A Comparison of Drought Assessment by the Aggregate Drought Index (ADI) and the Standardized Precipitation Index (SPI)

The Aggregate Drought Index (ADI) is a new index for measuring the severity of drought conditions, and is based on principal components analysis of monthly hydrologic data (Keyantash and Dracup 2004). The Standardized Precipitation Index (SPI) is a widely-used drought index that assesses the severity of meteorological drought based on probabilistic interpretation of precipitation shortages. Both indices may be used to describe drought over multiple timescales. The commonalities and differences for these two drought assessment approaches are compared for two climate divisions in Oregon-representing moist and dry environments-for water years from 1976 to present.