Baseflow-related phosphorus transport in an agriculture-dominated, human-influenced watershed: Quantification using...

Accurate quantification of total phosphorus (TP) load associated with baseflow is important for understanding phosphorus transport in agriculture-dominated, human-influenced watersheds. However, such estimates are subject to considerable uncertainty, largely stemming from the sensitivity of outcomes to the choice of baseflow separation method. To address this methodological uncertainty, we performed a comparative evaluation of multiple baseflow separation methods in the Sihe River watershed of northern China. The results showed clear differences among the methods in baseflow magnitude, hydrograph characteristics, and performance under different evaluation metrics, with whole-period baseflow index (BFI) values ranging from 0.244 to 0.453. Considering its watershed-specific parameterization and recursive formulation, ECK was retained as the working scenario for subsequent estimation of baseflow-related TP loads, whereas the other three methods were used to characterize sensitivity to the choice of baseflow-separation method. By integrating this separation with the LOADEST model, the mean annual baseflow-related TP load during 2006-2020 was estimated at approximately 8196 kg yr^-1, with a mean annual contribution of about 21%. Baseflow-related TP export exhibited a distinct seasonal decoupling: while the absolute load peaked during high-flow periods such as August, baseflow accounted for a larger proportion of the total load during low-flow seasons. These findings indicate that baseflow-related TP export constitutes an important and persistent component of riverine TP transport. The proposed framework quantifies baseflow-related TP export and its sensitivity to the choice of baseflow separation method, providing a basis for understanding phosphorus transport processes in complex human-influenced watersheds.