As developers in the Southwest fill in arroyos, we may hear talk of planning for 100-year storms. Many people, unfortunately including developers, don't understand what that really means, and it may mean scrimping on drainage. In the first place, when we talk of 100-year storms, we are basing this on an estimate—an estimate based on a very limited weather database as to how often severe storms can be expected in an area.
A second point seldom appreciated, though, is that even if such
estimates are correct, that does not mean that if we had a 100-year rain 10 years ago
that it will be 90 years before another such storm will occur. We might get such a
downpour tomorrow or it might be 150 years until the next one. The 100-year bit is an
average for over an extended period of time. Just as a .200 hitter in baseball might
get six hits in a row, there is no principle that says we can't get two 100-year
storms in the same year.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.