Hydrological and hydrochemical drought responses across ten solutes in a pre-alpine headwater catchment

Abstract. There is growing evidence that droughts affect stream water quality in multiple ways, often degrading it and thereby exacerbating water scarcity. However, our understanding of the hydrological and biochemical processes driving these changes is limited, due to a lack of high-frequency measurements across solutes covering pre-drought, drought and post-drought conditions. In this study, we analyzed the hydrological and hydrochemical responses to drought as compared to pre- and post-drought, in a forested pre-alpine catchment in Switzerland using high-frequency concentration data for ten different solutes. During the dry summer of 2018, discharge and groundwater table depth continued to decrease. The decrease in discharge slowed with increasing dryness, and flow never ceased entirely. Compared to normal summer conditions, a smaller fraction of rainfall converted into discharge, further illustrating the depletion of catchment storages. All solute concentrations exhibited significant breakpoints in their relationships with discharge and groundwater table depth. Mostly, they exhibited more chemostatic patterns at lower discharge (i.e., during drought) than during normal summer conditions. Groundwater table depth served as a complementary indicator for the disconnection of the hydrological and hydrochemical drought response. Overall, this observed divergence can be attributed to the fact that old groundwater was the only source of stream water during the drought, while shallower source areas, such as the catchment soils, were hydrologically disconnected from stream discharge. Our results also highlight the role of biochemical processes that alter the overall availability and mobility of different solutes, such as changes in redox conditions and nutrient uptake rates. In summary, our findings confirm the impact of drought on catchment water quality and demonstrate that the catchment's water quality response to drought cannot be explained by discharge dynamics alone. Rather, a detailed assessment of both hydrological and biochemical processes is necessary to identify the underlying drivers.