Different Concepts and Terminology of the Residence Time

Abstract

　Although residence time is a key concept in hydrological studies, some confusion has arisen about similar terms. Here, we re-examine various concepts related to residence time and present a definition of each term and their characteristics in different systems. Then we recommend limited use of the term ‘residence time’ as general term involving ‘turnover time (i.e., conventional residence time),’ ‘age,’ ‘mean age,’ ‘transit time,’ and ‘mean transit time.’ We should distinguish these terms clearly when their values are shown. Additionally, we clarify correspondence among estimation methods and concepts/terms, and state remarks in interpreting estimation results. Finally, as ways forward in residence time studies, we emphasize the following: (1) combined use of tracers and numerical models and (2)appropriate selection and multi-faceted application of simple and well-defined indices of residence time.