One explanation that I found was rainfall but it is not apparent that rainfall could contribute to such volume of water.
In: Geology
If there’s enough land higher than the spring, rain soaking the ground can come out well up a hill.
Otherwise, you get get [Artesian Aquifers](https://en.wikipedia.org/wiki/Artesian_aquifer) while are pressurized from higher land, possibly some distance away. A well or passage into the aquifer can yield water, even at the top of a hill.
More commonly, this is an illusion – if you’re at the bottom of a steep slope, observing a water coming down the slope from apparently the top, you may be completely unaware that there’s a large, less steeply sloped upland area behind it, which collects water.
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