Natural/Small Water Retention Measures (NSWRMs) can help mitigate the conflicts between agricultural water uses (e.g. plant production, animals) and other human and environmental demands for water, including drinking water or maintaining environmental flow. This is crucial, since these conflicts will be probably exacerbated by an increasing number of extreme events such as droughts and heavy rainfall. A more careful management of head watersheds will significantly contribute to a more resilient agriculture and society. Moreover, NSWRMs are contributing simultaneously to the achievement of different Sustainable Development Goals and environmental targets formulated in several water- and agriculture-related European Union policies. Despite a comprehensive set of techniques available to increase water retention on both catchment and farm levels, knowledge is still lacking on the effectiveness of different scale- and region-specific measures across various soil-climatic regions and agricultural systems, especially under changing climate conditions. OPTAIN aims to (i) identify efficient techniques for the retention and reuse of water and nutrients in small agricultural catchments across Continental, Pannonian and Boreal biogeographical regions of Europe, taking into account potential synergies with existing drainage-irrigation systems, and - in close cooperation with local actors - (ii) select NSWRMs at farm and catchment level and optimize their spatial allocation and combination, based on environmental and economic sustainability indicators. By building on existing knowledge and addressing these objectives, OPTAIN will improve the Technological Readiness Level of NSWRMs for the benefit of both humans and ecosystems. All gained knowledge will be translated into a learning environment allowing analysis of trade-offs and synergies between multiple values/goals in the management and design of NSWRMs.