Characterization of event water fractions and transit times under typhoon rainstorms in fractured mountainous catchments: Implications for time-variant parameterization

Abstract. Transit time with its indicative significance in regulating rainfall-runoff mechanism is a key factor for understanding many biogeochemical processes, but is rarely investigated in steep and fractured mountainous catchments. Mountainous catchments in Taiwan are characterized by active endogenic tectonics and exogenic typhoons and thus provide opportunities to explore the hydrodynamic systems over time. In this study, the hydrometrics and δ¹⁸O in rain and stream water were sampled by ~ 3-hour interval for six typhoon events in two mesoscale catchments. The TRANSEP (transfer function hydrograph separation model) and global sensitivity analysis was applied for estimating mean transit time (MTT_ew) and fraction (F_ew) of event water and identifying the chronosequent parameter sensitivity. Results show that TRANSEP could satisfactorily simulate the streamflow and δ¹⁸O change with the efficiency coefficients of from 0.85 to 0.97 and from 0.61 to 0.99, respectively. The MTT_ew and F_ew varied from 2 to 11 h and from 0.2 to 0.8, respectively. Our MTT_ew in the meso-scale catchments is similar with that in micro-scale catchments, showing a fast transfer in our steep catchments. The mean rainfall intensity which negatively controls on the MTT_ew and positively on the F_ew is a predominant indicator which likely activates preferential flow paths and quickly transfers event water to the stream. Sensitivity analysis among inter- and intra-events suggested that parameter sensitivity is event-depend and time-variant, affirming a nonlinear behavior in event water transfer function and time-variant parameterization should be particularly considered when estimating the MTT_ew in steep and fractured catchments.