Heya everyone, today in school we were talking about water and the water cycle and the book said that the approximate time for one drop to go through the whole water cycle, as in from the ocean to the clouds to the ground and back to the ocean again is 42 days so we were all wondering how did they measure that time? Is it true and possible?
I don’t have any data myself, but it seems plausible. Though this is only an average for one particular place, of course. Some drops will take tens of years, some will evaporate immediately after they fall as rain. And the average will be different in different places.
I imagine what they did was take a valley. From weather records, you have a pretty good idea of the rainfall. You can measure the flow of the river leaving the valley and have a pretty good idea of the water flowing out. The difference is the amount of water evaporating, so you have a pretty good idea of that.
Then you look at the water table and depth to bedrock over the valley, the length and size of the river, and so forth, and come up with an estimate of the total amount of water sitting in the valley, above and below ground.
Now, since you know how much is there, and how much comes in and out, you can calculate how often, on average, it cycles.
The particular study the authors use may have been more complicated (accounting for the travel time of the water leaving by the river, how much time the vapor spends in the air, and so on), but that’s the gist of it.
The Yellowstone rangers tell us that water from the various thermal springs has a much longer cycle, though. For a particular droplet spewing from Old Faithful, the estimate is that it last saw the light of day five hundred years previous.