Why do pink rain lilies flower only after rains, and not from being watered?
It looks like sustained or consistent moisture might be (at least part of) the phenological cue for flowering:

The wikipedia article mentions that consistent humidity will induce flowering in at least some species and cites Fernández-Alonso &amp; Groenendijk (2004), which says:


  It generally flowers after the rainy periods, but in humid pastures and under cultivation conditions, colonies show staggered flowering during the whole year.


There's a reference in Lange et al. 2013 to Zephyranthes flowering after "watering of at least 10 h[ours]", but I'm not able to find the original citation in the google books preview (which is given at "Nitsch 1965").



Edit: I tracked down Nitsch 1965, who cites 2 sources on the topic. He reports Kerling 1949 as showing that Zephyranthes rosea after a certain period of development goes into dormancy, which is broken when two conditions are met: low temperature and high water, which must be extended over at least 10 hrs. He adds that Holdsworth (1961) presents evidence showing that Zephyranthes citrina and Z. tubispatha are primarily triggered by soil moisture.



Fernández-Alonso, J. L. &amp; J. P. Groenendijk (2004). A new species of Zephyranthes Herb. s. l. (Amaryllidaceae, Hippeastreae), with notes on the genus in Colombia. Rev. Acad. Colomb. Cienc. 28: 177-186

Holdsworth, M. (1961) The flowering of rain flowers. Journal of the West African Science Association 7︰28-36.

Kerling, L.C.P (1949) The gregarious flowering of Zephyranthes rosea Lindl. Annals of the Botanic. Gardens. Buitenzorg 51: 1-41

Nitsch, J. P. (1965). Physiology of flower and fruit development. pp 1537-1647 in Encyclopedia of Plant Physiology Vol XV Part 1, W. Ruhland (ed.)

O. L. Lange, P. S. Nobel, C. B. Osmond, H. Ziegler (2013). Physiological Plant Ecology III: Responses to the Chemical and Biological Environment. Springer.