The Tank Cascade System (TCS) is an ancient, man-made rainwater harvesting and irrigation system unique to Sri Lanka’s dry zone. An ecosystem in itself, the TCS consists of an intricate network of small to large ‘tanks’ positioned along a gradient and connected through a series of canals. Within the system, paddy fields and dense forests coexist providing habitats for socio-ecologically significant species. Historically, the system has been crucial in drought and flood mitigation. Furthermore, fully functioning TCSs harvest copious amounts of rainwater, which is primarily used for irrigation to enable year-round crop production by the farming communities of the cascade landscape. The system’s important role in food security, rural livelihoods and local culture led to its designation as a Globally Important Agricultural Heritage System in 2017. Despite the TCS’s significance, the system has been deteriorating, and its sustainability is threatened by widespread tank neglect, rapid land use changes and biodiversity loss –the impacts of which are exacerbated by the effects of climate variability. There is national interest in safeguarding the TCS, though. The system has been recognized in Sri Lanka’s 2016-2025 National Adaptation Plan for Climate Change Impacts and within the 2021 Nationally Determined Contributions as an important Nature-based Solution (NbS) for strengthening national climate resilience. However, there is little evidence of this national support translating into on the ground action and of effective solutions to the challenges threatening TCS sustainability. This case study aims to fill these evidence gaps by sharing findings from research and project activities carried out under the Healthy Landscapes project. With a special focus on mainstreaming biodiversity and strengthening cascade ecology, the project rehabilitated and promoted the sustainable management of the TCS. This case study will highlight pathways for TCS rehabilitation to strengthen its function as a NbS, including associated challenges and further opportunities. As the cascade landscape community plays an important role in climate adaptation and resilience within the country's dry zone, we also emphasize the importance of investigating their perceptions of current national policy and formulating localized adaptation strategies that benefit climate resilience, food security and rural livelihoods.

In April 2021, the Sri Lankan Government banned imports of agrochemicals, including chemical fertilizers, to make Sri Lanka the first fully organic and chemical fertilizer-free country globally. The ban was justified by human and environmental health concerns, such as many cases of kidney failure in the central parts of Sri Lanka. While previous policies had envisioned a stepwise transition, the sudden ban jolted the agriculture sector. However, it was aligned with the emerging national economic crisis with drastically declining foreign exchange reserves that restricted the import of commodities, including fertilizer for distribution at subsidized prices. The ban was also opportune because fertilizer prices peaked on international markets in 2022. Without any transitional time, the thrust for organic fertilizers failed to satisfy demand or obtain the required crop nutrients resulting in severe agricultural losses. After the first data on decreasing yields were revealed, the government lifted the chemical fertilizer ban on December 1, 2021, but it was too late as the cropping season had arrived. Without financial reserves to import fertilizer, the donor community was urged to assist. This paper addresses: (1) justification of the ban, (2) the feasibility of transitioning to organic fertilizers based on the available biomass to replace chemical fertilizers; and (3) the related cost implications. The scenarios focus on irrigated paddy rice and the plantation sector that underpin the national economy. Undervalued nutrient sources are also considered as well as the constraints to and implications of such a transition beyond Sri Lanka’s frontiers.