The residence time is effectively the average length of time that an entity, in this case a water molecule, will remain in a reservoir. It is really only meaningful for a reservoir that is at or near a steady state condition. By definition, the residence time is the amount of material in the reservoir, divided by either the inflow or the outflow they are equal when the reservoir is at equilibrium. If there are multiple inflows or outflows, then we use the sum of the outflows or inflows to determine the residence time.
For our bathtub system here, the residence time is thus 10 liters divided by 1 liter per second, which is equal to 10 seconds. On the other hand, water trapped within the pores of sediment, formation water, is, at least in the short term, isolated from the water cycle. This can either be water that was originally trapped in the sediments during their formation, or water that percolated into the rocks later.
Formation water is usually saline, mainly because most sediments are marine and the water trapped in the sediments would be seawater. All parts of the hydrosphere store water temporarily, and are called reservoirs. The study of water movement upon and beneath the ground and the physics and chemistry of the water is called hydrology.
What are the main ways in which water is transferred between the various reservoirs of the hydrosphere shown in Figure 2. Evaporation and transpiration , precipitation , runoff from land and underground flow of water to the ocean.
Water resides for different lengths of time in different reservoirs. The average length of time that water stays in a reservoir before moving to another is called the residence time for that reservoir Table 2. A hydrosphere reservoir empties and replenishes at the same rate, and the residence time is calculated from the rate of replenishment in relation to the volume of the reservoir.
There may be several ways in which water is transferred to and from a reservoir Figure 2. For example, the residence time for rivers, assuming the only significant transfer from rivers is by runoff:. Residence time is a concept that can be applied to any cyclical process, not just the water cycle.
Figure 2. The residence time gives an indication of how quickly water in a hydrosphere reservoir can be renewed. The shortest residence time , 11 days, is for water vapour in the atmosphere, which is continually renewed by evaporation from the oceans and the land, and is lost by precipitation. This is a rapid subcycle of the water cycle. Subcycles involving the oceans, the ice caps and underground water are much slower Table 2. There is a large volume of fresh water locked up in the polar ice caps Table 2.
It is not economic to transport this water at the moment, but it may become so in the future. Apart from the oceans and ice caps, the greatest volume of water is underground, stored in porous rocks below the Earth's surface. The shallower underground water moves quite quickly through the cycle and is fresh water, so it can be used for water resources.
But it is only a small proportion of the total underground water, and its residence time is relatively short, ranging from a few weeks to a few years. Underground water below a depth of a few hundred metres moves more slowly through the cycle, and residence times are much longer, up to ten thousand years Table 2.
Much of this water is saline either because it has had time to dissolve salts from the rocks, or because it originates from seawater. The world's lakes contain large volumes of water Table 2. However, over half of these lakes are saline Figure 2.
Rivers are very useful for water resources. Although they store very little water Table 2. This water is distributed very unevenly, as can be appreciated when we hear about the extensive water shortages and droughts in many parts of the world. In order to understand the problems of availability and distribution of water in more detail, we shall now look at the processes in the water cycle that transfer water between the reservoirs of the hydrosphere.
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