Exploring the relations between sequential droughts and stream nitrogen dynamics in central Germany through catchment-scale mechanistic modelling

Winter post-droughts amplify extreme nitrate concentrations in German rivers

“…Although instream processes can increase nitrogen removal, especially during low flows [19], we did not identify descriptors that specifically pinpointed these processes (e.g. drainage density) as primary drivers of the spatial variability of nitrate responses during winter droughts, possibly because of lower instream removal during winter [11].…”

“…However, the fraction of wastewater input did not affect summer droughts (figure S12). During the warm summer months, biogeochemical removal in the soil and streams increases, which might obscure the effects of wastewater sources [11].…”

“…While point source contamination (e.g. wastewater) is often diluted by discharge into streams, nitrate from diffuse sources such as agriculture is mobilized through fast and shallow hydrological pathways that are activated under wet soil conditions [11]. In addition to transport, interactions between sources and biogeochemical processes in the soil and streams that remove nitrate can lead to diverse concentration-discharge relationships [12].…”

“…However, in agricultural catchments, the responses can vary. Reduced hydrologic connectivity between sources and streams reduces transport [11,13] and enhances the biogeochemical removal of nitrate owing to longer subsurface residence times in the soil [17,18] and streams [19], promoting lower nitrate concentrations. Changes in runoff generation processes can also alter nitrate concentrations during drought.…”

“…Changes in runoff generation processes can also alter nitrate concentrations during drought. Zhou et al [11] demonstrated that within a catchment with mixed land use during droughts, enhanced instream removal processes, combined with a more pronounced contribution of runoff from forested upland areas with a lower nitrate influx, led to a decrease in nitrate concentrations at the catchment outlet. Conversely, lower nitrate dilution in catchments with contaminated groundwater during droughts can increase the concentration of nitrate in streams [1,20,21].…”