[White paper] Water quality metrics derived from satellite observation can complement conventional water sampling, particularly to achieve much improved spatial and temporal coverage of medium (several square kilometres) and larger waterbodies. Thus, it has the potential to enhance confidence in WFD ecological status classification, firstly by quantifying elements of environmental status that are currently not or under-reported by Member States, such as the frequency, onset, duration and extent of phytoplankton blooms. Second, confidence in ecological status assessment would improve with increased representativeness of the natural diversity of waterbodies that are monitored, their inter-annual variability and water quality trends within larger waterbodies. Moreover, using standardised approaches, it would allow better comparison and standardization of water quality assessment across Member States, facilitating the management of transboundary waters in particular. Finally, by increasing spatial and temporal coverage, satellite observation is expected to enhance the effectiveness of the Programme of Measures (PoM) through early detection of deterioration, improving knowledge of the potential extent of an impact, improving monitoring of the effectiveness of PoMs and providing information to support more strategic in situ sampling. The European Union and European Space Agency currently boast the most advanced suite of satellite-based instruments designed to observe optical water quality. The Copernicus framework of sensors and services has had significant investment in recent years. Therefore, the vast majority of the cost associated with satellite-based monitoring of surface waters has already been invested. To promote and support the use of satellite-based water quality metrics in WFD statutory monitoring and reporting activities, we make the following recommendations, particularly in light of the ongoing revision of the WFD: Recognition of satellite observation as an assessment method in the context of the revision of the Water Framework Directive. Explicit encouragement to use satellite-based monitoring to complement national and statutory monitoring and reporting, such as already exercised by a limited number of countries (examples in this paper) and available from existing academic, governmental and private sector capabilities, will provide a clear signal to Member States that its use is supported. In particular, satellite products that enhance confidence in the classification of phytoplankton biomass (typically measured by chlorophyll-a) by vastly improving spatial and temporal coverage should be considered, since these are already highly mature and can support quality elements that have thus far been considered too costly to include using conventional methods, such as assessing the frequency and intensity of algal blooms in lakes and coastal waters. Create a satellite observation expert group to harmonise metrics across countries and advise member states on best practises. The satellite observation expert group will: ensure harmonisation of the applicable satellite observation methods and their comparability with nationally-approved and intercalibrated methods; establish guidelines on how observation uncertainties should be reported; and ensure close collaboration with the Water Framework Directive Common Implementation Strategy working group on ecological status (WG ECOSTAT). An expert group advising on best practise is necessary because satellite-based observation capabilities continue to improve over time whereas water quality management relies on stable and transparent methodologies. The expert group should work towards self-certification in the industry, including representation from the downstream Earth observation service sector, and be led by an independent research and policy advisory body (such as the Joint Research Centre). Reference the use of satellite-based Earth observation metrics in the Reporting Guidance (Annex 5) of the revised Water Framework Directive. Particular reference to the use of satellite-derived water quality indicators in assessing phytoplankton biomass (by proxy of chlorophyll-a) and the frequency and intensity of phytoplankton blooms in Annex 5 as well as national and international standards will ensure the provision of monitoring data of equivalent scientific quality and comparability. Convene a conference for EC, Member States, WFD authorities to agree on recommendations of common practices and reporting standards when using satellitebased water quality metrics to support the Water Framework Directive. Provide an opportunity for policy makers to recognise and proactively support the use of already available satellite-based Earth observation derived metrics and capitalise on already established networks of national Earth observation scientists applying relevant derived metrics to the Water Framework Directive requirements.

This project has received funding from the European Union's Horizon 2020 research and innovation programme (grant agreements 730066 and 776384).