Variable Source Zones Controlling Flow, Water Age, and Nitrate Export Dynamics in a Tile‐Drained Lowland Watershed

Abstract Linking hydrograph dynamics with transport velocities and biogeochemical responses remains a central challenge in watershed hydrology, yet it is critical for understanding nitrogen reactive transport and its implications for water quality. In this study, we combined high‐frequency measurements of stable water isotopes and nitrate concentration with a StorAge Selection (SAS) function‐based flow‐transport model to simultaneously quantify storage dynamics, flow path activation, water mixing patterns, and nitrate export regimes in a tile‐drained, mesoscale lowland watershed. The high‐frequency tracer measurements revealed characteristic variability across event to seasonal timescales, reflecting complex hydrologic and biogeochemical responses to changing wetness conditions. Incorporation of these observations into the flow‐transport model identified root zone soils as a critical hydrologic filter that produces threshold‐like flow responses and delivers very young recharge to groundwater. The model also revealed a nested inverse storage effect in streamflow age, likely driven by the progressive activation of preferential flow networks facilitated by tile drainage. Event‐scale nitrate concentrations and export regimes were strongly correlated with stream water age. Decomposing stream nitrate concentrations into flow and water age endmembers further demonstrated substantial variability in nitrate in both young and old water, and showed that their mixing during flow generation underlies the observed range of nitrate export behavior. Building on these insights, we propose a conceptual model in which dynamic expansion and contraction of subsurface source zones govern coupled flow, age, and nitrate export responses. This framework provides new implications for constraining water transit times and nitrogen reactive transport in tile‐drained lowland agricultural watersheds.