Multiple Timescale Comparison of the Aggregate Drought Index (ADI), the Standardized Precipitation Index (SPI), and Tree Rings in Southern California

The Aggregate Drought Index (ADI) [Keyantash and Dracup, 2004] is a drought index computed using the principal components of selected hydrological variables such as precipitation, evaporation, streamflow, reservoir storage, soil moisture, and alpine snowpack. Like the widely-used Standardized Precipitation Index (SPI), the ADI possesses the ability to simultaneously assess the shortage/abundance of water over a variety of desired timescales (e.g., 1 month, 12 month, 24 month, etc.). In this paper, the ADI is compared against the SPI, over multiple timescales, in the San Jacinto and Santa Ana River basins of Southern California. The comparisons occur between water years 1962 and 2011, an interval which spans three historic Southern California droughts (1976-77, 1987-1992, and 2007-2009). In both river basins, the drought indices are also compared against tree ring reconstructions of Big Cone Douglas Fir (Pseudotsuga macrocarpa), during a common crossover period in the latter 20th century. For historical perspective, the oldest tree ring reconstruction extends back to the year 1375.