Residence times of groundwater and spring water were estimated using the concentrations of chlorofluorocarbons (CFCs) as tracers at four mountainous headwater catchments, and the mechanisms of determination for these residence times were discussed. It is mainly discussed that residence times are controlled by the topographic factors in many studies, although it is still a controversial subject. On the other hand, we did not find a clear relationship between the topographic factors and the residence times at our observation sites. Then, we considered the relationship between the residence times and the groundwater dynamics in each catchment, and found that the contribution of groundwater from deeper soil or bedrock layers and/or differences of bedrock geology are important controlling factors for the residence times. The detailed discussions about the mechanisms at small headwater scales will be important information to clarify the control factors of the residence times as one of the parameters of scaling issues of the hydrological processes.
