Seasonal variation of transit time distribution and associated hydrological processes in a Moso bamboo watershed under the East Asian monsoon climate

•The first application of the StorAge Selection (SAS) method in a bamboo watershed.•Preferential release of younger water to summer discharge in wetter conditions.•Evapotranspiration tends to extract older water than streamflow.•SAS-derived young water fraction has a lower uncertainty than median age. Understanding seasonal variation of the transit time distribution of stream discharge and evapotranspiration is crucial in exploring the flow pathways and the rainfall-storage-streamflow processes in a watershed of interest . Here, both time-variant and time-invariant StorAge Selection (SAS) functions were adopted to estimate the transit time distribution in a Moso bamboo watershed under the East Asian monsoon climate. Daily stable isotopic compositions (δ18O, δ2H) of precipitation and streamflow at the watershed outlet from 1 October 2017 to 31 December 2020 were measured to calibrate the model parameters. The time-variant SAS method considering watershed wetness performs slightly better than the time-invariant SAS method, implying adding a watershed wetness indicator in the time-variant model results in only a minor improvement. Young water fraction and median age as two streamflow age measures were derived from the time-variant SAS model. The median age decreases abruptly within several days at three periods (one is in January 2020, and the other two periods are in Meiyu season) with larger uncertainty, while the estimated young water fraction varies gradually with much smaller uncertainty. The young water fraction derived from the time-variant SAS method appears to be more reliable than that estimated by the tracer cycle damping method. For stream discharge, the storage preferentially releases younger water to discharge under wetter conditions in the East Asian summer monsoon (EASM; June-September) period, while this phenomenon does not occur in the East Asian winter monsoon (EAWM; October-May) period. The larger hourly rainfall intensity for the whole EASM period (mean intensity 2.08 mm/h) likely enhances direct contribution of event water to streamflow than that in the EAWM period (mean intensity 0.75 mm/h). The evapotranspiration is estimated to extract older water than stream discharge in both the EAWM period and the EASM period. That is likely related to the occurrence of ecohydrological separation, and root water uptake below the soil-bedrock interface.