Exchange and transit of water and material in natural reservoirs and between the reservoirs and their environments occur all the time.The characteristics of different reservoirs vary.In the paper,some concepts to describe the temporal characteristics of these reservoirs are analyzed.The similarities and dissmilarities of different concepts are compared.The calculations of replacement period and re newal time are introduced especially and their relationship is ana-lyzed by formula(6)and figure1.On the basis of predecessors' studies,the relationships among replacement period,average transit time,average age and average residence time are analyzed when the locations of inlet and outlet of water bodies are different.As an application,the re-placement periods for middle and lower reaches of the Yellow River are computed.The change trends of the monthly replacement periods are analyzed.The results show that during less runoff period,the values for replace ment period are larger than values during more runoff period.The relationship between replacement period and flow are researched also.Negative correlation be-tween them is found.