Advancing water quality model intercomparisons under global change: perspectives from the new ISIMIP water quality sector

Water pollution poses widespread risks to ecosystems, human health, and water users more broadly. Furthermore, the interplay of future hydroclimatic changes and socioeconomic developments will strongly impact the quality status of freshwaters across the globe. Innumerable pollutants are increasingly entering water bodies, potentially creating hotspots at various spatial and temporal scales and with implications for different water-dependent sectors. While it is recognized that proactive solutions to protect and improve water quality are key for the achievement of Sustainable Development Goal 6.3 (clean water for all), deficiencies in our understanding of the current and future quality status pose significant challenges. Water quality models help bridge the gaps in our understanding of water quality due to limited observations, but they vary in terms of pollutants, spatial-temporal resolution, and structure. While such diversity poses various challenges, it also presents an opportunity to design a multi-dimensional framework for water quality model intercomparison projects (WQ-MIPs) that focus on three distinct aspects: multi-pollutant, multi-scale, and multi-sector. The water quality sector has been launched within the ISIMIP initiative to help facilitate these multi-dimensional WQ-MIPs. In this paper, we present community insights on WQ-MIPs. We first synthesize the diversity found among water quality models and then propose an ISIMIP intercomparison framework aimed at enhancing our understanding of uncertainties in pollution levels and identifying robust pollution hotspots, sources, and impacts across multiple sectors, pollutants, and scales. To this end, we use four illustrative examples of WQ-MIPs. Finally, we outline a future agenda for advancing WQ-MIPs that are essential for developing effective solutions to preserve future water quality under global change.