Analyzing impacts of seasonality and landscape gradient on event scale nitrate-discharge dynamics based on nested high-frequency monitoring

Strom event-scale analysis provides insights into nitrate transport dynamics at catchment scale. Investigating different hysteretic relationships between nitrate and discharge can disentangle catchment nitrate functioning both spatially and temporally. In this study, we explored seasonality and landscape gradiemt effects on nitrate concentration-discharge (C-Q) hysteresis patterns based on six-year (2012-2017), high-frequency (15 min) data in the Selke catchment (central Germany). The Selke catchment exhibits heterogeneous combinations of meteorological, hydrogeological, and anthropogenic conditions. Three nested gauging stations were built along the main Selke River, capturing discharge and nitrate concentration from the dominant uppermost mixed forest and arable land, middle catchment pure steep forest and lowland arable and urban land areas, respectively. Amongst the 227 storm events that have been detected, anticlockwise and accretion of C-Q relationships accounted for 76.6% and 75.3%, respectively, while the proportions decreased with the increasing areal share of arable land during summer season. Accretion pattern predominated forest areas (e.g., the middle catchment) throughout the whole year suggesting higher nitrate concentration in dominating interflow than baseflow. In contrast, dilution pattern was almost exclusively observed in lowland areas (dominated by arable and urban areas) in dry periods, indicating lower nitrate concentration in quick runoff components like surface runoff. We further investigated the consistency and variability of hysteresis patterns from upstream to downstream based on shared events. Results indicated hysteresis patterns seemed to be consistent at the three stations when discharge was high enough. Moreover, we found that nitrate load contributions from the upper and lower areas changed seasonally, albeit the dominant share of runoff volume from the upper area throughout the whole year. Such a comprehensive analysis (i.e., clockwise vs. anticlockwise, accretion vs. dilution) enables in-deep discussion of the plausible mechanisms of nitrate dynamics under different landscape conditions. We are also aware of limitations of such statistical data analysis, which can likely be tackled by mechanistic modelling at higher temporal resolutions.