Impacts of climate change on the dilution capacity of perennial and non-perennial European rivers.

Rivers provide ecosystem services, such as water purification and drinking water supply, which depend on the river's capacity to dilute effluents from wastewater treatment plants (WWTPs). There are no large-scale analyses of the dilution capacity of rivers that take into account the differences between perennial and non-perennial river reaches, even though more than half of the world's river and stream reaches are estimated to be non-perennial. We evaluated the actual and future capacity of perennial and non-perennial river reaches in Europe to dilute WWTP effluents. Future (2071-2100) climatic conditions were compared to reference (1985-2014) conditions, considering average and extreme streamflow conditions (i.e., driest months or years). The dilution factor (DF), i.e., the ratio of streamflow to cumulated upstream WWTP discharge, was annually, on average, 4.6 times lower in non-perennial than in perennial reaches under reference conditions, and decreased up to 10.8 times lower in August. Annual DF is projected to decrease significantly in two-thirds of the reaches under the high emissions scenario (corresponding to Representative Concentration Pathway RCP8.5). Reaches in the Mediterranean and semi-arid regions of Europe have low dilution capacity and are the most vulnerable to WWTP discharges. Therefore, they are particularly sensitive to climate change-induced streamflow decreases, which will further reduce DF. In contrast, under the low emissions scenario (corresponding to RCP2.6), DF will remain largely unchanged, highlighting the importance of emissions reductions in preserving the dilution capacity of the European rivers.