A new metric for determining the importance of transient storage

A review of various metrics used to characterize transient storage indicates that none of the existing measures successfully integrate the interaction between advective velocity and the transient storage parameters (storage zone, area, storage zone exchange coefficient). Further, 2 existing metrics are related to mean travel time, a quantity that is independent of the storage zone exchange coefficient, a. This interaction and the effect of a on travel time are important considerations when determining the mass of solute entering the storage zone within a given reach. A new metric based on median reach travel time is therefore proposed. Median reach travel time due to advection-dispersion and transient storage, and median reach travel time due solely to advection-dispersion are computed based on numerical simulations. These 2 travel times are used to determine F-med, the fraction of median travel time due to transient storage. Application of the new metric to 53 existing parameter sets indicates that transient storage accounts for 0.12% to 68.0% of total reach travel time. Rankings of storage zone importance based on the new metric are substantially different from rankings based on storage zone residence time, storage exchange flux, and the hydrological retention factor. These differences result from the ability of the new metric to characterize the interaction between advective velocity and transient storage, and the resultant effects on reach travel time and mass transport.