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How long does it take for water to be evaporated then to be put back on Earth as precipitation?
Question Date: 2005-04-18
Answer 1:

You could actually calculate this yourself. The time that water (or any other substance) spends in one particular location (or reservoir) is called its residence time. You can calculate residence times for water in a bay, a river, the atmosphere, a glacier, the surface ocean, the deep ocean, etc. You can think of a residence time as the average time a molecule of water will spend in that location, or reservoir. You calculate residence times by dividing the total volume of water in that location by the rate of inflow or outflow of water to/from that location (assuming the inflow and outflow are equal and the volume isn't changing).

For example, to calculate the average time that a water molecule will spend in the atmosphere, you'd divide the total volume of water vapor in the atmosphere by some average evaporation or precipitation rate. Obviously, this will change depending on where you are in the world, but we can use average rates to get an estimate.
There's a couple good diagrams of the hydrologic (water) cycle at:Hydrosphere andHydrologic Cycle.
According to these web sites, the average time a water molecule spends in the atmosphere is 8-9 days. This is pretty fast! So on average, it takes just 8-9 days for a water molecule to evaporate, enter the atmosphere, and then leave it again as rain. Since the ocean is the largest source of water, it also the largest sink of water (most water vapor in the atmosphere comes from evaporation off the ocean surface, and most of the precipitation from the atmosphere falls back into the ocean).

The residence time for water in the ocean will be much longer. The second web site lists the residence time for water in the ocean as 3,000-4,000 years. This must be just the surface ocean, because I know that the average residence time for water in the entire ocean (surface plus deep) is more like 10,000 years. Can you think why water in the deep ocean would have a longer residence time than water at the surface of the ocean?

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