Assessing Rainwater Risks and Rainwater Harvesting Opportunities for the New Capital City of Indonesia

Abstract

In the context of planning and construction of the new capital city of Indonesia, referred to as Ibu Kota Negara (IKN), this article addresses the spatial risks and opportunities of rainwater resources in the area where IKN is planned. The article relies on an inventory of various physical data, which were used to derive a flood susceptibility map, as well as rainfall data derived from public and open sources. The geospatial study drew on geospatial software (ArcGIS Pro, 2.1.) and the Google Earth Engine platform (GEE). After this analysis, we followed a management design, which took IPCC climate change scenarios into account. The results demonstrated that the southern coast has higher precipitation than the northern coast in the IKN area. To enhance the efficacy of rainwater management planning, a grid is proposed to mitigate the flood risk and to harvest rainwater. Although rainwater varies throughout the IKN area, and may vary even more with different climate change predictions, it is possible to capture rainwater and create a system to reduce reliance on traditional water sources, alleviate stormwater runoff and mitigate the impact of urban flooding. While IKN will be developed by both regulated planning and other population-driven developments, monitoring and reflecting on existing plans will still be necessary to make IKN sufficiently resilient and sustainable.

1. Introduction

The construction of a new capital city of Indonesia in eastern Kalimantan, referred to as Ibu Kota Negara (IKN) and/or Nusantara, is a project with many uncertainties. It was designed to overcome the existing challenges and risks in the capital city, Jakarta, where, due to large population densities, poor water run-off management from rivers, and groundwater extraction, the area is no longer safe. Significant land subsidence is occurring [1,2], and there are increased risks of flooding, depletion of ground and drinking water sources, and pollution of water [3]. This makes Jakarta no longer suitable for a reliable and safe government location. While the problems in Jakarta are partly the result of physical changes, they are also the result of inadequate urban planning and uncontrolled urban expansion. As a result, the water authorities and systems are no longer able to handle the water from both the incoming rivers and the sea level rises [4]. The IKN Authority, therefore, aspires to make IKN safe against any water-related risks. One of the main city design challenges, however, is how to manage and contain water resources, including rainwater. On the one hand, rainwater is a potential threat and risk for cities, as superfluous rain and poorly planned drainage infrastructure may lead to floods, damage and casualties. On the other hand, rainwater can be a vital resource for drinking water and wastewater if it is properly harvested, captured, stored and distributed. Hence, the design itself poses both an opportunity to use the water in a sustainable manner, as well as challenges in managing risk.

The designated location of IKN/Nusantara does not have any water-related problems so far and it is safer from natural disasters because it is not located in a volcanic or tectonic zone, or in a river delta area. Nonetheless, if the authorities mirror the inadequate planning and monitoring used in Jakarta, similar problems might occur in the future. Hence, it is important to develop suitable water management systems in the new city to handle any water-related problems, prevent flooding and make optimal use of the available water. How this can be accomplished and where potential problems could occur is the main focus of this study. The focus is on rainwater, specifically, accentuating the questions of how and where the area might be prone to floods, in general; how much and where the IKN area is currently affected by rainwater; what the trends are regarding rainwater in the IKN area; and how this rainwater can be appropriately and sustainably managed to prevent floods and maintain the rainwater as a valuable resource for drinking water or wastewater in the IKN area. The study does not aim to analyze the current conditions and problems in Jakarta but aims to set the requirements and boundaries for a new city design (IKN) so its construction and development can yield a green city, which balances rainwater’s potential and its risks in an appropriate manner.

We start by defining what rainwater is and exploring how to manage rainwater based on the literature sources. After this, we provide the presentation of the methods and materials used to collect and analyze data specifically for the IKN area, followed by a presentation of the results. The article continues by presenting several reflections on the limitations and significance of the study and concludes by addressing the next steps for further research.

2. Defining Rainwater and Methods to Drain, Harvest or Retain Rainwater

2.1. Rainwater

Managing rainwater requires first understanding what is meant by ‘rainwater’ and what needs to be managed. Technically, rainwater is the process's physical outcome whereby water droplets condensing from atmospheric water vapor fall under gravity. Yet, definitions vary in the literature depending on the scientific or practical context. First, ‘rainwater’ and ‘stormwater’ are often used interchangeably in scientific papers to explain relevant data and findings, but with a different purpose. When the problems relate to flooding risk mitigation, the term ‘stormwater’ usually describes the water source of flooding events. However, ‘rainwater’ is generally referred to when papers explain urban water management, especially when dealing with the design or assessment of rainwater utilization and facilities. Regardless of these different foci, flooding is often the most relevant context in which rainwater is discussed. Moreover, given that IKN still needs to be developed and constructed, it is important to note that any increase in the land ceiling, which comes with the construction of buildings and infrastructure, could further expand flooding risks. The work in [5] found that flooding increases with increased road densities and population growth. Hence, one needs to estimate flood susceptibility based on the physical aspects of where a new city is built and where these aspects currently lead to flood susceptibility. It is, thus, crucial to understand how to design strategies for urban rainwater facilities that can support reducing flooding susceptibility on the one hand and that can benefit from rainwater as a resource on the other hand.

Determining the locations and volumes of rainwater precipitation is normally possible through rainfall measurements, either directly by measuring the millimeters of water using rain gauges or by estimating the millimeters using weather radars [6]. Estimating spatial variations as well as trends requires a sufficient number of measuring gauges [7]. Currently, the number of gauges in the IKN area is still limited. For this reason, one must rely on other data sources, such as satellite-based data. Satellite-based precipitation measurements provide uninterrupted precipitation estimates at certain spatial–temporal resolutions.

2.2. Rainwater Management

Design criteria for rainwater drainage systems vary across countries. Countries often adopt particular design standards for rainwater drainage systems derived from established best practices but adapted to specific local climate conditions and the environmental and economic context. From an engineering perspective, it helps to choose a longer return period in a drainage system process to ensure a reduced risk of failure in case of flooding events, with the trade-off of accepting a higher associated cost. Therefore, achieving an optimal balance between construction cost and flooding risk becomes important for engineering decision making in planning and implementing drainage systems in the city establishment. Examples of rainwater management designs in similar climates and geographical environments are given in [8,9]. For example, in Cairo, the capital city of Egypt, recharge wells to store rainwater in groundwater is observed to significantly increase the groundwater recharge and influence the regional water cycle [10]. Green roof retrofitting is estimated to effectively help reduce the average runoff in Beijing, the capital city of China [11]. Sustainable rainwater management research is also conducted in Delhi [12] and other capital cities. Some of the specific features of the design challenges and possible solutions of rainwater drainage systems in urban areas for similar countries and regions as the IKN area are shown in

Table 1. Design standards for rainwater drainage systems in different countries.

Country (Region)	Return Period (1 in N Years)	Location	Standard
EU	1 in 5	City centers/industrial/commercial areas	EN 752 (2008) [13] DWA-A118 (2006) [14]
USA	1 in 2–15, normally 1 in 10	Residential and commercial development	ASCE/EWRI 45-05 (2006) [15]
China	1 in 5–10	Important areas in city center of big cities	GB 50014, (2021) [16]

(based on [13,14,15,16]).

To reduce the potential flooding risk and make good use of rainwater resources in urban areas, several rainwater harvesting and utilization methods exist for urban development. One such strategy is using a strategically planned network of natural and semi-natural areas with other environmental features designed and managed to deliver a wide range of ecosystem services such as water purification, air quality, space for recreation, and climate mitigation and adaptation [17]. Physical artifacts of this strategy to handle the rainwater appropriately constitute the use of green roofs, street trees, and permeable pavers, utilize soil, vegetation, and other permeable surfaces to retain, detain, and infiltrate urban rainwater runoff on decentralized sites distributed throughout urban watersheds, diverting runoff and pollutants away from engineered collection systems and alleviating local flooding [18,19,20]. Moreover, when cities rely on these technologies and methods to handle the rainwater, there is additionally an effective cooling effect [21], which can reduce the urban heat island effect in the surroundings. A larger green area is beneficial to improving air quality and creates a more sustainable living environment. Similarly, in Cairo, the capital city of Egypt, recharge wells to store rainwater in groundwater are observed to significantly increase the groundwater recharge and influence the regional water cycle [22]. Green roof retrofitting is estimated to effectively help reduce the average runoff in Beijing, the capital city of China [11]. Sustainable rainwater management research is also conducted in Lisbon [23], Delhi [12] and other capital cities. Cities worldwide increasingly embrace green infrastructure (GI) practices to foster sustainability, resilience, and enhanced living environments, from which we can see both advantages and problems.

Table 2. Rainwater management problems and solutions applied in different (capital) cities (based on [24,25]).

GI	City	Advantage	Problem
Green roofs	Beijing, China [11]	Ease the urban heat island effect Improve air quality	Limited research at a local level to find suitable native plants for the optimum performance of green roofs Initial high construction cost and require consistent maintenance Improper installation increases the probability of leakage and can even lead to structural failure of buildings Requires structural reinforcement for added weight Risk of leakage
Rainwater harvesting units	Cairo, Egypt [10]	Simple, easy to set up and operate Cost saving	The quality of stored water deteriorates after a prolonged period of time Space Requirements Maintenance for debris or contaminants
Permeable pavements	Shanghai, China [26]	Reduce runoff and flooding risk Water quality improvement by efficient trapping of suspended solids and pollutants Groundwater recharging Ease the urban heat island effect	Higher construction costs due to specialized materials and maintenance costs than conventional pavements Application has mostly been restricted to parking spaces and low-volume roads because of sediment clogging Risk in freeze–thaw conditions
Detention ponds	Semarang, Indonesia [27]	Easy to design and less maintenance needed Sedimentation, improving water quality Support biodiversity Recreational use	Requires a considerable amount of space, making it unsuitable for densely populated areas Mosquito breeding Limited effectiveness in heavy rain
Retention ponds	North Carolina, U.S. [28]	Add an aesthetic value Sedimentation, improving water quality Recreational use	Safety and health concerns Mosquito breeding
A combination of two or more GI	Delhi, India [12]	Better performance in rainwater sustainable management, increased resilience and effectiveness Allows for the tailoring of solutions to specific urban contexts	Higher construction cost Difficult in designing and maintenance Space requirement Difficult for reconstruction in old cities

presents an overview of the benefits, and problems of green infrastructure (GI) applied in different cities.

3. Materials and Methods

The approach to collect and analyze data relevant to making any prediction or assessment of rainwater risks and opportunities consisted of two parts. First, we investigated the flood susceptibility based on standard methods provided by the literature. This relied on a GIS analysis using 7 factors. After this, we specifically zoomed in on specific rainwater characteristics of the new capital city area. Both steps allowed the development of a set of rainwater management (i.e., mitigation and harvesting) requirements for the IKN area in view of possible rainwater scenarios based on different climate change predictions.

Determining the areas from which we needed to collect and analyze rainwater locations and volumes depended on the geospatial demarcation of the IKN area and the rainwater data available or deducible for this area. The geospatial boundary of the IKN study area can be derived from both the law declaring the IKN area and its boundaries (Law of the Republic of Indonesia number 3 of 2022 on National Capital), as well as from the administrative boundaries specified in Google Earth Engine (GEE). GEE is a cloud-based geospatial analysis platform developed by Google that allows users to access and analyze vast amounts of satellite imagery and geospatial datasets. GEE has access to several geospatial resource files, such as streams, waterbodies, and administrative areas, and has defined the geometry of the IKN area, as depicted in Figure 1.

Compared to Jakarta, the IKN area is less prone to flood risk. From a qualitative or descriptive point of view, one can note that the IKN area lies on the east coast of Borneo, boasts a hilly landscape and a coastal line extending eastward to the Makassar Strait and southward to Balikpapan Bay, featuring four nearby islands. This hilly terrain suggests significant altitude variation within IKN, potentially offering higher elevation points than Jakarta, which is predominantly flat with an average altitude of about 8 m above sea level. Additionally, the river influence in IKN, with rivers flowing into Balikpapan Bay, contributes to shaping its hydrology and landscape, differing from Jakarta’s deltaic setting dominated by the Ciliwung River. The hilly nature of the IKN area may provide a natural protection against sea level rise compared to Jakarta, which is susceptible due to its low-lying delta location. Geographically, this region lies along the Makassar Strait, which separates Borneo and Sulawesi islands. The Makassar Strait is a crucial water passage connecting the Pacific Ocean and the Indian Ocean, also known as the Indonesian Throughflow, which significantly influences the climatic conditions in the IKN area through ocean surface temperature changes. Additionally, the topography of IKN exhibits some complexity, particularly in the southwest, where the maximum elevation reaches 812 m above sea level (ASL) [30]. These topographic distinctions highlight the potential advantages of IKN’s terrain for sustainable urban planning and climate resilience in developing Indonesia’s new capital city. The development will be on the administrative regions of North Penajam Paser Regency and Kutai Kartanegara Regency, East Kalimantan [31].

To measure and locate the flood susceptibility, we considered seven factors to derive a first flood susceptibility map, following the recommendation of [32]: slope, elevation, land use, and land cover, soil, precipitation, distance from the drainage, and drainage density. The data for each of these factors were derived from international databases. Land use and land cover were obtained from the Esri Living Atlas, the rainfall map from the historical data from WorldClimate, and the soil map was acquired from the Harmonized World Soil Database from the FAO. Topographical factors, including slope, elevation, and hydrological factors, including distance from the drainage and drainage density, were deduced from the DEM. The downloaded datasets were processed in ArcGIS Pro and visualized in different thematic maps according to the flowchart depicted in Figure 2.

To analyze rainfall patterns, the Global Precipitation Measurement (GPM) satellite mission provides rainfall estimates through the Integrated Multi-satellite Retrievals for GPM (IMERG) algorithm. These data can be used in GEE for geospatial analyses and visualizations to study precipitation patterns, monitor changes, and support decision making for water resource management and environmental monitoring at various spatial scales [33]. Additionally, Ramadhan, et al. [34] analyzed 20 years of IMERG version 6 data to investigate rainfall trends in the IKN and two buffer cities, which can aid in identifying rainfall-related risks and forecasting potential occurrences. However, the data do not provide guidance on capturing rainfall or building rainwater harvesting or retention areas.

Another data source was the data of the Agency of Meteorology, Climatology, and Geophysics (BMKG) in Indonesia. Their official website (https://iklim.bmkg.go.id/en/climate-change/) (accessed on 6 October 2024) provides historical trends and future projections of precipitation in Indonesia, which can be used to find and select historical trends and future predictions. For this study, we zoomed in on the data in the regions Kutai Kartanegara and Penajam Paser Utara in Kalimantan Timur to assess when, where, and how much rainfall can be expected in the IKN area. A critical comment hereby is perhaps that acquiring rainfall data from this portal is not generating rainfall data at a detailed urban scale, and the location and amount of rainfall itself may change when urban areas are being created or are growing. For the moment, this is, however, the only rainfall data available, so we had to rely on this facility to move forward.

Based on the flood susceptibility map and the further physical data on rainfall and topography, the next step involved formulating strategic decision making to determine appropriate flood protection and rainwater management measures. This includes identifying where and how flood protection is needed and selecting the most effective types of protection. This step involved reviewing which technical flood protection and rainwater utilization measures are documented in the literature, followed by identifying how these could be applied in which areas of significant rainfall or flood susceptibility.

The subsequent step was to identify future possible trends in rainfall that might affect the IKN area. For this, we relied on the predictions and scenarios of the Intergovernmental Panel on Climate Change (IPCC) [35]. Climate change may result in more floods and extreme rainfall, which the Panel captures in so-called Representative Concentration Pathways (RCPs). Each of these RCPs represents the outcomes of one or more crucial changes in weather. Examples of IPCC scenarios are labeled with numbers, such as RCP2.6, RCP4.5, RCP6, and RCP8.5. These specific RCP references represent predictions of possible rainfall and flood in the future [36]. For our study, we selected the RCP4.5 and 8.5. scenarios, following the recommendations of [37]. In essence, this choice is partially discretionary, as the goal was not to predict climate change but to show how possible variations would impact the requirements for city planning. The IPCC describes RCP 4.5 as an intermediate scenario. It forecasts that greenhouse emissions will peak around 2040 and then decline. Contrastingly, RCP8.5 is a worst-case climate change scenario.

4. Results

Following the three-step approach described in the methods section, this section provides the results in three parts.

4.1. Flood Susceptibility Map of the IKN Area

Based on the seven thematic maps derived from the available geodatabases, a flood susceptibility map of the IKN area was possible. We used the methods of [32] to create such a map, which derives a map showing categized classes representing different degrees of flood susceptibility (from very high to very low). In this way, one can make a first interpretation of which locations are at most risk. Figure 3 is the map derived from this.

Figure 3 displays the degrees of flood susceptibility. The category of “very low flood susceptibility” does not appear, largely due to the area's topography, which contains a relatively high number of rivers. A visual scan of Figure 4 shows that, in general, the flood susceptibility is higher in the southern area of IKN than in the northern area. This is partly due to the lower amount of rainfall and partly due to higher elevation and lower density of drainage infrastructures. Remarkably, the west–east variation in susceptibility is relatively limited. Zooming into specific extremes, smaller pockets of very high susceptibility are closer to the southern coastal area.

The LULC within the orange and dark red areas varies. It is dominated by vegetation, but rangelands, crops, and built-up areas also lie within those flood-prone areas. Soil and distance to drainage have a negligible impact on the map, as their relative impact on the flood susceptibility only accounts for around 7% together. Overall, the study area is characterized by a moderate to high level of flood susceptibility whereby the pattern changes from north to south throughout the study area, with the highest levels of flood susceptibility found in the south and the lowest levels of flood susceptibility in the north.

The research of [38,39,40] discusses the possible urban growth scenarios and associated land use changes that might occur in the IKN area. Based on analogies with other newly developed capital cities in relatively remote areas, one can derive three possible scenarios: ghost town (no development), urban explosion (development beyond plans), and reasonable development (gradual development according to plans). In all scenarios, the most likely development will be in the southeastern part of the area and partly in the western part of the area. This has to do with higher elevated regions on the one hand (due to which expansion will be harder), the available road network (due to which ribbon expansion will take place), and the closeness to the coast (due to the ease with which resources can be brought in). Especially for the southern tip of the area, this would immediately increase the flood susceptibility.

4.2. Rainwater Conditions of the IKN Area

This flood susceptibility map was a basis for how and where the area is at risk and how and where (current and future) rainfall might affect or increase flood susceptibility. For this purpose, we relied on additional rainfall data from weather stations that were localized and analyzed through the GEE platform. With GEE, several feature collections (geometry, district border, province border, stream, regency) were initialized based on geographic data stored in different tables. The associated GEE code adds these feature collections as map layers with distinct colors and labels (IKN, district border, province border, stream, regency) to visually represent different administrative or geographic boundaries within the IKN area. The resulting map view can be centered via (Map.centerObject(geometry, 10)) based on the geometry feature collection at a zoom level of 10, ensuring that the entire specified region is displayed. Additionally, with NOAA’s ETOPO1 data, it was possible to derive the Automatic Weather Station's (AWS) locations.

Based on the published data from 1990 to 2020, the annual rainfall in the IKN area varies from 2584.0 mm/year to 2925.2 mm/year, following [32]. Rainfall increases as the location gets closer to the coastal sea. Over the last two decades, there has been a decreasing trend in rainfall. The highest decrease in annual rainfall is observed in offshore areas, which correspond to high annual rainfall values. The negative trend of yearly rainfall in the IKN can reduce clean water reserves in the region in the next few decades [34]. Rainfall at IKN also has seasonal and diurnal variations. The peak rainfall occurs in November–December and March–April, while the driest conditions occur in August–October. El Nino can exacerbate this dry season, which increases the potential for droughts and forest fires, as in 2016 [34].

The overall rainfall pattern in the IKN area is that it often rains in the early morning, leading to high precipitation volumes. This daily pattern is influenced by factors like how long it rains, the landscape, and the rainfall movement between land and sea during the day. Short-lasting rain (less than 3 h) is common on the coastline, making up 60% of the rainfall. Inland areas experience more moderate-duration rain; longer-lasting rain is more common offshore [34]. Marzuki et al. [41] demonstrate that there has been a slight decrease in extreme rainfall in the IKN area for the past two decades, particularly in Sepaku and Semboja sub-districts. However, there is an increase in the intensity of more extreme precipitation events. The overall trend suggests drier conditions in IKN, and specific precipitation frequency-based indices indicate wetter conditions during certain regional periods. This pattern aligns with global rainfall trends, where the rainy season is becoming wetter and the dry season is becoming drier [41].

From the BMKG portal, one can further derive several historical trends. The average change in seasonal cumulative precipitation is 0.1878 mm/y in December-February and 4.451 mm/y in March-May. In both time periods, the trend of cumulative precipitation is positive. In December-February, there is a bigger positive change in coastal areas than inland areas in the IKN area. In March-May, the precipitation changes are almost negative in the IKN area and show no obvious difference in spatial distribution. The highest recorded historical cumulative precipitation data are in the year 2000, with 1432.1 mm in the concerned areas (Figure 4). It is also clear that, on average, the peak rainfall is in November-December and March-April in the IKN area. This information is crucial for assessing the rainwater management risks and opportunities, which led to the decision to focus during the management design on the available data for these specific periods, December-February and March-May.

The BMKG data show that in East Kalimantan, the seasonal changes in the daily intensity of precipitation are minimal. It ranges from −0.7mm to 0.2mm under the RCP4.5 and RCP8.5 scenarios. For the RCP8.5 scenario, BMKG predicts a percentage change in the seasonal total precipitation in the IKN area. The forecast changes apply from March to April and December. For March to April, the change is −2.898%, indicating a decrease in seasonal precipitation. The change is 1.653% from December to February, indicating increased seasonal rainfall. There are no spatial differences within the IKN area. Figure 5 displays the BMKG predicted flood risk in the area on a seasonal basis considering variations throughout the year. December to February is the peak flood risk season with high flood risk, characterized by heightened water levels and potential overflows of rivers or other water bodies. The other months of the year have a relatively low flood risk.

Combining the existing scientific research results with the predictions from the BMKG portal confirms a prediction of peak rainfall in the IKN area in December and March to April, which yields a medium flood risk. As for the spatial distribution, there is no significant difference in specific locations, but in general, one has higher rainfall in the vicinity of coastal seas. Therefore, more detailed information about the spatial distribution of the rainfall would be needed for a more accurate prediction. With GEE, the spatial distribution of rainfall from December 2022 to April 2023 could be visualized. With GEE (through directly using and/or adapting the code), it is possible to explore and visualize the precipitation data and provide valuable insights into rainfall patterns and intensities within the IKN region.

Finishing the map visualization, the code loads an image collection containing precipitation data from the NASA GPM dataset for a specific date range (1 January 2022 to 30 April 2023). It selects the ‘precipitationCal’ band from this image collection and computes the maximum precipitation value across all images within the specified date range. To focus on significant precipitation events, a binary mask is created where pixel values in the precipitation image that are greater than 0.25 are retained, while others are masked out. For visualization, a custom color palette represents different precipitation intensities, ranging from blue (low) to red (high). This palette is applied to the precipitation image using specified visualization parameters such as minimum (0.0) and maximum (35.0) precipitation values. Finally, the processed precipitation image is added as a layer (‘Precipitation’) to the map using the defined visualization parameters (precipitationVis), allowing us to visually interpret and analyze the distribution and magnitude of precipitation over the IKN area during the specified period. The resulting map (Figure 6) visually represents areas with different precipitation, derived from GEE, but adapted visually to enhance the rainfall features and distribution thereof within the IKN area. The darker red colors indicate higher precipitation and greener and yellow colors indicate lower precipitation.

The darker colors in the northern part of the city and some areas in the middle suggest higher precipitation levels during the specified period, which should be considered when designing the urban green infrastructures, as for the scientific statement that rainfall increases as the location gets closer to the coastal sea, the graphic shows that the southern coast has a higher precipitation than the northern coastal area in the IKN area. However, the GPM data's approximately 10 km × 10 km pixel size is not ideal for a city-scale analysis. This resolution is relatively low and may not capture the spatial variations in precipitation within urban areas where microclimates and diverse land use patterns influence rainfall patterns at a more detailed level. When planning to protect the city from possible flood risk, localized precipitation intensity can impact drainage and rainwater management, which should also be considered. Therefore, more specific localized rainfall data are considered valuable as a reference for further urban planning of rainwater management in IKN.

4.3. Design of Method for Rainwater Management for the IKN Area

Given that currently, the resolution of rainfall data collection remains limited (due to very few collection points in the entire area), any management strategy for mitigating and harvesting is dependent on this resolution. We decided, therefore, to use the data collection resolution as a basis for creating a rainwater management system. The current resolution of the rainfall data is the 10 km × 10 km grid, which would lead to a spatial basis for different rainwater management strategies (namely, a separate management strategy for each square in this grid). Figure 7 displays the resulting rainwater management units, whereby the minimum size is 10 km × 10 km, but also some spatial units are consolidated, either because they fall partly outside of the anticipated IKN area or because the rainfall characteristics show a very low degree of variety, and thus can rely on a single rainwater management strategy.

These spatial rainwater management units each have different levels of rainfall intensity and hydrological characteristics. Characteristics of these units include the location, topographic features, and hydrological connectivity, which are listed in

Table 3. Characteristics of land units in IKN.

Land Unit No.	Maximum Rainfall Intensity	Topographic Features	Hydrological Connectivity
1	Medium	Hill, higher elevation than plain area	Including upstream of a river
2	Medium—high	Hill, higher elevation than plain area	Including upstream of a river
3	Medium—high	Hill, higher elevation than plain area	No connection to waterbody
4	High	Hill, higher elevation than plain area	Including upstream of a river
5	Medium—high	Plain, lower elevation	Including waterbody
6	Medium—high	Plain, lower elevation	No connection to waterbody
7	Low—medium	Plain, lower elevation	Coastal area
8	Medium	Hill, higher elevation than plain area	Including upstream of a river
9	Medium—high	Hill, higher elevation than plain area	Upstream merge into main stream
10	Medium	Plain, lower elevation	Including upstream of a river
11	Medium—high	Plain, lower elevation	Including upstream of a river
12	Low—medium	Plain, lower elevation	Upstream merge into main stream, river runs into ocean
13	Low	Plain, lower elevation	Coastal area, river runs into ocean
14	Medium—high	Hill, higher elevation than plain area	Including upstream of a river
15	High	Plain, lower elevation	Upstream merge into main stream
16	Medium—high	Plain, lower elevation	Coastal area, river runs into ocean
17	Medium—high	Plain, lower elevation	Upstream merge into main stream
18	Low	Plain, lower elevation	Including upstream of a river
19	High	Plain, lower elevation	Including upstream of a river
20	Low	Plain, lower elevation	Coastal area
21	Medium	Plain, lower elevation	Coastal area
22	Low—medium	Plain, lower elevation	Including upstream of a river
23	Medium	Plain, lower elevation	Coastal area
24	Medium	Plain, lower elevation	Coastal area
25	Medium	Plain, lower elevation	Coastal area
26	Medium—high	Plain, lower elevation	Coastal area

. Combining these characteristics with current land use and land cover can help to design more specific rainwater management measures per spatial unit.

The selection of appropriate strategies for rainwater management often involves multiple criteria, such as costs, environmental performance, safety, ecological risks, and community perception [24]. For this study, we relied on how a multi-criteria analysis was used for evaluating flood risk within the Marikina River Basin, Philippines [42].

Table 4. Properties of resulting flood risk map based on hazard, exposure and vulnerability [42].

Risk Level	Hazard	Exposure		Vulnerability
		Population Density	Building Density	% of Dependent Age	% of Low-Income Level	% of Low Education Level	% of Dependency to Locality	Access to Water Supply
Very high (5)	Level 5 (>3 m flood depth)	High to very high	High to very high	Mostly medium to very high	Low to medium	Mostly medium to very high	Mostly medium to very high	Mostly uncovered
High (4)	Mostly level 4 and 5 (2–3 m and >3 m)	Medium to very high	Mostly low to very high	Low to very high	Low to high	Low to very high	Low to very high	Covered and uncovered
Moderate (3)	Mostly level 2 and 3 (highly susceptible & below 2 m)	Mostly medium to very high	Mostly low to high	Low to high	Low to medium	Mostly low to very high	Low to very high	Covered and uncovered
Low (2)	Level 1 and 2 (low to high susceptible)	Very low to high	Very low to low	Medium to high	Very low to low	Low to high	Mostly low to medium	Mostly covered
Very low (1)	Level 2 (high susceptible)	No data	No data	No data	No data	No data	No data	covered

is based on these criteria. In Table 4, population and building density express exposure; local population and water supply are mentioned to assess vulnerability. Although these criteria are typically used for flood risk assessment within existing urban areas, the IKN area is a completely new urban area that is still being planned; due to the lack of available data on potential population numbers and building areas, one must make a number of ‘educated’ assumptions based on previous experiences. This is because no decision has been made on where exactly which buildings and infrastructure in the IKN will be created. Once this is done, one can make more accurate predictions on rainfall volumes and locations because any land ceiling will influence the rainfall patterns. How and where is still unknown. Hence, further monitoring at a higher resolution than the entire IKN area is necessary. For this reason, we suggest monitoring and managing rainwater within the specified geographic grids. However, how rainfall patterns change with the gradual land ceiling remains a topic for further research. Therefore, we note this fact already to warn the authorities to consider this aspect when they plan.

In view of this, we relied on how and where risks emerged in other cities and derived from these insights risk levels that could be used for the design of a rainwater management strategy. Unlike reconstructing facilities and taking measures to reduce flood risks in established urban areas, IKN can mitigate flood risk through strategic urban planning according to an analysis of local natural conditions (

Table 5. Proposed multi-criteria analysis method.

Risk Level	Precipitation mm/yr	Maximum Rainfall Intensity	Average Elevation	River Distance (m)	Width Channel River (m)	Soil Classification (Quality)	Slope (%)
Very high (5)	-	-	-	-	-	-	-
High (4)	-	-	-	-	-	-	-
Moderate (3)	-	-	-	-	-	-	-
Low (2)	-	-	-	-	-	-	-
Very low (1)	-	-	-	-	-	-	-

). Based on the results of flood risk analysis considering natural conditions, corresponding adjustments should be made to urban land planning. Critical infrastructure such as government offices, schools, and hospitals should not be in high-flood-risk areas. Similarly, residential and commercial areas with high population densities should be situated in units with lower flood risks.

In addition to strategic spatial planning and allocation of land, constructing rainwater management facilities plays a crucial role in sustainable urban development, particularly in regions with seasonal rainfall patterns and flooding risks. Based on the existing conditions, the spatial distribution of rainwater management facilities can be preliminarily planned on maps adapted from how these were generated by GEE (Figure 8). Rainwater management involves rainwater harvesting and rainwater utilization. This section briefly introduces rainwater management methods, highlighting their importance in water conservation, flood mitigation, and sustainable development initiatives. A better understanding of these practices can help find the potential of rainwater as a valuable resource to meet water demands, mitigate the impacts of floods, and promote environmental sustainability.

Rainwater harvesting systems typically consist of a collection surface, such as green roofs or permeable sidewalks, gutters or channels to direct the water flow, and a storage tank to hold the collected rainwater. The collected rainwater then enters the urban drainage system. The drainage system can rely on combined systems or separate sewer systems [43]. When deciding which system to use, [43] argues that separate systems cause less pollution and fewer sanitary risks than combined systems and are more suitable for more developed urban areas. Usually, it is costly to convert a traditional system to a separate one, as such a conversion may cause serious disruptions to citizens' daily lives. However, this problem does not exist in IKN, as it is a new city. In IKN, separate sewer systems can be initially designed and planned in residential and commercial areas. Special attention should be paid to avoiding pollution from initial rainwater. The appropriate distribution of facilities, such as rainwater pumping stations, retention tanks, water treatment plants, and discharge outlets, can also be reasonably set up during the planning stage.

5. Discussion

This data demonstrated that there are differences in rainfall patterns between the north and south. Although the focus of this research was not to find and test the root causes for these differences, meteorological and climate literature would suggest several factors that might play a role. Henn et al. [44] would argue that the influence of significant elevation differences (mountain ranges) and closeness to maritime or river-based resources might create different rainfall patterns. This factor is probably minimal because the IKN area does not have major elevation differences. The closeness to water and maritime resources might be of more influence. This is also confirmed by high-resolution measurements in other tropical areas in the adjacency of sharp land–sea thermal contrasts [45]. Another cause might be the changes in monsoon winds due to climate changes [46]. The IKN area is mostly affected by the northwesterly Australian monsoon patterns [47], which are not significantly changing, but do lead to possible differences to the north and south (if more moisture is captured from the northwestern areas).

Given the specifics of the rainfall variation, the implications for rainwater management are that certain areas are more vulnerable to rainwater-induced flood and thus require protective measures and/or retention areas. In contrast, other areas are more suitable for rainfall harvesting. As urban design is gradually progressing and being implemented, these conditions will likely change. Construction of vital buildings will reduce the possibilities for retention, for example, but enhance the need for nature-based solutions. The harvested rainwater can be utilized for various purposes, including landscape irrigation, toilet flushing, laundry, and even potable water with appropriate treatment. Existing wetlands and natural floodplains can be preserved, and additional planned wetlands can be constructed to act as natural buffers against flooding by absorbing rainwater from urban areas through pump stations.

The capture of rainwater introduces new forms of urban water management that currently do not yet exist at the given scale of the anticipated new city. Compared to conventional systems, these systems help reduce reliance on traditional water sources, alleviate stormwater runoff, and mitigate the impact of urban flooding. Additionally, rainwater harvesting promotes water conservation and resilience to droughts, making it a valuable component of sustainable water management strategies. Yet, these systems also require new behavioral patterns and attitudes from inhabitants, planners, and managers of cities. It requires, among other things, policy actions that stimulate a cultural change toward rainwater harvesting [48], more data transparency [49] and collective action [50]. These aspects need to be addressed by the new authorities.

6. Conclusions

The contribution of this paper lies in the specific rainwater management requirements when developing a new city from scratch. Instead of using historical data on existing operational processes followed by revitalizing, maintaining, or renovating existing structures, starting a rainwater management strategy can only rely on proxy scenarios, and experiences in similar environments and new city designs. These insights are necessary to assess whether there are potential risks for flooding caused by rainwater and to set up the requirements for adequate rainwater management—as a valuable resource for drinking water or wastewater.

The spatial analysis reveals unequal flood susceptibility and heterogeneous rainfall and rainfall predictions across the IKN area. These insights from the study should guide decisions about where to plan for flood protective measures and harvest rainwater for both drinking water and wastewater possibilities. Additionally, certain areas may be unsuitable for rainwater utilization. By mapping the rainwater distribution and using the IPCC scenarios, it is possible to optimize where to construct the water management facilities and make the most effective use of the rainwater. Though rainwater varies throughout the IKN area and may vary even more with different climate change predictions, it is possible to capture rainwater and create a system to reduce reliance on traditional water sources, alleviate stormwater runoff, and mitigate the impact of urban flooding. While the IKN will develop by both regulated planning and other population-driven developments, monitoring and reflecting on existing plans will still be necessary to make the IKN sufficiently resilient and sustainable.

This study has derived several recommendations for how and where to construct rainwater protection facilities or avoid construction due to flood risks from rainwater. Despite the derivation of spatial information, we need to mention that the data on precipitation remain incomplete and possibly insufficiently precise or localized. Part of this problem can be directly attributed to the current spatial technology tools. The precision of precipitation data used in GEE is insufficient to accurately monitor rainfall changes at smaller scales. Precipitation data at a more detailed scale than 10 km × 10 km would most likely be more useful to more accurately determine where exactly a major or minor flood risk exists. Therefore, for proper urban management, especially in these initial phases of the IKN development, the planning authorities of the IKN area should seek more comprehensive and precise data on rainfall from diverse sources to achieve better future flood prediction results.

A second deliberation at these early phases of urban planning concerns the dilemma of capturing or distributing rainwater. It is obvious that capturing more rainwater is easier where more rainwater falls, but this is not necessarily where the rainwater can be reused as a drinking water or wastewater management resource. The coastal areas are more suitable for rainwater harvesting, but the actual need is more inland, where most settlements and government buildings are planned. This requires, therefore, a re-distribution system.

A third concern is where and how to decide on the trade-offs between monetary and financial resources and sustainability concerns. Urban sustainability demands substantial economic investments for construction and maintenance. However, as IKN is still in its early stages of urban development with no or limited direct income generation from taxes or water management fees, there may be limited resources available in the IKN area itself other than central government allocations. Economic growth should also become a priority in spatial planning for a new city to develop more sustainably. Stakeholders may hesitate to invest in environmentally friendly projects lacking immediate economic returns, despite the long-term benefits of initial substantial investments in environmental initiatives. Therefore, developing relevant laws and regulations is necessary for the sustainable development of IKN.

A recommendation for further research is to evaluate the effectiveness of the possible rainwater management strategies. However, this is difficult, as the IKN area is still in the early stages of construction. Nevertheless, it would be helpful to construct a comprehensive indicator framework that would allow a quantitative analysis. This should enable us to evaluate the cost, environmental, and social benefits of different strategies in the specific context of the IKN development. Although qualitative frameworks have been developed for the overall relocation of the capital city area (such as [38,51]), a more quantitative management framework is still necessary to conduct both ex ante and ex post evaluations. Additionally, the main focus of this article was not to explain or deduce the causes of rainfall patterns but more to take the rainfall patterns as a given and derive relevant and appropriate risks and opportunities from the given rainfall. The root causes for rainfall varieties in the IKN area and the possible changes which might occur because of urban construction can be further researched.