Event-Scale Nutrient Export Dynamics Revealed by High-Frequency Monitoring in a Semi-Arid River Basin

The dynamics of rainfall-runoff processes, driven by climate change, impact the timing and magnitude of nutrients mobilization from land to river. However, understanding of concentration-discharge (C-Q) relationships of nutrients in event scale from high-frequency observations remains insufficient. This study analyzes event-scale concentration-discharge (C-Q) relationships for total phosphorus (TP) and total nitrogen (TN) using two-year high-frequency monitoring data from seven nested sub-catchments in the Wei River Basin. Based on 92 identified rainfall-runoff events, an advanced power-law C-Q model was applied to derive concentration intercepts, slopes, and hysteresis indices, with controlling factors examined via K-means clustering and Support Vector Machine classification. Results indicate that daily direct flow positively correlates with TP concentration but negatively with TN. Notably, 88.6% of TP C-Q slopes were positive, demonstrating that runoff processes substantially enhance TP export, whereas only 31.1% of TN slopes were positive, primarily in northern catchments. Cluster analysis revealed that TP response patterns (dilution, facilitation, and lagged-facilitation) are largely governed by rainfall/runoff duration and antecedent flow, whereas TN patterns (dilution, weak-function, and lagged-facilitation) are predominantly controlled by rainfall characteristics such as peak intensity, peak ratio, and antecedent rainfall. In conclusion, this research highlights distinct export mechanisms and drivers for TP and TN during hydrological events,  providing a process-based framework for predicting nutrient responses under changing climate conditions in semi-arid river basins.