2). Home to large swathes of rainforest and rare flora and fauna, Borneo is currently largely undeveloped. The new capital city is planned to be built entirely from the ground up with thousands of hectares of rainforest and natural spaces having to be clearcut to allow for the new infrastructure (Yusriyah et al., 2020). Additionally, the region is currently home to several Indigenous tribes, accounting for approximately 20,000 people who will become displaced as a result (Washington and Hasibuan, 2023). Environmental activists and Indigenous rights advocates alike have expressed high levels of concern over the project and remain deeply opposed (Yusriyah et al., 2020).

There remain questions about whether this drastic move will improve water-related issues for all citizens of Indonesia or whether it will only serve to further deepen the inequalities between the rich and poor. At a projected cost of over $30 billion USD, the new city will become home to government officials only, accounting for fewer than 2 million people (Beech, 2023). The fate of the 28 million remaining residents in Jakarta is unclear in relation to land subsidence and the high risk of flooding disasters. It is also uncertain what resources will be made available, if any, for those left behind to address the current dearth of freshwater availability. 

How did the water crises in Jakarta begin? 

In the first half of the 20th century, Jakarta was a relatively small city, home to an estimated 150,000 people (Rukmana, 2018). Since that time, Jakarta has undergone rapid urbanization to become the most populous city in Southeast Asia, home to nearly 30 million people (Rustiadi et al., 2021). This massive influx of people came with extreme changes in land use, ultimately transforming rainforests into high-rises, fields into pavement, and swamps into businesses. An estimated 97% of the city is now concrete or asphalt, with natural spaces few and far between (Kimmelman, 2017). The impact of this urbanization is continuing to be felt, with several significant implications for water management within the city.  

The urbanization in Jakarta pushed the existing water infrastructure, namely the piped water supply and sewer network, well beyond its intended coverage. For example, the piped water network was first built in 1918 and was intended to serve only a fraction of the population that now exists in Jakarta. Currently, the piped network only provides for 40% of the city, with that supply being primarily concentrated in the richer neighbourhoods of south and central Jakarta, as well as the business districts (Ardhianie et al., 2022). Similarly, the sewer network only covers approximately 4% of the city (KPPIP, 2019). There have been staggering levels of river pollution in Jakarta as a result, as waste is haphazardly discarded into rivers and other surface waters (Figure 3). For example, solid waste and faecal matter from overflowing or leaking septic tanks, effluent from households, businesses, and industries, as well as chemicals from agriculture have all contributed to extremely high levels of heavy metals, nitrates, and pathogens in Jakarta’s rivers (Apip et al., 2015; Furlong and Kooy, 2017). The levels of pollution have rendered the local rivers to be well beyond the point of safe consumption, resulting in the loss of a vital water source. 

With access to piped water being scarce and surface waters having been polluted beyond potability, there remain few sources of clean water for everyday use. Thousands of citizens and businesses across the city have therefore turned to digging personal-use wells as a result, with nearly 5,000 undocumented wells currently in use (Figure 4) (Batubara et al., 2023). These wells, however, are unregulated by water authorities and therefore are not subject to any form of extraction monitoring. Over-extraction is a massive concern, given that several million people rely on groundwater as a source of daily water supply (Ardhianie et al., 2022).

The recharging of these aquifers is very limited. Although shallow aquifers may be recharged naturally by rain, the vast areas of impervious concrete within the city are preventing this from happening. Rather, excess rain accumulates in the streets, increasing the risk of urban flooding (Le Jallé et al., 2021). This is in addition to the innate risk of flooding that already exists from land subsidence and sea level rise. Concurrently, shallow aquifers are increasingly contaminated by polluted surface waters and saltwater intrusion from rising sea levels, forcing wells to be dug even deeper (Kagabu et al., 2013). Deep groundwater aquifers, however, are even slower to recharge, a process that sometimes takes hundreds of years to occur (Luetkemeier et al., 2022). The cumulative effect is that groundwater is being consumed at a rate far greater than it is being replenished, consuming this precious and finite resource, while also contributing significantly to land subsidence (Furlong and Kooy, 2017)

The Indonesian government has made several attempts at controlling over-extraction over the years such as stopping groundwater extraction by public buildings and placing progressively more stringent abstraction limits on regulated wells. Unregulated wells, however, remain a huge challenge, as there exist thousands of them that are not subject to abstraction limits. It is also difficult to fully restrict the digging of undocumented wells when other sources of clean water in the city are severely limited. People require water to live, there is simply no way around it. It is no wonder then, that problems will persist until everyday citizens have an equally convenient, cheap, and easy alternative source of water. 

Data from space: How do we actually know this is happening? 

Groundwater over-extraction, land subsidence, and sea level rise are chronic and formidable threats facing Jakarta. How do we measure and track these changes over time? All three of these phenomena occur very slowly - much too slowly to be tracked by the naked eye. Indeed, they frequently go unrecognized until the damage is irreversible. They can also be difficult and expensive to track on a regional scale from the ground itself (Wada, 2015). Because of this, space-based technologies are essential in the monitoring of such changes, given the ability to track minuscule changes over large areas with consistency. 

Measuring changes in groundwater storage

The chronic overuse of groundwater is at the heart of the land subsidence issues in Jakarta. Scientists can utilize space-based technologies in conjunction with in situ measurements to monitor changes in groundwater storage over time. Space-based measurements are particularly helpful because they allow for routine data collection from very remote regions of the world, negating the need for constant sampling (Pamungkas and Chiang, 2021; Rodell et al., 2009). NASA’s Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) satellites do this by tracking water on land, known as Terrestrial Water Storage (TWS) (Figure 5). TWS is defined as the sum of the water on the land surface and subsurface – particularly it includes surface water, groundwater, soil moisture, and snow/ice (Girotto and Rodell, 2019). 

GRACE and GRACE-FO orbit the Earth together, as a pair of twin satellites, at approximately 220 kilometres apart (Rasmussen, Buis, and Zaragoza, 2016). Their location relative to each other is altered by the gravitational pull of the mass on Earth’s surface and subsurface. The process hinges on this principle: the larger the concentration of mass of an object, the larger its gravitational pull. The gravitational pull of a mountain range, for example, is larger than the gravitational pull of flat ground (Figure 6) (NASA, 2023a). 

Measuring the gravitational pull from the concentrated mass of terrestrial water is essentially no different. The interesting thing about water, however, is that its positioning can change significantly over time due to its fluid nature. This may be in response to seasonal changes, storms, droughts, other weather and climate effects, or the consumption of groundwater (Rasmussen, Buis, and Zaragoza, 2016). As the mass of water moves around the Earth, it causes regional changes in gravitational pull and therefore impacts the positioning of the GRACE satellites ever-so-slightly (Frappart and Ramillien, 2018). 

Scientists compare the changes in these measurements over time to track the movement of all terrestrial water. Given that terrestrial water is comprised of surface water, soil water, snow and ice, and groundwater, scientists can isolate groundwater specifically, given the following formula:  

ΔTWS = ΔWSurface water + ΔWSoil water + ΔWSnow + ΔWGroundwater

ΔWGroundwater = ΔTWS − ΔWSurface water − ΔWSoil water − ΔWSnow

Outputs from other hydrological models or in situ measurements are needed to determine ΔWSurface water, ΔWSoil water and ΔWSnow (Frappart and Ramillien, 2018). Hydrologists input this data to solve for ΔWGroundwater, thereby determining a change in the mass of groundwater in a given place at a given time. 

Keeping track of long-term datasets of the mass of groundwater in a specific location can reveal trends in extraction over time. For example, a recent study compared monthly GRACE data from Indonesia over the year 2020. The data suggested low groundwater availability in January and February and higher availability throughout the rest of the year (Figure 7) (Julzarika and Nugroho, 2022). While groundwater has been historically difficult to monitor over large regions, the data maps produced by the GRACE and GRACE-FO satellites allow for a deeper understanding of how and where groundwater is being consumed across time and across the globe. As technologies continue to improve, it is hoped that the resolution of the imagery produced from the GRACE and GRACE-FO satellites also improves, as the current spatial resolution is limited to 300-400 km grids (Mehta et al., 2020). 

Measuring land movement

Land subsidence was first documented in Jakarta in 1926. Since then, several innovative technologies have emerged to track the movement of land (Abidin et al., 2005). One such technology, known as interferometric synthetic aperture radar, or InSAR, has been developed and used to track changes in land subsidence over large spatial areas with a high degree of accuracy and resolution, down to centimetre and millimetre level (USGS, 2023; Xiao and He, 2013). The premise of InSAR is to compare two images of the same area, taken from the same vantage point, at different points in time. The system relies on Synthetic Aperture Radar (SAR) imagery, which is produced by transmitting microwave signals towards the Earth’s surface. Depending on the patterning of the energy that is reflected back to the radar sensor, scientists are able to detect the physical properties of the Earth. Essentially, if the ground has moved either towards or away from the satellite, a slightly different portion of the wavelength is reflected back to the satellite (Figure 8). 

When two SAR images are overlaid, scientists can produce what is known as an ‘interferogram’, otherwise known as a phase-shift map. Interferograms utilize a repeating colour scale to visually represent ground displacement. They also display a direction of displacement, meaning either land subsidence or land uplift (USGS, 2018). In essence, they can be interpreted by knowing what measure of displacement a given cycle of colours represents, and how many cycles of colour occur. For example, data taken from the greater Jakarta area shows that land is subsiding at a rate of up to 26 cm per year, with the coastal areas of Penjaringan and Cengkareng being particularly affected (Figure 9) (Ng et al., 2012).

One clear benefit of InSAR is the ability for microwaves to penetrate clouds, meaning that imagery can be collected in any weather, day or night, regardless of visibility. This is particularly important in regions with high amounts of cloud cover or when land subsidence relates to events like volcano eruptions, where visibility from ash may prevent other forms of imaging from occurring.  At the same time, there are limitations to InSAR as well, such as the effect of the atmosphere, mainly the troposphere and the ionosphere, on data accuracy. The waves that InSAR relies on are slowed down as they travel through the troposphere, which has been shown in the past to impact data accuracy (Ding et al., 2008). Oftentimes, the combination of technologies, such as combining InSAR data with GPS data, can provide a more accurate depiction than either technology could provide on its own. 

Measuring sea surface heights  

To compound the land subsidence issues in Jakarta, sea levels are also rising. In fact, global sea levels have increased approximately 16 to 21 cm since 1900 (Doyle, 2023). Measuring sea level rise is more complicated to measure, however, given that sea levels can vary significantly across the globe due to the differences in geography, gravity, temperature, ocean currents, salt content, and tides (Bolles, 2023). Despite these challenges, scientists have been able to measure changes in sea level from space since the early 1990s. This involves using instruments called radar altimeters. In essence, they work by bouncing radio waves off the surface of the ocean. The satellite times how long it takes the returning wave to reach the satellite, as well as measures its incoming intensity. With these two metrics, scientists can calculate the distance between the satellite and the ocean’s surface at a given time and at a given location. Broadly speaking, the faster and stronger the return signal is, the higher the sea level (Lindsey, 2022). 

Although the process may seem complicated, it is actually quite elegant: 

1. Find the distance between the satellite and the sea surface (Figure 10). 
2. Find the distance between the satellite and the centre of the Earth (Figure 11). 
3. Subtract A from B to determine the distance from the centre of the Earth to the sea surface, also known as the sea surface height (Figure 12). 

NASA’s Surface Water and Ocean Topography (SWOT) mission, for example, was launched on 16 December 2022. A prime directive of this mission has been to repeatedly capture high-resolution elevation measurements of the ocean’s surface (NASA, 2023b). Another example is the Jason-3 satellite, which measures sea levels across the entire planet every 10 days, taking measurements every 30 km. The radar altimeter onboard can measure variation in sea levels to within 3.3 cm – an astounding feat considering the satellite orbits 1300 kilometres above Earth (NASA JPL, 2023). Tracking this data over years and decades has allowed scientists to understand how sea level heights are changing worldwide. For example, sea levels have increased an estimated 21-24 cm globally since 1880 (Lindsey, 2022). To compound this, it is expected that global mean sea surface heights will continue to rise between 43 cm to 84 cm by the end of the century (Intergovernmental Panel on Climate Change, 2019). Low-lying coastal cities like Jakarta will therefore be placed at an increasing risk of flood as time goes on. The data is clear – serious interventions will need to be taken in order to prevent devastating flooding in coastal cities. 

What socioeconomic changes will result from the Nusantara construction project?  

There has been significant backlash from activists worldwide over the decision to move the capital city of Indonesia. Environmental activists, for example, have voiced high levels of concern over the future consequences on orangutan populations. The island of Borneo, and the East Kalimantan region specifically, is one of two places in the world where orangutans live in their natural habitats (WWF, 2023). With only 50,000 - 65,000 orangutans left in the wild, many experts believe that these animals will be extinct in the next 50 years at the current rate of loss (The Orangutan Conservancy, 2023). The last several decades have already involved increased levels of deforestation and increased agriculture on Borneo, destroying orangutan habitats in the process. Despite this, the proposed construction plan is set to deforest up to 256,000 hectares more, in the very centre of orangutan habitat (ASEAN Post, 2022). The impact on the local orangutan population will be undoubtedly devastating (Figure 13). 

The construction of the new city will also cause the removal of the local Indigenous population, totalling approximately 20,000 people (Suntoro et al., 2023). As a form of compensation, the government has offered money to Indigenous landowners in exchange for their eviction. It remains unclear, however, how the government is calculating the payouts. Several Indigenous leaders have reported being unsure if they received a fair price for their land – a tale as old as time. Some families have received as little as $3000 in exchange for their houses and farmlands (Washington and Hasibuan, 2023). Those without land ownership rights have received nothing and are fighting to keep their ancestral lands out of the hands of the government. 

Beyond the impacts on wildlife and Indigenous groups, there are also concerns for the well-being of the 28 million Jakartans that will be left behind. While Jakarta is expected to remain the centre of business, finance, trade, and services, it is not yet apparent what level of care or support the citizens will receive in the fight for clean water (Putri, 2019). It is also unclear what efforts will be made to mitigate the high risk of devastating floods, particularly in coastal areas of the city. With the new capital city of Nusantara set to be inaugurated in August of 2024, there remains much uncertainty over what the next several years will hold for Jakartans. 

What can be done? 

The challenges facing Jakarta are seemingly overwhelming. Is slowing, or even reversing, the sinking of a city even a possible task? Remarkably, it has been done before. Tokyo, for example, faced similar land subsidence challenges in the 1940s when factories began utilizing enormous amounts of groundwater. Underground aquifers were drained and caused land subsidence of up to 4.5 metres in some areas – a number very similar to the land subsidence in Jakarta (Cao et al., 2021). Once the severity of the problem was recognized, city officials began by disseminating information on groundwater usage to raise awareness of the issue. New policies were adopted and laws enacted to limit or even prohibit groundwater usage, particularly for industrial purposes (Sato et al., 2006). The government invested heavily in infrastructure, including several seawalls, an extensive drainage system, pumping stations, floodgates etc., to help mitigate the risk of flooding from sea level rise. Perhaps most importantly, however, the government acted to provide an alternative water source for domestic and industrial use, at a similar cost to groundwater (Sinaga, 2020). Although eliciting these changes required substantial investment from the Japanese government, Tokyo has since established itself as a model of how to manage land subsidence in the face of crisis. 

Looking at Jakarta, water management is similarly complex, with multifaceted challenges and no clear or easy solution. There remain enormous challenges in the enforcement of groundwater usage given that no alternative natural water sources are viable at the present time. Cleaning up the city’s natural waterways would likely require retrofitting the entire city, both domestic and industrial, with a sewage network to prevent further river pollution, a venture that would cost hundreds of billions of dollars (Prevost et al., 2020). Although building infrastructure to prevent sea level rise may be possible, similar to the proposed Great Garunda Sea Wall, it will only be a temporary solution if other issues, such as groundwater over-extraction or surface water pollution, are not also addressed. 

Despite these challenges, what seems undeniably clear is that the project to move the capital city is riddled with even bigger problems. Do the ends justify the means? That is to say, does building a new capital city for the government administration to operate justify vast deforestation, the risk of infringing on Indigenous rights, and the imminent endangerment of the local orangutan population, particularly when it comes with a price tag of USD $30 billion? Once the wells finally run dry, what efforts will be made to supply daily water for all inhabitants of Jakarta? How can the country prepare for devastating floods that will destroy swathes of Jakarta’s infrastructure?  It is not clear, as of yet, what will be done to prevent or mitigate water-related disasters. What is clear, however, is that the current water crisis in Jakarta will only continue to worsen if action is not immediately taken.