Guidelines for optimal implementation of NSWRM and their combinations in the specific European biogeographical region of interest (Pannonian, Continental, Boreal) across various agroecosystems, terrain, soil, climatic conditions. Deliverable D6.3 of the EU Horizon 2020 project OPTAIN.

Deliverable report D6.3 of the EU Horizon 2020 Project OPTAIN (Grant agreement No. 862756) Summary The purpose of the OPTAIN D6.3 was to explore, which NSWRM are the most efficient in the Continental, Boreal and Pannonian biogeographical regions of Europe (EBR), focusing environmental, socio-economic and policy dimensions. To reach this, the OPTAIN researchers and CS leaders identified nearly 30 key research questions. Building on these inputs, the deliverable’s shared goal was to determine whether the NSWRMs can be recommended for retaining water, sediment and nutrients in small agricultural catchments.The key methodological approach used in the OPTAIN project was the multi-actor approach, adopted to ensure a harmonised process of stakeholder engagement and modelling across case studies and governance levels. To enable meaningful and efficient stakeholder involvement, multi-actor reference groups were established (WP1). Through a series of strategically designed workshops, these groups actively shared descriptions of good practices in a globally standardised way, using the WOCAT NSWRM catalogue (WP2).The data collection and harmonisation for model-based assessment (WP3) laid the foundation for one of the project’s most significant outcomes: a major breakthrough in creating the evidence base on the environmental and economic efficiency of measures implemented in selected case studies, achieved using the SWAT+ and SWAP models (WP4). This evidence supported the development of a shared understanding of optimised spatial combinations of measures using the COMOLA tool to explore the stakeholder-preferred outcomes (WP5).Through a series of surveys, stakeholders on various levels shared their opinions on the efficiency and sufficiency of policies and approaches for promoting the NSWRM, as well as on issues and possible solutions for improving NSWRM uptake. The results capture perspectives of stakeholders ranging from supra national, through (international) river basin, and catchment, all down to local, field scale level, and provide a valuable and comprehensive policy overview to further guide NSWRM implementation (WP6). Finally, the OPTAIN findings were distilled and have been put forward to the OPTAIN Learning Environment (LE) platform where the evidence-base, presented through various carefully designed outputs, maintains accessible after the project to support multi-actor learning in real-life contexts (WP7).The harmonised modelling in OPTAIN helps understand the efficiency of (i) land management measures (LMMs), (ii) structural linear measures (SLMs), and (iii) structural areal measures (SAMs) across the EBRs. In the Continental EBR multiple case studies report significant hydrological and agronomic benefits. LMMs reduced sediment and nutrient losses while increasing soil moisture; in some cases this came with slight grain yield reductions. SAM showed strong potential to mitigate high flows, improve low-flow conditions, reduce soil loss, and decrease nitrogen loads. SLMs reduced in-stream nitrogen loads; however, a trade-off was observed in some simulations with more days below low-flow thresholds, while crop-yield effects were minor.In the Boreal EBR, the LMMs were the most effective in reducing nitrogen and phosphorus losses and increasing early-summer soil-water content. SLMs contributed to sizeable reductions in sediment loss and declines in nutrient loads. In areas prone to spring waterlogging, retention measures improved trafficability. The ability of LMMs to enhance water retention while sharply lowering phosphorus losses suggests they can address both nutrient pollution and seasonal water-management challenges in this region. In the Pannonian EBR, NSWRM implementation delivered substantial benefits for erosion control and water retention, with mixed effects on crop production. LMMs were the most effective for reducing sediment and phosphorus losses and for increasing soil-moisture storage; they also reduced nitrogen loss. Measures that remove arable land decreased total grain production, reflecting land-take. SLMs showed limited catchment-scale benefits. Yield responses varied: LMMs increased winter crop yields in some cases but had negligible effects on other crops.Based on the findings explaining environmental and partially socioeconomic dimension to NSWRMs relevance, the OPTAIN D6.3 provides indications for governance improvements that would likely lead to improved uptake and implementation of NSWRMs in the respective EBRs. Policy and implementation surveys of WP6 show that the ambitions of the European Green Deal, Farm to Fork Strategy, and Water Framework Directive, policies must go beyond incremental improvements and actively support systemic change.The most pressing changes show that future NSWMRs policies need to improve coordination of governance across scales, ensure robust and sustained financial mechanisms, simplifiy administration, work more towards effective instead overwhelming knowledge transfer, and, most importantly, focus on improved stakeholder engagement. NSWRMs are – here we put forward the synthesis of the available evidence – a strategic tool for advancing water quality and quantity goals, climate resilience, and sustainable agricultural production. However, their full potential will only be realised through integrated, well-designed, and adequately supported governance systems.Why this work matters in terms of practical application, policy integration, and sustainability? D6.3 provides robust evidence that NSWRMs can drive a shift from incremental improvements to systemic transformation in European agriculture, directly advancing the goals of the Green Deal, Farm to Fork Strategy, and Water Framework Directive. At the same time, it highlights gaps between the effectiveness of certain measures and the existing policy instruments supporting the uptake of NSWRMs, clearly pointing to where governance improvements and targeted interventions are most needed. By building this evidence base, D6.3 lays the foundation for the co-creation of the next-generation of incentives – the focus of OPTAIN D6.4.