A dual-threshold framework for antecedent wetness controls on hydrological connectivity and nitrate source dynamics in a subtropical agricultural watershed

Le résumé fourni par la source

Season-based hydrological classifications often fail to resolve the short-term wetness controls that reorganize connectivity and nitrate transport in monsoon-influenced agricultural catchments. To address this issue, daily precipitation and discharge records (1980–2023) were combined with monthly hydrochemical monitoring and dual isotopes of water (δ 2 H-H 2 O, δ 18 O-H 2 O) and nitrate (δ 15 N-NO 3 – , δ 18 O-NO 3 – ; 2014–2023) from six monitoring sites in a subtropical agricultural watershed in eastern China. A 7-day antecedent precipitation index (API 7 ) and the daily baseflow index (BFI) identified two operational thresholds: 34.3 mm for initial hydrological activation and 133 mm for saturated connectivity. These thresholds partitioned the hydrologic record into Baseflow, Transition, and Storm regimes, representing 68.4 %, 29.2 %, and 2.4 % of the record, respectively. Water isotopes showed that the first threshold mainly reflected increased discharge through displacement of stored, evaporatively enriched pre-event water, whereas the second threshold marked a stronger contribution of event water and a pronounced decline in specific conductance. In contrast, riverine NO 3 – concentrations changed only modestly at most sites. However, NO 3 – isotopes and MixSIAR revealed marked regime-dependent source reorganization in spite of the weak concentration response, with lower δ 15 N-NO 3 – values and shifts in the relative contributions of fertilizer-, soil-, and sewage-derived NO 3 – across human-impacted sub-catchments. Random Forest analyses further showed that specific conductance and δ 15 N-NO 3 – remained consistent predictors of NO 3 – variability, whereas antecedent wetness, dissolved oxygen, and δ 18 O-H 2 O became more influential under Storm conditions. Overall, antecedent wetness governed not only runoff partitioning but also when cryptic NO 3 – source switching became hydrologically expressed, providing a process-based alternative to conventional dry/wet seasonal classification.

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