Nature-Based Solutions in Cities—A View from a Water Supply Perspective ^†

Abstract

Nature-Based Solutions (NBSs) are decentralised and planted system elements with multiple benefits, requiring sufficient irrigation during dry weather periods to ensure plant health. In this work, the effects of the large-scale implementation of NBSs in the city centre of Klagenfurt from a water supply perspective are investigated, combining hydraulic analysis with water resource availability. As the large-scale implementation of NBSs in public squares shows, a coordinated NBS implementation strategy is required to ensure compatibility with the city’s water resources and infrastructure. This also emphasises the importance of alternative water sources for sustainable operations.

1. Introduction

In urban water management, Nature-Based Solutions (NBSs) are decentralised and planted system elements for innovative rainwater management on site, including green roofs, infiltration trenches, raingardens or trees on building and street level [1]. By reinstating the natural water cycle, NBSs mitigate stormwater runoff in the sewer network by enhancing rainwater infiltration into the ground. Additionally, NBSs yield positive effects on the urban climate through shading and increased evapotranspiration and water storage (e.g., urban cooling). Consequently, these elements are also frequently employed as a countermeasure against urban heat islands [2].

However, ensuring sufficient and effective irrigation during dry weather periods is essential for the functionality and efficiency of plants to prevent drought stress [3]. Possible sources for covering irrigation demands include drinking water, rainwater harvesting, and water reuse. However, withdrawing from the drinking water supply could potentially lead to conflicts of use with other urban water users, affecting domestic and commercial water requirements. On the other hand, strategically withdrawing irrigation water to reduce stagnation in water supply pipes can also have positive effects on water supply performance.

Therefore, the objective of this work is to investigate the effects of the large-scale implementation of NBSs in urban areas from a water supply perspective.

2. Materials and Methods

2.1. Implementation of Irrigation-Based NBSs

The number and locations of NBSs within the city, compatible with the city’s water resources and infrastructure, are determined in a three-step approach by combining hydraulic analyses with water resource availability for irrigation:

In the first step, the case study is categorised into various surface types (e.g., roof areas, traffic areas, green areas) and combined with land classification data to identify suitable implementation areas for NBSs.

Second, different degrees and types of NBS implementations are defined. As the aim is to achieve optimum cooling effects, it is assumed that the irrigation demand corresponds to the amount of evaporation, which is calculated in a simplified way by using the Hargreaves equation [4].

Afterwards, potential irrigation concepts involving drinking water, rainwater, and water reuse are developed across a range of future scenarios (including climate change and population growth scenarios). If the concept involves the use of drinking water, the required amount is allocated as an additional withdrawal quantity to the nearest demand node, incorporating the hydraulic analysis of the water distribution network in the assessment.

2.2. Case Study

The city of Klagenfurt, located in Austria, has 103,000 inhabitants and covers an area of 120 km². For this work, freely available data sets including spatial (e.g., land classification, OpenStreetMap) and point (e.g., weather stations, climate change forecasts) data were mainly utilised. Additionally, projections of future potable water demand were used for this case study, considering urbanisation and climate change, and a calibrated EPANET model of the water distribution network and a surface classification of the city of Klagenfurt were used, available from a previous work.

3. Results and Discussion

3.1. Implementation Areas for NBS

The city of Klagenfurt has a (historic) town centre with dense development and public squares, while the outer areas consist mainly of single-family houses with large green spaces. Subsequently, the city centre area is primarily considered for the densification of NBSs for urban cooling, including two possible scenarios: (1) public squares in the city centre are retrofitted with NBSs, including different types and combinations of NBSs, and (2) (commercial, public, residential) buildings and suitable streets in the city centre are equipped with NBSs.

3.2. Irrigation Demand and Impact on Water Distribution Network

As an exemplary evaluation, the public squares in the city centre from Scenario (1) are equipped with NBSs. For the year 2022, the evapotranspiration in the irrigation season (from 21.03. to 23.09.) is calculated to be 549 mm, corresponding to an irrigation demand of 549 L/m² for NBS. The public squares in the city centre cover a total area of approximately 254,600 m². As a result, the irrigation required is between 28,000 and 70,000 m³, depending on the implementation level of NBSs, which varies between 20 and 50%. From a water resource perspective, the additional irrigation demand accounts for around 1% of the yearly drinking water demand for the case study and could be covered entirely by drinking water, as there are sufficient drinking water resources available. Additionally, urban water infrastructure components are usually in operation for several decades, and the overall water demand of the case study will increase in the future due to climate change and population growth [5]. Consequently, the amount of drinking water available for irrigation purposes is set to continuously decrease in the future, emphasising the importance of including alternative water resources for irrigation (e.g., rainwater harvesting, wastewater reuse) (Figure 1a).

Further, high irrigation demands usually occur at the same time as decisive drinking water peak demands, hydraulically stressing the system. The irrigation demand for a typical NBS was calculated to be 5.69 L per m² NBS for the hottest day in 2022. Under the assumption that the irrigation demand is entirely covered by drinking water (the additional extraction volume from the water distribution network would be 509 L/s for an implementation degree of 20%, and there is an assumed average withdrawal rate of 0.01 L/s per m² NBS), this can influence the pressure conditions in the water distribution network (Figure 1b). Therefore, in particular, the city centre shows a clear reduction in pressure due to the additional extraction volumes, thus influencing other urban water users (e.g., commercial and domestic users).

In the next steps, potential irrigation concepts including drinking water, rainwater, and water reuse are developed across a range of future scenarios (including climate change and population growth). Furthermore, the impact on water resource availability and water distribution networks is investigated for different implementation strategies.

4. Summary and Conclusions

In urban water management, Nature-Based Solutions (NBSs) are being applied more frequently. NBSs are decentralised and planted system elements for rainwater management on site, providing multiple benefits for cities (e.g., providing improved stormwater management and a countermeasure against urban heat islands). However, a sufficient irrigation during dry weather periods is essential to maintain the desired functions.

In this work, the effects of potential large-scale implementations of NBSs on water supply services were investigated. The city of Klagenfurt was used as a case study, and hydraulic analyses were combined with research on the availability of water resources for irrigation to determine the number and locations of NBSs within the city. As an exemplary scenario, the public spaces in the city centre are equipped with NBSs, resulting in an additional water demand of 28,000 to 70,000 m³ for a degree of implementation of 20% and 50% in public spaces. While the drinking water resources and technical capacity of the water distribution network are currently sufficient in the present scenario, future population growth and climate change will lead to further emphasis being placed on the importance of alternative water sources for sustainable operations.

This information can subsequently be utilised by stakeholders for further urban development with NBSs and to ensure a sustainable water supply in the future (e.g., identifying areas requiring alternative irrigation concepts for NBSs).