I am wondering how one fills a mikve for the first time?
Most of the works I consulted on the topic (e.g., the very good document here or Artscroll Mishna Mikvaos) mention "forty seah of rainwater are gathered into a bor" but without details on how to accomplish this practically. Remember that for a kosher mikve, water has to flow naturally and cannot be drawn from containers.
I know it is possible to fill a mikve with ice (under very strict conditions) but assume that ice is not available everywhere, especially in summer months. And since I have seen opinions ranging from 575-1000 liters for forty seah, I assume waiting for rainfall to occur naturally during construction might take a long time, and not work at all for months in hot regions (e.g., Israel).
So what is practically being done?