Costa Rica is already experiencing the effects of climate change (CC), principally in the northern region of the country. CC scenarios suggest that by 2080 the annual area rainfall will be reduced by up to 65% in the region. In the short term, rainfall is predicted to decrease 15% by 2020 and 35% by 2050. These extreme conditions will exacerbate climate and water stress in some areas, recreating conditions that are typical of semi-arid areas. If CC-driven pressures are not addressed, the region will continue to experience significant water shortages that will have a severe economic impact on the livelihoods of local communities and the productive sectors. In Costa Rica, rural aqueduct associations (ASADAS), which are locally organized groups of men and women from the user-communities delegated by the National Institute of Aqueducts and Sewers (AyA), provide potable water and sanitation services to 28.7% of the country's population, reaching communities in suburban and rural areas. Most ASADAS in the region must develop the necessary skills and have access to knowledge and tools, as well as adequate investment, in order to address the scarcity of the water supply due to CC. Existing aqueduct infrastructure is often outdated and overloaded, causing inefficient water delivery, which in turn complicates the collection of fees from end users. Instability of fee collection leads to financial uncertainty, which impedes the ASADAS and the AyA's ability to plan for and implement targeted improvements and new investments, including adaptation to CC. AyA investment plans lack community-based or ecosystem-based adaptation measures. If the ASADAS do not strengthen their capacities to cope with CC, the vulnerability of rural populations of the northern region of Costa Rica will only increase. The long-term solution to mitigate the prevailing threats of water shortages to local livelihoods is to establish a holistic approach to managing the water supply and demand that takes CC into account. The objective of this five-year project is to improve water supply and promote sustainable water practices of end users and productive sectors by advancing community- and ecosystem-based adaptation measures in ASADAS to address projected climate-related hydrological vulnerability in northern Costa Rica. This will be achieved through community- and ecosystem-based measures in rural aqueduct associations (ASADAS) to address projected climate-related hydrological vulnerability. The interventions are targeted in the northern region of Costa Rica (Guanacaste and Alajuela provinces). However, the following barriers limit the achievement of the normative solution: a) lack of knowledge and access to finance for resilient infrastructure, efficient household-level water use technologies, and aquifer mapping to effectively manage water demand and usage and design strategies to conserve water during periods of drought; b) limited capacity and knowledge among local stakeholders to adopt sustainable water use practices and reduce their vulnerability to CC; c) incomplete hydroclimatological network and deficient climate early warning and information system (CEWS) that limit the ability of rural ASADAS and local communities to implement timely mitigation measures; d) lack of awareness among policy and decision-makers about the social, economic, and environmental implications of water resources vulnerability to CC; and e) lack of economic incentives for the livestock and agricultural sectors for adopting water conservation production practices to reduce their vulnerability to CC.The theory of change underpinning this project includes building community-based infrastructure and technical capacities to address projected changes in water availability (Component 1) and mainstreaming ecosystem-based adaptation measures into public and private sector policies and investments in the target area (Component 2). First, SCCF resources will be used to strengthen the infrastructure and technical capacity of ASADAS to cope with CC impacts Second, the capacities of ASADAS end users to mainstream CC adaptation into their livelihood systems will be strengthened through a community-based CC-training program with a gender focus and which includes indigenous communities. Third, hydrometeorological information will be integrated into land use and production practices and planning processes to increase the resilience of rural communities to water variability Fourth, ecosystem-based CC adaptation measures will be integrated into public and private sector policies, strategies, and investments related to rural community water-sourcing infrastructure and services, Finally, the purchasing and credit policies of at least 20 agricultural and livestock trading companies and five financial institutions in the target region will integrate incentives to promote adoption of ecosystem-based CC adaptation measures by farmers, and a knowledge management system will be developed allowing dissemination of data, information, and toolkits to foster and mainstream ecosystem-based adaptation practices in other water-intensive productive sectors across the country.