Average residence time? - Answers

The average residence time is a measure of the time it takes for the total stock or supply of the material to be cycled through a system. To compute the average residence time (T; assuming constant size of the system and constant rate of transfer), we take the total size of the stock (S) and divide it by the average rate of transfer (F) through the system: T=S/F. Example if a reservoir holds 100 million cubic meters of water, and both the average input from streams entering the reservoir and the average output over the spillway are 1 cubic meter per second, then the average residence time for a cubic meter of water in the reservoir is 100 million seconds, or about 3.2 years.