We have pink rain lilies (Zephyranthes carinata) in the garden. In my personal experience, I've found that they always flower, without exception, shortly after rains. However, there are roughly ten totally dry months each year. During this time, even though I water them regularly with well-water, they never flower. I've tried copious amounts of water, and frequent watering, but to no avail. They seem to depend on rain.
The rains here come at different times each year. We just had a totally unexpected rainstorm in May, causing them to flower. We are inland in southern India. It's very hot here, and the climate is quite dry in the periods when it's not raining.
How can this be? What's going on inside the plant, biologically speaking? How can it "know" whether it's getting rain water or well water? I'm looking for a description of the biological processes inside the plant which cause these different responses to rains versus watering.
Also, (minor, secondary point) is there any evolutionary reason why a plant would develop this trait?
I prefer answers that cite their sources for the information.
Edit: A user added some quite relevant personal observations in the form of an answer. I'm adding it here, as the observations are quite relevant to the question.
I live in central India which is marked by a heavy rainy season from mid June to September and almost dry rest of the year. This behavior of rain lilies has been a mystery to all of us for generations.
No matter what season, what temperature, the lily bush will give you at least one bloom even if rains for a mere few minutes. Rest of the time, any amount of watering does not help. I've seen the plant bloom with just a bit of drizzling in summers when temps cross 40 degrees celsius.
Whereas, a friend is growing them in a pot in a water lily pond. Basically, in shallow water where the rain lilies are just dipping their feet in water all the time. So basically, for a large portion of the year, the plant is meeting the low temperature, high water criterion. It doesn't work. The plants bloom only and only when it showers.
By the way, the behavior is not limited to pink rain lilies. It's the same with yellow and white lilies.