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 10 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 δ 18 O 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 (MTTew) and fraction (Few) of 15 event water and identifying the chronosequent parameter sensitivity. Results show that TRANSEP could satisfactorily simulate the streamflow and δ 18 O change with the efficiency coefficients of from 0.85 to 0.97 and from 0.61 to 0.99, respectively. The MTTew and Few varied from 2 to 11h and from 0.2 to 0.8, respectively. Our MTTew 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 MTTew and positively on the Few is a predominant indicator which likely activates preferential flow 20 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 MTTew in steep and fractured catchments. IntroductionTransit time chronicling the elapsed time of water from entering a system, traveling through the system to its exit of the system 25 (Stewart and Mcdonnell, 1991) implies information about sources, pathways, and water storage. It also indicates how catchments retain and release water and solutes associated with biogeochemical processes (Stark and Stieglitz, 2000), chemical weathering (Maher and Chamberlain, 2014) and contamination transportation (Kirchner, 2003;Kirchner et al., 2000) Therefore, accurately estimating mean transit time could provide a physical basis for understanding the hydrological behaviors and consequently the biogeochemical effects of anthropogenic disturbances (Turner et al., 2006) and landcover change (Burns et 30