Palmer Drought Severity Index (PDSI)
Index name: Palmer Drought Severity Index (PDSI).
Ease of use: Yellow.
Origins: Developed in the 1960s as one of the first attempts to identify droughts using more than just precipitation data. Palmer was tasked with developing a method to incorporate temperature and precipitation data with water balance information to identify droughts in crop-producing regions of the United States. For many years, PDSI was the only operational drought index, and it is still very popular around the world.
Characteristics: Calculated using monthly temperature and precipitation data along with information on the water-holding capacity of soils. It takes into account moisture received (precipitation) as well as moisture stored in the soil, accounting for the potential loss of moisture due to temperature influences.
Input parameters: Monthly temperature and precipitation data. Information on the water-holding capacity of soils can be used, but defaults are also available. A serially complete record of temperature and precipitation is required.
Applications: Developed mainly as a way to identify droughts affecting agriculture, it has also been used for identifying and monitoring droughts associated with other types of impacts. With the longevity of PDSI, there are numerous examples of its use over the years.
Strengths: Used around the world, and the code and output are widely available. Scientific literature contains numerous papers related to PSDI. The use of soil data and a total water balance methodology makes it quite robust for identifying drought.
Weaknesses: The need for serially complete data may cause problems. PDSI has a timescale of approximately nine months, which leads to a lag in identifying drought conditions based upon simplification of the soil moisture component within the calculations. This lag may be up to several months, which is a drawback when trying to identify a rapidly emerging drought situation. Seasonal issues also exist, as the PDSI does not handle frozen precipitation or frozen soils well.
Alley, W.M., 1984: The Palmer Drought Severity Index: limitations and assumptions. Journal of Applied Meteorology, 23: 1100–1109.
Palmer, W.C., 1965: Meteorological Drought. Research Paper No. 45, US Weather Bureau, Washington, DC.