The hydraulic retention time (HRT), also known as hydraulic residence time or t (tau), is a measure of the average length of time that a compound (ex. water) remains in a storage unit (ex. lake, pond, ocean).
Hydraulic retention time is the volume of the storage unit divided by the influent flowrate:
where using SI Units Volume is in [m3] and Influent flowrate is in [m3/h]. HRT is usually expressed in hours (or sometimes days).
Calculate HRT
References
Hydraulic retention time Wikipedia(Text) CC BY-SA