Water scarcity under droughts and heatwaves: understanding the complex interplay of water quality and sectoral water use
Authors: van Vliet Michelle , Cardenas Belleza Gabriel , Graham Duncan , Jones Edward 
Published: March 2024

Droughts and heatwaves pose serious challenges for water management and severely increase water scarcity in many regions of the world. It is increasingly recognized that water scarcity represents more than just a physical lack of water, referring to the imbalance between the supply and the demand of water of suitable quality for different uses. Changes in both climate and socioeconomic systems influence the availability, use and quality of water resources. Water scarcity thus amplifies when either one or more of the following three driving mechanisms intensify: 1) decreasing water availability; 2) increasing sectoral water use, and 3) deterioration of water quality resulting in unsuitability for use. Droughts and heatwaves are particularly critical as they adversely affect all three driving mechanisms, which are also highly interrelated1. However, limited understanding exists regarding the complex interplay, particularly between water quality and sectoral water use. Here we show responses in sectoral water use and water quality under droughts and heatwaves based on reported data for 1980-2019 globally and discuss a global assessment framework to unravel water scarcity and its drivers under these hydroclimatic extremes.Our results show that heatwaves and compound drought-heatwave events increase water use mainly for domestic and irrigation water use sectors2. River water quality tends to deteriorate during droughts and heatwaves in most cases as demonstrated based on a global literature survey3 and analyses of river water quality records of 314,046 water quality monitoring stations globally4. This showed for instance on average a 17% decrease in dissolved oxygen and 24% increase in river salinity under droughts and heatwaves over 1980-2019 globally4. Increasing sectoral water use, deterioration of water quality and decreasing water availability each amplify water scarcity in their own right, but more so together due to important interactions. For instance, a decline in water availability during a drought increases water scarcity directly, but also indirectly as less water is available to dilute pollutants, thereby leading to a deterioration of water quality3,4. This may result in higher water scarcity, when water quality thresholds for certain uses are temporary exceeded (e.g., increased salinity for irrigation). Increases in sectoral water use, such as for domestic use and irrigation2, result in higher water scarcity directly, but also indirectly due to water quality impacts. We propose a new integrated modelling framework building on the PCR-GLOBWB2 hydrological model coupled to the DynQual global surface water quality model5 to quantify water scarcity under droughts and heatwaves. Here we consider the two-way interactions between sectoral water use, water quality and water availability to improve understanding of the complex interplay between these water scarcity drivers, and test solutions options towards sustainable water management.&#160;1 van Vliet, M.T.H. (2023) Nature Water 1, 902&#8211;9042 C&#225;rdenas Belleza, G.A., M.F.P. Bierkens, M.T.H. van Vliet (2023) Environ. Res. Lett. 18 1040083 van Vliet, M.T.H. et al (2023) Nature Reviews Earth Environ. 4, 687&#8211;7024 Graham D.J., M.F.P. Bierkens, M.T.H. van Vliet (2024), J. of Hydrology 629, 1305905 Jones, E.R., M.F.P. Bierkens, N. Wanders, E.H. Sutanudjaja, L.P.H. van Beek, M.T.H. van Vliet (2023) Geosci. Model Dev. 16, 4481&#8211;45