Reservoir residence time

Other approximate reservoir residence times include 10 years for lakes and reservoirs, 2 weeks for rivers, and 10 to thousands of years for ice caps and glaciers. 

An important point to consider is that for a given amount and density of air or water pollution, if the pollution can only be remediated by dilution (mixing) and dispersion, the time it takes for nature to clean up stable chemical pollutants will be directly proportional to reservoir residence time. You might think about this idea as it applies to air pollution and surface- versus groundwater pollution. 

Fig. 7. The principal reservoirs in the hydrological cycle R, reservoirs in units of 10 metric tons (10 km ) E, fluxes in units of 10 km /yr T, residence time, yr. R/F = volume /input—output. Fluxes (flows) are approximate. For range of estimates, see Ref. 8.

The intrinsic drawback of LIBS is a short duration (less than a few hundreds microseconds) and strongly non-stationary conditions of a laser plume. Much higher sensitivity has been realized by transport of the ablated material into secondary atomic reservoirs such as a microwave-induced plasma (MIP) or an inductively coupled plasma (ICP). Owing to the much longer residence time of ablated atoms and ions in a stationary MIP (typically several ms compared with at most a hundred microseconds in a laser plume) and because of additional excitation of the radiating upper levels in the low pressure plasma, the line intensities of atoms and ions are greatly enhanced. Because of these factors the DLs of LA-MIP have been improved by one to two orders of magnitude compared with LIBS. 

Fluxes are linear functions of reservoir contents. Reservoir size and the residence time of the carbon in the reservoir are the parameters used in the functions. Between the ocean and the atmosphere and within the ocean, fluxes rates are calculated theoretically using size of the reservoir, surface area of contact between reservoirs, concentration of CO2, partial pressures of CO2, temperature, and solubility as factors. The flux of carbon into the vegetation reservoir is a function of the size of the carbon pool and a fertilization effect of increased CO2 concentration in the atmosphere. Flux from vegetation into the atmosphere is a function of respiration rates estimated by Whittaker and Likens (79) and the decomposition of short-lived organic matter which was assumed to be half of the gross assimilation or equal to the amount transferred to dead organic matter. Carbon in organic matter that decomposes slowly is transferred... 

The dynamics of these models depend strictly on carbon fluxes, but the fluxes are poorly measured or are calculated from carbon reservoir size and assumptions about the residence time of the carbon in the reservoir. In addition, model fluxes are linear functions while in reality few, if any, probably are linear. 

The residence time is the time spent in a reservoir by an individual atom or molecule. It is also the age of a molecule when it leaves the reservoir. If the pathway of a tracer from the source to the sink is characterized by a physical transport, the word transit time can also be used. Even for a single chemical substance, different atoms and molecules will have different residence times in a given reservoir. Let the probability density... 

X 10 years old, this implies that the content of the reservoir today is about half of what it was when the Earth was formed. The probability density function of residence time of the uranium atoms originally present is an exponential decay function. The average residence time is 6.5 x 10 years. (The average value of... 

It can be shown that for a reservoir in steady state. To is equal to t, i.e. the turnover time is equal to the average residence time spent in the reservoir by individual particles (Eriksson, 1971 Bolin and Rodhe, 1973). This may seem to be a trivial result but it is actually of great significance. For example, if tq can be estimated from budget considerations by comparing fluxes and burdens in Equation (1) and if the average transport velocity (V) within the reservoir is known, the average distance (L = Vxr) over which the transport takes place in the reservoir can be estimated. 

The concept of average residence time, or turnover time, provides a simple macroscopic approach for relating the concentrations in ocean reservoirs and the fluxes between them. For the single box ocean in Fig. 10-17 the rate of change of the concentration of component n can be expressed as... 

The definition of turnover time is total burden within a reservoir divided by the flux out of that reservoir - in symbols, t = M/S (see Chapter 4). A typical value for the flux of non-seasalt sulfate (nss-SOl"") to the ocean surface via rain is 0.11 g S/m per year (Galloway, 1985). Using this value, we may consider the residence time of nss-S04 itself and of total non-seasalt sulfur over the world oceans. Appropriate vertical column burdens (derived from the data review of Toon et ai, 1987) are 460 fxg S/m for nss-801 and 1700 jig S/m for the sum of DMS, SO2, and nss-S04. These numbers yield residence times of about 1.5 days for nss-S04 and 5.6 days for total non-seasalt sulfur. We might infer that the oxidation process is frequently... 

The atmospheric reservoir (8) represents P contained on dust particles. Because the mean residence time of dust in the air is very short, the standing stock of P in the atmosphere is rela-... 

The total burden, sum of inputs or exports, and average residence times for the reservoirs are listed in Table 14-5. As discussed in Chapter 4, the residence time of an element within a reservoir reflects the reactivity and exchange of that element with other reservoirs. A short residence time suggests that removal processes are rapid and significant over short time scales compared to the amount in the reservoir. 

Table 14-5 Summary of reservoir amounts, total fluxes, and residence time...

Reservoir A (mol X 10 I fluxes (mola X 10 ) Residence time (years)... 

Fig. 6 Comparison of results of the AF model solved with and without streamflow trend for wet and dry years, expressed as years above and below the mean water residence time in the reservoir (0.25 years)...

Obviously, these examples do not cover all possible management options, but they highlight the fact that a sound knowledge of the hydrodynamics is of paramount importance for a proper water quality oriented management. Reservoirs are very dynamic systems that often stratify in a multilayered fashion to produce layers of water with huge differences in their residence time [60]. In impaired reservoirs, the best option is to increase residence time of anoxic layers, and reduce it for surface water. The presence of outlets at different depths is essential in reservoirs devoted to water supply. 

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