We were once sitting in the mountains in Russia and my colleague raised an interesting question - where does all the water in the underground rivers we investigated come from?
I thought all the water in rivers ultimately comes from rain - that's at least what I remembered from geography lessons decades ago. My friend argued rain wouldn't be enough...
Now I recalled that question and made a quick fact check.
The basin area of river Nalchik (that's where we were camping) is 440 km2. An average flow in the river is 2.7 m3/s. An average rainfall in the Kabardino-Balkar Republic is ~200 mm. All that is public data published by Rosgidromet.
Now if we take that 200 mm of rain per year on area of 440 km2, it makes roughtly 1.1e8 m3 of water. The water then flows for a year (365*24*3600 s) which makes... 2.7 m3/s.
Minor quibbles:
0.2 * 440e6 = 0.88e8 != 1.1e8
0.2 * 440e6 /(365*24*3600) = 2.79 != 2.7
But close enough :). It does seem like enough rain for the flow.
Thanks, that 1.1 was a typo when I copied from WolframAlpha! The final mismatch is understandable averages may be calculated for different periods for example, and these are natural processes which change over time. I was just curious if even the *order of magnitude* of these flow would match, and it seems it's matching much closer than I expected!
