Where I live, Florida, we get about six feet of rain a year, significantly more than most regions. I think the only place that may get more precipitation might be tha Cascades, but I'm not really sure. Either way, if I weree to put a capture system on my roof, I could probably supply four or five families with water all year. Naturally that would just be drinking water, but you get the idea. In most areas, you can shoot a well point down fifty or sixty feet and hit clean, potable water.
The problem with water is one of storage. Where there is an abundance of water, much of it simply drains away, never to be used. While we cannot trap all water, we could probably trap more than we do now. In the Carribbean, cisterns are a common sight, since a lot of areas actually have a deartyh of rain. I lived on Antigua Air Ststion during a year long project and we had very, very little rain. The government ran a de-salination plant just to supply drinking water.
All of that said, my water investment plan is 55 gallon poly barrels under my downspouts to collect rain for drinking. When TSHTF, I will not be thinking about "investments" too much.