"Drought is a natural hazard with potentially significant economic, social, and ecological consequences. History suggests that severe and extended droughts are inevitable and part of natural climate cycles. Drought has for centuries shaped the societies of North America and will continue to do so into the future. The likelihood of extended periods of severe drought and its effects on 21st-century society in the United States raise several issues for Congress. These issues include how to respond to recurrent drought incidents, how to prepare for future drought, and how to coordinate federal agency actions, among other policy choices. [...] This report discusses why drought occurs in the United States; how drought is defined (e.g., why drought in one region of the country is different from drought in another region); and how droughts are classified and reported (e.g., what is meant by moderate, severe, extreme, and exceptional drought). The report briefly describes periods of drought in the country's past that equaled or exceeded drought conditions experienced during the 20th century, and the effects of those droughts on major western rivers such as the Colorado. Lastly, the report discusses the possible influence of human-induced climate change on drought, and current limits on our understanding for forecasting droughts."
CRS Report for Congress, R43407
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html