Palmer Drought Severity Index (PDSI)

Overview:  The PDSI is a soil moisture algorithm which includes terms for water storage and evapotranspiration. The index is calibrated for relatively homogeneous land surfaces.

Who uses it:   Many U.S. government agencies and states rely on the PDSI to trigger drought relief programs.

Advantages:   A 'standardized' nature facilitates the quantitative comparison of drought incidence at different locations and at different times of the year. The PDSI is the first comprehensive drought index developed in the United States.

Disadvantages:   The PDSI may lag emerging droughts by several months. The empirical relationships used to define the PDSI were determined by observations taken from just nine US climate stations. The limited nature of the original PDSI source training data brings the general applicability of the PDSI into question.

Developed by:  W. C. Palmer in 1965.

Calculation:  A clear and detailed description of the steps required to calculate the PDSI may be found in Alley (1984).

References:

1. Palmer, W. C. 1965. Meteorological Drought. Research Paper No. 45. US Weather Bureau, Washington, D.C. 58pp.
   
2. Alley, W. M. 1984. The Palmer Drought Severity Index: Limitations and assumptions. Journal of Climate and Applied Meteorology, 23, 1100-1109.

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