Nature Based Solutions

NBS are typically defined as are actions to protect, sustainably manage and restore natural or modified ecosystems, that address societal challenges effectively and adaptively, simultaneously providing human well-being and biodiversity benefits (Cohen-Shacham, et al., 2016). NBS covers a wide range of practices including, forest landscape restoration, ecosystem-based adaptation and mitigation, and ecosystem-based disaster risk reduction. Solutions often enhance existing natural or man-made infrastructure and spur long-term economic, social, and environmental benefits.

NBS that contribute to water security can vary widely and have multiple benefits. They can improve water availability and water quality, whilst simultaneously reducing water-related risks and generating additional social, economic, and environmental co-benefits. Examples of the types of water-related NBS include source water protection; wetlands restoration, protection, and construction; water harvesting; agricultural best management practices; soil conservation; afforestation; and protecting mangroves.

The terminology ‘green infrastructure’ is also often used in relation to NBS and is a subset of NBS. Green infrastructure for water refers to the natural or semi-natural systems that provide water resources management options with benefits that are equivalent or similar to conventional grey (built/physical) water infrastructure (World Water Development Report, 2018).

There are several nature-based solutions that vary in ecosystem condition, ranging from natural ecosystems to managed or modified ecosystems to novel or artificial ecosystems, as well as in scale, focal purpose, and implementing actors. A constructed or artificial wetland, for instance, could be used to address a local water quality issue, whereas the protection of intact ecosystems could be adapted to generate climate, biodiversity, recreational, food production, and human health benefits for communities. However, all NBS seek to maximise nature's capacity to provide ecosystem services that aid in addressing a human challenge, such as climate change adaptation, food production, or disaster risk reduction (Matthews et al, 2019). Here are some key groups of NBS in relation to water management:

NBS in Agricultural Water Management: Agricultural irrigation is driving the majority of water scarcity issues in high-risk basins threatening food systems, community water supplies, and ecosystem health. To sustain the future of food systems and, by extension, human life, agricultural producers are aligning with and transitioning into a new field of practice known as "regenerative agriculture". NBS for agricultural water management can be grouped into three main categories: (1) solutions aiming to increase the water storage in the soil root zone, (2) solutions to protect watercourses and field boundaries, and (3) water sowing and harvesting, including water treatment (Basch et al., 2022). These are cost-effective interventions that can increase the resilience of agricultural and food production while mitigating climate change and improving the environment.

NBS for Sanitation/Wastewater Treatment: The fundamental objective of NBS in wastewater treatment is to create engineered systems that mimic and benefit from functioning ecosystems with minimal reliance on mechanical and engineered components. This type of NBS utilises plants, soil porous media, bacteria, and other natural elements and processes to remove pollutants such as suspended solids, organics, nitrogen, phosphorus, and pathogens from wastewater (Kadlec and Wallace, 2009). To achieve the desired treatment efficiency for removing emerging contaminants such as steroid hormones and biocides (Chen et al., 2019), personal care products (Ilyas et al., 2020), or pesticides (Vymazal and Bezinová, 2015), it is possible to combine various NBS. For instance, in Croatia, a combination of a constructed wetland, and a sludge-drying reed was applied to treat locally generated wastewater treatment (as shown in Figure 1). Furthermore, in Seoul, an innovative sanitation system named “Resource Circulation Sanitation (RCS)” has been developed to provide public hygiene services in remote suburban areas. These systems are designed to divert urine and offer an alternative to conventional sanitation systems with high water and energy consumption. See case studies and resources for more examples and illustrations.

Figure 1: Constructed Wetlands (Croatia, Case Study)

NBS for Flood and Drought Management: Water managers have traditionally approached flood and drought management through a grey infrastructure-based approach, i.a., building dams. Over the years, there has been a growing understanding on the limitation of grey infrastructure however, e.g, costly to build and maintain, being inflexible, and assuming static hydroclimatic conditions, Hence, there is a gradual transition to green infrastructure solutions for managing floods and droughts, especially those that are less expensive and more flexible. NBS interventions for flood and drought management range from minimal or no interventions, such as protection and conservation, to management approaches to develop ecosystems and optimise the generation of selected ecosystem services, such as planning agricultural landscapes to minimise drought, to highly intensive management approaches, including those aimed at the creation of entirely new ecosystems, such as greening buildings or creating new green spaces. Examples include the adoption of integrated wetland restoration and smart quarrying in the Mindanao region of the Philippines, managed aquifer recharge in the Kumamoto region of Japan, and forest conservation and management in the Konya and Seyhan basins of Turkey, among others (ADB, 2020; OECD, 2020).

NBS for Urban Water Management (Sponge Cities): The idea behind the sponge city concept is simple, capture water when there is too much and release it when there is too little. Sponge city refers to the range of systems, grey and green infrastructure built to promote sustainable urban water management. Sponge cities provide a range of water services including flood control, water conservation, water quality improvement, and protection of natural ecosystems.

It was launched as a new approach to urban water management in China in 2013–14 as a commitment to guiding the transition from rapid rainwater drainage of traditional cities to multi-objective whole-process integrated rainwater management of cities via methods of infiltration, stagnation, storage, purification, use, and discharge (Liu, et al., 2017). The Sponge City can be implemented in macro-scale and micro-scale scenarios in other developing nations, such as China, where most urban areas have experienced high density population growth, intensive expansion of impermeable roads and rooftops, and pressures from climate change-related water flood disasters (Chan et al., 2018).

There are four main principles concepts vital to the Sponge City as shown in Figure. 2. The first is to improve the ability of the city's surface to absorb and store rainwater for water supply and to mitigate stormwater runoff, which can cause flooding. The second principle relates to the management of water ecology via water self-purification systems and the provision of environmentally friendly waterfront design. The third principle is concerned with the application of green infrastructure to purify, restore, adjust, and reuse stormwater, thereby preventing water and soil pollution in cities. This will reduce the urban heat island effect and contribute to sustainable urbanisation. The fourth principle is the use of permeable pavements in urban road construction.