2025/07/01 by Songjun Wu, Doerthe Tetzlaff, Xiaoqiang Yang +2 · 1 voice · 1 citation
Earth and Planetary Sciences · Environmental Science · #Groundwater and Isotope Geochemistry #Soil and Water Nutrient Dynamics #Hydrology and Watershed Management Studies
paper · pdf · doi:10.1029/2025wr040525
Abstract Distributed water quality modeling often involves considerable uncertainty due to complexity and spatial heterogeneity of hydrological and biogeochemical processes. To address this, stable water isotopes were coupled into a water quality modeling framework (EcH 2 O‐iso‐nitrate) for 30‐year hydrological and NO 3 ‐N simulations in a mixed‐land‐use catchment (Demnitz Mill Creek, DMC, Germany). The isotope‐aided model effectively simulated the (dis)connection of different flow paths and related biogeochemical transformations. Results highlight the role of transient hydrological states in NO 3 ‐N cycling in addition to static landscape characteristics. First, hydrological connectivity controls N transformations by regulating soil moisture and available NO 3 ‐N for processing from upstream inflows, which was demonstrated by the spatially‐explicit estimates of water ages and Damköhler number. Second, hydrological pathways determine where , when , and which NO 3 ‐N storages were connected, thereby dictating NO 3 ‐N concentrations in different flow paths. In DMC, subsurface flows were the primary pathway, but transition to near‐surface flow occurred in specific riparian “hot‐spots” due to the development of soil saturation along flow paths. Such transition was transient but exerted disproportional impacts on NO 3 ‐N dynamics by creating a preferential “fast‐track” toward channel network. This explains why DMC, as a groundwater‐dominated catchment, has flashy NO 3 ‐N peaks in stream flow. To further unveil the role of hydrological connectivity in management planning, scenario analysis was conducted to identify optimal locations for wetland construction, showing riparian hotspots with better connectivity had the greatest potential for NO 3 ‐N removal. These findings underscore the necessity of considering hydrological heterogeneity in modeling and management planning, which can be revealed by tracer‐aided modeling.