Long-Term Variations in Spring Water Mean Transit Time in a Forested Headwater Catchment in Japan

In this study, we applied a new methodology that utilized stable isotopic concentrations of water and SF6 concentrations of the air to investigate temporal variations in the spring water mean transit time (MTT) of a forested headwater catchment in Japan. We monitored the area from June 2010 to November 2021, including the forest thinning that occurred in October 2011. The SF6 concentrations were used to estimate the apparent age of the spring water, which was used to initiate a parameter calibration that determined the best possible MTT for each study period. The MTT was evaluated by estimating the d-excess variations of the spring water stable isotopes using the exponential piston flow model. The MTT ranged from 40 to 55 months and tended to increase soon after the forest was thinned. In contrast, the MTT was shorter by approximately 6 years after thinning occurred. The results indicate that the characteristics of the hydrological processes in the catchment underwent long-term changes after the forest was thinned. This study demonstrates that using combined tracer methods to investigate the hydrological response to forest management practices improved the results and can be used for better forest and subsurface water resource management.