Hydrological connectivity drives intra- and inter-annual variation in water quality in an intermittent stream network in a mixed land use catchment under drought

• Streamflow intermittency in a groundwater-fed, mixed land use lowland catchment in Germany increased during recent drought years. • Water quality reflects a eutrophic lowland catchment and spatial variation is related to catchment soils and land use. • Intra-annual variation in water quality is related to groundwater controls on streamflow generation with distinct connecting, connected and disconnecting phases. • Inter-annual variation in water quality is related to hydroclimate and catchment wetness with more solute flushing in wet years. We monitored the spatio-temporal variation of connectivity and linked water quality (WQ) in an intermittent stream network draining a mixed land use, lowland catchment in NE Germany. The monitoring period (2018–2022) coincided with four years of variable hydroclimate, though all years had negative rainfall anomalies compared to the long-term average. Correspondingly, streamflow became more intermittent (in terms of both the longevity and frequency of no-flows), with prolonged periods of no surface water flow in the summer and autumn. Despite inter-annual variation in hydroclimate and length of no-flow periods, in each of the four years the catchment showed three distinct seasonal phases of hydrological connection and disconnection in the channel network which has important implications for WQ. Autumn and early winter were characterised by a connecting phase as spatially variable streamflows were initialized in response to rising water tables following increased rainfall and reduced evapotranspiration as temperatures declined. The winter and early spring were charactered by a fully connected phase of the channel network where streamflows increased at the time of lowest temperatures. The late spring and early summer were characterized by a disconnecting phase as flow gradually ceased and the channel network began to fragment. A wetland in the centre of the catchment saw both the earliest and latest expression of streamflow, with the lower catchment downstream of this taking the longest to connect. The WQ is typical for a eutrophic lowland catchment and spatial variation is primarily related to soils and land use. During the connecting phase, stream WQ reflected that of groundwater though mobilization of solutes from the rewetting riparian area and channel bed also occurred. During the fully connected phase , streamwater was enriched by NO 3 from soilwater and agricultural drainage. During the disconnecting phase , lower flows and higher temperatures increased the intensity of in-stream biogeochemical interactions with mobilization of P, Fe and Mn associated with declining oxygen levels and release of dissolved organic carbon (DOC) concentrations. Inter-annual variations in WQ related to how hydroclimate and antecedent catchment wetness regulated the initiation, longevity and cessation of connection each winter. Future climate change is likely to drive increasing intermittency in streamflow in many lowland regions with implications for local and downstream ecosystem services.