Palmer drought severity index

(Abbreviated PDSI.) An index formulated by Palmer (1965) that compares the actual amount of precipitation received in an area during a specified period with the normal or average amount expected during that same period.

The PDSI is based on a procedure of hydrologic or water balance accounting by which excesses or deficiencies in moisture are determined in relation to average climatic values. Values taken into account in the calculation of the index include precipitation, potential and actual evapotranspiration, infiltration of water into a given soil zone, and runoff. This index builds on work by Thornthwaite (1931, 1948), adding 1) soil depth zones to better represent regional change in soil water-holding capacity; and 2) movement between soil zones and, hence, plant moisture stress, that is, too wet or too dry.
Palmer, W. C. 1965. Meteorological Drought Research Paper 45. U.S. Weather Bureau, Washington, D.C.
Thornthwaite, C. W. 1948. An approach toward a rational classification of climate. Geogr. Rev.. 38. 55–94.
Thornthwaite, C. W. 1931. Climates of North America according to a new classification. Geogr. Rev.. 21. 633–655.

Any function of time f(t) is periodic with period t if f(t) = f(t + τ) for all times t, where τ is the smallest number for which this equality holds.