There have been periodic droughts in the American southwest for millenia.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/roman-era-drought-southwest-usa
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/environment-news-network
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/united-states-america-us
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/natural-disasters
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/drought
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/pine
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/trees
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/history