A comment on the use of flushing time, residence time, and age as transport time scales

Abstract

Abstract-Applications of transport time scales are pervasive in biological, hydrologic, and geochemical studies yet these times scales are not consistently defined and applied with rigor in the literature. We compare three transport time scales (flushing time, age, and residence time) commonly used to measure the retention of water or scalar quantities transported with water. We identify the underlying assumptions associated with each time scale, describe procedures for computing these time scales in idealized cases, and identify pitfalls when real-world systems deviate from these idealizations. We then apply the time scale definitions to a shallow 378 ha tidal lake to illustrate how deviations between real water bodies and the idealized examples can result from: (1) non-steady flow; (2) spatial variability in bathymetry, circulation, and transport time scales; and (3) tides that introduce complexities not accounted for in the idealized cases. These examples illustrate that no single transport time scale is valid for all time periods, locations, and constituents, and no one time scale describes all transport processes. We encourage aquatic scientists to rigorously define the transport time scale when it is applied, identify the underlying assumptions in the application of that concept, and ask if those assumptions are valid in the application of that approach for computing transport time scales in real systems.In aquatic systems, most of the living biomass and masses of nutrients, contaminants, dissolved gases, and suspended particles are carried in a fluid medium, so it is essential to understand hydrodynamic processes that transport water and its constituents. A first-order description of transport is expressed as ''residence time'' or ''flushing time,'' which we conceive as measures of water-mass retention within defined boundaries. Aquatic scientists often estimate retention time and compare it to time scales of inputs or biogeochemical processes to calculate mass balances or understand dynamics of populations and chemical properties. Boynton et al. (1995) argue that residence time is such an important attribute that it should be the basis for comparative analyses of ecosystem-scale nutrient budgets.The classical empirical model of lake eutrophication (Vollenweider 1976) describes algal biomass as a function of phosphorus loading rate scaled by the hydraulic residence time. Since Vollenweider's recognition that the biogeochemical processing of phosphorus varies with residence time, variable water retention or flushing has been used to describe variability of lake thermal stratification (Hamilton and Lewis 1987), isotopic composition (Herczeg and Imboden 1988), alkalinity (Eshleman and Hemond 1988), dissolved organic carbon concentration (Christensen et al. 1996), elemental ratios of heavy metals (Hilton et al. 1995) and nutrients (Hecky et al. 1993), mineralization rates of organic matter (den Heyer and Kalff 1998), and primary production (Jassby et al. 1990). The mechanistic explanation of low plankton abun...

On the surface circulation in the western Coral Sea and residence times in the Great Barrier Reef

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On the estimation of transport timescales – case study: the Dez reservoir

“…There are many biological and chemical implications about these timescales, yet engineers and scientists have widespread misconceptions and confusion doi: 10.2166/hydro.2010.161 about suitable methods for the determination of these timescales (Monsen et al 2002). These confusions are the result of ignoring the underlying concept used (e.g.…”

“…The major assumption for the CSTR model is that any entrance of mass is instantaneously and evenly mixed throughout the system, so the concentration of a constituent exiting the system is equal to the concentration everywhere inside the CSTR (Monsen et al 2002). However, in…”

“…Residence times and flushing times are measures of water-mass retention within defined boundaries. These timescales are a first-order description of a set of multiple processes involved in transport and are the key parameters controlling the system behavior (Monsen et al 2002). By estimating the retention time, the fate of the substances transported within the water can be understood.…”

Changes in water types under the regulated mode of water level in Three Gorges Reservoir, China

“…These physical processes not only determine the spatial and temporal patterns of dissolved and suspended substances such as nutrients and solids, but they also regulate the ecological conditions for the occurrence of biogeochemical processes (Rueda et al, 2006). As a first order description of transport and mixing processes occurring in aquatic systems, residence time was often employed by limnologists to explain the variability in lake characteristics such as thermal stratification, nutrient concentration, primary production and trophic status (Monsen et al, 2002;Rueda et al, 2006;Xu et al, 2010a). Many examples published in international journals have illustrated that applications of residence time are pervasive in hydrologic, geochemical and ecological studies.…”