How can I calculate the PDSI (Palmer Drought Severity Index)?
Short answer: the PDSI is not a formula you can drop into one Excel cell. It uses a two-layer soil-moisture water balance that needs monthly precipitation and temperature, the soil's available water holding capacity (AWC), potential evapotranspiration, and locally calibrated climatic coefficients, all solved month by month (Palmer, 1965). Because of that bookkeeping, a reliable spreadsheet is impractical — DMAP V2.1 computes the PDSI and 18 other drought indices automatically and exports the severity classes and charts. You can also request a calculation service.
The Palmer Drought Severity Index (PDSI) (Palmer, 1965), developed by Wayne C. Palmer in the early 1960's, was one of the first procedures to demonstrate success at quantifying the severity of droughts across different climates. The following explanation is based directly on Palmer's paper (Palmer, 1965), which describes in detail how to calculate the PDSI on a one-month time step. Each month of every year, four values related to the soil moisture are computed along with their complementary potential values. These eight values are evapotranspiration (ET), recharge (R), runoff (RO), loss (L), potential evapotranspiration (PE), potential recharge (PR), potential runoff (PRO), and potential loss (PL). The potential evapotranspiration is calculated using Rosenberg's description of Thornthwaite's method (Rosenberg, 1974). The calculation of these values depends heavily on the available water holding capacity (AWC) of the soil. The PDSI itself depends on a two-stage modeling of the soil. The top layer of soil is assumed to be able to hold one inch of moisture. The amount of moisture that can be held by the rest of the underlying soil is an input value, characteristic to that location, which must be fed into the program. Ref
