The residence time of river water in reservoirs

The average length of time water remains within the boundaries of an aquatic system is one of the key parameters controlling the system's biogeochemical behavior. This time scale, which is generally referred to as the hydraulic residence time, provides a first order description of multiple and complex processes that create transport. This manuscript reviews the procedures to estimate transport time scales in reservoirs and explores, through the analysis of numerical simulations, the links between these time scales and the underlying hydrodynamic processes in a reservoir in North-Eastern Spain. The mean residence time scales undergo dramatic variations in time and in general, are comparable to the time scales of the systems' variability itself, such as those associated with seasonal changes in stratification, allowing complex patterns of intermittent mixing events to determine residence time scales. We demonstrate that the temporal variations of mean residence times occur not only at seasonal time scales, but also at shorter scales. The time scales are closely related to mixing and transport processes occurring within the reservoir, at the inflow sections and at the watershed.