OPTAIN - Optimal strategies to retain and reuse water and nutrients in small agricultural catchments

The increasing frequency of droughts and heavy rainfall is intensifying conflicts between agricultural water use and other human and environmental demands. Natural/Small Water Retention Measures (NSWRMs) can help mitigate these conflicts by enhancing water quality, improving agricultural resilience, and contributing to sustainable development goals. However, there are knowledge gaps about the effectiveness of these measures across different regions, scales, and climate conditions. The EU Horizon 2020 project OPTAIN aims to address these challenges in 14 European case studies. The project involves local stakeholders through Multi-Actor Reference Groups, which have identified and documented 235 potential NSWRMs, of which 66 from 29 categories have been selected for further evaluation. These measures are catalogued in collaboration with the WOCAT and NWRM.eu databases. To assess the impact of these NSWRMs at field and catchment scale, OPTAIN applies the SWAT+ model with a fully distributed routing scheme, accompanied by further field-scale simulations using SWAP in areas of high data availability. The project developed protocols and R scripts to standardize data preparation, model calibration, and evaluation across case studies, ensuring consistent analysis. Initial simulations in the German case study demonstrate positive effects of NSWRMs, such as low tillage and grassed waterways, in reducing peak water flows, increasing low flows, and enhancing nutrient and sediment retention. Furthermore, the project linked SWAT+ with an economic model using the CoMOLA platform to optimize NSWRM allocations based on environmental and economic criteria. Policy analysis is another important component of OPTAIN, with local and regional policies being reviewed to identify gaps and opportunities for harmonizing water and agricultural policies across Europe. Interim findings, shared through policy briefs, emphasize the need for better integration of agro-environmental policies, increased intersectoral collaboration, and awareness-raising among stakeholders. OPTAIN's overarching goal is to improve the acceptance and implementation of NSWRMs by harmonizing data, methods, and policies across the 14 case studies. While there are significant differences between countries, which pose challenges for comparative studies, the project is working to address these through data standardization and model improvements. The R scripts developed by the project will assist future SWAT+ users worldwide in setting up and calibrating models to evaluate the effectiveness of NSWRMs in water and nutrient retention. Ultimately, OPTAIN aims to optimize the spatial allocation and combination of NSWRMs, ensuring they are both environmentally and economically sustainable, while also promoting policy alignment at local, national, and EU levels.