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This study aims to build and test the adaptability and reliability of the Soil and Water Assessment Tool hydrological model in a small mountain forested watershed. This ungauged watershed covers 184 km2 and supplies 90% of blue water for the Brașov metropolitan area, the second largest metropolitan area of Romania. After building a custom database at the forest management compartment level, the SWAT model was run. Further, using the SWAT-CUP software under the SUFI2 algorithm, we identified the most sensitive parameters required in the calibration and validation stage. Moreover, the sensitivity analysis revealed that the surface runoff is mainly influenced by soil, groundwater and vegetation condition parameters. The calibration was carried out for 2001–2010, while the 1996–1999 period was used for model validation. Both procedures have indicated satisfactory performance and a lower uncertainty of model results in replicating river discharge compared with observed discharge. This research demonstrates that the SWAT model can be applied in small ungauged watersheds after an appropriate parameterisation of its databases. Furthermore, this tool is appropriate to support decision-makers in conceiving sustainable watershed management. It also guides prioritising the most suitable measures to increase the river basin resilience and ensure the water demand under climate change.

The current challenges that society faces call for actions fostering climate change adaptation and long-term sustainability grounded in effective policies. Consequently, to raise environmental resilience and sustain human wellbeing, we need to overhaul the policy-making process towards a ‘holistic approach’. The nexus approach has proven a valuable tool for identifying gaps and incoherencies in current policies, for understanding major interlinkages across sectors and scales, and for promoting the Sustainable Development Goals (SDGs). Nevertheless, a ‘silo approach’ to decision making currently prevails, thus achieving cross-sectoral and cross-scale harmonization still remains a challenge in policy-making. In response to this challenge, the next step in the science–policy–practice interface is to integrate the nexus approach in the ‘climate services’ arena, in order to support integrated policies. Climate services embody a bridge between researchers and stakeholders, and by supporting integrated policies they will ensure synergies between sectors and scales, reduce potential trade-offs, and enable co-benefits.

The water-energy-land nexus requires long-sighted approaches that help avoid maladaptive pathways to ensure its promise to deliver insights and tools that improve policy-making. Climate services can form the foundation to avoid myopia in nexus studies by providing information about how climate change will alter the balance of nexus resources and the nature of their interactions. Nexus studies can help climate services by providing information about the implications of climate-informed decisions for other economic sectors across nexus resources. First-of-its-kind guidance is provided to combine nexus studies and climate services. The guidance consists of ten principles and a visual guide, which are discussed together with questions to compare diverse case studies and with examples to support the application of the principles.

The present study evaluates the applicability performance of the Soil and Water Assessment Tool (SWAT) in small forested watersheds (less than 1000 km2). This open-source software is widely used in investigations focused on water availability and quality. Overall, SWAT model performance ranges between satisfactory to good. Normally, underestimates daily peak discharges. The limitations of the model are related to the accuracy of climate data used and to the time period used for validation. Watershed area, forest cover and streamflow spatial distribution have an important influence on modeling processes. Overall, from the analyzed studies, we observed for discharge a decreasing tendency, more pronounced towards the end of the 21st century (up to −54%). For surface runoff, was noticed the same decreasing tendency up to 41%. Regarding sediment yield, the results vary within very wide limits. These findings vary according to watershed location, scenarios adopted, and the eligible period of time.