Impacts of droughts and heatwaves on river water quality worldwide

Droughts and heatwaves have a major impact on river water quality worldwide. However, previous studies of river water quality under these climate extremes are limited to a small number of river basins and regions, mainly located in North America, Europe or Australia. In this study, we estimate the large-scale effects of droughts, heatwaves and compound drought-heatwave events on river water quality for a total of 314,046 water quality monitoring stations worldwide over the period 1980-2021. We focus on 16 water quality constituents grouped into physical (e.g., temperature, salinity), chemical (e.g., pharmaceuticals, pesticides) and biological constituents (e.g., biochemical oxygen demand, faecal coliform). Further, we analyse the response of each constituent to droughts and heatwaves in relation to climate type, land use and level of wastewater treatment. We find a general deterioration in river water quality under droughts and heatwaves globally for most rivers and water quality constituents considered. For example, there is on average a 27 % increase in river water temperature, 17 % decrease in dissolved oxygen and 24 % increase in salinity under droughts and heatwaves. In addition, we find that climate type, land use and level of wastewater treatment have a significant effect on the magnitude of response in each water quality constituent during these extreme events. The median increase in river water temperature under compound drought-heatwaves is strongly driven by climate zone with higher warming rates at the polar climate zone (+4.5 degrees C) compared to the tropical zone (+2.1 degrees C). Increases in salinity under droughts are on average two times larger in irrigated regions compared to non-irrigated regions. The concentrations of nutrients (P and N) in rivers can either increase or decrease under droughts, depending on the nutrient form (dissolved vs. particulate) and land use (urban vs. rural). Higher levels of wastewater treatment contributed to a greater decrease in pathogenic concentrations under droughts and heatwaves (as indicated by faecal coliform). Pharmaceuticals show mixed responses mainly depending on the persistence of the constituent in surface waters, with for instance declines in diclofenac concentrations under droughts and heatwaves due to increased decay under higher water temperatures. The results of this study provide a broader understanding of how droughts and heatwaves affect river water quality compared to previous local and regional-scale analyses. In addition, this study could form the basis for large-scale modelling of river water quality under droughts and heatwaves.