Evaluating Long-term Drought in Arizona

In the past 130 years, Arizona has experienced three long-term droughts, with the current drought lasting the longest. Establishing and evaluating the magnitude of Arizona’s most recent drought was directed by the governor’s Drought Declaration in 2007. Arizona has an arid and semi-arid climate, and its statewide temperature has increased more than 1°C since 1895. Consequently, establishing drought levels in Arizona requires more than evaluating precipitation deficits, such as found with the Standardized Precipitation Index (SPI).

To account for the influence of radiative input on water loss in vapor form, long-term drought evaluation in Arizona analyzes the Standardized Precipitation Evapotranspiration Index (SPEI). From this evaluation, it was determined that the current Arizona drought began in 1994. Not every year since 1994 has been excessively dry in Arizona; however, out of the last 30 years, 20 years have experienced below average precipitation and higher temperatures, leading to SPEI deficits over multiple months.