Past and future global surface water quality modelling using DynQual
Authors: Jones Edward R. , Bierkens Marc F.P , van Vliet Michelle T.H. 
Published: March 2024

Good surface water quality is essential for safeguarding human water use activities and maintaining ecosystem health. Yet, our quantitative understanding of past developments in surface water quality is mostly predicated upon observations at monitoring stations that are discontinuous in both space and time. Furthermore, very little is known about how both climate and societal change will impact surface water quality in the future.Process-based models provide unique opportunities to simulate both past and future surface water quality with a consistent spatial and temporal resolution. Representing one of the attempts in the emerging field of large-scale surface water quality modelling, we have developed the dynamical water quality routing model (DynQual) to simulate water temperature (Tw) and total dissolved solids (TDS), biological oxygen demand (BOD) and fecal coliform (FC) concentrations at 5 arc-minute (10km) spatial resolution and with a daily timestep. The model is open-source (https://github.com/UU-Hydro/DYNQUAL) and is coupled to the global hydrological model PCR-GLOBWB2, although hydrological input can alternatively be prescribed as a forcing. The model also incorporates a high-resolution wastewater treatment dataset1 to more realistically account for the impact of these practices on pollutant delivery to surface waters, compared to country-level or regional average rates.DynQual has been applied and validated against observed in-stream concentrations for the historic period (1980 &#8211; 2019) using input from ISIMIP3a2, and used to project future surface water quality (up to 2100) under (uncertain) climate change and socio-economic developments using input from ISIMIP3b3. Based on these modelled results, we assess the spatial patterns, temporal variations and long-term trends in surface water pollutant concentrations to evaluate global water quality dynamics.Our results show that surface water quality issues exist across all world regions, with current multi-pollutant hotspots especially prevalent in northern India and eastern China. Recent trends towards surface water quality deterioration are most profound in the developing world, particularly Sub-Saharan Africa and southern Asia. Conversely, in highly developed economies, organic (BOD) and pathogen (FC) pollution have decreased over time primarily due to expansions and improvements in wastewater collection and treatment. Simulations of future water quality indicate that pollution will increasingly and disproportionately affect people living in developing countries, with a widening gap in exposure rates between rich and poor countries. In particular, the combination of surface water quality deterioration and demographic changes in Sub-Saharan Africa will establish this region as a new global hotspot of surface water pollution. &#160;&#160;References&#160;1 Jones, E.R., M.T.H. van Vliet, M. Qadir, M.F.P Bierkens (2021) Country-level and gridded estimates of global wastewater production, collection, treatment and re-use, Earth Syst. Sci. Data, 13, 237&#8211;254, https://doi.org/10.5194/essd-13-237-2021&#160;2 Jones, E.R., M.F.P. Bierkens, N. Wanders, E.H. Sutanudjaja, L.P.H. van Beek,&#160; M.T.H. van Vliet (2023), DynQual v1.0: A high-resolution global surface water quality model, Geosci. Model Dev., 16, 4481&#8211;4500, https://doi.org/10.5194/gmd-16-4481-2023&#160;3 Jones, E.R., M.F.P. Bierkens, P.J.T.M. van Puijenbroek, L.P.H. van Beek,&#160; N. Wanders, E.H. Sutanudjaja, M.T.H. van Vliet (2023) Sub-Saharan Africa will increasingly become the dominant hotspot of surface water pollution, Nature Water, 1, 602&#8211;613, https://doi.org/10.1038/s44221-023-00105-5