So I started out with an observational exercise: looking at the difference in precipitation levels across the US over the past half century. For being so simple, it's actually pretty telling stuff, and it at least seems to match with observations people have made in the Southeaster-ly parts of the country, namely that many bodies of water were once regularly higher than they are today after our near-annual droughts.
Caveats: the precipitation data from the PRISM project is part of a research project that models precipitation for entire areas based on a small number of actual measurements; while that idea makes me put on my skeptical face, PRISM is apparently considered some of the best data out there. However, the data is not awesomely labeled; I believe the numbers in all the data files were in millimeters but wouldn't stake my life on it based on the lack of metadata. Furthermore (and here's the iffiest part).. the data files are averaged over a period of years. There's a file of averages from 2005-2009, 2000-2004, 1995-1999, 1990-1994, and.. 1961-1990. Yeah.
So the difference displayed by this map is between average precipitation levels during 2005-2009 and average precipitation levels during 1961-1990. No fancy intermediate averages or anything like that, just comparing the average levels during those two sets of time. Open it up and have a look!
